Analysis of the influence of thermal energy storage on the optimal management of a trigeneration plant

Safety, security, and sustainability of energy supply chains are among the main concerns of industrialized countries, and, therefore, distributed generation has significantly increased its share of the energy market, thanks to the possibility to simultaneously meet electrical, thermal and cooling demand, thus increasing the overall source-to-final-use conversion efficiency. The efficiency of a distributed generation system is influenced both by the individual performance of the plant components as well as by their interconnection, and is very sensitive to the control strategy adopted in the different plant sections. This last remark is particularly relevant for distributed generation systems, that are subject to rapid gradients in both the thermal and electrical loads, and in the values of the energy vector. In this respect, the introduction and the correct management of energy storage systems is a key point for trigeneration plants. In fact, energy storage brings on the one side advantages as for the reduced components sizes, but more importantly allows for a substantial decoupling of the thermal and electrical demands, making load following less of a stringent requirement. An optimization methodology, based on energy fluxes simulation, and on the application of the graph theory as in previous works by some of the authors, is used to identify the optimal set-points for each component. The optimization algorithm searches for the plant management envelope that minimizes a prescribed objective function. Specifically, two different optimization criteria are considered: i) economic optimization that minimizes the total daily operating cost and ii) primary energy use optimization, that minimizes the total daily amount of primary energy used by the plant. Since the paper focus is on the effects of energy storage, the trigeneration plant behavior will be analyzed both in terms of economical results and in terms of efficiency and primary energy use