Direct vs indirect evaporation in Organic Rankine Cycle (ORC) systems: A comparison of the dynamic behavior for waste heat recovery of engine exhaust

Abstract

Abstract Organic Rankine Cycle (ORC) is a prominent technology for the recovery of waste heat from internal combustion (IC) engines, in particular exhaust waste heat. An important challenge with IC engines, specially mobile, is the highly dynamic conditions and thus high variability of the waste heat thermal power which can lead to chemical decomposition of the ORC fluid or expander damage due to liquid droplets. The heat from the exhaust can be transferred to the ORC working fluid directly in one heat exchanger unit or indirectly through a heat transfer fluid. Compared to indirect evaporation, direct evaporation poses a higher risk to the integrity of the system due to a lower thermal damping capability. However, direct evaporation, is an attractive option in mobile applications due to its considerable lower footprint and potential of higher thermal efficiencies. In this paper, a methodological comparison of the dynamics of the two evaporation options in ORC is presented. The dynamic behavior of an indirect as well as two different direct evaporation options are simulated in a case study of exhaust waste heat recovery from a 240\u202fkW Diesel Engine. The expected variability of the heat source is broken-down to relevant frequencies and amplitudes of fluctuation based on a standard engine transient cycle. The geometry of the heat exchangers for direct evaporation is chosen based on a methodology to achieve a desired thermal inertia. The results show that a non-conventional direct evaporator designed for high thermal inertia can protect the fluid from fluctuations of up to 20\u202fkW of amplitude and not slower than 0.003\u202fHz even without any control measure in place. The weight and volume is reduced by 88% and 70% respectively compared to indirect evaporation structure. Such a design can enable direct evaporation by avoiding the requirement for a very fast-acting control system, while still allowing for the advantages of direct evaporation such as reduced footprint and potential for a higher thermal efficiency.