Testing of a 40-kWth Counterflow Particle-Supercritical Carbon Dioxide Narrow-Channel, Fluidized Bed Heat Exchanger

Abstract

Particle-based primary heat exchangers (HXs) must deliver sCO₂ fluid temperatures above 700°C to couple particle-based concentrating solar receivers and thermal energy storage (TES) sub-systems with efficient sCO₂ power cycles. Particle-sCO₂ HX designs have struggled to meet DOE cost targets (≤ $150/kW_th) due to the amount of expensive nickel alloys necessary for manufacturing full-scale, particle-sCO₂ HXs. Our team has demonstrated that mild bubbling fluidization of falling particles in a counterflow narrow-channel fluidized bed can reduce required HX surface area and thus, costs by increasing particle-wall heat transfer coefficients h_T,w > 800 W m^-2 K^-1. This paper reports on the fabrication and testing of a stainless steel, particle-sCO₂ HX with 12 fluidized-bed channels approximately 10.5 mm deep spaced between diffusion-bonded, micro-channel sCO₂ plates. The HX with a core length of ≈0.56 m is fed with CARBOBEAD HSP particles through a short, fluidized freeboard zone just above the core. Testing to date in the National Solar Thermal Test Facility (NSTTF) at Sandia National Laboratories has shown that parallel bed fluidization maintains uniform particle inventory across the instrumented channels. Heat transfer thermal duty between the particle and sCO₂ flows exceeds 30 kW_th with sCO₂ inlet temperatures of 200ºC and particle inlet temperatures up to 440ºC and mass flow rates of 0.2 kg s^-1 fluidized by counterflowing gas flow rates of 0.005 kg s^-1. Tests at higher particle and sCO₂ inlet temperatures (600ºC and 400ºC respectively) are targeted to achieve > 40 kW_th with model-predicted overall heat transfer coefficients U > 400 W m^-2 K^-1.