Design and Simulation of an Absorption Diffusion Solar Refrigeration Unit

B. Chaouachi and S. Gabsi Research Unit: Environment, Catalysis and Analysis of Processes National School of Engineers of Gabès (E.N.I.G)- Omar Ibn ElKhattab Street -6029 Gabès- Tunisia.

Abstract: The purpose of this study was the design and the simulation of an absorption diffusion refrigerator using solar as source of energy, for domestic use. The design holds account about the climatic conditions and the unit cost due to technical constraints imposed by the technology of the various components of the installation such as the solar generator, the condenser, the absorber and the evaporator. Mass and energy conservation equations were developed for each component of the cycle and solved numerically. The obtained results showed, that the new designed mono pressure absorption cycle of ammonia was suitable well for the cold production by means of the solar energy and that with a simple plate collector we can reach a power, of the order of 900 watts sufficient for domestic use.

Key words: Solar energy, simulation, absorption refrigeration, mono pressure, design

INTRODUCTION * Separator Under operation the rich solution water-ammonia, If the solar energy possesses the advantage to be coming from the storage tank, passes in the tubes of the "clean", free and renewable, this last is probably, solar collector (1), where it will be heated by the solar considered like an adapted potential solution, that flux in this collector. A vaporization of ammonia is answers in even time at a economic preoccupation and produced in this tubes and the product vapor take the ecological problems. poor solution to the separator. Among the main done currently research is the use The generated ammonia vapor containing a small of this free source to make operate system of water vapor quantity will be purified in the rectifier. refrigeration. The condensate water vapor returns in the separator (6) The diffusion absorption refrigerator cycle whereas the ammonia vapor penetrates in the condenser invented in the 1920s is based on ammonia (refrigerant) (7) in which it is liquefied. After that the liquid and water (absorbent) as the working fluids together ammonia passes from the condenser to the evaporator with hydrogen as an auxiliary inert gas[1,2]. Since there (8) where it comes into contact with a hydrogen gas are no moving parts in the unit, the diffusion absorption flux (9) facilitating its evaporation by absorbing a refrigerator system is both quiet and reliable. The quantity of heat from the medium to be cooled. The system is, therefore, often used in hotel rooms and mixture gas of hydrogen-ammonia (11), which produced at the evaporator temperature, comes into offices[3]. indirect contact with the poor solution, coming from the The absorption diffusion refrigerating machine is separator, in the heat exchanger where it absorbs a designed according to the operation principle of the quantity of heat of the latter. At the exit exchanger (13), refrigerating machine mono pressure invented by [2-10] the cooled weak solution penetrates in the absorber PLATERN and MUNTER . This machine uses three where it absorbs ammonia of the ammonia-hydrogen operation fluids, water (absorbent), the ammonia gas mixture. This absorption is favored at low (refrigerant) and hydrogen as an inert gas used in order temperature and it will be almost total. Insoluble to maintain the total pressure constant. The simplest hydrogen in water goes up in top of the storage tank (9) configuration of the designed machine, represented on and returns in the evaporator whereas the rich solution Fig. 1, is composed of the principal following elements: water-ammonia enters in the solar collector tubes and a * An evaporator new cycle start again. * A condenser * An absorber Theoretical analyzes: The evaluation of the * A heat exchanger refrigerating machine performance criteria requires the * Solar collector knowledge of momentum, energy and mass balance. * Rectifier The liquid and vapor flow rates are determined by

Corresponding Author: B. Chaouachi, Research Unit: Environment, Catalysis and Analysis of Processes. National School of Engineers of Gabès (E.N.I.G)- Omar Ibn ElKhattab Street -6029-Gabès-Tunisia. Tel: 00216 75 392 380, Fax: 00216 75 392 190 85 Am. J. Applied Sci., 4 (2): 85-88, 2007 developing a bubble pump model based on the Condenser (Currents 7 and 8): developed model in the literature [4, 11, and 12]. General mass balance equation: • • 7 mm78= (11) Rectifier Condenser Energy balance 4 6 5 •• Sep arat o r 10 −+mm7 = (12) hh788 Q C 11 Exchange Evaporator Gen erat o r 3 2 Evaporator (Currents 8, 9 and 11):

13 9 General mass balance equations: L 12 • • • Absorber 14 Storage m10 = m8 + m9 (13) Tank H • • 1 m10 = m11 (14) Ammonia mass balance equation:

Fig. 1: Schematic diagram of the absorption diffusion- • • m8 = m11ψ (15) refrigerating machine 11,NH 3 Energy balance The coefficient of performance of this refrigerating • • (16) machine with absorption diffusion is defined as m9 (h11,H − h9 ) + m8 (h11,NH 3 − h8 ) = QE follows: Heat Exchanger (Currants 5, 11, 12 et 13): General mass balance equation: QE COP = (1) • • • • Q G m12 + m13 = m11 + m5 (17) Ammonia mass balance equation: Mass and energy balances: In this paragraph, we will • • have to establish the mass and energy balance equations m13 ξ13 = m5 ξ5 (18) for the various elements of the refrigerating cycle. The • • • hydrogen-ammonia mixture is supposed an ideal m ψ = m 12 ψ = m (19) mixture. 11 11,NH 3 12 ,NH 3 11,NH 3 Bubble pump (Currents 1, 2 and 3): Energy balance General mass balance equation: • • m11,H (h − h ) + m11,NH 3 (h − h ) = Q (20) • • • 12,H 11,H 12,NH 3 11,NH 3 12−11 • m3 + m2 = m1 (2) (21) Ammonia mass balance equation: m 5 ( h13 − h 5 ) = Q 5−13 • • • Absorber (Currents 12, 13 and 14): m3 ξ 3 + m2 ψ2 = m1 ξ1 (3) General mass balance equation: Energy balance •• • •• • mm12+= 13 m 14 (22) −+mh m h + m2 h = Q (4) 113 3 2 G Ammonia mass balance equation: Separator (Currents 2, 3, 4, 5 and 6): ••• m ψ + mmξ= ξ (23) General mass balance equation: 12 12 13 13 14 14 ••••• Water mass balance equation: • • • mm4236+=++ mmm (5) 5 m (1 − ξ ) − m = m (1 − ξ ) (24) Ammonia mass balance equation: 14 14 9 13 13 ••••• Energy balance mm5326+=++ mm m (6) ξψξψξ543264 Energy balance • • • • • m13 h13 + (m12 − m 9 )h12,NH 3 − (m14 − m9 )h14 (25) ••••• −−+mh mh mh + mh + mh = 0 (7) • 544326 5326 + m11,H (h12,H − h14,H ) = QA Rectifier (Currents 4, 6 and 7): Storage tank (Currents 1, 9 and 14): General mass balance equation: General mass balance equation: ••• • • • mm64+= m (8) 7 m9 + m1 = m14 (26) Ammonia mass balance equation: Ammonia mass balance equation: •• • • mm7 = 4 ψ (9) 4 m14 ξ14 = m 1 ξ 1 (27) Energy balance Energy balance ••• • • • 74 (10) −−+m764 m6 m =Q (28) hhhR (m14 − m 9 )( h14 − h1 ) + m 9 (h14 − h9 ) = 0 86 Am. J. Applied Sci., 4 (2): 85-88, 2007

RESULTS AND DISCUSSION 1,2 1,1 1 The operating boundaries of the system were 0,9 examined by conducting simulations for various values 0,8 0,7 of the generator temperature, TG, the evaporator

PU (KW) 0,6 temperature, TE, the pressure of the system, P and the 0,5 generator heat input, QG. The operation ranges were 0,4 found to be: 0,3 0,2 5

TE=263 K TE=268 K TE=273 K 0.17 TE=278 K TE=283 K 0.15 0.13 Fig. 5: QE vs. to the evaporator temperature (ξr = 0.4, 0.11 P=12.5 bars, H/L=0.6) COP 0.09 0.07 Figure 2 presents the COP vs. the generator 0.05 330 340 350 360 370 380 390 400 410 temperature for different pressures of the system for a TG (K) fixed rich concentration and evaporator temperature. It P=5 bars P=7,5 bars P=10 bars shows that the COP decreases as the generator

P=12,5 bars P=15 bars temperature increases and it increases when the pressure increases too. This is may be explained by the Fig. 2: COP vs. to generator temperature for various fact that a smaller amount of ammonia was separated pressures of the system (ξr=0.45, Te=273K) from the ammonia-water solution and thus more solution had to be circulated so as to maintain the refrigerant flow rate in the condenser. It thus 0.15 recommended that pressure of the system as high as 0.14 possible. 0.13 The dependence of COP on the generator input at 0.12 0.11 fixed rich concentration, evaporator temperature and 0.1 pressure of the system shows that the COP decreases COP 0.09 when the generator heat input increases (Fig. 3). This is 0.08 may be explained by the same manner have Fig. 2. 0.07 The Fig. 4 shows that the COP decreases as the 0.06 0.05 generator temperature increases. It was also found that 0246810 the higher the evaporator temperature, the higher COP, QG (kW ) i. e. that more heat was absorbed in the evaporator. There are thus opposing demands for the evaporator Fig. 3: OP vs. to generator power (ξr = 0.45, temperature; on the one hand, it should be high enough Te=273K, P=12.5 bars) (depending on the desired cooling capacity) to yield a higher COP. An investigation of dependence of QE on the 0,15 generator temperature for different evaporator

0,145 temperatures at fixed pressure and rich concentration

0,14 showed that QE increased as the generator temperature. This is can be explained by the fact that more quantity 0,135 COP of ammonia was generated when the generator 0,13 temperature increases. 0,125 0,12 CONCLUSION 360 365 370 375 380 385 390 395 TG (K) TE=263 K TE=268 K TE=273 K A design and simulation of an absorption diffusion TE=278 K TE=283 K refrigerator using solar as source of energy, for domestic use was done and the system performances Fig. 4: OP vs. the evaporator temperature (ξr = 0.4, were analyzed parametrically by computer simulation. P=12.5 bars, H/L=0.6)

87 Am. J. Applied Sci., 4 (2): 85-88, 2007

It appears that best performance in terms of COP would 4. Koyfman, A., M. Jelinek, A. Levy and I. Borde, be obtained when we work with low generator 2003. An experimental investigation of bubble temperature and high pressure. In the other hand, the pump performance for diffusion absorption values of COP remain weak and depend of the power refrigeration system with organic working fluids. solar babble pump. Appl. Therm. Engg., 23: 1881–1894. 5. Mejbri, K.H., N. Ben Ezzine, Y.Guizani and A. Nomenclature Bellagi, 2005. Discussion of the feasibility of the COP: coefficient of performance Einstein refrigeration cycle. Intl. J. Refrigeration, Q: heat transfer power (J/s) 29: 60-70. h: enthalpy (kJ/kg) 6. Florides, J.A., S.A. Kalogirou, S.A. Tassou and H/L: submergence L.C. Wrobel, 2003. Design and construction of . LiBr-water absorption machine. Energy m : mass flow (kg/s) Conversion and Management, 44: 2483-2508. P: pressure (bar) 7. Srikhirin, P., S. Aphormatana and S. T: temperature (K) Chungpaibulpatana, 2001. A review of absorption ξ: mass fraction of cooling agent in the solution refrigeration technologies. Renewable and ψ: mass fraction of cooling agent in a mixture gas Sustainable Energy Reviews, 5: 343-372. 8. kang, Y.T., Y. Kunugi and T. Kashiwagi, 2000. Subscripts Review of advanced absorption cycles: 1, 2, 3…system’s point designation Performance improvement and temperature lift A: Absorber enhancement. Intl. J. Refrigeration, 23: 388-401. C: Condenser 9. Chekir, N., K. Mejbri and A. Bellagi, 2006. E: Evaporator Simulation d’une machine frigorifique a` G: Generator absorption fonctionnant avec des mélanges H: Hydrogen d’alcanes. Intl. J. Refrigeration, 29: 469-475. NH3: Ammonia 10. Zohar, A., M. Jenilik and A. Levy, 2005. R: Rectifier Numerical investigation of a diffusion absorption r: rich refrigeration cycle. Intl. J. Refrigeration, 28 : 515- 525. REFERENCES 11. Collier, J.G., 1982. Convective Boiling and Condensation. Mc Graw Hill International Book 1. Srikhirin, P. and S. Aphornratana, 2002. Company, pp: 26-58. Investigation of a diffusion absorption refrigerator. 12. Stewart, S.W., 2001. Bubble Pump Design and Appl. Therm. Engg., 22: 1181–1193. Performance. Thesis Presented to the Academic 2. Shelton, S.V., 1999. Design analysis of the Faculty, Georgia Institute of Technology, USA. Einstein refrigeration cycle. Ph. D. Georgia Institute of Technology, USA. 3. De Francisco, A., R. Illanes, J.L. Torres , M. Castillo, M. De Blas, E. Prieto and A. Garcia, 2002. Development and testing of a prototype of low power water–ammonia absorption equipment for solar energy applications. Renewable Energy, 25: 537–544.