04 - R ational Efficiency of a P owerplant 4.1 The influence of fuel properties on thermal efficiency The thermal efficiency of a cycle has been

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Rational Efficiency of a Powerplant 4.1 The influence of fuel properties on thermal efficiency The thermal efficiency of a cycle has been defined previously, in terms of specific quantities, as w,t 9th = - qin where wnet = net work output from the cycle per unit mass of fluid, and = energy addition to the cycle per unit mass of fluid. In this case is the energy transfer to the working fluid, and does not take into account any losses in the boiler or heat transfer device. Equation (4.1) can be rewritten for the whole powerplant, including the boiler or heat transfer mechanism, as (4.2) W",t %'qBVth=- - Ah0 where l;lo = overall efficiency of powerplant, l;lB = efficiency of boiler, qth = thermal efficiency of cycle, wWt = net work output from the cycle per unit mass of fuel, Ah,, = specific enthalpy of reaction of fuel. This might be considered to be an unfair, and possibly misleading, method of defining the efficiency because the energy addition cannot all be turned into work, as was shown when considering exergy and availability. Another definition of efficiency can be derived based on the Second Law, and this relates the work output from the cycle to the maximum work output obtainable. The efficiency of the powerplant has been related, in eqn (4.2), to the amount of energy that has been added to the cycle by the combustion of the fuel. In the past this has been based on the enthalpy of reaction of the fuel, or usually its calorific value, Qi. It was shown previously (Chapter 2) that this is not the energy available for the production of work, and that the maximum available work that can be obtained from the fuel is based on
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Rational efficiency 65 the change of its exergy at the dead state conditions. Hence, the maximum available work from unit mass of fuel is -Ago = g, - Ago = Ah0 - To(s, - SPo> (4.3) This is related to the enthalpy of formation by the equation (4.4) It was shown in Chapter 2 that I I could be greater than or less than I Aho 1 , and the difference was dependent on the structure of the fuel and the composition of the exhaust products. The efficiency of the powerplant can then be redefined as (4.5) Wnet a=- where wmt = actual net work output from the cycle per unit mass of fuel, - Ago and = change of Gibbs function caused by combustion, = maximum net work obtainable from unit mass of fuel. Equation (4.5) is often referred to as the Second Law Efficiency, because the work output is related to the available energy in the fuel, rather than its enthalpy change. The actual effect on thermal efficiency of using the change of Gibbs function instead of the enthalpy of reaction is usually small (a few per cent). 4.2 Rational efficiency When the efficiencies defined in eqns (4.2) and (4.5) are evaluated they contain terms which relate to the 'efficiency' of the energy transfer device (boiler) in transferring energy from the combustion gases to the working fluid. These effects are usually neglected when considering cycles, and the energy added is related to the change in enthalpy of the working fluid as it passes through the boiler, superheater, etc. Actual engine cycles will'be considered later. First, a general heat engine will be considered (see Fig 4.1). For
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This note was uploaded on 03/09/2010 for the course MECHANICAL ME9802701 taught by Professor Prof.william during the Spring '10 term at Institut Teknologi Bandung.

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04 - R ational Efficiency of a P owerplant 4.1 The influence of fuel properties on thermal efficiency The thermal efficiency of a cycle has been

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