Practice Versus Theory

Given the first law of thermodynamics, all of the energy that goes into an operating system is work or heat. The first law treats heat and work as being interchangeable, though some qualifications must apply. Work and all other forms of energy can be wholly converted to heat, but the converse is not generally true.

The second law of thermodynamics shows that given a source of heat, only a portion of the heat can be converted to work in a heat-work cycle. The rest must be rejected to a heat sink. Thus, it is impossible to have a heat engine that is 100% efficient and even the most perfect cycle must be less than perfectly efficient. Further, the ideal performance of a given power cycle is always less than the Carnot efficiency. During an ideal (reversible) process, it is theoretically possible to reach the maximum potential efficiency for the specific process. Cycle design and operational improvements are initiated in an effort to approach the theoretical limits of the Carnot cycle. There are, however, factors that render the process irreversible.

The practical limit of power cycle efficiencies is set by metallurgical limits or strength of available materials (ability to operate under high temperatures and pressures) and by the ambient temperature of the heat sink. In addition, all practical applications of cycles and any other thermal process will be subject to energetic or heat losses resulting from friction, sustained expansion, convection, and conduction. Thus, reversible thermodynamic processes exist in theory only, defining only the limiting case for heat flow and work processes.

From the comparison of reversible and irreversible processes and cycles, it becomes clear that a critical concern in evaluating various cycles is theoretical and practical cycle efficiency. Theoretical efficiency shows the maximum efficiency that could, in theory, be attained from any given cycle. Practical efficiency shows what can be expected from a particular system operating on a given cycle. In the ensuing chapters covering prime mover technologies, thermal efficiency and other expressions of performance are presented first in theory and then in a practical context that can be applied to actual applications.

The following series of expressions are used through out the various chapters that present combined heat and power system technologies and applications. They are used to present practical thermodynamic and economic performance measurements, based primarily on fuel and heat input and work and heat output.

Guide to Alternative Fuels

Guide to Alternative Fuels

Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.

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