Abstract
As a part of the change towards a higher usage of renewable energy sources, which naturally deliver the energy intermittently, the need for energy storage systems is increasing. For the compensation of the disturbance in power production due to inter-day to seasonal weather changes, a long-term energy storage is required. In the spectrum of storage systems, one out of a few geographically independent possibilities is the use of heat to store electricity, so-called Carnot-batteries. This paper presents a Pumped Thermal Energy Storage (PTES) system based on a recuperated and recompressed supercritical CO2 Brayton cycle. It is analysed if this configuration of a Brayton cycle, which is most advantageous for supercritical CO2 Brayton cycles, can be favourably integrated into a Carnot-battery and if a similar high efficiency can be achieved, despite the constraints caused by the integration. The modelled PTES operates at a pressure ratio of 3 with a low nominal pressure of 8 MPa, in a temperature range between 16 °C and 513 °C. The modelled system provides a round-trip efficiency of 38.9 % and was designed for a maximum of 3.5 MW electric power output. The research shows that an acceptable round-trip efficiency can be achieved with a recuperated and recompressed Brayton Cycle employing supercritical CO2 as the working fluid. However, a higher efficiency would be expected to justify the complexity of the configuration.
Highlights
As the demand for renewable energy is increasing, so is the importance of its reliability
The proposed cycle is recompressed and employs a double recuperation because this layout is reaching the highest efficiencies for sCO2 cycles and, generally, has a great potential for recuperation, as stated by McTigue et al [30], whereas other research focused on transcritical CO2 Cycles or simple non-recuperated supercritical CO2 cycles [33, 34]
The lower efficiency of the cycle, while being integrated into a Pumped Thermal Energy Storage (PTES) system as its discharging cycle, could be explained with the boundary conditions applied to the cycle through the attached storages and due to the unavoidable rejection of the thermal energy to the environment as well as through the desired optimum between the high COP of the heat pump and the high efficiency of the power cycle
Summary
As the demand for renewable energy is increasing, so is the importance of its reliability. Due to the possible storage time of several hours and days, it is feasible for handling the offset between the supply and the demand caused by the unstable nature of renewable energies sources like wind and solar radiation (as can be seen in Figure 1 and Figure 2). It is still in its concept phase [11], with the first prototypes being tested. There are several other projects of Carnotbattery pilot plants already under development, in construction or being tested
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