This passive, hermetically sealed, and highly efficient TES system scales to many megawatt hours of energy storage and can be integrated with various power conversion systems and other energy storage applications
THE GREAT TES INNOVATION
The utilization of TES to avoid the major intermittency issues associated with PV power generation is the key differentiator for CSP systems. Molten salt TES systems that store sensible heat energy in multiple tanks, which is later extracted over a range of temperatures, are now standard with utility scale CSP. They are functionally practical but are costly and complex subsystems that require significant maintenance. Phase-change salt TES has many advantages, including a high latent heat of fusion energy storage at a single temperature. Several groups have attempted to implement phase-change TES, but the low thermal conductivity and large volume increase of the salt during melting have prevented practical solutions for all but very small capacity systems. GREAT TES eliminates those problems with a patented approach (White, M., & Brehm, P., Patent No. 8,464,535, Systems, Apparatus and Methods for Thermal Energy Storage, Coupling and Transfer.) that uses a sodium pool boiler directly integrated with the TES salt. This passive, hermetically sealed system scales to many megawatt hours of energy storage and can be integrated with various power conversion systems. It is described further below.
GREAT TES DESCRIPTION
A schematic of an FPS engine integrated with GREAT TES is illustrated in Figure 6. The engine heater head is welded into the top of the TES containment vessel to always remain above the salt and sodium below. Since the solid salt is typically about 30% more dense than the liquid, any liquid salt will float above the solid. The Na is much less dense than the liquid salt, so it floats on the liquid salt surface. At the sodium/salt interface, the salt transfers its heat of fusion to the sodium, which solidifies the salt and vaporizes the sodium. In the insulated vapor space above the sodium layer, the engine heater head where heat is being extracted is the coolest part, so the sodium condenses on the heater head to function as the engine heat source. The liquid condensate then returns by gravity to the sodium pool. Once all the salt has solidified, the process stops, and the engine will cool down unless solar or combustion heating has resumed to begin re-melting the salt.
The GREAT TES concept was successfully integrated with an Infinia 3.5-kW FPS engine and demonstrated in the lab. In the laboratory test module shown in Figure 7, a NaF/NaCl eutectic salt that melts at 680 C was used. One identified issue is the tendency for a crystalline surface layer to form and attach to the container wall during cooldown. This prevents the remaining molten salt below that level from properly interacting with the sodium layer. It was shown that problem could be eliminated by using trace heating to maintain the circumference of the TES vessel above the TES melt temperature in the region that can have a liquid/solid salt interface during the heat extraction mode.
This approach can also be readily adapted to hybrid operation using hydrogen, natural gas, or other fuels to enable continuous power production during periods of extended cloudy conditions. Lithium salts generally have a significantly higher heat of fusion than other salts, but those are tentatively avoided for utility scale storage so Li cost or availability will not be an issue. The high melting temperature of the salt used in the demonstration unit requires relatively high-temperature containment materials. There are many other salt eutectics available in the range of 550 C to 650 C that can use more common lower cost containment steels, with some compromise in system efficiency due to the lower operating temperature. Choices will be made based on overall system optimization and LCOE considerations.
Figure 6 - Schematic illustration of a Stirling engine integrated with GREAT TES
Figure 7 - Laboratory test unit for GREAT TES concept
GREAT TES SOLAR CONCEPT IMPLEMENTATION
Delta Stirling engines integrated with GREAT TES can be installed with parabolic dish concentrators or used in conjunction with small central receivers (aka power towers) under the Central Receiver Comparisons tab. A conceptual TES tank configuration designed to be heated by a small CR with heliostat mirrors focused on the tower receiver is conceptually illustrated in Figure 8. A state-of-the-art molten salt receiver can be used with a molten salt heat transport system to melt the salt in a ground mounted TES tank, but the preferred approach is a two-phase liquid metal receiver and heat transport system.
A wide range of capacity levels are feasible. A conceptual design for a 1-MW installation with 12 hours of TES was conducted. The TES tank is 20’ in diameter by 18’ high with 22 48-kW delta engines using current commercial Qnergy 8-kW alternators installed on top as 11 stacked pairs. This would be capable of producing 1-MW continuously by using fuel for heating during cloudy periods. Future versions are expected to use fewer engines with higher capacity to reach the same power level while reducing manufacturing cost.
