Avoiding malfunction of ORC-based systems for heat recovery from internal combustion engines under multiple operation conditions

Abstract

A large number of electric generators driven by Internal Combustion Engines (ICEs) are in operation in developing countries where the national electric energy transport infrastructure is poor. An ORC-based system is considered here for heat recovery from such ICEs which in practice operate under partial load and in time-dependent conditions. A critical aspect of the transient operation is the possibility for the working fluid to enter the expander of the ORC-based system as wet vapor. The paper focuses on a control strategy to avoid such situations. A time-dependent model for the operation of ORC-based systems is developed. Measurements obtained on an experimental setup consisting of a 37 kW diesel fueled ICE and a heat recovery ORC-based system are used. The transient model allows to emphasizing the thermal inertia of the ORC-based system. For instance, decreasing the heat flux at the evaporator and keeping constant the working fluid mass flow rate needs to shut down the heat recovery system in a certain amount of time (33 s for the case studied here). There are two ways of keeping safe transient operation, i.e. controlling the mass flow rate of the cooling water at condenser or controlling the mass flow rate of the working fluid. The strategy has been used to control the operation of the ORC-based system for a specific time-dependent operation of the ICE. The controlled mass flow rate of the working fluid and the output power mimic the time variation of the waste heat flux entering the evaporator while the thermal efficiency of the ORC-based system is almost constant in time.