SOLID-STATE FUEL SYNTHESIS FOR SEASONAL ENERGY STORAGE

One of the main challenges for cyclical seasonal energy storage systems, such as batteries, is that affordable economics depend on extensive cycling of the system, which is not an option for seasonal storage. Instead, low-cost fuel synthesis provides a pathway to seasonal energy storage where affordable economics is based on the percentage of time the system is in operation.

A unique thermochemical process for synthesizing re-usable magnesium metal oxide-based zero carbon solid-state fuel (SoFuel), which can be used for seasonal energy storage is described. High temperature (1450°C) heating of the processing furnace can be driven by renewable electricity. The very simple fuel synthesis concept is based on a tubular falling bed reactor with countercurrent oxygen-depleted gas flow for complete heat recuperation. A sophisticated control strategy using pulsed width modulated gas flow and an L-valve maintains continuous magnesium metal oxide (Mg-X-O) particle flow up to 1450°C. The measured energy charge of the Mg-X-O (X denotes a metallic element) particles is more than 90% of the fully reduced state at equilibrium. Thermal to chemical storage efficiencies of 96% have been achieved.

The reversible SoFuel reduction and oxidation (redox) energy storage reactions are as follows: \[\text{Reduction (charge)}:\qquad \text{MgXO}_{3}\text{(s)} + \text{heat} \rightarrow \text{MgXO}_{2}\text{(s)} + \dfrac{1}{2}\text{O}_2\text{(g)}\] \[\text{Oxidation (discharge)}:\qquad \text{MgXO}_{2}\text{(s)} + \dfrac{1}{2}\text{O}_2\text{(in air)} \rightarrow \text{MgXO}_{3}\text{(s)} + \text{heat}\] These reactions allow extremely high energy storage densities of about 1600 MJ/m³ in the form of chemical bonds. The energy storage material can tolerate some impurities, which allows it to be produced at low cost from abundant earth materials magnesium oxide and metal oxide (i.e., there is no need for expensive purification steps).

The production of SoFuel occurs within a tubular falling pellet bed reactor heated via renewable electricity or via a cavity receiver that captures concentrated solar radiation from a solar field as shown in Fig. 1. Oxidized, highly reactive magnesium metal oxide (Mg-X-O) pellets descend through the tubular reactor's recuperation zone (A) where it picks up heat from air in counterflow leaving the reduction zone (B). In the reduction zone Mg-X-O pellets undergo thermal reduction at temperatures exceeding 1350°C. The thermally reduced SoFuel pellets are quenched (C) by a counter-current flow of oxygen depleted air, entering the reactor at the bottom. The reduced (charged) fuel is stored within a low-cost storage bin until needed. Oxidation of SoFuel (discharge) provides 1000°C heat that can be applied to a power block or used for industrial purposes, after which the particles are returned to the electrically heated furnace to be re-charged.

Due to high intermittency and seasonal variability in solar and wind energy supply, many countries cannot rely on these renewable energy resources to meet aggressive decarbonization targets without a means for low cost and long duration energy storage. Long duration storage at grid scale is challenging because commercially available storage technologies either do not have seasonal storage capability or are prohibitively expensive. The rechargeable solid-state fuel and its charging method presented here is a potential solution to meet the demand for grid scale, lower cost, and long duration energy storage at the seasonal time scale. The magnesium metal oxide based solid state thermochemical fuel can store energy for indefinitely long periods at less than a tenth of the cost of currently available commercial batteries and has the potential to enable deep decarbonization strategies for many geographical locations that would otherwise rely on more expensive green hydrogen when renewable energy production is not available.

Based on WIPO patent filing WO 2022/035672 A1