Energy analysis of BMW:s car test facility in Arjeplog Erik Olsson Master of Science Thesis in Energy Engineering. Umeå Institute of Technology (löpnr. som tilldelas)
Sammanfattning Detta examensarbete i energiteknik är genomfört på uppdrag från BMW och Icemakers AB på BMW s vintertestanläggning i Arjeplog. Testanläggningen består av ett flertal uppvärmda testbanor, byggnader tillhörande BMW samt ett antal byggnader som tillhör Icemakers AB. Uppvärmning av testbanor och byggnader sker från en gemensam värmeanläggning bestående av två gasolpannor med en effekt på 1 MW vardera. I dagsläget är enbart gasolförbrukningen samt energianvändningen av Icemakers byggnader kända, det finns ingen mätning av energiflöden till BMW s byggnader och testbanorna. Projektets huvudsyfte har varit att utreda potentiella energieffektiviserande åtgärder med målet att reducera anläggningens gasolförbrukning med 10 % på årsbasis samt föreslå åtgärder för en ökad kunskap om energianvändningen. Inledningsvis genomfördes en energianalys baserad på testsäsongen 2008-2009 med syfte att kartlägga hur energianvändningen är fördelad mellan byggnader och testbanor. Genom energisimuleringar av BMW s byggnader erhölls energibalanser och därmed kunde energireducerande åtgärder för byggnaderna undersökas. Vidare genomfördes energisimuleringar på uppvärmda testbanor för att undersöka uppvärmnings- och avkylningsförlopp för att identifiera möjligheter att reducera energianvändningen under testfria perioder. Under senaste året, dvs. testsäsong 2008-2009 förbrukades 169 ton gasol, motsvarande 2169MWh, för uppvärmning av anläggningens banor och byggnader. Energianalysen visar att 71% av den förbrukade energin användes för uppvärmning av testbanor, 21% för uppvärmning av BMW s byggnader samt 8 % för uppvärmning av Icemakers byggnader. För att öka kännedomen om energianvändningen föreslås energimätning till såväl banor som BMW s byggnader. Energisimuleringar på BMW s byggnader i BV2 visade att deras energibehov håller sig inom ramarna för Boverkets rekommendationer med ett specifikt energibehov på 130 kwh/m 2, år. Genom att justera byggnadernas inomhustemperatur till en rekommenderad nivå kan energiförbrukningen minskas med 13% vilket motsvarar 2,8% av anläggningens totala energianvändning. Genom att utöver temperatursänkning tilläggsisolera byggnadernas tak kan anläggningens totala energiförbrukning minskas med motsvarande 5,1% Tilläggsisolering kan dock inte anses ekonomiskt lönsamt på grund av en beräknad återbetalningstid på 23 år. En rad energisimuleringar genomfördes på de uppvärmda testbanorna. Resultaten visade att energibehovet för uppvärmning är kraftigt beroende av rådande vindförhållanden. Genom att reducera vindens påverkan på den uppvärmda ytan finns stora möjligheter att reducera energibehovet för uppvärmning av testbanorna. Förändringar i styrsystemet föreslås för att ta hänsyn till vindens inverkan på uppvärmningsbehovet. Ett alarmsystem bör installeras för att uppmärksamma om väderförhållanden som kan betyda att uppvärmningsbehovet överstiger systemets begränsningar. Dynamiken i uppvärmning och avkylning av testbanan har studerats och resultatet kan användas som referensvärde för tidsåtgång och effektbehov vid avkylning och uppvärmning av testbanorna. För att reducera energianvändningen under testfria perioder som nätter och helger är det rekommenderat att täcka över banorna med en betongtäckmatta vilket halverar energibehovet för att hålla en motsvarande temperatur i testbanan.
Abstract This Master of Science thesis in energy engineering is performed for BMW and Icemakers AB at BMW s winter test facility in Arjeplog. The test facility contains a number of heated test tracks, buildings belonging to BMW and a number of buildings belonging to Icemakers AB. The heating system consists of a common heating facility including two Liquefied petroleum gas (LPG) boilers of 1 MW heating power each. As of today only the LPG usage and the energy usage for heating of Icemakers buildings are known. There is no measuring of the energy flows to BMW s buildings and the heated test tracks. The main purpose of this project has been to investigate possible actions taken to reduce the LPG consumption at the test facility with 10% on a yearly basis and propose solutions for increased overview of the energy usage. Initially an energy analysis was performed based on test season 2008-2009 in order to map the energy distribution between the buildings and the heated test tracks. By energy simulations on BMW s buildings their energy balance was obtained and possible improvements to reduce the buildings energy demand were analyzed. Finally the heated test tracks were simulated in order to investigate their energy demand, visualize the process of heating and cooling and analyze solutions to reduce the heat losses during test free periods. During the last year, i.e. the test season 2008-2009, 169 tons of LPG, equals 2169 MWh, was consumed for heating of the test tracks and the buildings. The energy analysis shows that 71% of the energy usage is for heating of the test tracks, 21% is for heating of BMW s buildings and 8% is for heating of Icemakers buildings. In order to increase the awareness of the energy usage it is recommended to measure the energy flow to the heated test tracks as well as BMW s buildings. Energy simulations in BV2 reveals that the energy demand for heating of BMW s buildings are within the recommendations from the Swedish national board of housing, building and planning, with a specific energy demand of 130 kwh/m 2,year. By adjusting the indoor temperature to according to recommendations the energy usage can be reduced by 13% which equals 2,8 % of the total energy usage at the test facility. If in addition to the decreased indoor temperature the roof insulation is increased the total energy usage can be reduced by 5,1 %. However the additional insulation is not considered as economically justified due to a pay off time estimated to 23 years. A number of energy simulations have been performed on the heated test tracks. The result shows that the energy demand for heating is significantly affected by the present wind conditions. By reducing the air flow over the heated surfaces there is a great potential to reduce the energy demand for heating of the test tracks. Modification in the surface temperature control system is proposed in order to compensate for changes in wind conditions. An alarm system should be installed to alert when the weather conditions are rough and the maximum power of the heating system might not be sufficient to keep the test tracks in proper conditions. The dynamics of heating and cooling of the test tracks have been studied and the result can be used as a reference point for time and rate of heating required for heating and cooling of the test tracks. In order to reduce the energy demand during test free periods it is recommended to cover the tracks with an insulating blanket, which results in a halved energy demand to keep the tracks at a similar surface temperature in comparison to an uncovered surface.
Acknowledgements I would express my gratitude to the following people who have helped me during this thesis: My supervisor at Mattias Jonsson at Icemakers AB who have been a great support during my thesis. My supervisor Ronny Östin at the department for applied physic and electronics at Umeå University. He has been an excellent guidance and always taken time to give me useful feedback and support during my work. Lennart Lindström at Icemakers AB. He took time to answer all my questions and gave me a good insight in the operation of the test facility. Benkt Wiklund and the Office for external relations at Umeå University who provided this thesis and a great office for me during my work. Finally I would like to express my gratitude to BMW and Icemakers AB for giving me the opportunity to perform this thesis at BMW s test facility in Arjeplog. May 2010 Erik Olsson
1. INTRODUCTION... - 7-1.1 BACKGROUND... - 7-1.2 PURPOSE AND GOALS... - 7-1.3 SCOPE OF PROJECT... - 8-2. DESCRIPTION OF THE TEST FACILITY... - 9-2.1 BUILDINGS...- 10-2.1.2 Ventilation... - 12-2.2 HEATED TEST TRACKS...- 13-2.2.1 Straight and Curve... - 13-2.2.2 Heated slopes... - 14-2.2.3 Ice surface... - 14-2.2.3 Construction of heated test tracks... - 14-2.3 HEATING SYSTEM...- 15-2.3.1 Control of the heated test tracks... - 15-3. THEORY... - 18-3.1 HEAT TRANSFER...- 18-3.1.1 Heat transfer by conduction... - 18-3.1.2 Heat transfer by convection... - 19-3.1.3 Heat transfer by radiation... - 22-3.2 BUILDINGS ENERGY USAGE...- 23-3.2.1 Transmission losses... - 24-3.2.2 Ventilation... - 24-4. METHOD... - 26-4.1 ANALYSING THE ENERGY USAGE SEASON 2008-2009...- 26-4.2 BUILDINGS ENERGY USAGE SIMULATED IN BV2...- 27-4.2.1 Simulations in BV2... - 27-4.2.2 Climate profile... - 27-4.2.3 Construction... - 27-4.2.4 Internal heat sources.... - 28-4.2.5 Ventilation system... - 28-4.2.6 Hot water demand... - 29-4.3 SIMULATIONS ON HEATED TEST TRACKS IN COMSOL MULTIPHYSICS...- 29-4.3.1 Creating a model of the test tracks... - 30-4.3.2 Simulation plan... - 31-5. RESULT AND DISCUSSION... - 33-5.1 ENERGY USAGE SEASON 2008-2009...- 33-5.1.1 Calculated energy usage for heated test tracks... - 33-5.1.2 Energy distribution... - 36-5.1.3 Sources of errors... - 37-5.2 ENERGY BALANCE IN BMW S BUILDINGS...- 38-5.2.1 Reducing buildings energy usage... - 39-5.2.2 Error sources... - 40-5.3 ENERGY SIMULATIONS ON TEST TRACKS...- 42-5.3.1 Convective heat transfer coefficient... - 42-5.3.2 Wind and temperature dependence... - 43-5.3.3 Limiting weather conditions... - 46-5.3.4 Heating and cooling processes... - 48-5.3.5 Heat losses to the ground... - 52-5.3.6 Reduction of the surface temperature during non testing periods... - 52-5.3.7 Covering the tracks... - 53-6 SUGGESTIONS FOR IMPROVEMENTS... - 56 -
6.1 IMPROVED ENERGY AND TEMPERATURE MEASURING...- 56-6.1.1 Improved LPG measure... - 56-6.1.2 Measure energy flow to tracks and buildings... - 56-6.1.3 Improved placing of surface temperature sensors... - 56-6.2 IMPROVED SURFACE TEMPERATURE CONTROL...- 57-6.3 FLUE GAS HEAT RECOVERY...- 57-6.4 REDUCE BUILDINGS ENERGY USAGE...- 57-6.5 EXTREME WEATHER CONDITIONS AND WIND SENSOR...- 58-6.6 REDUCE WIND INFLUENCE ON THE HEATED TRACKS...- 58-6.7 COVER THE TRACKS DURING TEST FREE PERIODS...- 59-7 CONCLUSION... - 60 - REFERENCES... - 62 - APPENDIX... - 63 - APPENDIX I. SIMULATED HEATING POWER DEMAND...- 63 - APPENDIX II, HEATING OF THE TRACKS...- 64 - APPENDIX III COOLING SCENARIOS...- 70 - APPENDIX IV CONSTRUCTION PROPERTIES FOR BUILDING MODELS...- 76 - APPENDIX V CONSTRUCTION PROPERTIES FOR THE HEATED TEST TRACKS...- 77 -
1. Introduction This thesis is a project for the Master of Science program in energy engineering at Umeå University. The thesis is performed on behalf of BMW and Icemakers AB at BMW s car test facility in Arjeplog. 1.1 Background BMW owns a winter testing facility at Nåttiholmen in Arjeplog designed to provide car testing during winter condition. The test facility contains a number of buildings and land based test tracks, hence some test tracks are heated in order to provide the desired testing conditions. This facility is manufactured by Icemakers AB. Icemakers AB in Arjeplog has been a leading company in the vehicle testing industry since 1974 and provided service to many well known companies such as BMW, Bosch, Mercedes, MAN, Micheline, PSA, Renault and Toyota. Their service includes prepared ice tracks on lakes and on land adjusted to satisfy the costumer s request. In addition to this Icemakers also takes care of other service and organisation to allow their customers to fully focus on their testing. The area around Arjeplog is popular to the car testing industry because they can almost guarantee good winter conditions during a long period from November to April. This makes this area perfect for testing cars and systems in winter conditions. 1.2 Purpose and goals The purpose of this project is to map the energy distribution between the different end-users and investigate solutions to reduce the liquefied petroleum gas (LPG) usage at the test facility. The goal is to identify improvements to reduce the LPG usage at the test facility by 10 % on a yearly basis. In additions suggestions to improve the overview of the energy usage shall be presented. In order to determine the energy distribution at the test facility an energy analysis will be performed. This analysis will be based on data from the test season 2008-2009. The energy balance of BMW s buildings will be determined by energy simulations in order to estimate their energy demand. Possible improvements to reduce the energy usage for heating of BMW s buildings should be analyzed. The heating of the test tracks will be investigated by simulations in order to indentify the variations in heating power demand due to changes in outdoor conditions and determine when the heating power system is insufficient to keep the heated test tracks in desired testing conditions. Possibilities to reduce the heat losses from the test tracks during test free periods such as nights and weekends will also be investigated. - 7 -
1.3 Scope of project The scope of this project will be focused on the LPG heating system and the tracks and buildings that are heated, in order to reduce their energy demand. The installed track cooling system will not be included. No calculations will be done regarding electric power consumption for appliances or lighting. - 8 -
2. Description of the test facility BMW s facility contains a number of test tracks, both land based and tracks prepared on surrounding lakes. In figure 1 an overview of the test facility can be seen. The land based tracks are constructed for testing at varying road surface conditions. In order to provide proper testing conditions parts of the tracks are heated in order to create dry asphalt surfaces, other parts consist of polished ice. A cooling compressor is installed at the facility in order to guarantee good ice conditions. There are a several heated tracks at the test area, one long straight divided in different patterns of asphalt and ice surfaces, in addition there are also four heated slopes with different inclination The test facility contains a number of buildings, including one office building, workshops and a heated garage. The facility is only used during the winter test season, from November to April. The rest of the year the buildings are kept at a low inside temperature and the tracks are not used. The heated test tracks and the buildings use the same heating facility, which is composed of two LPG boilers of 1 MW each. It requires a lot of energy to keep the tracks above the freezing temperature during the winter, and a lot of heat goes to waste during periods when the tracks are not used for testing such as nights and weekends. During one year this facility uses around 162 tons of LPG which equals 2073 MWh for heating of the different tracks and buildings. When the weather conditions is though with low outdoor temperatures and strong wind the LPG consumption can reach up to 3 tons (38 MWh) on a single day and sometimes this is not enough to keep the heated tracks free from ice. At the present the only available information about the energy consumption is the amount of LPG provided to the boilers, a record is kept of the level in the LPG tank. There is no information available about where the energy goes from the boilers since there is no measuring of energy flows to the different end-users. There is also a lack of knowledge of the dynamics of heating and cooling the test tracks which can be useful to decide if the heating can be reduced during time periods when there is no testing in order to save energy. - 9 -
Figure 1. Overview of BMW's winter test facility in Nåttiholmen with land based tracks and buildings. The test season usually begins in the middle of November and continues to the end of March or beginning of April with a few weeks break over Christmas. During a normal week car testing takes place between 06.00 and 20.00 Monday to Saturday. 2.1 Buildings The construction of the test facility was finished before the test season 2005-2006. The buildings were built according to modern standards with 3-glass windows and ventilation heat recovery. The building complex seen in figure 2 is in fact one large connected building but in order to separate the different parts of the building they are referred to as different buildings. A brief introduction to the buildings follows: - 10 -
Figure 2. Overview of BMW s building complex at the test facility in Nåttiholmen. Building A Building A can be defined as the main building. This building contains four open landscape offices, a number of conference rooms, rest rooms, expedition and a few offices for the local staff. This is also where the main entrance is located. Heating Heated area 1466 m 2 Hot water radiators Building B Building B is mostly a passage building, but it also contains an electronic lab, dressing rooms, ventilation rooms and appliance rooms. Heating Heated area 444 m 2 Hot Water radiators Building C Building C is a workshop building. In addition to a big workshop building C also consist of a special hydrogen car room and a washing hall. The main gate for passing in and out to the workshop is constructed as a sluice-gate. It consists of two gates where the inner gate cannot be opened while the outer gate is open in order to reduce heat losses due to traffic in and out of the building. - 11 -
Heating Floor heating Heated area 1478 m 2 Building D Building D is a workshop building. In similarity to building C the main gate is constructed as a sluice-gate. Heating Floor heating Heated area 1427 m 2 Building E Building E is a heated garage to store cars. The ventilating system in this building is by separate exhaust air and supply air fans without heat recovery. Heating Floor heating Heated area 473 m 2 During the test season 2008-2009 buildings were kept at an average indoor temperature according to table 1. Table 1. Average indoor temperature during season 2008-2009 Building Indoor temperature [ C] A 21 B 21 C 18 D 18 E 16 2.1.2 Ventilation The ventilation system for building A-D includes three different ventilation appliances with exhaust and supply air and heat recovery, named LB1-LB3. LB 1: Provides ventilation to building A LB 2: Provides ventilation to parts of building B and building C LB 3: Provides ventilation to parts of building B and building D - 12 -
The ventilating system operates to keep a predefined pressure in the ventilation channels and the fans have rotation engine speed control. During night time and weekends when there is no activity in the buildings the ventilating system is shut down in order to reduce the heat losses. 2.2 Heated Test tracks The test facility contains several land-based test tracks. Parts of these tracks are heated to provide testing on varying road surface condition. The main purpose to perform tests on partially heated tracks is to simulate shifting road surface grip in order to test stabilisation system such as anti-lock braking system (ABS) and anti-spin system (ESP). The construction of these tracks was completed 2005 and test season 2005-2006 was the first year of service. 2.2.1 Straight and Curve The straight is the largest heated track with a total length of 276 m combined asphalt and ice surfaces. In addition to the straight a small curve is also heated by the same heating system. The straight consists of four different parts as shown in figure 3. Figure 3. Illustration of the heated straight with difference sections The runway The entry to the heated track, 45 meters long and 12,5 meters wide with heated asphalt. The mue-split The mue-split is created for testing on different road surface materials. The heated part is 160 meters long and 6,5 meters wide. On both sides of the heated zone there is a part consisting of polished ice 2,5 and 3,5 meters wide giving a total with of 12,5 meters. The mue-split was reconstructed during the summer of 2009 due to cracks in the concrete. - 13 -
The checker-board The checker board is designed to provide changing road surface conditions, it is constructed like a chess table with both heated asphalt and polished ice. The checkerboard is 46 meters long and 10 meters wide. The emergency brake-field The emergency break zone is the final part of this long heated straight, 25 meters long and 25 meters wide with heated asphalt. Total heated area of the straight is 2411 m 2 and the designed peak heating power is 965 kw The heated curve is a part of a large non heated track, the heated area is 40 m 2 and the designed peak heating power is 16 kw. Hence the straight/curve will be referred to as the straight with a total designed peak heating power of 981 kw. 2.2.2 Heated slopes In addition to the large heated straight there are also four partially heated slopes. The slopes consist of entry-zone, split zone and exit zone. Each slope is 25 meters long and 5,5 meters wide. All slopes are constructed in the same way but with different inclination. The total heated area of the slopes is 500 m 2 and the designed peak heating power is 200 kw 2.2.3 Ice surface The ice surfaces are polished to give the desired surface conditions for good testing. These parts are also constructed with a compressor cooling system that is designed to guarantee good ice quality irrespective of the weather conditions. The cooling system has not been used during the last season and will not be included in this project. 2.2.3 Construction of heated test tracks Figure 4. Cross-section of a heated test track to illustrate the construction principle Figure 4 illustrates a cross-section of the construction of the heated test tracks. The blue area represents insulation which is placed underneath the tracks to reduce ground heat losses. The light gray area represents concrete; the concrete contains pipes for heating or cooling of the tracks. The dark gray area is cellular glass insulation that is placed between heated and cold - 14 -
parts of the track to prevent heat transfer. On the road surface the black area represents heated asphalt and the white are represents ice. The material thicknesses are similar for all tracks except the mue-split. When it was reconstructed in the summer of 2009 the insulation layer was improved and the concrete layer was made thinner than before. 2.3 Heating system The heating system consists of two LPG boilers with a capacity of 1MW each. These boilers have an efficiency of 91% without flue gas heat recovery. An overview of the heating system is presented in figure 5. Figure 5. Overview of the heating plant at the test facility from the control system. The boilers produce hot water that is used for heat supply to tracks, BMW s buildings and three buildings belonging to Icemakers. The LPG usage is measured by keeping track of the level in the LPG tank, there is no digital measuring. Generally there is no equipment for measurements of energy flows except for the Icemakers buildings since they are not included in BMW s facility. The tracks are heated by two separate circulation systems, one for the slopes and one for the straight powered by pumps with constant flow. Heat from the boilers is transferred to the hot water circulation systems by heat exchangers and the temperature difference over the heat exchanger is measured. 2.3.1 Control of the heated test tracks The heated test tracks are operated with the goal to keep the asphalt dry in order to provide good testing conditions. The surface temperature needs to be kept at about 3 C or higher during testing to guarantee good surface quality. During test free periods, such as nights and - 15 -
weekends, the surface temperature is reduced to just below 0 C in order to prevent snow from melting on the surface which complicates the preparations of the tracks. In the morning, a few hours before testing begins the tracks are prepared for the day. Eventual snow is removed from the track. A so called pre-heat function is used to increase the surface temperature and melt ice on the surface. During the track preparation water is used to prepare the ice surfaces and the pre-heat also helps to keep the asphalt surface dry. When the pre-heat is used the rate of heating to the straight can be as high as 1300 kw since the boilers are operated with maximum heating power during the pre-heat. For the slopes the rate of heating during the pre-heat is limited to about 200 kw which is similar to the designed peak heating power. The heating of the tracks is controlled by the temperature curve shown in figure 6 Figure 6. Temperature curve used to control the exit temperature in the circulation system to the heated tracks as a function of the outdoor temperature. A similar temperature curve is used both for the straight and the slopes. This curve controls the exit water temperature in the circulation system depending on the outdoor temperature. There is no measuring of energy flows to the tracks and no consideration is taken to different wind conditions in the control system. In order change the heating power to the tracks the staff dislocates the temperature curve due to observed surface conditions. During night and weekends the exit temperature of the circulation water is reduce in order to lower the surface temperature. - 16 -
To increase the overview of the heating process temperature sensors are placed in the track as shown in figure 7 for the straight. Figure 7. Illustration of the placement of temperature sensors in the heated straight. The temperature sensors are placed in the concrete underneath the asphalt surface thus they do not give a good measurement of the surface temperature. The sensor marked GT7 in figure 7 is an exception, when the mue-split was reconstructed the sensor were placed in the asphalt and gives a better view of the actual surface temperature. - 17 -
3. Theory Chapter three covers the fundamental theory of heat transfer and buildings energy usage. 3.1 Heat transfer Heat is a form of energy and can be transferred between different systems with a temperature difference acting as the driving force. Heat transfer can only occur when there is a temperature difference between the involved systems; otherwise they are at a state of thermal equilibrium. There are three different principles for heat transfer, conduction, convection and radiation, a more covering theoretical description can be found in [1]. 3.1.1 Heat transfer by conduction Heat transfer by conduction takes place when energy is transferred from particles with a high energy to adjacent particles with lower energy. Conduction can occur in solids, liquids and gases. There are several variables that affect the rate of heat conduction trough a medium, such as geometry, thickness, material properties and temperature difference. The formula for heat transfer by conduction is given by Fourier s law of heat conduction. P cond T = ka (1) x Where k Thermal conductivity [W/mK] A Surface area [m 2 ] T Temperature difference [K] x Material thickness [m] Figure 8 illustrates an example of heat transfer trough a wall. - 18 -
Figure 8. Illustration of heat transfer by conduction trough a wall. According to equation 1 heat is conducted in the direction of decreasing temperature, as can be seen in figure 8. The thermal conductivity is a material property and can be described as a measure of the materials ability to conduct heat. 3.1.2 Heat transfer by convection Convection appears when heat is transferred between the surface of a solid to an adjacent fluid such as a liquid or gas. Convective heat transfer is a combination of conduction and fluid motion, without fluid motions there would be a case of heat transfer by pure conduction. When cold air is blown over a hot surface heat will be transferred to the air close to the surface, the air movement in the boundary heat away from the surface. With increased air speed the rate of heat transfer by convection heat will increase. There are two forms of convection: Forced convection: Occurs when a fluid is forces to flow over a surface by an external force, such as a pump, fan or wind motion. Natural convection: Occurs when movements in the fluid occurs due to density differences in the fluid caused by a temperature difference. The rate of heat transfer by convection is described by Newton s law of cooling: - 19 -
P conv = ha ( T T ) s s (2) Where h Convective heat transfer coefficient [W/m 2 K] A s Surface area [m 2 ] T s T Surface temperature [K] Ambient temperature [K] In equation 2 the surface area A s represents the surface from which the convective heat transfer takes place, the ambient temperature T is the temperature of the surrounding fluid at a distance where it is not influenced by boundary layer. Even though equation 2 gives a simple expression for the rate of heat transfer by convection the difficult part appears in determination of the convective heat transfer coefficient, h. In contrast to the thermal conductivity the convective heat transfer coefficient is not a property of the fluid or the surface. The h-value depends on the geometry of the surface, the fluid motion, and fluid parameters such as density, viscosity, thermal conductivity and specific heat capacity. Thus when dealing with convective heat transfer problems a large number of variables are involved. To reduce the total number of variables it is common to create dimensionless numbers, i.e. variables consisting of the input variables in order to create a simpler dimensionless equation that solves the problem. The Nusselt number is referred to as the dimensionless convective heat transfer coefficient and is defined as: hlc Nu = (3) k Where h Convective heat transfer coefficient [W/m 2 K] L c k Characteristic length [m] Thermal conductivity [W/mK] When a fluid flows over a surface the fluid layers closest to the surface will be affected, this zone is called the velocity boundary layer. Depending on the length of the surface and the velocity of the fluid the flow can take different characteristics. When a fluid flows smoothly, i.e. the stream lines are parallel, the flow is defined as a laminar flow. A more chaotic flow is defined as a turbulent flow. The Reynold s number, Re, is used to characterize the type of fluid flow. ρvlc Re = (4) µ - 20 -
Where ρ Fluid density [kg/m 3 ] V Fluid velocity [m/s] L c Characteristic surface length [m] µ Dynamic viscosity [kg/ms] By obtaining a value for the Reynold s number according to equation 4 the flow can be determined to be laminar, turbulent or in between which is called transient flow. As a fluid flows over a heated surface the fluid closest to the surface will be heated, this area is defined as a thermal boundary layer. The thickness of the thermal boundary layer at a certain point is defined as the distance from the plate where the fluid temperature equals 0,99(T s -T ). The relative thickness of the velocity and thermal boundary layer depends on fluid properties and is described by the dimensionless Prandtl number defined in equation 5: µc p Pr = (5) k Where µ Dynamic viscosity [kg/ms] c p k Specific heat capacity [kj/kgk] Thermal conductivity [W/mK] The Nusselt number can be expressed in a general form using the Prandtl number and Reynold s number such as: Nu m n = C Re L Pr (6) Where the constant C depends on the geometry of the object and m and n are exponents determined by experiments. By solving the equation for the Nusselt number the h-value can be obtained from equation 3. Regarding parallel flow over flat plate constants in equation 6 for the average Nusselt number has been determined. Turbulent flow: - 21 -
hl Nu = k 0.8 1/ 3 = 0.037 Re L Pr (7) Both laminar and turbulent flow on a flat smooth surface: hl k 0.8 1/ 3 Nu = = ( 0.037 Re L 871) Pr (8) Equation 7 is used when the size of the laminar zone is negligible in proportion to the turbulent zone. When the laminar zone is a significant part of the total length of the plate equation 8 is applied. The above equations for the Nusselt number are based on perfect conditions; an empirical expression of the h-value is given by [2], such as: h = 5,8 + 3, 9c (9) Where c Average wind speed [m/s] Equation 9 gives an alternative way to calculate the h-value as a function of the average wind speed regarding flow over a flat surface. 3.1.3 Heat transfer by radiation The rate of heat transfer by radiation between two surfaces separated by a gas is given by the Stefan-Boltzmann s law of radiation: P rad 4 4 = εσ A ( T T ) (10) s s surr Where ε Surface emissivity [dimensionless] σ Stefan-Boltzmann constant [W/m 2 K 4 ] A s Surface Area [m 2 ] T s T surr Surface temperature [K] Temperature of surrounding surface [K] - 22 -
Heat transfer from an outdoor surface surrounded by air will be by a combination of convection between the surface and the air and radiation between the surface and the atmosphere. The rate of heat transfer by radiation depends on atmospheric conditions, e.g. clear sky gives a lower surrounding temperature than cloudy sky thus a higher rate of heat transfer. A combined heat transfer coefficient including heat transfer by convection and conduction is usually defined in order to simplify the calculations, such as: P total = h A ( T T ) combined s s (11) Where h combined Combined heat transfer coefficient [W/m 2 K] 3.2 Buildings energy usage The purpose of heating a building is to bring about a good indoor climate by compensating for the heat losses. The use of heat energy equals the amount of heat supply required to ensure a sufficient indoor temperature and satisfy the hot water demand. Use of electric power to electronics apparatus and house- hold appliances are not included even though the heat generated is included in the heat balance[3]. Recommended indoor temperature differs depending on the activities in the building according to table 2. Table 2. Recommended indoor temperature in buildings depending on indoor activity given by [4]. Type of building Recommended indoor temperature [ C] Factories: heavy work 13 Stores, appliance rooms 15 Factories: light work 16 Class-rooms, hospitals 18 Offices, laboratories 20 Living rooms, public rooms 21 The energy balance for a building during a period of time is given by [3]: Q req = Q + Q + Q + Q + Q Q (12) t v l hw op sun Q int Where Q req Energy demand Q t Transmission losses Q v Ventilation losses Q l Losses due to air leakage Q hw Hot water heating demand Q op Energy demand to operate pumps and fans - 23 -
Q sun Q int Solar heat gain Internal heat sources 3.2.1 Transmission losses Heat will be transmitted through walls, roof and floor in a building due to a combination of radiation, convection and conduction. In order to determine a total heat resistance for the different parts of the building equation 1 and 2 can be rewritten in terms of thermal resistance which is described in [1] like: LA R cond = (13) k 1 R conv = (14) ha This provides the possibility to summarise the thermal resistances in analogy with electrical resistance and the heat transfer equation for transmission losses can be written as: P t = UA T (15) Where UA= 1 R tot (16) Equation 15 expresses the total heat transfer trough a wall or roof including convective and conductive heat transfer. 3.2.2 Ventilation Buildings can be ventilated in different ways. It can be naturally ventilated which means all air movements takes place naturally trough doors, windows and ventilators. A more modern way is mechanical ventilation by fans. The reason to ventilate a building is to keep a god indoor climate by replacing old air with fresh outdoor air. From an energy aspect the rate of air exchange should be kept as low as possible in order to keep the heated air inside the building. By installing an air to air heat recovery system, energy from the exhaust air can be transferred to the supply air and the heat loss due to ventilation can significantly reduced. Heat loss due to ventilation is calculated according to [5] by: - 24 -
P v ( η ) n c V ( t t ) = ρ 1 p i u (17) Where η Heat recover efficiency [%] n Air changes [1/h] ρ Air density [kg/m 3 ] c p Specific heat of air [kj/kgk] V Indoor air volume [m 3 ] t i t u Indoor temperature [K] Outdoor temperature [K] - 25 -
4. Method This chapter contains a brief introduction to the method to perform the energy analysis on the test facility. 4.1 Analysing the energy usage season 2008-2009 In order to determine the energy usage for the different parts of the facility it is divided into four different energy consumers: heated straight, slopes, BMW s buildings and Icemaker s buildings. In order to calculate the heating power to the straight and the slopes equation 18 is used. P=& mc p T (18) Where m& Mass flow [kg/s] c p T Specific heat capacity [kj/kgk] Temperature difference [K] The mass flow in the circulation systems is held constant by VS2-P1 and VS2-P2, see figure 5. By using stored data from the control system regarding the temperature difference (VS2- GT1 to VS2-GT4 regarding the straight in figure 5) over the heat exchanger the rate of heating at a certain point of time can be determined. Using data with small time steps and summarising the energy usage is obtained. This calculated value equals the energy distributed to the track, not the energy used by the boilers to supply the tracks. In order to determine the required energy amount heat losses in the boilers as well as in the heat exchangers have to be included. Information regarding mass flow and properties of the heating medium in the circulation system is compiled in table 3. Table 3. Properties of the heating medium in the circulation systems to the straight and the slopes. Medium Mass flow Cp [kg/s] [kj/kgk] Straight 40% Ethylen-glycol 18,17 3,6 Slopes 40% Ethylen-glycol 3,704 3,6 The energy flow to the Icemakers buildings is easily determined since it is measured. When the total amount of energy usage for the heated tracks and Icemakers buildings are known the remaining energy over the year is assumed to be for heating of BMW s buildings. - 26 -
4.2 Buildings energy usage simulated in BV2 Since there is no energy measurement of the energy demand to BMW s buildings the energy profile has to be determined by simulations according to [6]. In order perform energy simulations on the buildings the BV2-computer software [8] is used. 4.2.1 Simulations in BV2 BV2 is a computer program developed by CIT energy management AB to simulate buildings energy balance. In order to create a model in BV2 the design properties of the simulated building must be defined in a number of steps: 1. Choose a climate profile 2. Define construction properties 3. Define internal heat sources 4. Define ventilating system 5. Define hot water demand When all input data is defined BV2 calculates the buildings energy balance over one normal year. 4.2.2 Climate profile There are a number of predefined climate profiles from different parts of Sweden in BV2 but none comparable with the climate in Arjeplog. After contacting CIT they provided a climate profile for Gällivare which is used in the simulations. The climate in Gällivare can be considered as similar as in Arjeplog according to [13]. 4.2.3 Construction Building A is constructed with a normal timber ginger construction. The walls are insulated with 190 mm mineral wool insulation. The roof in building A is insulated with 350 mm mineral wool insulation and the ground is insulated with 100 mm extruded cell plastic. Building B-D is constructed with insulated building panels from Plannja. These buildings have 320 mm mineral wool insulation in the roof and 200 mm extruded cell plastic insulation in the ground. Construction input data regarding the buildings are compiled in appendix IV. To make simulations on the building s energy balances three models were created. One model for building A alone and one model for building E alone. In order to simulate building B,C,D a common model were created since they share ventilating system, they are also constructed similarly. Building B has a higher indoor temperature than building C and D but in the simulated model the indoor temperature is set according to building C and D:s indoor temperature. BV2 does not provide the possibility to divide a model in different temperature zones. This will result in a small reduction of the total energy usage but the approximation will not have a great effect on the result since building B is relatively small. - 27 -
4.2.4 Internal heat sources. The internal heat sources that is included in simulations on the buildings are lighting, personal heating and computers. The internal heat sources by lighting were estimated from test samples collected in parts of the buildings. The total number of visitors during the test season was determined from visitor list and a daily average of visitors during the test period was calculated. Each visitor is estimated to spend 5 hours a day inside the buildings. 70% is assumed to be located in building A and 30 % in buildings B,C,D according to [7]. It is also assumed that each person brings one laptop when he is inside the building. The estimated heating gain from one person is 70 W according to [8] and from a laptop 90 W according to [9]. Table 4. Internal heat sources in the buildings included in the simulation models. Building model Lighting Person heating Computers [W/m 2 ] [W/m 2 ] [W/m 2 ] A 9,5 1,9 2,4 B,C,D 9,5 0,32 0,41 E 9 0 0 Table 4 shows a summary of the internal heat sources included in the simulated models. During the test free period of the year the internal heat sources is assumed to be negligible since there is no activity at the facility. 4.2.5 Ventilation system The ventilation in building E is defined as an exhaust air system. Table 5 shows a summary of the input data for the ventilation systems in the model for building A and building B,C,D. Table 5. Technical specifications for the ventilation systems in BMW s buildings. Building Air flow SFP Heat exchanger efficiency [m 3 /s] [kw/(m 3 /s)] [%] A 2,25 2,08 70 B,C,D 6,62 2,39 71 The supply air is kept at an equal temperature as the indoor temperature when the outdoor temperature is -20 C or lower. The temperature decreases linearly to be 2 C below the indoor temperature when it is 0 C outside. - 28 -
4.2.6 Hot water demand BV2 contains a predefined model that calculates the hot water demand for local buildings. This model is used to estimate the hot water demand for one year and the result is divided by three since the building are in service for about four months. 4.3 Simulations on heated test tracks in Comsol multiphysics The Comsol Multiphysics is a powerful simulation tool designed for simulations of different engineering problems using partial differential equations. When a Comsol model is created the work can be divided in the following steps: Defining the model The first step is to choose what type of physical problem that should be investigated. There are several different choices, e.g. Fluid dynamics, heat transfer, electromagnetism, fluid thermal interaction. It is also possible to create a multiphysics model by involving several physical processes in the model. In this case a model with heat transfer will be created in order to simulate the heated test tracks. The most accurate way to simulate the circulation system would be to create a combined heat transfer and fluid flow model but due to the complexity of the model this would not be feasible. Drawing the model Comsol provides the possibility to create models in 1, 2 or 3 dimensions. It is desirable to create the model in as few dimensions as possible in order to reduce the size of the model and thus the time required to solve the model. By identifying symmetrical patterns in the model the dimensions can be reduced. When the geometry is drawn in Comsol it is done by using a CAD-tool. Models are created in real size thus it is possible to involve all material layers with accurate proportions. Defining physical properties When the model is created the material properties that are relevant for the type of physical problem simulated has to be defined. In heat transfer models physical properties for the included materials such as thermal conductivity, specific heat capacity and density has to be defined. The properties of external surfaces also have to be defined, i.e. what type of heat transfer will take place at the surface, if the surface is kept at a constant temperature or if the surface should be considered as thermally insulated. Meshing Comsol divides the geometry into finite elements with simple shape. The size of the meshed elements affects the goodness of the simulations. With large elements the solution will not become as accurate as if the size of the finite elements is decreased. It is possible to control the meshing and adjust the size of the finite elements in areas of the model that can be considered as important for the purpose of the simulation such as external boundaries and intersections between domains. - 29 -
Solving the model When the model is solved Comsol uses the meshed elements to calculate a solution. The process of solving a model is demanding, difficulties can occur if the model consists of too many finite elements or a large number of small domains. A physical problem can be solved in two different ways. One way is to solve it in a steady state form, i.e. for a heat transfer problem a solution when the model has reached a state of thermal equilibrium. The other way to solve the problem is by using a time dependent solver, this gives the opportunity to study heating and cooling in different parts of the model over a period of time. A time dependent model is much more demanding in comparison to a steadystate model and the simulations require a lot of time. When the model is solved the result can be studied by a number of post processing tools, e.g. temperatures can be plotted in certain points or over a surface and energy flow through a boundary can be integrated. 4.3.1 Creating a model of the test tracks One model that represents the straight and one model that represents one slope is created. Since the heated tracks consist of materials with different thicknesses and heating sections, it has to be divided into smaller parts. The straight are divided into four models, the entry, the mue-split, the checker-board and the brake zone, the curve is not simulated. All parts except the checker-board are created as a 2D model since they are symmetric in the length of the track. The checker board is created as a simplified 3D model containing a part of the total area in order to reduce the size of the model. The slopes are divided into two 2D models, one representing the exit/entry, which is similar, and one representing the split. The models are constructed according to blue prints, the heating power distribution between the different parts of the straight and the slopes and physical properties for the included materials can be sees in appendix V. The heating pipes in the model they are implemented as internal heat sources. The heating power to the different parts of the models is distributed over the total volume of the heating medium in the pipes. Figure 9 illustrates how the geometry of the heated sections of the tracks is modelled in Comsol. - 30 -
Figure 9. Illustration of how the heated part of the test tracks are modelled in Comsol with the asphalt surface, concrete containing heating pipes and ground insulation. The heating pipes inside the concrete can be seen as the white circles in figure 9 surrounded by the concrete with asphalt above and insulation underneath. In order to reduce the total number of domains in the model the pipe is not included since the thermal resistance of the pipes is negligible. The simulations will be focused on the mue-split for the straight and the split for the slopes. In order to assure that the result are conformable with the other parts of the tracks the remaining models will be simulated to control the result by using the same heating power and compare the temperature distribution and heating process. The heating power distribution between the different parts of the tracks will be used to obtain the total rate of heating for the track. The heating power added to the tracks will be assumed to be similar to the output heating power in the circulation system including distribution heat losses. The circulation systems of the test track may be compared with a district heating system. Heat losses in a typical Swedish district heating system are estimated to about 10 % according to [5]. Since these circulation systems are small in comparison to a district heating system the heat losses is assumed to be negligible. 4.3.2 Simulation plan The following simulations will be performed on the heated test tracks: Simulations to determine how the heating power demand is affected by the present temperature and wind conditions. The result will be used make an estimation of when the weather conditions cause a heating demand that exceeds the available heating power for the tracks. Heating and cooling of the tracks. As of today the time and rate of heating needed to raise the temperature of the tracks are not known, neither the process of cooling the - 31 -
tracks. Simulations will be done to illustrate the dynamics of heating and cooling processes during different weather conditions. Heat losses due to transmission in the ground will be investigated. The possibility to reduce the energy demand for the heated test tracks by covering the surface during test free periods will be estimated. - 32 -
5. Result and discussion In chapter 5 the result of the energy analysis and the calculations on buildings and test tracks is presented. 5.1 Energy usage season 2008-2009 The average LPG consumption during one year at the facility is about 162 tons, during season 2008-2009 the LPG consumption is estimated to 169 tons [10]. The average price for one ton of LPG is estimated to 8000 SEK [11]. Resulting in a total LPG cost of 1 352 000 SEK over season 2008-2009 5.1.1 Calculated energy usage for heated test tracks Temperature data was downloaded from the control system in order to determine the energy usage for the straight and slopes. When the downloaded data was analysed a period of data losses was detected, from 4th march to 12th march. In order to estimate the energy consumption during this period a liner approximation of the heating power as a function of the outdoor temperature was made based on the calculated heating power for the rest of the test season. y = -18,214x + 234,9 R 2 = 0,6081 Straight track and curve 800 700 Heating power 600 500 400 300 200 100 0-30 -25-20 -15-10 -5 0 5 Outside temperature [C] Figure 10. Linear approximation of the average daily heating power for straight track and curve as a function of the outside temperature. - 33 -