American Journal of Applied Sciences 2 (5): 914-919, 2005 ISSN 1546-9239 © Science Publications, 2005 Thermodynamic Analysis of an Absorption/Compression Refrigeration System Using Geothermal Energy L. Kairouani and E. Nehdi Unité de Recherche Energétique et Environnement Ecole Nationale d’Ingénieurs de Tunis, BP 37, Le Belvédère 1002 Tunis Abstract: This article presents the potential use and exploration of geothermal energy for cooling applications using a combined absorption/compression system. The considered system uses R134a for the compression part and the cool water-ammonia for the absorption part of the installation. The geothermal temperature source is in the range 343-349K, the condensation temperature is 308 K, and in order to produce ice, the R134a evaporation temperature is 263 K. The COP is about 5.4. Therefore, based on the typical geothermal energy sources in Tunisia which present a refrigeration potential power of 9.1 MW, the quantity of ice that could be produced is about 82 tons per hour. The greenhouse gas emissions should thus be reduced by about 5884 tons of CO 2 per year, which represents (59%). Key words: Absorption, Ammonia, Geothermal Energy, Refrigeration, R134a INTRODUCTION Geothermal wells which show the most favorable thermodynamic characteristics, such as the highest Geothermal energy accounts for 2% of the world temperatures and flows, are localized in the area of power consumption [1]. It is used in multiple Chotts: Kebili, Tozeur and El Hamma (south of applications and more than 60 countries exploit this Tunisia). energy [2-3] , 12% of this energy is used as a source for The thermodynamic characteristics and the generators of the heat pumps [3]. physicochemical analysis of the water are indicated in Considering the importance of this renewable Table 1 and 2. energy several research tasks are carried out in the world, for relevant exploitation. This exploitation Description of the Combined Cycle: Hybrid depends closely on the thermodynamic and absorption/compression installations using geothermal physicochemical data of water to emergence (dry energy have been studied by several authors [15-18] , and the Pressure, Temperature, flow, residues...). most used couples are H O/NH and LiBr/H O[12,19, 20] . Among these applications, we find the heat pumps 2 3 2 with absorption [4-10] , air-conditioning with absorption [11, In order to produce cold at negative temperatures, 12] and other passive applications. the studied cycle is formed by the association of In Tunisia, up to the year 2000, hydraulics and ammonia-water absorption loop, supplied with wind energies represented only 2% of the national geothermal water, and of vapor compression loop, power consumption, even though Tunisia has a using R134a like refrigerant (Fig. 1). substantial geothermal potential. These sources are For the absorption loop, the generator temperature mainly exploited in sericulture, in the heat of the TG is fixed by the geothermal temperature source TF [13] buildings and in aquiculture and in order to irrigate (Tg<TF). the neighboring areas with drinkable and industrial The temperature of the considered wells is in the irrigation water, the geothermal water of Chott Fdjej range 342-348 K. By taking account of the variation (south of Tunisia), which temperature exceeds 343 K, is ∆TFg between the fluid of heating (geothermal water) [14] cooled in cooling towers . and the weak solution in the generator, the generator temperature is fixed to 335 K. Tunisian Geothermal Resources: Much investigating By taking account of the climatic conditions of the work was carried out in the south of Tunisia, in order to area of the Chott, the condensation and absorbable determine the physicochemical characteristics of temperatures T and T are taken equal to 308 K. drilling water in this area. c a To locate them, the Directorate-General of the According to the Oldham diagram, if the triplet ( Tg, Water Resources (DGRE) tried to divide Tunisian Tc= Ta), and the mass concentration difference between territory into maps, where each one contains several the strong and weak solutions, ∆xrp , are fixed, the [21] geothermal wells. evaporation temperature Tev is also fixed . 914 American J. Applied Sci., 2 (5): 914-919, 2005 For the absorption loop and the indicated Evaporator: The refrigerating power obtained on the conditions, the mass concentrations are given from the evaporator of the absorption installation and which will [22] Merkel diagram . xr = 0.66 and xp = 0.56. be used to cool the condenser of the associated In order to produce ice we associate a compression installation is given by: vapor cycle using R134a like refrigerant, functioning ɺ = − between 2.01 bars (Tf =263 K) and 6.08 bars (Tcc =295 Q mɺ (h h ) (5) ev NH 3 5 4' K), (Fig. 1). The thermodynamic data relating to the combined By neglecting the pump work, the COP ev of the installation are indicated in Table 3. absorption loop is defined by: For the absorption system, the energy balances are established on the basis of the following assumptions: Qɺ = ev (6) COP ev Qɺ * No imperfections in both generating and absorber. g * The generator and evaporator pinch temperature is fixed at ∆T = 5 K The obtained potential could be exploited in air- pin conditioning, but considering the socioeconomic * At the generator exit, the ammonia vapor is pure. conditions of these areas, it is desirable to produce ice, that will be used in the storage of the agricultural Energy and Mass Combined Installation Balances produce and the fishery products. Generator: The energy and mass balances are It is thus convenient to associate the absorption established to evaluate the generator energy installation with the vapor compression installation consumption and the solution mass which provides one using R134a (Fig. 1). kg of pure refrigerant (Fig. 1). It is to be noticed according to Table 3, that the R134a Vapor Compression Installation: The vapor ammonia mass vapor concentration is approximately compression circuit (Fig. 1), functions between the 0.997, which educes the needs for a rectification of this evaporation temperature Tf = 263 K and the vapor. condensation temperature Tcc = 295 K. These data make The specific strong solution circulation is given it possible to evaluate the various enthalpies, by using [22] from the mass balance and it represents the mass of the Mollier diagram . solution per kg of vapor refrigerant leaving the The energy balances are established on the basis of generator: the following assumptions: − * At the entry of the compressor, the vapor is x '' xp saturated. f = (1) x − x * At the entry of the adiabatic throttling valve the r p liquid is also saturated. * The isentropic and electric efficiency of the The calorific generator power is given by the compressor are given by [23] : following expressions: ηis =0.874- 0.0135 τ (8) Qɺ = mɺ (h + ( f − )1 h − f h ) (2) g NH 3 2 3 ''1 ηe=0.9 (7) ɺ − + ∆ Q = mɺ FCPF (TF (Tg Tpin )) (3) Produced refrigerating power: g Qɺ = mɺ (h − h ) (9) The ammonia mass flow rate is deduced from the f R134a 6 '8 expressions (2) and (3): Power released on the condenser: mɺ C (T − (T + ∆T )) ɺ = mɺ (h − h ) (10) = F PF F g pin (4) Qcc R134a 7 8 mɺ NH 3 + − − (h2 ( f )1 h3 f h ''1 ) Mechanical power consumed by the compressor: Expression (4) will be used for the determination h − h Wɺ = mɺ '7 6 (11) of the realizable refrigerating potential by each R134a η geothermal well according to its mass flow rate ɺ and is mF its temperature TF. Electrical power consumed by the compressor: 915 American J. Applied Sci., 2 (5): 914-919, 2005 Fig. 1: Schematic View of the Combined System (A: Absorber, C: Condenser, CO: Compressor, EC: Evaporator/Condenser, EV: Evaporator, Ex: Exchanger, G: Generator, P: Pump, TV: Throttling Valve) − ɺ (h7' h 6 ) Table 1: Thermodynamic Characteristics of the We = mɺ (12) R134a η η is e Considered Sources (Source DGRE) Label Drilling Location T Mass flow rate -1 The coefficient of performance of the vapor (K) (kg s ) 1 El Mahassen Hamma Djerid 348 100 compression loop is noted, COP R134a : 2 Degache CI2 Hamma Djerid 346 22 ɺ 3 Chott Fdjej 2 El Hamma 343 70 COP = Qf (13) R134a ɺ 4 Debbacha Menchia 344 100 We 5 Seftimi NC17 Kebili 345.6 70 6 Zaouiet Echourfa Menchia 343.2 90 Combined Installation: In the adiabatic 7 Zaouia NCI5 Menchia 343.9 70 evaporator/condenser, the expressions (5) and (10) are 8 Menchia NCI6 Menchia 344 70 equal: 9 Limaguess NCI8 Kebili 345.5 70 916 American J. Applied Sci., 2 (5): 914-919, 2005 Table 2: Physicochemical Analysis of the Water (Source DGRE) 2+ 2+ + + 2- - No. Ca Mg Na K SO 4 Cl HCO 3¯ PH Dry Kg m ¯3 Kg m ¯3 Kg m ¯3 Kg m ¯3 Kg m ¯3 Kg m ¯3 Kg m ¯3 residual Kg m ¯3 1 0.332 0.038 0.278 0.073 0.988 0.362 0.183 7.15 2.220 2 0.308 0.048 0.292 0.072 0969 0.340 - - 2.020 3 0.280 0.120 0.328 0.034 0.931 0.674 0.117 7.47 2.700 4 0.228 0.145 0.265 0.028 0.816 0.568 0.123 7.47 2.160 5 0.249 0.098 0.303 0.039 0.595 0.674 0.111 - 2.700 6 0.232 0.097 0.310 0.040 0.662 0.674 0.114 7.5 2.360 7 0.252 0.108 0.317 0.039 0.768 0.710 0.117 7.85 2.340 8 0.248 0.087 0.312 0.037 0.739 0.674 0.090 7.6 2.444 9 0.280 0.091 0.340 0.036 0.806 0.639 0.128 - 2.320 Table 3: Thermodynamic Data and Numerical Values 1 1 Tg =335 K ∆Tpin =5 K h3=50 kJ kg ¯ h7=418.4 kJkg ¯ 1 1 Tc= T a =308 K PC =12.