Exergy analysis of a medium size LNG tanker steam power plant

Abstract

Exergy is the maximum work which can be obtained from a quantity of heat Q at temperature T1 received by a system interfacing with a surrounding environment at temperature T0. In other words from the received heat energy ‘Q’ a small portion fails to convert into useful mechanical work and is thus lost which we call exergy destroyed. The loss of exergy in this heat work exchange process lowers overall thermal efficiency of the plant. Exergy destruction is actually the result of irreversibility in various processes of the plant and can be estimated from the second law of thermodynamics as the product of ambient temperature T0 and the positive entropy change Δsi.e. (T0Δs). The common irreversibility which cause entropy rise are mechanical or hydraulic friction, heat transfer with a finite temperature difference and diffusion with a high gradient of concentration etc., as most of these are present in a physical plant. Exergy analysis gives an insight into the way energy flows in the system and helps to locate components of the plant with high irreversibility to effect design improvements. In this paper the authors conduct energy and exergy analysis of 30 MW dual fuel fired marine steam power plant of a typical medium capacity LNG tanker and identify components responsible for major exergy destruction. Effects of steam reheating to reduce irreversibility and potential utilization of the cold energy of boil off gas in the thermodynamic cycle has been studied with the result showing 2-4 % improvement in the overall exergy efficiency of the plant.