Investigation of Molten Salt/Air Direct-Contact Heat Exchangers for the Storage of Thermal Energy in Power Plants

The integration of renewable-based molten-salt thermal energy storage into conventional fuel-firing power plants is an attractive approach to reduce the consumption of fossil fuels and increase the flexibility in the power sector. With respect to economic viability, molten salt/air direct-contact heat exchanger is proposed for the integration to maximize the utilization of existing facilities in power plants, all the while minimizing additional investments. The main idea of this concept is to supplement a part of the combustion air with hot air from the storage system to reduce fossil fuel consumption. This introduction of molten salt/air direct-contact heat exchangers would eliminate the need to invest in molten salt steam generators. In this study, packed and spray direct-contact heat exchangers are both investigated and compared, in terms of technical and economical performances. For this purpose, a model for the molten salt/air direct-contact heat exchangers is developed and validated by using empirical correlations and experiments from literature. On the one hand, overall volumetric heat transfer coefficients and gas pressure drops can be predicted with this model, on the other hand, optimum dimensions and operating limits can be found. The simulation results suggest that the packed and spray direct-contact heat exchangers can offer high overall volumetric heat transfer coefficients. They are comparable to indirect-contact heat exchangers such as a finned tube heat exchanger that require large amounts of steel. However, both direct-contact heat exchangers have different operating concerns. Risk of flooding is the main concern for the packed columns. On the other hand, the problem of entrained molten salt droplets in the outlet air flow can occur in the spray columns which are operated with excessive air velocities. Considering the economic aspect, the packed columns are estimated to be more expensive than the spray columns operated with very fine molten salt droplets in most of the cases. In particular, the spray direct-contact heat exchangers operated with very fine molten salt droplets are expected to be more competitive than the packed direct-contact heat exchangers for the integration of the thermal energy storage into conventional power plants