The increasing penetration of variable renewable energies poses new challenges for grid management. The economic feasibility of grid-balancing plants may be limited by low annual operating hours if they work either only for power generation or only for power storage. This issue might be addressed by a dual-function power plant with power-to-x capability, which can produce electricity or store excess renewable electricity into chemicals at different periods. Such a plant can be uniquely enabled by a solid-oxide cell stack, which can switch between fuel cell and electrolysis with the same stack. This paper investigates the optimal conceptual design of this type of plant, represented by power-to-x-to-power process chains with x being hydrogen, syngas, methane, methanol and ammonia, concerning the efficiency (on a lower heating value) and power densities. The results show that an increase in current density leads to an increased oxygen flow rate and a decreased reactant utilization at the stack level for its thermal management, and an increased power density and a decreased efficiency at the system level. The power-generation efficiency is ranked as methane (65.9%), methanol (60.2%), ammonia (58.2%), hydrogen (58.3%), syngas (53.3%) at 0.4 A/cm², due to the benefit of heat-to-chemical-energy conversion by chemical reformulating and the deterioration of electrochemical performance by the dilution of hydrogen. The power-storage efficiency is ranked as syngas (80%), hydrogen (74%), methane (72%), methanol (68%), ammonia (66%) at 0.7 A/cm², mainly due to the benefit of co-electrolysis and the chemical energy loss occurring in the chemical synthesis reactions. The lost chemical energy improves plant-wise heat integration and compensates for its adverse effect on power-storage efficiency. Combining these efficiency numbers of the two modes results in a rank of round-trip efficiency: methane (47.5%) > syngas (43.3%) ≈ hydrogen (42.6%) > methanol (40.7%) > ammonia (38.6%). The pool of plant designs obtained lays the basis for the optimal deployment of this balancing technology for specific applications.

可变可再生能源的日益普及对电网管理提出了新的挑战。如果电网平衡电厂仅用于发电或仅用于电力存储，则其经济可行性可能受到年度运行时间短的限制。具有power-to-x功能的双功能发电厂可以解决此问题，该发电厂可以在不同时期发电或将过量的可再生电力存储为化学物质。这种工厂可以通过固体氧化物电池堆来唯一启用，该堆可以在同一堆燃料电池和电解之间切换。本文研究了这种类型的电厂的最佳概念设计，以电力到电力到电力的过程链为代表，其中x是氢，合成气，甲烷，甲醇和氨，关于效率（较低的发热量）和功率密度。结果表明，电流密度的增加会导致氧气流量的增加以及在堆栈级进行热管理时反应物的利用率降低，并且功率密度的增加和系统级效率的降低。在0.4 A / cm时，发电效率排名为甲烷（65.9％），甲醇（60.2％），氨（58.2％），氢（58.3％），合成气（53.3％）^{如图2所示}，由于通过化学重整而进行的热化学能转换以及通过稀释氢而使电化学性能下降的优点。储能效率为0.7 A / cm ^2时的合成气（80％），氢气（74％），甲烷（72％），甲醇（68％），氨气（66％），主要是由于共电解的好处和化学合成反应中发生的化学能损失。损失的化学能改善了工厂内部的热集成，并弥补了其对电力存储效率的不利影响。将这两种模式的效率数字结合起来，得出往返效率的排名：甲烷（47.5％）>合成气（43.3％）≈氢（42.6％）>甲醇（40.7％）>氨（38.6％）。获得的工厂设计库为针对特定应用最佳地部署此平衡技术奠定了基础。