Güneş enerjisinin depolanması

Abstract

] 1 1. ?-. ÖZET Dünyada enerji tüketimi, uygarlık düzeyinin yükselme siyle hızla artarken, insani iğin.en güncel ve başta gelen so runu; artan enerji gereksinmelerini karşılayabilecek yeni enerji kaynaklarının bulunmasıdır. Güneş enerjisi; teknolojisinin hızla gelişmesi, çok çeşitli uygulama alanları bulması ve günlük yaşantımıza konut ısıtılmasından elektrik üretimine kadar değişik alanlarda girmesiyle yeni enerji kaynakları arasında en ön sırayı al maktadır. Şu andaki en önemli sorun, sürekli ve yoğun bir enerji türü olmaması nedeniyle uzun süre depolanmayı gerek- tirmesidir. Bu nedenle maliyeti düşük ve verimi yüksek topla yıcı ve depolayıcı.teknolojinin geliştirilmesine gerek var dır. Günümüzde güneş enerjisinin toplanması ve depolanması bakımından geliştirilmiş sistemler arasında en uygun olanı, yüzeyine gelen güneş enerjisinin %20-30'unu depolayabilen gü neş havuzlarıdır. Literatürden ve sunulan bu çalışmadaki de neylerden, içindeki akışkan yoğunluğunun derinlikte arttığı derişiklik gracjyenli bir güneş havuzu uygun olarak tasarımm- lanırsa, yüzeyden dibe doğru üst taşımmlı, ara taşınımsız ve alt taşımmlı olmak üzere üç karakteristik bölgeden oluşabi leceği görülmüştür. Bu tip havuzların veriminin arttırılabilmesi, güneş havuzunun dibinde soğurulan güneş enerjisinin taşınım yoluyla- iv - y-izeyden kaybına engel olan ve yalıtıcı gibi vazife gören ara taşınmışız bölge içinde molekülsel düşey harekete mani olacak derişiklik gradyamnın oluşturulması ile mümkündür. Literatür araştırmalarımız sonucu; güneş havuzlarının temel maddesi olan su içinde güneş enerjisinin soğurulması ve havuzlardaki kararlılık problemleri hakkında çok az sayıda çalışmaya rastlanmış ise de özellikle güneş havuzlarının ta şınmışız ara bölgesinde geçerli olan kararlılık koşulları kriteri eriyle ilgili bir çalışmaya rastl anı lamamı ştır. Sunulan çalışmanın teorik bölümünde ilk olarak, güneş enerjisi spektrumunun su içindeki dağılımına ait eksponansi- yel bir bağıntı önerilmiştir. Bu bağıntının, deneysel bulgu lar ile iyi bir uyum içinde olduğu görül müştür{34}. Teorik bölümün ikinci kısmında ise güneş enerjisinin tuzlu su içinde soğurulması incelenmiş ve bir boyutlu güneş havuzunun matema tik modeli çıkarılmıştır. Elde edilen genel enerji denklemi, probleme ait başlangıç ve sınır koşulları için, sonlu farklar metodu ile bilgisayar yardımıyla çözülmüştür. Teorik bölümde üçüncü olarak, kararlı halde taşınım! ı iki bölge arasında ısı taşınımına engel olan ara taşınmışız bölgenin kararlılık ko şulları Galerkin yöntemi kullanılarak elde edilmiştir. Taşı nmışız ara bölgede güneş ışınımının; soğurulmadığı durumda kararlılık koşulu için: RT'< [Pr/(Pr-l.î]Rc soğurulduğu durumda kararlılık koşulu için,- v- - RT(1+A ) < [Pr/(Pr+l[[Rr ı r ^* bağıntılarının geçerli olduğu bulunmuştur. Çalışmanın deneysel bölümü, Potasyum Nitrat (KNC^» Pr=3, Le=100) tuzunun çeşitli derişiklikteki çözeltileri ile doldurulmuş laboratuvar havuzunun, devamlı bir ışınıma ve pe riyodik olarak ışınım ve karanlığa maruz bırakılması şeklinde gerçekleştirilmiştir. Başlangıçta havuz yüzeyi ile dibi ara- 3 sında tesis edilen 10 ve 60 kg/m lük derişiklik farkları, zaman ve derinlikle artan sıcaklığın ara taşınımsız bölgede başlattığı taşınım hareketini engelliyememiş ve kararlılık 3 bozulmuştur. Buna karşılık, 100, 150 ve 200 kg/m lük başlan gıç derişiklik farkları için yapılan deneylerde ise ara taşı nımsız bölgenin kararlılığı bozulmamış ve alt taşınımlı böl gede enerji depolanmıştır. Bu sonuçlar, deneylerden elde edi len verilerin kararlılık koşullarında kullanılmasıyla da elde edilmiştir. Taşınımsız ara bölgenin kararlılığının bozulmadı - ğı derişiklik farklarında (en az 100 kg/m ), derişiklik Ray- leigh sayısı R 'nin mertebesi yaklaşık olarak 10, ısıl Ray- o leigh sayısı Ry'nin mertebesi ise 10 olarak, bulunmuştur. Sunulan çalışmaya en yakın şekliyle deneysel olarak yapılan konvektif stabilite çalışmalarından elde edilen bul gular, bu çalışmadaki deneysel ve teorik sonuçlarla karşılaş tırıldığında, uyum içinde oldukları görülmüştür.- vı -. Bunlara ilaveten, yapay havuzların tasarımında, yüzey le dip arasında tesis edilerek, ara taşınmışız bölgenin ka rarlılık koşullarını sağlayacak başlangıç derişiklik farkının yaklaşık olarak tespit edilebilmesi için gerekli diyagramlar hazırlanmıştır.

- VI ı - SUMMARY The attractiveness of solar energy as a renewable source of energy available world Wide is self-evident in these times of world energy shortage. But the harnessing of solar energy on a large scale is confronted with two intrinsic difficulties arising from two fundemental characteristics of solar radiation: Low energy-density and irregularity. Low energy-density means that collecting solar energy in commercial quantities would require a collecting apparatus of very large dimensions. Such large collector systems in volve large investments both in money and materials and ex plain why even the simplest solar collectors are not viable in an era of cheaper fuels. Further problems arising from the large areas of collection include: a) Bringing the energy collected over a large area to a central point of use: Processes resulting in considerable losses of energy enroute besides their initial costs. b) Keeping such large areas clean. * The solar radiation reaching any point on earth's sur face exhibits a regular cylic character defined by sun-earth geometry plus superimposed irregularity caused by atmospheric conditions. In the vast majority of solar energy applications the time pattern of energy demand is not the same as the time pattern of insolation. Some form of energy storage or an auxiliary energy supply is needed for such instances when collected s^lar energy cannot meet demand. Alternatively excess collected solar energy must be `dumped` when it?vııı- exceeds demand. Thus, for example, harnessing solar energy for heating buildings in winter when the solar energy is at a minimum, would be greatly facilitated if a viable long term storage system that could exploit summer sun shine for winter use was available. There has thus been a very strong incentive to produce a solar collector system that would be large both in area and built-in energy storage capability. The non-convecting solar pond (The solar salt-gradient pond) is the product of an effort directed towards this end. Before describing a solar pond, we must review briefly what happens in an ordinary pond e.g a garden pond. Part of the sunlight incident on the pond is absorbed in the water, and part is absorbed on the bottom of the pond. The latter absorption leads to the heating of the water in the lower part of the pond. Being warmer, and hence of lesser density than the cooler water above it, the heated water begins to rise and sets up convection currents that eventually lead to the dissipation of the absorbed heat from the surface of the pond. A solar pond is designed to suppress this convection and retain the heat at the bottom of the pond. The solar pond is a solar collector and seasonal heat storage device whose structure is shown schematically in Figure 1.1. The solar salt gradient pond is a still body of water consisting of two convective layers and an insulating layer (non-convective zone) in between. The upper convective layer consists almost wholly of fresh water. The bottom con vective layer is a concentrated salt solution. It is covered fc^`- ıx - by the insulating layer which has a salt gradient increasing with depth. Since the hotter but saltier water at the bottom of, the gradient will be denser than the colder and less salty water above it, there will be no convection in the insulating layer when heat is absorbed on the bottom, if the salt gradient of the insulating layer is large enough. Also, as water is transparent to visible light but opeque to infrared radiation, the heat which reaches the darkened bottom in the form öf sunlight is absorbed there, and can escape only by conduction. Accordingly the pond is always insulated at the bottom to prevent heat loses there. Because the thermal con ductivity of water is moderately low and the insulating layer is thick enough, heat dissapation through the insulating layer is very slow. »This makes the solar pond not only a thermal collector but also a seasonal heat storage device. Storage capacity is increased by increasing the thickness of the hot convective bottom layer. The growth of this layer is related to the intensity of the incident solar radiation and the salt concentration difference between the surface and the bottom of the pond. '? Although, in the case of increased radiation inten sity, the growth of the bottom convective zone leads to an increase in the amount of energy to be stored, this may also lead to a decrease in the thickness of the insulating layer and hence the starting of a convection current. When convec tion starts in the insulating layer which becomes unstable, a continuous loss of heat from the system occurs. To prevent convection in the insulating layer the initial salt concent ration difference between the surface and the bottom of the- x - ? pond must be calculated beforehand for the verification of stability criteria of the insulating layer. Research done on available literature on solar ponds has revealed that: a) The amount of research conducted on the absorption of solar energy in concentrated salt water solutions is scarce, b) Almost no stability criteria exists on the insulat ing layer ( Non-con vective zone) of solar ponds. In this study the derivation and experimental proof of the stability criteria of the insulating layer has been aimed at rather than the analytical solution of temperature and concentration distributions in solar ponds. However, time- dependent temperature and concentration distributions at varying salt concentrations have also been experimentally investigated as shown in Figures 3.7 - 3.21. In the theoretical part of the study the absorption of solar radiation in salt water has been examined and a one dimensional mathematical model of a solar pond has been worked out. Although some empirical formulas for absorption and distribution of solar radiation in salt water have been presented by Bryant and Colbeck, RabT and Nielsen and others { 4 } { 1 4 } { 2 9 } { 3 5 }, these analytical forms do not conform with Schmidt's data{34}, as can be seen in Figure 2.2. Therefore, in this study, a formula to conform with Schmidt's data, which is shown in Table 2.1, has been developed-by prior knowledge and proof that the use of an inorganic salt, when added to water, does not radically affect optical properties- XT of water: where: Y(xi)= 2 S_ exp(-K, x')+ z n exp(-K x') J t İl U O M i lit bm O n=l n m=l li! x 3* n K ı K u Vertical coordinate from the water surface. Percentage of the incident radiation flux at any depth x~. Fraction of long-wave portion of solar spectrum. Fraction of visible portion of solar spectrum. Extinction coefficient for long-wave solar radia tion absorbed by water. Extinction coefficient for short-wave solar radia tion absorbed by water. The values of $. Ku. nm and KSm are given at Tables -, n un m 3m 3 2.2 and 2.3. Additionally stability criteria of the insulat ing layer were obtained by the application of the Gal erkine Method-with only the transfer of heat by convection under consideration-to the cases of absorption and non-absorption in the pond. The equation, Pr R_(l + A ) £ J^-r R T r Pr+1 for the absorption case, and the equation RT, Pr R Pr+1 c for the non-absorption case were derived. The second equation was found to agree well with the study of Veronis{45}. In these two equations:- xn - RT Thermal Rayleigh number.: = Absorption effect of solar energy in water Pr = Prandtl number R = Salinity Rayleigh number In the experimental part of the study a 290x190x240 mm insulated laboratory pond which was subjected to a constant incident heat-lamp radiation was used (1. and 2 in Figure 3.1). An 290x190x2 mm copper-oxide plated aluminium plate was pla ced at the bottom of the pond for the purpose of providing a selective' surface. The pond was filled with one layer of pure water and four layers of potassium Nitrate (KNOo) solu tions (Pr=3) with concentrations ranging from o `to 10, 0 to 60, 0 to 100, 0 to 150 and 0 to 200 kg/m3. In all experiments the bottom layer, consisting of the most concentrated solu tion, was filled first and the top layer, being of pure water was filled last. Time-dependend temperature profiles were measured by two vertically movable copper-constantant thermocouples (5 in Figure 3.1). The incoming heat-lamp radiation was measured to 2 be 1200 W/'m at the surface of the pond through the use of an Epply Black solarimeter. The time-dependend concentration profiles were obtained through the use of a probe which con tained 13 pairs of gold plated sensors (Figure 3.5). The func tion of the probe was to measure the electrical conductivity of salt water at different depths. In the experiments conducted with 0. to 10 and 0 to 60 kg/m` concentrations, the stability of the system was observ ed to deteriorate. Also, the data obtained from these experi-- XI 11 ` men ts` when applied to the stability criteria of the systems, yielded identical results (Figure 4.1), However when the experiments were conducted with 0 to 100, 0 to 150 and 0 to 200 kg/m, concentrations the stability of the system (insu lating layer) was observed to be steady. Again, when the data obtained from these experiments were applied to the stability criteria of the system, identical results were obtained (Figure 4.1 ). An examination of Figure 4.1 will show that the ther mal Rayleigh numbers calculated from the data of experiments conducted with concentrations of 0 to 100, 0 to 150 and 0 to 3 200 kg/m are smaller than the salinity Rayleigh numbers 10 8 (Rr * 10, RT * 10 ) and that the consequent results fall within the stable region. On the otherhand the thermal Ray- leigh numbers calculated from the data of experiments conduct- I,8 3 ed with concentrations of 0 to 10 and 0 to 60 kg/m are greater than the salinity Rayleigh numbers (Rr - 10 10 RT ~ 10 ) and the consequent results fall within the unstable region. The convective stability problem in solar ponds was experimentally examined by Styris, Leshuk and others with methods showing a close similarity to the ones used in this study{3}{16}{22}{27}{29}{39}. As seen in Figure 4.1, the results achieved by the application of the data established in experiments conducted by the above named researchers to the stability criteria for the case of absorption of radia tion display a complete agreement with the results obtained in this study.-XIV ~ To recapitulate, for a salt gradient solar pond com posed of water and inorganic salts to store the solar energy it absorbs, an insulating layer (Nonconvecting zone) must be present in this pond, The presence of this insulating layer is dependent on the establishment of a minimum of 100 kg/m initial salt concentration difference between the surface and the bottom of the pond. In the design of the artifical solar ponds the initial concentration difference verifying the stabi lity criteria can be derived from Figures 3.26, 3.27, and 4.1.