Thermodynamic Limits of the Use of Solar Energy for Cold Production

THERMODYNAMIC LIMITS OF THE USE OF SOLAR ENERGY FOR COLD PRODUCTION Lillo, I; Silva, M. Escuela Superior de Ingenieros de Sevilla Avenida de los Descubrimientos, s/n 41092. Seville (SPAIN) e-mail: isidorolil[email protected] ABSTRACT: The cold production with solar energy may be performed by absorption refrigeration cycles based on thermal solar energy collectors and using compression refrigeration cycles with photovoltaic solar energy systems. Which of the two procedures is thermodynamically more efficient? Which one would require the lowest collection surface to satisfy a given cooling demand? It has been determined, the minimum required surface ratio to meet the same cooling demand on the same site through the use of solar thermal or photovoltaic energy, depending on the yield of the respective facilities. It has been found that, taking into account the thermodynamic limits of compression refrigeration and absorption refrigeration cycles, the photovoltaic and solar thermal area ratio required only depends on the ratio between the hot source temperature of the refrigeration cycle, and the thermal fluid temperature of the solar thermal installation being independent of the cold source temperature relationship, besides the performance of each solar installations. We conclude that PV technology requires less collection surface than solar thermal technology of low temperature for the production of cold on a very wide range of values of photovoltaic and thermal installation efficiency. Keywords: Refrigeration, PV System, Energy Options. 1 INTRODUCTION The use of solar radiation, with or without concentration for cold production is mainly done by absorption cooling cycles (LiBr-Water, NH3-Water) with low temperature solar thermal energy and using compression refrigeration cycles with photovoltaic solar energy. [1,2,3]. In the case of solar thermal energy, there are different technologies that in turn can be applied to different absorption and cooling cycles [4]. From the standpoint of thermal solar energy, on one hand, there are the facilities that do not concentrate the solar radiation, by using flat plate or vacuum tube collectors and on the other hand, the technologies that concentrate the solar radiation with the use of fresnel lenses, parabolic trough, parabolic dishes, etc.. In the latter case, the temperature reached by the coolant TT is greater than in the case of no concentration of solar radiation and is directly related to the level of concentration of the solar radiation. Similarly, in the case of photovoltaic systems, different technologies of photovoltaic cells (crystalline silicon, amorphous, CIS, multifunction cells, gallium arsenide,...) may be applied to a compression refrigeration cycle, with or without concentrating the solar radiation. [5-11]. There are even applications that combine the use of solar thermal and photovoltaic solar energy for cold production or even the use of hybrid collectors. Cold production has three major widespread uses depending on the required cooling temperature Tcool. For climatization, cooling temperatures, Tcool, ranges in the vicinity of 291K-295K. For food preservation, Tcool, ranges in 273K-285K and for freezing temperatures varies in the vicinity of 255K. This process occurs over a range of ambient temperatures T0 and of condensation temperatures Tb, which ranges between 265K and 323K approximately. 2 OBJECTIVE When comparing the potential use of solar radiation for producing cold using solar photovoltaic technology and solar thermal technology means there are a number of advantages and drawbacks of one technology over the other. In a first approximation, the photovoltaic have today worse energy efficiency, , (varies between 5%- 25% approximately) that solar thermal installations,   (varies between 30-50% approximately). Furthermore, the current performance of the compression refrigeration cycle (EERc approximately varies from 2 to 5) is greater than the efficiency in absorption cycles (EERA varies between approximately 0.5 and 2), affecting the thermal level to their performance. In addition, the exergy efficiency of a PV system η (varies between 5% and 13% approximately) is usually higher than that of a solar thermal η whose value depends on the temperature levels. In the end, the aim of this work is to determine which of the two technologies (thermal and photovoltaic) would require less solar collection area to achieve a certain refrigeration effect, both for systems with and without concentration of the solar radiation according to three situations Case 1: Actual situation: In this case it is considered the current yields values of absorption and compression cycles and the current values of the yields of the photovoltaic and solar thermal technologies. Case 2: Potential from a thermodynamic cycle point of view: In this case, current yield values of the solar technologies are considered s and regarding the maximum thermodynamic efficiency of compression and absorption cycles. Case 3: Potential from the simultaneous thermodynamic cycle and solar technology point of view: In this case we consider the maximum thermodynamic 28th European Photovoltaic Solar Energy Conference and Exhibition 3863 efficiency of absorption and compression cycles and maximum performance possible for solar technologies. 3 HIPOTHESIS This paper answers this question under the following assumptions: a.- It is assumed that the compression cycle works only on solar energy and it is only considered the energy consumption of the compressor neglecting all other possible consumptions such as those associated with fans, lighting, cameras, etc. .. b.- It is assumed that the absorption cycle works only with solar thermal energy and it is only considered the energy consumption of the generator neglecting all other possible consumptions such as those associated with pumps between the absorber and the generator. It is recognized that the temperature of the heat input to the cycle is just the heat production temperature of the solar thermal installation TT. c.- It was considered that the photovoltaic and thermal installation operate without auxiliary power supply. d.- There has not been taken into account any environmental or economic aspect at the design of the facilities that in reality affect to the choice of installation. e.- It is indicated that both the energy collected by the PV installation and the solar thermal systems and the temperature of the solar collector vary significantly during the day. The aspects derivate from the thermal energy supply in quantity and quality at every moment for the solar installation are neglected. f.- The value of the condenser cooling temperature Tb is usually the ambient temperature value T0 for aircooled cycles or water temperature for water cooled cycles. In this paper we have considered a range of Tb of 265K-323K. The value of the temperature of heat input to the cycle TT has been considered in the range of 313-373 K, for systems without concentration of solar radiation (flat plate or vacuum tubes) and in the range of 373-1000 K for systems with concentrated of solar radiation. For this reason, we have considered a range of the ratio   between 0.75 and 0.95 for the case of solar thermal systems without concentrated solar radiation and a range of the ratio   between 0.4 to 0.75 for solar thermal systems with concentration. g.- For systems without concentration of solar radiation, both with photovoltaic and thermal, the global solar radiation H is considered over the plane of the module or collector of area A, whereas when there is concentration of the direct normal solar radiation, it is considered on the reflective surface Hd or collector aperture area Ap h.- The actual overall performance of a solar photovoltaic installation , is variable depending on the system configuration, concentration of solar radiation and cell technology. It has been considered a range of 5%-25%, while the current performance of a solar thermal installation  significantly depends on the temperature TT and the concentration level. For higher temperatures of about 373 K it is common to use, concentrator systems, depending on the energy efficiency of these facilities on the concentration and Technology (Fresnel, parabolic trough, parabolic dish, etc…). It has been estimated, a range between 30-70%, decreasing when TT temperature increases. Accordingly, for this study, we used a range of the relation   between 0.1 and 0.95. i.- From the energy point of view the energy efficiency of the photovoltaic and thermal installation cannot be greater than 1. At this upper limit must subtract the inevitable losses of the process of transformation. According to information provided by different authors, for systems without concentration it is obtained a range of  which depends on the solar cell technology and ranges between 30% and 68.2%. For systems with concentration, the maximum yield   depends on the level of concentration of the solar radiation and the technology, and whose current values range around 40% to 86.8% for maximum concentration (45900X) and infinite cell multifunction. Moreover, for systems without concentration of solar radiation by using sensors or flat thermal vacuum tubes, the maximum achievable yield , oscillates in maximum values at around 60-90%, decreasing the maximum value increasing the temperature TT. TT operating temperatures are in the range of 313K-400K. In the case of systems with concentration of solar radiation (Fresnel, parabolic trough parabolic disc, etc.) the maximum achievable yield   depends primarily on the concentration level of radiation, which at the same time is related to the temperature TT which increases when the concentration level increases and whose values are reflected in the following table: Therefore, for this study, we used a range of     between 0.2 y 0.9 for a ratio Tb/TT between 0.4 y 0.75. j.- Actual values of the compression performance EERC are between 3 and 7, while the actual performance values of the absorption refrigeration cycle EERA are around 0.5-0.8 for one-stage systems and for two-stage systems it ranges between 0.9 to 1.3. Consequently, it has been considered a ratio EERC/ EERA between 2 and 15. 4 THEORETICAL BACKGROUND 4.1 Compression refrigeration cycle with solar photovoltaic energy A schematic view of the solar electric-vapor compression refrigeration (SE-VCR) system is shown in Fig 1. As shown in this figure, the solar–mechanical system consists mainly of photovoltaic panels and an electrical cooling device which works according to the vapor compression cycle. The biggest advantage of using solar electric panels for cooling would be their simple construction and high overall efficiency when combined with a conventional vapor compression system because of its commonly higher coefficient of performance which is more than 1. 28th European Photovoltaic Solar Energy Conference and Exhibition 3864 Figur e vapor co m The p r b e perfor m batteries, a the PV s y installatio n W   Wher e A PV is the radiation o electrical e The e n cycle EE R EER  To e compressi maximum reversible Thermod y equations Q         Substi maximum EER   There f Q coolPV , t h cycle effi c installatio n surface, a c Q   4.2 Comp r energy. The g e a solar th e e 1: A schem m pression refri g r oduction of el e m ed directly ( w a ffecting in thi s y stem. The el e n in a period is :    HA   e  PV is the ov e photovoltaic c o ver the plane e nergy produc e n ergy efficienc y R C is defined as :      stablish the on refrigerati o performance process. Appl y y namics, accor d 1 and 2 respec t   W  Q      tuting equatio n performance o         f ore, for a gi h e relationship c iency and th e n , together w c cording to Eq s   EER    r ession refrige r e neral scheme o e rmal system is atic view of t g eration (SE-V C e ctricity from t h w ithout battery ) s case to the o v e ctrical energ y :  e rall efficiency ollection surfa c of the modul e e d in a certain p y of a compre s : thermodynami o n cycle an d EER  ,it y ing the First a n d ing to Schem e t ively.  n s 3 and 4 int o o f the cycle, EE R    󰇛   󰇜 ven cooling e between this, e efficiency o f ith the p hoto v s . [1] and [2] w o   HA  r ation cycle w o f operation o f represented in he solar elect r C R) system h e PV system c ) or by the use v erall efficienc y y supplied by t [1] of the PV syst e c e; H is the glo b e s and W PV is t eriod. s sion refrigerat i [2] c limit of t d to obtain t is assumed n d Second La w 1 there result t [3] [4] o equation 2, t R  is: [5 ] e ffect refriger a the refrigerat i f the photovol t v oltaic collect i o uld be: [6] w ith solar ther m a heat pump w Figure 2. r icc an e of y of the e m, bal the i on the the a w of the the ] a to r i on t aic i on m al w ith s o i n t h o v F i pu t h 1 0 r e e f c o 1 0 a c m s o m t h t h t h Figure 2: A o lar thermal sy s The thermal n a certain perio Q     H Where A T i s h e global radiat i v erall efficienc y Equations 8 i rst and Seco n u mp in a re h ermodynamic l Q  Q               The E E EER        Substit u 0 , it results the m EER   Therefore, f lationship be t f ficiency of the o llection photo v 0 , would be: Q  EE In the case w c cording to th e m aintaining the o lar thermal col Q  EE In the limi t m aximized ac c h ermodynamics h e maximum a c h e minimum ar e Q  EE schematic vie w tem. e nergy provide d , Q T , is: A  s the solar the r i on over the pl a y of the solar t h and 9 result fr n d Law of Th v ersible proc e l imit, accordin g  Q      0 E R A of the mac h   u ting equation m aximum cycl e         󰇛   󰇛  fo r a given c o t ween the c y solar thermal i n v oltaic surface , R    HA  w here the cycl e e second law o performance o l ecting area A  R    H t case where c ording to and the perfo r cording to dif f e aA  : R     e w of a heat p u e d by the solar i r mal collectin g a ne of collector h ermal system. r om the applic a h ermodynamics e ss that esta b g to Scheme 2. h ine is defined n s 8 and 9 int o e efficiency, E 󰇛     󰇜     󰇜 o oling effect Q y cle efficienc y i nstallation, alo n , according to  e efficiency is o f thermodyna m o f the solar t h   woul d H A  the cycle e f the second r mance of the P f erent authors, i  HA  u mp with a i nstallation, [7] area, H is s; T is the a tion of the to a heat b lishes the [8] [9] like: [10] o Equation E R  : [11] Q coolPV , the y and the n g with the Eqs. 7 and [12] maximized m ics, while h ermal, the d be [13] ff iciency is law of P V syste m , i t would be [14] 28th European Photovoltaic Solar Energy Conference and Exhibition 3865 4.3. Exergetic analysis The maximum exergetic efficiency of the transformation process up to a given amount of solar radiation H, per unit area, [12] in a cooling effect Qcool is indeed defined by equation 14, where one side is the exergy of solar radiation itself η and secondly the performance of the process of obtaining the cooling effect η, η ηη 󰇛 󰇜 󰇛 󰇜 [15] Resulting a value for T0 between 263-373K In addition, the exergy efficiency of the photovoltaic EXPV and solar thermal EXT installation in a given period would be: η   W  HApv󰇛1TT 󰇜 η  Q󰇛1TT 󰇜 HAt󰇛1TT 󰇜 The exergy efficiency of the compression refrigeration cycle EXC and the absorption refrigeration cycle EXA in a given period would be η  Q󰇛TT 1󰇜 W  η  Q󰇛TT 1󰇜 Q󰇛1TT 󰇜 5 COMPARATIVE ANALYSIS It is presented, a comparative analysis of what would be the required solar collecting area if using a PV front of a solar thermal system for the same refrigerator effect, in three scenarios: Case 1. - Actual situation Assuming the current yields of the cooling cycle both with compressionEER and with absorption EERand the current efficiency of photovoltaic  and thermal  technologies according to the set scenario. In this case it will be attached a photovoltaic collecting area A, and a solar thermal collecting area A without concentration and a photovoltaic collecting area A, and a solar thermal collecting area Awith concentration. Substituting in the equations it results: Q EERHA [16] Q EERHA [17] For the same cooling effect, Q Q, it results:      [18] Similarly to the case of concentrated solar radiation is an expression: A A EER EER    According to the ranges indicated above, the curve representing   1, is shown in figure 3: Figure 3: Evolution of the N/N as a function of the ratio EERC/EERA for an area ratio of 1. The figure 3 separates the regions where the area required for the same cooling effect is lower in solar thermal than in photovoltaic based on the EERC/EERA ratio. The lower zone of the curve corresponds to an area lower for photovoltaic than for thermal. For example, for absorption machines where EERA is about 0.7, and for compression machines with a EERC of 4, it results a EERC/EERA=5.71. For this value, we find that the limit value of the efficiency ratio   is 0.18 Today considering the current efficiency values of 10% in photovoltaic and a 40% in low thermal temperature, it would result in a ratio of 0.25, which would require lower photovoltaic area than thermal. Specifically, according to Eq. 19, for every m2 of photovoltaic surface, it would be needed 1.427 m2 of solar thermal collectors. However, for absorption machines where the EERA is around 1.3 and compression machines with a EERc of 4, it result a value of EERC/EERA=3.07, considering the same photovoltaic and thermal yields it results a ratio   0.7675 in with case it would be less thermal than photovoltaic surface required. Specifically, according to Eq. 19, for each m2 of thermal collection surface it would be required 1.3 m2 of photovoltaic surface. We conclude that with the current efficiencies of the cycles of the solar technologies, the photovoltaic installations would require less collecting surface than the low-temperature thermal installations (without concentration), while it would be required more area than in high temperature (with concentration) because using concentrators, the EERA is higher. Case 2.- Potential only from the point of view of the thermodynamic cycle. In this case the maximum yields are the ones of the compressionEERand absorption refrigeration cycles EERare assumed according to the Second Law of Thermodynamics and simultaneously assuming current yields both for the photovoltaic  and solar thermal technologies. In this case it will be attached a photovoltaic collecting area A, and a solar thermal collecting area A without concentration and a photovoltaic collecting area A, and a solar thermal collecting area Awith concentration. 28th European Photovoltaic Solar Energy Conference and Exhibition 3866 Substi Q   Q   For t h substituti n      Simil a an expres s      In bo t thermal s u      The e dependen t depend o n the conde n Figur e same cool thermal ba The fi required p hotovolt a ratio assu m cycles. For e x have a va l it is obtai n (equivale n Toda y 10% in temperatu r which w o area. Spe c p hotovolt a thermal c o By co n of 6.66 c tuting in the e q   EER     EER   h e same cooli n n g on equations  󰇧     󰇨     a rly to the case s ion:  󰇧     󰇨       t h cases, it is u rface if it is m e 󰇡1    󰇢 e xpression sh o t on the coolin g n the ratio bet w n ser Tb and the e 4: Regions w ing effect is lo w a sed on the Tb/ T fi gure 4 separ a for the sam e a ic than in so l m ing the maxi x ample, consid e l ue of 0.85 (th e n ed that the ef f n t to a quotient y considering t photovoltaic r e, it would re s o uld require l o c ifically, accor a ic surface, it w o llectors. ntrast, maintai n c onsidering th e q uations it resul t     HA     HA   n g effect, Q   5 and 11, it re s of concentrate d required less p e t: o ws that the a g temperature T w een the cooli n heat input te m w here the are a w er in photov o T T . a tes the region e cooling eff e l ar thermal ba s mum yields o f e ring that the c u e actual range i s f iciencies limit EE R  /EER t he current ef fi and a 40% s ult in a ratio o o we r p hotovol t ding to Eq.21 , w ould be neede d n ing a ratio E E e same photo v t s:   [19]  [20]   Q   s ults: [21] d solar radiatio n [22] p hotovoltaic t h [23] a rea ratio is n T cool , while it d o n g temperature p erature, T T . a required for t o ltaic than in s o s where the a r e ct is lower s ed on the Tb / f the refrigerat i u rrent    ratio c s between 0.75 value     is 0    of 6.66) fi ciency values in low ther m o f     0.25>0. t aic than ther m for every m 2 d 1.66 m 2 of s o ER  /EER   v oltaic yiel d ,   , n is h an n ot o es of the o lar a rea in / T T i on c an 5 -1) .15 of m al 15, m al of o lar   for r e w C a s o c o c y L a c u   c o A  a r c o c o s u E E T b t h r a c o d e h o w h s u v a quiting less s ould need an e f a se 3.- Potenti o lar technology In this ca s o mpression  E y cles EER   a w of Thermo u rrent maxim u   and sola r In this cas e o llecting area  A   without co n r ea A , and o ncentration. Substituting o ncentration of Q   E Q  E E For the sa m u bstituting on ER  , it res u     󰇧     For a range b /T T entre 0.4 y Similarly to h ere is an expre s     󰇧     Substituting a nges it resul t o ncentration of Figure 5: E e signated maxi m It is n o rizontal line h ere the p h o u rfaces than th e a cuum tubes c o s olar thermal f ficiency highe r a l of the ther m simultaneousl y e the maxi m E E R  and  are assumed d ynamics and u m yields bo t r thermal    e it will be A , and a so l n centration an d a solar therma l in the equati o the solar radiat i E E R     ER     m e cooling effe equations 5 a u lts:   󰇨     of       b et w y 0.75, it result s the case of co s sion:   󰇨       the designat e s the figure the solar radiat i E quation 27 s o m um efficienc y o ted as the o p     =1 c o o tovoltaic tec h e thermal techn o llectors, wher e surface, this r than 66.6% m odynamic cy c y m um yields a absorption r e according to t simultaneousl y t h for the p   technologies . attached a p l ar thermal col l d a photovoltai c l collecting are a o ns in the ca s t ion it results:  HA     HA  e ct, Q   a nd 11 the E w een 0.2 y 0. 9 s the attached g r o ncentrated sol a e d maximum 5 in the ca s t ion o lved by subs t y ranges. p erating points o rrespon d s to h nology requ i n ology. For fla t e there is no c o installation c le and the a re of the e frigeration t he Second y assuming hotovoltaic . hotovoltaic l ecting area c collecting a A  with s e of non-  [24] [25]  Q  , E R  y [26] 9 and for a r aph: a r radiation [27] efficiency s e of nont ituting the above the the region i res fewer thermal or ncentration 28th European Photovoltaic Solar Energy Conference and Exhibition 3867 of solar resulting p hotovolt a increased increases a 6 CON C We h a to meet t through t technolog y respective p erforma n At th e absorptio n ratio of t cycle, Tb thermal i temperatu r     Solve d Figur e graphic fo In the consideri n p hotovolt a smaller th a The p roductio n from the t e system i n radiation, w Even p hotovolt a p hotovolt a thermal i n than abou t This s u solar phot o thermal po In the collectors auxiliary t input flui d the efficie n radiation,     that it wou l a ic in ranges o f the ratio    a n a s well. C LUSIONS a ve determined t he same coo l t he use of s o y , depending facilities a n n ce. e area ratio f n cycles, it wa s t he hot spot t e and thermal i nstallation T T r e of the col d s p  󰇧     󰇨.     d for different s e 6: Equation r the different p case of a rel a n g Tb=300 K , a ic than solar t a n 399.9 K, 42 8 obtained res u n , the solar P V e chnological p o n which there when T T can n e with the curr e a ic and therm a a ic installation n stallations op e t 340 K. u ggests that it o voltaic power o wer in coolin g case of using s and vacuum t hermal energ y d temperature a n cy of the cycl e   is usually h l d be p refer a f     between n d the ratio     , the minimum a l ing demand o o lar thermal on the per f nd the corr e f or reversible s found that on l e mperature of fluid tempera t T , b eing ind e p ot. s ituations on fi g 28 solved in a p roposed scena r a tion     of , it would t hermal surfac e 8 .57 K and 60 0 u lts show t h V has a greater o int of view, t h is no conce n e ver be high. e nt values of a l systems, a may be requir e e rates at temp e should be enc o versus the low g production. s olar thermal i n tubes for c o y should be us e a t the cycle in e . h igher than 0. 7 a ble the use 0.01-0.25. W h the area AT A M a rea ratio requi r o n the same s and photovol t f ormance of t e sponding c y compression a l y depends on t the refrigerat i t ure of the s o e pendent to t [28] g ure 6: a three dimens i r ios. 0.25; 0.3; 0.5 a b e required l e as long as T T 0 K respective l h at, for cool i ability to ben e an a solar ther m n tration of s o efficiency of t lower surface e d when the s o e ratures T T lo w o uraged the us e temperature s o n stallations of f o oling product i e d to increase t order to incre a 75, of h en M AX r ed s ite t aic the y cle a nd the i on o lar the i on a nd ess T is l y. i ng e fit m al o lar the of o lar w er e of o lar f lat i on the a se 7 1. o p o f 2. L . s y 8 4 3. M i n m S o 4. s o a d ( 2 5. o f S o 6. L . te m E n 7. m E n 8. pe d o R e R E 9. B e r e a m 1 0 P h s o 5 6 1 1 E n s y P V h u 1 2 t h S c REFERENC E - Kim, D.S., I p tions – a stat e f Refrigeration 3 - Fong, K.F., . S., Compar a y stems for bui l 4 , 227–244. 20 1 - J.A. 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