Harnessing the power of the sun to produce hydrogen, oxygen and heat

In this study, we demonstrate an integrated photoelectrochemical device which utilizes solar concentration to produce green hydrogen at the kilowatt scale. This pilot-scale demonstrator highlights the potential of such high-efficiency technologies as a pathway to a more sustainable future.
Harnessing the power of the sun to produce hydrogen, oxygen and heat
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Solar energy is a truly vast primary energy resource where roughly 3,850,000 exajoules reach the surface of the earth every year. If we could effectively harness a tiny fraction of this immense resource we could technically meet the entire energy demand of humanity (~600 exajoules). However, technological and economic challenges prevent widespread implantation at scale.

One notable technical challenge of solar energy is that it is intermittent energy source. The rotation of the earth leads to daily cycles of sunlight, and the tilted spin axis of the earth with respect to its orbital plane leads to seasonal variations. In nature, photosynthesis evolved as a method to convert light energy into chemical energy, and taking inspiration from this, there is increasing amount of research into producing a solar energy derived fuels that can bridge the energy gaps.

Photoelectrochemical production of hydrogen has been identified as a promising route, but at present are typically restricted to small-scale demonstrations. In this work, we demonstrate an encouraging pathway to scale an integrated photoelectrochemical device utilizing solar concentration. The synergistic thermal integration of concentrated photovoltaic and electrolysis leads to a reduction in the total components required and results in a useful low temperature heat stream. A high utilisation rate of the input solar energy is achieved through this simultaneous co-generation of fuel and heat.

A demonstration plant was built on EPFL campus and includes a 7 m-diameter solar parabolic dish, an integrated photo-electrochemical reactor placed in the focal point of the dish and various auxiliary components such as pumps and heat exchangers. An annotated overview of the pilot plant and photographs of the system during operation is shown in Figure 1. A number of engineering challenges were overcome in order to realise this innovative design which included careful process design and control strategies to ensure the safe operation during start up, operation and shut down.

Figure 1 – Overview of the integrated photoelectrochemical hydrogen pilot plant. a, Annotated overview of the pilot plant. b, Close up details diagram of reactor unit. c, Photograph of the system during operation.

An experimental campaign was completed under a variety of conditions during summer and winter and demonstrated high performance without degradation. Furthermore, the system was advantageously responsive to disturbances in solar light and during system start-up. The pilot plant achieved an instantaneous H2 production rate of up to 0.9 Nm3 hr−1 and a reactor outlet temperature of up to 70 °C during periods of sunny conditions. On average, the overall system fuel efficiency was 6.6% (based on higher heating value, 286 kJ mol−1) and the overall system heat efficiency was 35.3%. In order to demonstrate the future potential for this technology when the solar concentrator area is matched to the reactor power, the average solar-to-hydrogen efficiency was assessed on a reactor-level basis and found to be 24.4% (HHV).

In order to assess the feasible performance improvements of this technology, a system optimization was performed. Firstly, a theoretical model of the system was built and the results validated to the experimental campaign. Then, this ‘digital twin’ was used to optimise parameters such as water flowrate, the reactor to concentrator area power matching, and the number of electrolysers in series. This optimisation leads to a theoretical improvement in system fuel production efficiency of greater than 19% (HHV). Furthermore, this model allowed us to demonstrate the effect of continuous water flow-rate control in order to capture thermal energy at higher temperature at the start and end of the day, and this control strategy leads to an average system output temperature improvement over 5.0 °C.

In conclusion, we present the successful scaling of a solar-concentrating photo-electrochemical cell, utilising thermal integration, to a kW-scale pilot plant. If you would like to learn more about this work, please see the original article at:

https://www.nature.com/articles/s41560-023-01247-2

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