Waste heat recovery using (s-CO2) power cycle - applications for maritime industry

Abstract

The predominant source of power in a ship is the diesel engine which has evolved as a highly efficient means of generating necessary power for propulsion and auxiliary uses. However, only less than 50% of the fuel energy is transformed into useful work the rest being losses. It is widely recognized that about 30% of the total energy converted in a Diesel engine is rejected in the exhaust gas. The recently mandated EEDI [1] system for large ships gives credit to ship design for any recoverable energy. While some of the energy saving devices being contemplated, use wind and solar power, it is being recognized that waste heat recovery from the engine exhaust gases and cooling water can still be tapped to generate power resulting in improved energy efficiency of the plant. One of the ways of recovering heat energy from exhaust gas is to transfer the heat to a medium from which the energy can be recovered. On large ships the medium used is water and steam thus produced is used to heat fuel oil or for electrical energy production through a turbine. In this paper an alternate fluid (supercritical carbon dioxide) is presented as a means for recovering energy through a closed loop gas turbine cycle (Brayton Cycle) It operates significantly at lower temperatures and is non-corrosive, non-toxic, non-flammable and thermally stable. In supercritical state, the s-CO2 has a high density which results in reducing the size of the components such as the turbine. Supercritical CO2 gas turbine can generate power at a high cycle thermal efficiency even at modest temperatures of 550oC. The cycle can operate at wide range of pressures 20 to 80MPa. A case study of the amount of energy recoverable from the exhaust gas of a typical engine installed in an offshore supply vessel is presented along with theoretical calculations for the heat carried out by the exhaust gas and extraction of power which could be generated by the supercritical CO2 gas turbine plant from the engine