Tecnologie per l'industria e l'agricoltura
Combined Heat and Power
Autors: Antonio Di Nardo, Marco Cavana, Pierluigi Leone
In Italy, the gross electricity and heat production from combined heat and power (CHP) plants (also referred to as heat and electricity co-generation plants) in 2019 was respectively 107.3 TWhe (declining to 100.5 TWhe in 2020 due to the Covid-19 pandemic) and 65.4 TWht (useful heat), with a gross capacity of about 26 GWe. Internal combustion engines are the most common technology in terms of the number of units, but combined cycles are the dominant technology in terms of power production. In 2018, the total CHP capacity that applied to the GSE regulatory for qualification as High Yield Cogeneration (CAR) was 13.4 GWe, of which 36.2% relating to CHP plants for district heating, with a production of 57.8 TWhe (electricity) and 35.6 TWht (useful heat), both enabling a primary energy saving of 11.1% if compared to separate production. However, only 49.6% of this amount of electricity can actually be classified as High Yield Cogeneration.Steam-driven power plants have capacities ranging from 0.5 to a few hundred MWe. In order to ensure heat production, steam expansion in the turbine is incomplete and electrical efficiencies are relatively low, ranging from 10 to 20%, with a total efficiency between 60% and 85%. Gas turbine power plants also have variable size from a few hundred kWe (typically, simple cycle plants with electrical efficiencies of 25 to 35%) to a few hundred MWe (typically combined gas cycles with electrical efficiencies in cogeneration of 40-50% and total efficiency of 70-90%). More recent are gas-fired micro-turbines with power ranging from 30 to 250 kWe, electrical efficiency of 20-30% and total efficiency of 70-80%. Internal combustion engines, in particular gas-fired Otto cycle engines, are very common for small size installation (although they can also reach a few MW) and have electrical efficiencies from 30% to 45% and a total efficiency of 60-85%. Biomass-fired systems (either by direct combustion or biomass gasification) with efficiency depending on the prime mover they are coupled with are also currently used in regions with large biomass availability. Lastly, among most innovative cogeneration systems, it is worth mentioning the Stirling engine systems in the 1-50 kWe capacity range, with electrical and total efficiency of 10-30% and 70-90%, respectively, and the fuel cell-based systems, with capacity between 5 kWe and 2 MWe and electric and total efficiency of 35-55% and 70-90%, respectively.