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Abstract
Analysis of global energy efficiency of thermal systems is of practical importance for a number of reasons. Cycles and processes used in thermal systems exist in very different configurations, making comparison difficult if specific models are required to analyze specific thermal systems. Thermal systems with small temperature differences between a hot side and a cold side also suffer from difficulties due to heat transfer pinch point effects. Such pinch points are consequences of thermal systems design and must therefore be integrated in the global evaluation. In optimizing thermal systems, detailed entropy generation analysis is suitable to identify performance losses caused by cycle components. In plant analysis, a similar logic applies with the difference that the thermal system is then only a component, often industrially standardized. This article presents how a thermodynamic “black box” method for defining and comparing thermal efficiency of different size and types of heat engines can be extended to also compare heat pumps of different apparent magnitude and type. Impact of a non-linear boundary condition on reversible thermal efficiency is exemplified and a correlation of average real heat engine efficiencies is discussed in the light of linear and non-linear boundary conditions.
Highlights
When optimizing a thermal plant, using a heat driven power cycle or a heat pump, practical experience indicates that one seldom have the luxury of choosing the components in any of the systems considered
Global in this approach means that the power cycle or heat pump is treated as a “black box” with global efficiency defined by the real boundary conditions dictated by the plant in which the “black box” operates
Thermodynamic of a reversible heat pump can be viewed as aboundary symmetricconditions mirror of ausing heat equations available in standard literature
Summary
When optimizing a thermal plant, using a heat driven power cycle or a heat pump, practical experience indicates that one seldom have the luxury of choosing the components in any of the systems considered. The plant designer often needs to choose between preexisting, industrially standardized machines Such preexisting thermal systems will have characteristics almost according to the designer’s preferences, but seldom exactly. Unless the providers make a complete and unique model available of each potential thermal system, plant optimization has to rely heavily on assumptions. Since such unique models have a tendency to become biased, the problem remains regardless. We propose a different approach to comparing thermal systems on a global level Global in this approach means that the power cycle or heat pump is treated as a “black box” with global efficiency defined by the real boundary conditions dictated by the plant in which the “black box” operates
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