Güneş Enerjili Aktif Isıtma Sistemlerinin İncelenmesi, Projelendirilmesi Ve Modellenmesi

Abstract

Güneş enerjili ısıtma sistemleri ve güneş evleri, sıcak iklim şartlarına sahip bölgelerde güneş enerjisinden optimum faydalanılarak tüm ev ihtiyaçlarını karşılamayı hedefleyen sistemlerdir. Daha önce bu konuda yapılan çalışmalar göstermiştir ki, güneş enerjili sistemler, neredeyse yardımcı bir enerji kaynağına ihtiyaç duymadan bir evin tüm ısıl ihtiyaçlarını karşılayabilmektedir. Bu da güneş enerjili ısıtma sistemlerinin ne kadar ekonomik olduğunun kanıtıdır. Gerçekleştirilen bu çalışmada, öncelikle aktif güneş enerjili ısıtma sistemlerinin günümüze kadar ne gibi gelişmeler gösterdiği incelenmiştir. Daha sonra genel olarak güneş enerjili ısıtma sistemlerinin yapıları ve çalışma prensipleri anlatılmıştır. Türkiye meteorolojik şartlarına göre aktif güneş enerjili ısıtma sistemlerinin uygulanabilirliğini incelemek için bir matematik model oluşturulmuş ve İzmit iline bağlı, Gebze ilçesi sınırları içerisinde bulunan bir ev için uyarlanmıştır. Evin ısı kayıplarını azaltmak için izolasyon tedbirleri artırılmış, buna bağlı olarak sistemin ısı yükü hesaplanmıştır. Sistemin daha iyi çalışmasını sağlamak için güneş kollektörü alanı ve depo hacmi mümkün olduğu kadar büyük seçilmiştir. Sistemin çalışmasını incelemek için bir bilgisayar programı yazılmıştır. Bu program vasıtası ile, her saat için bir yıl boyunca, değişen dış ortam sıcaklığı ve ışınım değerlerine bağlı olarak depo suyu sıcaklığı (Ts), evden depoya gelen su sıcaklığı (Ti), evin ısı ihtiyacı (Qt), depodan ortama ısı kaybı (Qı), kollektörden elde edilen ısı (Qu), eve gönderilen ısı (Ls) ve yardımcı ısı kaynağının büyüklüğü (Qa) hesaplanmıştır. Bilgisayar programı yardımıyla yapılan bu hesaplamalar neticesinde bulunan sonuçlar, Türkiye şartlarında güneş evlerinin kullanılabilirliğini göstermiştir. Azalan yeraltı enerji kaynakları ve kirlenen çevre göz önünde tutulduğunda; temiz, kendi kendini yenileyebilen ve ekonomik olan bu enerji kaynaklı sistemin uygun bir çözüm olduğu kanısına varılmıştır.

Abstract Today, we know that the energy sources in the world are not aboundant. This phenomenon forced us to work on the renewable energy sources. One of the most powerful and the cheapest renewable energy source is certainly the sun. We should say that It is not a new subject. There have been numerous articles published on solar heated houses since 1970's. In our work we have searched the publications about the solar houses.The main aim of this study is to investigate the feasibilty of solar houses in northwestern Turkey's geographical conditions. Therefore, a solar house is designed at Gebze which is a small town located at the northwestern section of Turkey. The possibility to supply all the energy needed for the house was investigated using real meteorological data. Optimum values of storage tank volume, insulation thicknesses and the solar collector surface area were searched to realise our goal. Introduction There have been numerous investigations on solar heated houses since 1970's. A. Debosscher built and tested a solar house system. In his work there was a heat pump assisted solar energy air heating system. He made experiments to understand the needs of a house heated by the solar energy. He worked on the heat pump assisted solar heating system with one storage reservoir. Another thing that he studied was the effects of the parameters of heating systems such as heat pump size, collector area, storage capacity, etc... In his experimental house he used air as a transporting fluid and used auxiliary heating. H. Buchberg and J. R. Roulet also designed combined solar collector and storage system for house heating. They made integration and optimisation to minimise the unfavourable economics. Their study comprises annual simulation of system performance including the house, a flat plate solar collector, a water heat storage unit and an auxiliary heater. And overall there was an optimisation to achieve a maximum allowable collector cost. Y. Jaluria and S. K. Gupta were interested in the stratification of the storage tank. They made an experimental study of the temperature decay in a thermal stratified water body. The water body was initially stratified by the recirculating flow of hot water dischange and also statically, by the addition of hot water at the top of cold fluid. J. H. Davidson, D. A. Adams and J. A. Miller formed a dimensionless coefficient to characterise the level of mixing in the tank called MIX number, which was based on the height-weighted energy, or moment of energy. Its range was 0 to 1, that 0 representing a perfectly stratified (unmixed) tank and 1 representing a fully mixed tank. P. B. L. Chaurasia investigated the insulating materials in solar water storage system. He tested two insulating materials for their relative performance for retaining solar heated hot water during the whole night. The materials that he tested were fibreglass wool and sawdust. Mahmoud S. Audi developed a compact solar air heating unit for space heating using four types of Jordanian rocks in their natural forms as the heat storage medium. W. Stahl, K. Voss and A. Goetzberger built a self-sufficient solar house. They showed that it was possible to supply all the energy needed in household throughout the year only from the sun, under the Central European climatic conditions. Generally in all basic systems, there are a solar collector, a heat exchanger and a storage tank. By this schematic formation it would be possible to heat a house if there were enough solar radiation. Now we will examine our prototype as a sample. Solar House System Operation The house we wanted to heat was two storey house with a penthouse and an underground floor. The solar collectors were mounted on the roof. The net collector area was 100m2 covering the whole roof and its heat loss coefficient was SWmfC. Also the coefficient of the collector was 0.8. The storage tank was on the underground floor. It had 3mts height, 5mts width and 10mts length. The total volume of the tank was 150m3 and the surface area of it was 190m2. It had nearly 150,000kgs mass. We have four basic elements in our system. The first element is the solar collector, the second one is the heat exchanger, the third one is the storage XI tank, and the last one is the auxiliary heating source. The system works in the following manner: a) System Turned-Off Mode There is no need of energy for the dwelling. Since the ambient temperature is high, the heat that is absorbed from the solar collector is conveyed through heat exchanger to the storage tank. Thus, the temperature of the tank starts to increase. During this time the valve M1 that controls the flow of the water that is to be transported to the dwelling is in turned-off position. b) Solar collector - Heat exchanger - Storage tank - Dwelling Mode When the temperature of the fluid that comes from the collector is high enough to activate the heat exchanger, i.e. it has higher temperature than the storage tank loop of the heat exchanger, the temperature of the storage tank starts to increase. In this case the valve M1 is in turned-on position and A1, A2, D1, D2 three-way valves are in 1 position. Thus, the energy need of the dwelling is directly supplied by the water that comes from the storage tank. c) Storage tank - Dwelling Mode When the temperature of the fluid that comes from the collector is not high enough to activate the heat exchanger, i.e. it does not have higher temperature than the storage tank loop of the heat exchanger, it is not possible to absorb heat from the solar collector. In this case the temperature of the water in the storage tank is high enough to supply the dwelling. The valve M1 is in turned-on position and A1, A2, D1, D2 three-way valves are in 1 position. d) Storage tank - Auxiliary heating - Dwelling Mode When the temperature of the fluid that comes from the collector is not high enough to activate the heat exchanger, i.e. it does not have higher temperature than the storage tank loop of the heat exchanger, it is not possible to absorb heat from the solar collector. In this case the temperature of the water in the storage tank is warm but not high enough to supply the dwelling. Therefore, the water that comes from the storage tank is transported to the dwelling after having been heated by the help of the auxiliary heating. The valve M1 is in turned-on position and A1, A2 three- way valves are in 0 position and B1, B2, D1.D2 three-way valves are in 1 position. e) Auxiliary heating - Dwelling Mode When the temperature of the fluid that comes from the collector is not high enough to activate the heat exchanger, i.e. it does not have higher temperature than the storage tank loop of the heat exchanger, it is not possible to absorb heat from the solar collector. In this case the temperature of the water in the storage tank is not high enough to supply the dwelling. Therefore, the water circulated in the system is heated only by the auxiliary XII heating and directed to the dwelling. The valve M1 is in tumed-off position and A1, A2, B1, B2, D1,D2 three-way valves are in 0 position. The fluid in the collector loop of the heat exchanger is a kind of glycol mixture. The Structure and the Mathematical Model of the System It has a heat capacity of 3350J/kgC. At the tank loop of the heat exchanger we use water as the transfer fluid. It has a heat capacity of 4190J/kgC. The important thing in heating the glycol mixture to sufficient temperature is the area of the collector. We mount the solar collectors all around the roof. It has a 100m2 surface area. The glycol mixture and water at both loops have the same rate of 5kg/s. The efficiency of the exchanger is assumed to be 0.7. To calculate the net energy, which is transferred from glycol to water, the heat exchanger's coefficient must be known at first. The heat exchanger coefficient is given by Equation 1.1. Fr=Frx 14 (AcxFRxU,) Kp)c X P'min 1-1 )_ (1.1) Absorbed solar radiation is given by Equation 1.2. Qu=AcxFR'x[S-U,x(T8-Ta)r (1.2) Formula (1.2) represents the net energy that the collector can transfer to the water. In this formula heat exchanger's effect is included FR' coefficient. Net absorbtion S is given by Equation 1.3 S=Ixaxyx$ (1.3) XIII a, y and 8 are the (»efficients to calculate the net absorbtion by the solar radiation. The most important point during the system design is the insulation thickness. Insulation of the water tank is very important to keep it warm during autumn and winter days. Also house insulation system obviously influences the heat loss of the house system. To minimize the heat loss, effective insulation is a vital part of the solar heated houses. As mentioned previously, the system a huge storage tank which is located in the basement of the house. Cross section of the water tank is shown above which contains three layers; outside layer is concrete, middle layer is stone wool and the water container is a stainless steel tank which is the third layer. The heat loss of the storage tank is given by Equation 1.4. Q,=u.xa.x(t,-iî) (1.4) Insulation of the house is another important thing to keep the dwelling warm. There are four different parts of the dwelling to be insulated; the walls, the windows, the roof and the basement. All the walls of the house are made of one layer of Ytong brick, one layer of Foamboard 1500, one layer of Dupan wall panel. All the windows are made of Isıcam glass. The roof of the house is made of one layer of Mertek roof cover and one layer of oak-wood roof material. The other place, which is insulated, is the basement of the underground floor. It is made of one layer of stonewool. The total heat loss of the dwelling is given by Equation 1.5. Qt=12.1x|R1x(20-Ta)+R2x(20-Tj] f1-6) The value Qt is the heat value that is needed to keep the dwelling at 20°C. The heat loss of the dwelling is subject to the seasonal ambient temperature (Ta) and the ground temperature (Tg) as shown by Equation 1.5. For the areas that are in contact with the ambient air, the value R1 is the sum of the XIV multiplication of the total wall and the windows areas by the total heat loss coefficients. For the areas that are in contact with the ground, the value R2 is the sum of the multiplication of the total wall and the windows areas by the total heat loss coefficients. The temperature of the water that comes from the dwelling and returns to the storage tank is shown by T|. In order to calculate the value of Tt, we must work out the heat amount that is supplied to the dwelling. The total heat loss of the dwelling is given by Equation 1.6. Ls=(mC)x(Ts-T,) (1.6) If we combine all the above given equations, we can calculate the hourly temperature changes in the storage tank as in Equation 1.7: r=T + ( 1 \ p/sy 'A0xFR'x[S^x(T.-T.)r- UsxAsx[Ts-Ta']-(mCp)x(Ts-T,) (1.7) Also we can easily find out whether the system needs an extra auxiliary- heating source or not, as in Equation 1.8. a=a-L (1.8) Conclusion The result of the calculation proves that it is reasonable to heat a house in Gebze with such a system under these conditions. Also a medium unit of an auxiliary source can be used in the system to make it work better in winter months. We can observe that the storage tank gets hot in the summer time. The temperature reaches its highest value in August. The storage tank keeps its warm condition until the end of January. During February and March, the temperature of the storage tank has approximately the same level of the dwelling that is required to be kept at 20°C. xv The maximum solar radiation for summer period is nearly 2.5 MJ/rrfh. We can only reach this value in June and July. And the monthly maximum average ambient temperature is nearly 23°C. Also it effects the solar gain. We must heat the house from September to April. The heat need of the house reaches its peak point in December. However, the house has no need to be heated. The gain of the collector is in its peak in May. And during October, November and December it decreases to 0°C. However, during September and January these values are so low that we can neglect them. The reason for the decrease in the solar gain is due to the fact that the temperature of the storage tank is too high to be increased any more. The solar gain in March is higher than the one in August. The reason for that is the fact that the temperature of the storage tank decreases while supplying the dwelling during winter. Thus, we can obtain solar gain due to the radiation in March and we can increase the temperature of the storage tank. The critical time for the house is naturally the winter months. Especially December is the month in which the solar gain value is zero. On the other hand, in December, the energy need of the house is in its peak level. The critical question is whether the energy taken from the tank is enough to supply the dwelling or do we need any extra auxiliary heating source. We do not need any extra auxiliary heating source until January. From January to April we absolutely need an extra auxiliary heating source. The highest heat loss occurs in August because the temperature of the storage tank reaches its highest level in August. The reason for that is fact that the difference between the temperature of the storage tank and the temperature of the ambient air of the storage tank is quite high. Since the temperature of the storage tank is very low in February and in March, the heat loss is nearly zero. All the results indicate that we must use an auxiliary heating from January to April and this is shown in the ninth diagram. The month in which we need the highest amount of auxiliary heating is February. The results that have been obtained through the computer assisted simulations have showed that such a solar house which will be located in Gebze can be heated by a solar asisted heating system. When compared to the classical heating systems the solar asisted heating systems will certainly provide considerable energy savings. And these savings in energy will naturaly lead to savings in financial savings.