Off-design comparison of subcritical and partial evaporating ORCs in quasi-steady state annual simulations

Abstract

The subcritical ORC (SCORC) is considered the industry standard due to its simple configuration, acceptable efficiency and low cost s. However , it is known that alternative ORC configurations have the potential to increase efficiency. A cycle modification which closely resembles the SCORC is the partial evap orati ng ORC (PEORC), where a two -phase mixture of liquid -vapour enters the expander instead of superheated vapour. In theoretical studies at design conditions , higher power outputs are achieved for the PEORC compared to the SCORC. This work aims to go a step further by investigating the performance of the SCORC and PEORC under time -dependent operating conditions. A direct comparison between the SCORC and PEORC is made for identically sized systems using as input the waste heat stream of a waste incinerator plant and the changing ambient conditions . Performance maps of both cycle configurations are compiled and the benefit of an expander operating at variable speed is briefly discussed . The results indicate that for the specific case under investigati on, the PEORC has an increased annual ly averaged net power output of 9.6% compared to the SCORC. Use of annually averaged input conditions result s in an overestimation of the net power output for both the SCORC and PEORC, and furthermore , the relative improvement in power output for the PEORC is reduced to 6.8%. As such, the use of time -averaged conditions when comparing cycle architectures should preferably be avoided.