Cogeneration systems with electric heat pumps: Energy-shifting properties and equivalent plant modelling

In this paper, a novel approach to energy and CO2 emission modelling of cogeneration systems coupled to electric heat pumps is presented. The specific objectives are to identify the relevant parameters and variables involved in the analysis of such composite systems, and to provide a synthetic and indicative assessment of the energy and environmental benefits potentially brought with respect to conventional energy systems. To this purpose, the concept of equivalent cogeneration plant is introduced, characterized by equivalent electrical and thermal efficiencies obtained by transforming the classical cogeneration efficiencies through the energy-shifting characteristics of the heat pump. The latter point is further highlighted by introducing equivalent cogeneration ratios and a thermal multiplication factor (TMF) indicator. This formulation allows investigation of the performance of the cogeneration-heat pump system by using the equivalent efficiencies as the input entries to equivalent energy saving and emission reduction indicators. The conditions at which energy and emission benefits occur, and their extent with respect to classical generation means, are illustrated through various numerical examples, highlighting the generality and effectiveness of the models introduced. In particular, equivalent cogeneration systems for small-scale distributed applications can exhibit energy saving and emission reduction of the order of up to 50% with respect to benchmark generation means within a typical Italian energy context. (C) 2009 Elsevier Ltd. All rights reserved.