Thermal Storage Research and Development
As part of its research in concentrating solar power (CSP), the U.S Department of Energy (DOE) sponsors research and development (R&D) for thermal storage. The key goals for thermal storage R&D are to
- Support industry in developing and deploying advanced heat-transfer fluids and thermal storage systems
- Develop and characterize advanced heat-transfer fluids and thermal-storage materials and systems to reduce storage costs
- Update and integrate thermal storage cost and performance models into CSP system models.
This page summarizes key thermal storage R&D activities by the National Renewable Energy Laboratory (NREL) and Sandia National Laboratories to attain the above goals in three areas:
Storage Systems R&D
Storage systems R&D focuses on systems analysis issues that relate to evaluating and improving the performance and cost of thermal storage systems for parabolic trough power plants. Representative projects in this area include the following:
Specially developed computer software helps to model and analyze the cost for CSP, including the impact of thermal energy storage.
Thermal Energy Storage Cost Analysis
We are developing a cost model for storage that determines the capital costs of thermal energy storage based on the quantities of commodity materials used to construct and operate the system. Capital costs for the systems can then be updated simply by updating the costs of the required commodity materials. The method considers the practical limits of tank size, including stress constraints and commercial availability.
Thermal Energy Storage Modeling
We model heat-transfer and fluid dynamics within thermal storage systems to better evaluate operation and overall system performance. Comparative modeling of heat transfer and fluid dynamics of the two-tank and thermocline storage systems helps to determine relative performance benefits and to optimize storage configurations and operational strategies. The analysis considers total storage requirements, charging and discharging power requirements, storage-vessel size limitations, vessel orientation, and aspect ratio. Our goal is to determine the optimal storage configurations for likely solar field sizes that minimize the levelized cost of electricity.
Advanced Heat-Transfer Fluids R&D
Advanced heat-transfer fluids are essential for improving the operation of CSP systems. Historically, trough systems have used synthetic heat-transfer oils, power towers have demonstrated direct steam generation and molten-salt working fluids, and dish/engine systems have used hydrogen and helium as working fluids. We are developing heat-transfer and storage fluids that provide improved efficiency and lower cost for CSP systems. Representative projects in this area include the following:
Nitrate Salt Formulations
Sandia has developed and characterized a new low-freezing (100°C) nitrate salt formulation. Researchers are now characterizing the properties and performance of their new nitrate salt formulation for use in parabolic trough power plants.
Aluminum nanoclusters can be tailored for unique thermochemical properties.
Nanofluids
Solutia Inc.'s Therminol VP-1 is the current heat-transfer fluid of choice in parabolic trough solar fields. It is thermally stable to about 400°C and has a low freezing point of 12°C. NREL is studying a new class of nanofluids that possess enhanced thermophysical properties including thermal conductivity, heat capacity, freezing point, boiling point, and high-temperature thermal stability. One goal is to develop heat-transfer fluids with higher operating temperatures, which allow a plant to operate with greater efficiency. A second goal is to develop storage fluids with higher heat capacities, which allow the use of much less storage fluid for the same amount of storage capacity.
Non-Nitrate Molten-Salt Fluids
We are pursuing non-nitrate salts and other inorganic substances as heat-transfer and storage fluids. We are identifying candidates that possess the thermophysical and fluid properties of the current nitrate salts, but without the potential corrosive properties of some nitrate salts.
Materials Laboratory for Thermal Storage
We are establishing a materials laboratory to develop and characterize advanced heat-transfer fluids, advanced storage fluids, and advanced phase-change materials for thermal storage applications. These materials will lead to increased thermal storage capacity, thermodynamic efficiency, and dispatchability, as well as lower levelized energy costs for CSP power plants.
Laboratory capabilities will support experiments and validate molecular dynamic modeling studies of nanophase materials and will allow the study of novel phase-change materials. Such materials are essential for implementing thermal storage that is compatible with direct steam generation—which is being considered as the next-generation heat-transfer fluid for parabolic trough power plants.
Industry Support
NREL and Sandia provide technical testing and evaluation support to its industry partners. DOE also coordinates industry support for CSP thermal energy storage as it collaborates with national laboratories in Europe—CIEMAT in Spain and DLR in Germany.
Some of DOE's technical activities support the awardees of financial opportunities for concentrating solar power. The complete list of DOE-funded CSP projects is available on the Concentrating Solar Power Industry Projects page that specifically includes the thermal storage projects.