Combined vs separate heat and power production - Primary energy comparison in high renewable share contexts

Natural Gas Combined Cycle (NGCC) units are currently the most efficient power plants based on fossil fuels. When used for Combined Heat and Power (CHP) production, serving District Heating (DH) systems, they have been usually promoted by stating their lower primary energy consumption compared to separate production of power and heat with conventional technologies. However, a significant increase of the share of Renewable Energy Sources (RES) in power generation and Heat Pumps (HP) for heat production in buildings could undermine this assumption. This paper evaluates a case study in Northern Italy, by comparing the real operation of three NGCC plants serving a DH network against the separate production of power (from real data of the National electricity mix) and heat (considering two scenarios based on natural gas boilers and heat pumps). The analysis is performed on hourly data over a two-years' time frame, to highlight the variations across the hours of the day and the seasons. To perform a comprehensive analysis, the entire system performance is considered, by comparing the useful energy supplied to the users to the primary energy consumption. The results show how the primary energy savings of fossil CHP technologies are strongly related with the available alternatives, which have been going through a significant evolution in last years. The separate production of heat and power can now be performed with competitive technologies, which benefit from the high share of RES in electricity production. Therefore, the comparison between combined and separate production is influenced by the high variability of the electricity generation mix, which needs to be carefully considered.