Troubleshooting a Counterflow, Direct Contact, Solid-Air Heat Exchanger Inspired by Fluidized Beds

High temperature thermal energy storage (TES) is a valuable technology for long-duration energy storage (LDES). Particle-based TES is a potentially cost-effective storage medium; however, there are underlying challenges in particle-to-working fluid heat transfer. Presented here is a direct contact, counterflow, shallow, fluidized bed (FB) heat exchanger (HX). The particles tested belong in Geldart Group B and have a mean diameter of 625..mu..m. There are fundamental tradeoffs between the behavior of a fluidized bed and desired temperature gradients and thermal stratification commonly found in counterflow heat exchangers. It is shown that with the constraints imposed by establishing suitable fluidization conditions, there is a narrow window of operation for FB HX with this geometry and particle type. Operating this FB HX with a fluidizing air flow rate of 24.2 SCFM (0.0140 kg/s) yields a mixed bed condition whereby the bottom of the bed is nominally fluidized (u/umf>1) but operates more similarly to a packed particle bed due to u/umf<1 for the upper portion of the bed. At a fluidization air flow rate of 24.9 SCFM (0.0144 kg/s), the FB HX is fully fluidized and heat transfer between air and particles is significantly less than in the mixed bed condition. This illustrates the sensitivity of the FB HX to the fluidization air flow rate, and the tradeoff between fluidization behavior and HX performance.