THEORETICAL STUDY ON SOLAR POWERED ABSORPTION COOLING SYSTEM

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1 THEORETICAL STUDY ON SOLAR POWERED ABSORPTION COOLING SYSTEM B. CACIULA, V. POPA 2, L. COSTIUC 3 CRIOMEC SA, GALAŢI, 2 DUNĂREA DE JOS UNIVERSITY GALAŢI, 3 TRANSILVANIA UNIVERSITY, BRAŞOV Abstract. This paper investiates the relative performance of a thermally activated environmentally friendly coolin system: a NH3-H2O sorption system. This system can by activated with relatively low heat source temperatures such as those achieved by solar collectors. The study explores the relative thermal performance, i.e. the performance coefficient and refrieration capacity of the system and a qualitative comparison based on the type of solar collector, i.e. compound parabolic collector. The eoraphical functionin location of the system was chosen for the city of Braşov, Romania for July. Firstly, the solar system collectors was investiated: the efficiency of the solar collectors as a function of the ambient temperament, the solar radiation and heatin medium temperature, daily irradiance on a fixed plane, daily irradiance on a 2-axis trackin plane, averae daily temperature profile. The absorption system is analyzed. The mathematical model developed is based on the enery and mass conservation equations. The thermodynamic model of ammonia-water binary mixture was used in the calculations. The advantae of this model is iven by the hih evaluation accuracy of the state points, compared with the common method, with the enthalpy-concentration diaram, where the readin accuracy of the values is relative. The results show an improvement to the desin parameters currently used when calculatin these types of refrieratin installation. In addition, a minimum number of requirements for the refrieratin cycle have been identified in order for this to be able to work within the desined parameters and with acceptable values for the performance coefficients. The thermodynamic model of the ammonia-water binary mixture, once completed, enables different types of absorption thermodynamic cycles to be studied with the purpose of improvin their efficiency. Keywords: absorption, solar refrieration, sustainable coolin, environmentally friendly technoloy.. INTRODUCTION The manaement of fuel and enery use in commercial and industrial fields are based usually on electrical enery. As modern methods on enery savin and decreasin the CO2 emissions is the use of the enery from recoverable resources from the technoloical processes. Many requirements for enery use purposes such as: producin hot-water, heatin, air conditionin, refrieration etc., are based on electrical enery but one promisin way is the use of the solar enery combined with heatin and coolin plants. For commercial, industrial, technoloical refrieration or for comfort technoloical air conditionin the refrieration plants with vapor compression, which enerally use electrical enery, they can be replaced with absorption refrieratin systems. These systems use directly the recoverable enery resources savin hence savin electricity. The absorption refrieratin systems assume the eneretic potential of the recoverable enery resources to be bier than the enery needed for the coolin production and the simultaneously existence of a heatin source and coolin user. 2. RECOVERABLE RESOURCES WITH LOW THERMAL POTENTIAL The absorption refrieration system was desined, built and analyzed in terms of usin the hot water provided from the solar compound parabolic collector located in Braşov, Romania. The Brasov climatic data for solar irradiance and averae daily temperature for July are reported in Fiures -3 []. The maximum solar radiation is about 00 Wm 2, while the ambient temperature reaches its maximum of 25 C at 4:0 h. 3. THE SOLAR COLLECTOR The heat source consists of an enhanced compound parabolic concentrator (CPC) developed by Solarfocus-GmbH with a ross area of 2.42 m 2. An increase of 20% in efficiency is claimed by the manufacturer for these new CPCs compared to classical ones. The efficiency η of the CPC as a 30 TERMOTEHNICA /203

THEORETICAL STUDY ON SOLAR POWERED ABSORPTION COOLING SYSTEM function of the ambient temperature, the solar radiation and the heatin medium temperature is iven by manufacturer s data with the

2 THEORETICAL STUDY ON SOLAR POWERED ABSORPTION COOLING SYSTEM function of the ambient temperature, the solar radiation and the heatin medium temperature is iven by manufacturer s data with the relation [] : F 4.6 F () whith F ( T, T ) / I hw mean amb 2 where T hw,mean is the heat transfer fluid mean temperature: T HW = 0.5(T hw,out + T hw,in ); T amb the ambient temperature and I is the solar radiation. The enery balance for the solar collector is iven by the followin equation: I dt M C dt A c ( T T hw p, hw hw, in hw, out ) (2) Fi. 2. Daily irradiance on a 2-axis trackin plane where M, C, T are mass, heat capacity and temperature of the collector, and subscripts hw are related to the hot water circulated in collector, the mass flow, heat capacity, inlet and outlet temperature respectively. Fi. 3. Averae daily temperature profile Fi.. Daily irradiance on a fixed plane. 4. ABSORPTION REFRIGERATION SYSTEM A absorption refrieratin system (Fi. 4) is made by a vapor Generator, an Absorber, Heat exchaner (economizer), an Evaporator and solution circulation Pump. Condenser 3 2 Evaporator Expansion valve 4 Generator 0 9 Absorber Stron solution Weak solution 5 6 Heat exchaner P p Pump a Fi. 4. Schematic of absorption refrieration plant. TERMOTEHNICA /203 3

3 B. CACIULA, V. POPA, L. COSTIUC This confiuration makes the installation more voluminous in comparison with the simple compression refrieratin system. The main advantae of the absorption refrieratin system is the fact that the movin parts are involved just in Pump parts. The other absorbtion plant components are not containin such as movin parts. In fact, that means the maintenance of the installation is much more facile on lon term period. The vapor absorption processes in the Absorber and the vapor producin process in the Generator can take place at pressure values, bein dependent of the temperature. The pressures p 0 and p F can be calculated from thermodynamic properties at saturation values for the NH 3 -H 2 O mixture. Thus, problems like tihtenin, dimensionin, pump construction are simplified and the thermal potential of the heatin aent in the vapor enerator can be reduced makin possible the use of the recoverable enery resources with a lower thermal potential than the one used on absorption refrieratin systems. 5. THERMODYNAMIC MODELING OF THE ABSORPTION REFRIGERATION SYSTEM The absorption refrieration modelin is based on the enery and mass conservation equations. In order to analyze the absorption refrieration system, mass, component and enery balance must be performed for each system part like below. For evaporator: 3 4 ref (3) e ref h 4 h 3 (4) For the expansion valves: For the enerator: 2 m 3 m ref, 2 h3 9 0, 9 h0 h (5) h (6) () () m m m x x h h h (9) From the equations () and (), the stron solution and the weak solution mass flow rate can be obtained: x x (0) m x x x () From equation (), the circulation ratio can be obtained: For the absorber: f m x x x (2) (3) h h (4) a h5 Dividin by 4, it results that: h h f h (5) q a h5 where: q a represents the heat dissipated per unit mass; f the mass flow ratio. The first term of the riht side represents the phase chane and the second the coolin of the mixture. For the pump: P p For the condenser: (6) 5 m 6 5h6 h5 () () m 2 m ref h h 2 (9) c For stron solution-weak solution heat exchaner: 6 9 (20) h h6 h h9 (2) The performance coefficient of the system is: 6 e. (22) The enery efficiency, ex, is an important criteria for performance evaluation of the absorption refrieratin system. The exeretic efficiency is calculated as: Ex( e) e Tamb Tom Tom ex, (23) Ex( ) T T T Fm amb where: T om is the averae temperature of the solution in the evaporator: T om = 0.5(T om,in +T om,out ); T Fm the averae boilin temperature of the solution in the vapour enerator: T Fm = 0.5(T + T 6 ). The absorption refrieratin system calculation was made under the followin conditions: the minimal boilin temperature of the ammonia-water solution in the evaporator t 0min = 3 C ; Fm 32 TERMOTEHNICA /203

4 THEORETICAL STUDY ON SOLAR POWERED ABSORPTION COOLING SYSTEM the initial temperature of the absorber coolin water t wr = +30 C; the ammonia-water solution temperature at the absorber and condenser exit t 9 = t 3 = +30 C. the refrieration power of the system e 00 kw ; The followin variable parameters were also considered: the heatin aent temperature in the vapor enerator and the maximum boilin temperature of the ammonia-water solution t F = t 6 = C ; the boilin temperature of the ammoniawater solution in the evaporator t 0 = 3 +6 C ; of the evaporator temperature is increasin and is increasin when the boilin temperature increased in the temperature rane reater than 90 C for enerator. An interestin result is the fact that, for values of boilin temperature between 90 C and 05 C, this flux presents minimum values which are more pronounced for small values of the evaporator temperature, values below - C. ex 0,22 0,2 0, 0,6 0,4 T vap=-3 [ C] ex 0, 0,65 0,6 0,55 0,5 6. DIUSSIONS 0,2 0,45 A thermodynamic model of the H2O-NH3 [2],[3],[4],[6] binary mixture was used in the calculations. The advantae of usin this model is iven by the hih evaluation accuracy of the state points, compared with the common method, with the h- diaram [5],[], were the readin accuracy of the values is relative. The calculations were made for condenser temperature t cond = +30 C, takin into consideration that the heatin aent temperature of the vapor enerator is more important than the maximum work temperature in the refrieratin system. The performance evaluation of the resorption refrieratin system is presented for: the heat flow at the boiler, Q GV ; the exeretic efficiency, ex ; the performance coefficient,. Qen [kj/time] T en [ C] Qen (t vap =+6 C) Qen (t vap =-3 C) Fi. 5. Generator heat flux vs. boilin temperature and evaporator temperature. Accordin to Fiure 5 the heat flux from the vapor enerator, Φ, is decreasin when the value ex 0, 0, Ten [ C] Fi. 6. The exeretic efficiency, ex and the performance coefficient, in function of boilin temperature in enerator and at evaporator temperature t vap =-3 C. 0,2 0, 0,6 0,4 0,2 0, T vap = +6[ C] 0,3 0,2 0, 0,69 0,6 0,6 0,66 0,0 0, Ten [ C] ex Fi.. The exeretic efficiency, ex and the performance coefficient, in function of boilin temperature in enerator and at evaporator temperature t vap = +6 C. Fiures 6 and show the influence of the evaporator temperature and of the boilin temperature (correspondin to the enery level of the source of the recoverin heat) upon the exeretic efficiency of the friorific cycle. It can be noticed that when the boilin temperature t en rises from 0 C to 50 C, the exeretic efficiency decreases for the evaporator temperature t vap = +6 C, in despite of the variation for the evaporator temperature t vap = 3 C where the exeretic efficiency have a maximum at t en = C. The performance coefficient,, dependin on the boilin temperature presents a maximum around the value of 90 C (Fi. 6) for the evaporator temperature t vap =-3 C, after which it uniformly 0, TERMOTEHNICA /203 33

5 B. CACIULA, V. POPA, L. COSTIUC decreases. The behavior is the same for the evaporator temperature t vap = +6 C, whith a maximum value, but at lower boilin temperature which is about t en =4 C.. CONCLUSION This paper has presented an adaptation of the absorption refrieratin installation followin the thermodynamic model of the absorption refrieratin cycle and that of the H 2 O-NH 3 binary mixture with a solar system with CPC for the purpose of improvin the overall performance. The results show an improvement to the desin parameters currently used when calculatin these types of refrieratin installation. In addition, a minimum number of requirements for the refrieratin cycle have been identified in order for this to be able to work within the desined parameters and with acceptable values for the performance coefficients. REFERENCES [] Clausse, M., Alam, K.C.A., Meunier, F.: Residential Air Conditionin and Heatin by Means of Enhanced Solar Collectors Coupled To an Adsorption System. In: Solar Enery 2 (200): [2] Franks, R.G.E.: Modelarea si simularea în inineria chimică (Modelin and Simulation in Chemical Enineerin). Bucureşti. Editura Tehnică, 99. [3] Iosifescu, C., Gheorhiu, C., Badea, C.: Equations for the Properties of the NH 3 -H 2 O Solution. In: Analele Universităţii din Galaţi, Fascicula IV, 2/94:. 5-. [4] Miron, V, Costiuc, L., Costiuc, I.: Valorificarea resurselor eneretice recuperabile de potenţial termic scăzut în scopuri eneretice (The use of the recoverable low potential resources as useful enery). In: Analele Universităţii din Galaţi, Fascicula IV, : [5] Reid, R.C., Prausnitz, J.M., Sherwood, Th.K.: Properties of Gases and Liquids. 3rd Edition. N.Y. Mc Graw Hill, 9. [6] Schulz, S.: Equations of State for System Ammonia-Water for Use with Computers. In: Proc. XIIth Int. Con. Refri., Washinton, 9, Vol.2:. 43. [] Vilcu, R.: Termodinamica chimică (Chemical Thermodynamics). Bucureşti. Editura Tehnică, 994. [] Accessed: TERMOTEHNICA /203

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