1Thermal ratcheting is a critical phenomenon associated with the cyclic operation of dual-

A thermocline tank is a low-cost thermal energy storage subsystem for concentrating solar power plants that typically utilizes molten salt and quartzite rock as storage media. Longterm thermal stability of the storage concept remains a design concern. A new model is developed to provide comprehensive simulation of thermocline tank operation at low computational cost, addressing deficiencies with previous models in the literature. The proposed model is then incorporated into a system-level model of a 100 MW e power tower plant to investigate storage performance during long-term operation. Solar irradiance data, taken from measurements for the year 1977 near Barstow, CA, are used as inputs to the simulation. The heliostat field and solar receiver are designed with DELSOL, while the transient receiver performance is simulated with SOLERGY. A meteorological year of plant simulation with a sixhour capacity for the thermocline tank storage yields an annual plant capacity factor of 0.531. The effectiveness of the thermocline tank at storing and delivering heat is sustained above 99% throughout the year, indicating that thermal stratification inside the tank is successfully maintained under realistic operating conditions. Despite its good thermal performance, structural stability of the thermocline tank remains a concern due to the large thermal expansion of the internal quartzite rock at elevated molten-salt temperatures, and requires further investigation.

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Molten-salt thermocline tanks are a low-cost option for thermal energy storage in concentrating solar power (CSP) systems. A review of previous molten-salt thermocline tank studies is performed to identify key issues associated with tank design. Discharge performance improves with both larger tank height and smaller internal filler diameter due to increased thermal stratification and sustained outflow of molten salt with high thermal quality. For wellinsulated (adiabatic) tanks, low molten-salt flow rates reduce the axial extent of the heatexchange region and increase discharge efficiency. Under non-adiabatic conditions, low flow rates become detrimental to stratification due to the development of recirculation zones inside the tank. For such tanks, higher flow rates reduce molten-salt residence time inside the tank and improve discharge efficiency. Despite the economic advantages of a thermocline tank, thermal ratcheting of the tank wall remains a significant design concern. The potential for thermal ratcheting is reduced through the inclusion of an internal insulation layer between the molten salt and tank wall.

Molten-salt thermocline tanks are a low-cost energy storage option for concentrating solar power plants. Despite the potential economic advantage, the capacity of thermocline tanks to store sufficient amounts of high-temperature heat is limited by the low energy density of the constituent sensible-heat storage media. A promising design modification replaces conventional rock filler inside the tank with an encapsulated phase-change material (PCM), contributing a latent heat storage mechanism to increase the overall energy density. The current study presents a new finite-volume approach to simulate mass and energy transport inside a latent heat thermocline tank at low computational cost. This storage model is then integrated into a systemlevel model of a molten-salt power tower plant to inform tank operation with respect to realistic solar collection and power production. With this system model, PCMs with different melting temperatures and heats of fusion are evaluated for their viability in latent heat storage for solar plants.Thermocline tanks filled with a single PCM do not yield a substantial increase in annual storage or plant output over a conventional rock-filled tank of equal size. As the melting temperature and heat of fusion are increased, the ability of the PCM to support steam generation improves but the corresponding ability of the thermocline tank to utilize this available latent heat decreases. This trend results from an inherent deconstruction of the heat-exchange region inside the tank between sensible and latent heat transfer, preventing effective use of the added phase change for daily plant operations. This problem can be circumvented with a cascaded filler structure composed of multiple PCMs with their melting temperatures tuned along the tank height. However, storage benefits with these cascaded tank structures are shown to be highly sensitive to the proper selection of the PCM melting points relative to the thermocline tank operating temperatures.3