Zhen Yang

Review of Molten-Salt Thermocline Tank Modeling for Solar Thermal Energy Storage

Molten-salt thermocline tanks are a low-cost option for thermal energy storage in concentrating solar power systems. A review of previous experimental and numerical thermocline tank studies is performed to identify key issues associated with tank design and performance. Published models have shown that tank 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 well-insulated (adiabatic) tanks, low molten-salt flow rates reduce the axial extent of the heat-exchange region and increase discharge efficiency. Under nonadiabatic conditions, low flow rates become detrimental to stratification due to the development of fluid 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 thermal insulation layer between the molten salt and tank wall to diminish temperature oscillations along the tank wall. Future research directions are also pointed out, including combined analyses that consider the solar receiver and power generation blocks as well as optimization between performance and economic considerations.

Thermomechanical Simulation of the Solar One Thermocline Storage Tank

The growing interest in large-scale solar power production has led to a renewed exploration of thermal storage technologies. In a thermocline storage system, heat transfer fluid (HTF) from the collection field is simultaneously stored at both excited and dead thermal states inside a single tank by exploiting buoyancy forces. A granulated porous medium included in the tank provides additional thermal mass for storage and reduces the volume of HTF required. While the thermocline tank offers a low-cost storage option, thermal ratcheting of the tank wall (generated by reorientation of the granular material from continuous thermal cycling) poses a significant design concern. A comprehensive simulation of the 170 MWht thermocline tank used in conjunction with the Solar One pilot plant is performed with a multidimensional two-temperature computational fluid dynamics model to investigate ratcheting potential. In operation from 1982 to 1986, this tank was subject to extensive instrumentation, including multiple strain gages along the tank wall to monitor hoop stress. Temperature profiles along the wall material are extracted from the simulation results to compute hoop stress via finite element models and compared with the original gage data. While the strain gages experienced large uncertainty, the maximum predicted hoop stress agrees to within 6.8% of the maximum stress recorded by the most reliable strain gages.

Rarefied Gas Flow in Microtubes at Different Inlet-Outlet Pressure Ratios

A model is developed for rarefied gas flow in long microtubes with different inlet-outlet pressure ratios at low Mach numbers. The model accounts for significant changes in Knudsen number along the length of the tube and is therefore applicable to gas flow in long tubes encountering different flow regimes along the flow length. Predictions from the model show good agreement with experimental measurements of mass flow rate, pressure drop, and inferred streamwise pressure distribution obtained under different flow conditions and offer a better match with experiments than do those from a conventional slip flow model.

Thermocline Energy Storage in the Solar One Power Plant: An Experimentally Validated Thermomechanical Investigation

The growing interest in large-scale solar power production has led to a renewed exploration of thermal storage technologies. In a thermocline storage system, heat transfer fluid (HTF) from the collection field is simultaneously stored at both excited and dead thermal states inside a single tank. A granulated porous medium included in the tank provides thermal mass for storage and reduces the amount of HTF volume required. While the thermocline offers a low-cost storage option, thermal ratcheting of the tank wall (generated by filler material reorientation from continuous thermal cycling) poses a significant design concern. A comprehensive simulation of the 170 MWht thermocline tank used in conjunction with the Solar One pilot plant is performed with a multi-dimensional two-temperature computational fluid dynamics model. In operation from 1982 to 1986, this tank was subject to extensive instrumentation, including multiple strain gages along the tank wall to monitor hoop stress. Temperature profiles along the wall material are extracted from the simulation results to compute hoop stress via finite element models and compared with the original gage data. While the strain gages experienced large uncertainty, the stresses computed from the simulation agree reasonably well with the experimental measurements. The maximum predicted hoop stress agrees to within 6.8% of the maximum stress recorded by the most reliable strain gages.Copyright