Chemical looping electricity storage

Developing grid-scale energy storage technologies is the key element for broader deployment of renewable sources of energy. This is due to bench-mark technologies like pumped hydro and compressed air being geographically restricted, i.e. require large reservoirs to store air/water, new storage solutions must be found. Hydrogen and pumped thermal storage have emerged as options, but hydrogen suffers from low round-trip efficiency and pumped thermal has a relatively low capacity. In a pumped thermal electricity storage system, electricity is converted to heat using either an electrical heater or a heat pump. This heat is stored and converted back to electricity using a heat engine. In these storage schemes, heat is mostly stored as sensible heat which leads to a low storage capacity. In this regard, this Dissertation examines a simple cycle which makes use of a thermo-chemical store, with a view to achieving high storage capacity by using the chemical looping concept. To do so, a simplified model of a packed bed reactor is developed enabling faster analysis of different layouts and materials. The objective is to find layouts and material properties that are optimising the performance of the storage system including round-trip efficiency and capacity. Round-trip efficiency describes the proportion of electricity put into the storage system during charge returned to the user during discharge. Capacity specifies the size of the storage system and is generally proportional to the capital cost. Results show that a Chemical Looping Electricity Storage (CLES) system can achieve a high capacity, in the range of 250-350 kWh/m3, second only to hydrogen electricity storage systems. Its round-trip efficiency (40-55%) is potentially higher than that of the hydrogen electricity storage systems. By achieving a higher capacity than pumped thermal energy storage and higher round-trip efficiency than that of hydrogen systems, CLES has the potential to fill out the gap between these two grid-scale storage technologies. Thus, this system may play an important role in our future energy mix. In early schemes, a heat pump is employed to convert electricity to heat, but its operating temperature is limited and only those solid oxides capable of releasing oxygen at low temperatures (below 900 K) are feasible. Therefore, ways of using materials with a higher decomposition temperature, i.e. the commonly used materials in chemical looping systems, are investigated. Two methods are proposed: using a vacuum pump to reduce the charging pressure or an electrical heater to increase the charging temperature. Results show using a vacuum pump to be infeasible whereas a simplified charging cycle only comprising of an electrical heater and a recuperator is deemed optimal. This system capacity can be as high as 600-800 kWh/m3 with round-trip efficiency in the range of 40-55%. Finally, a detailed model of a packed bed reactor is developed to study the performance of the CLES for one cycle of charge and discharge. This model of the packed bed is better equipped to capture the transient response of the system to changes in the operating parameters. Previous findings showed that a combination of manganese and copper oxide may have high potential. This stage led to three important outcomes and helped redirecting the future work. First, it showed that the initial simple model of the system is able to capture the dynamic nature of the system to an acceptable degree and therefore it can be used for further investigation of other materials. Second, it showed that a mixed oxide performance can be explained by its constituents. Copper manganese oxide capacity was higher than that of the manganese oxide and lower than copper oxide. Similarly, its efficiency was higher than that of the copper oxide and lower than manganese oxide. This is important because it helps directing the future search for potential materials best suited for electricity storage. Third, the practicality of the concept is ensured by studying the system with more accurate material properties, e.g. reaction kinetics, and a transient model of the reactor.