Using phase change materials for thermal energy storage

Abstract

The demand for electricity has increased drastically owing to population growth in South Africa. Because of urbanisation, the amount of energy used by residential and commercial buildings is increasing for water heating, space heating, or cooling. Eskom (the country’s power utility) is not coping with the demand; hence a shift to available renewable energy is needed which will alleviate the problem and also result in a clean and efficient way of generating power. Solar power, converted to electricity or used for water heating, presents a potential solution to the energy demand by the residential and commercial building sectors. The main difficulty in utilising solar energy is that its availability is not continuous, and sometimes unpredictable. In order to alleviate the peak demand time for electricity for heating and cooling, storage of thermal solar energy is a possible solution. A thermal storage unit using phase change materials (PCMs) can be used to supply energy to conventional active space heating and cooling systems at peak energy demand times. The experimental investigation focused on using a test apparatus to study the melting and solidification processes of paraffin wax as a PCM. The PCM was stored in a vertical annular space between a concentrically placed outer shell and an inner coiled tube through which water, the heat transfer fluid (HTF), was channelled. Various thermo-physical properties have been used to select PCMs due to their high energy storage capacity, availability, being economical, etc. RT25, RT27 and RT35 were selected for this project because of their melting points which are between 15 ºC and 90 ºC and thus applicable to solar energy applications. Tests were performed for the rate of heat absorbed and released for the three chosen PCMs, measuring the temperature difference across the test section over time for three different flow rates (200, 400 and 600) litres/hour of the HTF. The effect of increasing the flow rate of the HTF was a directly proportional increase to the rate of heat absorbed and released, and inversely proportional to the time taken by the three PCMs tested. The best results were obtained from RT35, with efficiencies of 77%, 82% and 85% for the three respective HTF flow rates, compared with RT27 and R25 in that order.