Second law analysis: An alternative indicator of system efficiency

Abstract

The most commonly used indicator for the efficiency of energy conversion processes is the ratio of the output of useful energy (heat or work) to the total energy input. This ratio is called first law efficiency, because it is based on a quantitative accounting of energy which reflects recognition of the first law of thermodynamics and the conservation of energy. Growing awareness of limited energy supplies has prompted renewed interest in conservation. One aspect of this renewed interest has been the search for better indicators of the efficiencies of energy conversion processes. Second law analysis, which examines the efficiency with which available energy is consumed, represents one avenue of this search. As is well known, the second law of thermodynamics defines the availability of energy more restrictively than the first law. Comparisons of first and second law efficiencies as measures of effectiveness, described in a wide range of literature on thermodynamics and energy management, are reviewed and summarized in this paper. Principally, first law efficiency is silent on the effectiveness with which availability is consumed. Analyses in terms of the second law of thermodynamics more nearly describe the effectiveness with which systems or processes use available energy. For example, when high quality (low entropy) energy sources are used to provide low quality energy (low grade heat), the waste of potentially useful work is expressed by second law efficiency but not by first law analysis. Many conversion processes used in space heating and cooling are of this nature. Second law analysis allows attribution of losses to design and operating parameters. For example, reducing irreversibility implies larger units of equipment for the same overall heat flow at lower thermal gradients. This introduces tradeoffs between equipment costs and operating costs. First law efficiency does not provide enough information to support a comprehensive conservation ethic. Because it measures quantities of work and energy and ignores the quality of energy use, it can tell how much energy is needed to perform a particular task, but not how well that energy is used. It can only support bulk allocation arguments. Second law analysis goes beyond allocation, and provides insights needed to apply energy resources to uses which produce less entropy per unit of useful heat or work. The authors review extant second law analyses of energy conversion components and cycles, and provide some original comparative analyses. These comparisons form the basis for conclusions which augment the traditional first law measure of efficiency by second law analysis.