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

Abstract

SUMMARYUp 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 (MgCl 2 ·6H 2 O), magnesium nitrate hexahydrate (Mg(NO 3 ) 2 ·6H 2 O), 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.

Comparison of constant volume energy storage systems based on compressed air

“…There are many ways of storing energy: pumped storage (PHS), compressed air energy storage (CAES), 1 liquid air energy storage (LAES), flywheels (FES), chemical energy storage (eg, hydrogen), 2 chemical energy storage ‐ power to gas PtG 3 /power to fuel, 4 battery energy storage (BES), 5,6 flow batteries (VRFB), superconducting coils (SMES), supercapacitors (UC), heat storage (sensible 7 and latent 8 heat), and cold storage 9 . Pumped energy storage is widely used all over the world, but is constrained by geographical requirements: topology, geology, and water conditions.…”

Inverse design methodology to optimize sensible thermal energy storage systems working as rectifiers

“…Soda and Beyene assessed different PCMs to be used in an organic Rankine cycle. The authors emphasized that the advantage of latent heat storage in respect to sensible one relies on its capacity to deliver constant fluid output temperature, a desirable quality whenever steady‐state regimes are to be performed, but highlighted handicaps as inflammability, hysteresis, and cost.…”

Influence of alkaline chlorides on thermal energy storage properties of bischofite

“…Latent heat TES materials are also known as phase change materials (PCMs). They can be sub‐classified as organics (fatty acids, paraffin, mineral oil and synthetics oils) and inorganics (salts, salt hydrates and mixtures) . To become a TES material that can be used at industrial scale, it must have low cost.…”