Abstract
In the first part of this work, combined heat and power (CHP) criteria pertaining to energy, exergy, environmental (pollutant emission) and economic aspects, have been investigated and compared. Although the constraints in legislation usually refer to energy efficiency, primary energy savings and greenhouse gas savings, other criteria should also be taken into account in order to obtain a better evaluation of a cogeneration plant. Here particular attention has been paid to saving indexes for both an individual CHP-unit and for a CHP-system, that is the complete system with all the cogeneration units and the auxiliary plants necessary to cover the users’ demand. Five indexes, named potential indexes, have been introduced to evaluate the cogeneration potential: one for energy saving, one for exergy, two for environmental aspects (global and local scale) and one for economic aspects. Finally, some indexes analysed in the paper have been applied to a case study concerning a district heating cogeneration system, and the different behaviour of the energy-exergy, environmental and economic aspects has been discussed.
Highlights
Several criteria have been proposed, over the last few decades, to evaluate the advantages of cogeneration and different aspects have been taken into account
Because of the useful heat that is produced by a combined heat and power (CHP)-unit, the users do not use fuel in local boilers ( Q η th, Separated Heat andPower (SHP) ) and this fuel could be used in a central power plant to obtain a surplus of electricity
When CO2 emissions are analysed, a further simplification can be made by introducing two hypotheses: the same fuel is used for CHP and SHP, and a complete combustion of the fuel is assumed, in this way, the emission only depends on the fuel characteristics
Summary
Several criteria have been proposed, over the last few decades, to evaluate the advantages of cogeneration and different aspects have been taken into account. Seven factors are considered to assess their index: efficiency, installation costs, fuel costs, electricity costs, heat costs, CO2 emissions and energy footprint These factors are normalised with the weight coefficients and sixteen systems, with different CHP technologies and plant sizes, are compared. In [8], the same authors have introduced a new environmental index, trigeneration CO2 emission reduction, which can be used to compare a trigeneration plant with conventional separate production of heat and power, and they have considered CHP as a subcase of the trigeneration analysis They presented case studies, based on current technologies (microturbines, internal combustion engines, gas turbines, combined cycles), and discussed CO2 emission reductions: the results mainly depend on the technology that was used for the combined production and on the composition of the energy generation mix. The main indexes analysed in the paper have been applied to a case study concerning a district heating cogeneration system, and the different behaviour of the saving and potential index has been evaluated and discussed
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