Electrically charged thermal energy storage systems for grid-level electricity storage

Abstract

Unlike most other commodities, electricity produced at any given time must match the electricity being consumed or the stability of the electric grid is jeopardized. Electricity demand changes throughout the day result in required generation ramp-ups that strain power plants, reduce cycle efficiency and increase CO2 emissions. This problem is exacerbated when renewable sources such as wind and solar are integrated into the grid, due to their intermittency. A change in methods of energy production globally that allows synergistic coupling of renewable and fossil fuels is needed. Currently, pumped hydroelectric and compressed air energy storage are the two most common methods of storage, but are highly geographic dependent systems and thus of limited applicability. There exists a strong demand for grid-scale energy storage that are cost-effective and without geographic constraints. In this thesis, storage systems that are charged by electricity and discharged to produce electricity at times of high demand, are theoretically evaluated. Various types of storage such as chemical, thermal, and mechanical, are reviewed to determine the most ideal method for grid-level energy storage. Thermal energy storage systems using phase change materials are most attractive on a cost and energy density basis. Two system designs are evaluated that can couple to both existing and future power plants since they are electrically charged, via joule heating for example, and later discharged to produce electricity using the plant's turbomachinery. Described within is a novel system in which silicon is used as the storage medium and energy release is predominantly through radiative heat transfer. Another design based on the eutectic alloy Al0.88 Si0.12 and other sensible energy storage materials is also evaluated. As an example, the energy storage systems are coupled to a power plant operating according to a supercritical Rankin cycle, and their performance is compared to that of a boiler. Additionally, system cost is compared to existing storage technologies. Although storing electricity as heat and back to electricity is thermodynamically unfavorable, we present an analysis to show that this approach can be cost competitive and provides a segue from fossil fuels to renewable energy.