Low-Cost Particle-to-CO2 Moving Bed Heat Exchanger

A key advantage of particle-based solar receivers is that the solar radiation directly interacts with the heat transfer medium (the particles), thus eliminating the typical tubular heat exchanger receiver of a molten salt or gas-phase heat transfer fluid. However, a challenge with particle-based CSP systems is the transfer of thermal energy into the power cycle working fluid, expected to be sCO2 for the Gen3 CSP systems. Particle-to-fluid heat exchangers have been developed for lower-temperature and lower fluid pressure applications using a “pillow plate” construction approach in a Moving Bed Heat Exchanger (MBHE). However, for the high temperatures planned for Gen3 systems, a more capable approach is needed. To date, the higher pressure of sCO2 cycles (25-30 MPa) has been theoretically accommodated by using diffusion bonded “Printed Circuit Heat Exchanger” (PCHE) technology in place of the pillow plate. While PCHEs have been used successfully, including by EPS, for sCO2 service, their use in MBHEs has several disadvantages: The particle bed flows through narrow channels to enable efficient heat transfer between the particles and the solid surface. A common mode of failure in particle flowfields is known as “bridging”, where multiple particles form a quasi-stable “bridge” between two surfaces. Not only does this interrupt flow locally, but in the long channels of the PCHE-based MBHE, these bridges increase the risk of entirely blocking the flow to one or more channels and degrading the performance of the heat exchanger. A PCHE-MBHE uses a monolithic block of material. Above 600°C, the reduction in yield strength and creep resistance of austenitic stainless steels precludes their use in pressure-containment, requiring high-cost nickel-based alloys such as INCONEL alloy 740H. However, the highest- strength material available for diffusion-bonded PCHE service is INCONEL alloy 617, which has a 50% lower allowable stress at 700°C. For the proposed project, Echogen will team with a commercial heat exchanger manufacturer, Super Radiator Coils, and an expert in bulk material flow handling, Greg Mehos, to develop a finned tube MBHE for Gen3 particle receiver conditions. The finned tube arrangement is commonly used in gas-fluid heat exchangers, with heat transfer coefficients on the gas side that are even lower than the particle- side coefficients of the MBHE. Full flexibility over fin spacing, tube arrangement, and material selection will be utilized to define a lower-cost MBHE for the G3P3 program. By using individually-finned tubes, bridging issues would be limited to local field issues only. Finned tube heat exchangers can use INCONEL alloy 740H or Haynes 282 for the tubing material, and stainless steel for the fins, substantially reducing the raw material costs of the heat exchanger. And finally, conventional tube-forming and fin-attachment technology can be utilized, avoiding the high cost of chemically etching and diffusion bonding sheets of INCONEL alloy 617.