Test and improvement of the cyclic stability of oxalic acid dihydrate for thermal energy storage

Oxalic acid dihydrate (OAD) which has the high initial phase transition enthalpy is a promising PCM for thermal energy storage (TES). However, large degradations of thermophysical properties over time have restricted further application of OAD. Cyclic stability of OAD, which means thermal cycling stability, needs to be improved to give full play to its potential for TES. In this paper, cyclic stability of OAD was determined by using ordinary aluminum pans and high pressure gold-plated pans in DSC measurement. In addition, PEG (with different molecular weights) was added to improve cyclic stability of OAD. Results show that the containers with good sealing performance and strong corrosion resistance are vital for the application of OAD. Meanwhile, adding 2.5% PEG 6000 gives better cyclic stability to OAD without affecting its melting temperature neither decreasing significantly its heat of fusion.