Thermodynamic Analysis of Combined Vapor Compression and Vapor Absorption Refrigeration System

Two of the popular refrigeration cycles, VC (Vapor Compression), and VA (Vapor Absorption) are used extensively for refrigeration purposes. In this paper, a system is proposed that works using both cycles powered by an IC (Internal Combustion) engine, where mechanical energy is used to run the VC cycle while exhaust gasses are used to operate the VA cycle. The VC cycle works on R12 refrigerant while LiBr-H2O combination is selected for operation of VA cycle. Firstly, the refrigeration system is modeled, followed by a parametric study to investigate the impacts of various operating parameters on the system performance. The results exhibit that for maximum chilling and overall performance, the condenser and evaporator pressures in the VC cycle are obtained as 710 and 340 kPa, respectively, whereas generator and absorber temperatures in VA cycle are 85 and 20oC, respectively.

are being studied for designing energy efficient systems [2]. Various solar-assisted VA and cascade refrigeration cycles have been studied with different working fluid combinations [3][4][5]. A number of researchwork is devoted to thermodynamic, economic and environmental analyses of VA refrigeration cycles [6], ejector-based cascade refrigeration cycle [7] and VC cascade cycle with different refrigerant combinations [8]. A general theoretical analysis is performed on a VC-VA cascade system with different fluids without considering the particular source of operation [9].
The literature review related to the cascade refrigeration shows a limited analysis of cascade refrigeration systems considering only components of refrigeration cycles, ignoring the prime movers which operate the systems. In this paper, a novel cascade refrigeration system is proposed that is processed for energy analysis by including a gas engine as the prime mover along with the refrigeration cycles.

THERMODYNAMIC MODELLING
In this section, thermodynamic modeling of the system is presented which is mainly based on the continuity equation and first law equation for control volumes as given below: To apply each of these equations to model different components of the system, following assumptions are made: (1) All processes are under steady-state condition.
(2) The change in kinetic energy and change in potential energy of the flowing fluids are negligible. (3) The compressor work of VC cycle is isentropic.
Chilled water inlets at 18 o C, which is theoretically been kept constant to calculate the cooling capacity of the system. Now with these assumptions, Table 1 is generated which exhibits the application of basic equations to various system components.

Performance of Combined System
The thermal efficiency of the IC engine is given as: The generator of VA cycle is extracting 50% of the heat energy provided to the engine by fuel, according to assumption 11 above. Therefore, heat received by generator is given as: CoP of the cycles is given by:   The heat absorbed by chilled water from the refrigerated space (i.e. cooling capacity) is given as:

MODEL VALIDATION
Results for heat transfers and COP of VA cycle as reported by Anand and Kumar [3], Kaushik and Arora [10], and Sarkar and Basu [11] are compared with the simulated values in order to validate the model. Fig. 2  of COP is also significant, which has a mean difference less than 0.3%.

RESULTS AND DISCUSSION
In this section, results of the parametric analysis have been discussed. The effects of condenser and evaporator pressures of VC cycle and the generator and absorber temperatures of VA cycle on the system's overall performance and cooling capacity have been investigated.

Effect of Condenser and Evaporator Pressures of VC Cycle
The effects of condenser pressure on the cooling capacity and overall performance of

Effect of Generator and Absorber Temperatures of VA Cycle
Figs. 6-7 exhibit the effects of generator and absorber temperatures on cooling capacity and overall performance of the combined system, respectively, which reflect a similar trend in variation. The cooling capacity and overall performance increase with a decrease in the absorber temperature while decrease initially then starts to grow with an increase in the generator temperature. As the reaction in the absorber is exothermic, a lower temperature increases the absorbing capacity of the absorbent for the refrigerant, which causes generation of the refrigerant in the generator at a higher rate. It results in an increase in the refrigerating effect and overall performance. Similarly, higher generator temperatures cause same effects since more refrigerant is generated in the generator. Notably, it is shown in Figs. 6-7 that both quantities decrease with increase in the generator temperature up to 105 o C and then starts to grow.

CONCLUSION
A combined VC-VA refrigeration system powered by IC engine for refrigeration applications is proposed. The VC is by far the most widely used refrigeration system due to high performance with absorption units of importance if a cheap source of thermal energy is available. The significance of the proposed model is to obtain both these benefits with the help of an IC engine whose mechanical energy is transferred directly to the VC cycle while its waste heat to the VA cycle. A thermodynamic model of the system was developed from the first law point of view. The developed model admirably predicts results available in the literature. The energy-based parametric analysis was performed to assess the effects of condenser and evaporator pressures of VC cycle and generator and absorber temperatures of VA cycle on the system cooling capacity and overall performance. According to the results, cooling capacity and overall performance increase with a decrease in the condenser pressure and increase in the evaporator pressure of VC cycle while both decrease with absorber temperature and generator temperature of VA cycle until some minimum values and increase afterward. Furthermore, the system attained optimal cooling capacity and overall performance with 570 kW and 2.83, respectively, corresponding to VC cycle's condenser and evaporator pressures of 710 and 340 kPa, respectively under the base-case operating values. Similarly, with respect to the VA cycle's generator and absorber temperatures, maximum capacity, i.e. 480 kW and overall performance, i.e. 2.4 were attained corresponding to 85 and 20 o C, respectively with base-case conditions.

NOMENCLATURE
m  Mass flow rate of fluids.
x Concentration of LiBr in LiBr-H2O solution.
T Temperature of fluids.
h Enthalpy of fluids.
Q  Heat transfer rate.
W  Work transfer rate.