OPTIMIZING THE GENERATOR AND ABSORBER THERMAL LOAD OF COMBINED ABSORPTION-EJECTOR REFRIGERATION SYSTEM

Transcription

1 OPTIMIZING THE GENERATOR AND ABSORBER THERMAL LOAD OF COMBINED ABSORPTION-EJECTOR REFRIGERATION SYSTEM Azher M.Abed,,, M.A.Alghoul,, Ali Najah Al-Shamani,,K. Sopian, Solar Energy Research Institute (SERI), UniversitiKebangsaan Malaysia, 436 Bangi, Selangor Malaysia Department of Air conditioning and Refrigeration, Al-Mustaqbal University College, Babylon, Iraq Abstract: - The thermal load of generator and absorber is still subject for further optimization in the combined ejector- absorption cycle operated with NH 3 /H O as a working fluid. A new configuration of the combined cycle is proposed and analysed. The mathematical results show that the new configuration indicates a significant thermal load reduction at the generator (%) and absorber (7%) compared to the cycle before modification. Moreover, the results show that as the generator temperature increases the thermal load of the condenser and the evaporator remains constant and the entrainment ratio ω as well. It is also found that by increasing the effectiveness of solution heat exchanger (SHE), the thermal load of the generator or the absorber decreased about 38%, with increased coefficient of performance (COP) about 8 % compared to the first configuration. This improvement in the overall COP is found due to improve energy utilization efficiency significantly. Key-Words: -Combined ejector-absorption system, energy balance, improves components efficiency, enhanced COP.. Introduction Traditionally, vapour compression systems are the common on board ships refrigeration systems. These systems are powered using electric energy produced through burning of increasingly more expensive fuel. The challenges of environment protection and energy economy have led to a growing interest in non-conventional refrigeration systems such as absorption refrigeration. The advantage of the absorption cycles is their much lower consumption of electricity achieved by replacement of a compressor by so called a thermochemical compressor. The main benefits of the absorption cycles are the following: very low consumption of electricity (only up to % of the cooling capacity), possible utilizing of thermal energy obtained from renewable sources (solar radiation, biomass combustion) or whatever source of waste heat energy with temperature above 8 C for the production of cooling. Another positive effect of the absorption cycles is utilizing of ammonia as refrigerant. Disadvantage of the absorption cycles is the overall performance of the absorption cycle in terms of cooling effect per unit of energy input is generally poor. For this reason, the researchers have been devoted to improvement the absorption cooling systems. However, the COP of the NH3/HO systems is limited due to various constraints posed by the component temperatures and pressures. Combining of the ejector with the single stage absorption cycle showed potential improvement in the COP of the cycle []. The advantage of using an ejector refrigerator cycle can utilize thermal energy at temperature levels upward from 333 K []. Heat energy at these temperatures is available from the absorber of the absorption refrigerator. Previous researches demonstrated that the COP of absorption and ejector refrigerators are in the ranges..8,while the basic absorption cycle..6 [3, 4]. Many researchers have improved the performance of the single effect absorption cooling system in recent years [, 6]. It was showed further enhancement by adding flash tank to the cycle with the presence of the ejector. Moreover, experimental studies shows that COP can be increased up to 6% when a solution heat ISBN:

2 exchanger is used [7]. The effect of the generator absorber temperature on the system performance has been studied [8-]. They informed that the system has a low generator temperature limit which that the cycle cannot be operated at generator lower than its limit. The optimum generator temperature determines the optimum solution concentration leaving the generator. Therefore, there is thermal energy wasted within the generator and the absorber, when the solution temperature at the generator higher than its optimum value. From this reasons, for thermal energy saving, the modifications are made in the system to save the energy wasted and to improve the COP of the cycle. In the present study, the effect of streamline of strong solution and heat exchanger effectiveness on the thermal loads of components, entrainment ratio (ω) and coefficient of performance are investigated. Qg ammonia vapor from the rectifier are mixed and passed to the condenser (state ). Exp.valve S.H.E 3 Generator Absorber 7 PUMP 6 Rectifier High Pressure level 9 8 Low Pressure level 6 Ejector booster ' Flash tank Condenser 3 4 3' Evaporator Exp. valve Qc. System Description Figure shows schematically the main part of (combined ejector absorption refrigeration system) was done by [3] and []. The schematic diagram of the proposed new design is shown in Figure. In this new design the ammonia mixture, leaves the absorber ( state 7) as a saturated solution at the low pressure with a relatively high ammonia concentration. It is pumped to the system high pressure (state 8) and divided into two parts. : One part flows to the solution heat exchanger (state 9B) it used to recovers the heat from the returning weak liquid solution in the recovery heat exchanger before entering the generator, mainwile the remaining part flows to the rectifier. In the rectifier the vapor is cooled by the rich solution pumped from the absorber to the generator (state 9A).As the generator operates from high temperature source to separate the binary solution of water and ammonia (strong solution comes from absorber), the basic solution is partially heated to produce a two phase mixture : a liquid (state ), which is relatively weak in ammonia, and a vapor (state ) with a high concentration of ammonia. This two-phase mixture is separated, and the weak liquid transfer heat to the high concentration stream before it is throttled to system low pressure and sprayed into the absorber. The rectifier cools the saturated ammonia vapor (state ) to condense out any remaining water (state 6). It is assumed that the ammonia vapor concentration leaved the rectifier passed to the ejector is 99.6%. On the ejector, the secondary flow (ammonia vapor from the mixing of the evaporator and flash tank (state 4) and the primary flow of Figure.First configuration of combined cycle with common solution heat exchanger. Qg Exp.valve S.H.E 3 Generator Absorber B 9B 8 PUMP 7 6 A Rectifier High Pressure level 9A Low Pressure level 6 Ejector 7 4 Weak solution line Strong solution line 8 booster ' Flash tank Ammonia Vapor line Ammonia liquid line Condenser 3 4 3' Evaporator Exp. valve Figure.Second configuration of combined cycle - through utilizing the streamline of strong solution. 3. Thermodynamic analysis of absorption cooling system The mass balances, energy balances, and the equations of state for NH3/HO solution, and refrigerant at each component involved in the cycle are required for system study. In order to calculate the heat and mass balance for the proposed cycle, the thermodynamic properties (pressure, temperature, concentration, enthalpy and density) are necessary for the simulation. The binary mixture of NH3/HO and pure NH3 are used in the Qc ISBN:

3 proposed system. The detailed thermodynamic property equations of NH3/HO are found by [3] and [4]. To evaluate the COP for the cycle, the governing equation required through the each component. - Mass balance and Energy balance () = - () whereq K is the heat added to component K at temperature Tk. - Pump The electrical power needed for the pump w p = m 7v 7 (P 8 -P 7 ) (3) η hed η mec w p = m 8h 8 m 7h 7 (4) - Heat exchanger The solution heat exchanger performance, expressed in terms of an effectiveness ε. T = T 9B ε SHE + (-ε SHE )T, X = X, () P =P g, h = h s (T, X ) The absorbed liquid strong solution is pumped into two parts. One part is pumped into the rectifier, whereas the second part is pumped into the heat exchanger. Therefore, the mass flow rate on states (9A)and (9B) are as follows: m 8=m 9A+m 9B, m 8=m 9A, m 9B = m 9A (6) The component's thermal loads of the combined ejector cooling system per unit of refrigerant mass are expressed as follows: Q g =m h +m 4 h 4 ( m Ah A +m 6h 6 + (7) m Bh B ) Q e =m (h ' h ) (8) Q c = ( + ω)m (h h 3 ) (9) Q a =m 6h 6 +m 3h 3 -m 7h 7 () The coefficient of performance COP is defined as Q COP= E () Q G +w p - Equation of state Finally, The thermodynamic properties at states ()- (6),7,8,4 and () in Figure are determined by NH 3 and other properties at states (7)-(6) can be calculated based on the binary mixture of NH 3 /H O which are given by the correlations of [3] and [4]: EES software is used to evaluate the performance of the system using NH3/HO as working fluid. The operation temperatures range of the proposed cycle are selected as Tgen= 6 o C, T cond =T abs = - o C, T evp =-- o C, and mass flow rate of refrigerant =.66 kg/s. The effectiveness of the solution heat exchanger is assumed.. 4. Evaluation of utilizing the streamline of strong solution In this work, the effects of the streamline of strong solution on the performance of the proposed cycle are analysed. The effects of the streamline of strong solution on the thermal loads of generator and absorber are shown in Figs.3and 4.It can be seen that the heat load of the generator decreases with an increase temperature of the strong solution entering the generator. Similarly, with a decrease in the weak solution temperature entering the absorber, the heat rejected from the absorber also decreases. For this reason, the reduction ratios of both generator and absorber thermal loads increase with increases the generator temperature by about % and 7% respectively as shown in Figure3. Also it is found that the decreasing ratios in both generator and absorber thermal loads increase with the increases of SHE effectiveness and reach up to 38% as shown in Figure 4. Reduction in thermal loads (%) Tcon= 3 C Tcon= C Tevp= - C 8 9 Generator Temp. Figure 3.Reduction in thermal loads versus the generator temperature. 4. Results and Discussion ISBN:

4 Reduction in thermal loads (%) 4 3 Tgen = 9 C Tcon= 3 C Tcon= C Tevp= - C...7 SHE effectiveness Figure 4.Reduction in thermal loads versus the effectiveness of SHE As it can be seen from Figure, when the generator temperature increases, the thermal loads of the generator and absorber decrease. The optimum generator temperatures corresponding to the minimum thermal loads of the generator and absorber after this optimum value the thermal loads remain unchanged. If the generator temperature gets higher, the concentration of the solution leaving the generator increase, and hence, the circulation ratio decreases. The thermal loads of the condenser and the evaporator remains constant, as the generator temperatures increases. Moreover, the entrainment ratio ω remains almost constant under generator temperature range. Thermal Load (kw) Generator Temp. ( C) Figure.Variation of different components thermal loads versus generator temperature at (Tc=3 oc, Ta= oc,te=- oc ). w Qcond Entrainment raito 4. Comparison between the first and second configuration Figures 6 a & b illustrate the variation of the coefficient of performance with generator temperature for five scenarios of different operating conditions (condenser, absorber and evaporator temperature) for combined cycle before and after the modification. These five scenarios can be used to evaluated the COP of the systems under generator temperature range (7- o C), condenser temperature range ( -4 o C) to operate under water cooled or air cooled condenser. The range of the evaporator temperature is (- to o C) that shows the best cooling effect under the employed generator and ambient condensation temperature ranges. The overall COPs are found higher at the new modified combined cycle (second configuration) than that of combined cycle (first configuration). This improvement in the overall COP is found due to minimize the energy consumption at the generator, and absorber. COP COP Generator temperature( C) 6, a). Cycle before modification (first configuration). Tc=Ta=,Te=- Tc=Ta=3,Te=- Tc=Ta=3,Te= Tc=Ta=4,Te= Tc=Ta=4,Te= Tc=Ta= C,Te=- C Tc=Ta=3 C,Te=- C Tc=Ta=3 C,Te= C Tc=Ta=4 C,Te= C Tc=Ta=4 C,Te= C Generator temperature( C) 6, b) Cycle after the modification (second configuration). ISBN:

5 . Conclusions In this study energy balance analysis is performed to predict the potential optimization of the thermal load of generator and absorber of the combined ejector-absorption cooling cycle. For this purpose, a new cycle configuration is proposed. The results show that a significant reduction of the required generator and absorber thermal loads by about % and 7% respectively is obtained. Moreover, by increasing the effectiveness of solution heat exchanger (SHE), the thermal load of the generator or the absorber decreased about 38%. The thermal loads of the condenser and the evaporator remains constant, as the generator temperatures increases. Moreover, the entrainment ratio ω remains almost constant under generator temperature range. The overall COPs are found higher at the new modified combined cycle than that of ordinary combined cycle. This improvement in the overall COP is found due to utilize the internal heat recovery in the new modified cycle. Acknowledgements The authors would like to acknowledge the support of Solar Energy Research institute, National University of Malaysia. References : [] S. Aphornratana, I.W. Eames, Experimental investigation of a combined ejector-absorption refrigerator, International journal of energy research, (998) 9-7. [] D.-W. Sun, I.W. Eames, S. Aphornratana, Evaluation of a novel combined ejectorabsorption refrigeration cycle I: computer simulation, International Journal of Refrigeration, 9 (996) 7-8. [3] P. Srikhirin, S. Aphornratana, S. Chungpaibulpatana, A review of absorption refrigeration technologies, Renewable and Sustainable Energy Reviews, () [4] D.S. Ward, Solar absorption cooling feasibility, Solar Energy, (979) [] R. Sirwan, M.A. Alghoul, K. Sopian, Y. Ali, J. Abdulateef, Evaluation of adding flash tank to solar combined ejector absorption refrigeration system, Solar Energy, 9 (3) [6] R. Sirwan, M.A. Alghoul, K. Sopian, Y. Ali, Thermodynamic analysis of an ejector-flash tank-absorption cooling system, Applied Thermal Engineering, 8 (3) [7] S. Aphornratana, Theoretical and experimental investigation of a combined ejector-absorption refrigerator, in, University of Sheffield, Department of Mechanical and Process Engineering, 99. [8] Y. Kang, A. Akisawa, T. Kashiwagi, An advanced GAX cycle for waste heat recovery: WGAX cycle, Applied Thermal Engineering, 9 (999) [9] Y.T. Kang, W. Chen, R.N. Christensen, Development of design model for a rectifier in GAX absorption heat pump systems, Ashrae Transactions, (996) [] G. Grossman, R.C. DeVault, F.A. Creswick, Simulation and performance analysis of an ammonia-water absorption heat pump based on the generator-absorber heat exchange (GAX) cycle, in, Oak Ridge National Lab., TN (United States), 99. [] D.-W. Sun, Computer simulation and optimization of ammonia-water absorption refrigeration systems, Energy Sources, 9 (997) [] D.-W. Sun, Comparison of the performances of NH3-HO, NH3-LiNO3 and NH3-NaSCN absorption refrigeration systems, Energy Conversion and Management, 39 (998) [3] P. Bourseau, R. Bugarel, Absorption-diffusion machines: comparison of the performances of NH3HO and NH3NaSCN, International Journal of Refrigeration, 9 (986) 6-4. [4] J. Pátek, J. Klomfar, Simple functions for fast calculations of selected thermodynamic properties of the ammonia-water system, International Journal of Refrigeration, 8 (99) ISBN: