Thermodynamic analysis

Thermodynamic analysis

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The purpose of this chapter is to provide the means for evaluation of the performance of cogeneration and trigeneration systems from the point of view of useful energy products, primary energy consumption and primary energy savings. The correct definition and calculation of efficiencies and the primary energy savings are important in order not only to obtain a clear picture of what cogeneration can achieve but also to reveal whether a particular system is eligible for economic incentives provided in several countries for promotion of cogeneration, such as subsidy on investment, guaranties of origin and special tariff for electricity coming from high efficiency cogeneration. A cogeneration system may operate at a variety of loads and external conditions. Thus, an analysis on the design point only may lead to overestimation of its thermodynamic performance. If these results are then used for evaluation of its economic performance, they may give a wrong picture of the economic viability of the investment. For this reason, an example with off-design performance will also be presented in this chapter.

Chapter Contents:

  • 7.1 Introduction to thermodynamic analysis of cogeneration systems
  • 7.2 Indexes of thermodynamic performance
  • 7.2.1 Efficiencies based on energy
  • 7.2.2 Efficiencies based on exergy
  • 7.2.3 Electricity to heat ratio
  • 7.2.4 Primary energy savings
  • Primary energy savings of a cogeneration system
  • Primary energy savings of a trigeneration system
  • 7.3 Procedure for determination of electricity cogenerated with useful heat
  • 7.3.1 What is the issue?
  • 7.3.2 Distinction between cogeneration systems withoutloss and with loss of work production due to useful heat production
  • 7.3.3 Splitting the cogeneration unit in CHP and non-CHP parts
  • 7.3.4 Procedure to calculate the cogenerated electricity and related parameters
  • Definitions
  • Calculation of the power-to-heat ratio in full cogeneration mode, CHP electricity, non-CHP electricity, and fuel consumptions
  • Calculation of the primary energy savings of the CHP and non-CHP parts
  • Nomenclature
  • Appendix 7.A Fundamentals of exergy
  • 7.A.1 Definition of exergy
  • 7.A.2 Exergy of work and heat
  • 7.A.3 Exergy of a closed system
  • 7.A.4 Flow exergy
  • 7.A.5 Physical flow exergy of ideal gas and of mixture of ideal gases
  • 7.A.6 Physical flow exergy of incompressible fluids
  • Appendix 7.B Power-to-heat ratio in full cogeneration mode
  • References
  • Further Reading

Inspec keywords: power generation economics; thermodynamics; cogeneration; energy consumption; trigeneration; energy conservation

Other keywords: investment; cogeneration performance evaluation; primary energy consumption; trigeneration systems; energy products; thermodynamic analysis; economic performance evaluation; economic viability; primary energy savings; economic incentives

Subjects: Power system management, operation and economics; Thermal power stations and plants

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