Wimolsiri Pridasawas, M. Sc., Department of Energy Technology, Royal Institute of Technology, Sweden

Per Lundqvist, Ph. D., Assoc. Prof., Department of Energy Technology, Royal Institute of Technology, Sweden

Abstract

The TRNSYS-EES simulation tool is used to simulate the characteristic of the solar- driven ejector refrigeration system with a flat-plate, double-glazed solar collector. Butane is used as a refrigerant in the cooling subsystem and water is used as a heating medium in a solar-collector subsystem. The performance of the system is shown in terms of coefficient of performance (COP) for the refrigeration subsystem and system thermal ratio (STR) for the whole system. The simulation results show the performance of the system, the annual electricity usage by the pumps and the auxiliary heater at different solar collector area, storage tank volume and water flow rate. The system performance depends on the solar radiation and the operating temperatures in the refrigeration subsystem. The STR is high when the solar radiation is high. The maximum STR that can be obtained is about 0.25 at a COP of

0. 55. The optimum solar collector area for the average cooling load 4 kW is about 50 m2. The system operates only during daytime, thus the volume of the well-mixed storage tank does not significantly affect the performance and the electricity usage of the system.

Introduction

Refrigerators and air-conditioning systems are mostly driven by electricity and account for about 15% of the world’s electricity consumption (Lucas, 1998). Solar energy can be converted to both thermal and electrical energy, both of which can be used to drive refrigeration systems. The demand for cooling is generally high when the solar radiation is high. The performance of an electricity-driven refrigeration system is quite high but it requires photovoltaic panels, which are expensive and have low efficiencies. These systems, however, can be built in small sizes, making them suitable for applications such as vaccine transportation or cooling boxes. An air-conditioning system is used to control temperature and humidity for human thermal comfort. The demand for this application is high in a densely populated area such as big cities. The solar thermal-driven refrigeration systems are more suitable for air-conditioning applications due to the lower installation cost, furthermore it can provide high cooling capacity.

A solar-driven ejector refrigeration cycle is quite a reliable and simple system. An interesting advantage that can be noticed is its ‘low temperature heat supply’ that allows it to be integrated with a simple solar collector such as a flat-plate solar collector. Furthermore, this system is easy to install, design and operate. Several research groups have studied the ejector refrigeration cycle in different perspectives but only a few of the solar-driven systems have been presented. Huang (1998), has developed a solar ejector cooling system using R141b as the refrigerant; the overall COP is about 0.22 at a generating temperature of 95°C, an evaporating temperature of 8°C, and solar radiation of 700 W m-2. Several simulation models are found in the literature of Dorantes (1996), Sokolov (1992) and Al-Khalidy (1997). Chlorinated refrigerants such as R142b (Dorantes, 1996), R114 (Sokolov, 1993) or R113 (Al-Khalidy, 1997) were recommended due to a high performance. These refrigerants, however, have negative environmental effects.

Some environmentally benign refrigerants for solar-driven ejector refrigeration systems are introduced in the literature of Pridasawas (2003) including a comparison of the technical feasibility and performance of each refrigerant.

In this paper, a TRNSYS-EES simulation tool was used to model and analyse the performance of a solar-driven ejector refrigeration system using butane as a refrigerant. TRNSYS is a transient systems simulation program with a modular structure (Klein, 2000). It is widely used for analysis of time dependent systems such as solar systems, low energy buildings and HVAC systems. The Engineering Equation Solver program or EES is generally used for solving a set of algebraic equations and initial value differential equations (Klein, 2002). It provides built-in mathematical and thermophysical property functions suitable for cycle simuations. The whole system is simulated by using TRNSYS but the model of the ejector refrigeration sub-system is developed in EES. The weather data from Bangkok, Thailand is chosen to represent the warm climate for this simulation.

The system’s performance mainly depends on the solar radiation and the operating temperature. The performance decreases in inverse proportion to the condensing temperature but it increases when the generating temperature increases. High generating temperature requires high outlet solar collector temperature but the efficiency of the solar collector decreases at the high outlet solar collector temperature. The optimum operating condition for the highest system performance should be considered. The optimum generating temperature, solar collector area and storage tank volume were studied by using TRNSYS-EES tool as mentioned above.