A new pinch based method for simultaneous selection of working fluid and operating conditions in an ORC (Organic Rankine Cycle) recovering waste heat

ORC (Organic Rankine Cycles) are widely used in low temperature waste heat recovery which can lead to considerable energy saving. Huge amounts of waste heat need to be cooled to the ambient temperature in refineries. The target temperature of this type of waste heat can be as low as the ambient temperature. The selection of working fluid and operating conditions exert significant influence on the performance of an ORC system. In this paper, a new method is proposed to simultaneously determine the working fluid and operating conditions in an ORC system, which recovers this type of waste heat in refineries. PPP (Preheating Pinch Point) and VPP (Vaporization Pinch Point) are newly introduced. The method is based on a newly defined parameter (PREDICTOR) that can predict the Pinch position between the waste heat carrier and the working fluid accurately, calculate the mass flow rate of working fluid and the amount of heat recovered easily, and determine the optimum working fluid and corresponding operating conditions simultaneously. Two illustrative examples are used to demonstrate the effectiveness of the proposed method. The waste heat can be recovered completely and the power output reaches the maximum with the following conditions: (1) An appropriate positive temperature difference between the waste heat inlet temperature and the working fluid critical temperature exists. (2) Working fluid evaporates in its near critical region. However, if the working fluid critical temperature is close to the waste heat inlet temperature, the working fluid evaporating in the near critical region cannot provide maximum heat recovery and power output.

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