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.

The sector integration of large-scale heat pumps (LSHP) into existing district heating networks (DHN), most of which are operated by combined heat and power plants (CHP), is observed to grow rapidly over the past few years. Due to significant lower operational and initial costs, the predominant configuration in DHN is a CHP+LSHP combination, where the LSHP is operated by internal power supply, provided by the CHP-plant. In a dynamic simulation based on aggregated data over the course of one year, specific CO2-emissions are calculated according to the exergetic Carnot methodology, fulfilling the first and sec-ond law of thermodynamics. In order to determine and quantify the ecologic benefit of a generic sector integrated LSHP, the specific CO2-emissions of the heat of the most common CHP-plants are compared to a LSHP operated by internal (CHP+LSHP) and external power supply (electrical grid and volatile renew-able energy sources (RES)). The analysis shows, that a generic LSHP operated by internal power supply out of every examined CHP-plant, seasonally independent generates heat with higher specific CO2-emissions over the heat from the CHP-plant alone. In contrast, the heat provided by the LSHP operated with external electricity from RES, achieves 5 to 16-fold lower CO2-emissions showing, that with growing share of power from RES in the electric grid, regulation to ensure that external power supply is economically feasible will be mandatory in order to achieve ecologic beneficial operation of sector integrated LSHPs.

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.