The performance investigations of subcritical ORC supercritical ORC and trilateral Rankine cycle (TLC) power plants with the heat source temperatures of 210-250°C were conducted and compared in this study. Several working fluids were evaluated. A MATLAB code was developed and used in this study. The thermodynamic properties of the working fluids were calculated by using NIST REFPROP program. The justification of the code was validated with a result taken from the literature. The optimal operating conditions were searched using the golden-section technique. The maximum net output power of 141.72 kW was obtained when using the supercritical ORC plant with R141b as its working fluid and the heat source temperature was at 250°C. The corresponding cycle efficiency was 16.25%. When using the subcritical plant at the same heat source temperature the maximum net output power of 133.40 kW and cycle efficiency of 15.70% are obtained when pentane is used as the working fluid. Furthermore the net output power of 133.82 kW and cycle efficiency of 14.90% are obtained when using R141b as the working fluid in the TLC power plant. According to the off-design simulations an appropriate adjustment of the working fluid flow rate can mitigate the variation of the net output power.

The ORC power plant is a promising technology to generate electricity from low-temperature heat sources. However implementation of the ORC for low capacity electricity generation is unattractive at the commercial level. Scroll compressors are massively produced to be used in refrigeration and air-conditioning applications. Using a scroll compressor in reverse as an expander can reduce the investment cost of low-capacity ORC power plants. In this study 2 scroll compressors that are available in the automotive air-conditioning market of Thailand were modified and used as the expander of a 1 kW ORC power plant. The performances of these 2 scrolls were compared and the plant performance was investigated. A heat source temperature from 100°C to 150°C was supplied to the plant. It was found that the gross power is higher by 18-35% for the larger expander (110 cc/rev) than for the smaller expander (85.7 cc/rev). The larger one provides the isentropic efficiency of 37-76% and the gross power of 806-1926 W. In addition the effects of check valve installation were examined and compared with those of the tests without check valve installation. It was revealed that the isentropic efficiency increases by 18% while the gross power decreases by 4-22% when the check valve was installed.