High temperature latent heat storage with a screw heat exchanger: Design of prototype

An important option for increasing the efficiency in Concentrated Solarthermal Power (CSP) is the development of systems generating steam directly in the solar field. To produce dispatchable and affordable electrical power, the integration of an efficient and economical thermal energy storage is an attractive option. To (dis)charge a storage with steam, latent heat storages are particularly suitable for the use with two-phase heat transfer media. Due to low heat conductivities of the phase change materials, the heat exchanger design is crucial. Up to now no latent storage is commercially deployed in CSP. A new latent storage approach is being developed at Fraunhofer ISE; a Screw Heat Exchanger (SHE) is employed for heat transfer. With a SHE, material can be transported during the phase change. With this innovative concept, the size of the heat transfer area and the storage capacity can be decoupled and it seems possible to develop an economically viable latent storage.To understand the dynamics of the phase change and the heat transfer characteristics in the SHE, testing will be done with a laboratory prototype. The phase change material (PCM) used is an eutectic mixture of NaNO3 and KNO3 with a melting point at Tm = 221 °C. It is a salt mixture with low price and high availability, as is the requirement for the development of a low cost thermal energy storage.A proof of concept for the storage concept has been obtained. The problematic crystallization process in the SHE was successfully performed. A thermal power of Q = 11 kW was transferred to a salt mass flow of about 50 kg/h. It showed the feasibility of the transport of a PCM during the phase change due to the self-cleaning effect of the screws. The heat exchanger has been designed, constructed and equipped with measurement devices. Peripheral transport components have been selected to allow semi-automated storage operation. The storage system will be tested to gain experience for the design of process equipment necessary for the eventual use in large scale solarthermal power plants. Parallel to the commissioning, the heat exchanger design was assessed and improved. The heat transfer characteristics of the storage is dependent on parameters as the PCM mass flow, the rotational speed of the screw shafts, the temperature difference of PCM to the heat transfer media, the inclination of the heat exchanger and the granule size. The correlation between these factors will be examined in further experiments to assess their influence on operation strategies.

► Specific cost of latent heat storage materials suited for CSP applications.
► Screw heat exchangers allow an increase of capacity without increasing the heat transfer area.
► Phase change during transport of salt inside screw heat exchanger is possible.