EPFL researchers have built a pilot-scale solar reactor that produces usable heat and oxygen, in addition to generating hydrogen with unprecedented efficiency for its size.

This is the first system-level demonstration of solar hydrogen generation. Unlike typical lab-scale demonstrations, it includes all auxiliary devices and components, so it gives us a better idea of the energy efficiency you can expect once you consider the complete system, and not just the device itself.

With an output power of over 2 kilowatts, we’ve cracked the 1-kilowatt ceiling for our pilot reactor while maintaining record-high efficiency for this large scale. The hydrogen production rate achieved in this work represents a really encouraging step towards the commercial realization of this technology.

—Sophia Haussener, head of the Laboratory of Renewable Energy Science and Engineering (LRESE) in the School of Engineering

(a) Technical illustration of the overall site showing key components such as the solar parabolic concentrator dish, reactor and ancillary hardware and cabinets. (b) Close-up of the integrated reactor showing the assembly of the shield, homogenizer, PV and enclosure. (c) A simplified process and instrumentation diagram of the system showing material and energy flows. The key input/output/intermediate energy streams are composed of the PV-generated electrical work available for electrolysis, heat output from the heat exchanger and the external work required for water pumping. W and Q stands for work and heat respectively and sensors are denoted by a circle (T = temperature sensor, P = pressure sensor, H₂ = hydrogen concentration sensor). Holmes-Gentle et al.

The work builds on preliminary research demonstrating the concept on the laboratory scale, using LRESE’s high-flux solar simulator, which was published in Nature Energy in 2019. Now, the team has published the results of their scaled-up, efficient, and multi-product process under real-world conditions in an open-access paper in the same journal.

The LRESE parabolic dish © LRESE EPFL

A close-up of the LRESE solar reactor © LRESE EPFL

After the dish concentrates the sun’s rays, water is pumped into its focus spot, where an integrated photoelectrochemical reactor is housed. Within this reactor, photoelectrochemical cells use solar energy to split water molecules into hydrogen and oxygen. Heat is also generated, but instead of being released as a system loss, this heat is passed through a heat exchanger so that it can be harnessed—for ambient heating, for example.

In addition to the system’s primary outputs of hydrogen and heat, the oxygen molecules released by the photo-electrolysis reaction are also recovered and used, for example, in medical applications, Haussener says.

The system is suitable for industrial, commercial, and residential applications; LRESE-spinoff SoHHytec SA is already deploying and commercializing it. The EPFL start-up is working with a Swiss-based metal production facility to build a demonstration plant at the multi-100-kilowatt scale that will produce hydrogen for metal annealing processes, oxygen for nearby hospitals, and heat for the factory’s hot-water needs.

With the pilot demonstration at EPFL, we have achieved a major milestone by demonstrating unprecedented efficiency at high output power densities. We are now scaling up a system in an artificial garden-like setup, where each of these ‘artificial trees’ is deployed in a modular fashion.

—SoHHytec co-founder and CEO Saurabh Tembhurne

The system could be used to provide residential and commercial central heating and hot water, and to power hydrogen fuel cells. At an output level of about half a kilogram of solar hydrogen per day, the EPFL campus system could power around 1.5 hydrogen fuel cell vehicles driving an average annual distance; or meet up to half the electricity demand and more than half of the annual heat demand of a typical four-person Swiss household.

With the artificial photosynthesis system well on its way to scale-up, Haussener is already exploring new technological avenues. In particular, the lab is working on a large-scale solar-powered system that would split carbon dioxide instead of water, yielding useful materials such as syngas for liquid fuel, or the green plastic precursor ethylene.

Resources

• Holmes-Gentle, I., Tembhurne, S., Suter, C. et al. (2023) “Kilowatt-scale solar hydrogen production system using a concentrated integrated photoelectrochemical device.” Nat Energy doi: 10.1038/s41560-023-01247-2