The current work discusses means to utilize low-grade small-scale energy in vehicle exhaust gases, to reduce the vehicle's fuel consumption and to make it run more environmental friendly. To utilize the energy in the exhaust gas, a CO₂ bottoming system in the vehicle's engine system is proposed. Several basic cycles-according to the different design concepts-are presented, and the efficiencies are calculated using Engineering Equation Solver (EES).¹ Several thermodynamic models in EES show that after system optimization, in CO₂ Transcritical power cycle with a gas heater pressure of 130 bars and 200 °C expansion inlet temperature, about 20% of energy in the exhaust gas can be converted into useful work. Increasing the pressure in the gas heater to 300 bars and with same expansion inlet temperature, about 12% of exhaust gas energy can be converted. When raising the pressure both in the gas cooler and in the gas heater, the cycle runs completely above the critical point, and the efficiency is about 19%. Besides, in the CO₂ combined cycle, the system COP is 2.322 and about 5% of exhaust gas energy can be converted.

The organic rankine cycle (ORC) as a bottoming cycle1The expression "bottoming cycle" refers to the power cycle that uses waste industrial heat for power generation by supplementing heat from any fossil fuel.¹ to convert low-grade waste heat into useful work has been widely investigated for many years. The CO₂ transcritical power cycle, on the other hand, is scarcely treated in the open literature. A CO₂ transcritical power cycle (CO₂ TPC) shows a higher potential than an ORC when taking the behavior of the heat source and the heat transfer between heat source and working fluid in the main heat exchanger into account. This is mainly due to better temperature glide matching between heat source and working fluid. The CO₂ cycle also shows no pinch limitation in the heat exchanger. This study treats the performance of the CO₂ transcritical power cycle utilizing energy from low-grade waste heat to produce useful work in comparison to an ORC using R123 as working fluid. Due to the temperature gradients for the heat source and heat sink the thermodynamic mean temperature has been used as a reference temperature when comparing both cycles. The thermodynamic models have been developed in EES2EES - Engineering equation solver. The thermodynamic properties for carbon dioxide in EES are calculated by the fundamental equation of state developed by R. Span and W. Wagner, A new equation of state for carbon dioxide covering the fluid region form the triple-point temperature to 1100 K at pressures up to 800 MPa, J. Phys. Chem. Ref. Data, Vol. 25, No. 6, 1996. http://www.fchart.com/ees/ees.shtml.² The relative efficiencies have been calculated for both cycles. The results obtained show that when utilizing the low-grade waste heat with the same thermodynamic mean heat rejection temperature, a transcritical carbon dioxide power system gives a slightly higher power output than the organic rankine cycle.