Active heat exchange system development for latent heat thermal energy storage

Abstract

This topical report describes active heat exchange concepts for use with thermal energy storage systems in the temperature range of 250 0 C -350 0 C, using the heat of fusion of molten salts for storing thermal energy. 'It identifies over 25 novel techniques for active heat exchange thermal energy storage systems. Salt mixtures that freeze and melt in appropriate ranges are iden tified and are evaluated for physico-chemical, economic, corrosive and safety characteristics. Eight active heat exchange concepts for heat transfer during solidification are conceived and conceptually designed for use with selected storage media. The concepts are analyzed for their scalability, maintenance, safety, technological development and costs.A model for estimating and scaling storage system costs is developed and is used for economic evaluation of salt mixtures and heat exchange concepts for a large scale application. The importance of comparing salts and heat.exchange con cepts on a total system cost basis, rather than the component dost basis alone, is pointed out. Comparison of these costs with current state-of-the-art systems should be avoided due to significant differences in developmental status. The heat exchange concepts were sized and compared for 6.5 MPa/.281 0 C steam condition and a 1000 MW(t) heat rate for six hours. A cost sensitivity analysis for other design conditions is also carried out.The study resulted in the selection of a shell and coated-tube heat exchanger concept and a direct contact-reflux boiler heat exchange concept. For the storage medium, a dilute eutectic mixture of 99 wt% NaNO 3 and 1 wt% NaOH is selected for use in experimenting with the selected heat exchanger concepts in subsequent tasks.