Comparison of mini Organic Rankine Cycle plants for waste heat recovery Free

Improving energy efficiency in every sector (e.g., building, industrial, transport) is a priority to reduce the effects of the human impact on the environment. Among the techniques which can be applied, waste heat recovery has received growing interest in the last few decades. Normally, waste heat can be utilized for power production or process heating. In the first case, recovery plants based on Organic Rankine Cycles (ORCs) are a common solution. Even though ORC systems are considered a consolidated technology, their application is not always economically convenient, especially for small- and medium-scale plants. This is due to the low temperature of the heat source made available to the ORC system, which inevitably implies a low thermodynamic cycle efficiency. Therefore, given a reference thermal source available for the ORC plant, particular attention has to be paid to the layout arrangement, in particular to the selection of some key components among which there is the expander. The paper deals with the comparative analysis of three small-scale plant layouts designed for the same reference waste heat source. In particular, three different expanders have been taken into consideration for electrical power production <100 kWe, i.e., 1) one-stage radial turbine, 2) twin-screw expander, and 3) two-stage radial turbine. Each expander imposes several constraints on the cycle and, consequently, the overall ORC plant performance is affected by the preliminary choice of such a machine. The three cases have been preliminarly designed and comparatively analyzed. Namely, they were compared from the view point of economic performance, to highlight potential trade-offs between improved thermodynamic efficiency and resulting costs. Moreover, a life cycle analysis completed the comparison by evaluating the overall environmental impact of the three cases in order to establish their benefits and drawbacks in terms of sustainable electricity production.