Pumped thermal electricity storage for active distribution network applications

This paper introduces a new model for Pumped Thermal Electricity Storage (PTES) devices as an emerging thermal storage technology. PTES devices are capable of reaching higher capacities than battery storage devices and therefore are suitable for grid-scale storage at the distribution voltage levels. The new model captures the inherent thermal characteristics, such as the variable efficiency, of the PTES device, yet it is not computationally burdensome for integration into non-linear optimisation problem formulations. It therefore makes it suitable for operational planning studies in active distribution networks. The new model uses a two-stage regression of a detailed thermodynamic model of PTES to capture the approximate behaviour. The salient feature of this reduced model is that the variable efficiency is a function of the energy content — the state of charge — of the device. The new model is tested on a medium-voltage 33-bus distribution network within a dynamic optimal power flow formulation for day-ahead operational planning. The main objective has been to minimize daily cost of buying energy from the external grid. The results have been compared with the same test network without any storage devices and with storage models with fixed round-trip efficiency. In both cases the results clearly show the suitability and prowess of the new model in producing accurate operational cycles for the device and its benefits in terms of significant savings in operational costs when using large-scale PTES devices.