Compared to conventional technology, power cycles that use supercritical carbon dioxide (sCO2) as working fluid promise more effective energy conversion, very low dimension, and high flexibility, especially at high temperatures. These abilities are very suitable for use with advanced small modular reactors (SMRs). The concept of an SMR based on a fluoride salt (FLiBe) coolant and a sCO2-based conversion cycle, called ‘Energy Well’, is being developed by Centrum výzkumu Řež (CVR).

During the research and development of the Energy Well nuclear reactor and long-term activities of CVR in the field of sCO2 technologies, a loop-type experimental facility working with FLiBe was developed, assembled, and operated. The facility was coupled with another unique infrastructure, the sCO2 loop, to support the development of a potential interface between the FLiBe coolant and an innovative conversion cycle fluid (sCO2). This facility intends to test the key components and operational procedures important in the application of similar technologies on a larger scale. The key components of the FLiBe loop are the main circulation pump, the heat exchanger (FLiBe/sCO2) and the electrical heater, which represents the core of a molten salt reactor. All components were developed and assembled by CVR. Some aspects of the design of these components were validated using CFD tools.

A description of the first operational experience and experimental data are provided in this paper. The resulting outlet temperatures, heat flux and FLiBe mass flow rate were compared with experimental measurements and the overall heat transfer coefficient was estimated.

As part of the experimental campaign, the circulation pump for FLiBe was first tested separately. After this successful test, the experimental FLiBe circuit was connected to the existing sCO2 loop, which is operated by CVR, via the heat exchanger.

During the experimental campaign, data were obtained from the operations of both coupled devices, including from the commissioning procedure of the FLiBe circuit and the controlling of the two devices simultaneously. Experimental data were processed and evaluated. Unfortunately, large uncertainties were found in the FLiBe temperature measurements and, consequently, the resulting mass flow rates.

The European Commission aims at achieving a net-zero greenhouse gas emissions economy until 2050. For this reason, renewable electricity generation is expected to increase up to 69% by 2030. However, the intermittent nature of solar and wind power generation requires sustainable storage systems, in order to compensate for the mismatch between energy supply and demand. Large-scale thermal energy storage in combination with supercritical carbon dioxide (sCO₂) power cycles is a promising solution to address this issue.

The EU-project CEEGS (Novel CO₂-based Electrothermal Energy and Geological Storage System) aims to develop a highly efficient, cost-effective and scalable energy storage technology. When excess renewable electricity is available, a compressor drives a heat pump cycle to increase temperature and pressure. The hot CO₂ heats a hot water storage and cools down before entering an expansion turbine. The low-temperature CO₂ cools a cold-water storage tank before entering the compressor. In the discharge cycle, CO₂ is pumped from the geological reservoir into the surface components and gets heated by the stored thermal energy through a heat exchanger, before entering the turbine to generate electricity. The low-pressure CO₂ is liquified by entering a condenser, which is cooled by the cold-water reservoir, and pumped back into the subsurface reservoir through an injection well to extract heat from the subsurface.

In order to demonstrate the transcritical cycle of the CEEGS concept and to validate the surface components, a 20 kW demonstrator was designed, built and operated at the Helmholtz-Zentrum Dresden-Rossendorf. In the present contribution, the design of the facility and the operation of the discharge cycle at CO₂ temperature and pressure of up to 250 °C and 235 bar will be presented and discussed. The safety and measurement concept of the facility is presented. The design of the main components, such as the high-temperature heat exchanger (HXW), the low-temperature heat exchanger (HXI) and the CO₂-pump, as well as the cycle behavior are presented. The experimental results show a lower Reynolds number at higher cycle pressures, due to the more closed valve position. Particular attention is paid to the performance of the high-temperature heat exchanger in terms of overall heat transfer coefficient. Convective heat transfer on the CO₂ side plays a dominant role in the thermal resistance of the heat exchanger. The thermal efficiency of the cycle during discharge is strongly influenced by the maximum cycle pressure. In fact, at higher pressures, higher cycle efficiency is achieved due to greater expansion work.

The experimental results will be used by the project partners to validate the numerical models. Future experimental campaigns will investigate the dynamic operation of the facility.