Modeling and Simulation of Solar-Assisted Absorption Cooling System

Teclemariam G. Nemariam, Royal Institute of Technology, Dept. of Energy Technology, Div. of Applied Thermodynamics and refrigeration, Brinellvagen 60, SE-100 44 Stockholm Prof. Per Lundqvist, Royal Institute of Technology, Dept. of Energy Technology, Div. of Applied Thermodynamics and refrigeration, Brinellvagen 60, SE-100 44 Stockholm

Abstract

In this paper an analytical study is performed on solar energy utilization in space cooling of a building using a solar driven single-effect absorption refrigeration system. It is modeled with a transient modeling tool TRNSYS, Transient Simulation Program. The main components of the system are, solar collector, hot water storage tank, auxiliary heater, absorption chiller, and different components of building. Two types of solar collectors: double-glazed and evacuated tube collectors are used in this study. Two different locations: Assab, Eritrea and Nicosia, Cyprus are chosen to see how system performance and efficient vary.

The effect of collector area and hot water storage tank volume on the solar energy extraction is studied and discussed. The effect of hot water and cooling water temperatures on the performance of the absorption machine is studied.

The effect of sizes of insulation thickness, shading devices, overhang and wing wall on cooling load of the building is calculated and discussed.

The highest system efficiency is obtained when evacuated tube solar collector is used in both locations, but is higher for Assab for a given collector area. The generator inlet water temperatures for Assab are 86°C and 85°C when evacuated and double-glazed collectors are used respectively, while for Nicosia they are 86°C and 83°C. The yearly cooling load for Assab is 265 MJ and for Nicosia it is 78.6 MJ. The highest cooling load for both locations is obtained during July and for the non-insulated building it is 30.5 MJ for Assab and 16.02 MJ for Nicosia. The lowest cooling load is obtained when 0.2 m insulation thickness and 1.5 m overhang and wing wall is added. The cooling load reduces 34% for Assab and 25% for Nicosia in the first addition of 0.05 m insulation thickness.

Introduction

Electricity and some natural gas are the common energy sources for air conditioning systems. Alternative energy sources are needed for near future since the demand of air conditioning and the cost of energy is increasing. Solar energy is one of the possible alternative energy sources for cooling systems and one of the advantages of using solar energy, as energy source is that the maximum energy is obtained when the cooling load is at its peak.

Absorption refrigeration system is one of the available technologies, which use solar energy as heat source. Most solar-powered absorption cooling projects to date have used single-effect lithium bromide absorption systems. Single-effect absorption system gives best results in the temperature range of 80 to 100oC and limited in COP to about 0.6 to 0.8 [1]. If one considers an absorption refrigeration application system to include the solar collectors, storage tank auxiliary heater, building pumps, piping systems etc, it is not only the absorption chiller, solar collectors and other components which must minimize energy usage, but the cooling load of the building must also reduce as much as possible.

Constructing a hardware model of a building cooling system and performing a test in order to obtain all parameters that are needed for a complete design of a system consumes more time and money compared to that of computer modeling and
simulation. Many researchers have studied and modeled different solar assisted air conditioning systems. Comparison has been made between conventional and solar cooling systems [2], between solar assisted single-effect, double-effect and triple­effect cooling systems [3], between flat plate solar collector, evacuated tube collector and compound parabolic collector [4], and between cooling systems of various combinations of solar collectors and absorption cycles [1]. Most of them were interested in energy supply and cooling systems and the application of each model is limited to a particular condition. The cooling load minimization has not been taken into consideration.

The purpose of this work is to model and simulate a solar assisted air conditioning system for two locations; Assab, Eritrea and Nicosia, Cyprus and comparison has made in terms of optimum collector slope, solar fraction, system efficiency, hot water inlet temperature and cooling water temperature. In addition, the cooling load of the building is calculated with and without insulation, overhang and wing walls. The system is modeled for one year using a TRNSYS program together with meteorological weather data of both locations. System performance of two different collector models: evacuated solar collector and double-glazed selective surface flat plate collector has been done. The optimum system efficiency and solar fraction of each system is calculated and compared based on the appropriate area and slope of collector, size of storage tank, insulation thickness, overhang and wing walls.

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