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Solar thermal energy storage (TES) has the potential to significantly increase the operating flexibility of solar power. TES allows solar power plant operators to adjust electricity production to match consumer demand, enabling the sale of electricity during peak demand periods and boosting plant revenues. To date, TES systems have been prohibitively expensive except in certain markets. Two of the most significant capital costs in a TES system are the storage medium (typically molten salt) and the storage tanks. Thermocline storage is a relatively unproven TES method that has the potential to significantly reduce these costs. In a thermocline system, approximately 75% of the required storage medium is replaced with an inert quartzite rock, and only one storage tank is required instead of the two typically needed for high-temperature TES. This report includes preliminary designs and cost estimates for molten salt thermocline systems with capacities ranging from pilot scale to commercial scale. Thermal
and system level modeling was conducted to determine the performance of these systems.
ObjectiveThe potential benefits of TES are significant; however, experience is limited and costs and performance of the various technology options remain uncertain. The intent of this study was to develop a basic design for the thermocline technology as well as detailed process flow diagrams, heat and material balances, and detailed equipment lists that provide a starting point to develop this technology at pilot scale and later at full commercial scale. One key objective was to provide a meaningful comparison to the current state-of-the-art two-tank TES technology to determine whether the thermocline cost and performance might be competitive.
ApproachThe approach was to define and optimize parameters for several design cases and develop AACE Class 4 project estimates for each. The main components of the Class 4 estimate are the design basis, process flow diagrams, and equipment lists. These design details allowed the project team to work with vendors and obtain quotes for necessary equipment. These data were used to complete an EPC estimate for the construction of the thermocline systems. In parallel, thermodynamic and system models were developed to determine the thermal stability of the system under different operating scenarios and examine the performance of thermoclines.
ResultsThe study determined the application areas in which thermoclines might be economically competitive. Cost estimates were developed for the construction of thermocline systems, and similar estimates were developed for the corresponding state-of-the-art two-tank storage systems for comparison. Both parabolic trough indirect storage and central receiver direct storage systems were evaluated, ranging from 100 to 3500 thermal megawatt-hours (MWht) in size. The results confirm that the thermocline offers the lowest installed capital cost over the two-tank system at each design capacity.
Application, Value and UseAlthough the cost of solar energy is still high compared to traditional generation options, this cost is expected to decrease as technologies mature and deployment increases. Thermal energy storage presents a unique opportunity to reduce the levelized cost of electricity while providing increased plant operating flexibility and energy value. This report shows that thermocline systems might offer a lower cost option for a wide range of solar technologies and storage applications. The results of this study will be beneficial to any energy company or project developer considering a solar thermal project.
EPRI PerspectiveThe first utility-scale concentrating solar thermal power plants in the world were built in southern California in the 1980s, and several new large-scale plants are currently under development in the United States, Spain, and other locations throughout the world. Although a two-tank molten salt system is now operational in Spain, there has been limited RD&D in the United States to implement the multi-tank molten salt technology and develop next-generation TES technologies. There is also a need to determine the ideal operation and integration of those technologies into electric grid operations. The thermocline process has the potential to significantly reduce the costs of thermal energy storage without compromising performance, which could in turn greatly increase the utility of solar power and lead to wide-scale adoption of the technology. Continued research along with the operation of the first utility-scale thermal energy storage units in the next few years will provide more concrete data by which to compare thermal energy storage systems. This work supports a long-term vision for a broad generation portfolio that includes renewable energy as a cost-competitive option.
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