Optimal Integration of a Solid-Oxide Electrolyser Cell into a Direct Steam Generation Solar Tower Plant for Zero-Emission Hydrogen Production
Electrolysis systems have a great potential for CO2-free hydrogen production because its relative simplicity facilitates the interconnection with renewable electrical energy systems like wind and solar. Additionally, this hybridization makes it possible to interconnect power and fuel networks (natural gas and transport fuel). Thus, hydrogen could be used as energy carrier or a temporary energy storage contributing to strengthen the national energy system by allowing higher penetration of renewables, decreasing external energy dependence and improving energy security. Based on the promising attributes of solid-oxide electrolysis cells (SOEC) to reduce electrical consumption by 20-25 % versus current alkaline and proton exchange membrane electrolysers, and the capacity of concentrating solar thermal power plants to produce electricity and heat under stable conditions, their hybridization stands for a promising system of large-scale carbon-free hydrogen production process. This study evaluates the integration of a SOEC plant into a direct steam generation solar tower plant in terms of energy and cost. In a first step, both subsystems have been implemented and optimised independently. The evaluation of the implemented quasi steady-state model of a SOEC process reveals that temperature variation along the cells is lower when the electrolyser is operated under thermoneutral conditions along the whole power range. Additionally, this temperature gradient is reduced further when the area specific resistance of the cells varies notoriously with temperature. As consequence of this effect, the SOEC system is capable to operate from 5-13 % to 100 % of its maximum capacity. Furthermore, under constant steam conversion operation, the global efficiency of the implemented SOEC electrolysis plant follows a very uniform curve in the range of 92-95 % vs. the high heating value of hydrogen. Concerning the solar power plant, a comparison between saturated and superheated direct steam generation plants has been done. The simulations show that the advantages of higher performance of solar collectors in saturated steam plants are counterbalance with the increase of the Rankine cycle efficiency with temperature. Nevertheless, in the megawatt range, direct superheated steam dual-receiver solar tower plants are competitive with respect to saturated steam solar plants only at working temperatures higher than 550 ºC. Having optimised both subsystems, several possible configurations of the hybrid plant have been analysed aiming at minimizing the penalties over the solar thermal plant, and maximizing the electrolysis performance. By operating the SOEC system at high pressure, compression work can be reduced drastically and oxygen can be obtained as co-product of the process. Nevertheless, the hybrid plant efficiency is maximized operating the SOEC system at ambient pressure because penalties over the solar plant are minimised. The most efficient integration scheme takes process steam from the low pressure turbine and uses electrolyser's outlet hot streams to preheat the Rankine cycle feed water. Finally, annual yield simulation and economic analysis have been done based on the levelised cost of electricity and hydrogen. The results reveal that the stand-alone solar plant achieves a levelised cost of electricity of 207.3 €/MWhe. Based on this electricity cost, the resulting levelised cost of hydrogen for the optimal hybrid plant configuration is 11.7 €/kg. Further analyses of the SOEC system consuming electricity from the grid and heat from a waste hot stream from an industry, reveals a cost reduction above 45 %, 6.2 €/kg. Thus, a final hybrid plant where process steam is taken from the solar plant while electricity is consumed from the grid has been implemented and evaluated, obtaining a levelised costs of hydrogen of 6.4 €/kg.
Tesis Doctoral leída en la Universidad Rey Juan Carlos de Madrid en 2015. Director de la Tesis: Manuel Romero Álvarez. Co-director: Javier Muñoz Antón. Tutor: Raúl Sanz Martín
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