Experimental Investigation of Efficiency Maximization in Solid Oxide Electrolysis Systems by Internal Steam and Heat Recovery

Solid oxide electrolysis cells (SOEC) recently experienced increasing attraction in the context of producing hydrogen from renewable energy. SOEC systems provide one of the highest efficiencies for hydrogen production from water and electrical power. With current cell-technology they can be operated at thermal neutral condition, which means that the cell itself generates no waste heat at all. Therefore the efficiency of the solid oxide electrolysis system is determined by the systems capability to recover heat and steam from the off-gas. The key component for heat recovery in an SOEC system is the steam generator because it is by far the largest consumer of energy beside the solid oxide electrolysis cell stack itself. In this contribution, the development of a scalable steam generator (Figure 1) and results with a prototype for a reversible solid oxide cell (rSOC) system with 40 kW of power in electrolysis mode will be presented. The steam recovery in this design is accomplished indirectly by reusing the heat of vaporization from condensation of the remaining steam in the off-gas to preheat the feed water (in HX1). The design is based on our long term experience with different types of steam generators for SOEC and a multiphase, multi-component pinch analysis. The pinch analysis shows clearly that a high degree of energy recovery requires focusing on the water preheating and steam superheating. Besides evaluating the optimal heat and steam integration, the prototype allows to investigate other design goals as stability, low steam contamination and high dynamic responsiveness. Furthermore this contribution discusses the benefits and drawbacks comparted to other approaches and the combination with off-gas recirculation. In the studied configuration a reduction in electrical energy for supplying superheated steam of about 20% is expected. That means at 70% steam utilization a reduction of about 5% of the specific energy demand of the SOEC system. Besides the cost reduction for cells and stacks, the development of a simple but highly efficient system designs is the key requirement to match the forthcoming demand for viable hydrogen production solutions based on SOEC. The steam supply is a major building block in that respect. Figure 1. Steam generator with heat and (indirect) steam recovery Figure 1