A novel Pumped Thermal Electricity Storage (PTES) system with thermal integration

Power to heat technologies are becoming more and more important due to the extreme need of energy storage solutions to help manage the mismatch between supply and demand of electric power in grids with a large penetration of intermittent renewable energy systems. Several Electric Energy Storage (EES) technologies have been proposed in the literature and some of them have been built as pilot or commercial plants, with different characteristics in terms of storage capacity, response time and roundtrip efficiency. In this paper, the attention was focused on Pumped Thermal Electricity Storage (PTES), which is a technology that stores electric energy as heat by means of Heat Pumps (HP) and converts it again to power with a Heat Engine (HE). In this study, a hybrid PTES application was studied, which took advantage of a low-grade heat source to boost the electric round-trip efficiency of the system beyond 100%. The main idea was to exploit the heat source to reduce the HP operational temperature difference; this thermal integration boosted the HP COP and thus the electric efficiency of the whole system. A Matlab numerical model was developed, using the thermodynamic properties of the Coolprop data base, and the steady state operation of a PTES system composed by a vapor-compression HP and an Organic Rankine Cycle (ORC) we simulated. Heat source temperature values ranging from 80 °C to 110 °C and different working fluids were studied. Among the refrigerants, which comply with the latest European environmental legislation, the most promising fluid was R1233zd(E): with such fluid a maximum round trip-efficiency of 1.3 was achieved, when the heat source temperature reaches 110 °C and the machinery isentropic efficiencies is 0.8, the heat exchangers pinch points is 5 K and the ORC condensation temperature is 35 °C.