Concentrating Solar Systems as Heat and Electricity Source for a Solid Oxide Electrolyzer

Abstract

Steam electrolysis carried out in Solid Oxide Electrolysers has the advantage of considerably higher efficiency than conventional alkaline water electrolysis when producing hydrogen from water using solar concentrating systems. The solar-to-fuel efficiency of the alkaline electrolyzer process is lower since the available heat first has to be converted into electricity. By contrast, a SOE can directly use high temperature heat from concentrated solar power (CSP) leading to a significantly higher process efficiency. In the EU project ADEL (2011–13) the integration of suitable heat and power sources is analyzed by a European consortium. Flowsheeting of the process has been carried out. Operating the process with direct steam from a CSP tower has the benefit of using a heat transfer fluid well suited to generate electricity in a Rankine cycle as well as feed steam for the HTE. Moreover, this process scheme permits the use of steam as sweep gas for the anode in order to maintain reducing conditions. This steam can be easily separated at the outlet of the electrolyzer by condensation, which offers the benefit of producing oxygen as a valuable byproduct. A flowsheet of a plant in MW-range was elaborated and the process was simulated with the commercial software tool Aspen Plus. Alternative process concepts (Figure 1, left) based on the use of air as working medium and as a sweep gas for the SOE are analysed as well and compared to the one based on steam. Those activities are complemented by a DLR internal project, which is currently carried out to study the integration option of direct steam from a solar tower in more detail. Main emphasis is put on the development of a new solar test receiver for steam generation (Figure 1, right) accompanied by modeling of this receiver.