Today, demand for energy has been increasing and the environment pollution has become serious with the rapid growth of economy day by day. The reality urgently requires reduction of fossil fuel consumption and wide use of renewable energy resources.
Geothermal energy, a kind of the renewable energies such as solar energy and waste heat from the industry, has been widely used for power generation as an energy source unaffected by seasons and environment. Organic Rankine cycle (ORC) is recognized as a practical solution for small power production using low grade geothermal resources thanks to its simplicity, reliability, and flexibility.
Selection of the working fluid, cycle configuration and operation parameters most suitable for given temperature of heat source is a key process to improve the performance of ORC. For this reason, many researchers have reviewed the optimal combination of ORC configurations, working fluid and cycle parameters under the conditions of heat source above 100℃ and below 100℃. However, it is difficult to identify the optimal ORC configuration, working fluid and cycle parameters for particular areas from the reported data since the best working fluid and cycle parameters vary with the temperatures of heat and cooling source and the system configuration.
Ri Hung Nam, a section head at the Faculty of Heat Engineering, has performed thermodynamic optimization of ORC based on the net work output and exergy efficiency in areas with different geothermal and cooling water temperatures.
He determined optimal ORC system configuration, best working fluid, optimal evaporation temperature and the optimum area through comparative analysis of 8 different ORC systems using 8 working fluids in the two areas. He proposed a multi-objective function combined with the net work output and exergy efficiency to evaluate the overall thermodynamic performances of ORC.
As a result, he found that even though the exergy efficiency is relatively low in the area with lower temperatures of geothermal and cooling water, the net work output is greater and the overall thermodynamic performance is more powerful. It was shown that the area where geothermal and cooling water temperatures are relatively low is the optimal one in terms of overall thermodynamic performance of ORC.
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