Supercritical CO2 cycle for sensible thermal energy storage and power generation applications

The necessity of energy independency, political and climatic concerns is driving the global attention towards sustainable energy sources. The European commission targets carbon-neutrality with net zero greenhouse gas emission by 2050 [1], which results in a decarbonization of the energy sector by renewable energy sources. Nevertheless, a significant challenge arises from the fluctuating power infeed of wind and solar based plants, which may result in grid instability, overstrain and mismatch in supply-demand. Thus, energy storage systems (ESS) are required to decouple electricity generation from the demand and to enhance the reliability of the energy system. Furthermore, a successful implementation of such a storage technology on a large scale requires cost-effective cycles for energy storage and reconversion.
Therefore, a Power-to-Heat-to-Power system is proposed, based on electrical charged high temperature ceramics and a supercritical CO2 (sCO2) power cycle for discharge. The high temperature sensible thermal energy storage is manufactured from waste material of the aluminum production, which is a low-cost storage solution. As a result, such a thermal storage combines an important contribution to the circular economy with an economic solution for large scale, location-independent energy storage. CO2 becomes supercritical when temperature and pressure are higher than 31 °C and 74 bar. In this phase the viscosity is low and the density as well as the heat capacity are high. Hence, the installed components, in particular the turbomachinery of the sCO2 power cycle are highly compact, which results in low investment costs and low thermal losses. Furthermore, high temperatures up to 600 °C can be utilized to generate electricity at high thermal efficiency.
An inhouse code was used to study various heat transfer fluids, materials and designs for the thermal storage vessel during the charge and discharge cycle. Furthermore, the heat exchanger transferring the heat from the thermal storage cycle to the sCO2 power cycle was numerically investigated, to optimize compactness and thermal efficiency. Finally, the experimental facility CARBOSOLA was designed in cooperation with the Siemens Energy AG, the TU Dresden and the DLR, to investigate the involved components of the power cycle as well as the cycle performance. Various questions will be addressed, such as material resistance at temperatures up to 650 °C, optimal heat exchanger designs, part load operation of the facility, cost effective volume flow measurement, compression and expansion processes and the favorable integration of the thermal storage cycle. Furthermore, the mentioned partners perform techno-economical-analysis of the described systems and the facility will be used to validate these models.
References
[1] G. Subbaraman et al., “ZEPS TM Plant Model: A High Efficiency Power Cycle with Pressurized Fluidized Bed Combustion Process,” 2nd Oxyfuel Combust. Conf., pp. 2–5, 2011.