Cooling system for solar housing in the Middle East A design study based on a concept Master of Science Thesis in the Master’s Programme Structural engineering and Building Performance Design MATTHIEU MAERTEN AUDE TAN Department of Energy and Environment Division of Building Services Engineering CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg, Sweden 2011 Master’s Thesis 2011:09 MASTER’S THESIS 2011:09 Cooling system for solar housing in the Middle East A design study based on a concept Master of Science Thesis in the Master’s Programme Structural engineering and Building Performance Design MATTHIEU MAERTEN AUDE TAN Department of Civil and Environmental Engineering Division of Building Services Engineering CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg, Sweden 2011 Cooling system for solar housing in the Middle East A design study based on a concept Master of Science Thesis in the Master’s Programme Structural engineering and Building Performance Design MATTHIEU MAERTEN AUDE TAN © MATTHIEU MAERTEN AUDE TAN 2011 Examensarbete/Institutionenförbygg- ochmiljöteknik, Chalmerstekniskahögskola 2011:09 Department of Energy and Environment Division of Building Services Engineering Chalmers University of Technology SE-412 96 Göteborg Sweden Telephone: + 46 (0)31-772 1000 Cover: Drawing of the Solar House by Marcus Rydbo, Norconsult, 2006. Department of Energy and Environment, Göteborg, Sweden 2011 VII Cooling system for solar housing in the Middle East A design study based on a concept Master of Science Thesis in the Master’s Programme Structural engineering and Building Performance Design MATTHIEU MAERTEN AUDE TAN Department of Energy and Environment Division of Building Services Engineering Chalmers University of Technology Abstract In the 21th century, energy use and resources have become a major issue. This is even tougher in the Middle East, one of the first producers of oil in the world, and also a high consumer. Air conditioning is a huge part of the consumption. On top that, no renewable energy is used at the moment in the region. The solar house concept studied in the following thesis is a direct answer to this concern. It includes an energy-efficient building using a low-consumption air conditioning system. This system comprises a chiller and a water tank to create and store cooling for the house on a daily basis. The chiller is directly linked to photovoltaic cells. The aim of this work is to investigate how much electricity can be spared for a family house cooling system with such a design. Simulations are made using a modelling tool to determine the cooling loads and the cooling system behaviour. Especially, the main interest is to get the area of solar panels needed, to balance the energy used to run the cooling system for one year. This study is made for three buildings for two climates. The results show that the highest heat gain source is the solar radiations. Care should be taken to reduce the windows areas. Besides, the choice of chiller and the strategy used can highly influence size of the water tank, although the solar panels area is reasonable. Both might be further decreased, if the strategy used is more precisely defined throughout the year. Key words: Cooling system, solar house, storage tank, fan coil, chiller, energy efficient, vapour- compression VIII Système de climatisation pour un projet de « maison solaire » au Moyen-Orient Une étude basée sur un concept Master of Science Thesis in the Master’s Programme Structural engineering and Building Performance Design Matthieu MAERTEN Aude TAN Department of Energy and Environment Division of Building Services Engineering Chalmers University of Technology Résumé Au 21 eme siècle, l’utilisation des ressources du sol, la production et la consommation d’énergie sont devenus des problèmes majeurs. Cela est d’autant plus vrai au Moyen- Orient, première région productrice de pétrole. Etant donné l’abondance et le faible coût de cette ressource, cette région en est aussi une importante consommatrice. Les systèmes de climatisation, dans ces pays aux températures élevées, participent pour beaucoup dans cette consommation. De plus, aucune énergie renouvelable n’est utilisée à ce jour dans ces pays. La maison solaire « Solar house », étudiée dans cette thèse de master, est une réponse directe à ce problème. Elle est composée d’un bâtiment à faible demande énergétique, utilisant un système de climatisation basse-consommation. Ce système comprend une machine frigorifique, pour créer du froid, et un réservoir à eau, pour le stocker et le réutiliser pendant la nuit, et cela pour des cycles journaliers. La machine frigorifique est alimentée par des panneaux photovoltaïques. Le but de cette thèse de master est d’analyser et de calculer la quantité d’électricité consommée pour la climatisation par une famille avec ce concept. Les simulations sont réalisées avec un outil de modélisation. Elles permettent de déterminer la demande de froid et d’obtenir ainsi les conditions intérieures idéales, mais aussi de calculer la surface de panneaux solaires nécessaire pour compenser l’électricité utilisée à faire fonctionner la machine frigorifique. Les simulations sont toujours réalisées sur une année. L’analyse est faite sur trois maisons et pour deux climats différents. Les résultats montrent que la source qui apporte le plus de chaleur au bâtiment vient des radiations solaires. La surface vitrée de la maison doit donc être considérée avec attention. Le choix de la machine frigorifique, la taille du réservoir à eau et la surface de panneaux solaires dépendent les uns des autres. En fonction de la stratégie utilisée pour produire la quantité de froid nécessaire, ces différents paramètres sont optimisés. Mots clés : Système de climatisation, maison solaire, réservoir à eau, machine frigorifique, bâtiment basse-consommation, ventilo-convecteur, réfrigérateur à compression de vapeur. CHALMERS Energy and Environment, Master’s Thesis 2011:09 IX Contents ABSTRACT VII RESUME VIII CONTENTS IX PREFACE XIII NOTATIONS XV 1 INTRODUCTION 1 1.1 Background 1 1.2 Scope 3 2 THE SOLAR HOUSING CONCEPT 5 2.1 The background 5 2.2 Solar photovoltaic, HVAC and energy storage system 5 2.3 The cooling system 6 2.3.1 The vapour-compressor system 6 2.3.2 First arrangement of the coolant feeding device 7 2.3.3 Benefits of the system 9 3 INDOOR AND OUTDOOR CONDITIONS, BUILDING DESIGN 11 3.1 Indoor climate 11 3.1.1 Metabolic rate 11 3.1.2 Clothing insulation 11 3.1.3 Factors for thermal sensation 12 3.1.4 Humidity 13 3.1.5 Draft-air speed 13 3.2 Outdoor climate 14 3.2.1 Outdoor temperature 14 3.2.2 Solar radiations 16 3.2.3 Long-wave radiations 18 3.2.4 Wind conditions 19 3.2.5 Equivalent surface temperature 20 3.2.6 Ground temperature 21 3.2.7 Moisture risks 22 3.3 Building characteristics 23 3.3.1 Building 1 - Traditional building techniques 23 3.3.2 Building 2 – Insulated walls 24 3.3.3 Building 3 – Optimization of the windows 25 3.3.4 Characteristics of the buildings to be compared 26 4 NEED FOR COOLING 29 CHALMERS, Energy and Environment, Master’s Thesis 2011:09 X 4.1 Principles 29 4.1.1 Heat gains through the envelope 29 4.1.2 Solar gains 29 4.1.3 Air leakages 30 4.1.4 Need of cooling calculation 31 4.1.5 Comparison of the heat gains 32 4.2 Definition of the ventilation system 32 4.3 Cooling load 32 4.4 Comparison of the cooling demands 33 4.5 Climate without active cooling 35 4.6 Thermal capacity 36 5 AIR COOLING SYSTEM 39 5.1 Cooling strategy 39 5.2 Cooling recovery 40 5.3 Mixing of air 42 5.4 The fan coil 42 5.5 Indoor Moisture 44 5.6 Choice of fan coils 45 5.7 Resulting indoor climate 45 6 WATER SYSTEM DESIGN 47 6.1 Solar panels and their behaviours in this climate 47 6.1.1 Angle of the panels 47 6.1.2 The reduction factors of the supply electricity 49 6.2 Chiller capacity and choice of chiller package unit 53 6.3 The cooling strategy 55 6.3.1 Estimation of the tanks losses to the ground 55 6.3.2 Evaluation of the cooling needed for one day 56 6.4 Off-grid/On-grid 58 6.4.1 Strategy 1: variable work chiller 58 6.4.2 Strategy 2: one-step work compressor 59 6.4.3 Results 59 6.5 Detailed design of the distribution pipes 65 6.5.1 Day mode (chiller on) 66 6.5.2 Night mode (chiller off) 68 6.5.3 Flows through the system 69 6.6 The water tank 70 6.6.1 Filling of the tank 71 6.6.2 Temperatures in the tank 73 7 DISCUSSION 75 CHALMERS Energy and Environment, Master’s Thesis 2011:09 XI 7.1 Same system, other alternatives 75 7.2 Modification of the system 76 7.2.1 Active cooling systems 76 7.2.2 Passive cooling systems 77 7.3 Conclusion 78 8 REFERENCES 79 LIST OF FIGURES 83 LIST OF TABLES 87 APPENDICES 89 A. Architectural drawings 89 B. Simulink programmes 91 C. Moisture properties of light-weight concrete 92 D. Jeddah further results 93 a. Solar radiations on facades 93 b. Temperature and Relative Humidity at different points of the air distribution system for Building 2 94 c. Building 1 95 d. Flows through the system for building 2 97 e. Building 3 97 E. Chiller characteristics 100 F. Riyadh results 102 a. General results 102 b. Building 1 103 c. Building 2 104 d. Building 3 106 G. Absorption cooling 109 a. Working principle 109 b. Advantages 111 c. Disadvantages 111 d. Other possibilities 111 H. Desiccant cooling 112 a. Working principle 112 b. Solid desiccant cooling 112 c. Liquid desiccant cooling 113 d. Advantages 113 e. Disadvantages 113 CHALMERS, Energy and Environment, Master’s Thesis 2011:09 XII CHALMERS Energy and Environment, Master’s Thesis 2011:09 XIII Preface The study, carried out from January to June 2011, is part of a bigger housing project in Saudi Arabia, directed by the company Norconsult. This thesis was enabled thanks to the support from the companies Norconsult and Agera, as well as the Department of Civil and Environmental Engineering, Chalmers University of Technology, Sweden. Also, we would like to thank our supervisors Marcus Rydbo from Norconsult and Mattias Larsson from Agera, for their support during this master thesis. We highly appreciate working in Norconsult’s office, as part of the company. For the technical support, Kent Barry from Agera has been really helpful all along the project with his advice, especially when it came to HVAC design. We are also very grateful to Angela Sasic Kalagasidis for her general guidance and specific advice on how to model the building, as well as her help to get the different climate data. Finally, we are thankful to our examiner Jan Gustén for his participation to the elaboration of this report and his connection to Chalmers. Göteborg May 2011 Matthieu Maerten Aude Tan CHALMERS, Energy and Environment, Master’s Thesis 2011:09 XIV CHALMERS Energy and Environment, Master’s Thesis 2011:09 XV Notations Roman upper case letters area [m 2 ] windows area [m 2 ] width [m] thermal capacity [-] coefficient of performance [-] view factor between the surfaces [-] view factor between the sky and the given surface [-] G indoor moisture production [kg/h] irradiation of the surface due to long-wave radiations [W/m 2 ] irradiation of the surface due to sun radiations [W/m 2 ] length [m] heat flow rate [W] RH Relative Humidity [%] T temperature [K] ̅ mean temperature of two surfaces [K] annual average outdoor temperature [K] indoor temperature [K] thermal transmittance [W/m 2 .K] ̇ volume flow [l/s] V volume [l] compressor work [W] Roman lower case letters specific heat capacity [J/kg,K] specific heat capacity [J/ m3.K] thickness[m] periodic penetration depth [m] dh height of the shadow [m] dhm length of the overhang [m] density of moisture flow rate [kg/s] enthalpy [J/kg] hp height of the wall [m] air exchange rate [h -1 ] heat flow rate [W/m 2 ] reflected solar radiation [-] t temperature [°C] sky temperature [°C] air temperature [°C] equivalent temperature [°C] ground temperature [°C] outdoor temperature [°C] time period [s] CHALMERS, Energy and Environment, Master’s Thesis 2011:09 XVI wind speed [m/s] moisture content in the material [kg/m 3 ] moisture content in the air [kg/m 3 ] moisture at saturation in the air [kg/m 3 ] Greek lower case letters absorptivity of the solar radiations through a material [-] convective coefficient [W/m 2 .K] total heat transfer coefficient at the surface [W/m 2 .K] heat transfer coefficient due to long wave radiation [W/m 2 .K] vapour permeability of the material [mm 2 /s] emissivity of the surface [-] γ height of the sun [rad] thermal conductivity [W/m,K] temperature efficiency [%] moisture efficiency [%] density of the material[kg/m 3 ] Φ azimuth of the sun [rad] transmittance for solar radiation [-] θ incidence angle of the sun [rad] CHALMERS, Energy and Environment, Master’s Thesis 2011:09 1 1 Introduction In this part will be described first the background leading to this project, then the scope of this thesis. 1.1 Background Saudi Arabia is one of the biggest oil producers in the world. This resource is abundant and cheap there. Artificially low power price has increased the demand on electric utilities (averaging 5 to 7 percent annual growth). On the graph below, there is a real increase in electricity consumption (twice as big for the last ten year), and it is just a start. With the ever increasing life standard in this developing country, and the fast demographic growth (2% per year), the electricity demand will be much higher in the next few years. Saudi Arabia's Water and Electricity Ministry estimates that the country will require at least 35 Gigawatts (GW) of additional power generating capacity by 2023-2025 – more than double the capacity in 2005. Also, building new power plants cannot be done fast enough to cope the demand of the country. Moreover, these only run on non-renewable sources, like oil and gas. Figure 1 Electricity [GWh] generated in Saudi Arabia (IEA, 2011) Saudi Arabia is, with Japan, one of the country in which the highest amount of electricity is produced from oil resources (116 TWh in 2008). As to avoid producing even more energy in the next few years and building power plants all over the country, something has to be done. This is even more needed as energy transportation and electricity supply to each and every building create huge losses due to the big size of the country. http://www.mowe.gov.sa/ENindex.aspx CHALMERS, Energy and Environment, Master’s Thesis 2011:09 2 Table 1 Countries with the highest electricity production from oil during 2008 (IEA i.e., 2011) Table 2 Electricity production and consumption of electricity in Saudi Arabia during 2008(IEA, 2011) Moreover, ventilation and air conditioning systems represent 65% of the total electricity consumption in buildings (Syed Hasnain, 1999). Especially in summer, the demand to cool a building can be really high (peak load). The electricity production in Saudi Arabia can hardly follow this increase in consumption and, often in summer, shutdowns of the network occur. More power plants should be created to face the demand for these specific days. The electricity production company in Saudi Arabia, SCECO has created a load demand reduction programme to limit the amount of electricity used for air conditioning of office buildings between 13h00 and 17h00 (the peak load period) during the summer. (SM Hasnain, 2000). Each building should create its own electricity to minimise its transportation and all the effects from the increase in electricity demand. Therefore, photovoltaic panels to create electricity are a very suitable solution. Even more, using photovoltaic panels to cool a building will match together the electricity production and consumption. For now, renewable energy production does not exist in Saudi Arabia, even if there is a high potential with solar energy, due to the typical weather condition in this area. There are two ways to cool a building with solar energy. One is to use solar thermal collectors to heat up circulating water which will be utilized for cooling purpose (absorption or desiccant cooling). The other is to use electricity to run a cooling machine (vapour compression). Electricity Unit: GWh Production from: - oil 116238 - gas 87962 - other sources 0 Total Production 204200 Energy Industry Own Use 16668 Losses 17472 Final Consumption 170060 Industry 21144 Residential 96687 Commercial and Public Services 48529 Agriculture / Forestry 3461 Other Non-Specified 239 CHALMERS, Energy and Environment, Master’s Thesis 2011:09 3 1.2 Scope In this part will be defined all the structure of the master thesis, among which, the scope, outlines, expectations and limitations. In this master thesis, the focus is made on a project developed by 2 companies (Norconsult and Agera) to create residential houses with a good indoor thermal comfort in Saudi Arabia. In this project, a system has been developed and is patent pending, using photovoltaic panels (PV) to cool a building. As these PV panels can only work during the day, a storage system has been created to be able to cool the building even at night. This also leads to reducing the electricity demand during the peak load. As seen in Table 2, there is a real need of this kind of system, as most of the electricity produced in Saudi Arabia is used by residential buildings (96687 GWh on 170060 GWh available from the production plant). This energy source will also be environmental-friendlier than oil or gas. The amount of carbon dioxide rejected to the atmosphere to produce electricity will be much lower. It is to be mentioned that part of this project has already been considered before. Norconsult have already established the house design. Together with Agera, they have also defined the cooling system, transforming solar energy to cool the air. Some drawings, explanation texts and cost calculations have been made, and can be used further as ground for this thesis. The project has also been presented in Saudi Arabia. Yet, no deep analysis has already been done. Therefore, this thesis is the opportunity to investigate further, in 20 weeks, the whole system comprising the energy-efficient design of the house and the cooling system as described earlier. The aim is to design the cooling system as a viable solution. This work will be organised as following. To start, the concept is expounded, as to understand its principles, advantages and possible improvements. Then, each part of it will be designed in detail. First, the input data will be determined: the indoor climate, the outdoor climate and the building characteristics. Two different climates are considered, both in Saudi Arabia: Riyadh, with hot and dry weather, and cool nights; and Jeddah, hot and humid weather with warmer nights. As mentioned before, the building has been clearly designed by Norconsult’s architects. For this reason, its shape or layout will not be modified. The current drawings will be used as grounds for future calculations. Yet, as explained later, alternative buildings will also be studied: they have exactly the same layout, and only the materials will be changed to compare the results for different thermal characteristics. For this part, it could have been possible to investigate other optimisation means, such as the use of passive systems or of natural ventilation, but these possibilities will only be discussed in the last part. Second, the need of cooling will be determined. It is one of the most important parameters for this project. Indeed, reducing the need of cooling will reduce the energy use of the house, and thus decrease the costs of the cooling machine and storage tank. All parameters used will be clearly exposed. However, some parameters may not be mentioned: only the ones with the highest impact on the results and the cooling system design are taken into consideration. CHALMERS, Energy and Environment, Master’s Thesis 2011:09 4 Third, the design of the cooling system will be made in detail for the air part, describing the strategy used. Fourth, the whole concept for cooling system as imagined by the companies will be analysed. The different components will be designed depending on the cooling load. The interactions between the solar energy, which can only be available during the day to create electricity, and the need of cooling at night, are optimised. The tank to store the cold water, the chiller and its electricity need are dimensioned. It also induces the solar panel characteristics. In this part will be calculated the amount of solar panels needed to decrease the electricity consumption peak load. Different strategies could be used depending on the choice and schedule for using grid or solar electricity. Two different strategies will be compared. Finally, the results will be analysed and discussed. This part will be an extension to further research on the subject. Also, this project is of main importance, as it could be extended to other kinds of buildings. Norconsult is also willing to apply it to a United Nation project in Soudan if possible. The system can be used in different countries where the need of cooling is important and the outside temperature high. All the calculations made in this thesis are done using the MATHWORKS simulation tool SIMULINK. This means that only the assumptions and formulas implemented in the tool are used. This report will follow the work structure. Chapter 2 is a description of the concept as defined by Norconsult. In chapter 3, all the initial conditions are defined: the indoor climate, the outdoor climate in Saudi Arabia, the building characteristics. In chapter 4, the need of cooling is calculated. Chapter 5 defines exactly the different components of the air cooling system and their efficiency. The detailed design of the water cooling system is done in chapter 6. In the last part of this report, chapter 7, the different upgrading and difficulties of the cooling system are discussed, as well as some other suitable solutions for cooling purposes in Saudi Arabia. CHALMERS, Energy and Environment, Master’s Thesis 2011:09 5 2 The solar housing concept In this chapter, the innovative concept of the SolarHouse will be explained, with more focus on how the cooling system works. 2.1 The background In the Middle East, the hot outdoor climate induces a need for cooling inside the buildings. Especially, the solar radiations are significant throughout the year. Nowadays, air conditioning is very common in this region, and sometimes overused, leading to high peak loads for the electricity grid. Many parameters can help reducing this peak load. First, reducing the internal loads will lower the need for cooling. Especially, protecting the inside from solar radiations helps keeping low temperatures indoors. Meanwhile, one can notice that in summer, the air conditioning load is highly related to the solar radiation. Thus, the idea is to use the energy from the solar radiations to run the air conditioning system and reduce the peak demand on the electrical grid. Also, when using solar panels, it is most preferable to immediately use the solar energy than to keep it for later. Therefore, as to avoid using much electricity from the grid at night to run the chillers, the idea is to produce and store during the day enough cold to keep a good indoor climate for the whole night. 2.2 Solar photovoltaic, HVAC and energy storage system The system developed by Norconsult consists in four main points. First, the building should be well insulated, with low thermal transferring windows, roof and walls. Second, the building will be equipped with solar photovoltaic panels (PV) integrated to the climate shell of the roof. The electricity produced will be first used for the air conditioning system, and then for the house appliances if possible. The first one is the main energy consumer and should be designed efficiently. Third, the compressor is oversized so that during the day, enough cooling capacity is produced to cool the building for a whole day and night. The coolant flows through the fan coil unit when needed, and the surplus is stored in a water tank for later, when the cells are not producing any electricity. This is much cheaper than using a battery to run the chiller at night. Also, the house will be quieter when the compressor is off at night. Last, a managing device regulates the need for cooling at any time, depending on the indoor temperature. The advantage of using PV panels is that excess energy produced is exported to the grid and may be credited to the owner. Reversely, in case there is not enough sun radiation during the hottest days, energy can be provided to the compressor from the grid. A patent is pending (No. PCT/SE2006/001301) for the combination of this storage system with the improved building characteristics (mentioned below). CHALMERS, Energy and Environment, Master’s Thesis 2011:09 6 Figure 2 Sketch of the Solar House concept as defined by Norconsult 2.3 The cooling system The studied cooling system is a vapour-compressor type, as explained in the following part. The compressor in the cool generating system is powered by electricity from the solar panels built in the roof. An inverter converts the electrical power from the solar cells into suitable electrical power for the compressor (from DC to AC).The excess can be transferred to an external or internal supply network (batteries). To reduce energy losses during the transportation, the converter is placed as close as possible to the solar panels. The evaporator enables energy transfer from a coolant to the refrigerant. The coolant circulates within the coolant feeding system, which comprises a tank and a cooling device (fan coils). Different arrangements can be made for the coolant feeding device. In this report, only the first arrangement will be studied, as called in the patent pending. 2.3.1 The vapour-compressor system In the following part, the numbers refer to the elements noted on Figure 4. The vapour-compressor cooling system comprises a compressor (2), a condenser (5), a check valve (4) and an evaporator (3). A refrigerant flows during the day through all these units in a closed loop. The compressor is used to increase the pressure of the vapour refrigerant flowing from the evaporator. The refrigerant at high pressure and high temperature then gets to the CHALMERS, Energy and Environment, Master’s Thesis 2011:09 7 condenser where it condenses, rejecting heat. The condenser is located outside the house so that the exhaust heat does not affect the in-house environment. The expansion device diminishes the pressure and the temperature of the refrigerant, until it gets to the evaporator. In there, the refrigerant is evaporated, absorbing heat to change phase. In the evaporator, the coolant flows like in a heat exchanger, releasing heat for the refrigerant to absorb it. The refrigerant and the coolant flow in opposite directions, and at the output, the refrigerant is hot (and in vapour form) and the coolant cold. Figure 3 Compression cooler (Eicker, 2003) The compressor is the component that uses most energy in this system, so it must be powered with solar energy from the PV panels. 2.3.2 First arrangement of the coolant feeding device In the following part, the numbers refer to the elements noted on Figure 4. In the first arrangement, there are a valve (10) and 2 pumps (11 and 12). The control switches at the points I to III can be shunt-like devices, in the form of diverters and relief valves. The fluid control system is then controlled using an algorithm. During the day, the system works as following, as can be seen on the left part of Figure 5. The cold coolant flows from the energy transfer unit (3) to the cooling device (7) (a-b-c). If there is a lower need of cooling than the capacity of the cooling system, the shunt at the connection point I will deviate part of the coolant to the tank (f), while keeping enough coolant for the cooling coil to meet the indoor climate demands (c). The temperature of the water flowing to the fan coil (7) can be adjusted by mixing the cold coolant (b) with the warm liquid (g) coming out of the fan coil (7), thanks to the valve 10. The warm water coming out from the fan coil unit (d) principally flows back to the energy transfer unit (e), to get cooled down by thermal exchange with the refrigerant in the condenser. CHALMERS, Energy and Environment, Master’s Thesis 2011:09 8 1 inverter 10 shunt a first conduit 2 a compressor 11 first pump b second conduit 3 energy transfer unit 12 second pump c third conduit 4 a check valve d fourth conduit 5 a condenser I first connection point e fifth conduit 6 tank II second connection point f sixth conduit 7 cooling device III third connection point g seventh conduit 8 solar cell system h eight conduit Figure 4 First arrangement of the cooling system Figure 5 Day and night utilisation of the first arrangement of the cooling system. See legend on Figure 4 CHALMERS, Energy and Environment, Master’s Thesis 2011:09 9 During the night, there is no energy from the solar panels. So, the cool generating device is shut down (2, 3, 4, 5). Then, there is no need to use pipes a and e and to take the coolant through the condenser (refer to the right sketch on Figure 5). At connection points I and III, the shunts shut down the access to pipes a and e. Instead, the cold water is directly taken from the tank to the cooling device through pipes f, b and c. During the night, the cooling demand should be less than during the day, and the coolant flowing out of the cooling device (d) should not be so much warmer than the temperature of the coolant flowing in. It can thus be sent back to the tank (h). In case of high desired cooling effect, the coolant should flow faster through the cooling device. The tank may have a device separating warmer water from the cooling device, from the cold water accumulated earlier. Pipe g might be used to control the temperature of the coolant in pipe c. 2.3.3 Benefits of the system One benefit of the invention is that the electricity used comes from a harmless source for the environment. The solar cells also produce free electricity for the owner of the house, which is a long-term savings means. Yet the area of the solar cells has to be designed correctly as to correspond to the power consumption of the cool generating device. Also, as setting the use of the cool generating device only for daytime, this induces the system to run almost continuously when turned on. Even if the compressor changes its efficiency with the change of inclination of the sun, it works during long cycles as the coolant is almost fed continuously to the tank during the day. The system then runs at optimum efficiency when in use, and is switched off at night time. This increases the life expectancy of the compressor and thus of the whole cooling system. From a financial point of view, it is cheaper to have a tank for the coolant than a battery to store energy. Also, the tank is an easy and silent way to provide a cool environment during night time in the house. Of course, the tank and the pipes are insulated to avoid accidental heating of the fluid during transportation through the different devices. Another benefit is the fact that water can be used as coolant, which makes it a cheap and harmless solution. Other possible liquid fluid can be ethylene glycol, especially if the system should create higher cooling loads. Nevertheless, some electricity needs to be used at night to drive the cooling system. Some ideas are to use batteries or store electricity from the solar panels or from the wind, or either use potential energy to create the flow in the system. CHALMERS, Energy and Environment, Master’s Thesis 2011:09 10 CHALMERS, Energy and Environment, Master’s Thesis 2011:09 11 3 Indoor and outdoor conditions, building design In the following part will be analysed the given conditions for further use in the calculations. First, the choice for the indoor climate will be discussed. Then, outdoor climate data will be defined for the studied cities. Last, the building used for the studies will be precisely defined. 3.1 Indoor climate To define the indoor condition in a residential building in Saudi Arabia, the ASHRAE standard 55P will be used. This is the American national standard created by the American society of heating, refrigerating and air-conditioning engineers. Saudi Arabia does not have any such standard, so it is common to use the most international one. The six factors that define the indoor climate conditions and its feeling by people are: metabolic rate, clothing insulation, air temperature, radiant temperature, air speed, and humidity. The operative temperature is the average of the air temperature and the radiant temperature. 3.1.1 Metabolic rate It is defined depending on the activities of the people living in the building. It measures the energy produced by organisms. Sleeping: 40 W/m 2 (0,7 Met) Seated: 60 W/m 2 (1 Met) Standing: 70W/m 2 (1,2 Met) Walking, cooking: 100W/m 2 (1,7 Met) When the metabolic rate increases above 1,0 Met, the sweating depending of the activity will have a bigger and bigger influence on the thermal sensation. 3.1.2 Clothing insulation It has to be defined as to know the ideal indoor temperature. Saudi Arabia is a warm climate, so people do not wear so many clothes. So, for an ensemble of a thin trousers and short sleeve shirt, the Icl=0,45clo. When sleeping, it is hard to know the clothing insulation due to the use of sheets, which provide thermal insulation. CHALMERS, Energy and Environment, Master’s Thesis 2011:09 12 Figure 6 Optimal operative temperature depending of the activity and the clothing (ASHRAE, standard 55P, 2003) According to this graph the optimal operative temperature would be 25,5°C in summer. 3.1.3 Factors for thermal sensation To have an idea of the temperature felt by people living in the building, two factors will be used. PMV (Predicted mean value) is a scale to quantify people thermal sensation. It is from -3 to +3. (+3 hot, +2 warm, +1 slightly warm, 0 neutral, -1 slightly cool, -2 cool, -3 cold) PPD (Predicted percentage dissatisfied) defines the percentage of people who do not have a good thermal sensation in a specified indoor climate. It is always connected to the PMV like on the graph below. For a PMV of -0,5, 10% of the persons are dissatisfied. Figure 7 Correlation between PPD and PMV CHALMERS, Energy and Environment, Master’s Thesis 2011:09 13 Three classes of comfort are defined. In this design study, class B will be used. It means that the PPD should be below 10 % and the PMV between -0,5 and 0,5. 3.1.4 Humidity For a Met-value between 1 and 1,3 and clothes insulation around 0,5, the graph below shows the thermal comfort zone in which the PMV is between +0,5 and -0,5. For an operative temperature of 26 °C, all the humidity ratios are accepted. It will not create any discomfort for people living in the building. Yet, it should not be higher than 0,012kg/m 3 . That means that the relative humidity should not be higher than 57%. Figure 8 Acceptable range of operative temperature and humidity of the indoor climate (ASHRAE, standard 55P, 2003) If it is assumed that the relative humidity of the air in the house is 40%, to obtain a good thermal comfort with a PMV of 0,5, the operative temperature will be 24°C- 27°C in summer (0,4 clo). The mean radiant temperature takes into account the radiation received by people. It will increase the temperature felt by them. It could come from the solar radiations or walls. If a mean radiant temperature of 26°C-29°C is considered – which is reasonable because the windows in the house are protected by overhangs and the walls inside the house are at the ambient temperature – it is better to include the ambient temperature between 22°C and 25°C. 3.1.5 Draft-air speed To avoid too high turbulence in the air and to create draft (unwanted local cooling by air movement), the mean air velocity should be restricted. A turbulence intensity of CHALMERS, Energy and Environment, Master’s Thesis 2011:09 14 20% is a good design. So with a local temperature of 23,5 °C, the mean velocity can reach 0,23 m/s as a maximum to avoid discomfort. Figure 9 Allowable mean air speed as function of air temperature and turbulence intensity (ASHRAE, standard 55P, 2003) 3.2 Outdoor climate The outdoor climate has a major impact on the energy use in a building. There are several factors related to the energy performance, including the outdoor temperature, the solar radiations, the long-wave radiations and the wind conditions. The indoor comfort is also affected by the outdoor humidity and the indoor surface temperatures. For this thesis, two cities are studied in Saudi Arabia: Riyadh and Jeddah. 3.2.1 Outdoor temperature All data for the outdoor temperature is taken directly from the software METEONORM. The values are given on hourly basis and are directly used as input in SIMULINK. Riyadh (24° 38′ 0″ N, 46° 43′ 0″ E), the capital city of Saudi Arabia is situated in the middle of the country. Jeddah (21° 32′ 36″ N, 39° 10′ 22″ E), is on the Red Sea coast in the west side of the country. CHALMERS, Energy and Environment, Master’s Thesis 2011:09 15 Figure 10 Map of Saudi Arabia (al-Islami) In Riyadh, the climate is arid and dry with hot days and cool nights. The temperature difference between the summer and winter is important. The hottest day is the 7 th of August with a temperature of 46,6°C. The moisture varies all year long too, but will never be very high: between 60% relative humidity in winter and 10% in summer. Figure 11 Temperature (-5 to 50°C) and relative humidity (10 to 100%) in Riyadh for one year (8760h) In Jeddah, the climate is defined as hot and humid throughout the year. There is nearly no moisture difference between summer and winter (50-75% relative humidity). But sometimes it can reach 100% RH. The hottest day is the 13th of July with 42,2°C.The peak temperature is lower than in Riyadh, but the temperature is high all year long. The mean temperature on a year is 28,1 °C. CHALMERS, Energy and Environment, Master’s Thesis 2011:09 16 The climate in Jeddah is close to the climate on the Arabian Gulf coast, in the west of Saudi Arabia in cities like Dhahran. The cooling characteristics in Jeddah and this other part of Saudi Arabia are then comparable. Figure 12 Temperature (-5 to 50°C) and relative humidity (10 to 100%) in Jeddah for one year (8760h) 3.2.2 Solar radiations In Saudi Arabia, the solar radiation may have a huge impact on the energy use of a building. Indeed the solar constant, which is the intensity of the solar radiation that reaches the upper limit of the atmosphere, has a mean value of 1,39 kW/m². With some dispersion and reflection through the atmosphere, its intensity is decreased when reaching the earth surface, but still has a high value for the cities in study. Heating to the building is provided by both direct and diffuse solar radiations. Actually, the solar radiation is directed in 3 different ways when reaching a surface: - One part is reflected back. It is noted . - One part is absorbed by the material, creating heat. Convection, conduction and radiation will then transport this heat away, some amount going back to the outside of the building, and some to the inside. The absorptivity of the solar radiations through a material is noted . - One part is transmitted through the material (mostly for windows) to the inside of the building. The transmittance for solar radiation is noted . It is dependent on the angle of incidence of the incoming solar radiation. Yet, as to simplify the model, this angle will not be taken into account in the calculations. Doing this for a building in Saudi Arabia is taking the worst case, when the solar transmittance is the highest. It is to be noted that: ( 1 ) CHALMERS, Energy and Environment, Master’s Thesis 2011:09 17 The solar radiation on each surface is calculated from:  The irradiance of the beam. It is the direct radiation received by a surface perpendicular to the sun radiations.  The diffuse radiation received by an horizontal surface  The orientation of the wall (North, East, South or West)  The inclination of the surface (90° for the walls,0° for flat roof)  The longitude  The latitude  The reflectivity of the ground (0,2 for sand)  The local standard time meridian (45° for Saudi Arabia). It is the real position of the sun on earth compared to the time zone (+3h GMT) These data give the direct solar radiation, the diffuse solar radiation using the different angles of sun in the sky. The sum of all this will give the total solar radiation. To calculate the solar radiation depending on the facade, a MATLAB program is used. It has been created by Toke Rammer Nielsen in 2000 and using the method explained in the report “All-weather Model for sky luminance distribution- preliminary configuration and validation” written by Richard Perez, Robert Seals and Joseph Michalsky from the state University of New-York in USA in 1993. To be more accurate, the shadow created by the overhangs on the façade has also been calculated. The overhangs will protect the walls from direct sun radiations. Thus, it will reduce the peak solar load. To calculate this shadow, different angles of the sun in the space are used: Figure 13 Shadow on a wall from an overhang and different inclinations of the sun (Centre Scientifique et Technique du Bâtiment, 2008) With: θ [rad] the incidence angle. It is the angle between the direct sun radiation and the normal to the surface. Direct Sun radiation CHALMERS, Energy and Environment, Master’s Thesis 2011:09 18 γ [rad], the height of the sun. It is the angle between the direct sun radiation and the horizontal projection. Φ [rad], the azimuth. It is the angle between the horizontal projection of the direct sun radiation and the normal of the wall. dhm [-], the length of the overhang. dh [-], the height of the shadow. hp [-], the height of the wall. The height of the shadow is calculated by: ( 2 ) In Appendices D.a. and F.a., the total solar radiation for different surfaces are shown for Jeddah and Riyadh. The radiation on the north façade is really small (from 200 to 300 W/m 2 ); on the south façade, the radiations are high in winter (900W/m 2 ) but low in summer (250W/m 2 ). There are two main reason of this. First the sun is higher in the sky in summer so it has a big γ angle, the shadow on the wall is important. The highest radiation received by the wall is the normal radiation, when γ and ϕ are equal to 0. The radiation level is reduced by the angle made with the normal to the surface. These angles are higher in summer. The west and east façade received the highest radiation level (600 to 900 W/m 2 ). These façades receive more sun (the height of the sun is lower than on the south) in summer and during a longer time. As to reduce the solar heat gain through the window, it is crucial to decrease its solar factor. Different types of coatings can be used for this purpose by decreasing the direct transmittance. Furthermore, a low-emissivity coating on the inner side of the outer glazing will reduce act as a better thermal insulation by reflecting long-wave radiations to the outside and letting short-wave radiations pass to the inside. 3.2.3 Long-wave radiations Long wave radiations from the sky to the outer parts of the building change its surface temperature. To determine the surface temperature, the “sky temperature” or temperature of the atmosphere is consider, which takes into account the temperature of radiation temperature of the sky. As defined by (Hagentoft, 2001), the sky temperature is calculated as following: [°C] for a horizontal surface, clear sky ( 3 ) [°C] for a vertical surface, clear sky ( 4 ) [°C] for a cloudy sky ( 5 ) Where [°C] is the air temperature. CHALMERS, Energy and Environment, Master’s Thesis 2011:09 19 Both Jeddah and Riyadh will be taken as clear sky regions. In reality, Jeddah should be considered as cloudier, especially due to its high relative humidity, leading to a foggy sky. For this report, it was interesting to determine the impact of the long-wave radiations on the air temperature, therefore the skies were assumed to be clear. (WeatherReports.com). As the long-wave radiations are taken from the software METEONORM, the long- wave radiation heat flux to a specific surface can be derived using this formula: [W/m 2 ] ( 6 ) Where: [-] is the view factor between the sky and the given surface [-] is the emissivity of the surface considered [W/m 2 ] is the irradiation of the surface due to long-wave radiations, as taken from METEONORM. The cloudiness of the city is considered as already taken into account in this data. W/m 2 .K 4 is Stefan-Boltzman constant 3.2.4 Wind conditions The wind conditions influence the heat transfer by convection of the building envelope. The convective coefficient [W/m 2 .K] can be calculated from the known wind speed, as taken from METEONORM. (Hagentoft, 2001) on a windward side with m/s ( 7 ) on a leeward side with m/s ( 8 ) on a parallel side with m/s ( 9 ) Where [m/s] is the wind speed. In Jeddah the wind speed and wind direction are very constant through the year. Its values range between 0,5 and 1 m/s and the direction is always from the North. It is to be noted that these values are very low, so the wind has almost no influence on the cooling by convection of the building. Table 3 Wind conditions in Jeddah Façade: North East South West Roof Wind blowing from: North Windward Parallel Leeward Parallel Parallel Min [W/m 2 .K] 7,215 8 5,75 8 8 Max [W/m 2 .K] 9,36 10 6,5 10 10 CHALMERS, Energy and Environment, Master’s Thesis 2011:09 20 Table 4 Wind conditions in Riyadh Façade: North (0°) East (90°) South (180°) West (270°) Roof Wind blowing from: North (0-45, 315-360°) Windward Parallel Leeward Parallel Parallel Min [W/m 2 .K] 6,3374 7,2 5,45 7,2 7,2 Max [W/m 2 .K] 7,6496 8,4 5,9 8,4 8,4 Wind blowing from: East (45-135) Parallel Windward Parallel Leeward Parallel Wind blowing from: South (135-225°) Leeward Parallel Windward Parallel Parallel 3.2.5 Equivalent surface temperature As described in the previous paragraphs, the heat transfer between the building and the surroundings is increased by several factors. Especially, the sun irradiation of the envelope, the exposure to long wave radiations and the convective heat transfer at the external surface can change the heat transfer magnitude of the building envelope. To take into account these processes, it is possible to determine an equivalent temperature for the walls and roof of a building. As on the inside, there is no or little exposure to sun or long-wave radiations, the heat transfer on the inside can be neglected compared to the heat transfer between the outer side of the envelope and the exterior. Therefore, the surface temperature will be approximated to the equivalent outdoor temperature. Also, in the present case, latent heat is disregarded. Hagentoft then gives the equivalent temperature as following: [°C] ( 10 ) Where: [W/m 2 ] is the heat due to solar radiation per surface area [W/m 2 .K] is the total heat transfer coefficient at the surface [W/m 2 .K] ( 11 ) In this equation is the heat transfer coefficient due to long wave radiation. It is calculated as following for 2 surfaces ̅ [W/m 2 .K] ( 12 ) Where: ̅ [K] is the mean temperature of the surfaces as calculated in Equation ( 13 ) [-] is the emissivity of a surface [-] is the view factor between the surfaces [m 2 ] is the area of a surface CHALMERS, Energy and Environment, Master’s Thesis 2011:09 21 Here, one surface will be a wall or the roof, and the other one is the sky. Therefore the mean temperature will be: ̅ ̅ [K] ( 13 ) Where [°C] is considered equal to the outdoor air temperature Also, in this case, [-] is the view factor between the sky and the surface. It is equal to 0,5 for the walls. A simplification is made for the roof: instead of using the view factor corresponding to the real pitch angle of the roof of 18°, the view factor will be considered equal to 1 as if the roof was horizontal. Finally, when considering the sky as one surface, the heat transfer coefficient due to long wave radiations can be simplified, as the area of the sky is infinite ̅ [W/m 2 .K] ( 14 ) Where: [-] is the emissivity of the wall or the roof. For a white painted surface, it is taken equal to 0,85. For a window, it is equal to 0,92. 3.2.6 Ground temperature The heat transfer to the ground also differs from the heat transfer through another part of the envelope to the exterior. This is due to the value of the ground temperature as well as other factors. In the present case, the ground is considered as a semi-infinite region filled with sand. The steady-state temperature in the ground below the centre of the slab is given hereafter as a function of the shape of the slab and the degree of thermal insulation, as well as the annual average outdoor temperature and the indoor temperature. [K] ( 15 ) Where: [K] is the annual average outdoor temperature [K] is the indoor temperature [m] is the equivalent insulation thickness of the soil, calculated as , with [W/m.K] the thermal conductivity of the sand and [m 2 .K/W] the total thermal resistance of the floor structure, from the soil surface to the interior of the boundary temperature. It includes the interior surface resistance and the thermal resistance of the floor structure. [-] is the center temperature of the ground. It depends on the shape of the rectangle slab ( [m] is the width, and [m] the length). This temperature is taken from the plots in the handbook written by Hagentoft (2001), p.191. CHALMERS, Energy and Environment, Master’s Thesis 2011:09 22 3.2.7 Moisture risks At the same time, moisture problems in the wall are checked. In Jeddah, the outdoor humidity can be really high. The software 1D HAM is used. It analyses the heat and moisture transport in multi layers walls. The analysed wall is composed of 3 layers: 20 cm of light weight concrete, 5 cm of polyurethane insulation, 20cm of light weight concrete. The input data are: the equivalent temperature on the outdoor surface of the wall ( ), the indoor temperature, the indoor and outdoor moisture content in the air, and the solar radiations on the surface. Below, the different characteristics of the materials are set. and are characteristics points on the sorption isotherms of the materials (see in Appendix C, the characteristics of the light weight concrete). is the upper limit of the hygroscopic region. , or in 1D Ham, is the vapour permeability of the material. Figure 14 Characteristics of the materials used in 1D-HAM The purpose is to divide the structure of the wall in different cells (until 100). The numerical model is based on finite difference technique. The transfer of moisture to and from the cells is governed by the humidity by volume in the cells and the humidity at the boundary. The increase of the moisture content, w (kg/m3), of cell number , with the width due to the net flow rate of moisture ((kg/m2·s)), is given by: ( 16 ) And ( 17 ) Here Δt is the time step considered. In this simulation, it is equal to one day. For the simulation, there indoor relative humidity was set to 40% and the temperature to 23°C, with a moisture production of 1,2 kg/h. In the fan coil design part, calculations are made with a moisture production of only 0,45 kg/h. The results show that there is no moisture problem even in the worst case, when there is a lot of moisture production inside. The wall is vapour permeable: the light-weight concrete and the polyurethane layers do not act as vapour barriers. CHALMERS, Energy and Environment, Master’s Thesis 2011:09 23 Figure 15 Moisture and temperature distribution through the wall (simulation using 1D-HAM). The outdoor surface of the wall is on the right and the indoor one on the left. 3.3 Building characteristics In the following part will be described the three building studied. It came originally from the building designed by Norconsult for the patent. The size, orientation and architecture of the buildings are exactly the same. 3.3.1 Building 1 - Traditional building techniques The main idea behind the choice of materials for the building, as designed by Norconsult, is that the building techniques used should be close to the traditional ones, thus making it easy to build and to maintain for local workers in the Middle East. Therefore, blocks of light-weight concrete were chosen for the walls, as the technique to erect them is well-known. Columns and light-weight concrete blocks will be used for the façade. Light-weight concrete material was preferred to common concrete for its thermal properties. Table 5 Thermal characteristics of light-weight concrete (YtongThermopierre) Minimum Maximum Density 365 kg/m 3 425 kg/m 3 Thermal conductivity 0,09 W/m,K 0,14 W/m,K Thermal capacity 1000 J/kg,K 1300 J/kg,K Vapour diffusion resistance coefficient 5 36 Light-weight concrete Light-weight concrete Polyurethane CHALMERS, Energy and Environment, Master’s Thesis 2011:09 24 For the roof structure, the use of concrete and foamglas was preferred. Indeed, foamglas is both a mechanically resistant material and a good insulation material. Table 6 Thermal characteristics of foamglas (Foamglas T4) Minimum Maximum Density 100 kg/m 3 165 kg/m 3 Thermal conductivity 0,038 W/m,K 0,050W/m,K Specific heat capacity 840 J/kg,K 1000 J/kg,K Vapour diffusion resistance coefficient 36 38 3.3.2 Building 2 – Insulated walls Regarding the current situation in Saudi Arabia, especially concerning oil use, it is assumed that a law will be soon voted to compel to use a certain amount of thermal insulation material in buildings. As light-weight concrete is not considered as an insulation material, other materials had to be chosen. In this sub-part will be discussed the choice of materials for the walls, roof and windows, as to reduce further on the need for cooling. To stick with the architectural idea of the Solar House, the only change in the design was to add insulation as a “sandwich wall” with 2 layers of light-weight concrete blocks and insulation in-between. 3.3.2.1 Polyurethane Life Cycle Cost According to Ahmad (December 2002), polyurethane is the best solution regarding Life Cycle Costs benefits. Indeed, for Riyadh city, the study shows that the optimum insulation thickness for polyurethane is 5,0 cm regarding both thermal resistance and total cost (average insulation cost and estimated cost for energy use to get the desired indoor climate). 5 cm thickness polyurethane costs about 7,8 US$ per square meter (2002) and thus has a payback period of about 2,1 years. Another study shows that using 5 cm polyurethane can reduce the building annual energy use by up to 45% for residential buildings in Riyadh, compared to non-insulated buildings; the impact on the peak cooling load is up to 37% reduction in the same conditions (Al-Houmoud, 2003). Thermal characteristics Polyurethane insulation is made of small closed gas cells having a lower thermal conductivity than air. Thus, it has a very good thermal conductivity ranging from 0,021 W/(m,K) to 0,028 W/(m,K). It is usually produced as foam within 2 aluminium foils about 50 μm thick. Those ensure the air tightness of the insulation as well as keeping the good thermal properties of the insulation through the age. (ISOVER, 2008). CHALMERS, Energy and Environment, Master’s Thesis 2011:09 25 Table 7 Thermal characteristics of polyurethane (Kingspan Insulation Kooltherm K8) Minimum Maximum Density 30 kg/m 3 50 kg/m 3 Thermal conductivity 0,021 W/m,K 0,028 W/m,K Specific heat capacity 1400 J/kg,K Vapour diffusion resistance coefficient 36 38 3.3.2.2 Extruded Polystyrene Extruded polystyrene is only used for insulating the ground. This is valid for all three buildings studied. Table 8 Thermal characteristics of extruded polystyrene (DOW Styrofoam IB) Minimum Maximum Density 25 kg/m 3 40 kg/m 3 Thermal conductivity 0,029 W/m,K 0,035W/m,K Specific heat capacity 1300 J/kg,K Vapour diffusion resistance coefficient 100 As a result, “sandwich” type of wall will be used, including 200 mm of light-weight concrete, 50 mm of polyurethane insulation, 200 mm of light-weight concrete. Indeed, according to the studies of Al-saadi (May 2006), using polyurethane with light-weight concrete blocks (Siporex) is the best association regarding thermal issues. 3.3.3 Building 3 – Optimization of the windows When studying the buildings and their needs of cooling, it was noticed that the sun constituted the main heat gain source (refer to chapter 3.2.2 and 4.4). Also, to decrease the need of cooling, the best way was to improve the design of the windows for the house. Optimizing the size, location and characteristics of the windows influence very much the indoor climate. Preferably windows should be avoided on the western façade, as it is the most difficult ones to protect from direct sunlight during summer afternoons, inducing high solar heat gains. On the southern side, it is also very useful to protect the windows with roof eaves and solar protected glazing. Yet, there should be enough windows to get natural light to the building, and avoid using additional electrical appliances. According to ISOVER (2008), the total windows area should be around 20% of the heated area to reach a good thermal comfort, while keeping good lighting. As defined on the architectural plans, the studied building has a roof eave of about 0,5m. There will be no windows on the southern façade, 44 m² on the northern façade, 14 m² on the eastern façade, and 30 m² on the western façade. CHALMERS, Energy and Environment, Master’s Thesis 2011:09 26 For the choice of glazing, the main characteristics considered are: the U-value and the shading coefficient. The most energy-efficient choice at the moment is triple-glazing windows with a U-value of 0,7 W/m².K and a solar shading coefficient of 17%. It should be noted that this U-value is for the window only. When taking the frame into account, the U-value is increased. In the following study, it will then be considered equal to 1,0 W/m².K 3.3.4 Characteristics of the buildings to be compared In the Appendix A, the drawings of the house are shown. It is a building with two floors with no windows on the south façade. Building 1 presented in the table below, is the one already designed. All its characteristics are shown. The two other buildings keep the same shape but some parameters are changed. Table 9 Characteristics of the 3 types of buildings Building 1 Building 2 Building 3 Solar House (basic design) Insulated building Solar House (improved design) Area [m²] Composition U[W/m 2 K] Area [m²] Composition U [W/m 2 K] Area [m²] Composition U [W/m 2 K] N Walls 63,12 50 cm LWC 0,231 63,12 20 cm LWC 5 cm PUR 20 cm LWC 0,170 62,16 50 cm LWC 0,231 Windows 48,88 SHGC 0,32 0,70 48,88 SHGC 0,32 0,70 49,84 SHGC 0,32 1,0 E Walls 44,4 50 cm LWC 0,231 44,4 20 cm LWC 5 cm PUR 20 cm LWC 0,170 48,24 50 cm LWC 0,231 Windows 18,17 SHGC 0,15 0,70 18,17 SHGC 0,15 0,70 14,33 SHGC 0,15 1,0 S Walls 88 80 cm LWC 0,146 88 20 cm LWC 20 cm PUR 30 cm LWC 0,072 88 80 cm LWC 0,146 W Walls 24,77 50 cm LWC 0,231 24,77 20 cm LWC 5 cm PUR 20 cm LWC 0,170 56,82 50 cm LWC 0,231 Windows 37,33 SHGC 0,15 0,70 37,33 SHGC 0,15 0,70 5,28 SHGC 0,15 1,0 Roof 222 20 cm C 20 cm FG 0,179 222 20 cm C 10 cm PUR 0,197 222 20 cm C 50 cm FG 0,074 Ground 214,5 20 cm C 50 cm XPS 0,061 214,5 15 cm C 10 cm XPS 0,289 214,5 15 cm C 10 cm XPS 0,289 Middle slab 150 20 cm C 150 20 cm C 150 20 cm C Equivalent U- value[W/m 2 K] Without ground: 0,2858 Including ground: 0,2225 Without ground: 0,2639 Including ground: 0,2708 Without ground: 0,2512 Including ground: 0,262 Heat capacity 53870 Wh/K 54025Wh/K 54248Wh/K C Concrete XPS Extruded Polystyrene FG Foamglas LWC Light-weight concrete PUR Polyurethane CHALMERS, Energy and Environment, Master’s Thesis 2011:09 27 Figure 16 Walls and windows distribution for the different facades of the three buildings. CHALMERS, Energy and Environment, Master’s Thesis 2011:09 28 CHALMERS, Energy and Environment, Master’s Thesis 2011:09 29 4 Need for cooling For the following chapters, the focus is put on building 2 for Jeddah’s climate. The results for the other buildings are put in the annex part. In this chapter, the need for cooling to the building will be calculated. 4.1 Principles The need for cooling is defined as the surplus heat provided to the building, and that is to be got rid of to reach good indoor thermal conditions. In the following part, several sources of heat gain to the building will be considered: gains through the envelope, solar gains and leakages. Solar heat gains to the building can be transmitted in two ways: through the windows and by heating of the wall surfaces. The heating of the wall surfaces gives the equivalent temperature. 4.1.1 Heat gains through the envelope The heat gains through the envelope are calculated as following: [W] ( 18 ) There are different values for , depending on the orientation of the façade, the roof or the slab. has a different value in each case (see the data about the characteristics of the house). [°C] is taken as for the walls and the roof (depending on the orientation of the façade too). For the slab, it will be equal to . For the whole envelope, the transmission losses are summed up, giving: [W] ( 19 ) 4.1.2 Solar gains The heat gains by solar radiations through the windows can be calculated with: [W] ( 20 ) Where: [-] is the solar factor of the window pane, or solar heat gain coefficient: it is the part of the radiation due to direct transmittance and re-radiated radiation from the absorbed part, as seen on [m 2 ] is the windows area [W/m 2 ] is the solar radiation on the surface considered CHALMERS, Energy and Environment, Master’s Thesis 2011:09 30 Figure 17 Insulating Glass Unit incorporating coated solar control glass (Pilkington, 2010) Sun radiations provide internal heat gain to the building during the day. For the whole building, [W] ( 21 ) To reduce the solar heat gains, the windows should be chosen with a low solar heat gain coefficient, reducing the solar transmission to the inside. Another solution is to reduce the windows area, especially the ones facing east and west. 4.1.3 Air leakages Air tightness is another factor that can increase the thermal transmissions between a building and its environment. Also, as described later, the building studied uses mechanical supply-and-exhaust ventilation with heat recovery. In that case, air leakages are of importance for the energy use, as in case of leakages the air short- circuits the heat recovery, thus requiring more energy use. Therefore, having an air tight building is a requirement in the design. Usually, the air tightness of a building is measured by a pressurisation test (the Blower Door test). There are regulations specifying the maximum pressure difference that should be obtained during this test. For the present building, the limit value is taken as 0,05 l/s,m 2 of heated area. Therefore the heat loss corresponding to these leakages is equal to: [W] ( 22 ) With: [m 2 ] the heated area of the building CHALMERS, Energy and Environment, Master’s Thesis 2011:09 31 J/m 3 .K the volumetric heat capacity of the air. 4.1.4 Need of cooling calculation With the previous equations, one can derive the need of cooling for the building: [W] ( 23 ) One can notice that the thermal capacity of the building is taken into account in this equation. It reflects how the envelope can buffer heat coming through the building. The thermal capacity is derived from: [J/K] ( 24 ) With , the periodic penetration depth, equal to √ [m] ( 25 ) , the time period of the variation, is 24h in this design, reflecting the daily temperature variations. The periodic penetration depth represents the thickness of the envelope on the indoor side for which the conditions will be influenced by the indoor temperature on 24 hours. It depends on the materials: - for light weight concrete, - for concrete, The walls inside the building and the middle slab are also included in the calculations for the thermal capacity. All of them act as buffer for the variations of the indoor temperature. CHALMERS, Energy and Environment, Master’s Thesis 2011:09 32 4.1.5 Comparison of the heat gains Figure 18 Heat gains to the building (-10 to 25W/m 2 ), for one year (8760h). In blue: the solar gain, in red: the transmissions through the building envelope, in green: the leakages, in yellow: the hygienic ventilation (Jeddah Building 1) (simulation using the modelling tool Simulink) Due to their low value compared to the other gain sources, heat gains provided by people activity and leakages will be disregarded from now on, leading to: ( 26 ) 4.2 Definition of the ventilation system After having determined the heat gains, it is possible to derive the cooling load. Also, it is important to know the cooling and ventilation system to get right load. The cooling provided to the building will be done using a fan coil. Therefore the air is taken from the rooms and directly cooled down inside the rooms. Yet, for every building there is a minimum amount of fresh air to be provided: the hygienic flow. For this study, the hygienic flow is considered as a certain amount of fresh air that does not provide cooling to the building. Therefore, it is considered to be an input of air at the indoor temperature. Nevertheless, there is a need of energy to bring this fresh air – at the outdoor temperature – to the indoor temperature. 4.3 Cooling load As regards to the previous definition, the heat gain is the amount of heat provided to the building through transmission through the walls and solar gains. [W] ( 27 ) CHALMERS, Energy and Environment, Master’s Thesis 2011:09 33 The cooling demand would then be the same amount of cooling needed to compensate this heat gain. [W] ( 28 ) The cooling load is the cooling demand to cool down the building, in addition to the cooling demand to cool down the hygienic air. ̇ [W] ( 29 ) With ̇ the hygienic flow, equal to 93 l/s. 4.4 Comparison of the cooling demands In this part, the need of cooling for the different situations presented above will be compared. Figure 19 Transmission heat gains through the walls for the different buildings (-15 to 15W/m 2 ) in Jeddah for one year (8760h) [W/m² floor area]. (simulation using the modelling tool Simulink) Buildings 1 and 3 are both made of light-weight concrete in the same dimensions and thus have the same transmission heat gains for the walls. Building 2 has insulation material to buffer the heat gains. Therefore, building 2 should have less heat gains than building 1 and 3. Yet, as there is a huge reduction in the windows area for building 3, it also improves the equivalent U-value of the walls and windows. Here, the gains are diminished by half (from about 10 W/m 2 for buildings 1 and 2 to 8 W/ m 2 for building 3). CHALMERS, Energy and Environment, Master’s Thesis 2011:09 34 Figure 20 Solar heat gains through the windows for the different buildings (0 to 25W/m 2 ) in Jeddah for one year (8760h) [W/m² floor area]. (Simulation using the modelling tool Simulink) Buildings 1 and 2 have the same windows characteristics. Building 3 has more than 50 % less windows. Also, the solar heat gains are reduced by half for building 3 (from 20 W/m 2 for buildings 1 and 2 to 10 W/m 2 for building 3). Figure 21 Total heat gains to the building (-15 to 35W/m 2 ) in Jeddah. In blue: building 1, in green: building 2, in red: building 3. for one year (8760h) [W/m² floor area]. (Simulation using the modelling tool Simulink) CHALMERS, Energy and Environment, Master’s Thesis 2011:09 35 Finally, from the graph above, the total heat gain of building depends a lot on the solar radiations (reduction from 30W/m 2 with the building 2 to 20W/m 2 with the building 3). The insulation has an influence on the diminution of the heat gains to the building, but a reduction of the windows area, a low solar heat gain coefficient g, or a better layout of them (avoiding west and east facades) is really the decisive factor in this kind of climate. 4.5 Climate without active cooling Without fan coil in the building to keep the indoor temperature between 22°C and 25°C, the latter varies with the external conditions. Equation ( 26 ) is used to calculate the indoor temperature. It is shown for both Riyadh and Jeddah and for the three different buildings, on the two figures below. Again, the comparisons made before are also valid here. Building 3, with the reduction of the windows area, has a much lower indoor temperature in summer (32,5°C in Riyadh, 34°C in Jeddah). The insulation layer, in building 2, reduces the summer temperature of 3°C compared to building 1, in both climates (49°C to 46°C for example in Jeddah). Of course, the indoor temperature cannot vary so much. It is interesting to see that it is very far from the comfort temperature of 25°C. That is why fan coils are necessary. In the next part all the cooling system is calculated. Figure 22 Temperatures in Riyadh without active cooling (20 to 50°C) for one year (8760h) [°C] (simulation using the modelling tool Simulink) CHALMERS, Energy and Environment, Master’s Thesis 2011:09 36 Figure 23 Temperatures in Jeddah without active cooling (20 to 50°C) for one year (8760h) [°C] (simulation using the modelling tool Simulink) 4.6 Thermal capacity Figure 24 Thermal capacity of the building. Comparison between the outdoor temperature (in green) and the indoor temperature (in blue) (28 to 46°C). (simulation using the modelling tool Simulink) The influence of the thermal capacity can be seen in the delay between the indoor and outdoor temperature variations. The following figure shows the indoor and outdoor temperature for Building 2 in Jeddah. Here, there is about 12 hours of delay. Also, the Delay Indoor temperature variation (low variations) Outdoor temperature variation (high variations) CHALMERS, Energy and Environment, Master’s Thesis 2011:09 37 thermal mass helps the building buffer the climate variations. While the outdoor climate varies in a range of 14°C, the indoor temperature only varies in a range of 2°C. CHALMERS, Energy and Environment, Master’s Thesis 2011:09 38 CHALMERS, Energy and Environment, Master’s Thesis 2011:09 39 5 Air cooling system 5.1 Cooling strategy The scheme below shows the strategy used to cool the house. Figure 25 Strategy for the air cooling system With: [°C] the temperature [g/m 3 ] the moisture content in the air ̇ [m 3 /s] the flow through the ducts The relative humidity (RH) [%] is equal to with: [g/m 3 ], the moisture content at saturation, represents the maximum amount of moisture that can be present in the air. It depends on the temperature. And: [g/m 3 ] ( 30 ) This formula is basically applied when the temperature is between 0°C and 30°C. But this formula still works for a temperature of 45°C with a margin of ±0,02 g/m 3 . The cooling is done in 2 parts:  The cooling recovery unit  The fan coil Mixing flows Cooling recovery unit Fancoil Water cooling House ̇ ̇ ̇ ̇ CHALMERS, Energy and Environment, Master’s Thesis 2011:09 40 There is mixing of the hygienic air with the indoor air before cooling by water. The moisture is considered in this part too. In the rest of the study, no more care will be taken for the cooling demand, as defined in paragraph 4.4. Also, when writing about the need of cooling, it will always refer to the cooling load. 5.2 Cooling recovery The weather in Saudi Arabia is not only hot, it can be wet too. Therefore, the cooling strategy will be to use a comprehensive moisture and heat recovery unit. The indoor condition will be around 23°C and 40% relative humidity. The outdoor condition can rise to 42-46°C and 60-80% relative humidity. In the current market, the efficiency of these cooling energy recovery unit with hygroscopic wheel made in aluminium is 80 % for temperature and 70% for moisture. They recover both sensible and latent heat. The efficiencies correspond to: Temperature: [-] ( 31 ) Moisture: [-] ( 32 ) This will reduce the energy consumption to dehumidification and cooling of the air. So it will reduce the investment cost for cooling, more specifically with the reduction of fan coils or heat pump and pipes. In Figure 26, the cooling capacity of the cooling energy recovery all along the year with different climates is shown. This cooling capacity can be 5 kW, especially in Jeddah, where the moisture content in the outdoor inlet air is high. The outlet temperature and moisture content after the cooling recovery is shown too. This air is used only as hygienic flow, which is needed for hygienic reasons inside the building: it is the air being renewed. As it is a house of 300 m 2 of floors with few people in it, and few activities, this flow is very low. It will correspond to 0,35 ACH, i.e. 0,35 times the volume of the house, which is 300m 2 ×3,2m. This flow will be 336 m 3 /h or 93 l/s. The cooling capacity of the cooling energy recovery is: ̇ [W] ( 33 ) Where [kJ/kg] ( 34 ) , the specific thermal capacity of the dry air, is 1kJ/(kg,K) , the specific thermal capacity of the water vapour, is 1,85 kJ/(kg.K) , the latent heat of vaporization of water at 0°C, is 2500 kJ/kg CHALMERS, Energy and Environment, Master’s Thesis 2011:09 41 Figure 26 Cooling recovery unit capacity (-6000 to 3000W), for one year (8760h). In blue: Jeddah, in green: Riyadh. (simulation using the modelling tool Simulink) On the graph above, the cooling capacity of the recovery unit is shown. The cooling capacity is higher in Jeddah (until -5000W) than in Riyadh (until-2000W) because there is a higher amount of moisture to be transferred between the exhaust air and the outdoor one, so the difference of enthalpy is higher. All the values are negative because the difference of enthalpy in the equation is negative. The hygienic flow temperature varies between 22°C and 28°C depending on the season. The temperature of the hygienic flow is shown in the Appendix D b. This is perfect to be used to cool the house. The moisture transferring part is a key part. Without this desiccant wheel, the moisture would be a big issue in the hygienic flow, because some condensation would occur with a hygienic flow temperature below 26°C, if the moisture content is the same as the one outside. The air would be saturated. On the graph in Appendix D b, it is possible to see that there will be no moisture problem anymore in the air after the cooling recovery. The relative humidity is small enough, between 40% and 60%. More in Riyadh, and a little bit in Jeddah, the recovery wheel will work as a heat recovery unit. The nights can be quite cold in Riyadh with this arid climate. The main purpose of the recovery unit is to reduce the difference between the hygienic flow conditions and the indoor conditions. That is why, on the graph for Riyadh, the cooling capacity is above 0W in winter. It means that the unit is not cooled but heated. The heat recovery unit will supply enough amount of energy to keep the temperature inside the house above 22°C. It should be noted that, at the time when the report is written, no cooling recovery unit (for temperature and moisture) exists for such a small air flow. The calculations are still made considering the same recovery efficiencies, but these are not based on data from a real unit. CHALMERS, Energy and Environment, Master’s Thesis 2011:09 42 5.3 Mixing of air The hygienic air coming from the outlet of the cooling exchanger is not redistributed directly to the rooms. It is mixed with the indoor air at the inlet of the fan coil to be cooled to the desired temperature through it. The indoor air flow through the fan coil is much bigger compared to the hygienic flow through it. This indoor air flow will be a fixed value all along the year and, as to meet the required cooling needed inside the house, it will be either equal to: - 0 m 3 /s or 0,8 m 3 /s in Jeddah - 0 m 3 /s or 1 m 3 /s in Riyadh By fixing this air flow, only the outlet temperature of the fan coil will vary. There will be a step change of the inlet flow in Riyadh and Jeddah. The step change is mostly due to the difference between day and night conditions. Sometimes the hygienic flow is enough to cool the building ( ̇ = 0 m 3 /s and ̇ =0,093 m 3 /s). Sometimes a much bigger flow is needed to create cooling ( ̇ = 1 or 0,8 m 3 /s and ̇ =0,093 m 3 /s). The sun radiations influence a lot the need of cooling inside the house. In Jeddah, the flow is always at the maximum during summer. To calculate the temperature and moisture of the mixed air, this common equation is used: ̇ ̇ ̇ ̇ [°C] ( 35 ) ̇ ̇ ̇ ̇ [g/m 3 ] ( 36 ) ̇ ̇ ̇ [l/s] ( 37 ) 5.4 The fan coil In the fan coil, the air will be cooled at the contact with the water system. The inlet and outlet temperature of the water is 8/14°C, which makes a mean temperature of 11°C. On the Mollier chart below, the fan coil cooling process is shown. It takes into account the latent and the sensible heat: a wet fan coil is used. The cooling capacity of the fan coil is written as: ̇ [W] ( 38 ) CHALMERS, Energy and Environment, Master’s Thesis 2011:09 43 Figure 27 Mollier chart for a hot outdoor climate There will be dehumidification during the process of cooling. That is why the cooling capacity equation with enthalpy difference is used. There is both sensible and latent cooling. Of course, there is a need to take care of the water obtained from the dehumidification in the fan coil, but this is always integrated in these fan coils, especially for this kind of climate. To get the required temperature conditions inside the building (between 22°C and 25°C), the fan coil needs to be parameter depending on the indoor temperature; the efficiency of the fan coil will vary. Figure 28 Correlation between the outlet fan coil temperature and the indoor temperature t min t in 22 25 t in t fan CHALMERS, Energy and Environment, Master’s Thesis 2011:09 44 To get a correct outlet temperature from the fan coil, it is adjusted linearly depending on the indoor temperature, as seen on the figure above. The maximum outlet temperature is equal to the indoor temperature, and the minimum outlet temperature depends on the need of cooling. Therefore, the outlet temperature is equal to: [°C] ( 39 ) With: set to 14°C in summer, and to 17°C in winter, to avoid moisture problems. On the figures in Appendix D b, the moisture and the temperature at the outlet of the fan coil are shown together with the outdoor conditions and after the cooling recovery unit. The temperature at the outlet of the fan coil varies between 16°C and 21°C, while the relative humidity varies between 45% and 65%. Figure 29 Fan coil cooling load (-7000 to 0W), in Jeddah, for one year (8760h). Building 1 in blue, building 2 in green, building 3 in red. (simulation using the modelling tool Simulink) On the figure above, the cooling capacity of the fan coil is shown. Depending on the building, the need of cooling is different; hence the fan coil cooling load is different. For building 3, with fewer windows, it reaches -4500 W in summer, whereas for building 2, with the insulation layer, it reaches -5500W and -6000W for building 1. All the values are negative because it is a need for cooling: the difference of enthalpy in the equation is negative. 5.5 Indoor Moisture There are many different moisture sources in a house. CHALMERS, Energy and Environment, Master’s Thesis 2011:09 45  Human produce moisture depending on the activity: 50g/h when sleeping, 90g/h when standing/walking.  Cooking: 2kg/day  Shower: 0.2kg/day  Laundry. 0.5kg/day Assuming 6 people inside the house, the total moisture production inside is: G = 0,45 kg/h To deal with moisture indoors, a transient equation with moisture source is used. ̇ [kg/h] ( 40 ) The rotating wheel in the recovery unit has a big effect on the reduction of the moisture. This is the key component of the design. 5.6 Choice of fan coils To simplify the calculations, a big fan coil has been studied, but in fact, it is better to have 10 of them distributed all over the house. For the case of the solar house, 10 fan coils with a cooling capacity of 1000W and an airflow of 396 m 3 /h, i.e. 0,11 m 3 /s would be a good system. It is possible to change the parameters of either the airflow through the fan coil, or the temperature of the water, or the fan coil outlet temperature to obtain the same cooling capacity. To be sure to avoid draft problems, the size of the ducts is important. With a local temperature of 23,5°C, the mean velocity can reach 0,23 m/s as a maximum to avoid discomfort, as it is written in chapter 3. With an airflow of 0,11 m 3 /s through the fan coil, a supply pipe with a radius of 40 cm, i.e. 0,5 m 2 section, creates an airflow of 0,22 m/s, below the discomfort value for draft air. 5.7 Resulting indoor climate The most important here is to reach the indoor conditions to have a good thermal comfort. No boundaries have been set on the indoor temperature and relative humidity. They depend on ̇ and . Thanks to the equations written before, these two graphs are obtained. They are the results of the simulation using the modelling tool Simulink. The initial condition (at the beginning of the year) has been set arbitrarily. Besides, the indoor relative humidity needs some time to be stabilized. On the graph below, the moisture inside the room is not critical. The indoor relative humidity varies around 40%. According to the ASHRAE, the critical value of thermal comfort value, with a PPD of 10%, is 12g/m 3 of moisture in the indoor air. Here, it is around 8g/m 3 , so way below. CHALMERS, Energy and Environment, Master’s Thesis 2011:09 46 Figure 30 Indoor temperature (22 to 26°C), in Jeddah, for one year (8760h) (simulation using the modelling tool Simulink) Figure 31 Indoor relative humidity (30 to 45%), in Jeddah, for one year (8760h) (simulation using the modelling tool Simulink) CHALMERS, Energy and Environment, Master’s Thesis 2011:09 47 6 Water system design In this chapter, the first part consists of obtaining the amount of cooling delivered by the chiller from the photovoltaic panels or the grid. Then the second part is the design of the connection between all the components: the chiller, the fan coil and the tank as for the chiller to be able to deliver the necessary cooling capacity to the fan coil, for day and night. Proceeding this way, the chiller cooling capacity will match the fan coil cooling demand. Optimising it leads to deriving the area of the solar panels, the chiller size, and the tank size. 6.1 Solar panels and their behaviours in this climate Figure 32 Part of the cooling system to be analysed: solar panels 6.1.1 Angle of the panels To optimize the electricity delivered by the solar panels, the right angle should be determined. The same MATLAB program as the one to calculate the irradiation on the façade and the roof is used. But this time, a south direction of the roof is set and its horizontal angle varies. The optimal angle is the one that can create the highest amount of electricity both all year long and during the peak period. The peak period is defined as the time of the year that requires the most important cooling need. It is possible to get it from the fan coil design chapter. For Jeddah, this period goes from the 15 th of May to the 15 th of September (hour n°3217 to 6192). For Riyadh, it runs from the 6 th of May to the 15 th of September (hour n°3001 to 6192). CHALMERS, Energy and Environment, Master’s Thesis 2011:09 48 Table 10 Irradiations on the solar panels for different periods, depending on the roof angle for Jeddah. Angle roof (°) Jeddah Irradiation panels for one year (Wh/m 2 ) Irradiation panels for the cooling peak period (Wh/m 2 ) 0 2,1714×10 6 8,3457×10 5 15 2,3034×10 6 8,1252×10 5 18 2,3160×10 6 8,0326×10 5 21 2,3238×10 6 7,9250×10 5 24 2,3268×10 6 7,8002×10 5 30 2,3184×10 6 7,5071×10 5 45 2,2174×10 6 6,5557 ×10 5 For Jeddah, the optimal angle to obtain the highest amount of irradiation all year long is 24°, but the angle to have the highest amount of irradiation during the peak period is 0°. An angle of 18° has been chosen because the variation of irradiations with the optimal angle, in both cases, is still small. Table 11 Irradiations on the solar panels for different periods, depending on the roof angle for Riyadh Angle roof (°) Riyadh Irradiation panels for one year (Wh/m 2 ) Irradiation panels for the cooling peak period (Wh/m 2 ) 0 1,9610 ×10 6 8,3211×10 5 15 2,0979×10 6 8,1738×10 5 20 2,1207×10 6 8,0366×10 5 25 2,1318×10 6 7,8574×10 5 27 2,1331×10 6 7,7763×10 5 30 2,1318×10 6 7,6446×10 5 45 2,0637×10 6 6,7961×10 5 For Riyadh, the optimal angle to obtain the highest amount of irradiation all year long is 27°, but the angle to have the highest amount of irradiation during the peak period is 0°. Therefore, an angle of 20° has been chosen. On the graph below, it is possible to see the path of the sun at the equinoxes and the solstices. The angle of the sun with a horizontal surface, the sunset and sunrise are visible. Because the two cities are close to the cancer tropics (23° 26' 16" latitude north), the sun is nearly vertical the 21 th of June. To remind it, the latitude in Riyadh is 24,43° and 21,41° in Jeddah. Therefore, the best roof angle for the peak load season is 0°. The sun has an angle with a vertical surface corresponding to the latitude of the cities at the equinoxes. This explains why the most efficient angle of the solar panels depends on the seasons. CHALMERS, Energy and Environment, Master’s Thesis 2011:09 49 Sun path Sunrise/sunset March Sunrise June 21 Sunset December 21 Annual variation Equinox (March and September) Figure 33 Sun path in the sky for Jeddah (on the left) and Riyadh (on the right) (Tukiainen, 2005-2011) 6.1.2 The reduction factors of the supply electricity All powers of the solar panels are measured under the standard test conditions (STC): an irradiation of 1000W/m 2 and a temperature of 25°C. Of course, this does not represent the real outdoor conditions. The power delivered by the solar panels varies depending on the outdoor conditions. Below, the efficiency of different panels is shown under the STC conditions. Figure 34 Comparison of different PV module efficiencies (Solar Navigator, 2008) CHALMERS, Energy and Environment, Master’s Thesis 2011:09 50 Figure 35 Mono and poly crystalline solar panels (Uni-solar) 6.1.2.1 Mono crystalline panels The most efficient solar panel is the mono-crystalline one with an efficiency between 14% to 18%. They are quite more expensive than the poly-crystalline ones, but they have a longer life-span (25 to 50 years), better performance and efficiency. All the panels have a reduced electricity production when the temperature of the panel increases. The thermal reduction coefficient for a mono-crystalline panel is 0,5% per degree above 25°C. This value is much bigger for a poly-crystalline panel. So if the equivalent temperature of the surface of the panels is equal to 60°C, the reduction coefficient would be (60-25)×0,005 = -17,5%. On Figure 36, it can be seen that this temperature can nearly reach 100°C in Jeddah, with high variations during the day. This temperature is slightly the same in Riyadh and reaches 100°C too. This is the most important reduction factor of the electricity produced by the PV panels. To calculate the equivalent temperature on the solar panels surface, the same equation ( 41 ) as before is used, considering the convection, the conduction and the radiation. But this time, the absorptivity of the mono-crystalline panels is 0,92 and the emissivity is 0,84 (Muller, 2010) CHALMERS, Energy and Environment, Master’s Thesis 2011:09 51 Figure 36 Equivalent temperature on the surface of the photovoltaic panel (-20 to 120°C), for one year (8760h) in Jeddah (simulation using the modelling tool Simulink) 6.1.2.2 Reduction factors Assumptions have to be made when it comes to reduction factors. All variables that can influence the efficiency of the solar panels are: - Weather data, equivalent temperature of the solar panels - Soiling (dust on the panels): a small area of the panels are shaded or soiled and their efficiency can be much reduced. Especially for crystalline panels, the electricity output can nearly be reduced to 0 if two cells are really shaded or soiled. - Shading (from a tree, another building...) - The tilt angle of the panels: seen above, the electricity produced by the panels depends on its inclination with the sun - Array mismatch: the voltage obtained from all the panels is usually an average of the voltage at maximum power of each panel. When the difference between these two values increases, the power delivered is reduced. This value is considered as 2% losses - Inverter efficiency: the inverter has a large influence on the reduction of the electricity produced. It goes from 6% to 10% reduction losses. If a 3 phases system is used, and it is the case in this design (see chiller electrical data in appendix E), there is an added 2.5% to 3% reduction to be considered - Distribution losses: quite small, can be considered as 1% - Wiring losses: one should choose a right wire size that limits the losses (I 2 ×R) to less than 3% (Allan Gregg) CHALMERS, Energy and Environment, Master’s Thesis 2011:09 52 6.1.2.3 Triple junction amorphous silicon cells solar panels Figure 37 Triple junction amorphous solar panels (Uni-solar) A triple junction amorphous silicon cells solar panel can be a good alternative. Its advantages are: - It has a better efficiency than crystalline panels at low irradiation of the panels (beginning and end of the day or cloudy weather) - It has a better efficiency when the equivalent temperature of the solar panel is high. The thermal reduction coefficient for an amorphous silicon cells panel is 0,21% per degree above 25°C. So when the panel’s temperature is equal to 60°C, the reduction coefficient is equal to (60-25)×0,0021 = -7,35%. - The panels are really flexible and lighter and can resist better to damages than the crystalline ones: they still produce most of their rated power. On the contrary, when the glass of a crystalline panel is broken, it becomes nearly inefficient and the changing of the glass can be really expensive. - They cost much less to produce because less material is needed to build one panel as they are very thin. - In case of soiling or shading, the power produced by the amorphous silicon cells is reduced of 9% only, because the cells are bigger (fewer cells on one panel) and each cell has a by-pass diode. Figure 38 Effect of shading or soiling on the efficiency of the PV panels (Uni-solar) CHALMERS, Energy and Environment, Master’s Thesis 2011:09 53 However, compared to crystalline panels, their efficiency is smaller (between 5% and 9%), so a bigger surface of solar panels is needed. At very high equivalent temperature of the panels, both kinds of panels have the same efficiencies. For poly- crystalline panels, which are less efficient than mono-crystalline ones, it is the case. (Meike, 1998) To conclude, both kind of panels have pros and cons. The amorphous silicon cells solar panels work better in the Saudi Arabia climate conditions, but are less efficient compared to the mono-crystalline panels. In this study, the mono-crystalline panels are used to produce electricity. But the people who will live in this building should be aware of the maintenance, like cleaning the panels every two weeks or checking damages. 6.2 Chiller capacity and choice of chiller package unit Figure 39 Part of the cooling system to be analysed: chiller To know the capacity of the chiller, this one has to produce the cooling required for one day and the next night. Connected to solar panels, the chiller will only work during day time. In this report, the main goal is not to focus on the chiller properties but more on the strategy between the different parts of this cooling system. Therefore, to know the work to be delivered to the chiller, the outdoor temperature should be taken into account. The higher the outdoor temperature, the less efficient the condenser, and the smaller the chiller’s COP (coefficient of performance). To calculate this COP, for a given refrigerant, the isentropic coefficient of performance is [-] ( 42 ) With: [-] the isentropic Carnot efficiency [K] the temperature of the refrigerant on the evaporator side [K] the temperature of the refrigerant on the condenser side [W] compressor power input [W] the cooling delivered by the chiller CHALMERS, Energy and Environment, Master’s Thesis 2011:09 54 This is not the real value of the COP of the chiller. This corresponds to an ideal system; the chiller process is not really isentropic. [-] ( 43 ) With , the electric motor coefficient of performance [-] the isentropic efficiency of the compression process [-] the combined efficiency of the electric driving motor and transmission [-] the electric Carnot efficiency. It is usually between 0,4 and 0,6. This equation gives a more accurate value. A deeper analysis can be carried out on this part, for another project,