Thermomechanical Simulation of the Solar One Thermocline Storage Tank

Flueckiger, Scott M.; Yang, Zhen; Garimella, Suresh V.

doi:10.1115/1.4007665

Cited by 38 publications

(12 citation statements)

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“…As a device that stores energy solely by sensible heat, the low energy density of the molten-salt and rock volume requires large tank diameters to store sufficient quantities of high-temperature heat (the tank height itself is constrained by the bearing capacity of the underlying soil). A large tank diameter is undesirable as it increases the potential for both maldistribution of fluid flow inside the porous bed and a structural instability known as thermal ratcheting [8,9]. A design modification that has been proposed for reducing the tank size (by increasing energy density) is a substitution of the internal filler rock with a phase-change material (PCM).…”

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confidence: 99%

Latent heat augmentation of thermocline energy storage for concentrating solar power – A system-level assessment

Flueckiger¹,

Garimella²

2014

Applied Energy

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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

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Latent heat augmentation of thermocline energy storage for concentrating solar power – A system-level assessment

Flueckiger¹,

Garimella²

2014

Applied Energy

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“…One disadvantage of DMT tanks is the possibility for mechanical failure by thermal ratcheting [14]. During cyclic operation, the tank wall and the internal filler material undergo differential thermal expansion and contraction.…”

Section: Introductionmentioning

confidence: 99%

“…This disparity can lead to separation between the wall and internal filler; reorientation of the filler bed to settle into this annular gap (i.e., interference) results in mechanical stress along the tank wall. If this stress is sufficient for the tank wall material to yield, repeated wall expansions (or "ratchets") may accumulate with each storage cycle until catastrophic failure [14]. Thermal ratcheting cannot occur in the SMT tank design due to the absence of a filler material.…”

Section: Introductionmentioning

confidence: 99%

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

Mira-Hernández

Flueckiger

Garimella

2015

Journal of Solar Energy Engineering

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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.

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“…Yang and Garimella [3] developed a multidimensional computational fluid dynamics (CFD) model to simulate mass, momentum, and energy transport inside the thermocline tank. Flueckiger et al [4] later extended this model to simulate hoop stress in the original Solar One thermocline tank and verify the prevention of thermal ratcheting phenomena. Van Lew et al [5] developed a solution based on the method of characteristics for energy transport inside the thermocline tank.…”

Section: Introductionmentioning

confidence: 99%

Economic Optimization of a Concentrating Solar Power Plant With Molten-Salt Thermocline Storage

Flueckiger

Iverson

Garimella

2013

Journal of Solar Energy Engineering

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System-level simulation of a molten-salt thermocline tank is undertaken in response to year-long historical weather data and corresponding plant control. Such a simulation is enabled by combining a finite-volume model of the tank that includes a sufficiently faithful representation at low computation cost with a system-level power tower plant model. Annual plant performance of a 100 MW e molten-salt power tower plant is optimized as a function of the thermocline tank size and the plant solar multiple (SM). The effectiveness of the thermocline tank in storing and supplying hot molten salt to the power plant is found to exceed 99% over a year of operation, independent of tank size. The electrical output of the plant is characterized by its capacity factor (CF) over the year, which increases with solar multiple and thermocline tank size albeit with diminishing returns. The economic performance of the plant is characterized with a levelized cost of electricity (LCOE) metric. A previous study conducted by the authors applied a simplified cost metric for plant performance. The current study applies a more comprehensive financial approach and observes a minimum cost of 12.2 ¢/kWh e with a solar multiple of 3 and a thermocline tank storage capacity of 16 h. While the thermocline tank concept is viable and economically feasible, additional plant improvements beyond those pertaining to storage are necessary to achieve grid parity with fossil fuels.

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Thermomechanical Simulation of the Solar One Thermocline Storage Tank

Cited by 38 publications

References 10 publications

Latent heat augmentation of thermocline energy storage for concentrating solar power – A system-level assessment

Latent heat augmentation of thermocline energy storage for concentrating solar power – A system-level assessment

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

Economic Optimization of a Concentrating Solar Power Plant With Molten-Salt Thermocline Storage

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