DOE OSTI2019
Thermochemical energy storage (TCES) systems offer several advantages over sensible heat storage via molten salts for concentrating solar power (CSP) including: energy density, cost, indefinite storage time, and no risk of freezing. SR’s TCES system utilizes a CaO-based carbonation reaction at high temperature in a fixed bed heat exchanger reactor. The SunShot APOLLO project focuses on the validation of the calcium oxide sorbent for capacity, durability, system economics, and demonstration of the system at a small pilot scale, including system control and operation in simulated on-sun conditions, discharging at 720°C. Under phase I SR completed evaluation of the sorbent loaded in a single-channel heat exchanger reactor. The major accomplishments of phase II are: Design, fabrication and commissioning of a 4 kWhth pilot plant (50x capacity increase from previous bench scale) with a closed-loop CO 2 system, and a commercially relevant heat exchange reactor. The heat exchange reactor has been designed with state-of-the-art modeling tools to ensure 1) sufficient lifetime of the Incoloy body and 2) performance of the packed bed reactor will be adequate to meet the milestones. The rest of the pilot system has been designed with off-the shelf components to maximize reliability and cost effectiveness. The system is highly instrumented and capable of time resolved, independent measurements of the heat and mass balances respectively. Major developments in the technoeconomic analysis (TEA) include 1) reduction of the cost of membrane gas storage for the near-term and future systems and 2) integration with liquid metal receivers and particle receivers for higher decarbonation temperatures and pressures which reduced the cost of gas compression and increased system efficiency. The result are three scenarios with costs bases of 21.9 dollars, 14.3 dollars, and 10.9 dollars/kWhth corresponding to a real LCOE of 59, 57, 56 dollars/MWhe, respectively. The basic operations of the energy storage system were demonstrated for carbonation (discharging) and decarbonation (charging) over short time periods. Excellent control of the CO 2 mass balance across the reactor was achieved with mass flow controllers and the basic operation of the level control of the CO 2 gas bladder was achieved. The system was apparently free from leaks and operated continuously for 18 days with no mechanical failures, thus meeting milestone 2.5.5. The major shortcomings of the system had to do with the controls which prevented us from deep cycling and or reaching high cycles. We were unable to decarbonate (charge) the system for more than a few minutes at a time, which was much shorter than the design target of 10 hrs. This issue could not be resolved due to facility availability and contract closure. One major finding is that CaO-based TCES is most cost efficient when charged with HTF (heat transfer fluid) temperatures >850°C to reach the round-trip exergy efficiency and system cost targets which are higher temperatures than those considered for Gen 3 CSP. Many of the balance of plant components do not exist today, however we are encouraged by our findings that in general with more work we are able to reduce the capital cost, for example with the membrane gas storage, which is an innovative component in itself. The CaO-based TCES system has potential to meet the latest SunShot 2030 goals of LCOE at 50 dollars/MWhe with capital costs of <11/kWhth.