Coupling Supercritical and Superheated Direct Steam Generation with Thermal Energy Storage

Two new concepts have been identified for direct steam generating solar fields that charge a two-tank molten salt storage system. The use of both supercritical and superheated subcritical steam is proposed, and discussed in practical terms and with regards to thermodynamic issues of heat transfer to the salt. Both concepts are found to allow significantly higher hot-tank temperatures than solar systems restricted by temperature limitations in thermal oil, with achievable temperatures comparable to solar tower systems that directly heat salt. This a distinct advantage for minimising the amount of salt required for a given amount of energy storage. Analysis of steam cycle efficiency using these results as a basis once again compared the supercritical and subcritical cases. For the power cycle, it was shown that there are advantages in using a supercritical power block when discharging from molten salt storage, because of the improved matching of the salt and steam cooling curves. Two examples were developed, and an increase in thermal conversion efficiency from 42.4% to 43.8% was found comparing the subcritical to the supercritical steam cases, were a suitable turbine available. Overall, energy transport and storage concepts are presented that maximise the advantages of high temperature that are achievable from high-concentration solar collectors such as solar dishes, in particular, minimisation of molten salt quantities in storage, and maximisation of thermal cycle efficiencies.