Influence of the heat storage size on the plant performance in a Smart User case study

The increasing diffusion of renewable energy sources are posing new challenges to the power grid due to their intrinsic unpredictability causing poor power quality, line congestion and unreliable and unsecure grid operations. In the future, power grid operators may require to the customers/producers a prescribed exchange profile, leading to a diffusion of storage systems or prime movers (especially small combined heat and power for distributed resources) able to balance renewable sources fluctuations. In this latter case, one of the major issue is the efficient use of the heat co-generated: the adoption of thermal storages appears to be necessary. In this paper, the energy performance of a Smart User, i.e. a dwelling with renewable energy sources, a combined cooling, heat, and power system, and heat and cooling back-up generators, is estimated by means of a purposely developed TRNSYS unsteady model. A virtual stand-alone operating condition is imposed for the analysed building as an arbitrary profile of power exchange with the grid. The balancing of renewable sources fluctuations imposes a non-negligible part of the heat from the prime mover to be wasted and a sensible consumption for auxiliary devices (i.e. the auxiliary boiler and the compression chiller). By applying a thermal storage, and increasing its capacity, the fraction of heat wasted and the supply from other devices is remarkably changed, showing different plant performance and efficiencies. The Smart User primary energy consumptions of the different configurations are compared to several options, showing appreciable differences in the performance.