| 1. |
- Bhikkaji, Bharath, et al.
(författare)
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Reduced order models for diffusion systems using singular perturbations
- 2001
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Ingår i: Energy and Buildings. - 0378-7788 .- 1872-6178. ; 33:8, s. 769-781
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we consider a special case of the one dimensional heat diffusion across a homogeneous wall. This physical system is modeled by a linear partial differential equation, which can be thought of as an infinite dimensional dynamic system. To simulate this physical system, one has to approximate the underlying infinite order system by a finite order approximation. In this paper we first construct a simple and straight forward approximate finite order model for the true system. The proposed approximate models may require large model order to approximate the true system dynamics in the high frequency regions. To avoid the usage of higher order models, we use a scheme similar to singular perturbations to further reduce the model order.
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| 2. |
- Cehlin, Mathias, et al.
(författare)
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Measurements of Air Temperatures Close to a Low-Velocity Diffuser in Displacement Ventilation Using Infrared Camera
- 2002
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Ingår i: Energy and Buildings. - : Elsevier. - 0378-7788 .- 1872-6178. ; 34, s. 687-698
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Tidskriftsartikel (refereegranskat)abstract
- The near zone of supply air diffusers is very critical for the indoor climate. Complaints of draft are often associated with low-velocity diffusers in displacement ventilation because the air is discharged directly into the occupied zone. Today, the knowledge of the near zone of these air supply diffusers is insufficient, causing an increased need for better measuring methods and representation of the occupied zone. A whole-field measuring technique has been developed by the authors for visualization of air temperatures and airflow patterns over a large cross-section. In this particular whole-field method, air temperatures are measured with an infrared camera and a measuring screen placed in the airflow. The technique is applicable to most laboratory and field test environments. It offers several advantages over traditional techniques; for example, it can record real-time images within large areas and capture transient events. The purpose of this study was to conduct a parameter and error analysis of the proposed whole-field measuring method applied to a flow from a low-velocity diffuser in displacement ventilation. A model of the energy balance, for a solid measuring screen, was used for analyzing the influence of different parameters on the accuracy of the method. The analysis was performed with respect to the convective heat transfer coefficient, emissivity, screen temperature and surrounding surface temperatures. Theoretically, the temperature difference between the screen and the ambient air was found to be 0.2–2.4 °C for the specific delimitation in the investigation. However, after applying correction the maximum uncertainty of the predicted air temperature was found to vary between 0.62 and 0.98 °C, due to uncertainties in estimating parameters used in the correction. The maximum uncertainty can be reduced to a great extent by estimating the convective heat transfer coefficient more accurately and using a screen with rather low emissivity.
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| 3. |
- Gustafsson, Stig-Inge, et al.
(författare)
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Life-Cycle Cost Minimization Considering Retrofits in Multi-Family Residences
- 1989
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Ingår i: Energy and Buildings. - : Elsevier BV. - 0378-7788 .- 1872-6178. ; 14:1, s. 9-17
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Tidskriftsartikel (refereegranskat)abstract
- When a building is to be renovated it is important to implement the optimal retrofit combination. If this strategy is neglected it might not be profitable to change the building in order to improve it as an energy system. This paper deals with energy retrofits and how the strategy can be optimized considering one specific building. The best solution is found when the life-cycle cost for the building is minimal, and building envelope, ventilation and heating system retrofits are combined. In order to solve the problem, a mathematical model, OPtimal Energy Retrofit Advisory (OPERA), has been developed. Energy balance calculations are used in which the free energy from solar radiation and from appliances is taken into proper account. The interaction between different retrofits is emphasized. Provided that the optimal solution is implemented, the retrofits in the combination will have a minor interaction which, for most cases, could be neglected. This will also imply that the order of implementation is of no, or minor, importance. A case study for a real building sited in Malmö, Sweden, and a sensitivity analysis for some critical input parameters are discussed.
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| 4. |
- Gustafsson, Stig-Inge, 1948-, et al.
(författare)
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Mixed 0-1 sequential linear programming optimization of heat distribution in a district-heating system
- 2000
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Ingår i: Energy and Buildings. - 0378-7788 .- 1872-6178. ; 32:3, s. 309-317
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Tidskriftsartikel (refereegranskat)abstract
- A district-heating system transports heat from the heat plant by using primary pipe network, via substation, to secondary pipe network where heat is finally distributed to buildings. When this system is designed its operational characteristics were selected to provide thermal comfort (TC) in all buildings served by this district heating system. After several years of operation, the system characteristics may change and TC in buildings deteriorates, some buildings are overheated and other buildings are underheated. The study investigates an optimum strategy to mitigate the problem caused by changes of three of system characteristics: hydraulic resistance of secondary pipe network, heat transmittance of radiators inside buildings, and heat transmittance of building envelope. A strategy of problem mitigation consists of the adjustment of hydraulic resistance of existing valves and retrofitting the local heating system with new substation heat exchanger and additional pumps. We used a steady state, bottom-up approach and mixed 0-1 sequential linear programming to find optimal mitigation strategy, i.e. optimum combination of valves' hydraulic resistances, new pumps placement and new size of substation heat exchanger. The results indicate that the calculated optimal strategy does not effectively improve TC in buildings only in cases when TC is deteriorated by higher than nominal values of heat transmittance of some building envelopes.
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| 5. |
- Gustafsson, Stig-Inge
(författare)
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Mixed integer linear programming and building retrofits
- 1998
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Ingår i: Energy and Buildings. - : Elsevier. - 0378-7788 .- 1872-6178. ; 28:2, s. 191-196
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Tidskriftsartikel (refereegranskat)abstract
- When a building is subject for refurbishment it is important to add only such measures that will reduce the Life Cycle Cost (LCC), for the building. Even better is to add measures that will, not only reduce the cost, but minimise the LCC. One means for such an optimisation is to use the so called Linear Programming (LP), technique. One drawback with LP models is that they must be entirely linear and therefore two variables cannot be, for example, multiplied with each other. The costs for building retrofits are, however, not very often linear but instead ‘steps’ are present in their cost functions. This calamity can, at least to a part, be solved by introducing binary integers, i.e., variables that only can assume 2 values, 0 or 1. In this paper it is described how to design such a Mixed Integer Linear Programming (MILP), model of a building and how different cost elements of the climate shield influence the optimal solution.
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| 6. |
- Gustafsson, Stig-Inge
(författare)
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Optimal fenestration retrofits by use of MILP programming technique
- 2001
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Ingår i: Energy and Buildings. - 0378-7788 .- 1872-6178. ; 33:8, s. 843-851
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Tidskriftsartikel (refereegranskat)abstract
- When buildings are subject for refurbishment, it is very important to add the optimal strategy at that very moment. If other solutions are chosen and implemented, it will no longer be possible to change the building at a later occasion with the same profitability. A suitable criterion for optimality is the point where the life-cycle cost (LCC) has its minimum value. This point can be calculated by using so-called mixed integer linear programming (MILP). This paper shows how building and possible fenestration retrofits are described in such a MILP program. Changing existing double-glazed windows to triple ditto will of course make the U-values lower, but at the same time less solar radiation is transferred through the glass panes. This must be properly addressed in the MILP model. Of vital importance are also the heating system and the energy tariff connected to it. Nowadays, time-of-use rates are common practice both for district heating and electricity. These facts make it unsuitable to write, optimise and solve the MILP model "by hand", and instead a computer program has been designed for writing the model in the form of a standard MPS data file. This file can in turn be scanned and optimised by MILP-solving programs available at the market today. © 2001 Elsevier Science B.V. All rights reserved.
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| 7. |
- Gustafsson, Stig-Inge, 1948-
(författare)
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Optimization of insulation measures on existing buildings
- 2000
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Ingår i: Energy and Buildings. - 0378-7788 .- 1872-6178. ; 33:1, s. 49-55
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Tidskriftsartikel (refereegranskat)abstract
- In Sweden, the activity on building new residences has been decreased for a number of years. The building stock as an average has therefore become older and in the future it will be subject for refurbishment. This paper deals with how to optimize retrofit measures, i.e. how to act in order to minimize the Life-Cycle Cost (LCC) of a building. Insulation measures are emphasized but also other retrofits are dealt with such as changing the heating system. It is shown that the heating system has a vital influence on the optimal amount of extra insulation which is to be applied. District heating is common in Sweden at least for larger buildings such as multi-family block of flats sited in urban areas. The tariffs for district heat must therefore be properly addressed in order to find out if extra insulation is profitable or not. As an example the Navestad area in Norrkoping is used. This residential area is now the subject for extensive retrofitting.
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| 8. |
- Larsson, Ulf, 1966-, et al.
(författare)
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Thermal analysis of super insulated windows (numerical and experimental investigations)
- 1999
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Ingår i: Energy and Buildings. - 0378-7788 .- 1872-6178. ; 29:2, s. 121-128
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Tidskriftsartikel (refereegranskat)abstract
- Windows are crucial for people's experience of the indoor climate, especially in the Nordic countries with cold climate and short days during the winter. This paper reports the first results from an ongoing research project focused on an improved integration of windows with the indoor air climate and people's perception of the windows. The thermal performance of a well-insulated window has been investigated both numerically and experimentally in a full scale test room. The window under consideration is a low-emissive triple-glazing window with two closed spaces filled with the inert gas krypton. An oxidised metal with low emissivity factor coats one pane in each space. Experimental and numerical investigations on the thermal performance of the window have been conducted for different winter cases. Temperature data obtained by direct temperature measurement using thermocouples and through numerical analysis are presented. The heat transfer through a window construction depends on three mechanisms i.e., conduction, convection and radiation. In this paper the convection-conducting mechanisms have been closely investigated. The numerical predictions agree well with the results from the measurements.
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| 9. |
- Olofsson, Thomas, 1968-, et al.
(författare)
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A method for predicting the annual building heating demand based on limited performance data
- 1998
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Ingår i: Energy and Buildings. - : Elsevier. - 0378-7788 .- 1872-6178. ; 28:1, s. 101-108
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we present an investigation of the possibility to use a neural network combined with a quasi-physical description in order to predict the annual supplied space heating demand (P) for a number of small single family buildings located in the North of Sweden. As a quasi-physical description for P, we used measured diurnal performance data from a similar building or simulated data from a steady state energy simulation software. We show that the required supplied space heating demand may be predicted with an average accuracy of 5%. The predictions were based on access to measured diurnal data of indoor and outdoor temperatures and the supplied heating demand from a limited time period, ranging from 10 to 35 days. The prediction accuracy was found to be almost independent of what time of the year the measurements were obtained from, except for periods when the supplied heating demand was very small. For models based on measurements from May and fo some buildings from April and September, the prediction was less accurate.
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| 10. |
- Olofsson, Thomas, 1968-, et al.
(författare)
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Energy load predictions for buildings based on a total demand perspective
- 1998
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Ingår i: Energy and Buildings. - : Elsevier. - 0378-7788 .- 1872-6178. ; 28:1, s. 109-116
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Tidskriftsartikel (refereegranskat)abstract
- The outline of this work was to develop models for single family buildings, based on a total energy demand perspective, i.e., building-climate-inhabitants. The building-climate part was included by using a commercial dynamic energy simulation software. Whereas the influence from the inhabitants was implemented in terms of a predicted load for domestic equipment and hot water preparation, based on a reference building. The estimations were processed with neural network techniques. All models were based on access to measured diurnal data from a limited time period, ranging from 10 to 35 days. The annual energy predictions were found to be improved, compared to models based on only a building-climate perspective, when the domestic load was included. For periods with a small heating demand, i.e., May-September, the average accuracy was 7% and 4% for the heating and total energy load, respectively, whereas for the rest of the year the accuracy was on average 3% for both heating and total energy load.
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