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 kW 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 kW of amplitude and not slower than 0.003 Hz
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.
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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 he...
»
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