Effect of air temperature variation on the performance of wet vapour organic rankine cycle systems

A multi-variable optimization program has been developed to investigate the performance of Wet Organic Rankine Cycles (WORC) for generating power from low temperature liquid dominated brines. This cycle model contains a detailed thermodynamic model of a twin-screw expander, and the methods used to match the operation of the expander to the requirements of the cycle are described. Optimum operating conditions are calculated for a particular design point, which specifies the required size of heat exchangers and the port geometry and operating speed of the expander. Performance at off-design conditions can then be optimized within these constraints. This allows a rigorous investigation of the effect of air temperature variation on performance of WORC systems. The capability of the cycle model has been demonstrated for the case of power generation from a brine heat source at 120°C, assuming typical air temperature conditions for Nevada, USA. There are two main findings from the paper. Firstly, optimization of the WORC system using the annual average air temperature of 10.5°C achieves maximum power output with 75% dry working fluid at the inlet to the expander. Secondly, analysis of the off-design performance of the system shows that positive net power output is possible for air temperatures up to around 40°C. The estimated average power output over the course of a year was only 3.4% smaller than the power generated at the average annual temperature of 10.5°C. This confirms that a single calculation of WORC system performance using the average temperature for the region gives a good estimate of the expected average annual power output of the system. For the resource conditions assumed, screw driven WORC systems can be built with net power outputs of the order of 600kW, using standard size machines