Abstract: In order to improve the thermal performance, reduce the irreversible loss and make full use of the low grade heat of the organic Rankine cycle (ORC) system, from reducing the temperature difference of the heat transfer into consideration, based on the first and second rules of thermodynamics, the new type ORC-bypass flash system is put forward and the thermodynamic model is established which regards the 363-463 K water as the heat source. The analysis and comparison with evaporation temperature under different heat source temperature of two type systems and the influence of fluids have been made by written the calculation program. When the heat source is not enough to make the refrigerant evaporates in the near critical temperatures, the bypass flash system can improve the performance, the exergy efficiency increase by 10.51%, 8.9% and 7.07% compare to the ORC system under the heat source of 363 K, 393 K and 423 K; At the same time, it is helpful in reducing the effect of working fluids on performance and which behaves similar to each other; but the optimum fluid changes with the heat source’s temperature, if the fluid is selected whose critical temperature less than the heat source about 15 K can make the system obtain the maximum performance.

Key words: bypass to flash, critical temperature, geothermal water, organic Rankine cycle(ORC), thermal performance