Heat extraction for the CSPonD thermal storage unit

Abstract

Three coiled tube heat exchanger prototypes were designed to extract heat from containers holding 0.5 kg, 2.3 kg, and 10.5 kg of Sodium Nitrate-Potassium Nitrate salt. All of the prototypes were left with an open surface free to undergo radiation losses and surface convection. The first objective was to measure the power extraction over time for each prototype. Coiled tube heat exchangers were modeled as a tube with a constant wall temperature. Air is used as the working fluid, with a maximum Reynolds number of 2000 at a maximum flow rate of 10 standard liters per minute (SLPM) at air flow temperatures above 900°C. The accuracy of the power extraction model for the three prototypes in increasing order: 46 %, 35 %, and 43 % of the measured data. The duration of power extraction with an open top container for the first (P-1), second (P-l1), and third (P-II) prototype respectively are: 14 min, 29 min, 45 min producing an average power of 22 W, 23 W, and 22 W respectively. To compare across the prototypes, the data provided is for bath bulk temperatures starting at 330°C and ending at 275' C. Prototype three produced 25 W for 123 minutes for the same temperature change in the bulk temperature (330° to 275°C) with the lights off and a thermal lid, to reduce radiation and surface convection losses. The thermal lid improved the extraction duration by a factor of four. The second objective was to characterize the thermal loss rate (W) of the each prototype. The thermal loss rate model is accurate within 28.9 % (P-1), 28.7 % (P-11), and 24.7 % (P-III) of the measured values. There is evidence of convection cells in prototype two and three. A high temperature Particle Image Velocimetry (PIV) system has been proposed to measure the magnitude of the convection cells, and a proof of concept setup has been tested. Particles native to the molten salt are illuminated using a Class 3b laser (power <5mW). The laser beam is converted into a plane using a polypropylene conical centrifuge tube filled with water.