Modelling and experimental validation of the heat-transfer processes of a direct vaporization micro-scale ORC-evaporator for thermal degradation risk assessment

Abstract

Demanding size and response time requirements imposed to ORC based micro-CHP systems, attempting to retrofit wall-hang domestic combi-boilers, suggests that the vaporization of the organic fluid should be done directly by the combustion gases. The evaluation of the risk that this option may put to the organic fluid thermal degradation requires the determination of the temperature of the heat-transfer surfaces with which the organic fluid is in contact. As its experimental measure is extremely difficult to accomplish, such determination requires the development of a detailed physical model of the combustion and heat-transfer processes in the ORC-evaporator.The development, calibration and validation of such model is presented in this paper. This model will allow a detailed evaluation of several key features of the combustion and heat-transfer processes as a function of some ORC operating parameters. Among those features is the temperature of the internal surface of the tubes with which the organic fluid is in contact. This temperature, which can be used to assess the risk of the thermal degradation of the organic fluid, has shown to be highly affected by the thermal resistances and by the combustion gases temperature. To reduce that risk, the operating conditions of the ORC should be those allowing the vaporization process to start as early as possible and reducing the superheating phase to the minimum possible.