Demand-based process steam from fluctuating energies: Experimental results from a 100 kW demonstrator based on the Rotating Drum Heat Exchanger

Kurzfassung

In Europe, the demand of process steam exceeds 600 TWh per year, accounting for approximately 5% of the total final energy demand. Currently, the supply of process steam heavily relies on fossil fuels. Within this contribution, a thermal energy storage system is presented based on the Rotating Drum Heat Exchanger, developed by the DLR, to enable demand-based process steam supply using fluctuating renewable energy sources. Thereby, a horizontally mounted rotating drum is partially immersed in liquid nitrate salt. As water evaporates inside the drum, the nitrate salt solidifies on the outer surface, effectively transferring both sensible and latent heat. A stationary scraper is used to remove the solidified nitrate salt from the drum's surface, enabling the separation of the liquid and solid phases of the storage material. While the storage material is similar to that used in commercial two-tank molten salt storage, the additional utilization of the phase change enthalpy allows for a 60% increase in storage density. This results in cost savings for both, the storage material and the required storage tanks. Additionally, the cold storage tank does not require any external energy to prevent unintentional solidification. Based on previous research, a demonstrator with a drum 1 m long and 0.36 m in diameter is currently being commissioned at the DLR for the provision of steam at a pressure of 8 bar and a temperature of 170 °C. The electrically charged thermal energy storage system, using the eutectic mixture of sodium and potassium nitrate as storage material, is designed for a thermal capacity of 50 KWh and a discharge power of 100 kW. The presentation will include experimental details and results of the demonstrator, published for the first time, along with a comparison to numerical simulations, followed by a discussion of scalability and commercial application.