Scaled Solar Fuel Production System Using Concentrated Integrated Photo-Electrochemical Device

Abstract

Direct conversion of solar energy and water (or other reactants such as CO2) into chemical energy via photo-electrochemical (PEC) processes is one viable route for renewable fuel processing and energy storage. Though, any practical implementation of such approach requires it to be efficient, robust, economically competitive and sustainable at the same time. Our detailed techno-economic analysis1 provided the proof that PEC devices operating with optical and current concentrations can satisfy all the four aspects for deployable solar fuel generation, even though being based on rare earth elements as photoabsorbing and catalytic materials. Furthermore, a close thermal and electronic integration of the photoactive and electrochemical component allows to achieve higher operating current densities while maintaining high efficiencies2. However, no integrated photo-electrochemical (IPEC) system operating under concentrated irradiation has been demonstrated, until recently when we presented3 successful design, fabrication and testing of such a device employing smart thermal management strategies in order to achieve a solar-to-hydrogen efficiency as high as 17.12% along with an electrochemical current density of 880 mA/cmEC 2 and a photovoltaic current density of 6.04 A/cmPV 2. This lab-scale proof of concept (Fig. 1a) was our first step towards a more practical device which eventually can be deployed in the market. To achieve this goal, such a device needs to be scaled, tested under long-term realistic conditions, and the economic competitiveness and capability factor needs to be increased. Here, design, installation and characterization of such a scaled-up system are discussed. The scaled-up prototype is constructed at EPFL Lausanne campus (system design and photo shown in Fig. 1(b)-(c)). The prototype has a collector area of 38.5 m2, with the potential to produce 10 kW of electrical power in addition to 10 kW of thermal power or equivalently around 100 kWh of electrical with 360 MJ of thermal energy. Equivalently, the prototype’s H2 generation potential is 1.3 kg/day (fully solar, only day-time operation and that also with simultaneous production of 10 kW of heat) and more (~1.3 kg/night for 10 hour off-peak grid operation) for combined grid electricity-based operation. The solar radiation is redirected by the parabolic dish and concentrated at the focus. The radiation is further homogenized in a flux homogenizer before reaching our IPEC which uses this energy to produce fuel (hydrogen and oxygen), electricity, and heat. The ratio between the three outputs can be tuned to the user’s demand. In the first phase of demonstration, we start with ~1/3rd capacity of the system i.e. with the photoabsorber area of 142 cm2 and a total electrochemically active area of 800 cm2 with an optical concentration of ~1000 suns. The photovoltaic component’s operating current density is expected to be 13.8 A/cm2 and that of electrochemical component to be 1.6 A/cm2 with system level (including concentrator and homogenizer losses) solar to hydrogen efficiency of 17% and device level solar to hydrogen efficiency of 23%. Overall functioning of the system comprises of the parabolic dish automatically starting/stopping the tracking (dual-axis) of the sun based on irradiation conditions. The tap water is pumped (at 1-2 bars), passing through a de-ionizer, to the IPEC reactor. The oxygen-water output (2-3 bars) from anodic stream is passed through a heat exchanger (evacuated heat to be used for residential heating or industrial processes) followed by a liquid-gas separator (separating the dry oxygen from water). The separated water is recycled back and fed back to the de-ionizer. Dry oxygen is passed through a compressor to be finally stored in cylinders (at 30 bars). Anodic stream has larger quantities of liquid water (non-stoichiometric) than cathodic stream (i.e. the latter has only water because of membrane cross-over). The hydrogen–water product leaves the cathodic side of the IPEC device at 30 bars and the dry hydrogen is directly (without further compression) stored in the cylinders. The stored hydrogen and oxygen are then utilized by a fuel-cell system to power a microgrid4. The implemented device opens a new pathway towards scalable and large-scale deployment of photo-electrochemical water splitting and demonstrates an efficient way of solar hydrogen processing.