Oxide particles have potential as effective heat transfer and thermal energy storage (TES) media capable of overcoming high- and low-temperature limitations of molten nitrate salts used in state-of-the-art TES subsystems. Alumina-silica particles offer a low-cost and robust storage media for temperatures above 700°C, which enables TES to be coupled to supercritical-CO~2~ (sCO~2~) cycles. Primary particle-sCO~2~ heat exchangers based on moving packed beds suffer from high particle-wall thermal resistance of 4-5 m^2^ K kW^-1^ \[1,2\], which increases heat exchanger size and TES subsystem costs. Recent studes on fluidized beds of net-downward flowing particles in narrow channels demonstrate much lower particle-wall thermal resistances as low as 1.0 m^2^ K kW^-1^ \[3\]. Such low particle-wall thermal resistances enable particle-sCO~2~ heat exchangers to take advantage of high heat transfer into microchannel sCO~2~ flows and therby greatly reduce primary heat exchanger size and costs that connect particle-based TES subsystems to sCO~2~ power cycles. This study explores the feasiblity of achieving these low particle-wall thermal resistances over longer beds and assesses how operatingconditions and particle properties impact average and variation of particle-wall heat transfer over the bed height.

The present work looks at how bed temperature, superficial gas velocity, particle mass flow rate, and particle diameter influences performance of the fluidized bed (0.25m tall × 0.10m wide × 0.018m deep). Both continuous particle flow mode and batch particle mode operation provide very similar particle-wall heat-transfer coefficients (*h*~T,w~) and fluidization characteristics. The test apparatus has interchangeable instrumented walls to enable various functionality including heating, cooling, and visualization. Heating is provided through an oxidized stainless-steel wall with embedded thermocouples and external radiation from near-IR quartz lamps capable of heat fluxes up to 20 W cm^−2^. Cooling walls convect heat to the ambient or a forced flow through an instrumented wall. Visualization for bed temperatures up to 250°C is provided through an FTO-glass window. Temperatures and pressures within the fluidized bed are monitored at various locations along the bed height.

Extensive particle-wall heat transfer measurements with Carbo Accucast ID50 particles (Sauter mean diameter of 260 um, bulk density of 3340 kg m^‑3^) demonstrate steady operability over a wide range of conditions. The fluidized bed was operated at temperatures up to 450°C, at superficial gas velocities to 0.6 m s^-1^, and particle flow rates to 20 g s^-1^. Fluidization becomes less stable at superficial gas velocities below 125% of the minimum fluidization velocity. Solid volume fractions decrease from 0.56 just above minimum fluidization down to 0.40 at the highest velocities tested. Heat transfer coefficient values increase with bed temperature primarily due to increases in gas thermal conductivity and increased radiative exchange at higher temperatures. In agreement with past studies \[4\], a shallow peak in *h*~T,w~ occurs at intermediate gas velocities, due to tradeoffs of higher particle turnover but reduced solid volume fraction with increasing gas velocities.

Results from this study provide a path for particle-to-sCO~2~ heat exchangers to achieve *h*~T,w~ above 1000 W m^-2^ K^-1^ (i.e. thermal resistances below 1.0 m^-2^ K^-1^ kW) with gas‑to‑particle mass flow ratios below 2% and bed temperatures of 600-700°C. A correlation for *h*~T,w~ fitted to these data is used to design a 40 kW~th~ counterflow particle-to-sCO~2~ heat exchanger, which can demonstrate this technology in a multi-channel heat exchanger at relevant scales for CSP plants and other high-temperature TES applications.

 

References

1.         Albrecht, K.J. and C.K. Ho*,* Solar Energy,178:331,2019.

2.         Watkins, M.F. and R.D. Gould*,* Journal of Heat Transfer-Transactions of the ASME,141(3),2019.

3.         Miller, D.C., C.J. Pfutzner, and G.S. Jackson*,* International Journal of Heat and Mass Transfer, 126:730,2018.

4.         Molerus, O.,Powder Technology,70(1):15,1992.


Effective Heat Transfer in Narrow-Channel Fluidized Beds for Primary Heat Exchangers in Thermal Energy Storage Subsystems

Division / Sponsor:
Advanced Energy Systems Division (AESD) & Solar Energy Division (SED)

Paper Proceedings: https://asmedigitalcollection.asme.org/ES/ES2020/volume/83631

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