Highly Efficient Solar Hydrogen Generation—An Integrated Concept Joining III–V Solar Cells with PEM Electrolysis Cells

Abstract

In this work, a concept of highly efficient solar–hydrogen generation by direct coupling of III–V multijunction solar cells with proton exchange membrane (PEM) electrolysis cells is presented. III–V solar cells under concentrated illumination feature voltages above 2 V enabling the generation of hydrogen by water electrolysis. The resulting “hydrogen concentrator” is called HyCon. The temperature‐dependent electrochemical behavior of a PEM electrolysis cell is analyzed and its current–voltage characteristics are presented. The distribution of the velocity of the water flow and pressure for different flow fields is simulated by fluidic simulation. An electrolysis cell featuring a porous fiber as a current collector without any flow‐field pattern shows no visible difference in performance compared to a cell with a flow field pattern. Outdoor measurements of a HyCon module with 6 cells show a maximum efficiency of 16.8 % for one of the HyCon cells. Higher currents at lower voltages are desirable. In this respect the III–V multijunction solar cells can be optimized to obtain a higher current by adapting the indium content of each sub cell. This will lead to an increase of the system efficiency. The improvements, which will be applied in the near future, are discussed.

An Integrated Device View on Photo-Electrochemical Solar-Hydrogen Generation

“…Since the inception of the concept of PEC solar-water splitting more than 40 years ago, several attempts to demonstrate functioning solar-fuel devices have been made (17,19,(24)(25)(26)(30)(31)(32)(33)(34)(35)(36). But to a large extent, the problem of building practical systems remains unsolved owing to their complex and simultaneously desired characteristics: (a) continuous, efficient, and stable productions of fuel under illumination; (b) long lifetimes; (c) economic viability; (d ) scalability of components (earth-abundance of materials can become a significant challenge for systems to reach the required terawatt scale); (e) net energy surplus over the device life cycle; and ( f ) operation under safe and environmentally benign conditions.…”

Solar Hydrogen Reaching Maturity

“…The best choice for the electrolyzer is a Polymer Electrolyte Membrane (PEM) electrolyzer, since this type can more easily handle fluctuating input currents [20]. It can be scaled much smaller than the PV modules, since sunshine is diffuse (100 mW cm À2 , corresponding to maximal photocurrents around 20-30 mA cm À2 ) and electrolyzers economically operate at high current densities of >1 A cm À2 [21].…”

“…On the other hand, Rau et al [20] reason that a PEM electrolyzer directly attached to the back of a PV module would cool down the module, so enhancing its efficiency. Building upon earlier work [23], they engineered integrated PV/electrolyzer modules without the need for a DC-DC converter.…”

“…In this work, two PV junctions were stacked to increase output potential and efficiency. The electrolyzer surface area was then adjusted as a function of the current output of the solar cell, to achieve operation near the maximal power point [20]. Jacobsson et al [25] went one step further and directly attached electrodes to the back of a solar cell, which was separated from the electrolyzer environment by a polymer film, glass and epoxy resin.…”

“…PV/electrolysis set-ups have demonstrated SolarTo-Hydrogen (STH) efficiencies from 10% up to 18% [20,[23][24][25]27]. Accounting for a DC-DC converter efficiency of 90% and 75% efficient electrolyzer, in theory STH efficiencies of up to 18.6% or 30% are possible using state-of-the art silicon or III-V solar cells, respectively [30].…”