The rotating drum heat exchanger for latent heat thermal energy storage

Abstract

Since the industrial revolution, the evaporation of water has been the most common method for providing industrially usable energy. As of today, important primary energy sources for the evaporation are limited and globally unevenly distributed fossil fuels, whose combustion releases greenhouse gases. For a transition in the energy system to the use of fluctuating renewable energies, such as wind and solar, suitable energy storages can be used to meet a demand-based energy supply. With regenerator type latent heat thermal energy storages, thermal energy can be stored at a constant temperature level by utilizing the phase change enthalpy of a phase change material (PCM). Since the evaporation of a liquid is also an isothermal process, a low and constant temperature difference can be realized when transferring heat from a PCM to an evaporating liquid; this can be used for increased exergetic efficiency. During the evaporation/discharging process of a latent heat thermal energy storage, a solid layer of PCM solidifies on the heat transfer surface, which reduces the heat transfer, as many PCMs have a low thermal conductivity. In the novel Rotating Drum Heat Exchanger studied in this thesis, a rotating drum is partially immersed in liquid PCM. While a liquid evaporates on the inner side of the drum, PCM solidifies on the outer side. The solidified PCM is removed by a stationary scraper with each rotation. Therefore, the average layer thickness of the solidifying PCM and thus the heat transfer can be controlled by the rotational speed. Furthermore, the solidified PCM can be stored separately from the liquid material, resulting in complete separation of power and capacity of a storage system based on the Rotating Drum Heat Exchanger. An experimental test rig developed as part of this work serves as proof of concept of the Rotating Drum Heat Exchanger, using the low-temperature PCM decanoic acid and water as the heat transfer fluid. Thereby, a surface specific heat transfer density of up to 6.8 kW∙m-2 could be measured at a temperature difference of 5 K between the melting point of the PCM and the temperature of the heat transfer fluid. The experimentally examined heat transfer is used to validate a developed numerical model, which reproduces the experimental heat transfer with an accuracy of 8 % on average. To obtain a freely scalable design, the so-called multiple-channel drum is developed for generating steam in the multi-megawatt range. Therein, nitrate salts are proposed as the PCM, similar to the material used in state-of-the-art two-tank molten salt storages, which utilize only the sensible heat storable in the liquid phase. The Rotating Drum Heat Exchanger is able to transfer both the latent as well as the sensible heat of the storage material. The thermal capacity of the material can thereby be increased by up to 60 % compared to the state-of-the-art, which might result in cost savings for the storage system.