Abstract
The increasing contribution of renewable energy sources (RES) to the power generation sector has motivated researchers to devise new methods for reliable energy storage systems. The significance of mechanisms for storing large amounts of electricity for longer time-scales is accentuated by the dynamics of renewable electricity generation that depend upon the season of the year, time of day, and the location. Conventional large-scale electricity storage technologies such as pumped-hydro and compressed-air storage systems are limited by geographical features (e.g., access to a cavern or high elevations). Traditional rechargeable batteries are limited by the fact that energy and power capacities do not scale independently. One of the promising and multipurpose techniques for storing electricity from these fluctuating renewable sources such as wind and solar, is conversion of the electric power to sustainable and carbon free gaseous fuel like hydrogen (power-to-gas (P2G)). In fact, owing to its diverse applications and worthwhile features such as high gravimetric energy density, hydrogen has become an attractive means of delivering renewable energy to many heavy-duty transport and industrial processes that are difficult to electrify. The renewable hydrogen produced from these zero emission electricity sources could be stored in caverns or injected into existing natural gas grids, initially at low concentrations as mixed with natural gas and ultimately piecewise replacing natural gas in the system. The stored hydrogen could be utilized in transportation for fuel cell electric vehicles (FCEV), in power generation for stationary fuel cells or in chemical industries as a feedstock for chemical synthesis of low-carbon fuels or other products, enabling the eventual decarbonization of all energy conversion.
One of the efficient and cost-effective techniques for electrochemical conversion of electricity to hydrogen is steam electrolysis using solid oxide electrolysis cells (SOEC). The steam electrolysis reaction is intrinsically an endothermic reaction for which the required energy is to be supplied as electricity or heat or a combination of both. High temperature electrolyzers, such as SOEC, benefit from both lower energy required for the electrochemical reactions and an ability to provide part of the endothermic energy demand with the heat content of the inlet streams. At high current densities, SOEC can operate exothermically, meaning that the heat generation due to the losses and electricity supply are greater than the endothermic energy demand. When the SOEC systems are connected to intermittent renewable sources like solar or wind, the varying electricity profile will cause the SOEC operation to dynamically switch between endothermic to exothermic modes and all levels of endothermicity and exothermicity in between. This transient behavior of the electrolyzer could lead to highly dynamic thermal response that could have detrimental effects on thermal stress and degradation.
In this work, an energy storage system (ESS) is designed in Matlab/Simulink which simulates the dynamic operation of an integrated SOEC system applied to an intermittent RES. The main focus of the research is the thermal management of the SOEC stack during highly dynamic operation throughout the entire operating regime from highly endothermic to thermo-neutral and to highly exothermic operation. An additional thermal management challenge is to keep the SOEC system hot during periods of non-operation (hot standby). These challenges are addressed in the current work by a novel system design that integrates a solid oxide fuel cell (SOFC) with the SOEC and combines several control strategies within the balance of plant (BoP). For thermal balance of the system the SOFC exothermicity is used to reduce the dependence of the system on electric heaters. The SOFC system oxidizes a fraction of the renewable hydrogen produced by SOEC and generates heat and electricity. A heat exchanger network is designed to convey the SOFC heat to the SOEC for a wide range of endothermic conditions. The main function of SOFC is to keep the SOEC warm when it is not operating or when it is operating in highly endothermic regions. The proposed configuration is integrated with 2 different patterns of electricity production from RES: one representing measured wind dominant RES such as that in Germany, and one representing solar dominant RES such as that in California. Dynamic analysis of the system in terms of the electric power generated by RES for these 2 conditions is presented. The response of different parameters in the system to keep the integrated system warm, self-sustaining, and within desired limits for lower thermal stress and degradation are presented. These parameters include required SOFC power, voltage and temperature profiles of both SOEC and SOFC, BoP parasitic consumption, system efficiencies, hydrogen production amounts, consumption, and storage levels during transient operation.