Session: 05-01 Thermochemical Energy Storage for CSP Applications

Paper Number: 114044

114044 - Startup of a Combined Moving-Fluidized Bed Oxidation Reactor for High Temperature Discharge of Solid-State Thermochemical Energy Storage Particles 

The intermittency of many renewable energy systems presents a challenge as they become more prevalent. Thermochemical energy storage (TCES) shows great potential as a low-cost complement to these renewable energy systems. Specifically, redox reactions using magnesium manganese oxides have been shown to exhibit excellent reversibility and reactive stability, high energy density, and high operating temperatures, making them well suited for power-block implementation. Furthermore, decoupling the charging and discharging processes enables the energy to be stored for long periods and transported between sites, which improves this technology’s value proposition. Due to slow reaction kinetics at low temperatures, startup of the discharge process requires an external heat input if the charging process is located elsewhere. There are multiple objectives associated with the startup procedure, such as minimizing energy input, minimizing time to reach steady state, and minimizing thermal oscillations while stabilizing the reaction. Since the discharge process is to be operated frequently (at minimum, daily), it is important to understand how to best fulfill these objectives.

To study startup, we experimentally investigate numerous control strategies with a 1-kW discharge reactor prototype. This prototype uses an exothermic oxidation reaction to release the stored energy from solid-state magnesium-manganese-oxide particles, employing ambient air as the other reactant. Heat exchangers within the reactor enable a working fluid, air, to absorb the released heat from the reactive bed. Since the working fluid is isolated from the bed, it can be sent into a turbine downstream without accompanying particle fines. CFD simulations predict that steady state will be achieved 3-4 hours after a cold start, at which point the prototype is predicted to operate at a bed temperature of 1000 °C with a working fluid outlet temperature of 800-900 °C. Over the duration of each startup experiment, we manipulate the resistance heater power, particle flow rate, air flow rate and oxygen partial pressure at the bed inlet, and air flow rate through the heat exchangers. Based on test results and simulation results, we present the best-performing startup strategies and discuss their tradeoffs.

Additionally, to predict cyclic performance in a utility-scale application, two methods of integrating this discharge process into a power block are modeled, using experimental data as an input. The first configuration is an air-Brayton cycle, where the plant receives all its heat from the discharge-reactor heat exchangers. The second configuration is a hybrid TCES/natural-gas-turbine plant, where the air exiting the TCES heat exchangers is combusted before flowing through the turbine. This hybrid plant can dynamically shift its thermal load between the heat exchangers and the combustion chamber. As the discharge reactor starts up and delivers more heat to the working fluid over time, the natural gas flow into the combustion chamber to maintain the turbine inlet temperature can be reduced, decreasing the carbon footprint. When the discharge reactor reaches steady state, the combustion chamber consumes a constant natural gas flow that is over 40% lower than a standard gas turbine with the same power output. This operational strategy permits baseload electrical production when the TCES discharge process is run cyclically. The hybrid plant configuration also has potential for low capital investment by utilizing pre-existing energy infrastructure.

Presenting Author: Owen Ramsey Oregon State University

Presenting Author Biography: Owen is a Master of Science student in mechanical engineering at Oregon State University. He is a graduate research assistant for Dr. Nick AuYeung, where he is experimentally investigating the operation of a high-temperature oxidation reactor for thermochemical energy storage using metal oxides. He works with researchers at Mississippi State University, Michigan State University, and RedoxBlox.