Comparative Analysis of Single- and Dual-Media Thermocline Tanks for Thermal Energy Storage in Concentrating Solar Power Plants

Abstract

A molten-salt thermocline tank is a low-cost option for thermal energy storage (TES) in concentrating solar power (CSP) plants. Typical dual-media thermocline (DMT) tanks contain molten salt and a filler material that provides sensible heat capacity at reduced cost. However, conventional quartzite rock filler introduces the potential for thermomechanical failure by successive thermal ratcheting of the tank wall under cyclical operation. To avoid this potential mode of failure, the tank may be operated as a singlemedium thermocline (SMT) tank containing solely molten salt. However, in the absence of filler material to dampen tank-scale convection eddies, internal mixing can reduce the quality of the stored thermal energy. To assess the relative merits of these two approaches, the operation of DMT and SMT tanks is simulated under different periodic charge/discharge cycles and tank wall boundary conditions to compare the performance with and without a filler material. For all conditions assessed, both thermocline tank designs have excellent thermal storage performance, although marginally higher firstand second-law efficiencies are predicted for the SMT tank. While heat loss through the tank wall to the ambient induces internal flow nonuniformities in the SMT design over the scale of the entire tank, strong stratification maintains separation of the hot and cold regions by a narrow thermocline; thermocline growth is limited by the low thermal diffusivity of the molten salt. Heat transport and flow phenomena inside the DMT tank, on the other hand, are governed to a great extent by thermal diffusion, which causes elongation of the thermocline. Both tanks are highly resistant to performance loss over periods of static operation, and the deleterious effects of dwell time are limited in both tank designs.

Challenges and Roadmap for Solar-Thermal Desalination

“…Solid-state packed-bed storage media experience a high degree of temperature stratification due to the large density difference of hot and cold fluid and can form a thermocline region between hot and cold regions which is susceptible to mixing. The thermocline region needs to be minimized to maximize high temperature storage. ,− …”

Wall impact on efficiency of packed-bed thermocline thermal energy storage system

“…The discharge efficiency of the thermocline tank was found to increase with the increasing 𝑅𝑅𝑅𝑅 , opposite to the trend under adiabatic condition. Mira-Hernández et al [32] reported that the heat loss under non-adiabatic condition could cause inhomogeneous internal flow for the whole tank due to the cooling of molten salt near the tank wall. But the total heat lost during charge/discharge the cycle was estimated to be negligible compared to the heat storage capacity.…”

Thermo-mechanical parametric analysis of packed-bed thermocline energy storage tanks

“…Air worked as HTF and no liquid option was contemplated. Another concept removes all the solid filler material, obtaining a single-media thermocline tank with fluid only [10]. Even though convective mixing flows are significant without the porous media, the thermal diffusivity lessens, so it can achieve a slightly better thermal function.…”