| 1. |
- A Monfared, Behzad, 1983-
(författare)
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Magnetic Refrigeration for Near Room-Temperature Applications
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Refrigeration plays a crucial role in many different sectors and consumes about 17% of the electricity produced globally. This significant energy consumption implies large share of refrigeration in primary energy consumption and other environmental impacts. In addition to the environmental impacts associated with energy consumption, the vapor-compression systems contribute in global warming due to the release of their gaseous refrigerants into the atmosphere. As an alternative technology for near room-temperature applications, magnetic refrigeration is proposed by some researchers to eliminate the release of gaseous refrigerants into the atmosphere and to reduce the energy consumption. This thesis is a compilation of a number of studies done on magnetic refrigeration for room-temperature applications.In the first study, the environmental impacts associated to magnetic refrigeration are looked at closely through a life cycle assessment. The life cycle assessment indicates that because of the environmental burdens related to the rare-earth materials used in magnetic refrigeration, the reduction in the environmental impacts is not guaranteed by switching to magnetic refrigeration technology. Accordingly to avoid the extra environmental impacts the magnetic refrigeration systems should use magnetic materials frugally, which requires an optimized design. In addition, operation with higher efficiency compared to vapor-compression systems is necessary to have environmental advantages, at least in some impact categories.A practical method to optimize the design of magnetic refrigeration systems, e.g. to have a compact design or high efficiency, is utilizing a flexible software model, with which the effect of varying different parameters on the performance of the system can be simulated. Such a software model of the magnetic refrigeration system is developed and validated in this project. In developing the model one goal is to add to the precision of the simulated results by taking more details into consideration. This goal is achieved by an innovative way of modeling the parasitic heat transfer and including the effect of the presence of magnetocaloric materials on the strength of the field created by the magnet assembly. In addition, some efforts are made to modify or correct the existing correlations to include the effect of binding agents used in some active magnetic regenerators. Validation of the developed software model is done using the experimental results obtained from the prototype existing at the Department of Energy Technology, KTH Royal Institute of Technology.One of the parameters that can be modified by the developed software model is the choice of the magnetocaloric materials for each layer in a layered active magnetic regenerator. Utilizing the software model for optimizing the choice of the materials for the layers reveals that materials with critical temperatures equal to the cyclic average temperature of the layers in which they are used do not necessarily result in the desired optimum performance. In addition, for maximizing different outputs of the models, such as energy efficiency or temperature lift sustained at the two ends of the regenerators, different choice of materials for the layers are needed. Therefore, in other studies seeking to improve one of the outputs of a system, the choice of the transition or critical temperatures of the materials for each layer is an additional parameter to be optimized.The prototype existing at the Department of Energy Technology, KTH Royal Institute of Technology, was initially designed for replacing the vapor-compression system of a professional refrigerator. However, it could not fulfil the requirements for which it was initially designed. The aforementioned developed simulation model is used to see how much the choice of the materials, size of the particles, and number of layers can enhance the performance while the operation frequency and flow rate of the heat transfer fluid are at their optimum values. In other words, in that study the room for improvement in the performance without applying major changes in the system such as the geometry of the regenerator, which implies redesigning the whole magnet assembly, is investigated. In the redesign process the effect of binding agent and the limitations associated to different properties of it is also investigated theoretically. Nevertheless, the study did not show that with keeping the geometry of the regenerators and the currently existing magnetocaloric materials the initial goals of the prototype can be achieved.In the next study more flexible choice of geometries and magnetocaloric materials are considered. In fact, in this study it is investigated how much the magnetocaloric materials need to be improved so that magnetic refrigeration systems can compete with vapor-compression ones in terms of performance. For the two investigated cases, the magnetic-field dependent properties of the currently existing materials are enough provided that some other issues such as low mechanical stability and inhomogeneity of the properties are solved. Nevertheless, for more demanding design criteria, such as delivering large cooling capacity over a considerable temperature span while the magnetic materials are used sparingly, the magnetic-field dependent properties need to be enhanced, as well.A less explored area in room-temperature magnetic refrigeration is the subject of another study included in the thesis. In this study, solid-state magnetic refrigeration systems with Peltier elements as heat switches are modeled. Since the Peltier elements consume electricity to pump heat, the modeled systems can be considered hybrid magnetocaloric-Peltier cooling systems. For such systems the detailed transient behavior of the Peltier elements together with layers of magnetocaloric materials are modeled. The mathematical model is suitable for implementation in programing languages without the need for commercial modeling platforms. The parameters affecting the performance of the modeled system are numerous, and optimization of them requires a separate study. However, the preliminary attempts on optimizing the modeled system does not give promising results. Accordingly, focusing on passive heat switches can be more beneficial.
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| 2. |
- Abuasbeh, Mohammad, 1988-, et al.
(författare)
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Aquifer Thermal Energy Storage Insight into the future
- 2018
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Rapport (refereegranskat)abstract
- Underground Thermal Energy Storage (UTES) systems, such as Aquifer thermal energy storage(ATES) are used in several countries. The regulation and research on the potential impacts of ATESon groundwater resources and the subsurface environment often lag behind the technologicaldevelopment of an ever-growing demand for this renewable energy source. The lack of a clear andscientifically supported risk management strategy implies that potentially unwanted risks might betaken at vulnerable locations such as near well fields used for drinking water production. At othersites, on the other side, the application of ATES systems is avoided without proper reasons. Thisresults in limiting the utilization of the ATES technology in many occasions, affecting the possibilityto increase the share of renewable energy use. Therefore, further studies to characterizegroundwater resources, performance monitoring and identification of environmental impacts areneeded to understand the advantages and limitations of ATES systems.The environmental impact and technical performance of a Low Temperature ATES (LT-ATES)system in operation since 2016 is presented. The system is called Rosenborg and is owned byVasakronan. It is located in the northern part of Stockholm, on a glaciofluvial deposit called theStockholm esker. The ATES system is used to heat and cool two commercial buildings with a totalarea of around 30,000 m2. The ATES consists of 3 warm and 2 cold pumping wells that are able topump up to 50 liters per second.Analysis of groundwater sampling included a period of 9 months prior to ATES operation as well asthe first full season of heating and cooling operation. The sampling was conducted in a group ofwells in the vicinity of the installation and within the system. Means of evaluation constituted astatistical approach that included Kruskal-Wallis test by ranks, to compare the wells before and afterthe ATES was used. Then principal component analysis (PCA) and clustering analysis were used tostudy the ground water conditions change before and after the ATES. Aquifer Variation Ratio(AVR) was suggested as mean to evaluate the overall conditions of the aquifer pre- and post- ATES.The results showed some variations in redox potential, particularly at the cold wells which likely wasdue to the mixing of groundwater considering the different depths of groundwater beingabstracted/injected from different redox zones. Arsenic, which has shown to be sensitive to hightemperatures in other research showed a decrease in concentration. A lower specific conductivityand total hardness at the ATES well compared to their vicinity was found. That indicates that theyare less subject to salinization and that no accumulation has occurred to date. It is evident that theenvironmental impact from ATES is governed by the pre-conditions in soil- and groundwater. ThePCA and clustering analysis showed very little change in the overall conditions in the aquifer whencomparing the ATES before and after operation. Temperature change showed negligible impact.This can be mainly attributed to the relatively small temperature change (+6 and – 5 degrees) fromthe undisturbed Aquifer temperature which is 10.5°C.Performance of Aquifer Thermal Energy Storage (ATES) systems for seasonal thermal storagedepends on the temperature of the extracted/injected groundwater, water pumping rates and thehydrogeological conditions of the aquifer. ATES systems are therefore often designed to work witha temperature difference between the warm side and cold side of the aquifer without riskinghydraulic and thermal intrusion between them, and avoiding thermal leakage to surrounding area, i.e. optimize hydraulic and thermal recovery. The hydraulic and thermal recovery values of the first yearof operation in Rosenorg weres 1.37 and 0.33, respectively, indicating that more storage volume(50500m3) was recovered during the cooling season than injected (36900m3) in the previous heatingseason.Monitoring the operation of pumping and observation wells is crucial for the validation of ATESgroundwater models utilized for their design, and measured data provides valuable information forresearchers and practitioners working in the field. After months of planning and installation work,selected measurements recorded in an ATES monitoring project in Sweden during the first threeseasons of operation are reported in this report.The monitoring system consists of temperature sensors and flow meters placed at the pumpingwells, a distributed temperature-sensing rig employing fiber optic cables as linear sensor andmeasuring temperature every 0.25 m along the depth of all pumping and several observation wells,yielding temporal and spatial variation data of the temperature in the aquifer. The heat injection andextraction to and from the ground is measured using power meters at the main line connecting thepumping wells to the system. The total heat and cold extracted from the aquifer during the firstheating and cooling season is 190MWh and 237MWh, respectively. A total of 143 MWh of heatwere extracted during the second heating season. The hydraulic and thermal recovery values of thefirst year of operation was 1.37 and 0.33, respectively, indicating that more storage volume(50500m3) was recovered during the cooling season than injected (36900m3) in the previous heatingseason. The DTS data showed traces of the thermal front from the warm storage reaching the coldone. Only 33% of the thermal energy was recovered. These losses are likely due to ambientgroundwater flow as well as conduction losses at the boundaries of the storage volume. Additionally,the net energy balance over the first year corresponds to 0.12 which indicates a total net heating ofthe ATES over the first year. It is recommended to increase the storage volume and achieve morehydraulic and thermal balance in the ATES system. This can enhance the thermal recovery andoverall performance. Continuous monitoring of the ATES is and will be ongoing for at least 3 moreyears. The work presented in this report is an initial evaluation of the system aiming to optimize theATES performance.Furthermore, data management and processing tool has been established for the ATES system in Rosenborg. Additionally, a conceptual model of the ATES area has been established. Current andfuture work is focussed on completing a full scale numerical model in FEFLOW and validated themodel (both hydraulically and thermally) with the available monitoring data. Furthermore,establishing recommendations for optimum design and operation of ATES system.
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| 3. |
- Abuasbeh, Mohammad, 1988-, et al.
(författare)
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Long term performance monitoring and KPIs' evaluation of Aquifer Thermal Energy Storage system in Esker formation : Case study in Stockholm
- 2021
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Ingår i: Geothermics. - : Elsevier BV. - 0375-6505 .- 1879-3576. ; 96
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Tidskriftsartikel (refereegranskat)abstract
- The majority of Aquifer Thermal Energy Storage (ATES) systems studies have been conducted in aquifer systems located in large sand aquifers. Esker formation present a more challenging geometrical complexity compared to typical sand aquifers. This study aims to conduct comprehensive and long term performance evaluation of doublet type ATES system in esker geological formation in Stockholm, Sweden. The total heating and cooling used from the ATES are 673 MWh and 743 MWh respectively during the first 3 annual storage cycles of operation. The licensed total amount of water extraction and injection is 50 liters per second with undisturbed groundwater temperature of 9.5 degrees C. Over the first three storage cycles, the average injection and extraction temperatures for the warm side are 13.3 degrees C and 12.1 degrees C, and for the cold side 7.6 degrees C and 10.5 degrees C. The average temperature differences across the main heat exchanger from the ATES side are 4.5 K during winter and 2.8 K during summer which is 4-5 degrees lower than the optimum value. The average thermal recovery efficiency over the first 3 storage cycles were 47 % and 60 % for warm and cold storages respectively. The data analysis indicated annual energy and hydraulic imbalances which results into undesirable thermal breakthrough between the warm and cold side of the aquifer. This was mainly due to suboptimal operation of the building energy system which led to insufficient heat recovery from the warm side, and subsequently insufficient cold injection in the cold wells, despite the building heating demand and the available suitable temperatures in the ATES. The cause of the suboptimal operation is the oversizing of the heat pumps which were designed to be coupled to larger thermal loads as compared to the ones in the final system implementation. As a result, the heat pumps could not be operated during small-medium loads. Additionally, the paper discusses the limitations of currently used energy and thermal key performance indicators (KPI) for ATES and propose an additional thermal KPI named heat exchanger efficiency balance (beta HEX) that connects and evaluate the optimum operational point of temperature differences from both the building and ATES prospective. In addition to ATES energy and hydraulic KPIs, beta HEX can contribute in providing more complete picture on the ATES-building interaction performance as well as highlights if the losses in energy recovery from ATES are due to the subsurface processes or building energy system operation which has been proven to be critical for the optimum ATES performance.
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| 4. |
- Acuña, José, 1982-, et al.
(författare)
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A novel coaxial BHE : Description and first Distributed Thermal Response Test Measurements
- 2010
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Ingår i: Proceedings World Geothermal Congress 2010. ; , s. paper 2953-
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Konferensbidrag (refereegranskat)abstract
- The thermal performance of a Borehole Heat Exchanger plays a significant role when defining the quality of heat exchange with the ground in Ground Source Heat Pumps. Different designs have been discussed and increased interest on innovation within this field has taken place during the last years. This paper presents the first measurement results from a 189 meters deep novel coaxial Borehole Heat Exchanger, consisting of an inner central pipe and an annular channel in direct contact with the surrounding bedrock. The measurements were taken during a distributed thermal response test using fiber optic cables installed in the energy well. Fluid temperature every ten meters along the borehole depth are presented and compared with similar measurements from a common U-pipe heat exchanger. A unique measurement of the borehole wall temperature in the coaxial collector illustrates how effective the heat transfer performance is through the annular channel.
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| 5. |
- Acuña, José, 1982-, et al.
(författare)
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Characterization of Boreholes : Results from a U-pipe Borehole Heat Exchanger Installation
- 2008
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Ingår i: Proceedings 9th IEA Heat Pump Conference 2008. - Zurich, Switzerland : International Energy Agency. - 9789185829811 ; , s. 4-19
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Konferensbidrag (refereegranskat)abstract
- Heat exchange with the bedrock for ground source heat pumps is commonly done with the help of U-pipe energy collectors in vertical boreholes. At the moment, there exist many uncertainties about how efficient the heat transfer between the rock and the collector is. For a complete performance analysis of these systems, a 260 m deep water filled borehole is characterized, by measuring the borehole deviation, the ground water flow and the undisturbed ground temperature. Significant attention is devoted to detailed temperature measurements along the borehole depth during operation providing a complete description of the temperature variations in time both for the secondary working fluid and for the ground water. The results show a deviated borehole from the vertical direction without any relevant ground water flow. The undisturbed ground temperature gradient varies from negative to positive at approximately half of the borehole depth. The transient response of the borehole during the heat pump start up is illustrated and it is observed that there does not exist any thermal short circuiting between the down and up-going pipes when the system is in operation.
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| 6. |
- Acuña, José, 1982-, et al.
(författare)
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Comprehensive Summary of Borehole Heat Exchanger Research at KTH
- 2010
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Ingår i: IIR/Eurotherm Sustainable Refrigeration and Heat Pump Technology Conference. - Stockholm : KTH Royal Institute of Technology. - 9789174156935 ; , s. 69-
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Konferensbidrag (refereegranskat)abstract
- A research project that aims at presenting recommendations for improving the COP of ground source heat pump systems by 10-20% through better design of Borehole Heat Exchangers (BHE) is described in this paper. Experiments are carried out with temperature measurements taken in different BHE types during heat pump operation conditions as well as during the thermal response tests. It is also expected to point out methods for having natural fluid circulation in the BHE, i.e. demonstrating that the heat carrier fluid can naturally circulate thanks to temperature induced density differences along the borehole depth, and thereby avoiding the use of electricity consuming pumps. A brief background presenting the most relevant work regarding BHE research around the world is first presented, followed by a comprehensive description of the current research at KTH. Some new measurements and obtained results are presented as an estimation of to what extent the project results have been achieved is discussed. An analysis on how the project results could allow reducing the borehole depth keeping today’s Coefficient of Performance is presented.
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| 7. |
- Acuña, José, 1982-, et al.
(författare)
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Distributed Temperature Measurements on a Multi-pipe Coaxial Borehole Heat Exchanger
- 2011
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Ingår i: IEA Heat Pump Conference. - : International Energy Agency. ; , s. 4.19-
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Konferensbidrag (refereegranskat)abstract
- The first experiences with a multi-pipe borehole heat exchanger prototype consisting of an insulated central pipe and twelve parallel peripheral pipes are described. Secondary fluid distributed temperature measurements along the borehole depth, being the only ones of its kind in this type of heat exchanger, are presented and discussed. The measurements are carried out with fiber optic cables during heat injection into the ground, giving a detailed visualization of what happens both along the central and peripheral flow channels. The heat exchange with the ground mainly occurs along the peripheral channels and an indication of almost no thermal short circuiting, even while having large temperature differences between the down and upwards channels, is observed.
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| 8. |
- Acuña, José, 1982-, et al.
(författare)
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Distributed Temperature Measurements on a U-pipe Thermosyphon Borehole Heat Exchanger With CO2
- 2010
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Ingår i: Refrigeration Science and Technology Proceedings. - Sydney, Australia : International Institute of Refrigeration.
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Konferensbidrag (refereegranskat)abstract
- In thermosyphon Borehole Heat Exchangers, a heat carrier fluid circulates while exchanging heat with the ground without the need of a circulation pump, representing an attractive alternative when compared to other more conventional systems. Normally, the fluid is at liquid-vapor saturation conditions and circulation is maintained by density differences between the two phases as the fluid absorbs energy from the ground. This paper presents some experimental experiences from a 65 meter deep thermosyphon borehole heat exchanger loop using Carbon Dioxide as heat carrier fluid, instrumented with a fiber optic cable for distributed temperature measurements along the borehole depth. The heat exchanger consists of an insulated copper tube through which the liquid CO2 flows downwards, and a copper tube acting as a riser. The results show temperatures every two meters along the riser, illustrating the heat transfer process in the loop during several heat pump cycles.
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| 9. |
- Acuña, José, 1982-, et al.
(författare)
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Distributed Thermal Response Test on a U-Pipe Borehole Heat Exchanger
- 2009
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Ingår i: Proc. Effstock 2009, 11th International Conference on Thermal Energy Storage. - Stockholm, Sweden : Academic Conferences Publishing.
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Konferensbidrag (refereegranskat)abstract
- In a Distributed Thermal Response Test (DTRT) the ground thermal conductivity and boreholethermal resistance are determined at many instances along the borehole. Here, such a testis carried out at a 260 m deep water filled energy well, equipped with a U-pipe borehole heatexchanger, containing an aqueous solution of ethanol as working fluid. Distributed temperaturemeasurements are carried out using fiber optic cables placed inside the U-pipe, duringfour test phases: undisturbed ground conditions, fluid pre-circulation, constant heat injection,and borehole recovery. A line source model is used for simulating the borehole thermal response.Fluid temperature profiles during the test are presented. The results show local variationsof the ground thermal conductivity and borehole thermal resistance along the boreholedepth, as well as a deviation of the latter as compared to the one resulting from a standardthermal response test.
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| 10. |
- Acuña, José, 1982-
(författare)
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Distributed thermal response tests : New insights on U-pipe and Coaxial heat exchangers in groundwater-filled boreholes
- 2013
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Konstnärligt arbete (övrigt vetenskapligt/konstnärligt)abstract
- U-pipe Borehole Heat Exchangers (BHE) are widely used today in ground source heating and cooling systems in spite of their less than optimal performance. This thesis provides a better understanding on the function of U-pipe BHEs and Investigates alternative methods to reduce the temperature difference between the circulating fluid and the borehole wall, including one thermosyphon and three different types of coaxial BHEs.Field tests are performed using distributed temperature measurements along U-pipe and coaxial heat exchangers installed in groundwater filled boreholes. The measurements are carried out during heat injection thermal response tests and during short heat extraction periods using heat pumps. Temperatures are measured inside the secondary fluid path, in the groundwater, and at the borehole wall. These type of temperature measurements were until now missing.A new method for testing borehole heat exchangers, Distributed Thermal Response Test (DTRT), has been proposed and demonstrated in U-pipe, pipe-in-pipe, and multi-pipe BHE designs. The method allows the quantification of the BHE performance at a local level.The operation of a U-pipe thermosyphon BHE consisting of an insulated down-comer and a larger riser pipe using CO2 as a secondary fluid has been demonstrated in a groundwater filled borehole, 70 m deep. It was found that the CO2 may be sub-cooled at the bottom and that it flows upwards through the riser in liquid state until about 30 m depth, where it starts to evaporate.Various power levels and different volumetric flow rates have been imposed to the tested BHEs and used to calculate local ground thermal conductivities and thermal resistances. The local ground thermal conductivities, preferably evaluated at thermal recovery conditions during DTRTs, were found to vary with depth. Local and effective borehole thermal resistances in most heat exchangers have been calculated, and their differences have been discussed in an effort to suggest better methods for interpretation of data from field tests.Large thermal shunt flow between down- and up-going flow channels was identified in all heat exchanger types, particularly at low volumetric flow rates, except in a multi-pipe BHE having an insulated central pipe where the thermal contact between down- and up-coming fluid was almost eliminated.At relatively high volumetric flow rates, U-pipe BHEs show a nearly even distribution of the heat transfer between the ground and the secondary fluid along the depth. The same applies to all coaxial BHEs as long as the flow travels downwards through the central pipe. In the opposite flow direction, an uneven power distribution was measured in multi-chamber and multi-pipe BHEs.Pipe-in-pipe and multi-pipe coaxial heat exchangers show significantly lower local borehole resistances than U-pipes, ranging in average between 0.015 and 0.040 Km/W. These heat exchangers can significantly decrease the temperature difference between the secondary fluid and the ground and may allow the use of plain water as secondary fluid, an alternative to typical antifreeze aqueous solutions. The latter was demonstrated in a pipe-in-pipe BHE having an effective resistance of about 0.030 Km/W.Forced convection in the groundwater achieved by injecting nitrogen bubbles was found to reduce the local thermal resistance in U-pipe BHEs by about 30% during heat injection conditions. The temperatures inside the groundwater are homogenized while injecting the N2, and no radial temperature gradients are then identified. The fluid to groundwater thermal resistance during forced convection was measured to be 0.036 Km/W. This resistance varied between this value and 0.072 Km/W during natural convection conditions in the groundwater, being highest during heat pump operation at temperatures close to the water density maximum.
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| 11. |
- Acuña, José, 1982-, et al.
(författare)
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Distributed Thermal Response Tests on a Multi-pipe Coaxial Borehole Heat Exchanger
- 2011
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Ingår i: HVAC & R RESEARCH. - London : Taylor & Francis. - 1078-9669 .- 1938-5587. ; 17:6, s. 1012-1029
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Tidskriftsartikel (refereegranskat)abstract
- In a distributed thermal response test, distributed temperature measurements are taken along a borehole heat exchanger during thermal response tests, allowing the determination of local ground thermal conductivities and borehole thermal resistances. In this article, the first results from six heat injection distributed thermal response tests carried out on a new, thermally insulated leg type, multi-pipe coaxial borehole heat exchanger are presented. The borehole heat exchanger consists of 1 insulated central and 12 peripheral pipes. Temperature measurements are carried out using fiber-optic cables placed inside the borehole heat exchanger pipes. Unique temperature and thermal power profiles along the borehole depth as a function of the flow rate and the total thermal power injected into the borehole are presented. A line source model is used for simulating the borehole heat exchanger thermal response and determining local variations of the ground thermal conductivity and borehole thermal resistance. The flow regime in the peripheral pipes is laminar during all distributed thermal response tests and average thermal resistances remain relatively constant, independently of the volumetric flow rate, being lower than those corresponding to U-pipe borehole heat exchangers. The thermal insulation of the central pipe significantly reduces the thermal shunt to the peripheral pipes even at low volumetric flow rates.
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| 12. |
- Acuña, José, 1982-, et al.
(författare)
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Distributed thermal response tests on pipe-in-pipe borehole heat exchangers
- 2013
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Ingår i: Applied Energy. - : Elsevier BV. - 0306-2619 .- 1872-9118. ; 109:SI, s. 312-320
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Tidskriftsartikel (refereegranskat)abstract
- Borehole Thermal Energy Storage systems typically use U-pipe Borehole Heat Exchangers (BHE) having borehole thermal resistances of at least 0.06 K m/W. Obviously, there is room for improvement in the U-pipe design to decrease these values. Additionally, there is a need for methods of getting more detailed knowledge about the performance of BHEs. Performing Distributed Thermal Response Tests (DTRT) on new proposed designs helps to fill this gap, as the ground thermal conductivity and thermal resistances in a BHE can be determined at many instances in the borehole thanks to distributed temperature measurements along the depth. In this paper, results from three heat injection DTRTs carried out on two coaxial pipe-in-pipe BHEs at different flow rates are presented for the first time. The tested pipe-in-pipe geometry consists of a central tube inserted into a larger external flexible pipe, forming an annular space between them. The external pipe is pressed to the borehole wall by applying a slight overpressure at the inside, resulting in good thermal contact and at the same time opening up for a novel method for measuring the borehole wall temperature in situ, by squeezing a fiber optic cable between the external pipe and the borehole wall. A reflection about how to calculate borehole thermal resistance in pipe-in-pipe BHEs is presented. Detailed fluid and borehole wall temperatures along the depth during the whole duration of the DTRTs allowed to calculate local and effective borehole thermal resistances and ground thermal conductivities. Local thermal resistances were found to be almost negligible as compared to U-pipe BHEs, and the effective borehole resistance equal to about 0.03 K m/W. The injected power was found to be almost evenly distributed along the depth.
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| 13. |
- Acuña, José, 1982-, et al.
(författare)
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Evaluation of a coaxial borehole heat exchanger prototype
- 2010
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Ingår i: Proceedings of the 14th ASME International Heat Transfer Conference. - : ASME Press. - 9780791849392
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Konferensbidrag (refereegranskat)abstract
- Different borehole heat exchanger designs have been discussed for many years. However, the U-pipe design has dominated the market, and the introduction of new designs has been practically lacking. The interest for innovation within this field is rapidly increasing and other designs are being introduced on the market. This paper presents a general state of the art summary of the borehole heat exchanger research in the last years. A first study of a prototype coaxial borehole heat exchanger consisting of one central pipe and five external channels is also presented. The particular geometry of the heat exchanger is analyzed thermally in 2-D with a FEM software. An experimental evaluation consisting of two in situ thermal response tests and measurements of the pressure drop at different flow rates is also presented. The latter tests are carried out at two different flow directions with an extra temperature measurement point at the borehole bottom that shows the different heat flow distribution along the heat exchanger for the two flow cases. The borehole thermal resistance of the coaxial design is calculated both based on experimental data and theoretically.
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| 14. |
- Acuña, José, 1982-, et al.
(författare)
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Experimental Comparison of Four Borehole Heat Exchangers
- 2008
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Ingår i: Refrigeration Science and Technology Proceedings. - Copenhagen : International Institute of Refrigeration. - 9788792127037 ; , s. SEC09-W1-09
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Konferensbidrag (refereegranskat)abstract
- The most common way to exchange heat with the bedrock in ground source heat pump applications is circulating a secondary fluid through a closed U-pipe loop in a vertical borehole. This fluid transports the heat from the rock to the ground source heat pump evaporator. The quality of the heat exchange with the ground and the necessary pumping power to generate the fluid circulation are dependent on the type of fluid and its flow conditions along the pipe. Four different borehole heat exchangers are tested using ethyl alcohol with 20% volume concentration. The fluid temperatures are logged at the borehole inlet, bottom, and outlet. The collectors are compared based on their borehole thermal resistance and pressure drop at different flow rates. The results indicate that the pipe dimensions play an important roll, spacers might not contribute to better heat transfer, and inner micro fins in the pipes improve the performance of the collectors.
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| 15. |
- Acuña, José, 1982-, et al.
(författare)
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First Experiences with Coaxial Borehole Heat Exchangers
- 2011
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Ingår i: Proceedings of the IIR Conference on Sources/Sinks alternative to the outside Air for HPs and AC techniques. - : International Institute of Refrigeration. - 9782913149847
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Konferensbidrag (refereegranskat)abstract
- Some experiences with coaxial borehole heat exchanger prototypes are discussed here. Four different designs are described as they have been part of a research project at KTH: two pipe-inpipe annular designs, one multi-pipe and one multi-chamber design. A special focus is given to two of the prototypes, a pipe-in-pipe design with the external flow channel consisting of an annular cross section and partly insulated central pipe, and a multi-pipe design with twelve parallel peripheral pipes and an insulated central channel. The secondary fluid temperature profiles at low volumetric flow rates are presented for these two prototypes, measured with fiber optic cables during thermal response tests and allowing a detailed visualization of what happens along the heat exchanger depth. It is the first time this is carried out in these types of borehole heat exchangers. The measurements indicate good thermal performance and point at potential uses for these heat exchangers in different ground coupled applications.
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| 16. |
- Acuña, José, 1982-
(författare)
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Improvements of U-pipe Borehole Heat Exchangers
- 2010
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The sales of Ground Source Heat Pumps in Sweden and many other countries are having a rapid growth in the last decade. Today, there are approximately 360 000 systems installed in Sweden, with a growing rate of about 30 000 installations per year. The most common way to exchange heat with the bedrock in ground source heat pump applications is circulating a secondary fluid through a Borehole Heat Exchanger (BHE), a closed loop in a vertical borehole. The fluid transports the heat from the ground to a certain heating and/or cooling application. A fluid with one degree higher or lower temperature coming out from the borehole may represent a 2-3% change in the COP of a heat pump system. It is therefore of great relevance to design cost effective and easy to install borehole heat exchangers. U-pipe BHEs consisting of two equal cylindrical pipes connected together at the borehole bottom have dominated the market for several years in spite of their relatively poor thermal performance and, still, there exist many uncertainties about how to optimize them. Although more efficient BHEs have been discussed for many years, the introduction of new designs has been practically lacking. However, the interest for innovation within this field is increasing nowadays and more effective methods for injecting or extracting heat into/from the ground (better BHEs) with smaller temperature differences between the heat secondary fluid and the surrounding bedrock must be suggested for introduction into the market.This report presents the analysis of several groundwater filled borehole heat exchangers, including standard and alternative U-pipe configurations (e.g. with spacers, grooves), as well as two coaxial designs. The study embraces measurements of borehole deviation, ground water flow, undisturbed ground temperature profile, secondary fluid and groundwater temperature variations in time, theoretical analyses with a FEM software, Distributed Thermal Response Test (DTRT), and pressure drop. Significant attention is devoted to distributed temperature measurements using optic fiber cables along the BHEs during heat extraction and heat injection from and to the ground.
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| 17. |
- Acuna, José, et al.
(författare)
-
Numerically generated g-functions for ground coupled heat pump applications
- 2012
-
Ingår i: Proceedings of the COMSOL Conference in Milan.
-
Konferensbidrag (refereegranskat)abstract
- In most ground-coupled heat pump systems, Borehole Heat Exchangers (BHE) represent the typical engineering solution for utilizing renewable energy from the ground. The design of a complex BHE field is a challenging task, due the inherent transient nature of the thermal interaction between the heat exchangers and the surrounding soil. A computation effective method for solving the 3D transient conduction equation describing the ground response to a variable heat load profile is the temporal superposition of pre-calculated temperature response factors or g-functions. In this study Comsol heat conduction models have been developed to calculate g-function values for a borehole field with 64 boreholes. The aim of the investigation is to get an insight on the numerical generation of temperature transfer functions and to some extent provide new information on the Finite Line Source method for analytically generated g-functions as well as on those existing behind existing design software such as EED. The results generally showed a good agreement in lower time ranges. Further in time, the Comsol model revealed to be influenced either by the domain dimensions or the simulation end time.
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| 18. |
- Ali, Rashid, et al.
(författare)
-
A Visualization Study During Flow Boiling of R134a In A Horizontal Microchannel
- 2010
-
Ingår i: ASME 2010 8th International Conference on Nanochannels, Microchannels, and Minichannels Collocated with 3rd Joint US-European Fluids Engineering Summer Meeting, ICNMM2010<em><em><em><em><em><em></em></em></em></em></em></em>. - 9780791854501 ; , s. 85-94
-
Konferensbidrag (refereegranskat)abstract
- In this paper, the experimental flow boiling visualization results of a microchannel are presented and discussed. A series of visualization experiments have been conducted in a horizontal, circular, uniformly heated microchannel, to record the two-phase flow patterns evolved during the boiling process and to study the ebullition process. A high speed camera (REDLAKE HG50LE) with a maximum of 100000 fps together with tungsten lights was used to capture the images along the test section. Microchannel was made of circular fused silica tube having an internal diameter of 0.781 mm and a uniformly heated length of 191 mm. Outside of the test tube was coated with a thin, electrically conductive layer of Indium Tin Oxide (ITO) for direct heating of the test section. Refrigerant R134a was used as working fluid and experiments were performed at two different system pressures corresponding to saturation temperatures of 25 degrees C and 30 degrees C. Mass flux was varied from 100 kg/m(2)s to 400 kg/m(2)s and heat flux ranged from 5 kW/m(2) to 45 kW/m(2). Visualization results show that the bubble growth is restricted by the tube diameter which results in very short existence of isolated bubbly flow regime except essentially restricted to a very short length of test tube. Flow patterns observed along the length were: Isolated bubble, elongated bubble, slug flow, semi annular and annular flow. Rigorous boiling and increased coalescence rates were observed with increase in heat flux. Bubble frequency was observed to increase with both heat and mass flux. A comparison with our previous flow boiling visualization studies, carried out for a test tube of 1.33 mm internal diameter, shows that the number of active nucleation sites is less while the bubble frequency is higher for the current study. Mean bubble length and bubble velocity during elongated bubble flow pattern have also been calculated from the images obtained during the tests.
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| 19. |
- Ali, Rashid, et al.
(författare)
-
Dryout Characteristics During Flow Boiling of R134a in Vertical Circular Minichannels
- 2011
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 54:11-12, s. 2434-2445
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, the experimental results of dryout during flow boiling in minichannels are reported and analysed. Experiments were carried out in vertical circular minichannels with internal diameters of 1.22 mm and 1.70 mm and a fixed heated length of 220 mm. R134a was used as working fluid. Mass flux was varied from 50 kg/m(2) s to 600 kg/m(2) s and experiments were performed at two different system pressures corresponding to saturation temperatures of 27 degrees C and 32 degrees C. Experimental results show that the dryout heat flux increases with mass flux and decreases with tube diameter while system pressure has no clear effect for the range of experimental conditions covered. Finally, the prediction capabilities of the well known critical heat flux (CHF) correlations are also tested.
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| 20. |
|
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| 21. |
- Ali, Rashid, et al.
(författare)
-
Experimental Investigation of Two-phase Pressure Drop in a Microchannel
- 2011
-
Ingår i: Heat Transfer Engineering. - : Informa UK Limited. - 0145-7632 .- 1521-0537. ; 32:13/14, s. 1126-1138
-
Tidskriftsartikel (refereegranskat)abstract
- Experimental results of two-phase pressure drop in a horizontal circular microchannel are reported in this paper. A test tube was made of fused silica having an internal diameter of 781 mu m with a total length of 261 mm and a heated length of 191 mm. The outer surface of the test tube was coated with an electrically conductive thin layer of ITO (indium tin oxide) for direct heating of the test section. Refrigerants R134a and R245fa were used as the working fluids, and mass flux during the experiments was varied between 100 and 650 kg/m(2)-s. Experiments were performed at two different system pressures corresponding to saturation temperatures of 25 degrees C and 30 degrees C for R134a and at three different system pressures corresponding to saturation temperatures of 30 degrees C, 35 degrees C, and 40 degrees C for R245fa. Two-phase frictional pressure drop characteristics with variation of mass flux, vapor fraction, saturation temperature, and heat flux were explored in detail. Finally, the prediction capability of some well-known correlations available in the literature, some developed for macrochannels and others especially developed for microchannels, was assessed.
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| 22. |
- Ali, Rashid, et al.
(författare)
-
Flow Boiling Heat Transfer Characteristics of a Minichannel up to Dryout Condition
- 2010
-
Ingår i: MNHMT2009, VOL 2. - New York : AMER SOC MECHANICAL ENGINEERS. - 9780791843901 ; , s. 25-34
-
Konferensbidrag (refereegranskat)abstract
- In this paper the experimental flow boiling heat transfer results of a minichannel are presented. A series of experiments was conducted to measure the heat transfer coefficients in a minichannel made of stainless steel (AISI 316) having an internal diameter of 1.7mm and a uniformly heated length of 220mm. R134a was used as working fluid and experiments were performed at two different system pressures corresponding to saturation temperatures of 27 degrees C and 32 degrees C. Mass flux was varied from 50 kg/m(2) s to 600 kg/m(2) s and heat flux ranged from 2kW/m(2) to 156 kW/m(2). The test section was heated directly using a DC power supply. The direct heating of the channel ensured uniform heating and heating was continued until dry out was reached. The experimental results show that the heat transfer coefficient increases with imposed wall heat flux while mass flux and vapour quality have no considerable effect. Increasing the system pressure slightly enhances the heat transfer coefficient. The heat transfer coefficient is reduced as dryout is reached. It is observed that dryout phenomenon is accompanied with fluctuations and a larger standard deviation in outer wall temperatures.
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| 23. |
- Ali, Rashid, et al.
(författare)
-
Flow Boiling Heat Transfer Characteristics of a Minichannel up to Dryout Condition
- 2011
-
Ingår i: Journal of heat transfer. - : ASME International. - 0022-1481 .- 1528-8943. ; 133:8, s. 081501-
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, the experimental flow boiling heat transfer results of a minichannel are presented. A series of experiments was conducted to measure the heat transfer coefficients in a minichannel made of stainless steel (AISI 316) having an internal diameter of 1.70 mm and a uniformly heated length of 220 mm. R134a was used as a working fluid, and experiments were performed at two different system pressures corresponding to saturation temperatures of 27 degrees C and 32 degrees C. Mass flux was varied from 50 kg/m(2) s to 600 kg/m(2) s, and heat flux ranged from 2 kW/m(2) to 156 kW/m(2). The test section was heated directly using a dc power supply. The direct heating of the channel ensured uniform heating, which was continued until dryout was reached. The experimental results show that the heat transfer coefficient increases with imposed wall heat flux, while mass flux and vapor quality have no considerable effect. Increasing the system pressure slightly enhances the heat transfer coefficient. The heat transfer coefficient is reduced as dryout is reached. It is observed that the dryout phenomenon is accompanied with fluctuations and a larger standard deviation in outer wall temperatures.
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| 24. |
- Ali, Rashid, et al.
(författare)
-
Flow Boiling Heat Transfer Of Refrigerants R134a And R245fa In A Horizontal Micro-Channel
- 2012
-
Ingår i: Experimental heat transfer. - : Informa UK Limited. - 0891-6152 .- 1521-0480. ; 25:3, s. 181-196
-
Tidskriftsartikel (refereegranskat)abstract
- Micro-channel-based evaporators are a promising option for high heat flux cooling applications. Micro-channels offer several advantages, including a smaller coolant inventory, superior heat transfer performance, compactness, lightness of weigh. Despite being attractive, the governing phenomena in micro-channels, especially during phase change, are less understood. This article reports the experimental flow boiling heat transfer results of refrigerants R134a and R245fa in a horizontal micro-channel. A series of experiments was conducted to measure the heat transfer coefficients in a circular micro-channel made of fused silica having an internal diameter of 781 mu m and a uniformly heated length of 191 mm. The outer surface of the test tube was coated with a thin, electrically conductive layer of indium-tin-oxide. The surface coating with the electrically conductive layer of indium-tin-oxide made it possible to visualize the flow boiling process simultaneously with uniform heating of the test section. R134a and R245fa were used as working fluids and experiments were performed at a system pressure of 7.7 bar for R134a and at 1.8 bar for R245fa, corresponding to saturation temperature of 30 degrees C. Mass flux was varied from 175 kg/m(2)s to 500 kg/m(2)s, and heat flux ranged from 5 kW/m(2) to 60 kW/m(2). A high-speed camera was used to capture the images in the case of flow boiling of R134a. The experimental results indicated that the heat transfer coefficient increased with heat flux while the mass flux proved to have a negligible effect on heat transfer coefficient.
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| 25. |
|
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| 26. |
- Ali, Rashid, et al.
(författare)
-
Flow Patterns and Flow Pattern Maps for Microchannels
- 2010
-
Ingår i: 2010 3rd International Conference on Thermal Issues in Emerging Technologies, Theory and Applications - Proceedings, ThETA3 2010. - 9781612842660 ; , s. 33-42
-
Konferensbidrag (refereegranskat)abstract
- Dense packaging of electronic components generates very high heat fluxes and therefore results in challenges for proper thermal management of such components. Microchannel based evaporators with phase changing liquids are regarded as a promising solution for such high heat flux cooling applications. Due to confinement of flow and differences in the relative importance of governing phenomena, the two-phase flow and heat transfer characteristics of microchannels have been shown to be different from those of conventional sized channels. The fact that microchannel is an attractive cooling option but at the same time there is a clear lack of understanding of related hydrodynamic and thermal transport phenomena which provides an impetus for microchannel research. This paper presents the flow patterns and flow pattern maps obtained for an experimental study of R134a during flow boiling in a horizontal microchannel. The microchannel was a fused silica tube, the outer surface of which was coated with thin, transparent and electrically conductive layer of Indium-Tin-Oxide (ITO). The microchannel was 781 m in internal diameter and 191 mm in heated length. Operating parameters during the experiments were: mass flux 100-400 kg/m2 s, heat flux 5-45 kW/m2, saturation temperature 25 and 30 °C. A High speed camera was used with a close up lens to capture the flow patterns evolved along the channel. Flow pattern maps are presented in terms of superficial gas and liquid velocity and in terms of Reynolds number and vapor quality plots. The results are compared with some flow pattern maps for conventional and micro scale channels available in literature.
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| 27. |
- Ali, Rashid, 1977-
(författare)
-
Phase Change Phenomena During Fluid Flow in Microchannels
- 2010
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Phase change phenomena of a fluid flowing in a micro channel may be exploited to make the heat exchangers more compact and energy efficient. Compact heat exchangers offer several advantages such as light weight, low cost, energy efficiency, capability of removing high heat fluxes and charge reduction are a few to mention. Phase change phenomena in macro or conventional channels have been investigated since long but in case of micro channels, fewer studies of phase change have been conducted and underlying phenomena during two-phase flow in micro channels are not yet fully understood. It is clear from the literature that the two-phase flow models developed for conventional channels do not perform well when extrapolated to micro scale. In the current thesis, the experimental flow boiling results for micro channels are reported. Experiments were conducted in circular, stainless steel and quartz tubes in both horizontal and vertical orientations. The internal diameters of steel tubes tested were 1.70 mm, 1.224 mm and the diameter of quartz tube tested was 0.781 mm. The quartz tube was coated with a thin, electrically conductive, transparent layer of Indium-Tin-Oxide (ITO) making simultaneous heating and visualization possible. Test tubes were heated electrically using DC power supply. Two refrigerants R134a and R245fa were used as working fluids during the tests. Experiments were conducted at a wide variety of operating conditions. Flow visualization results obtained with quartz tube clearly showed the presence of confinement effects and consequently an early transition to annular flow for micro channels. Several flow pattern images were captured during flow boiling of R134a in quartz tube. Flow patterns recorded during the experiments were presented in the form of Reynolds number versus vapour quality and superficial liquid velocity versus superficial gas velocity plots. Experimental flow pattern maps so obtained were also compared with the other flow pattern maps available in the literature showing a poor agreement. Flow boiling heat transfer results for quartz and steel tubes indicate that the heat transfer coefficient increases with heat flux and system pressure but is independent on mass flux and vapour quality. Experimental flow boiling heat transfer coefficient results were compared with those obtained using different correlations from the literature. Heat transfer experiments with steel tubes were continued up to dryout condition and it was observed that dryout conditions always started close to the exit of the tube. The dryout heat flux increased with mass flux and decreased with exit vapour quality. The dryout data were compared with some well known CHF correlations available in the literature. Two-phase frictional pressure drop for the quartz tube was also obtained under different operating conditions. As expected, two-phase frictional pressure drop increased with mass flux and exit vapour quality.
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| 28. |
- Ali, Rashid, et al.
(författare)
-
Study of flow boiling characteristics of a microchannel using high speed visualization
- 2013
-
Ingår i: Journal of heat transfer. - : ASME International. - 0022-1481 .- 1528-8943. ; 135:8, s. 081501-
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents the visualization results obtained for an experimental study of R134a during flow boiling in a horizontal microchannel. The microchannel used was a fused silica tube having an internal diameter of 781 lm, a heated length of 191mm, and was coated with a thin, transparent, and electrically conductive layer of indium-tin-oxide (ITO) on the outer surface. The operating parameters during the experiments were: mass flux 100-400 kg/m2 s, heat flux 5-45 kW/m2, saturation temperatures 25 and 30 °C, corresponding to saturation pressures of 6.65 bar and 7.70 bar and reduced pressures of 0.163 and 0.189, respectively. A high speed camera with a close up lens was used to capture the flow patterns that evolved along the channel. Flow pattern maps are presented in terms of the superficial gas and liquid velocity and in terms of the Reynolds number and vapor quality plots. The results are compared with some flow pattern maps for conventional and micro scale channels available in the literature. Rigorous boiling and increased coalescence rates were observed with an increase in the heat flux.
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| 29. |
|
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| 30. |
- Andersson, K., et al.
(författare)
-
Water to water heat pump with minimum charge of propane
- 2018
-
Ingår i: Refrigeration Science and Technology. - : International Institute of Refrigeration. - 9782362150265 ; , s. 725-732
-
Konferensbidrag (refereegranskat)abstract
- This paper describes the technology for an environmentally friendly ground source heat pump for a single family home, characterized by using pure water as coolant and propane as refrigerant. The objective was to build a test system, operating under realistic conditions, using less than 150 g of propane, providing at least 5 kW heating capacity with reasonable efficiency and without freezing the coolant water. The borehole heat exchanger was of coaxial type, providing about half the thermal resistance compared to a standard U-tube collector (Acuña 2010). The evaporator and condenser where asymmetrical plate heat exchangers with small channel height (< 1 mm) on the refrigerant side. They were developed and manufactured exclusively for this project with a new type of press pattern, including a special, small volume sub-cooling section at the end of the condenser. A DC-motor scroll compressor for AC in electric vehicles was used, characterized by small internal volumes, small oil charge and wide capacity range (800-9000 rpm). A PAG-type oil was used, which however seemed to cause some problems with heat transfer and pressure drop in the evaporator. The system also included a specially built mini channel liquid/suction line heat exchanger and a standard thermostatic expansion valve. The paper presents test results for a heating capacity range of 2-10 kW. The performance was reasonable in this range with a charge of 100 g of propane (-1
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| 31. |
- Anton, Raul, et al.
(författare)
-
Modeling of Air Conditioning Systems for Cooling of Data Centers
- 2002
-
Ingår i: Proc. 8<sup>th</sup> InterSociety Conf. on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm 2002), San Diego, CA, USA. - : IEEE Press. - 0780371526 ; , s. 552-558
-
Konferensbidrag (refereegranskat)abstract
- Cooling of data centers has emerged as an area of increasing importance in the field of electronics thermal management. As the packaging and power densities are steadily increasing, so will the need for efficient and reliable cooling systems.In this paper, a model of an air conditioning unit is described. The model offers considerable flexibility in terms of the ability to choose between different designs of heat exchangers (evaporators and condensers), and working media, and hence the model offers the possibility to optimize the design.The model is developed using EES (Engineering Equation Solver), a programming environment that provides the thermo-physical properties for the working media used. The heat transfer and pressure drop in the components are modeled using an integral approach, i.e. the overall behavior of each component is modeled. This approach has been proven to give adequate accuracy.
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| 32. |
- Antón Remírez, Raúl, 1977-
(författare)
-
Experimental and numerical study of the thermal and hydraulic effect of EMC screens in radio base stations : detailed and compact models
- 2006
-
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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| 33. |
- Anwar, Z., et al.
(författare)
-
Dryout characteristics of natural and synthetic refrigerants in single vertical mini-channels
- 2015
-
Ingår i: Experimental Thermal and Fluid Science. - : Elsevier. - 0894-1777 .- 1879-2286. ; 68, s. 257-267
-
Tidskriftsartikel (refereegranskat)abstract
- Experimental results on dryout of seven refrigerants (R134a, R1234yf, R152a, R22, R245fa, R290 and R600a) in small, single vertical tubes under upward flow conditions are reported in this study. The experiments were conducted under a wide range of operating conditions in stainless steel tubes (0.64-1.70. mm and 213-245. mm heated length). The effects of operating parameters like mass flux, vapor quality, saturation pressure and channel size are discussed in detail. In general, dryout heat flux increased with increasing mass flux, and with increasing tube diameter. No effect of varying saturation temperature was observed. The experimental findings were compared with well-known macro and micro-scale correlations from the literature and it was found that Wu's correlation (in modified form) quite satisfactorily predicted the whole database. A new correlation for prediction of heat flux at dryout conditions is also proposed.
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| 34. |
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| 35. |
- Anwar, Zahid, et al.
(författare)
-
Dryout characteristics of R1234yf in a uniformly heated vertical mini-channel
- 2013
-
Ingår i: UK Heat Transfer Conference.
-
Konferensbidrag (refereegranskat)abstract
- Two phase heat transfer in small channels has many practical applications like, miniature heatexchangers, high powered electronics, miniature refrigeration system. Flow boiling in these compactchannels offers many potential advantages like, cope with high heat flux, less fluid inventory,compactness in size. It is well known that two phase heat transfer is drastically reduced when theheater surface becomes partially dry, for any reason. Moving beyond the point where this happensresults in a sharp increase in the temperature of the heated surface and eventually leads towardsburnout. So the upper operational limit (from safety and efficiency point of view) is extremelyimportant to be able to predict.Experimental findings on dryout of Isobutane in a uniformly heated, vertical, stainless steel testsection (1.6 mm inside diameter and 245mm heated length) are reported in this article. Experimentswere conducted at two saturation pressures corresponding to temperatures of 27 and 32 oC, with fivemass fluxes in the range 50-350 kg/m2s and with vapor fractions at the outlet up till dryout conditions.Analysis showed that the dryout heat flux increased with increasing mass flux, while no effect ofvarying the operating pressure was observed. Experimental results were compared with differentcorrelations from the literature, Wu [5], Mikielewicz [6], Callizo [3] and Katto-Ohno [4] correlationsquite satisfactorily predicted the data.
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| 36. |
- Anwar, Zahid, et al.
(författare)
-
Dryout characteristics of R1234yf in a vertical mini-channel
- 2013
-
Ingår i: Eurotherm seminar on convective heat transfer.
-
Konferensbidrag (refereegranskat)abstract
- This article reports dryout characteristics of R1234yf in a single, uniformly heated vertical stainless steel channel (d=1.6mm, Lh=245mm). Tests were conducted at 27 and 32 oC saturation temperature with 100-500 kg/m2s. Results of various operating parameters (mass flux, vapor quality, saturation temperature) were discussed in detail. Comparison with R134a revealed lower (about 18%) critical heat flux values with R1234yf. Experimental findings were compared with various macro & micro scale correlations from the literature. Katto- Ohno and Wu’s correlations [7,8] accurately predicted the data from macro and micro scale models respectively.
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| 37. |
- Anwar, Z., et al.
(författare)
-
Flow boiling heat transfer and dryout characteristics of R152a in a vertical mini-channel
- 2014
-
Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 53, s. 207-217
-
Tidskriftsartikel (refereegranskat)abstract
- This article reports on flow boiling heat transfer and dryout characteristics of R152a in a vertical mini-channel. The experiments were carried out with a resistively heated stainless steel tube (1.60mm in diameter and 245mm heated length) at 27 and 32°C saturation temperature. Five mass fluxes in the range 100-500kg/m2s with heat fluxes from 5 to 245kW/m2 were tested. Under similar operating conditions experiments were repeated with R134a in the same setup to compare thermal performance of R152a. The results showed that the heat transfer was strongly influenced by the applied heat flux with insignificant convective contributions. The dryout heat flux increased with increasing mass flux but no effect of varying operating pressure was noticed. The experimental results for heat transfer and dryout heat flux were compared with well-known macro and micro-scale correlations from the literature.
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| 38. |
- Anwar, Zahid, et al.
(författare)
-
Flow Boiling Heat Transfer and Dryout Characteristics of R600a in a Vertical Minichannel
- 2015
-
Ingår i: Heat Transfer Engineering. - : Informa UK Limited. - 0145-7632 .- 1521-0537. ; 36:14-15, s. 1230-1240
-
Tidskriftsartikel (refereegranskat)abstract
- Refrigerant-related environmental concerns forced legislative bodies to phase out some types of refrigerants, namely, chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) and in the near future European legislation will be affecting hydrofluorocarbons (HFCs) as well. Natural refrigerants such as hydrocarbons can thus be expected to be more common as refrigerants in the future. Experimental findings on flow boiling heat transfer and dryout characteristics of isobutane (R600a) in a uniformly heated, vertical, stainless-steel test section (1.60mm inside diameter and 245mm heated length) are reported in this article. The experiments were conducted at two saturation pressures corresponding to the temperatures of 27 and 32 degrees C, with five mass fluxes in the range 50-350kg/m(2)-s and at outlet vapor qualities up to dryout conditions. Analysis showed that heat transfer was primarily controlled by the applied heat flux with insignificant effect of mass flux and vapor quality. The dryout heat flux increased with increasing mass flux; however, no significant effect of varying saturation temperature was observed. The experimental results (for heat transfer and dryout) were compared with different macro and microscale correlations from the literature.
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| 39. |
- Anwar, Zahid, et al.
(författare)
-
Flow boiling heat transfer of R600a in a uniformly heated smooth vertical minichannel
- 2013
-
Ingår i: Proceedings of the 13th UK Heat Transfer Conference Sept. 2-3, 2013, UKHTC2013. - 9780957229853
-
Konferensbidrag (refereegranskat)abstract
- Refrigerant related environmental concerns forced legislative bodies to phase out some types of refrigerants namely CFC’s and HCFC’s and in the near future European legislation will be affecting HFCs as well. Natural refrigerants such as hydrocarbons can thus be expected to be more common as refrigerants in the future. Experimental studies with these fluids are important in understanding their performance and potential. Experimental findings on flow boiling of Isobutane in a uniformly heated, vertical, stainless steel test section (1.6 mm inside diameter and 245mm heated length) are reported in this article. Experiments were conducted at two saturation pressures corresponding to the temperature of 27 and 32 oC, with five mass fluxes in the range 50-350 kg/m2s and at outlet vapour qualities up till dryout conditions. Analysis showed that heat transfer was primarily controlled by the applied heat flux with insignificant effect of mass flux and vapor quality. The experimental results were compared with different macro and micro-scale correlations from the literature, and Owhaib, Liu & Winterton and Mikielewicz correlations quite accurately predicted the heat transfer data.
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| 40. |
- Anwar, Zahid, 1981-
(författare)
-
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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| 41. |
- Anwar, Zahid, et al.
(författare)
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Flow boiling heat transfer, pressure drop and dryout characteristics of R1234yf : Experimental results and predictions
- 2015
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Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 66, s. 137-149
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Tidskriftsartikel (refereegranskat)abstract
- Flow boiling heat transfer, pressure drop and dryout characteristics of R1234yf in a vertical stainless steel test section (1.60mm inside diameter and 245mm heated length) under upward flow conditions are reported in this article. The experiments were carried out at 27 and 32°C saturation temperatures with five mass fluxes in the range of 100-500kg/m2s while the applied heat flux was in the range of 5-130kW/m2. The experiments were carried out with gradual increase of the applied heat flux til completion of dryout. Under similar conditions, tests were repeated with R134a in the same test setup to compare thermal performance of these two refrigerants. The results showed that boiling heat transfer was strongly controlled by the applied heat flux and operating pressure with insignificant dependence on mass flux and vapor quality. The frictional pressure drop increased with mass flux and vapor quality and decreased with increasing saturation temperature as expected. Signs of dryout first appeared at vapor qualities of 85%, with the values generally increasing with increasing mass flux. The effect of varying system pressure was insignificant. The experimental results (boiling heat transfer, pressure drop and dryout heat flux) were compared with the predictions from well-known correlations (for macro and micro-scale channels) from the literature.
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| 42. |
- Anwar, Zahid, et al.
(författare)
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Flow boiling of R1234yf in a uniform smooth vertical minichannel
- 2013
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Ingår i: Science et Technique du Froid. Comptes Rendus/Refrigeration Science and Technology. Proceedings, 2013, Vol. 3. - : Institut International du Froid. - 9782913149991 ; , s. 1-11
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Konferensbidrag (refereegranskat)abstract
- This study describes experimental findings on flow boiling heat transfer with R1234yf in a smooth, vertical stainless steel tube of 1.6 mm inner diameter and 245 mm heated length. Tests were conducted at two saturation pressures corresponding to saturation temperatures of 27 and 32 °C. Other operating parameters were: mass flux 100-500 kg/m²s with heat flux 3-65 kW/m² while quality change was up to 60%. The heat transfer coefficient appeared to be a strong function of the applied heat flux and insignificant effect of mass flux and quality was observed. Increase in saturation temperature/pressure increased the heat transfer performance. Experiments were repeated with R134a in the same test section to compare the two fluids, almost similar results were duplicated with R134a. Experimental results were compared with different correlations, Tran et al. (1996), Gungor and Winterton (1986) and Martín-Callizo et al. (2007) correlations accurately predicted the data.
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| 43. |
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| 44. |
- Baina, Fabiola, et al.
(författare)
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Analysis of a high-temperature heat exchanger for an externally-fired micro gas turbine
- 2015
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Ingår i: Applied Thermal Engineering. - : Elsevier BV. - 1359-4311 .- 1873-5606. ; 75, s. 410-420
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Tidskriftsartikel (refereegranskat)abstract
- The externally-fired gas turbine (EFGT) can convert fuels such as coal, biomass, biomass gasification gas and solar energy into electricity and heat. The combination of this technology with biomass gasification gas represents an interesting option for gasification, for which it has been difficult to find a conversion technology. In this system, the heat exchanger deals with the contaminants of biomass derived gas instead of the turbine itself. However, these contaminants can build a deposit layer in the heat exchanger that can affect its performance. The heat exchanger is important in externally fired gas turbines since the turbine inlet temperature is directly dependent on its performance. Several studies on heat exchangers for externally fired gas turbines have been carried out. However, very few detailed studies were found comparing the performance of heat exchangers for externally fired gas turbines considering the effect of deposit materials on the surfaces. In this regard, this work compares the performance of a corrugated plate heat exchanger and a two-tube-passes shell and tube heat exchanger considering the effect of thickness of deposit material with different thermal conductivities on pressure drop and effectiveness. The results show that the effectiveness of the corrugated plate heat exchanger is more influenced at larger thicknesses of deposit materials than the two-tube-passes shell and tube heat exchanger. There is an exponential increase in the pressure drop of the plate heat exchanger while a monotonic increase of pressure drop is seen for the shell and tube heat exchanger. The increase in the thickness of the deposit material has two effects. On one hand, it increases the resistance to heat transfer and on the other hand, it reduces the through flow area increasing the velocity and hence the heat transfer coefficient. Additionally, the effectiveness of the heat exchangers had a stronger influence on the power output than the pressure drop.
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| 45. |
- Behi, Mohammadreza, et al.
(författare)
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Evaluation of a novel solar driven sorption cooling/heating system integrated with PCM storage compartment
- 2018
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Ingår i: Energy. - : Elsevier. - 0360-5442 .- 1873-6785. ; 164, s. 449-464
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Tidskriftsartikel (refereegranskat)abstract
- Recently the interest in solar thermal cooling has been growing for Air Conditioning (AC) applications. This paper presents an applied experimental and numerical evaluation of a novel triple-state sorption solar cooling module. The performance of a LiCl-H2O based sorption module (SM) for cooling/heating system with integration of an external energy storage has been evaluated. The dynamic behavior of the SM, which can be driven by solar energy, is presented. Two PCM assisted configurations of the SM have been studied herein; (i) PCM assisted sorption module for cooling applications (ii) PCM assisted sorption module for heating applications. Initially, an experimental investigation was carried out to evaluate the charging/discharging process of the SM without external energy storage. Secondly, the initial experimental configuration was modeled with a PCM integrated storage compartment. The PCM storage compartment was connected to the Condenser/Evaporator (C/E) of the SM. The temporal history of the sorption module's C/E and PCM storage, the cyclic and average performance in terms of cooling/heating capacity, cooling/heating COP, and the total efficiency were experimentally and numerically investigated. Furthermore, PCM charging/discharging power rate and solidification/melting process of the PCM in the integrated storage compartment to the SM were predicted by the model.
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| 46. |
- Behi, M., et al.
(författare)
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Optimized energy recovery in line with balancing of an ATES
- 2014
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Ingår i: American Society of Mechanical Engineers, Power Division (Publication) POWER.
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Konferensbidrag (refereegranskat)abstract
- The present study explores the potential imbalance problem of the Aquifer Thermal Energy Storage (ATES) system at the Eindhoven University of Technology (TU/e) campus, Eindhoven. This ATES is one of the largest European aquifer thermal energy storage systems, and has a seasonal imbalance problem. Reasons for this issue may be the high cooling demand from laboratories, office buildings and the direct ATES cooling system. Annually, cooling towers use on average 250 MWh electricity for the removal of about 5 GWh of excess heat from the ATES to the surroundings. In addition, the TU/e uses a large amount of natural gas for heating purposes and especially for peak supplies. Recovering the surplus heat of the ATES, a CO2 Trans-critical Heat Pump (HP) system to cover particularly peak demands and total heating demand is proposed, modeled and optimized. The model is validated using data from International Energy Agency. Based on simulation results, 708294 nm3 of natural gas are saved where two different scenarios were considered for the ATES efficiency, cost saving and green house gas reduction. In scenario I, the COP of the ATES increased up to 50% by which K€ 303.3 energy cost and 1288.5 ton CO2 are saved annually. On the other hand, it will be shown that the ATES COP in Scenario II will improve up to 20%. In addition, the proposed energy recovery system results in a 606 ton CO2 -reduction and K€152.7 energy cost saving for the university each year.
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| 47. |
- Beier, R. A., et al.
(författare)
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Borehole resistance and vertical temperature profiles in coaxial borehole heat exchangers
- 2013
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Ingår i: Applied Energy. - : Elsevier BV. - 0306-2619 .- 1872-9118. ; 102, s. 665-675
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Tidskriftsartikel (refereegranskat)abstract
- Ground source heat pump systems are often coupled to the ground by circulating a fluid through vertical Borehole Heat Exchangers (BHEs). The design of a system requires estimates of the ground thermal conductivity and the borehole thermal resistance, which are usually determined by an in situ thermal response test on a completed borehole. The usual test interpretation methods average the inlet and outlet fluid temperatures and use this mean temperature as the average temperature along the borehole length. This assumption is convenient but does not strictly apply. For a coaxial heat exchanger this paper develops an analytical model for the vertical temperature profiles, which can be used instead of the mean temperature approximation to estimate borehole resistance. The model is verified with measured temperatures on a BHE, where an optical technique allows continuous measurements along a coaxial borehole during a distributed thermal response test. A sensitivity study shows that the proposed method corrects errors in the mean temperature approximation, which overestimates the borehole resistance in a coaxial borehole.
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| 48. |
- Beier, Richard A., et al.
(författare)
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Transient heat transfer in a coaxial borehole heat exchanger
- 2014
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Ingår i: Geothermics. - : Elsevier BV. - 0375-6505 .- 1879-3576. ; 51, s. 470-482
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Tidskriftsartikel (refereegranskat)abstract
- Ground-source heat pumps often use vertical boreholes to exchange heat with the ground. A transient heat transfer model has been developed for a thermal response test on a pipe-in-pipe coaxial borehole heat exchanger. The analytical model calculates the vertical temperature profiles in the fluid flowing through the pipes, which are coupled to the surrounding grout and ground. The model is verified against measured vertical temperature profiles in the circulating fluid during a distributed thermal response test. The comparison with measured data indicates that the proposed model gives a more accurate estimate of the borehole thermal resistance than the conventional analytical model that uses a mean temperature approximation. The model demonstrates how strongly the shapes of the temperature profiles are dependent on the thermal resistance of the internal pipe wall and the flow direction.
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| 49. |
- Beier, R. A., et al.
(författare)
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Vertical temperature profiles and borehole resistance in a U-tube borehole heat exchanger
- 2012
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Ingår i: Geothermics. - : Elsevier BV. - 0375-6505 .- 1879-3576. ; 44, s. 23-32
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Tidskriftsartikel (refereegranskat)abstract
- The design of ground source heat pump systems requires values for the ground thermal conductivity and the borehole thermal resistance. In situ thermal response tests (TRT) are often performed on vertical boreholes to determine these parameters. Most TRT analysis methods apply the mean of the inlet and outlet temperatures of the circulating fluid along the entire borehole length. This assumption is convenient but not rigorous. To provide a more general approach, this paper develops an analytical model of the vertical temperature profile in the borehole during the late-time period of the in situ test. The model also includes the vertical temperature profile of the undisturbed ground. The model is verified with distributed temperature measurements along a vertical borehole using fiber optic cables inside a U-tube for the circulating fluid. The borehole thermal resistance is calculated without the need for the mean temperature approximation. In the studied borehole, the mean temperature approximation overestimates the borehole resistance by more than 20%.
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| 50. |
- Bekele, Getachew, et al.
(författare)
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Feasibility study for a standalone solar–wind-based hybrid energy system for application in Ethiopia
- 2010
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Ingår i: Applied Energy. - : Elsevier BV. - 0306-2619 .- 1872-9118. ; 87:2, s. 487-495
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this paper is to investigate the possibility of supplying electricity from a solar-wind hybrid system to a remotely located model community detached from the main electricity grid in Ethiopia. The wind energy potential of four typical locations has been assessed in a previous article. The solar potential has also been investigated and the results are presented in detail in an accompanying article awaiting publication. For one of the sites, Addis Ababa, the results of the investigation are given here in detail. For the other sites, the results are given as sensitivity diagrams only. Based on the findings of the studies into energy potential, a feasibility study has been carried out on how to supply electricity to a model community of 200 families, which comprises 1000 people in total. The community is equipped with a community school and a health post. The electric load consists of both primary and deferrable types and comprises lighting, water pumps, radio receivers, and some clinical equipment. A software tool, Hybrid Optimization Model for Electric Renewables (HOMER) is used for the analysis. The result of the analysis is a list of feasible power supply systems, sorted according to their net present cost. Furthermore, sensitivity diagrams, showing the influence of wind speeds, PV costs, and diesel prices on the optimum solutions are also provided.
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