Working Fluid Selection for Low Temperature Solar Thermal Power Generation with Two-stage Collectors and Heat Storage Units

Organic Rankine Cycle (ORC) is named for its use of an organic, high molecular mass fluid that boils at a lower temperature than the water. Among many well-proven technologies, the ORC is one of the most favorable and promising ways for low-temperature applications. In comparison to water, organic fluids are advantageous when the plant runs at low temperature or low power. The ORC is scalable to smaller unit sizes and higher efficiencies during cooler ambient temperatures, immune from freezing at cold winter nighttime temperatures, and adaptable for conducting semi-attended or unattended operations [1]. Simpler and cheaper turbine can be used due to the limited volume ratio of organic fluid at the turbine outlet and inlet [2]. In the case of a dry fluid, ORC can be employed at lower temperatures without requiring superheating. This results in a practical increase in efficiency over the use of the cycle with water as the working fluid [3]. ORC can be easily modularized and utilized in conjunction with various heat sources. The success of the ORC technology is reinforced by high technological maturity of majority of its components, spurred by extensive use in refrigeration applications [4]. Moreover, electricity generation near the point of use will lead to smaller-scale power plants, and thus the ORC is particularly suitable for off-grid generation. The selection of the working fluid is of key importance in ORC applications. This is because the fluid must have not only thermophysical properties that match the application but also adequate chemical stability at the desired working temperature. There are several optimal characteristics of the working fluid: 1. Dry or isentropic fluid to avoid superheating at the turbine inlet, for the sake of an acceptable cycle efficiency; 2. Chemical stability to prevent deteriorations and decomposition at operating temperatures; 3. Non-fouling, non-corrosiveness, non-toxicity and non-flammability; 4. Good availability and low cost. However, not all the desired general requirements can be satisfied in a practical ORC. In the previous research, numerous theoretical and experimental studies have focused on ORC fluid selection with special respect to thermodynamic properties. Hung et al. studied waste