Inverted Brayton Cycle With Exhaust Gas Condensation

Abstract

Approximately 30% of the energy from an internal combustion engine is rejected as heat in the exhaust gases. An inverted Brayton cycle (IBC) is one potential means of recovering some of this energy, in order to improve the overall system efficiency. When a fuel is burnt, water and CO2 are produced and expelled as part of the exhaust gases. In an IBC, in order to reduce compression work, the exhaust gases are cooled before compression up to ambient pressure. If coolant with a low enough temperature is available, it is possible to condense some of the water out of the exhaust gases, further reducing compressor work. In this study the condensation of exhaust gas water is studied. The results show that the IBC can produce an improvement of approximately 5% in BSFC at the baseline conditions chosen and for a compressor inlet temperature of 310 K. The main factors that influence the power output are heat exchanger pressure drop, turbine expansion ratio, coolant temperature and turbine inlet temperature. A lower coolant temperature significantly increases power output, particularly when condensation occurs. Larger turbine expansion ratios produce more power and slightly lower the temperature at which condensation onset occurs. The system is very sensitive to heat exchanger pressure drop, as larger pressure drops increase the compressor pressure ratio whilst leaving the turbine expansion ratio unchanged. Higher turbine inlet pressures can also increase net power, but the higher exhaust backpressures may increase engine pumping losses. Finally, for conditions when condensation is possible, the water content of the exhaust gas has a significant influence on power output. The hydrogen to carbon ratio of the fuel has the most potential to vary the water content and hence the power generated by the system. If there is no condensation, water content has a small impact on performance. The effect on power in the condensing region is predominantly due to reduced mass flow in the compressor.

Techno-economic analysis of waste heat recovery by inverted Brayton cycle applied to an LNG-fuelled transport truck

“…But only recently this concept got a new wave of attention in the scientific literature. In particular, the application of IBC for the reciprocating engine bottoming is studied by Kennedy et al [12] and Di Battista et al [13] for light vehicles, or Di Battista et al [14] for heavy vehicles whose models show good results from thermodynamic point of view. Kennedy et al report about 5% decrease in brake specific fuel consumption.…”

“…The principle of operation of the IBC is described in details in [12]. Briefly, the hot exhaust gas leaving its source (reciprocating engine in the studied case) comes to the turbine of IBC with near atmospheric pressure.…”