Deep Eutectic Solvents as Designer Phase-Change Materials for Organic Rankine Cycle Applications

Organic Rankine cycles (ORCs) are one of the most prominent technologies for efficient energy use, from renewable or residual energy sources. These systems are widely used due to their simple design and flexibility. One interesting addition to an ORC is a thermal energy accumulator (TEA), which can improve the energy utilization efficiency. This contribution analyzes the thermal and exergetic efficiency changes when a TEA is added to an ORC. Seven novel deep eutectic solvent (DES)-based phase-change materials (PCMs) are studied and compared with paraffin wax used as a benchmark compound. The ORC uses ten different working fluids (WF), including fluorinated gases and hydrocarbons (HCs). The results show that R1233zd and n-pentane are the working fluids reaching the highest thermal and exergetic efficiency, while choline chloride plus suberic acid and choline chloride plus 4-hydroxybenzoic acid are the best PCMs, far exceeding the exergetic performance of paraffin wax. Furthermore, adding a TEA improves the quality of energy delivery, increasing the exergetic efficiency by up to 95%. These findings highlight the practicality of using hydrocarbons in ORCs in conjunction with TEAs. Additionally, the analysis indicates that PCMs with a higher-melting-temperature and fusion enthalpy offer the most promising solution for ORCs with TEAs, thereby providing valuable insights for the design and optimization of ORC systems.