Researchers at the Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) in Germany have developed a photovoltaic water electrolysis system that utilizes its own micro concentrator photovoltaics (micro-CPV) technology.
The scientists explained that prior approaches using dual- and triple-junction III-V concentrator cells reached up to 19.8% solar-to-hydrogen efficiency (SHT) outdoors and around 30% indoors, but required careful matching of voltage, current, and system configuration. Their new work demonstrated a four-junction concentrator system driving PEM cells outdoors, achieving a record 31.3% STH efficiency.
“We are still at low technology readiness level (TRL) and therefore it is hard to say how quickly we can get to a low levelized cost of hydrogen which is competitive. We first need partners to develop the system fully,” Frank Dimroth told pv magazine. “With Clearsun Energy, we try to create a startup to commercialize concentrating photovoltaics and this solar hydrogen module could be a future generation product for the company.”
The TRL measures the maturity of technology components for a system and is based on a scale from one to nine, with nine representing mature technologies for full commercial application. “I would say our system is a proof of concept which is TRL3,” Dimroth added. “Currently we have no funding to build a pilot system but of course this would be the next step.”
In the paper “Photovoltaic water electrolysis reaching 31.3% solar-to-H2 conversion efficiency under outdoor operating conditions,” published in communications engineering, the Fraunhofe ISE researchers explained the electrolysis sytem is driven by the propietary HyCon system, which consists of Fresnel lens arrays focusing light onto four parallel-connected 4-junction CPV cells with a size of 7 mm² each, which are in turn electrically and thermally linked to the anode and cathode of two proton exchange membrane (PEM) electrolyzer cells connected in series.
An aluminum frame holds a Fresnel lens array at an 80 mm focal distance from the CPV solar cells, with screw adjustment for fine-tuning alignment. The solar cells are mounted on copper (Cu) substrates fixed to a large copper baseplate, which also supports the overall thermal and structural integration. A series-connected PEM electrolysis stack is attached to the rear of the baseplate, electrically and thermally linked to the CPV system via titanium (Ti) screws and the Cu interface.
Image: Fraunhofer ISE, communications engineering, 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 4 J solar cell technology has demonstrated world record solar-to-electricity (STE) conversion efficiencies of up to 47.6% under the concentrated reference AM1.5 direct spectrum,” the scientists emphasized.
The PEM electrolyzer consists of two machined chlorinated polyvinyl chloride (PVC-C) plates that guide deionized water to the reaction chamber containing the membrane electrode assembly (MEA), which uses a 175 μm perfluorosulfonic acid (PFSA) membrane with a 1.13 cm² active area, coated with iridium at the anode and platinum at the cathode as catalysts. A titanium screw presses a titanium mesh onto the MEA to act as a porous transport layer and flow field for water distribution and product removal.
The whole system was designed to operate the electrolysis stage at elevated temperatures, ideally through thermal coupling with the CPV array.
In its current version, however, only limited passive heat transfer was achieved, so additional inlet water heating was required to sustain stable operation and maintain efficiency. “Hence, active heating will be avoided through an enhanced thermal coupling between the CPV and electrolysis cells in a future design,” the academics emphasized.
The conducted field testing of the CPV/PEM electrolysis system using a dual-axis solar tracker over 13 summer days in Freiburg, Germany, and found 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 literature which range between 20 and 30%,” the team said.
This peak performance corresponded to operating conditions where the CPV array and PEM electrolysis stack reached 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 in which our system went 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) below $3/kg.
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