Researchers from the German institute built a photovoltaic water electrolysis system based on photovoltaic microconcentrators coupled with proton exchange membrane electrolysis. Outdoor tests have shown a record solar-to-hydrogen efficiency of 31.3%, achieved by a four-terminal CPV system that controls two PEM cells in series under real operating conditions.
Researchers at the Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) in Germany have developed a photovoltaic water electrolysis system that uses its proprietary microconcentrator photovoltaic technology (micro-CPV).
The scientists explained that previous approaches using double- and triple-junction III-V concentrator cells achieved up to 19.8% solar hydrogen efficiency (SHT) outdoors and about 30% indoors, but required careful tuning of voltage, current and system configuration. Their new work demonstrated a four-junction concentrator system that powers outdoor PEM cells, achieving a record 31.3% STH efficiency.
“We are still at a low Technology Readiness Level (TRL) and therefore it is difficult to say how quickly we can achieve low levelized hydrogen costs that are competitive. We first need partners to fully develop the system,” said Frank Dimroth. pv magazine. “With Clearsun Energy we are trying to create a startup that commercializes concentrated photovoltaic solar energy and this solar hydrogen module could be a future generation product for the company.”
The TRL measures the maturity of technological components for a system and is based on a scale of one to nine, with nine representing mature technologies for full commercial deployment. “I would say our system is a proof of concept, which is TRL3,” Dimroth added. “Currently we don’t have funding to build a pilot system, but this would of course be the next step.”
In the newspaper “Photovoltaic water electrolysis achieves 31.3% solar energy in hydrogen2 conversion efficiency under outdoor operating conditions”, published in communication technologythe Fraunhofe ISE researchers explained that the electrolysis system is controlled by the proprietary HyCon system, which consists of Fresnel lens arrays that focus light onto four parallel-connected CPV cells with 4 junctions with a size of 7 mm² each, which in turn are electrically and thermally coupled to the anode and cathode of two proton exchange membrane (PEM) electrolytic cells connected in series.
An aluminum frame houses a Fresnel lens array at an 80mm focal length of the CPV solar cells, with screw adjustment for precise alignment. The solar cells are mounted on copper (Cu) substrates attached to a large copper base plate, which also supports overall thermal and structural integration. A series-connected PEM electrolysis stack is attached to the back of the base plate and electrically and thermally connected to the CPV system via titanium (Ti) screws and the Cu interface.
Image: Fraunhofer ISE, communication technology, CC BY 4.0
The CPV solar cells are built by wafer bonding of two dual-junction structures, namely gallium indium phosphide (GaInP)/gallium arsenide (GaAs) and gallium indium arsenide phosphide (GaInAsP)/gallium indium arsenide (GaInAs). “This 4J solar cell technology has demonstrated a world record solar to electricity (STE) conversion efficiency of up to 47.6% under the concentrated reference AM1.5 direct spectrum,” the scientists highlighted.
The PEM electrolyzer consists of two machined chlorinated polyvinyl chloride (PVC-C) plates that direct deionized water to the reaction chamber containing the membrane electrode assembly (MEA), which uses a 175 μm perfluorosulfonic acid (PFSA) membrane with an active area of 1.13 cm², covered with iridium at the anode and platinum at the cathode as catalysts. A titanium screw presses a titanium mesh onto the MEA and acts as a porous transport layer and flow field for water distribution and product removal.
The entire system is designed to operate the electrolysis stage at elevated temperatures, ideally via thermal coupling to the CPV array.
However, in the current version, only limited passive heat transfer was achieved, so additional heating of the inlet water was required to ensure stable operation and efficiency. “Hence, in a future design, active heating will be avoided through improved thermal coupling between the CPV and the electrolysis cells,” the academics pointed out.
Field testing of the CPV/PEM electrolysis system using a dual-axis solar tracker over 13 summer days in Freiburg, Germany, showed that the system can achieve hydrogen production with a solar-to-hydrogen (STH) efficiency of 31.3%. “This is 5% higher than the best photovoltaic/electrolysis systems reported in the literature, which are between 20 and 30%,” the team said.
This peak performance corresponded to operating conditions where the CPV array and PEM electrolysis stack achieved efficiencies of 34.7% and 91.1%, respectively. At this operating point, the system operated at a current density of 368 mA/cm² and a cell voltage of 3.25 V. “No degradation was observed during the 107 hours of operation during which our system was run through 13 dynamic cycles,” the researchers concluded, noting that increasing the capacity factor of the HyCon technology to 35% could enable a levelized cost of hydrogen (LCOH) of less than $3/kg.
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