Thermal degradation assessment study of a direct vaporization ORC based micro-CHP system under close-to-real operating conditions

Abstract

• Most of the thermal degradation assessment methods are far from real conditions. • The thermal degradation temperature is affected by the assessment method. • Static approaches can provide over-restrict thermal degradation temperatures. • Surface temperatures don’t need to be limited by values taken from static methods. • Degradation studies must quantify the effective time over the degradation limit. Organic Rankine cycles have the potential to be implemented at a micro-scale and to fulfil the technological gap that is preventing the retrofit of the wall-hang residential combi-boilers by cogeneration systems. Such ability requires the use of high-temperature combustion gases to vaporize the organic fluid and needs to deal with the risk of its thermal degradation. Limits to the fluid bulk temperatures are well known and easily controllable, nevertheless, the relevance of the thermal boundary layer and the temperature of the heat-transfer surfaces over the thermal degradation must be considered and analyzed in depth because of the significant temperature differences between the combustion gases and the organic fluid. With the objective of understanding the thermal degradation temperature limits presented in the literature, an extensive and updated review of the most relevant works was presented. Due to the dispersity of the information regarding the thermal degradation methods and procedures found, a new framework was developed to classify them according to their key essential features: the thermal stress, the degradation assessment and the determination of the limit temperature. The output of that classification is presented together with the degradation temperature value to expose their dependency. The review also allows identifying that the large majority of the thermal degradation temperature values were determined using operating conditions that are far from reality. Complementing the review analysis, this paper offers a first insight into the study of the effect of the thermal boundary layer on its chemical integrity. This is achieved by coupling a detailed comprehensive characterization of the system operating conditions, which comprise the determination of the heat-transfer surface temperatures in contact with the organic fluid, with the realization of dynamic thermal stress tests that include the experimental evaluation of the organic fluid thermal degradation. Operating conditions that lead to heat-transfer surface temperatures well above the threshold found in the literature are considered in this work. The absence of thermal degradation in this situation may indicate that i) the current threshold should not be used to impose upper bounds on the evaporator heat-transfer surface temperature and ii) the time (and not only the temperature) has a significant effect on the thermal degradation of the organic fluid in real operating systems.