Experimental Study of Thermodynamic Assessment of a Small Scale Solar Thermal System.

Abstract

In this study, a scaled solar thermal system, which utilises HFE 7000, an environmentally friendly organic fluid has been designed, commissioned and tested to investigate the system performance. The proposed system comprises a flat-plate solar energy collector, a rotary vane expander, a brazed type water-cooled condenser, a pump and a heat recovery unit. In the experimental system, the flat-plate collector is employed to convert HFE-7000 into high temperature superheated vapour, which is then used to drive the rotary vane expander, as well as to generate mechanical work. Furthermore, a heat recovery unit is employed to utilise the condensation heat. This heat recovery unit consists of a domestic hot water tank which is connected to the condenser. Energy and exergy analysis have been conducted to assess the thermodynamic performance of the system. It has been found that the collector can transfer 3564.2 W heat to the working fluid (HFE 7000) which accounts for the 57.53% of the total energy on the collector surface. The rotary vane expander generates 146.74 W mechanical work with an isentropic efficiency of 58.66%. In the heat recovery unit, 23.2% of the total rejected heat (3406.48 W) from the condenser is recovered in the hot water tank and it is harnessed to heat the water temperature in the domestic hot water tank up to 22.41 ºC which subsequently will be utilised for secondary applications. The net work output and the first law efficiency of the solar ORC is found to be 135.96 W and 3.81% respectively. Exergy analysis demonstrates that the most exergy destruction rate takes place in the flat plate collector (431 W), which is the thermal source of the system. Post collector, it is followed by the expander (95 W), the condenser (32.3 W) and the pump (3.8 W) respectively. Exergy analysis results also show that the second law efficiency of the solar ORC is 17.8% at reference temperature of 15 ºC. Parametric study analysis reveals that both increase in the expander inlet pressure and the degree of superheat enhances the thermodynamic performance of the solar ORC.