Güneş havuzları ile enerji depolanması
Solars ponds
- Tez No: 46552
- Danışmanlar: Y.DOÇ.DR. KORHAN BİNARK
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1995
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 68
Özet
The values of B_, K.., n" and KSm are given at Tables 2.2. and 2.3 Additionally stability criteria of the insulat ing layer were obtained by the application of the Galerkine Method-with only the transfer of heat by convection under consideration-to the cases of absorption and non-absorption in the pond. The equation. R_ (1+ A ) = -££- R T v x' Pr+1 c (2) for the absorption case, and the equation Rrp - Pr Pr+1 R (3) for the non-absorption case were derived. The second equa tion was found to agree well with the study of Veronis. In these two equations: Rrp Ar P? R_ Thermal Rayleigh number Absorption effect of solar energy in water Prandtl number Salinity Rayleigh number The first three experiment was completely naturally made following ways - With pure water and pool space open the atmosphere - With pure water and pool space is closed to atmos- pheer by transparent gloss - Solar pond, of which space is closed to atmosphere by transparent glass is fulled varrows concentration of KN03 Other experiment was made putting NaCl bottom of the suppond varraus variation, During these experiments effects of wind, speed, shodow and salt dose to the solar pond are observed. It's recognized that the temperature of upper connective zone is bigger when solar pond space is closed by the glass. It's absorved that shadow part of solar pond become a bit for from the solar pond temperature profile xixi
Özet (Çeviri)
SOLAR PONDS SüMMARY The attaractiveness of solar energy as a renewable soûrce 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 difficulti.es arising form two fundemental characteristics of solar radiation: Low energy-density and irregularity. Low energy-density means that collec.ting solar energy in commercial quantities would require a collecting appa- ratus of very large dimensions. Such large collector system involve large investments both in money and raate- rials and explain why even the simplest solar collectors are not viable in an area of cheaper fuels. Further problems arising from the large areas of colluction include: a)Bringing the energy collected över a large area to a central point of use: Processes resulting in consi- derabel losses of energy enroute besides their intial costs. b)Keeping such large areas clean, The solar radiation reaching any point on earth's surface exhibits a reqular cylic character defined by sun- earth geometry plus superimposed irregularity caused by atmospheric conditions. in the vast mojority of solar energy applications the time patten of energy demand is not the the same as the time pattern of insolation. Some form of energy storage ör an auxiliary energy supply is needed for such instances when collected solar energy cannot meet demand. Alternatively excess collected solar energy must be =dumped= when it exceeds demand. Thus, for example, harnessing solar energy for heating buildings in winter when the solar energy is at a minimum, vould be greatly facilitated if a viable long term storage system that could exploit summer sun shine for vinter use was available. xThere has thus been a very strong incentive to produce a solar collector system that vould be large both in aVea. and built-in energy storage capability. The non- convecting solar pond (The solar salt-gradient pond) is the product of an effort directed tovards this end. Beföre dascribing a solar pond, we must reviev bri~â£ly what happens in an ordinary pond e.g.a garden pond. Part of the sunlight incident on the pond is absorbed in the vater, and part is absorbed on the bottora of the poud. The latter absorption leads to the heating of the vater in the lover part of the pond. Being varmer, and of lesser density than the coolar above it, the heated water begins to rise and sets up convection currents that eventually lead to the^dissipation of the absorbed heat frora 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 stoarge device vhose structure is shovn schematically in Figüre 1.1. The solar salt gradient pond is a stili body of vater consisting of two convective layers and an insulating layer (non-convective zone) in between. The upper convective layer consists almost wholly of fresh vater. The bottom convective layer is a concentrated salt solution. it is covered by the insulating layer vhich has a salt gradient increasing with depth. Since the hotter but saltier vater at the bottom of the gradient will be denser than the colder and less salty vater above it there vill be no convection in the insulating layer vhen heat is absorbed on the bottom, if the salt gradient of the insulating layer is large enough. Also, as vater is transparent to visible light but opeque to infrared radiation, the heat vhich reaches the darkened bottom in the form of sunlight is absorbed there, and can escape only by conduction. Accordingly the pond is alvays insulated at the bottom to prevent heat loses there. Beç,ause the thermal conductivity of vater is moderately low( and the insulating layer is thick enough, heat dissâpation through the insulating layer is very slov. 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 coavective bottom layer. The grovth of this layer is rela.ted to the intensity of the incident solar radiation and the salt concentration djfference betveen the surface and the bottom of the pond. xıAlthough, in the case of increased radiation inten sity the growth of the bottom convective zone leads to an increase in the alount 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 convection 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 concentration difference between the surface and the bottom of the pond must be calculated beforehand for the verification of stability criteria of the insula ting 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-convective zone) of solar ponds. 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, Rabl and Nielsen and others, these analytical forms do not conform with Schmidt's data. Therefore, in this study a formula to conform with Schmidt's data, which is shown in Table 2.1 has been developed-by prior konwledge and proof that use of an inorganic salt, when added to water does not radically affect optical properties of water: Y(X3>= 1=1 VxP(“Ku x3>+m£=lnmexP(-Ks”x3> n m where; x, : Vertical coordinate form the water surface Y : Percentage of the incident radiation flux at any depth x- B : Fraction of long-wave portion of solar spectrum. n : Fraction of visible portion of solar spectrum. K : Extinction coefficient for long-wave solar radia tion absorbed by water K : Extinction coefficient for short-wave solar radia tion absorbed by water. XllThe values of B_, K.., n" and KSm are given at Tables 2.2. and 2.3 Additionally stability criteria of the insulat ing layer were obtained by the application of the Galerkine Method-with only the transfer of heat by convection under consideration-to the cases of absorption and non-absorption in the pond. The equation. R_ (1+ A ) = -££- R T v x' Pr+1 c (2) for the absorption case, and the equation Rrp - Pr Pr+1 R (3) for the non-absorption case were derived. The second equa tion was found to agree well with the study of Veronis. In these two equations: Rrp Ar P? R_ Thermal Rayleigh number Absorption effect of solar energy in water Prandtl number Salinity Rayleigh number The first three experiment was completely naturally made following ways - With pure water and pool space open the atmosphere - With pure water and pool space is closed to atmos- pheer by transparent gloss - Solar pond, of which space is closed to atmosphere by transparent glass is fulled varrows concentration of KN03 Other experiment was made putting NaCl bottom of the suppond varraus variation, During these experiments effects of wind, speed, shodow and salt dose to the solar pond are observed. It's recognized that the temperature of upper connective zone is bigger when solar pond space is closed by the glass. It's absorved that shadow part of solar pond become a bit for from the solar pond temperature profile xixiTo recapitulate, för a salt gradient solar pond com- posed of water inorganic salt to store the solar energy it absorbs, insulating layer (Nonconvecting zone) must be present in this pond. The presence of this insulating layer is dependent on the establishment of minimum of 100 kg/m3 initial salt concentration difference between the surface and the bottom of the pond. For NaCl this value is depen on 216 kg//m3 concentra¬ tion difference which must be supplied at the beginning, between the surface and the bottom of the pond. According to dotained results from the experiments. If solar pond, which is used to store the solar energy is correctly planned, storing this energy for olang time can be possible.
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