Constant-volume vapor-liquid equilibrium for thermal energy storage: investigation of a new storage condition for solar thermal systems

The sector of thermal energy storage shows a number of alternatives that could have a relevant impact on the future of energy saving as well as renewable energy technologies. Among these, latent heat thermal energy storage technologies show promising results. Technologies that exploit solid-liquid phase change have already been widely proposed, but those technologies show common drawbacks limiting their application, such as high cost, low energy storage density and particularly low heat transfer properties. This work proposes to exploit the liquid-vapor phase transition in closed and constant volumes because it shows higher heat transfer properties. Consequently, the objective is to assess its energy storage performances in target temperature ranges. With respect to previous activity by the authors, this work proposes an exergy analysis of these systems, gives a methodology their deployment, and proposes a comparison between a new storage condition for solar thermal domestic hot water systems exploiting vapor-liquid equilibrium and conventional technologies. The exergy analysis is performed in reduced terms in order to have a generalized approach. Three hypothetical fluids with increasing degree of molecular complexity are considered in order to have a complete overview of the thermodynamic behavior of potential heat storage fluids. The analysis shows that the increased pressure of liquid systems has a major impact on exergy, resulting in vapor-liquid systems having less than 50% of the exergy variation of pressurized liquid systems. This is proven to have no impact on thermal energy storage. For the case study, the proposed methodology indicates that water itself is a strong candidate as a heat storage fluid in the new condition. Comparison shows that the new condition has a higher energy storage capacity at same volume. The useful temperature range is increased by 108% by setting a 10.5% volume vapor fraction at ambient temperature. The resulting improvement gives a 94% higher energy storage, with a maximum operating pressure of the system of less than 5 bar.