Alternative solutions for the integration of solar heat production in the operation of an existing district heating system

The standard technical solution for the integration of solar energy within District Heating (DH) systems is the installation of water solar collectors that directly feed hot water within the network; this network is usually fed in parallel by other generation sources, usually fossil fuel based (i.e. natural gas in most cases). The present paper investigates alternative solutions for the integration of the solar source within an existing DH network; a key hypothesis is the constraint to cover the heat demand entirely by renewable sources through the integration of solar energy and wood biomass. Various technologies were considered for such integration, and six different plants layout have been analysed and compared in terms of deliverable energy (both thermal and electric) and energy conversion efficiencies. The results show the advantages of using the available solar radiation to operate a wood biomass dryer and consequently increase the quality of the fuel used in biomass boilers or ORC unit. The use of a solar dryer can decrease the wood biomass requested by the generation plant, resulting in higher thermal and electric efficiencies with respect to the integration with traditional solar water collectors. This configuration has the additional advantage of offering the opportunity of storing solar energy when the availability is higher than the demand, by using the wood biomass as heat storage medium.