| 1. |
- Antón Remírez, Raúl, 1977-
(författare)
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Experimental and numerical study of the thermal and hydraulic effect of EMC screens in radio base stations : detailed and compact models
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Today’s telecommunication cabinets use Electro Magnetic Compliance (EMC) screens in order to reduce electromagnetic noise that can cause some miss functions in electronic equipment. Many radio base stations (RBSs) use a 90-degree building architecture: the flow inlet is perpendicular to the EMC screen, which creates a complex flow, with a 90-degree air turn, expansions, compressions, perforated plates and PCBs. It is of great interest to study how the EMC screen interacts with the rest of components and analyze the total pressure drop and how much the flow pattern changes due to the placement of the screen. Velocity, pressure and temperature measurements as well as flow pattern visualizations have been carried out to gain good insight into the flow and heat transfer characteristics in a subrack model of an RBS. Furthermore, these measurements have been very useful for validating detailed CFD models and evaluating several turbulence models. Nowadays, industrial competition has caused a substantial decrease in the time-to-market of products. This fact makes the use of compact models in the first stages of the design process of vital importance. Accurate and fast compact models can to a great extent decrease the time for design, and thus for production. Hence, to determine the correlations between the pressure drop and flow pattern on the PCBs as a function of the geometry and the Reynolds number, based on a detailed CFD parametric study, was one objective. Furthermore, the development of a compact model using a porous media approach (using two directional-loss coefficients) has been accomplished. Two correlations of these directional loss coefficients were found as a function of the geometry and Reynolds number.
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| 2. |
- Anwar, Zahid, 1981-
(författare)
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Flow boiling heat transfer, pressure drop and dryout characteristics of low GWP refrigerants in a vertical mini-channel
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Two-phase heat transfer in mini/micro-channels is capable of meeting the high cooling demands of modern high heat flux applications. The phase change process ensures better temperature uniformity and control for local hot spots. Furthermore, these compact channels could be helpful in reducing the required charge and material inventories.Environmental concerns—mainly ozone depletion and global warming—have instigated a search for new alternatives in refrigeration industry. While new compounds are being developed to address stringent legislative demands, natural alternatives are also coming into prominence. A limited number of investigators have reported on thermal performance of such alternatives. The current study is therefore focused on saturated flow boiling heat transfer, pressure drop and dryout characteristics for three low global warming potential (GWP) refrigerants (R152a, R600a and R1234yf) in a vertical mini-channel.In this study experiments were carried out by uniformly heating a test section (stainless steel tube with 1.60 mm inside diameter and 245 mm heated length) at 27 and 32 oC saturation temperature with 50-500 kg/m2s mass velocities. The effects of various parameters of interest (like heat flux, mass flux, system pressure, vapor quality, operating media) on flow boiling heat transfer, frictional pressure drop and dryout characteristics were recorded. R134a, which has been widely used in several applications, is utilized as a reference case for comparison of thermal performance in this study.Experimental results for saturated boiling heat transfer showed strong influence of heat flux and system pressure with insignificant contributions from mass flux and vapor quality. Two phase frictional pressure drop increased with mass flux, vapor quality and with reduced operating pressure. The dryout heat flux remained unaffected with variation in saturation temperature, critical vapor quality in most cases was about 85%. The experimental results (boiling heat transfer, two-phase pressure drop and dryout heat flux) were compared with well-known macro and micro-scale correlations from the literature.
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| 3. |
- Arias, Jaime, 1957-
(författare)
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Energy Usage in Supermarkets - Modelling and Field Measurements
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis investigates a special type of energy system, namely energy use in supermarkets through modelling, simulations and field studies. A user-friendly computer program, CyberMart, which calculates the total energy performance of a supermarket, is presented. The modelling method described in this thesis has four phases: the first phase is the de-velopment of a conceptual model that includes its objectives, the envi-ronment and the components of the system, and their interconnections. The second phase is a quantitative model in which the ideas from the conceptual model are transformed into mathematical and physical rela-tionships. The third phase is an evaluation of the model with a sensitivity analysis of its predictions and comparisons between the computer model and results from field measurements. The fourth phase is the model ap-plication in which the computer model answers questions identified in the beginning of the modelling process as well as other questions arising throughout the work. Field measurements in seven different supermarkets in Sweden were car-ried out to: (i) investigate the most important parameters that influence energy performance in supermarkets, (ii) analyse the operation of new system designs with indirect system implementation in Sweden during recent years, and (iii) validate the computer model. A thorough sensitivity analysis shows a total sensitivity of 5.6 %, which is a satisfactory result given a 10% change in the majority of input parame-ters and assumptions, with the exception of outdoor temperatures and solar radiation that were calculated as extreme values in METEO-NORM. Comparisons between measurements and simulations in five supermarkets also show a good agreement. Measurements and simula-tion results for a whole year were not possible due to lack of data. CyberMart opens up perspectives for designers and engineers in the field by providing innovative opportunities for assessment and testing of new energy efficient measures but also for evaluation of different already-installed system designs and components. The implementation of new energy-saving technologies in supermarkets requires an extensive inte-grated analysis of the energy performances of the refrigeration system, HVAC system, lighting, equipment, and the total energy usage. This analysis should be done over a long period, to evaluate and compare the real energy performance with the theoretical values calculated by Cyber-Mart.
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| 4. |
- Bekele, Getachew, 1958-
(författare)
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Study into the Potential and Feasibility of a Standalone Solar-Wind Hybrid Electric Energy Supply System
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The tendency to use renewable energy resources has grown continuouslyover the past few decades, be it due to fear over warnings of globalwarming or because of the depletion and short life of fossil fuels or evenas a result of the interest which has developed among researchers doingscientific research into it. This work can be considered as joining any ofthese groups with an objective of giving electric light to the poorpopulation living in one of the poorest nations in the world.The aim of the work is to investigate supplying electric energy fromsolar-wind hybrid resources to remotely located communities detachedfrom the main grid line in Ethiopia. The communities in mind are one oftwo types; the first is the majority of the poor population residing in thecountryside; and the other is people relocated by the Government fromthe over used and dry regions to relatively productive and fertile ones inline with the long-term poverty reduction plan.The work was begun by investigating wind energy and solar energypotentials at four geographically different locations in Ethiopia bycompiling data from different sources and analyzing it using a softwaretool. The locations are Addis Ababa (09:02N, 038:42E), Mekele (13:33N,39:30E), Nazret (08:32N, 039:22E), and Debrezeit (8:44N, 39:02E).The results related to wind energy potential are given in terms of themonthly Average wind speed, the wind speed probability densityfunction (PDF), the wind speed cumulative density function (CDF), thewind speed duration curve (DC), and power density plots for all fourselected sites. According to the results obtained through the analysis, thewind energy potential, even if it is not exceptional, is irrefutably highenough to be exploited for generating electric energy.The solar energy potential, based on sunshine duration data collectedover a period of 7 - 11 years and radiation data obtained from differentsources, has been calculated using regression coefficients specific to thesites in question. Based on the sunshine duration data, the monthlyaverage daily sunshine amount for each of the places has also beencomputed and given in a form of plot. Through additional work on theresults of the calculations, the solar energy potential has been given inthe form of solar radiation plots for each of the selected sites. Asexpected, the results indicated an abundance of solar energy potential.It is based on the promising findings of these two energy resourcepotentials, wind and solar, that the feasibility study for a standalonesolar-wind hybrid energy supply system has proceeded, targeting thecommunity mentioned earlier. The hybrid system consisted of Windturbine, Photovoltaic panel, diesel generator and a bank of batteries, witha power conditioning converter included in the system.The hybrid standalone supply system is intended to provide electricity toa model community of 200 families with five to six family members ineach. The community is equipped with a primary load, a deferrable load,a community school and a health post. An electric load which includeslighting, water pumping, a radio receiver, and some clinical equipmenthas been suggested. Hybrid Optimization Model for ElectricRenewables, HOMER, software has been used for the analysis. Theaverage wind speed and average solar radiation calculated from the datafor all of the selected sites has been used to input into the software.The hybrid system design is approached in three different ways. The firstapproach is to include within the hybrid system those components whichare locally available, without giving special attention to their efficienciesand proceed with the design work. The second approach is tothoroughly search the market for the best and most efficienttechnological products and to select the best components for theanalysis. A third approach considered in an attempt of cost minimizationis to see if a self-contained type of design can be a better solution. Whatthis means is every household will have its own supply system that mayconsist of any combination of PV and wind turbine including converter,battery and charge controller.After running the simulations, lists of power supply systems have beengenerated, sorted according to their net present cost. Sensitivity variables,such as range of wind speeds, range of radiation levels and diesel pricehave been defined as inputs into the software and the optimizationprocess has been carried out repeatedly for the sensitivity variables andthe results have been refined accordingly.
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| 5. |
- Bitaraf Hgahighi, Ehsan, 1980-
(författare)
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Single Phase Convective Heat Transfer with Nanofluids : An Experimental Approach
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nanofluids (NFs) are engineered colloids of nanoparticles (NPs) dispersed homogenously within base fluids (BFs). Due to the presence of NPs, the thermophysical and transport properties of BFs are subject to change. Existing technologies for cooling electronics seem to be insufficient and NFs, as reported in several studies, might offer a better alternative to liquid cooling. The main purpose of this study, by choosing a critical approach to existing knowledge in the literature, is to investigate experimentally the potential for replacing BFs with NFs in single–phase flow. Several NFs (mainly water based metal oxide NFs) were synthesised, and different experiments (including thermal conductivity, viscosity, heat transfer coefficient, and shelf stability) were performed.The thermal conductivity and the viscosity of several NFs were measured at both near room and elevated temperatures; the results are reported and compared with some correlations. It is shown that the Maxwell model for thermal conductivity and the modified Krieger–Dougherty model for viscosity can be used to predict these properties of NFs within ±10% error, even at elevated temperatures.A screening setup, including a test section with d = 0.5 mm and L = 30 cm, was designed for measuring the heat transfer performance of NFs in laminar flow. In addition a closed–loop setup with a 3.7 mm inner diameter and 1.5 m length test section was also designed to measure the heat transfer coefficients in both laminar and turbulent flow with higher accuracy. Based on the results, classical correlations for predicting Nusselt number and friction factor in a straight tube are still valid for NFs within ± (10 – 20)% error provided that the correct thermophysical properties are used for NFs.Different methods of comparing cooling performance of NFs to BFs are then investigated. Comparison at equal Reynolds number, the most popular method in the literature, is demonstrated both experimentally and analytically to be misleading. However, if the most correct criterion (at equal pumping power) is chosen, a small advantage for some NFs over their BFs should be expected only under laminar flow. The investigation concludes with the proposition of a unique method and apparatus to estimate the shelf stability of NFs.
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| 6. |
- Björk, Erik, 1965-
(författare)
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Energy Efficiency Improvements in Household Refrigeration Cooling Systems
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is based on eight articles all related to the characteristics of the cooling system and plate evaporator of a household refrigerator. Through these articles, knowledge is provided that can be used to increase the operational efficiency in household refrigeration. Papers A, B and C focus on heat transfer and pressure drop in a commonly used free convection evaporator – the plate evaporator. Applicable correlations are suggested on how to estimate the air side heat transfer, the refrigerant side pressure drop and the refrigerant side heat transfer. Papers D, E and F hold a unique experimental study of the refrigerant charge distribution in the cooling system at transient and steady state conditions. From this cyclic losses are identified and estimated and ways to overcome them are suggested. In paper G the topic “charging and throttling” is investigated in an unparalleled experimental study based on more than 600 data points at different quantities of charge and expansions device capacities. It results in recommendations on how to optimize the capillary tube length and the quantity of refrigerant charge. Finally, Paper H holds a thermographic study of the overall cooling system operating at transient conditions. Overall, a potential to lower the energy use by as much as 25 % was identified in the refrigerator studied. About 10 % was found on the evaporator’s air side. 1-2 % was identified as losses related to the edge effect of the evaporator plate. About 8 % was estimated to be cyclic losses. About 5 % was found in cycle length optimization. It is believed that most of these findings are of general interest for the whole field of household refrigeration even though the results come from one type of refrigerator. Suggestions of simple means to reduce the losses without increasing the unit price are provided within the thesis
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| 7. |
- Bohdanowicz, Paulina
(författare)
-
A Study of Environmental Impacts, Environmental Awareness and Pro-Ecological Initiatives in the Hotel Industry
- 2003
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Throughout the millennia commercial lodging establishmentshave evolved to what is today known as the hospitalityindustry. Hotels constitute one of the main, and stillexpanding, pillars of this sector and are highly unique amongother commercial buildings. They are designed to providecomfort and services twenty-four/seven. The success of tourism,and indirectly the hotel industry, largely depends on theavailability of a clean natural environment. At the same time,the environment is the major recipient of negative impactscreated by the construction and operation of hotel facilities.For many years, the industry was very reluctant to admit itsinfluence on the natural surroundings, but recently attitudeshave started to change.An e-mail based survey of environmental attitudes has beenperformed among more than 4000 European hoteliers, yielding 610responses. A similar questionnaire has further been distributedamong the guests of chosen hotels resulting in 141 answers fromfour Scandinavian hotels. Surveys revealed that both Europeanhoteliers and Scandinavian hotel guests perceive theenvironment as an important factor in the development andsuccess of tourism. Hoteliers are typically aware that theirfacilities influence the natural surroundings, a view sharedalso by hotel patrons, although the magnitude of the impacts isoften underestimated. Hotel managers and staff do have acertain (though varying) level of environmental knowledge andare generally aware of measures that can be taken towardsgreater environmental responsibility.Despite the fear shared by many hoteliers, thatenvironmental practices may negatively affect customer comfortand satisfaction, more than 75% of hotel guests interviewedexpressed their support and willingness to participate in manysuch initiatives. As hotel patrons are gradually demanding“green alternatives”, hoteliers are becomingincreasingly motivated and willing to take steps towardsgreater environmental responsibility. Almost 25% of hotelguests interviewed declared their willingness to pay more foraccommodation in an eco-certified facility.More than 70% of hotels investigated have already introducedpro-ecological initiatives, primarily in the areas of energyand water conservation, as well as responsible wastemanagement. The areas targeted are typically those, which canyield company benefits in a relatively short time.The results of the survey confirmed that the possibility ofa significant decreaseof the operational costs, and theexistence of an obvious and continuous customer demand, maylikely prompt hoteliers to respond with greater environmentalresponsibility. There is undoubtedly a great need for enhancingof education and environmental awareness among hotelrepresentatives and the general public. Given its profile thehotel industry can indisputably become a significant venue ofsuch education by communicating its environmental commitment tocustomers and inter-related industries.Keywords:hotels, environmental impacts, mitigationmeasures, survey, hoteliers, hotel customers, environmentalattitudes, environmental awareness, pro-ecological activities,incentives.
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| 8. |
- Bohdanowicz, Paulina, 1976-
(författare)
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Responsible resource management in hotels : attitudes, indicators, tools and strategies
- 2006
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Hotels constitute one of the main, and still expanding, pillars of the tourism sector and are highly unique among other commercial buildings. Resource intensive and frequently inefficient systems and operational routines applied in the sector, result in considerable environmental impact and indicate an urgent need for more environmentally sound practices and products in the hotel industry. A certain level of activity in the area of reducing resource use has been observed for quite some time but the motivation for this was related to cost-benefit issues rather than the environment. Furthermore, most of the initiatives are still considered to be best practice case studies and not daily routines. The constantly increasing prices of basic commodities, such as energy resources and water, encourage the implementation of energy and water efficiency and conservation measures in hotel facilities. To this end, numerous guidelines and initiatives have been produced by hotel-related organisations. The study of attitudes among 610 European hoteliers indicated that, at the moment, the level of environmental awareness among hotel managers is not high enough to introduce significant changes, although attitudes differ depending on the country of origin and the corporate policy. The prospects of significant cost savings, as well as customer demand were identified as the most likely parameters to enhance environmental responsibility among hoteliers. The popularity of CSR reporting and sustainability indices, especially among the larger companies, leads the author to believe that the industry is preparing to change. Many of the companies running hotels are also reporting their environmental management goals and indicators of environmental performance, while benchmarking and all types of comparison league tables are gaining popularity. There is, however, no system or methodology of data collection and monitoring that is universally accepted or applicable throughout the hotel sector. Furthermore, the accuracy and validity of the published performance indicators and benchmarks is widely debated due to large variations in the figures reported. The analyses performed on data from over 180 upscale and mid-market European chain hotels (Hilton International and Scandic) indicated that even among relatively uniform (service-wise) hotels the amenities offered did influence consumption, further complicating the concept and applicability of uniform benchmarking and resource consumption modelling. It was thus concluded that, creating a uniform model for all hotels is almost impossible, or would require a significant amount of very detailed input data, and that the results could still be highly inaccurate. Instead, it was suggested that it might actually be more accurate to develop models for individual hotels. Such an approach would allow for the modelling of the behaviour of all types of hotels with no size, type of services or standard limitations. Multi-variate step-wise regression analyses performed on individual Scandic hotels in Sweden indicated that energy consumption was dependent on the outdoor air temperature/actual heating degree days, while water consumption was mainly influenced by the number of guest-nights sold. Experience gained by the author during the process of the Hilton Environmental Reporting system upgrade allowed for the formulation of a set of rules of thumb that ought to be followed in the design of similar schemes. The experience of various regions and numerous companies also suggests that well designed and implemented environmental and resource management programs bring significant benefits at an individual as well as a corporate level. This study provides an indication of the strategies that can be used by various stakeholders in the process of development and implementation of such programs. This research further suggests that future developments in environmental performance indicators and benchmarking may best be served by disaggregating hotels into modules (such as guest-rooms, catering outlets, conference centres) and developing consumption models and best practice indicators for these particular components. The individual efficiencies/performance indicators should then be combined and weighted to provide a just overall evaluation of a facility that could then be compared to a benchmark (developed in the same manner). In a similar manner, sustainability assessment schemes and indicators for destinations and communities could be developed. However, more accurate and differentiated knowledge of the individual performance of sub-systems is necessary to proceed with such an alternative. In addition, the methodologies for data collection and reporting procedures, at all levels of the company and sector, need to be standardized and detailed
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| 9. |
- Chen, Jianyong, 1983-
(författare)
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Investigation of Vapor Ejectors in Heat Driven Ejector Refrigeration Systems
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Refrigeration systems, air-conditioning units and heat pumps have been recognized as indispensable machines in human life, and are used for e.g. food storage, provision of thermal comfort. These machines are dominated by the vapor compression refrigeration system and consume a large percentage of world-wide electricity output. Moreover, CO2 emissions related to the heating and cooling processes contribute significantly to the total amount of CO2 emission from energy use. The ejector refrigeration system (ERS) has been considered as a quite interesting system that can be driven by sustainable and renewable thermal energy, like solar energy, and low-grade waste heat, consequently, reducing the electricity use. The system has some other remarkable merits, such as being simple and reliable, having low initial and running cost with long lifetime, and providing the possibility of using environmentally-friendly refrigerants, which make it very attractive. The ERS has received extensive attention theoretically and experimentally.This thesis describes in-depth investigations of vapor ejectors in the ERS to discover more details. An ejector model is proposed to determine the system performance and obtain the required area ratio of the ejector by introducing three ejector efficiencies. Based on this ejector model, the characteristics of the vapor ejector and the ERS are investigated from different perspectives.The working fluid significantly influences the ejector behavior and system performance as well as the ejector design. No perfect working fluid that satisfies all the criteria of the ERS can be found. The performance of nine refrigerants has been parametrically compared in the ERS. Based on the slope of the vapor saturation curve in a T-s diagram, the working fluids can be divided into three categories: wet, dry and isentropic. A wet fluid has a negative slope of the vapor saturation curve in the T-s diagram. An isentropic expansion process from a saturated vapor state will make the state after the expansion to fall inside the liquid-vapor area of the T-s diagram which will result in droplet formation. Generally, an isentropic expansion for a dry fluid will not occur inside the liquid-vapor area, and consequently no droplets will form. An isentropic fluid has a vertical slope of the vapor saturation curve in the T-s diagram and an isentropic expansion process will hence follow the vapor saturation curve in the T-s diagram, ideally without any droplet formation. However, when the saturation condition is close to the critical point, it is possible that the isentropic expansion process of a dry fluid and an isentropic fluid occurs inside the liquid-vapor area of the T-s diagram, resulting in formation of droplets. In order to avoid droplet formation during the expansion, a minimum required superheat of the primary flow has been introduced before the nozzle inlet. Results show that the dry fluids have generally better performance than the wet fluids and the isentropic fluid. Hence the thesis mostly focuses on the features of vapor ejectors and the ERS using dry fluids.Exergy analysis has been proven to be very useful to identify the location, magnitude, and sources of exergy destruction and exergy loss, and to determine the possibilities of system performance improvement. This method is applied to the ejector and the ERS. The ejector parameters are closely interacting. The operating condition and the ejector area ratio have a great impact on the ejector overall efficiency and system COP. The ejector efficiencies are sensitive to the operating conditions, and they significantly influence the system performance. A so-called advanced exergy analysis is adopted to quantify the interactions among the ERS components and to evaluate the realistic potential of improvement. The results indicate that, at the studied operating condition, the ejector should have the highest priority to be improved, followed by the condenser, and then the generator.Thermoeconomics, which combines the thermodynamic analysis and economic principles, is applied to reveal new terms of interest of the ERS. The economic costs of the brine side fluids (fluids that supply heat to the generator and evaporator and remove heat from the condenser) play very essential roles in the thermoeconomic optimization of the ERS. Depending on different economic conditions, the system improvement from a thermodynamic point of view could be quite different from the thermoeconomic optimization. The ERS is economically sound when using free heat sources and heat sink.An ejector test bench has been built to test the entrainment ratio of different ejectors. Although the experiments do not achieve the desired results, they could still be discussed. The insignificant effect of the superheat of the secondary flow found in the theoretical study is validated. The assumption of neglecting the velocities at the ejector inlets and outlet are confirmed. The quantification of the ejector efficiencies shows that they largely depend on the operating conditions and the ejector dimensions.
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| 10. |
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| 11. |
- Chen, Yang
(författare)
-
Novel cycles using carbon dioxide as working fluid : new ways to utilize energy from low-grade heat sources
- 2006
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This licentiate thesis proposes and analyzes three carbon dioxide novel cycles, namely: the carbon dioxide transcritical power cycle, the carbon dioxide Brayton cycle and the carbon dioxide cooling and power combined cycle. Due to the different characteristics of each cycle, the three cycles are suitable for different applications. The CO2 transcritical power cycle is suitable for harvesting energy from low-grade heat sources, near which a low temperature heat sink is accessible. The CO2 Brayton cycle is suitable for harvesting the energy from relatively high-grade heat sources when there is no low temperature heat sink available. The CO2 cooling and power combined cycle is suitable for applications, where both power and cooling are needed (e.g. automobile applications, in which the cycle can utilize the energy in the engine exhaust gasses to produce power and provide cooling/heating to the mobile compartment room at the same time). Several models have been developed using the software known as Engineering Equation Solver (EES)1 for both cycle analysis and computer aided heat exchanger design. Different cycle working conditions have been simulated and different working parameters’ influence on the cycle performance has been explained. In addition, Refprop 7.02 is used for calculating the working fluid properties and the CFD tool Femlab has been employed to investigate the particular phenomena influencing the heat exchanger performance.
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| 12. |
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| 13. |
- Claesson, Joachim, 1970-
(författare)
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Thermal and hydraulic performance of compact brazed plate heat exchangers operating as evaporators in domestic heat pumps
- 2005
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis investigates the performance of compact brazed plate heat exchangers (CBE) operating as evaporator in heat pump applications. The thesis, and the performances investigated, has been divided into three main sections; One zone evaporator performance; Two zone evaporator performance; and finally Local performance. The 'One zone evaporator performance' section considers the evaporator as one "black box". It was found that "approaching terminal temperatures" were obtained as low overall heat flux is employed. It was also found that the total area averaged film heat transfer coefficient was affected by changes of the brine mass flow rate. This indicates that the widespread Wilson plot method may not be used to determine flow boiling heat transfer coefficients. Further, it seems that co- and counter-current flow configuration performs equally well if the superheat is kept low. A numerical simulation of the above investigations indicates that a nucleate boiling model better predicts the performance compared to a convective evaporation model. Finally, the significant impact of the refrigerant inlet distributor design was illustrated using several CBEs with different inlet geometries but with identical heat transfer surfaces. The 'Two zone evaporator performance section' considers the evaporator as two "black boxes", i.e. the boiling and superheating boxes. Thermochromic liquid crystals (TLC) was used to determine the boiling heat transfer area. The resulting flow boiling heat transfer coefficient was found to correlate with heat flux. The superheated heat transfer area was then estimated using single phase correlations. It was observed that the TLC measurements and the predicted superheating area did not agree. Possible causes for this deviation were discussed. The most likely explanation found was the presence of mist flow at the higher vapor quality range in the boiling section of the evaporator. The 'Local Performance' section considers local pressure drop and flow boiling heat transfer. The Chisholm parameter was found not to be a constant and was found to correlate well with the kinetic energy per volume. The resulting predictions of the pressure drop were better than ± 10%. The resulting local flow boiling heat transfer coefficient, at different vapor quality, mass flux and heat flux, was compared to flow boiling correlations available in the literature. It was found that the saturated nucleate pool boiling correlation by Cooper (1984) and narrow channel flow boiling correlations (Tran 1999, Lazarek and Black 1982) predicted the experimental data better than several traditional flow boiling correlations, developed for larger tubes.
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| 14. |
- Fernando, W. Primal D., 1969-
(författare)
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Experimental Investigation of Refrigerant Charge Minimisation of a Small Capacity Heat Pump
- 2007
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Enormous quantities of heat are available in air, soil, water, exhaust air from buildings, and in waste water of any kind. However these heat sources are use-less for heating purposes since their temperatures are lower than the tempera-ture required for heating. Heat pumps can be used to extract heat from these sources with a small expenditure of additional energy and up-grade and deliver the energy as useful heat for room heating. The heat pump cycle employs the well-known vapour compression cycle. The amount of heat delivered by a heat pump is equal to the amount of energy extracted from the heat source plus the heat equivalent to the compression work of the heat pump. Heat pumps, of course, are being generally accepted as outstanding energy saving units due their coefficient of performance (COP). Heat pumps for house heating have been used extensively in many countries and are especially common in Sweden. The annual growth rate of heat pump usage in Sweden is the same as in rest of Europe. According to the Swedish heat pump association, between 1986 to August 2003, the number of installed heat pump units in Sweden was 332,309. The demand for heat pumps started to increase from the year 1995 and in the year 2002, approximately 40,000 heat pump units were installed. Among the many types available, single-family heat pumps providing heating capacity of about 5 kW are widely popular. The main drawbacks of heat pumps are the complexity of the systems, high cost, need of technical knowledge, safety hazards and environmental effects of certain refrigerants, etc. An efficient heat pump with small refrigerant charge would have less of some of these drawbacks and could be a competitive alterna-tive to other heating processes. In this study, methods of refrigerant charge minimisation without reducing the performance of a small capacity (5 kW) heat pump have been investigated. Work has been focused on finding refrigerant charge distribution in different components of the heat pump, on finding out the solubility of refrigerant (pro-pane) with different compressor lubrications oils, on testing different types of compact heat exchangers, on constructing new minichannel heat exchangers and on finding correlations for calculating the heat transfer of minichannel heat exchangers. The results included in this thesis have been presented in four con-ference papers and five journal papers of which two were published and three were submitted for publication.
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| 15. |
- Fischer, David
(författare)
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Integrating Heat Pumps into Smart Grids
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Transforming our energy system towards 100% renewable energy sources requires radical changes across all energy sectors. Heat pumps as efficient heat generation technology link the electricity and heat sector. From 2010 to 2015 between 750,000 and 800,000 heat pumps have been sold every year across Europe. Those heat pumps, when connected to thermal storage or using the buildings’ thermal inertia, have the potential to offer demand flexibility to the power system. In a renewable, interconnected and to a large extend decentralised energy system - the smart grid - heat pumps can be operated according to the need of the electric power system. This might impact heat pump system design, controls and operation, which is investigated in this thesis. The main objective of this thesis is to add knowledge and suggest methods to facilitate the transition towards a renewable and smart energy system, in which heat pump systems and their flexibility are used and designed in an optimal way. Therefore this thesis investigates the integration of heat pumps in a smart grid on three different system boundary levels. On each boundary level the focus is on different aspects and different methods are used. On the highest boundary level, the integration of heat pumps into a smart grid and the resulting requirements for heat pump system design are studied. Results of a literature study show, that currently discussed applications of heat pumps in a smart grid focus on the provision of ancillary services, the integration of renewable energy sources, and the operation under time variable electricity prices. Integrating heat pumps into the power system can be achieved by direct, indirect and agent based control strategies. The next level of investigation covers the aggregation of heat pumps into pools. For this purpose a stochastic bottom-up model for heat pump pools has been developed. This model accounts for the diversity of buildings, heat pump systems and occupants. Results of a simulation study of a heat pump pool highlight the fact that flexibility of heat pumps is not constant and is changing during the course of the day and year. A characteristic response of a heat pump pool towards direct load control signals is identified and shows three characteristic phases 1) charging/ activation phase, 2) steady state phase, 3) discharging/regeneration phase. It is found that the duration of the control signal and the load shift strategy implemented in the heat pump systems are decisive for flexibility. Further it is shown that flexibility might come at the cost of efficiency of the local heat pump systems. On the level of individual buildings this thesis explores to which extent the sizing of heat pumps, storage and back-up heater as well as system controls have to be adjusted when integrating heat pumps into a smart grid. Results of a structural optimisation study, targeting to minimise total cost of ownership, show that sizing of the heat pump unit and the electric back-up heater remain almost unchanged when PV and time variable electricity prices are introduced. However an increase in storage capacity is beneficial to profit from time variable prices or onsite photovoltaic (PV). It showed that the ways heat pumps and storages are sized in Germany today provided sufficient storage capacity for most of the investigated scenarios. Furthermore increasing storage leads to diminishing returns as investment costs and system losses increase with increasing storage size. This leads to the conclusion that local heat pump system efficiency as well as flexibility requirements of the power system should be considered, when designing heat pump systems. Improving the controls shows great potential for increasing heat pump system efficiency, reducing operation cost and scheduling heat pump operation along to match the requirements from the power system. A dynamic building simulation study, where rule-based, predictive rule-based and model predictive control approaches were compared, reveals that the use of model predictive controls can reduce annual electricity cost and increase PV self-consumption significantly, compared to tailored rule-based and predictive rule-based control approaches. When deciding upon a control strategy the following should be taken into account: complexity of design, robustness against changes in external conditions and computational resources. It is shown that operating heat pumps in a smart grid changes operating hours, temperatures, on/off cycles and seasonal performance compared to today’s heatdriven operation. It is shown that the goals to reduce operating cost, maximise system efficiency or increase PV self-consumption can be conflicting and are often impossible to achieve simultaneously. Not necessarily will operation in a smart grid increase the efficiency of individual systems, rather offers the possibility to increase efficiency of the overall energy system. It is found that sizing, controls and use-case are interconnected and should be considered simultaneously in the design process of heat pump systems. A goal for future research should be the design of optimum flexible heat pump systems, where the heat pump unit, the building, the hydraulic system, heat distribution, storage and controls are designed optimally for the flexibility requirements of both the end-users and the power system.
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| 16. |
- Furberg, Richard
(författare)
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Enhanced boiling heat transfer from a novel nanodendritic micro-porous copper structure
- 2006
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Following licentiate thesis is a summary of the advances made within the research project - Micro- and nano structured surfaces for enhanced boiling heat transfer – which is a collaboration effort between the Divi-sion of Applied Thermodynamics and Refrigeration and the Division of Materials Chemistry at the Royal Institute of Technology (KTH). The main objectives with this research project has been to: develop methods for producing highly efficient boiling surfaces with well defined micro- and nano-structured porous surfaces by the use of micro- and nano-manufacturing techniques. This objective has been achieved and the result is a novel micro-porous surface structure comprising dendritically ordered nano-particles of cop-per. The structure was fabricated by a high-current-density electrode-position process, in which the evolution of hydrogen bubbles serve as a dynamic masking template to the growth of the dendritic copper struc-ture. Important variables were identified that affect the production of the structure and its features, such as surface orientation during electrode-position, pressure and temperature of electrolyte, and a final heat treat-ment of the surface under reduced atmosphere, all of which have previ-ously not been reported on. Experimental tests have been conducted in a widely used refrigerant, R134a, where the micro-porous structure was shown to enhance the boiling performance of a copper surface over 15 times compared to a regular copper surface. The boiling characteristics of the structure were found to be dependent on controllable surface characteristics. The re-markably good boiling performance of the novel micro-porous en-hancement structure has been attributed to its high porosity ( ~94%), a dendritically formed and exceptionally large surface area, and to a high density of well suited vapor escape channels (>50 per mm2). A patent application, intended to protect the enhancement structure and its fabrication method, was submitted to the Swedish patent authorities (PRV) on March 1st, 2006.
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| 17. |
- Ghanbarpourgeravi, Morteza, 1985-
(författare)
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Investigation of Thermal Performance of Cylindrical Heatpipes Operated with Nanofluids
- 2017
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nanofluids as an innovative class of heat transfer fluids created by dispersing nanometre-sizedmetallic or non-metallic particles in conventional heat transfer fluids displayed the potential toimprove the thermophysical properties of the heat transfer fluids. The main purpose of this study is toinvestigate the influence of the use of nanofluids on two-phase heat transfer, particularly on thethermal performance of the heat pipes. In the first stage, the properties of the nanofluids were studied,then, these nanofluids were used as the working fluids of the heat pipes. The thermal performance ofthe heat pipes when using different nanofluids was investigated under different operating conditionsexperimentally and analytically. The influences of the concentration of the nanofluids, inclinationangles and heat loads on the thermal performance and maximum heat flux of the heat pipes wereinvestigated.This study shows that the thermal performance of the heat pipes depends not only on thermophysicalproperties of the nanofluids but also on the characteristics of the wick structure through forming aporous coated layer on the heated surface. Forming the porous layer on the surface of the wick at theevaporator section increases the wettability and capillarity and also the heat transfer area at theevaporator of the heat pipes.The thermal performance of the heat pipes increases with increasing particle concentration in all cases,except for the heat pipe using 10 wt.% water/Al2O3 nanofluid. For the inclined heat pipe, irrespectiveof the type of the fluid used as the working fluid, the thermal resistance of the inclined heat pipes waslower than that of the heat pipes in a horizontal state, and the best performance was observed at theinclination angle of 60o, which is in agreement with the results reported in the literature. Otheradvantages of the use of nanofluids as the working fluids of the heat pipes which were investigated inthis study were the increase of the maximum heat flux and also the reduction of the entropy generationof the heat pipes when using a nanofluid.These findings revealed the potential for nanofluids to be used instead of conventional fluids as theworking fluid of the heat pipes, but the commercialization of the heat pipes using nanofluids for largescale industrial applications is still a challenging question, as there are many parameters related to thenanofluids which are not well understood.
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| 18. |
- Grozdek, Marino, 1977-
(författare)
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Load Shifting and Storage of Cooling Energy through Ice Bank or Ice Slurry Systems : modelling and experimental analysis
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ice based Cool Thermal Energy Storage (CTES) systems have attracted much attention during last few decades. The reasons are mainly of economical and environmental nature. Compared to conventional refrigeration and air-conditioning systems without cool thermal energy storage, implementation of CTES will increase environmental standards and overall efficiency of the energy systems as it contributes to the phase-out of synthetic refrigerants and reduces peak loads in electricity grids. For the application of a cool thermal energy storages in refrigeration installations and HVAC systems in industry and building sector, it is necessary to have appropriate design tools in order to sufficiently accurate predict their performance. In this thesis theoretical and experimental investigations of two ice based cool thermal energy storage systems, namely static, indirect, external melt, ice-on-coil, i.e. ice bank system and dynamic, ice slurry cool thermal energy storage system are carried out. An ice bank storage technology for cooling purposes is known for a long time. The main drawbacks which are hindering its wider use are the system complexity, high first costs, system efficiency which is highly dependant on design, control and monitoring of the system, etc. On the other hand, ice slurry technology was not well studied until recently, while in the current scientific literature there are still differences between results and conclusions reported by different investigators. The aim of the present thesis is to extend the knowledge in the field of ice based CTES systems, thereby contributing in the development and wider utilization of those systems. In the first part of the thesis a computer application, named “BankaLeda” is presented. It enables simulation of an ice bank system performance. In order to verify developed simulation model an experimental evaluation has been performed. Field measurements have been conducted on a two module silo which was installed as a part of the refrigeration system in dairy and cheese factory “Antun Bohnec” in the city of Ludbreg in Croatia. Experimental findings were compared to the simulation model. The software „BankaLeda“ presents a strong optimization tool for designers and engineers in the field by providing a high degree of freedom in defining particular system design and operating parameters. It offers a basis for assessment and testing of a new energy efficient system arrangements and measures. Besides it will give decisionmakers the ability to test potential solutions in the process of CTES system design. In the second part of the thesis ice slurry pressure drop and heat transfer in horizontal straight tubes have been experimentally investigated. In particular a mixture of 10.3 % of ethanol and water with an initial freezing point of -4.4 °C was considered. It was found that the behaviour of ice slurry flow is changing with time and that ice slurry pressure drop is generally higher than for single phase flow. However for ice concentrations of 15 % and higher, for certain velocities ice slurry pressure drop is found to be of a similar value as for single phase fluid. Moreover, if ice slurry is to be used as a energy transport media it is recommended to keep the ice mass fraction at a level of 20 %. With tube geometry and thermophysical properties of a carrier fluid the heat transfer of ice slurry is generally a function of ice mass fraction and velocity. The imposed heat flux has no or has just minor influence on the heat transfer coefficient. Up to ice mass fraction between 10-15 % the mean heat transfer coefficient shows only slight (laminar flow) or no increase (turbulent flow) in comparison to single phase flow. Beyond that ice mass fraction the heat transfer coefficient is increasing significantly. The test data for pressure drop and heat transfer in laminar and turbulent regime was compared to several correlations from the literature. A new correlations for ice slurry pressure drop and heat transfer in the laminar flow regime, for 10.3 % ethanol and water mixture, were derived based on the present experimental data. The correlation for pressure drop predicts 82 % of the experimental data with ±15 % accuracy, while the correlation for heat transfer predicts 75 % of the data with the same accuracy. In order to investigate advantages and disadvantages of a dynamic, ice slurry system over a static, indirect, external melt, ice-on-coil CTES system and to assess their differences from economical aspects, a theoretical simulation model of an ice slurry CTES have been developed. It was found that the ice slurry based CTES systems posses higher economic and energy saving potential than static type systems. In the best case scenario the total energy consumption of dynamic CTES system was found to be approximately 25 % lower than for a static CTES system.
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| 19. |
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| 20. |
- Hägg, Cecilia, 1975-
(författare)
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Ice Slurry as Secondary Fluid in Refrigeration Systems : Fundamentals and Applications in Supermarkets
- 2005
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis summarises the work performed within the project known as ICE-COOL at the Royal Institute of Technology (KTH) on low tem-perature applications of ice slurry. The ICE-COOL project is a Euro-pean Community funded project within the 5th Framework Program. The task given to KTH was to find and characterize the best possible aqueous solution with a freezing point of -25 °C applicable for ice slurry with an operating temperature of -35 °C. The circumstances differ for low temperature ice slurry from medium temperature as a result of for example the change in thermo-physical properties due to increased addi-tive concentration and the lower temperature. Ice slurry is a mixture of fine ice crystals, water and freezing point de-pressant additives. The typical ice crystal size ranges between 0.1 to 1 mm in diameter. The main purpose of using ice slurries is to take advan-tage of the latent heat. More than a few requirements are to be fulfilled by an ideal ice slurry fluid. It should have good thermo-physical proper-ties, high heat transport and transfer abilities, as well as low pressure drop to facilitate small pumping power. The ice content of ice slurry af-fects all the mentioned abilities. Apart from the mentioned factors of the fluid, there are other aspects that have to be taken into consideration such as environmental pollution and toxicity, flammability, material compatibility, corrosion, handling security and cost. This thesis reports on the experiences accomplished and the initial ex-periments performed on low temperature ice slurry. The thesis also gives the background and fundamentals necessary for a discussion and com-parison of different aqueous fluids suitable for ice slurry in general and for low temperature in particular. In addition to the low temperature ap-plication, the thesis also reports on an initial energy consumption com-parison between using ice slurry and single-phase fluid in supermarkets by means of the simulation program CyberMart developed by Jaime Arias at the Department of Energy Technology at KTH.
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| 21. |
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| 22. |
- Jonsson, Hans, 1969-
(författare)
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Turbulent Forced Convection Air Cooling of Electronics with Heat Sinks Under Flow Bypass Conditions
- 2001
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Air cooling is still the dominant method for dissipating theheat produced by electronic components. In a typical electronicdevice, the components are situated on a Printed Circuit Board(PCB). For most applications natural convection is sufficientto cool the components. However in some cases forced convectionair cooling, which provides more effective cooling, has to beused. In cases with high heat fluxes the switch from natural toforced convection is not enough and in these cases the surfacearea of the components must also be increased by means of aheat sink, i.e. a finned surface. The size of the components isoften considerably smaller than the PCB, and in cases whereheat sinks are used, the heat sink seldom covers the air flowduct formed by two parallel PCBs. Since the air flow is seekingthe path of least resistance, some of it will bypass the heatsink. This air flow bypass has a major impact on theperformance of the heat sink, why knowledge of the amount ofbypass flow is of vital importance. The aim of this work has been to investigate the influenceof flow bypass on the performance of heat sinks used forelectronics cooling. The focus has been set on determining theparameters that influence the bypass, and to quantify theirrelative importance using experimental and numericaltechniques. Experimentaland numerical parametric studies have beenperformed. The influence of duct dimensions, air velocity, fintype, array arrangement, fin height, and fin-to-fin distance onthe thermal and hydraulic performance of plate fin, strip fin,and pin fin heat sinks have been investigated. The flow wasturbulent, and the Reynolds number was varied between 1 700 and16 000. Measurements have been performed in a wind tunnel. Intotal 42 different heat sinks were tested in seven differentduct configurations. For the plate fin heat sinks and thecircular pin fin heat sinks, numerical predictions have beenperformed by using the finite element code FIDAP and the finitevolume code PHOENICS, respectively. For turbulence modeling therevised RNG k-ε model and the Chen-Kim k-ε model wereused. The agreement between the results from the experimental andnumerical investigations was very good, suggesting that acombination of the two methods can be a powerful tool forpredicting the thermal and hydraulic performance of heat sinks.The experimental data can be used to validate the numericalmodel, which in turn can be used for parametric studies. The results of the investigations showed that the thermalperformance was most influenced by the duct height, and the airvelocity while the hydraulic performance was dependent on allthe investigated parameters. From the experimental and numerical results severalcorrelations have been developed to describe the relativeimportance of the investigated parameters. The agreementbetween the source data and the correlations is very good, aswell as the agreement between the correlations developed withdata from the experimental and the numerical investigationsrespectively. Keywords: forced convection, heat transfer, pressuredrop, turbulent, air cooling, bypass, plate, strip, pin, fin,heat sink, electronics, experimental,numerical, CFD
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| 23. |
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| 24. |
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| 25. |
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| 26. |
- Liu, Qingming
(författare)
-
Numerical study of flow boiling in micro/mini channels
- 2017
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Boiling phenomena in micro scale has emerged as an interesting topic due to its complexity and increasing usage in micro electronic and mechanical systems (MEMS). Experimental visualization has discovered five main flow regimes: nucleate boiling, isolated bubbles, confine bubbly flow, elongated bubbly (or slug) flow, and annular flow. Two of these patterns (confine bubbles and slug flow) are rarely found in macro channels and are believed to have very different heat transfer mechanisms to that of nucleate boiling.The development of a phenomenological model demands a deep understanding of each flow regime as well as the transition process between them. While studies in every individual flow pattern are available in literature, the mechanisms of transition processes between them remain mysterious. More specifically, how the isolated bubbles evolve into a confined bubbly flow, and how this further evolves into elongated bubbles and finally an annular flow. The effects of boundary conditions such as wall heat flux, surface tension, and interfacial velocity are unclear, too.The aims of this thesis are to develop and validate a new numerical algorithm, perform a comprehensive numerical study on these transition processes, uncover the transition mechanisms and investigate effects of boundary and operating conditions.Firstly, a sophisticated and robust numerical model is developed by combining a coupled level set method (CLSVOF) and a non-equilibrium phase change model, which enables an accurate capture of the two-phase interface, as well as the interface temperature.Secondly, several flow regime transitions are studied in this thesis: nucleate bubbles to confined bubbly flow, multi confined bubbles moving consecutively in a micro channel, and slug to annular flow transition. Effects of surface tension, heat flux, mass flux, and fluid properties are examined. All these regimes are studied separately, which means an appropriate initial condition is needed for each regime. The author developed a simplified model based on energy balance to set the initial and boundary conditions.
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| 27. |
- Mader, Gunda
(författare)
-
Economic analysis of air-water heat pump technologies with a screening method
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Early on in the process of product development a decision has to be made which technologies to focus on. Under a purely techno-economic viewpoint for a heat pump only these technologies should be considered which maximize heat pump performance for a given cost. Finding these optima is in practice far from trivial as the result is inuenced by variations in operating conditions, interactions between components, model assumptions and uncertain economic data. The overall objective of this thesis is therefore to develop a screening method for the evaluation and comparison of different technologies in regard of cost and efficiency with the aim to identify optimal heat pump designs.In practice the applicability of such a method depends on the required effort. Simple mathematical models and short computation times are as mandatory as reliable, coherent and sufficiently general results. To this end the screening method combines methods of simulation, annual performance calculation, metamodeling and optimization.The screening method is employed for air-water heat pumps in three utilization examples. In the first example the operational costs of maldistribution in evaporators are quantified. It is shown that for air-water heat pumps increasing the evaporator size is no liable option to counteract maldistribution effects. Two alternative technologies, an adjustment of the cycle layout and a change of the superheat control method, are evaluated under the aspect of total cost of ownership. Only with the second technology noteworthy savings can be achieved compared to the baseline. In the second utilization example on/o_ and variable speed control for regulation of the heat pump capacity are considered. Only for colder climates variable speed control pays o_ for the end consumer in a reasonable time. The study shows the comparison of the two control methods to strongly depend on compressor size. In the third example four different cycle layouts are compared. It is demonstrated that the result of the evaluation depends strongly on practical operating limits and on compressor characteristics. The most promising option is a staged layout with economizer heat exchanger.Two additional studies consider the modeling of operating conditions in more detail. In the first study the simple approach for calculating annual performance used in the screening method is compared with a comprehensive dynamic model. Optimization for both model approaches results in similar component sizes. In the second study the evaporator model is extended to include a dynamic frost growth model which is used to assess operational costs induced by frosting and defrosting of the evaporator coil. A method to reduce the number of defrosts without negatively affecting heat pump capacity is presented and its feasibility demonstrated with experiments.The screening method can be extended to include more optimization parameters than used in the presented examples. It can also be applied for other small scale vapor compression systems or it can be integrated in a comprehensive evolutionary optimization procedure to reduce computation time and increase numeric stability. Thus the screening method can be a valuable tool in the product development process for shortening the times until new technologies which safe energy and thereby reduce greenhouse gases are available on the market.
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| 28. |
- Maqbool, Muhammad Hamayun
(författare)
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Flow boiling of ammonia and propane in mini channels
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The environmental concerns in recent times have grown especially after signing Montreal protocol. In the last ten years, researchers have focussed mainly on understanding the boiling and condensation phenomena of HFC refrigerants in minichannels. As global warming concerns are growing day by day, due to high global warming potential, HFCs are not the ultimate option. In the near future, HFCs will probably be replaced by environmentally friendly refrigerants. Therefore, to find the potential replacements of HFCs and also to get a deeper understanding of the boiling phenomena in minichannels, more and more fluids having low GWP (Global Warming Potential) and ODP (Ozone Depletion Potential) should be tested. Recent efforts to protect the environment have led to a growing interest for natural refrigerants. However in the literature, flow boiling data of natural refrigerants in minichannels are scarce. To meet the environmental concerns and to understand the behaviour of natural refrigerants in minichannels and the performance compared to HFCs, flow boiling experiments in single circular vertical minichannels of internal diameters of 1.70 and 1.224 mm were performed using ammonia and propane as working fluids. Flow boiling heat transfer results of ammonia and propane with 1.70 mm channel showed that the heat transfer coefficient was a function of heat flux and the effect of mass flux was insignificant. The heat transfer coefficient of ammonia in 1.224 mm was dependent on heat flux at low vapour qualities then a clear dependence of the heat transfer coefficient on the mass flux was observed at higher vapour qualities. The heat transfer results of ammonia and propane were compared with well known correlations and among them Cooper (1989) correlation in case of ammonia and Liu and Winterton (1991) and Cooper (1984) pool boiling correlations in case of propane best predicted the experimental heat transfer data. Results of the two phase pressure drop studies of ammonia and propane showed that the two phase pressure drop increased with the increase of mass flux, with the increase of heat flux and with the decrease of saturation temperature. The comparison of the two phase pressure drop experimental data with well known predicting models showed that none of the correlations predicted the ammonia data well and that Müller Steinhagen and Heck (1986) was well in agreement with the propane data. Dryout of propane in 1.70 mm and 1.224 mm internal diameter channels was also investigated. Dryout heat flux was observed to increase with the increase of mass flux, with the decrease of vapour quality and with the increase of internal diameter. The effect of saturation temperature on the dryout heat flux was insignificant. The experimental dryout data were compared with macro and micro scale correlations and among them Bowring (1972) and Callizo et al. (2010a) gave best predictions. The heat transfer and pressure drop results of ammonia and propane and dryout results of propane were compared with R134a data taken on the same test rig by Owhaib (2007) and Ali (2010). The comparison of heat transfer showed that the heat transfer coefficient was a function of heat flux and the effect of mass flux was insignificant in all tested conditions except ammonia in 1.224 mm tube where the heat transfer coefficient was dependent on heat flux at lower vapour qualities and a clear dependence of mass flux was observed at higher vapour qualities. The heat transfer data of ammonia, propane and R134a were compared with correlations and among them Cooper (1989) correlation gave best predictions. The comparison of pressure drop results showed that the two phase pressure drop of all fluids was increased with the increase of mass flux, with the increase of heat flux and with the decrease of saturation temperature. At equal heat flux and mass flux, the two phase pressure drop of ammonia was increased with the decrease of internal diameter but the diametric effects of R134a were unclear. Müller Steinhagen and Heck (1986) and Zhang and Webb (2001) best predicted the experimental data of two phase pressure drop of ammonia, propane and R134a among the correlations considered for comparison. The dryout data of propane were also compared with dryout data of R134a and it was observed that the dryout heat flux of propane and R134a increased with the increase of mass flux, with the decrease of vapour quality and with the increase of internal diameter. The effect of saturation temperature on the dryout heat flux of propane and R134a was insignificant. The correlation of Bowring (1972) for conventional channels and the microscale correlation of Callizo et al. (2010a) were among the correlations which gave best predictions of experimental data of dryout.
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| 29. |
- Martin Callizo, Claudi, 1979-
(författare)
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Flow Boiling Heat Transfer in Single Vertical Channels of Small Diameter
- 2010
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Microchannel heat exchangers present many advantages, such as reduced size, high thermal efficiency and low fluid inventory; and are increasingly being used for heat transfer in a wide variety of applications including heat pumps, automotive air conditioners and for cooling of electronics.However, the fundamentals of fluid flow and heat transfer in microscalegeometries are not yet fully understood. The aim of this thesis is to contribute to a better understanding of the underlying physical phenomena in single-phase and specially flow boiling heat transfer of refrigerants in small channels. For this purpose, well-characterized heat transfer experiments have been performed in uniformly heated, single, circular, vertical channels ranging from 0.64 to 1.70 mm in diameter and using R-134a, R-22 and R-245fa as working fluids. Furthermore, flow visualization tests have been carried out to clarify the relation between the two-phase flow behavior and the boiling heat transfer characteristics. Single-phase flow experiments with subcooled liquid refrigerant have confirmed that conventional macroscale theory on single-phase flow and heat transfer is valid for circular channels as small as 640μm in diameter. Through high-speed flow boiling visualization of R-134a under non adiabatic conditions seven flow patterns have been observed: isolated bubbly flow, confined bubbly flow, slug flow, churn flow, slug-annular flow, annular flow, and mist flow. Two-phase flow pattern observations are presented in the form of flow pattern maps. Annular-type flow patterns are dominant for vapor qualities above 0.2. Onset of nucleate boiling and subcooled flow boiling heat transfer of R-134a has been investigated. The wall superheat needed to initiate boiling was found as large as 18 ºC. The experimental heat transfer coefficients have been compared to predictions from subcooled flow boiling correlationsav ailable in the literature showing poor agreement. Saturated flow boiling heat transfer experiments have been performed with the 640 μm diameter test section. The heat transfer coefficient has been found to increase with heat flux and system pressure and not to change with vapor quality or mass flux when the quality is less than ∼0.5. For vapor qualities above this value, the heat transfer coefficient decreases with vapor quality. This deterioration of the heat transfer coefficient is believed to be caused by the occurrence of intermittent dryout in this vapor quality range. The experimental database, consisting of 1027 data points, has been compared against predictions from correlations available in the literature. The best results are obtained with the correlations by Liu and Winterton (1991) and by Bertsch et al. (2009). However, better design tools to correctly predict the flow boiling heat transfer coefficient in small geometries need to be developed. Dryout incipience and critical heat flux (CHF) have been investigated in detail. CHF data is compared to existing macro and microscale correlations. The comparison shows best agreement with the classical Katto and Ohno (1984) correlation, developed for conventional large tubes.
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| 30. |
- Melinder, Åke, 1944-
(författare)
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Thermophysical Properties of Aqueous Solutions Used as Secondary Working Fluids
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Secondary working fluids (secondary refrigerants, heat transfer fluids, antifreezes, brines) have long been used in various indirect re-frigeration and heat pump systems. Aqueous solutions (water solu-tions) have long been used as single phase (liquid only) secondary working fluids for cooling in supermarkets, ice rinks, heat recovery systems, heat pumps and other applications. However, aqueous solutions are increasingly used also for freezers in supermarkets and other applications in low temperature refrigeration. Of importance when comparing different secondary working fluids for indirect systems are the following basic thermophysical properties: freezing point, density, viscosity, specific heat, and thermal conductivity. Reliable data of these properties are needed to choose suitable fluid and to make technical calculations of an indirect refrigeration or heat pump system. The main intention of this work has been to select thermophysical property data with good or acceptable technical accuracy of a number of aqueous solutions that can be used by the refrigeration and heat pump industry, rather than focusing on a limited number of property values or scientifically very accurate measuring techniques. A thorough literature search was in view of this made to find the most reliable property values of aqueous solutions. Detailed literature references are given for thermo-physical properties of the following aqueous solutions, without other additives: Ethylene and propylene glycol, ethyl and methyl alcohol, glycerol, ammonia, potassium carbonate, calcium, lithium, magnesium and sodium chlorides as well as potassium acetate and potassium formate. Some laboratory measurements were made of most of the fluid types when literature values were incomplete or deemed unreliable. Methods used are briefly described and results are given. Much of the work was reported on in the Engineering Licentiate Thesis: Thermophysical properties of liquid secondary refrigerants, A Critical Review on Literature References and Laboratory Measure-ments (Melinder 1998a). That material forms the basis for the charts and tables used in the IIR-publication Thermophysical properties of liquid secondary refrigerants (Melinder, 1997). The present thesis reports on an update made since 1998, including re-view work done on two additional fluids not covered in Melinder (1998a). The thesis describes how the selection of property values results in tables and charts intended for the industry. Coefficients for poly-nomial equations are generated from these property values using a Matlab program and this material is intended as a useful tool for computer treatment. Aqueous solution of ethyl alcohol is used as example to see how this process is made. This choice of fluid can also be seen as a test of this method, as the basic thermophysical properties of aqueous solutions of ethyl alcohol present more chal-lenges than the other fluids examined. A comparison is made of a few types of aqueous solutions used as secondary working fluids for two types of applications. The first example is bedrock heat pumps and the second is cooling cabinets in a supermarket. An effort is made to see how the additive con-centration affects the thermal performance. Most aqueous solutions used as single-phase secondary fluids can also be used as ice slurry, a fluid consisting of liquid and ice where small ice crystals are produced, usually with some type of ice generator. The ice crystals are then transported to the cooling object from which heat is removed when ice crystals melt. This results in less temperature change in the cooling object and makes it also possible to reduce the volume flow rate and to use smaller pipe dimensions in the system. In order to choose a secondary fluid for ice slurry use and to make correct technical calculations of the ice slurry system there is a need to examine and evaluate thermo-physical properties and other aspects of ice and of the aqueous solution used. For dimensioning purposes it is of interest to estimate ice mass fraction and enthalpy values and enthalpy-phase diagrams can serve that purpose. This thesis presents enthalpy-phase diagrams made by author that besides isotherms contain lines with ice fraction and lines connecting enthalpies at freezing point and 1, 2, etc. to 10 K below the freezing point curve.
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| 31. |
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| 32. |
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| 33. |
- Owhaib, Wahib, 1973-
(författare)
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Experimental Heat Transfer, pressure drop, and Flow Visualization of R-134a in Vertical Mini/Micro Tubes
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- For the application of minichannel heat exchangers, it is necessary to have accurate design tools for predicting heat transfer and pressure drop. Until recently, this type of heat exchangers was not well studied, and in the scientific literature there were large discrepancies between results reported by different investigators. The present thesis aims to add to the knowledge of the fundamentals of single- and two-phase flow heat transfer and pressure drop in narrow channels, thereby aiding in the development of this new, interesting technology with the possibility of decreasing the size of electronics through better cooling, and of increasing the energy efficiency of thermal processes and thermodynamic cycles through enhanced heat transfer. A comprehensive experimental single-phase flow and saturated flow boiling heat transfer and pressure drop study has been carried out on vertical stainless steel tubes with inner diameters of 1.700, 1.224 and 0.826 mm, using R-134a as the test fluid. The heat transfer and pressure drop results were compared both to conventional correlations developed for larger diameter channels and to correlations developed specifically for microscale geometries. Contrary to many previous investigations, this study has shown that the test data agree well with single-phase heat transfer and friction factor correlations known to be accurate for larger channels, thus expanding their ranges to cover mini/microchannel geometries. The main part of the study concerns saturated flow boiling heat transfer and pressure drop. Tests with the same stainless steel tubes showed that the heat transfer is strongly dependent on heat flux, but only weakly dependent on mass flux and vapor fraction (up to the location of dryout). This behavior is usually taken to indicate a dominant influence of nucleate boiling, and indicates that the boiling mechanism is strongly related to that in nucleate boiling. The test data for boiling heat transfer was compared to several correlations from the literature, both for macro- and mini-channels. A new correlation for saturated flow boiling heat transfer of refrigerant R-134a correlation was obtained based on the present experimental data. This correlation predicts the presented data with a mean absolute deviation of 8%. The frictional pressure drop results were compared to both macro- and mini channel correlations available from the literature. The correlation suggested by Qu and Mudawar (2003) gave the best prediction to the frictional two-phase pressure drop within the studied ranges. A unique visualization study of saturated flow boiling characteristics in a vertical 1.332 mm inner diameter quartz tube, coated with a transparent heater has also been conducted. The complete evaporation process in a heated circular mini-channel has been studied visually in detail using high speed CCD camera. The study revealed the developments of the flow patterns and the behavior from bubble nucleation to the dry out of the liquid film. The bubble departure frequency, diameter, growth rate, and velocity were determined by analyzing the images. Finally, a flow pattern map for boiling flow in microchannels has been developed based on the test data.
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| 34. |
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| 35. |
- Persson, Tomas
(författare)
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Elbesparing med pelletkaminer och solvärme i direktelvärmda småhus
- 2004
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of this study was to investigate how electricallyheated houses can be converted to using wood pellet and solarheating. There are a large number of wood pellet stoves on themarket. Many stoves have a water jacket, which gives anopportunity to distribute the heat to domestic hot water and aradiator heating system.Three typical Swedish houses with electric resistanceheating have been studied. Fourteen different system conceptsusing wood pellet stoves and solar heating systems have beenevaluated. The systems and the houses have been simulated indetail using TRNSYS. The houses have been divided in up to 10different zones and heat transfer by air circulation throughdoorways and open doors have been simulated. The pellet stoveswere simulated using a recently developed TRNSYS component,which models the start- and stop phases, emissions and thedynamic behaviour of the stoves. The model also calculates theCO-emissions. Simulations were made with one stove without awater jacket and two stoves with different fractions of thegenerated heat distributed in the water circuit.Simulations show that the electricity savings using a pelletstove are greatly affected by the house plan, the systemchoice, if the internal doors are open or closed and thedesired level of comfort. Installing a stove with awater-jacket connected to a radiator system and a hot waterstorage has the advantage that heat can be transferred todomestic hot water and be distributed to other rooms. Suchsystems lead to greater electricity savings, especially inhouses having a traditional layout. It was found that not allrooms needed radiators and that it was more effective in mostcases t use a stove with a higher fraction of the heatdistributed by the water circuit.The economic investigation shows that installing a woodpellet stove without a water jacket gives the lowest totalenergy- and capital costs in the house with an open plan (fortoday's energy prices and the simulated comfort criteria). Inthe houses with a traditional layout a pellet stove givesslightly higher costs than the reference house having onlyelectrical resistance heating due to the fact that less heatingcan be replaced. The concepts including stoves with a waterjacket all give higher costs than the reference system, but theconcept closest to be economical is a system with a bufferstore, a stove with a high fraction of the heat distributed bythe water circuit, a new water radiator heating system and asolar collector.Losses from stoves can be divided into: flue gas lossesincluding leakage air flow when the stove is not in operation;losses during start and stop phases; and losses due to a highair factor. An increased efficiency of the stoves is importantboth from a private economical point of view, but also from theperspective that there can be a lack of bio fuel in the nearfuture also in Sweden. From this point of view it is alsoimportant to utilize as much solar heat as possible. Theutilization of solar heat is low in the simulated systems,depending on the lack of space for a large buffer store.The simulations have shown that the annual efficiency ismuch lower that the nominal efficiency at full power. Thesimulations have also shown that changing the control principlefor the stove can improve efficiency and reduce theCO-emissions. Today's most common control principle for stovesis the on/off control, which results in many starts and stopsand thereby high CO-emissions. A more advanced control varyingthe heating rate from maximum to minimum to keep a constantroom temperature reduces the number of starts and stops andthereby the emissions. Also the efficiency can be higher withsuch a control, and the room temperature will be kept at a moreconstant temperature providing a higher comfort.
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| 36. |
- Phdungsilp, Aumnad, 1971-
(författare)
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Energy analysis for sustainable mega-cities
- 2006
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- ABSTRACT Cities throughout Asia have experienced unprecedented economic development over the past decades. In many cases this has contributed to their rapid and uncontrolled growth, which has resulted in a multiplicity of problems, including rapid population increase, enhanced environmental pollution, collapsing traffic systems, dysfunctional waste management, and rapid increases in the consumption of energy, water and other resources. The significant energy use in cities is not very well perceived in Asian countries. Although a number of studies into energy consumption across various sectors have been conducted, most are from the national point of view. Energy demand analysis is not considered important at the level of the city. The thesis is focused on the dynamics of energy utilization in Asian mega-cities, and ultimately aims at providing strategies for maximizing the use of renewable energy in large urban systems. The study aims at providing an in-depth understanding of the complex dynamics of energy utilization in urban mega-centers. An initial general analysis is complemented by a detailed study of the current situation and future outlook for the city of Bangkok, Thailand. An integrated approach applied to the study includes identification of the parameters that affect the utilization of energy in mega-cities and a detailed analysis of energy flows and their various subsystems, including commercial, industrial, residential and that of transportation. The study investigates and evaluates the energy models most commonly used for analyzing and simulating energy utilization. Its purpose is to provide a user-friendly tool suitable for decision-makers in developing an energy model for large cities. In addition, a Multi-Criteria Decision-Making (MCDM) process has been developed to assess whether or not the energy systems meet the sustainability criteria. A metabolic approach has been employed to analyze the energy flow and utilization in selected Asian mega-cities, including Bangkok, Beijing, Shanghai, and Tokyo. The approach is applied to measure the majority of indirect energy flows or the energy embodied in the flows of goods and services involving the residents of those cities. Since the function of cities is to serve the lives of the residents, indirect energy consumption could be regarded as being of equal importance as that of direct energy use. The essence of embodied energy is that an indirect reflection upon behavior following direct energy consumption. It can illustrate how a city relies on the outside, for example other cities, countries, etc. and provides some interesting information that cannot be easily drawn from the direct energy demand. The study reveals that the indirect energy demand is more significant than the direct energy demand in Bangkok, Shanghai, and Tokyo, while direct energy demand is greater than the indirect energy demand in Beijing. This can be explained by the fact that Bangkok, Shanghai, and Tokyo have a greater reliance upon the outside in terms of energy demand. The Long-range Energy Alternative Planning (LEAP) system has been selected to perform Bangkok energy modeling. In a Bangkok case study a range of policy interventions are selected and how these would change the energy development in Bangkok by the year 2025 is examined. Different policies can be grouped by the sectors analyzed. The only supply-side policy considered meets an existing target of having 10% of electricity generated from renewable sources. The study period for the model started in 2005 and ends in 2025, with the year 2000 taken as the base year. The proposed scenarios were evaluated using the MCDM approach to rate their sustainability. Team members found that this method provided a methodology to help decision-makers to systematically identify management objectives and priorities.
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| 37. |
- Pridasawas, Wimolsiri, 1976-
(författare)
-
Solar-driven refrigeration systems with focus on the ejector cycle
- 2006
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Interest in utilizing solar-driven refrigeration systems for air-conditioning or refrigeration purposes has grown continuously. Solar cooling is com-prised of many attractive features and is one path towards a more sus-tainable energy system. Compared to solar heating, the cooling load, par-ticularly for air-conditioning applications, is generally in phase with solar radiation. The objective of this thesis is to establish a fundamental basis for further research and development within the field of solar cooling. In this thesis, an overview of possible systems for solar powered refrigeration and air-conditioning systems will be presented. The concept of the ‘Solar Cool-ing Path’ is introduced, including a discussion of the energy source to the collector, and choice of cooling cycle to match cooling load. Brief infor-mation and comparisons of different refrigeration cycles are also pre-sented. The performance of solar cooling systems is strongly dependent on local conditions. The performance of a solar divan air-conditioning system in different locations will therefore be compared in this thesis. Solar cooling systems can be efficiently operated in locations where sufficient solar ra-diation and good heat sink are available. A solar-driven ejector refrigeration system has been selected as a case study for a further detailed investigation. A low temperature heat source can be used to drive the ejector refrigeration cycle, making the system suitable for integration with the solar thermal collector. Analysis of the solar driven ejector system is initiated by steady state analysis. System performance depends on the choice of working fluid (refrigerant), oper-ating conditions and ejector geometry. Results show that various kinds of refrigerants can be used. Also, thermodynamic characteristics of the re-frigerant strongly influence the performance of the cycle. An ejector re-frigeration cycle using natural working fluids generates good perform-ance and lower environmental impact, since traditional working fluids, CFC’s and HFC’s are strong climate gases. Further on, exergy analysis is used as a tool in identifying optimum operating conditions and investi-gating losses in the system. Exergy analysis illustrates that the distribu-tion of the irrervsibilities in the cycle between components depends strongly on the working temperatures. The most significant losses in the system are in the solar collector and ejector. Losses in the ejector pre-dominate over total losses within the system. In practice, the cooling load characteristic and solar radiation are not constant. Therefore, a dynamic analysis is useful for determining the characteristics of the system during the entire year, and dimensioning the important components of the solar collector subsystem, such as storage tanks. The final section of the thesis will deal with the ejector, the key compo-nent of the ejector refrigeration cycle. Characteristics of the actual ejector are shown to be quite complicated and its performance difficult to de-termine solely through theoretical analysis. Suggested design procedures and empirical equations for an ejector are offered in this thesis. Prelimi-nary test results for one fixed ejector dimension using R134a as the re-frigerant are also included.
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| 38. |
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| 39. |
- Sakellari, Dimitra, 1974-
(författare)
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Modelling the dynamics of domestic low-temperature heat pump heating systems for improved performance and thermal comfort : a systems approach
- 2005
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The present environmental concerns and the rising human requirement for solutions with better comfort and lower costs have resulted in an increased awareness for the energy use in the built environment. Technical advances in building structural systems and materials, heating and other comfort-providing systems and controlling strategies all lead to the integration of building technology with the function of buildings and the aesthetics. Therefore, in the process of improving the performance of energy systems and increasing the energy efficiency, integrated system approaches are of high importance. Performing the necessary energy analysis before any construction-installation occurs can help designers and decision makers reach guided solutions. Hence, a broad range of calculation tools for evaluating the operation of energy systems and the controls in buildings have been developed the latest years with different levels of complexity and angles of focus. However, research and development regarding holistic energy system designs and techniques are in their infancy. The standard tactic has been to isolate system parts, study them as stand-alone sub-systems and focus on optimising components or processes of a complex function. In the present study, it is demonstrated the necessity for uniting energy engineers, architects, installers and technicians regarding decision making upon the energy use for heating, ventilation and air-conditioning (HVAC) in the built environment. Systems approach has been employed for studying the research issue that is presented in the current thesis. An extended part of this treatise has been devoted to systems thinking in practice. The thesis demonstrates systematic methods of modelling and analysing certain, integrated, domestic, HVAC applications. The reference system boundaries enclose the building as a construction and as a dynamic function, a comfort-providing system based on a heat pump, a low-temperature hydronic heat distribution system and controls in a residential application. Obviously, these are not the only components met in a hydronic heating system. Numerous pieces of equipment, as piping, circulating pumps, expansion tanks, zone valves, relief valves and other essential elements are needed to make a safe and functional heating system. However, this study focuses on the analysis of the chosen reference system. Several models have been developed in the computational tools of TRNSYS and EES. These tools have been employed because they allow co-solving, hence the integrated system as well as the interaction between the different parts of the system can be studied. The foremost result of this study is that approaching the system as a whole provides a better picture of the operation of every system component and the interaction between them. Explanations are given for the parameters that have a significant impact on the system’s performance. The thesis shows the importance of factors that are not easy to predict, as well as the difference in the building’s behaviour under fast changing thermal loads when the incorporated thermal mass is altered. Finally, implementing sophisticated controls for reducing the energy costs without compromising thermal comfort is vital.
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| 40. |
- Sawalha, Samer, 1974-
(författare)
-
Carbon Dioxide in Supermarket Refrigeration
- 2008
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis theoretically and experimentally investigates different aspects of the application of CO2 in supermarket refrigeration. Theoretical analysis has been performed using computer simulation models developed to simulate CO2 indirect, NH3/CO2 cascade, CO2 trans-critical and direct expansion (DX) R404A systems. The models supported the selection of the CO2 system solutions to be tested experimentally and facilitated the design of NH3/CO2 cascade and trans-critical systems test rigs. Performance evaluation and systems’ optimizations have also been carried out. In order to verify the findings of the theoretical analysis an experimental evaluation has been performed whereby a scaled-down medium size supermarket has been built in a laboratory environment. NH3/CO2 cascade and trans-critical systems have been tested and compared to a conventional R404A system installed in the same laboratory environment. Experimental findings have been compared to the computer simulation models. In supermarket refrigeration applications, safety is a major concern because of the large number of people that might be affected in the event of leakage. Therefore, a computer simulation model has been developed to perform calculations of the resulting concentration levels arising from different scenarios for leakage accidents in the supermarket. The model has been used to validate some of the risks associated with using CO2 in the application of supermarket refrigeration. Results of the experiments and the computer simulation models showed good agreement and suggest that the NH3/CO2 cascade system is a more efficient solution than the analyzed conventional ones for supermarket refrigeration. On the other hand, CO2 trans-critical solutions have efficiencies comparable to the conventional systems analyzed, with potential for improvements in the trans-critical systems. From a safety point of view, the analysis of the calculations’ results clearly shows that using CO2 in supermarket refrigeration does not create exceptional health risks for customers and workers in the shopping area. Studies conducted in this thesis prove that the CO2 systems investigated are efficient solutions for supermarket refrigeration.
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| 41. |
- Simanic, Branko
(författare)
-
Energy Auditing and Efficiency in a Chain Hotel : the Case of Scandic, Järva Krog
- 2011
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Energy efficiency and conservation measures in hotels is an area of research at the Department of Energy Technology, Royal Institute of Technology in Stockholm (KTH), Sweden. As a continuation of the research work, this project was focused on a hotel's resource-use performance, considering both internal and external factors that influence their consumption. The work comprised a literature study on energy efficiency and conservation measures in the built environment, focusing on hotel buildings, as well as an analysis of existing building assessment tools. An auditing and monitoring process was applied at one Scandic hotel, Järva Krog, which was selected with the support of Scandic management. Another four Scandic hotels were included in this study, using their data on energy/water consumption for the past five years. The tourism industry is one of the world’s largest single industries, experiencing rapid expansion in recent decades. The number of international tourist arrivals increased from 441 million in 1990 to 763 million in 2004 (WTO, 2005). The hotel industry forms the largest part of the tourism industry with more than 300 000 hotels worldwide. Hotels offer their lodging capacities with the highest possible comfort and quality, without compromise. High demands on comfort and the level of services offered require an intensive utilization of energy and water. The price of energy has been increasing continuously during the last few years, with electricity increasing 6 % on an annual basis, which has made energy expenses increase rapidly during the past years. Adding high energy costs to concerns about CO2 emissions, highlights the importance of accurate information for energy management within any hotel facility. A need to decrease energy expenditure demands depending of knowledge relating to hotel facility consumption. The objective of this thesis is to understand energy performance and energy distribution within one typical chain hotel facility, and how this understanding can be applied to a selected group of similar hotels as well as to define the relationship between the factors behind energy expenditure. Such factors include outdoor climate as an external quantitative factor; HVAC system efficiency, lighting, occupancy rate and number of food covers sold as internal quantitative factors; and personnel behavior, operation/maintenance routines/levels as internal qualitative factors.
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| 42. |
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| 43. |
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| 44. |
- Yang, Chen, 2011-
(författare)
-
Thermodynamic Cycles using Carbon Dioxide as Working Fluid : CO2 transcritical power cycle study
- 2011
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The interest in utilizing the energy in low‐grade heat sources and waste heat is increasing. There is an abundance of such heat sources, but their utilization today is insufficient, mainly due to the limitations of the conventional power cycles in such applications, such as low efficiency, bulky size or moisture at the expansion outlet (e.g. problems for turbine blades). Carbon dioxide (CO2) has been widely investigated for use as a working fluid in refrigeration cycles, because it has no ozonedepleting potential (ODP) and low global warming potential (GWP). It is also inexpensive, non‐explosive, non‐flammable and abundant in nature. At the same time, CO2 has advantages in use as a working fluid in low‐grade heat resource recovery and energy conversion from waste heat, mainly because it can create a better matching to the heat source temperature profile in the supercritical region to reduce the irreversibility during the heating process. Nevertheless, the research in such applications is very limited. This study investigates the potential of using carbon dioxide as a working fluid in power cycles for low‐grade heat source/waste heat recovery. At the beginning of this study, basic CO2 power cycles, namely carbon dioxide transcritical power cycle, carbon dioxide Brayton cycle and carbon dioxide cooling and power combined cycle were simulated and studied to see their potential in different applications (e.g. low‐grade heat source applications, automobile applications and heat and power cogeneration applications). For the applications in automobile industries, low pressure drop on the engine’s exhaust gas side is crucial to not reducing the engine’s performance. Therefore, a heat exchanger with low‐pressure drop on the secondary side (i.e. the gas side) was also designed, simulated and tested with water and engine exhaust gases at the early stage of the study (Appendix 2). The study subsequently focused mainly on carbon dioxide transcritical power cycle, which has a wide range of applications. The performance of the carbon dioxide transcritical power cycle has been simulated and compared with the other most commonly employed power cycles in lowgrade heat source utilizations, i.e. the Organic Rankin Cycle (ORC). Furthermore, the annual performance of the carbon dioxide transcritical power cycle in utilizing the low‐grade heat source (i.e. solar) has also been simulated and analyzed with dynamic simulation in this work. Last but not least, the matching of the temperature profiles in the heat exchangers for CO2 and its influence on the cycle performance have also been discussed. Second law thermodynamic analyses of the carbon dioxide transcritical power systems have been completed. The simulation models have been mainly developed in the software known as Engineering Equation Solver (EES)1 for both cycle analyses and computer‐aided heat exchanger designs. The model has also been connected to TRNSYS for dynamic system annual performance simulations. In addition, Refprop 7.02 is used for calculating the working fluid properties, and the CFD tool (COMSOL) 3 has been employed to investigate the particular phenomena influencing the heat exchanger performance.
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| 45. |
- Zanki Alujevic, Vlasta
(författare)
-
Energy use and environmental impact from hotels on Adriatic Coast in Croatia : current status and future possibilities for HVAC systems
- 2006
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis analyses a specific type of energy usage system, “energy usage in hotels”, and how this system behaves. In order to evaluate the current state of energy use in hotels, an energy audit questionnaire was developed and conducted among 31,5% of hotels on the Adriatic coast. The energy audit was used as a tool to set a benchmark for energy consumption in hotels and to identify opportunities for increased energy efficiency measures in HVAC systems. The analysis has shown that the average energy consumption in hotels on the Adriatic coast is in the range of 159 to 180 kWh/m2 and 162 to 225 kWh/m2 for seasonal and non seasonal hotels respectively. In order to establish a relationship between different independent variables in the hotels (total floor area and number of rooms) and dependent variables, such as electricity and oil consumption, mathematical statistical methods, such as correlation and regression analysis, were implemented. The objective of this thesis was also to develop - from an energy, environmental and economical points of view - a methodology for the design and retrofit of HVAC systems in the hotels on the Adriatic coast. The methodology named HOTECO is based on a system thinking approach. With respect to the technological aspects, the first step was to analyse conventional HVAC system designs and to compare it with the three most promising alternative HVAC systems utilizing renewable energy sources for a typical hotel. Computer modelling in TRNSYS was used to assess energy consumption. TRNSYS software has been used for simulations for a number of years, is internationally recognized, and has been validated and verified. It was concluded that energy and environmental savings in hotels on the Adriatic coast could be achieved using readily available technologies. The HOTECO methodology also demonstrated a framework that supports decision making iv regarding system selection and operational strategies to limit environmental impact from HVAC systems in hotels. Four scenarios for energy consumption in hotels on the Adriatic coast with regards to current state of energy systems and improved retrofit solutions utilizing renewable energy source were given.
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| 46. |
- Öhman, Henrik, 1964-
(författare)
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Low temperature difference power systems and implications of multi-phase screw expanders in Organic Rankine Cycles
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- New and old data on screw expanders operating with 2-phase mixtures in the admission line has been combined to enable the first public correlation of adiabatic expansion efficiency as a function of entry vapour fraction. Although not yet perfected, these findings have enabled an entirely new approach to the design and optimisation of Organic Rankine Cycles, ORCs. By allowing a continuous variation of vapour fraction at expander entry optima for thermal efficiency, second law efficiency and cost efficiency can be found. Consequently one can also find maxima for power output in the same dimension.This research describes a means of adapting cycle characteristics to various heat sources by varying expander inlet conditions from pure liquid expansion, through mixed fluid and saturated gas expansion, to superheated gas. Thermodynamic analysis and comparison of the above optimisations were a challenge. As most terms of merit for power cycles have been developed for high temperature applications they are often simplified by assuming infinite heat sinks. In many cases they also require specific assumptions on e.g. pinch temperatures, saturation conditions, critical temperatures etc, making accurate systematic comparison between cycles difficult. As low temperature power cycles are more sensitive to the ‘finiteness’ of source and sink than those operating with high temperatures, a substantial need arises for an investigation on which term of merit to use.Along with an investigation on terms of merit, the definition of high level reversible reference also needed revision. Second law efficiency, in the form of exergy efficiency, turned out to be impractical and of little use. A numerical approach, based on a combination of first and second law, was developed. A theory and method for the above is described. Eventually low temperature power cycle test data was compiled systematically. Despite differences in fluid, cycle, temperature levels and power levels the data correlated well enough to allow for a generalised, rough correlation on which thermal efficiency to expect as a function of utilization of source and sink availability. The correlation on thermal efficiency was used to create a graphical method to pre-estimate key economic factors for low temperature site potential in a very simple manner. A major consequence from the findings of this thesis is the reduced dependency on unique choices of process fluid to match heat source characteristics. This development significantly simplifies industrial standardisation, and thereby potentially improves cost efficiency of commercial ORC power generators.
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