Abstract: Energy and exergy analysis of triple effect series and parallel flow absorption refrigeration systems with lithium bromide/water as working fluid pair is presented in this study. Thermodynamic properties, mass and heat flow rate and exergy destruction are evaluated in each component of the system. Furthermore, The Coefficient of Performance (COP) and exergetic efficiency (E) of the absorption systems under different operating conditions are estimated. The results show that the exergy destruction at the generator and absorber are more than that of the other components which was expected due to mixing in these components. The results also show that the COP of the system increase slightly with an increase of High Temperature Generator (HTG) and evaporator temperature. However, the exergetic efficiency of the system decreases as the HTG and evaporator temperature increase. The latter results apply for both the series and parallel flow systems. Another indication of the results is a relative preference of parallel-flow in comparison with series-flow. In addition it can be seen that the triple effect system has higher COP in comparison with double and single effect types.
INTRODUCTION
Recent developments in cooling systems show a growing interest in the application of absorption systems. Absorption refrigeration systems provide opportunities for energy saving because they can use heat energy to produce cooling, instead of electricity used by conventional vapour compression chillers. Furthermore, non-conventional sources of energy such as solar, waste heat and geothermal can be used as their primary energy input. In addition, absorption units use environmentally friendly working fluid pairs instead of CFCs and HCFCs, which deplete the ozone layer of the atmosphere (Chua et al., 2000). The first law analysis of double effect and triple effect lithium bromide-water absorption system has been done recently by some researchers (Xu et al., 1996; Arun et al., 2001; Kaita, 2002). Also Exergy analysis of single and double effect absorption refrigeration cycles with lithium bromide/water was carried out by some researchers (Talbi and Agnew, 2000; Arzu Sencan et al., 2005). However, there is a lack of data in the second law analysis of triple effect absorption refrigeration systems albeit of their higher COPs. In this study energy and exergy analysis is carried out for two types of Triple effect absorption refrigeration cycles, parallel and series flow, with lithium bromide/water as the working fluid pair. The study was performed for different operating conditions by means of the computer program.
MATERIALS AND METHODS
The series flow and parallel flow kinds of triple effect absorption refrigeration cycles shown schematically in Fig. 1 and 2 have been analyzed.
| Fig. 1: | Series flow cycle |
| Fig. 2: | Parallel flow cycle |
In the series-flow cycle, Fig. 1, the weak solution from the absorber is sent directly to High Temperature Generator (HTG) and returned to the absorber through Middle Temperature Generator (MTG) and Low Temperature Generator (LTG). In the parallel flow cycle in Fig. 2, the weak solution is sent first to LTG and then to MTG and at the end to HTG. For thermodynamic analysis of the absorption system the principles of mass conservation, first and second laws of thermodynamics are applied to each component of the system. Each component can be treated as a control volume with inlet and outlet streams, heat transfer and work interactions. In the system, mass conservation includes the mass balance of total mass and each component of the solution.
(1) |
(2) |
where,
(3) |
The exergy of a fluid stream can be defined as:
(4) |
where, ψ is the exergy of the fluid at temperature T and pressure P. h0 and s0 are the enthalpy and entropy of the fluid at environmental temperature and pressure. Irreversibility in each component is calculated by:
(5) |
Icv is irreversibility that occurred in the process. COP of the absorption system is defined as the ratio of control volume heat transfer in the evaporator to that of the generator.
(6) |
The exergetic efficiency, E, is defined as the ratio of the exergy gained at the evaporator to the exergy gained at the generator:
(7) |
For simplification purposes, the work input to the solution pump and the frictional losses inside the system are neglected.
RESULTS AND DISCUSSION
The initial conditions introduced to the program include the ambient conditions, heat exchanger effectiveness and temperatures of HTG, evaporator and condenser and also absorber exit mass flow rate. With the given parameters, the program calculates all the thermodynamic properties at any point of the cycle. The results are presented in Table 1 and 2 for parallel and series flow, respectively.
Both tables were obtained with Tgen = 200°C, Te = 5°C and Tc = 30°C.
A parametric study was performed for the performance of both systems, varying the Tg and Te. All these figures indicate and influence of above mentioned temperatures on first and second law efficiencies (Fig. 3-8). Figure 3 shows the variation of the COP of the parallel flow system. As, it can be shown from Fig. 3, in general the COP increases with an increase of Tg and/or Te. However, this increase is highly pronounced at lower Te and Tg. This can be explained by Fig. 4 and 5 that show the variation of HTG and Evaporator heat loud (Qhtg ,Qe) with increasing Tg and Te for parallel flow and take attention to the Eq. 6.
As can be seen from Fig. 4 and 5 the rate of increase in Qe is faster than Qhtg with increasing Tg and Te.
Figure 6 shows the variation of the second law efficiency of the parallel flow type with Tg and Te.
| Table 1: | State point data for triple-effect parallel flow cycle |
| Table 2: | State point data for triple-effect series flow cycle |
As, it shows, the exergetic efficiency (E) of the system decreases with increasing Tg and Te. This can be explained by the fact that an increase of Tg causes an increase of exergy entering the generator and also an increase of Te causes a decrease of exergy entering the evaporator. According to Eq. 7 these changes of input exergies both result in the exergy efficiency to decrease.
| Fig. 3: | Variation of the COP of the parallel flow type with the Tg and Te |
| Fig. 4: | Variation of Qe and Qhtg with Tg for parallel flow type |
| Fig. 5: | Variation of Qe and Qhtg with Te for parallel flow type |
| Fig. 6: | Variation of the exergetic efficiency (E) of the parallel flow type with the Tg and Te |
| Fig. 7: | Variation of the COP of the series flow type with the Tg and Te |
The results for series flow type (Fig. 7, 8) are similar to parallel flow type qualitatively. However, a relatively higher amount of COP and E is found for parallel system.
Irreversibilities in the absorber and generators are more than that of the other components of the system (Fig. 9, 10). This is due to the heat of mixing in the solution, which is not present in pure fluids.
Figure 11 shows the variation of the COP of triple effect parallel flow and series flow, double effect series flow and single effect systems with Tg when Tc = 30°C and Te = 5°C. As it can be seen from this figure, for the triple effect systems there is a good agreement between present results and those reported in the literature.
| Fig. 8: | Variation of the exergetic efficiency (E) of the series flow type with the Tg and Te |
| Fig. 9: | :Irreversibility at the various components of the parallel system |
| Fig. 10: | Irreversibility at the various components of the series system |
| Fig. 11: | Variation of COP with generator temperature (Tg) in single, double and triple effect absorption systems |
| Fig. 12: | Variation of exergetic efficiency (E) with generator temperature (Tg) in single, double and triple effect absorption systems |
The results also show that the COP of triple effect systems is about 50% more than that of the double effect series flow that obtained from present results and other researchers (Arun et al., 2000) work and is about 2.2 times more than that of the single effect systems. As it shows, exergetic efficiency (E) of triple effect parallel flow type is more than that of the series type (Fig. 12). Also in triple effect types the rate of decrease in E is lower in comparison with those of double and single effect systems.
CONCLUSION
For both triple effect series and parallel flow types there is an increase of COP and a decrease of E as HTG temperature (Tg) and/or evaporator temperature (Te) increases.
Considering both the first law and second law efficiencies, there is a relative preference of triple effect parallel flow type in comparison with series type.
The COP of triple effect systems is about 50% more than that of the double effect systems and it is about 2.2 times that of the single effect systems.
For a given value of ΔTg the variation of E for triple effect systems is less than that for double effect systems. This variation is the most for single effect systems.
Most of the irreversibilities occur in generators and absorber due to mixing losses in series and parallel flow types.
NOMENCLATURE
| ABS | = | Absorber |
| COND | = | Condenser |
| E | = | Exergetic efficiency |
| EVAP | = | Evaporator |
| H | = | Enthalpy (kJ kg-1) |
| HHX | = | High-temperature heat exchanger |
| HTC | = | High-temperature condenser |
| HTG | = | High-temperature generator |
| LHX | = | Low-temperature heat exchanger |
| LTG | = | Low-temperature generator |
| MHX | = | Middle-temperature heat exchanger |
| MTC | = | Middle-temperature condenser |
| MTG | = | Middle-temperature generator |
| = | Mass flow rate (kg sec-1) | |
| = | Heat load (kW) | |
| s | = | Entropy (kJ kg-1 K) |
| T | = | Temperature (K) |
| x | = | Mass fraction of lithium bromide (%) |
| COP | = | Coefficient of performance |
| = | Work (kW) | |
| ψ | = | Exergy (kJ kg-1) |
| ¤ | = | Irreversibility (kW) |
Subscripts
| c | = | Condenser |
| e, evap | = | Evaporator |
| g, gen | = | Generator |
| I | = | Inlet stream |
| o | = | Outlet stream |