Numerical Simulation of the Performance of a Solar Assisted Vapour Compression Refrigeration System

: Research has established that a considerable part of the electrical energy produced globally is been consumed by refrigerators and air-conditioning systems. In this area of cooling technology, research is therefore being geared towards energy reduction in the cooling devices. In addition to this approach, alternative sources of energy such as renewable energy, like solar is also being explored in this area of refrigeration and cooling technology. Studies have also been conducted generally both experimentally and numerically to simulate the performance of Vapor Compression Refrigeration System (VCRS) under different conditions. However, experimental study is often seems to be expensive and time consuming to carry out due the function of many variables. This study was therefore designed to numerically simulate the performance assessment of a solar assisted VCRS. A numerical model of a solar assisted vapour compression refrigeration system was developed using standard solar energy and thermodynamics relations of the major components of solar assisted refrigerating system such as solar power system, compressor, condenser, evaporator and an expansion valve to determine refrigeration effect, compressor work, and the Coefficient Of Performance (COP). Standard data of R134a refrigerant was utilized in the modelling. The model was then simulated on MATLAB source code with a CoolProp installed packages via python under two different simulation cases. In the first case, the evaporating temperature was varied for all while the condensing temperature was kept constant and a reversed condition was investigated in the second case.The results showed that both the refrigerating effect and the COP of the system increase as the evaporating temperature increased while the compressor work and the panel area decreased. Further, the refrigerating effect and the COP of the system were decreased as the condensing temperature was increased while the compressor work and the panel area remained constant. The coefficient of performance at evaporating temperature of -17.2 shows 2.91 giving a deviation of -5.37% from the literature.The viability of a solar assisted vapour compression refrigeration system was established. The vapour compression refrigeration system compressor powered with solar energy will perform well when the solar panel (PV) area is large.


INTRODUCTION
According to literatures, the most commonly used system in refrigeration and air-conditioning industry is vapor compression refrigeration system. A considerable part of the energy produced worldwide is consumed by refrigerators and it is crucial to manage the energy utilization of these devices and also provide cheap alternative source energy to power them especially in the case of a country that has a problem of epileptic power supply. In order to achieve this, cheap alternative sources of energy such as renewable energy sources like solar could be harnessed to power the compressor of a vapor compression refrigeration system. The refrigerant vapour at low temperature and low pressure enters the compressor (state 1) where it is compressed isentropically to high temperature and high pressure. The energy required by the compressor to carry out this activity is provided by the photovoltaic (PV) panel. The higher this amount of energy, the higher the PV panel area. The high temperature refrigerant vapor then enters the condenser (state 2) where it is condensed to high pressure liquid. The high pressure liquid refrigerant then enters into expansion device (state 3) where its pressure is decreased. Ultimately the liquid refrigerant will attain lower temperature. This low temperature liquid refrigerant absorbs the latent heat from the evaporator (state 4) undergoes phase change to vapor state. This high temperature vapor refrigerant then sucked by the compressor and the cycle continues which keeps the temperature in the evaporator much below the atmospheric temperature. In some cases flash chamber is used after the expansion device which separates the vapor refrigerant from the liquid and sends it to compressor inlet.
From the previously published works, it has been observed that the simulation and performance assessment of vapor compression refrigeration system has been carried out by many researchers with little attention on alternative source of energy to power the compressor. To my best knowledge, there is a dearth of research work on the combined simulation and performance assessment of a solar assisted vapor compression refrigeration system. This justifies the need for this study. Thus, the present study deals with the numerical simulation and performance assessment of a solar assisted vapor compression refrigeration system. The simulation will be carried out based on the specified system constraints such as the working conditions of the system. Each component parameters will be simulated and then combined to give the overall system. Further, the coefficient of performance of the system will be computed, the condensing and evaporating temperatures will be varied so as to observe the effects on the system properties and performance.

LITERATURE REVIEW
Reduction of energy consumption for refrigeration, however, cannot be relied solely on the improvement of efficiency. As cooling demand increases with the intensity of solar radiation, solar refrigeration has been considered as a logical solution. Therefore, a variety of solar refrigeration technologies have been developed and many of them are now available in the market at much cheaper prices than ever. This development has prompted scientists and engineers to make more research efforts on energy consumption and performance analysis of vapor compression refrigeration systems. Mehmet (2011) investigated a solar electric-vapor compression refrigeration (SE-VCR) system. In the study, the hourly variations of various parameters such as coefficient of the performance, condenser capacity and compressor power consumption were calculated. In addition, the minimum photovoltaic panel surface area was determined to meet the compressor power demand according to the hourly average solar radiation data. For evaporating temperature of 0 0 C, the maximum compressor power consumption was obtained as 2.53 kW at 15:00 PM on August 23. The required photovoltaic panel surface area was found to be around 31.26 m2. It was determined that the SE-VCR system could be used for home/office-cooling purposes during the day in the southern region of Turkey. Thangavel and Somansundaram (2013) studied on Part load performance analysis of vapour compression refrigeration system with hydrocarbon refrigerants. In this work, various input parameters like compressor input power, discharge temperature, heat rejected in the condenser, refrigeration effect and coefficient of performance were analyzed and presented at various loads and 2,4,6,8,10,12,14 kg of water were used in the evaporator. As compared to other loads, 10 kg of water gives economic mode to operate the system. Mishra (2014) investigated the methods of improving thermodynamic performance of vapour compression system using twelve eco-friendly refrigerants in primary circuit and nanofluid (water-nano particles based) in secondary circuit. The author pointed out that the COP of vapour compression refrigeration system can be improved either by increasing refrigeration effect or by reducing work input given to the system. Domanski and Didion (1994) studied Evaluation of suction-line/liquid-line heat exchange in the refrigeration cycle .The author discovered that adding extra internal heat exchanger in a single-stage cycles prior to expansion process sub cools the liquid refrigerant which consequently improves performance of the system. Memet (2014) investigated the performance analysis of a vapor compression refrigeration cycle working with R134a and RE170. The motivation of this comparison was based on the efforts done to solve the problem of high GWP of R134a, since RE170 shows a significantly lower one. The study was developed on the analysis of the effect of evaporation pressure on some of important factors which should be considered in the selection of a new refrigerant: evaporation pressure, pressure ratio, Coefficient of Performance, power per ton of refrigeration, volumetric cooling capacity. The evaporation temperature varied in the range (−20, +10) 0 C and the condensation temperature is kept constant, at 45oC. Since cycle performance can be improved by superheating and sub cooling inclusion, the two processes were considered in the cycle. Comparative results showed that RE170 could replace R134a, due to its low evaporation pressure and pressure ratio and also to its better COP and similar volumetric cooling capacity Vincenzo and Giuseppe (2011), have analyzed and compared the performance of a vapour compression refrigerating unit operating with R22, and with three new HFC fluids, substituting the former according to Regulation No 2037/2000. Here the plant working efficiency was first tested with R22 and then with three new HFC fluids; R417a, R422a and R422d. It is analyzed that the performance with the new tested fluids did not result as efficient as when using R22. Akhilesh and Kaushik (2008) elucidated on a detailed exergy analysis of an actual vapour compression refrigeration (VCR) cycle. A computational model was developed for computing coefficient of performance (COP), exergy destruction, exergetic efficiency and efficiency defects for R502, R404A and R507A. The investigation was done for evaporator and condenser temperatures in the range of −50 °C to 0 °C and 40 °C to 55 °C, respectively. The results indicated that R507A is a better substitute to R502 than R404A. The efficiency defect in condenser is highest, and lowest in liquid vapour heat exchanger for the refrigerants considered. Wang et al. (2012) studied a solar-assisted cascade refrigeration cycle system. The system consists of electricity-driven vapor compression refrigeration system and solardriven vapor absorption refrigeration system. The vapor compression refrigeration system is connected in series with vapor absorption refrigeration system. Refrigerant and solution reservoirs are designed to store potential to keep the system operating continuously without sunlight. The results indicated that the system obtains pretty higher COP as compared with the conventional vapor compression refrigeration system. COP of the new-type vapor compression refrigeration system increases as sunlight becomes intense. Dalkilic and Wongwises (2010), have studied the performance of a vapour Compression refrigeration system with refrigerant mixtures based on R134a, R152a, R32, R290, R1270, R600 and R600a was done for various ratios and their results were compared with R12, R22 and R134a as possible alternative replacements. The results showed that all of the alternative refrigerants investigated in the analysis have a slightly lower COP than R12, R22, and R134a for the condensation temperature of 50 °C and evaporating temperatures ranging between −30 °C and 10 °C. Refrigerant blends of R290/R600a (40/60 by wt. %) instead of R12 and R290/R1270 (20/80 by wt. %) instead of R22 are found to be replacement refrigerants among other alternatives. Praveen and Simhadri (2018) experimentally investigated the performance of HFC-134a-HC-600a and hydro carbon blends (propane/isobutane) using solar driven vapour compression refrigeration system. The author made it known that an attempt has been made to conduct the experimental investigation of HC and HFC blends to replace the R134a to reduce the global warming potential. The various parameters such as cop, mass flow rate, compressor work net refrigeration effect were calculated. The results showed that the of COP of R134a is better than R290/600a but the global warming potential of R290/600a is lower than that of R134a, hence, it is more environmental friendly than R134a. Further, the solar driven system minimize the energy crisis and maintain room cooling during day and night. The author concluded that solar refrigeration can be expected to play a significant role in meeting the needs of people in the rural areas of developing countries for refrigeration. In spite of the extensive number of studies on the simulation performance evaluation of vapor compression refrigeration system, little has been done yet on the simulation of the vapour compression refrigeration system using solar energy source both theoretically and experimentally. Thus, the present study deals with the numerical simulation and performance assessment of a solar assisted vapour compression refrigeration system (VCRS).

NUMERICAL ANALYSIS
The numerical simulation model of a vapour compression refrigeration system utilised the fundamental equations and many thermodynamic relations. It is simulated on the MATLAB platform.
The following assumptions are made for analysis of the system. 1. Clearance ratio of the compressor is 5% 2. Evaporator temperature ranges from -17.2 0 C to 32.8 0 C 3. Condenser temperature ranges from 45.3 0 C to 95.  is the efficiency of the solar panel. In the solarelectricity system, compressor power consumption (W) is aimed to be covered by the electricity (Wpv) produced from solar (i.e. photovoltaic) panels. On the other hand, sufficient photovoltaic panels depending on their properties should be provided to meet the whole energy demand of the system. The necessary total photovoltaic panel area of the system should be determined to cover daily power consumption of the compressor. Therefore, daily total power consumption of the compressor T W (kWh/day), daily total solar radiation T I (kWh/m2day) and the photovoltaic area A(m2) are related by the following In this work average solar irradiation of Nigeria as 5.5kwhr/m2/day (Ojosu, 1990) efficiency of the solar panel as 20% and the number of hours of use of the refrigeration system to be 12 hours per day were used. Due to the limited number of studies regarding the experimental study of Solar assisted vapor compression refrigeration system, Praveen and Simhadri (2018)'s experimental database will be used in the validation process of the proposed model.
The following thermodynamics relations were also used in this work: 1) Refrigeration Effect RE = h1 -h4 2) Work done by the compressor, Wc = h2 -h1 3) Coefficient of Performance of the system COP = (h1 -h4) / ( h2 -h1) Where, h1= enthalpy of refrigerant at the outlet of evaporator h2= enthalpy of refrigerant at the inlet of condenser h3= enthalpy of refrigerant at the outlet of condenser h4= enthalpy of refrigerant at the inlet of evaporator