Multiphase ultra-low grade thermal energy storage for organic Rankine cycle

Abstract

Up to 60% of thermal energy is wasted as low or ultra-low quality through exhaust systems. The organic Rankine cycle can be used to generate mechanical power and electricity from these low grade energy sources. A variety of organic working fluids are available to optimize the organic rankine cycle (ORC) for any target temperature range. Because the out of these sources is intermittent, it is desirable to have a storage system. Multiple types of low grade thermal energy storages have been explored including sensible, latent, and thermochemical. Targeting 86 °C, the operating temperature of our experimental ORC, multiple potential materials were explored and tested as potential phase change materials including magnesium chloride hexahydrate (MgCl2·6H2O), magnesium nitrate hexahydrate (Mg(NO3)2·6H2O), montan wax, and carnauba wax. The addition of graphite to augment heat transfer rates was also tested. Melting and solidification temperatures largely matched predictions. The magnesium salts were found to be less stable under thermal cycling than the waxes. Graphite was only soluble in the waxes. Mixtures of magnesium salts and waxes yielded a layered composite with the less dense waxes creating a sealing layer over the salt layer, which significantly increased the stability of the magnesium salts. Research into optimum heat exchangers and storage vessels for these applications indicates that horizontally oriented aluminum pipes with vertically oriented aluminum fins would be the best method of storing and retrieving energy. Fin spacing can be predicted by an equation based on target temperatures and phase change material characteristics. Copyright © 2015 John Wiley & Sons, Ltd.