SUMMARY

Thermal energy storage (TES) is rapidly becoming a popular tool to balance energy supply and demand as the energy sector transforms. Many forms of TES are available that vary by the temperature of storage. This work centers around low-grade thermal energy (<100°C) for building space conditioning. TES benefits the energy grid by storing excess electrical energy designated for space conditioning as thermal energy during the grid’s off-peak times and when space conditioning is not immediately needed. The TES then discharges the stored thermal energy when space conditioning is needed when the electrical grid’s demand is highest or ambient conditions are unfavorable for conventional space conditioning. Thus, TES may assist in leveling the energy demand curve by decoupling thermal energy generation from its use. Much like electric batteries, TES systems are crucial for the widespread deployment of renewable energy technologies. Phase change materials (PCMs) are a key component of many TES systems for their ability to store thermal energy nearly isothermally during a 1st order phase change. Inorganic salt hydrate PCMs are notable for their high volumetric storage density (45-130 kWh/m3) and low material thermal energy storage cost (<$10/kWh) for space heating applications (30-100°C phase change). However, many salt hydrate PCMs undergo incongruent melting processes thus losing much of the energy storage capacity upon repeated thermal cycling or long dwell periods in the liquid state. Salt hydrates have low thermal conductivity (<0.6 W/m-K) that limit thermal charging capabilities. And salt hydrates with melting temperatures useful for space cooling (<20°C) have higher material thermal energy storage costs (>$40/kWh). The goal of this work is to develop and characterize a high thermal conductivity, cyclically stable salt hydrate PCM that preserves the high volumetric storage density, maintains low material cost, and has a phase change temperature <20°C. And demonstrate the performance of such salt hydrate PCMs in a TES system. Lastly, a technoeconomic analysis will present the advantages of PCM-based TES for many climates around the US as a function of the PCM melting temperature. The broad scale impacts of the installation of such TES will be investigated using a life cycle analysis approach.