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Publication Title | Waste Heat Recovery Using (s-CO2) Power Cycle -Applications for Maritime Industry

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Waste Heat Recovery Using (s-CO2) Power Cycle -Applications for Maritime Industry

Mr.U.S.Ramesh

Chief manager at IMU Visakhapatnam campus,India

Mr.Y.Mahesh Babu

Scientist-B, at IMU Visakhapatnam campus,India

Mrs.T.Kalyani

Scientist-B, at IMU Visakhapatnam campus,India

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

Key words: Waste heat, s-CO2, water, Brayton cycle. I. INTRODUCTION

Most of the ships today use diesel engines for propulsion and for producing electrical power. The points of heat rejection form diesel engines which are normally considered to have practical potential for waste heat

recovery are the exhaust and the jacket coolant. Heat is usually recovered from the exhaust gas of a main propulsion engine of large ocean liners in the form of steam as it is the most preferred medium for fuel and cargo heating including heating required for domestic services. Heat from jacket cooling water is usually recovered in the form of fresh water generation. Waste heat recovery from auxiliary engines, till recently, was not considered economical and practical except in case of large passenger ships or ships operating with Diesel Electric propulsion system. The debate at the IMO and other international forums on the green house effect of emissions from shipping has changed the outlook of many with respect to looking at various options for enhanced waste heat recovery as a means of improving the overall efficiency of the ship .

In small ships such as the offshore supply and support vessels, the luxury of having space for such mechanisms is generally not available. Other ships generally operating in coastal trade do not generally use fuels which require heating due to their trading pattern hence do not recover energy from exhaust systems. It has been estimated that the global consumption of fuel on coastal ships alone is around 80 million tonnes in the year 2007[1]. Out of this the offshore vessel segment alone contributed around 6.6 million tonnes. The fuel consumption from propulsion engines contributed to about 4.5 million tonnes the rest being from Auxiliary engines [2]. It is also well known that the amount of CO2 released into the atmosphere is approximately 3.1 times the amount of fuel burnt. Therefore any energy recovered from exhaust gas will result in reduced fuel consumption and result in reduced emissions. This itself may be considered as an incentive to develop WHR systems which have high energy outputs with low footprint in terms of size and weight so that they can be installed on ships serving the offshore sector like supply vessels.

One of the means of recovery of heat from exhaust gases is to use a fluid in supercritical range. The use of supercritical fluids is not new in the power generation industry. Most of the thermal power plants use steam in supercritical stage (23.5 to 38 Mpa) to increase the thermal efficiency of the plant.Similarly nuclear power industry also uses the supercritical water-steam for generation of power. It is

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