Numerical investigation of a high temperature stratified storage with integrated steam generator

A stratified single tank is an alternative to a two-tank storage system for solar thermal power plants since it is promising cost reductions and shows more potential for further technical development. Direct integration of the steam generator is one of the possibilities. In this paper a higher order finite volume modeling approach for the storage is presented which reduces numerical diffusion but applies a turbulence factor which determines stratification. The discharge of the storage and the circulation of molten salt through the shell side of the internal steam generator by a forced flow is compared to natural circulation. The buoyancy driven flow strongly depends on the system design. The performance is improved with higher positioning of the heat sink in the storage system since a lower minimum charging level of the storage is required to obtain the design flow rate. The difference between the operation modes is more distinctive in the morning and the evening, when the charging level is relatively low and during days with low irradiation. With the chosen configuration both operation modes lead to the same circulation rate as soon as the charging level exceeds 24%. An annual simulation showed a reduction of total produced electricity by 3% for natural circulation compared to forced circulation. In case of forced circulation 95% of the cumulated charging power can be discharged, pure natural circulation achieves a discharge rate of 91%. To achieve the same performance the buoyancy driven flow needs to be supported by an impeller in 9% of the operational time