General numeric determination of ecological operation of heat pumps using power from cogeneration.

Abstract

The combination of a combined heat and power (CHP) system with a heat pump (HP), is growing popular as an innovative heating solution in residential buildings as well as decarbonization-solution in district heating networks. Due to significant lower operational and initial costs, the predominant configuration is a CHP+HP combination, where the HP is operated by internal power supply, provided by the CHP. In this paper, the CHP+HP combination was generally analysed under exergy analysis regarding its climate protection potential, thus taking the first and second law of thermodynamics into consideration. As a result, the common conception of CHP+HP combinations being ecologically advantageous in terms of low CO2-emissions, was disproved analytically. The new, theoretical COP limit, which is derived and presented in this work gives the COP-threshold that any HP operated with internal power supply from CHP has to achieve at minimum to provide heat at lower CO2-emissions than the CHP. The COP limit derived is a function of CHP temperatures only. The graphical representation of the results allows a simple determination of the energy system conditions to check whether the HP has a positive impact on the CO2-emissions of the combined CHP+HP-system under usage of internal power supply. The evaluation based on these results confirms that the heat from a HP using either of the 2 popular heat-sources surfaced-water or ambient air, inevitably has higher specific CO2-emissions in the CHP+HP combination than the CHP heat. Only when using a sufficiently hot and emissions-free heat source, the HP-heat from a CHP+HP combination can yield to maximum of half the CO2-emissions of the CHP. Therefore, power consumption with sufficiently low specific CO2-emissions out of external source is mandatory to achieve in case of a high temperature waste heat source fully capitalize on the ecological benefits through HP operation in CHP+HP systems.