A Comparative Study of Cooling Sources in Organic Rankine Cycle for Low-Temperature Geothermal Heat Sources

Abstract

Geothermal energy refers to ground heat sources exploited for many purposes (for example, generating electricity). A steam power plant, that uses water as a medium and operating on the Rankine cycle, is a prospective technology that can be used to generate electricity utilizing geothermal heat. However, due to the thermal properties of the applied working fluid, the steam power plant has the limitation that may not utilize cold source like liquefied dimethyl ether (DME) at the temperature of around -25 °C or the extreme one, like liquefied natural gas (LNG) at the temperature of around -160 °C. For this reason, it seems that RC using an organic working fluid (so-called ORC) is an appropriate technology to utilize heat sources of extremely low temperatures. Selected ORC working fluids are considered suitable to absorb the cold source. This paper reported the thermodynamic modelling analysis and a comparison of geothermal power systems exploiting air, water, LNG, and DME as the cooling medium of the ORC. The simple scheme of ORC is used for modelling, furthermore, the Trilateral Flash Cycle (TFC) is described as a comparative study. Thanks to the LNG technology, it is already mature enough, and the method of altering the phase into liquefied form is likely to store the energy (i.e., the power to liquid). In the liquefied form, natural gas can be easily distributed and transported at a certain distance. In this model system, the result shows that LNG and DME appear to be excellent options for increasing the operating range of ORC. The modelling result shows that combining propane (a working fluid inside the cycle) and LNG (a cooling source) has a wider operating range and is a good option to exploit low-temperature geothermal heat as a power generation system. Also, using DME for both cooling sources and the working fluid inside the geothermal power system employing ORC outperforms other ones. Taking advantage of using them as a cooling source is significantly boosting the potential deriving from low-temperature geothermal energy (i.e., below 90 °C) as promising sources in the future.