Integration of Heat Pump in Combined Heat and Power Plant – Comparison of Vapor Compression and Absorption Technology

A central combined heat and power plant based on extraction steam turbines may operate in a range of modes. The operating map is defined by boiler load and maximum (back-pressure mode) to zero (condensation mode) heat production. The paper investigates the potential of expanding the map by integrating a heat pump in thecycle of the Avedøreværket Power Station in Copenhagen to preheat the return of district heating.

The analysis was made for both vapor compression or absorption technology by numerical simulations of the plant. In both cases power was sacrificed to produce heat. For the vapor compression heat pump electricity from the generator was consumed, while for the absorption unit low pressure steam was used for the heat pump generator instead of producing power.

Both heat pumps extend the heating capacity of the plant. The vapor compression unit produced 85.9 MJ s−1 additional heat while sacrificing 17.1 MW power. The absorption heat pump produced 74.5MJ s−1 heat at a COP of 1.75, while sacrificing 8.9 MW power. Hence the compression heat pump produced a higher amount of heat, while the absorption heat pump provided a little better electric and exergy efficiency.

In addition to the better performance of the absorption heat pump, its lower investment meant that it was estimatedto have a payback period of 8.6 years compared to 16.8 years for vapor compression unit.