Thermodynamic optimization of a pumped thermal energy storage with integration of heat sources and heat sinks

Kurzfassung

As the share of fluctuating renewable energies in electricity generation continues to rise, storage technologies for electrical energy are becoming increasingly important. Pumped thermal energy storage (PTES) systems represent a promising approach without geological limitations to complement established large-scale storage technologies such as pumped hydro energy storage systems. PTES systems store surplus electrical energy in the form of thermal energy, which is converted back into electrical energy when it is required. The considered subcritical PTES system is based on a heat pump (HP) cycle, a high temperature thermal energy storage (HT-TES) system and a heat engine (HE) cycle. Depending on the operating mode, heat integration during evaporation in the HP or heat extraction during condensation in the HE is possible, thus achieving a sector coupling between electricity and heat. In this work, a seasonal thermal energy storage of a smart district heating network is used as a heat source and sink for the PTES system. By adding a PTES system, the smart district heating system is complemented to form an energy hub, which allows for the flexible and demand-orientated conversion and distribution of thermal and electrical energy. Initial findings from the CHESTER project revealed potential for increasing the systems efficiency as well as the need for modifications to match the requirements of the superordinated energy system. With the aim of a high temperature lift in HP and HE cycle, screening for a suitable combination of storage material and working fluid leads to an upper HT-TES temperature of 222 ◦C and cyclohexane as the working fluid. Simulations for three different operating modes are performed using EBSILON®Professional considering exergy losses during heat transfer and realistic efficiencies of machinery. Furthermore, this work investigates multi-stage evaporation in the HP and condensation in the HE cycle. Since the seasonal thermal energy storage is a sensible heat source and sink, a multi-stage system design promises a significant reduction of exergy losses. If the PTES system acts as an electricity storage without heat integration or heat extraction, the powerto-power efficiency of the system increases from 39.5 % for the single-stage system design to 46.3 % by using four evaporation stages in HP and three condensation stages in HE cycle (plus condensation to environment).