TAILIEUCHUNG - BATTERIES AND FUEL CELLS

The electrical efficiency Z of a galvanic cell or supercapacitor is defined as the ratio of the electrical energy that can be removed from it to the electrical energy supplied. Often the electrical efficiency of a galvanic cell or supercapacitor is also called energy efficiency as heat generation is never a useful by-product of the operation of galvanic cells or supercapacitors: | BATTERIES AND FUEL CELLS Contents Efficiency Lifetime Power Techno-Economic Assessments Efficiency H Wenzl Clausthal University of Technology Clausthal-Zellerfeld Germany 2009 Elsevier . All rights reserved. Definitions and Terms The electrical efficiency z of a galvanic cell or supercapacitor is defined as the ratio of the electrical energy that can be removed from it to the electrical energy supplied. Often the electrical efficiency of a galvanic cell or supercapacitor is also called energy efficiency as heat generation is never a useful by-product of the operation of galvanic cells or supercapacitors Electrical energy removed during discharge Electrical energy required during charging x 100 This definition may readily be extended to fuel cells provided that the electrical energy supplied refers to the enthalpy DH of the fuel. It is possible to define the theoretical electrical efficiency of a fuel cell thermodynamically. This is given by the formula _ DG _ _ TDS Z - DH - DH where DH is the enthalpy DG the Gibbs free energy and DS the entropy change of the reaction. For secondary or rechargeable electrochemical power sources this definition is not useful as it does not take the energy losses during charging into account. Obviously the theoretical electrical efficiency cannot be obtained in real systems and the reasons for this are given below. Heat is often a useful by-product of operation of fuel cells and therefore the term thermal efficiency is also important. In analogy to the electrical efficiency defined above the thermal efficiency is Thermal energy supplied Energy content of fuel x 100 The thermal efficiency may differ significantly from the electrical efficiency. In fuel cells used for combined heat and power CHP applications the ratio of electrical and thermal efficiency is an important design parameter for CHP plants. The total efficiency of a fuel cell or any other power generator for supplying heat and electricity to loads is Electrical energy .

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