Energy and exergy analysis of a double effect parallel flow LIBRH2O absorption refrigeration system
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Journal of Thermal Engineering 2016, Vol. 2, Issue 1, pp. 541-549; doi.org/10.62051/ytu.journal-of-thermal-engineering-energy-and-exergy-analysis-of-a-double-effect-parallel-flow-librh2o-absorption-r
Abstract
Keywords: Parallel flow; Absorption system; Exergy; Exergetic efficiency; Solution distribution ratio.
Introduction
Most absorption cooling systems adopt the double-effect cycle to increase the cooling performance of the system when the heat source available is at high temperature with water lithium bromide. However, when using series flow double effect system the range of operation comes close to the crystallization line of the LiBr solution and the absorption ability becomes weak. The parallel-flow type of double-effect cycle has been proposed to eliminate these difficulties compared with the series-flow type. Compared to the series-flow type, the range of operating conditions of the parallel-flow type is far away from the
various operating parameters on investment cost. Li et al. [19] did the performance analysis of solar air cooled double effect LiBr/H2O absorption refrigeration system.
Mathematical Modelling The thermodynamic analysis of the system involves the application of principles of mass conservation, species conservation, energy balance and exergy balance to individual components of the system. Mass and species conservation for each component can be written in general form as follows:
It is to be noted that in all the studies mentioned above the optimum solution distribution ratio is not computed corresponding to maximum exergetic efficiency. It is important to determine optimum solution distribution ratio corresponding to maximum exergetic efficiency because minimum irreversibility occurs in a thermal system corresponding to maximum exergetic efficiency.
Keeping this viewpoint, in the present communication the analysis of a parallel flow double effect absorption refrigeration system is performed to compute the optimum solution distribution ratio from the viewpoint of maximum cop and maximum exergetic efficiency. The effect of operating parameters such as generator, absorber and evaporator temperatures is also presented on optimum solution distribution ratio.
she1 Fig. 1 shows schematic diagram of a LiBr/H2O parallelflow, double-effect refrigeration system. It includes the most important components of the double-effect cycle: absorber (a), evaporator (e), HP generator (hpg), LP generator (lpg), condenser (c), HT heat exchanger (she2), LT heat exchanger (she1) and a solution pump (p). The strong (in absorbent) solution produced in the absorber is separated at the outlet of the LT heat exchanger and is distributed separately to the HP and LP generators. This is the main feature of the parallel-flow double effect absorption refrigeration system. The ratio of solution entering the HP generator to the solution leaving the solution pump is known as solution distribution ratio. The solution in the high-pressure generator is heated externally from solar energy or waste heat or any other heat source. The refrigerant vapour (steam) produced in the HP generator is used as a heat source for the low-pressure generator. The weak (in refrigerant) solution from the HP generator passes through the HT heat exchanger and mixes with the weak solution coming from LP generator through the solution throttle valve (stv2) at the inlet of the LT heat exchanger. The solution then enters the absorber and is diluted by absorbing the refrigerant vapour
Parallel FLOW Absorption System
Fig. 1. Schematic diagram of a Parallel Flow Double Effect Absorption Refrigeration System .
The energy balance of the system is specified by equation (3). .
coming from evaporator. The refrigerant vapour produced by the LP generator is condensed in the condenser, and flows to the evaporator. The refrigerant generated in LP and HP generators mix before entering the evaporator. In evaporator, liquid refrigerant extracts heat from the space to be cooled and evaporates, thereby producing cooling effect.
Properties at various state points are calculated using the equations (4) to (10). LiBr/H2O solution:
Refrigerant (water): f4 (h, P, T) = 0 f5 (s, P, T) = 0 f6 (h, Tsat) = 0 f7 (s, Tsat) = 0
. T . . T0 0 ED = m e − m e + Q 1 − − Q 1 − ± W ∑ ∑ i ∑ ∑ ∑ in out T in T out .
Exergy is the property of a system relative to a reference state, which gives the maximum power that can be extracted from the system when it is brought in to thermodynamic equilibrium with the reference state [20]. Exergy balance for a control volume undergoing steady state process is expressed as [13].
where ED i represents the rate of exergy destruction or the irreversibility occurring in the process. The first two terms on the right hand side represent exergy of streams entering and leaving the control volume. The third and fourth terms are the
exergy associated with heat transfer from the source maintained at constant temperature T and is equal to work obtained by Carnot engine operating between T and T0, and is therefore equal to maximum reversible work that can be
Q obtained from heat energy . The last term is the mechanical
work transfer to or from the control volume. Solution distribution ratio (R) is specified by equation (22). .
Second law performance of the system can be measured in terms of exergetic efficiency [21]. It is defined as the useful exergy or available energy gained from a system to that supplied to the system. For the double effect system under consideration, it is the ratio of the exergy of the cooling effect produced by the evaporator to the exergy of the heat supplied at the HP generator plus pump work supplied. .
The energy balance in each component of a parallel flow double effect system is given by the following equations: .
The above modeling procedure forms the basis for the equations given in the subsequent paragraph.
Mass, material and energy balance of parallel flow double effect absorption refrigeration system
The mass and material balance at high temperature generator, second effect generator and condenser are given by: 543
EDmixer8−16−17 = mw1 ((h8 −T0s8) + mw2 ((h16 −T0s16) − mw((h17 −T0s17) (46) .
Exergy destruction in each component of a parallel flow double effect absorption refrigeration system is furnished below.
EDmixer11−20−21 = mr1 ((h11 −T0s11) + mr2 ((h20 −T0s20) − mr ((h21 −T0s21) (47)
. . . . EDa = mr (h − T s ) + mw (h − T s ) − ms (h − T s ) 22 o 22 4 o 4 1 o1 . T − Qa 1 − o T a .
. . . T EDe = mr ((h21 − h22 ) − To (s21 − s22 )) + Qe 1 − 0 Tr (38) .
EDshe1 = ms ((h2 −h5) − To(s2 − s5)) + mw((h17 − h3) −To(s17 − s3)) (39) .
. T Q e 1 − 0 Tr ηex = . T . Q hpg 1 − 0 + W p T hpg
The present work is based on the following assumptions:1. Pressure and heat losses through the system components are negligible.
2. Solution leaving the absorber and the generator are assumed
to be saturated in equilibrium conditions at their respective temperatures and concentrations.
3. Refrigerant leaving the condenser and vapour leaving the
evaporator are assumed to be saturated at their respective saturation temperatures.
5. Non equilibrium states at the inlet to the HP generator, and
the absorber and states at outlet to the solution pump and the solution heat exchanger are taken to be at their actual conditions.
7. Throttling of the refrigerant is assumed to be isenthalpic and
non-isentropic while solution throttling is considered to be isothermal, isenthalpic and non-isentropic.
. . . T ED c = m r1 ((h9 − h10 ) − To ( s9 − s10 ) ) − Q c 1 − 0 Tc
Results And Discussion
EDlpg = ms1 (h7 − Tos7 ) − mw1 (h8 − Tos8 ) + mr2 (h18 − Tos18) .
Equation (12) can now be rewritten in the form as shown in equation (49).
EDt = EDa + EDhpg + EDlpg + EDc + EDe + EDshe1 + EDshe2 + EDrtv1 .
EDhpg = ms2 (h13 − To s13 ) − mw2 (h14 − To s14 ) − mr2 (h18 − To s18 )
+ EDrtv2 + EDstv1 + EDstv2 + EDstv3 + EDmixer8−16−17 + EDmixer11−20−21 (48) (34)
EDshe2 = ms2 ((h12 −h13) −To(s12 − s13))+ mw2 ((h14 −h15) −To (s14 − s15)) (40) 544
8. The temperature in high temperature heat source, medium
temperature heat sink and low temperature heat source are assumed to be constant while the fluid temperature varies in non-isothermal components due to different inlet/outlet solution concentrations.
9. The reference enthalpy (ho) and entropy (so) used for
calculating exergy of the working fluid are the values for water at an environment temperature (To) of 25°C. A computer program has been developed using Engineering Equation Solver (EES) software [22] for carrying out the energy and exergy analysis of the single effect and double effect absorption refrigeration systems. The properties of LiBr/H2O solution used for the analysis purpose have been obtained from correlations developed by Pátek and Klomfar [23]. Subroutines for calculating the properties of LiBr-H2O solution were linked to the library file of the EES. Following parameters are assumed for parametric computations:1. High pressure generator temperature (Thpg) = 120 °C – 170 °C
5. Temperature of the space to be cooled (Tr) = 17.2 °C
Fig. 2 illustrates the effect of variation in solution distribution ratio and HP generator temperature on the COP (Tc = Ta = 29.4 °C). The COP increases with reduction in HP generator temperature. The reason being that for same solution distribution ratio, the increase in HP generator temperature cause an increase in exergy destruction in HP generator, absorber, condenser, LP generator and total exergy destruction. The maximum value of COP varies between 1.39 and 1.42. The global maximum of COP is observed at 135°C HP generator temperature. Further with increase in solution distribution ratio, it is observed that total exergy destruction first decreases up to the optimum and then again starts increasing.
6. Cooling capacity of the system ( e ) = 100 kW
The results computed from the present work are compared with the results given in Riffat and Shankland (1993) and are detailed in Table 1. The difference in values of heat transfers in various components is less than 3%. The difference in value of COP is less than 0.6%.
Table 1. Comparison of present results with the results of Riffat and Shankland [6]
results
The optimum value of solution distribution ratio for maximum COP varies between 0.19 and 0.28 depending upon the HP generator temperature, indicating best results are achieved when most of the solution is distributed to the LP generator and simultaneously minimum total exergy destruction is observed corresponding to these values. Secondly, it is observed that with increase in HP generator temperatures (i.e. between 135 °C to 155 °C) the optimum solution distribution ratio decreases from
0.23. to 0.19 and for temperatures between 155 °C and 170 °C,
it increases to 0.28. Moreover lower values of Ropt show that circulation losses are also less when lesser amount of solution is entering the HP generator. The nature of the curve obtained in
Fig. 2. Effect of solution distribution ratio (R) and high pressure generator temperature on COP (Te=7.2 °C, Ta= Tc=29.4 °C).
the present analysis is similar to the curve obtained by Gommed and Grossman [5] in their analysis.
Fig. 4. Effect of solution distribution ratio (R) and high pressure generator temperature on exergetic efficiency (Te=7.2 °C, Ta= Tc=29.4 °C)
Fig. 3. Effect of solution distribution ratio (R) and high pressure generator temperature on COP (Te=7.2 °C, Ta= Tc=37.8 °C)
Fig. 3 presents the effect of increasing the absorber and condenser temperatures on the COP and optimum solution distribution ratio. It is observed that with increase in the absorber and condenser temperatures (from 29.4 to 37.8 °C), the COP and its maximum value drops for the range of generator temperatures considered. It happens because of increase in solution circulation ratio, with increase in absorber and condenser temperatures, and it causes increase in circulation losses and hence reduction in COP. Further the optimum solution distribution ratio is achieved at higher values of solution distribution ratio than before. It indicates that more solution is required to be pumped to HP generator at higher absorber and condenser temperatures to achieve maximum value of the COP. However increasing the HP generator temperature causes the optimum distribution ratio to shift toward lower values of solution distribution ratio. Fig. 4 depicts the effect of solution distribution ratio and HP generator temperature on exergetic efficiency. It is observed that increase in the HP generator temperature brings down the exergetic efficiency. The same reasons can be attributed for such a behaviour as already explained for the trend of COP curve. Secondly, with increase in solution distribution ratio, there is increase in exergetic efficiency up to optimum value of solution distribution ratio and beyond which there is drop in exergetic efficiency. The optimum value of solution distribution ratio lies between 0.23 - 0.19 for the HP generator temperatures between 135 °C and 155 °C, and for temperatures above 155 °C and up to 170 °C, it increases to 0.28.
Fig. 5. Effect of solution distribution ratio (R) and high pressure generator temperature on exergetic efficiency (Te=7.2 °C, Ta= Tc=37.8 °C).
The effects of increase in absorber and condenser temperatures on exergetic efficiency and optimum solution distribution ratio are represented in Fig. 5. The increase in the absorber and condenser temperatures is responsible for reduction of exergetic efficiency. It happens because of overall increase in solution circulation ratio with increase in absorber and condenser temperatures which causes increase in circulation losses and irreversibility in various components of the system increase and hence reduction in exergetic efficiency. The higher values of the exegetic efficiency are achieved at lower values of HP generator temperature for solution distribution ratio varying between 0.2 and 0.77. The optimum value of solution distribution ratio increases as compared to the case when absorber and condenser temperatures are lower.
Fig. 6. Variation of COP and exergetic efficiency with solution distribution ratio (R) (Thpg= 135 °C, Te = 7.2 °C, Ta = Tc = 29.4 °C)
Fig. 8. Exergy destruction in different components at various solution distribution ratios (R)
Fig. 8 represents the variation of exergy destruction in different components with solution distribution ratio at high pressure generator temperature of 140 °C and condenser and absorber temperature of 29.4 °C. It is observed that total exergy destruction is lowest at optimum solution distribution ratio and it increases on either side of the optimum solution distribution ratio on either side. The absorber is the component in which percentage exergy destruction is maximum followed by HP generator, mixing sections, LP generator and evaporator.
Fig. 6 depicts the variation of the COP and exergetic efficiency with solution distribution ratio. It is observed that the maximum value of the COP and exergetic efficiency occur for same solution distribution ratio. This happens because minimum irreversibility occurs at optimum solution distribution ratio. The similar results were also obtained for different values of generator temperature at other absorber and condenser temperatures. 1.4
Conclusions
In the present analysis, we have performed the energy and exergy analysis of a parallel flow double effect LiBr/H2O absorption refrigeration system. The conclusions drawn from this analysis are specified point wise below.
0.8 0.25 0.6 0.15 COP_max (Te = 7.22 °C) COP_max (Te = 5 °C) R_opt (Te = 7.22 °C) R_opt (Te = 5 °C) η_ex_max (Te = 7.22 °C) η_ex_max (Te = 5 °C)
1. The COP and exergetic efficiency increases with reduction in
HP generator temperature. Thus global maximum of COP is observed at 135°C HP generator temperature. The maximum value of COP varies between 1.39 and 1.42. The optimum value of solution distribution ratio for maximum COP varies between
0.19. and 0.28 depending upon the HP generator temperature.
The optimum value of solution distribution corresponds to minimum exergy destruction.
Fig. 7. Variation of maximum COP and exergetic efficiency and Ropt with Thpg for varying evaporator temperatures
2. Increasing the absorber and condenser temperatures cause the
COP, exergetic efficiency and their maximum values to drop. The optimum solution distribution ratio is achieved at higher values of solution distribution ratio.
Fig. 7 shows the effect of variation in the evaporator temperature on the optimum solution distribution ratio, maximum COP and maximum exergetic efficiency at condenser and absorber temperature of 37.8 °C. The overall system irreversibility increases with reduction in evaporator temperature and hence maximum value of the COP and the exergetic efficiency reduce.
4. The total exergy destruction is lowest at optimum solution
distribution ratio and it increases on either side of the optimum solution distribution ratio. The absorber is the component in
which percentage exergy destruction is maximum followed by HP generator, mixing sections, LP generator and evaporator.
Nomenclature
f h HP HT LP LT P s SCR R T X Subscripts 0 1, 2, 3… a aev C e hpg i in lpg max opt out p r rtv s she stv t
Specific exergy (kJkg-1), Exergy (kJ) Rate of energy (kW) Rate of exergy destruction (kW) Function Specific enthalpy (kJ kg-1) High pressure High temperature Low pressure Low temperature Mass flow rate (kg s-1) Pressure (kPa) Heat transfer rate (kW) Specific entropy (kJ kg-1 K-1) Solution circulation ratio Solution distribution ratio Temperature (K) Work transfer rate (kW) LiBr mass fraction
Weak Efficiency defect Effectiveness of exchanger(s) Efficiency
Acknowledgments
The support of Ministry of New and Renewable Energy (MNRE), Government of India is duly acknowledged.
Share and Cite
Arora, A.; Dixit, M.; Kaushik, S. Energy and exergy analysis of a double effect parallel flow LIBRH2O absorption refrigeration system. Journal of Thermal Engineering 2016, Vol. 2, pp. 541-549. https://doi.org/10.62051/ytu.journal-of-thermal-engineering-energy-and-exergy-analysis-of-a-double-effect-parallel-flow-librh2o-absorption-r