Operating strategies for a Solar Heat Supported Solid Oxide Electrolyzer Cell system under transient conditions

Abstract

The intermittent behavior of renewable energy sources induces heavy fluctuations in the electricity supply and causes large problems when integrated into the energy grid. Therefore, large scale electrolyser systems have drawn attention to balance, transport and store energy on the mega watt scale. Among different types of electrolyser cells, Solid Oxide Electrolyser Cell (SOEC’s) have been reported to be the most electrically efficient because of their high operating temperature. However, the high operating temperature of SOEC’s increases the heat demand of the system, among other for the evaporation of water (~ 17%). One way to fulfill this heat demand is by integrating parabolic trough collector (PTC) fields into the system. In this way, electric evaporators can be omitted boosting the electrical efficiency of the system. To produce hydrogen on a large scale, many different system designs and concepts can be taught of. In this research, the dynamic behavior of a 1 MW solar heat supported SOEC system is investigated by integrating a parabolic trough collector field into the system with two thermal energy storage tanks. The system is designed to operate 24h/day given an averaged irradiance pattern. The operating conditions are selected to be 830°C, 1 atm and 70% reactant conversion. Hot fuel electrode recirculation is included to include a 10% molar hydrogen content at the stack unit inlet and reduce the heat demand of the system. If no heat is available, the system is not able to produce steam and is switched to a stand by mode where forming gas is purged to prevent the cells from cooling down. Two scenarios are investigated to find the effect of individual parameters on the system performance while for three selected KPI, improved operating strategies are proposed. In the first scenario, the system is started from hot stand-by with sun rise. It is found that the main factors limiting the start up are the heat capacity of the stack unit in combination with a maximum allowed thermal gradient. Nevertheless, combining system parameters proved to operate the system at its design current 9 minutes earlier. Depending on the improved KPI selected, it is possible to increase the hydrogen production with 22% or the electric exergy efficiency with 2% when evaluated for the first three hours after sunrise. In the second scenario, an overcast period is assumed to block 90% of the incoming radiation during nominal operation. It is found that shortly switching to stand by mode is the most beneficial when the time away from the design current should be kept minimum. However, when operating endothermically and increasing the reactant conversion to 90% could increase the electric exergy efficiency or hydrogen production with 5.53% or 38% respectively. To find the most optimum operating strategies depending on the user case, more research should be done on different system designs with a longer time span, accompanied by a full sensitivity analyses for the operating strategies.