Dynamic analysis of off-grid systems with ORC plants adopting various solution for the thermal storage

Real time matching of electric power generation is a crucial aspect in off-grid systems as well as in case of use of renewable intermittent sources. In this paper, with reference to 1 MWel Turboden biomass ORC plants operating in off-grid systems, we study the possibility to store thermal energy in the form of sensible heat within a storage composed by a bunch of steel or cast iron pipes with variable thickness and different coating materials. Thermal power is taken in and out of the storage by a flow of thermal oil, heated by a biomass furnace, and eventually supplied and converted into electricity by the ORC plant running in an off-grid area. Starting from steady state conditions at 30% of the nominal power of the furnace, we assume an instantaneous increase to 100% of the electric power demand and we study the transient of the system in correspondence of the furnace power ramp. To this purpose, we developed a dynamic finite difference model, for the system composed by the furnace, the ORC plant, the furnace inlet and outlet piping and the thermal storage. It comes out that with a storage system properly designed as function of the furnace power ramp, it is possible to run the ORC at 100% during the analyzed transient, thus allowing a real time matching of the power demand.