Analysis of a cogeneration system based on solid oxide fuel cell and air source heat pump with novel heat recovery

Transition to energy systems with low greenhouse gases emissions is required by national and international organisations (e.g. EU Commission, Environmental Protection Agency and/or United Nations Framework Convention on Climate Change) with the aim of decreasing global warming. Different solutions have already been studied and proposed but more improvements could be achieved. In this paper a new heat recovery for micro-cogeneration system based on a Solid Oxide Fuel Cell and Air Source Heat Pump is presented with the main goal of improving efficiency on energy conversion for a residential building. The exhaust gases after the fuel cell are firstly used to heat water for heating/domestic water, and then they are mixed with the external air to feed the evaporator of the heat pump with the aim of increasing energy efficiency of the latter. This system configuration decreases the possibility of freezing of the evaporator as well, which is one of the drawbacks for air source heat pump in Nordic climates. Simulations are performed varying the external air temperature, air humidity and SOFC nominal power. COP increases up to more than 100 % when SOFC electric power is close to its nominal (50 kW), and/or inlet air has a high relative humidity (close to 100 %). Instead, the effect of mixing the exhausted gases with air may be negative (up to -25 %) when SOFC electric power is 20 kW and inlet air has 25 % relative humidity. Thermodynamic analysis is carried out to prove energy advantage of such a solution with respect to a traditional one, resulting to be between 37.5 % and 45 % in terms of primary energy.