Abstract
Waste heat recovery of the internal combustion engine (ICE) has attracted much attention, and the supercritical carbon dioxide (S-CO2) cycle was considered as a promising technology. In this paper, a comparison of four S-CO2 cycles for waste heat recovery from the ICE was presented. Improving the exhaust heat recovery ratio and cycle thermal efficiency were significant to the net output power. A discussion about four different cycles with different design parameters was conducted, along with a thermodynamic performance. The results showed that choosing an appropriate inlet pressure of the compressor could achieve the maximum exhaust heat recovery ratio, and the pressure increased with the rising of the turbine inlet pressure and compressor inlet temperature. The maximum exhaust heat recovery ratio for recuperation and pre-compression of the S-CO2 cycle were achieved at 7.65 Mpa and 5.8 MPa, respectively. For the split-flow recompression cycle, thermal efficiency first increased with the increasing of the split ratio (SR), then decreased with a further increase of the SR, but the exhaust heat recovery ratio showed a sustained downward trend with the increase of the SR. For the split-flow expansion cycle, the optimal SR was 0.43 when the thermal efficiency and exhaust heat recovery ratio achieved the maximum. The highest recovery ratio was 24.75% for the split-flow expansion cycle when the total output power, which is the sum of the ICE power output and turbine mechanical power output, increased 15.3%. The thermal performance of the split-flow expansion cycle was the best compared to the other three cycles.
Highlights
The internal combustion engine (ICE) has become a primary power source which has been widely applied in vehicles, industrial machineries, agricultural machineries and stationary power units [1].Improving the total thermal efficiency of the ICE has been widely researched since last century to reduce fossil fuel consumption and CO2 emissions
Compressor inlet pressure could be selected to achieve the highest exhaust heat recovery ratio, which critical pressure and increases with the rising of the turbine inlet pressure
For the recuperation of the supercritical CO2 (S-CO2) cycle considered in this study, there is the highest thermal efficiency and exhaust heat recovery ratio at a compressor inlet pressure which is slightly higher than the critical pressure
Summary
The internal combustion engine (ICE) has become a primary power source which has been widely applied in vehicles, industrial machineries, agricultural machineries and stationary power units [1]. Song et al [11] combined an ORC with the S-CO2 cycle for heat recovery to utilize the residual heat, leading to increased thermal efficiency. Various cycle layouts have been designed and researched to reduce the internal irreversible losses in the recuperator and increase thermal efficiency. The performance of single recuperated and recompression S-CO2 cycles for recovering low temperature waste gas heat was discussed by Mohagheghi [15]. The exhaust heat recovery ratio and cycle thermal efficiency were very important to the net output power. A discussion on the cycle design parameters for four different cycles was conducted, and the major influencing parameters on the thermal efficiency and exhaust heat recovery ratio of each cycle layout were analyzed
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