Solid-State Tunable Thermal Energy Storage for Building Envelopes

Thermal energy storage (TES) based on phase-change materials (PCMs) has many current and potential applications, such as climate control in buildings and thermal management for batteries and electronics. In all of these applications, the PCM works near the ambient temperature and is typically designed to operate at a fixed temperature. One fundamental challenge in the adoption of PCM-based TES is that there is limited tunability in the usage temperature, especially for the near-ambient applications. For example, the use temperature in buildings during the summer and winter months can vary significantly (there can be also significant diurnal variations). In reality, that translates into reduced use of the PCM, because either the PCM melts partially or does not melt at all. There has been significant interest in recent years in making thermal materials and devices tunable, such that their properties and performance can be dynamically changed. However, most of the focus has been on changing thermal transport properties, such as thermal conductivity, rather than thermodynamic properties, such as the melting/transition temperature (Tm) of a material. Changing the Tm of a material using an external stimulus such as pressure, an electric field, or a magnetic field, is a non-trivial task, as the required magnitude of the stimulus to achieve a sizable change in Tm is typically large, and the enthalpy change at Tm is only moderate for thermal storage applications. Besides, dealing with the liquid phase of PCMs during the phase change transition also prevents TES from practical application. To circumvent the aforementioned problems, here, we report on the solid-state tunable thermal energy storage of a shape-stabilized PCM. We achieve a transition temperature tunability up to 10 degrees C and shape stability up to 1 month, which may enable simpler and safer TES devices/system designs.