Efficiency Gains for Thermally Coupled Solar Hydrogen Production in Extreme Cold

Abstract

Hydrogen produced from water using solar energy constitutes a sustainable alternative to fossil fuels, but solar hydrogen is not yet economically competitive. A major question is whether the approach of coupling photovoltaics via the electricity grid to electrolysis is preferential to higher levels of device integration in ‘artificial leaf’ designs. Here, we scrutinise the effects of thermally coupled solar water splitting on device efficiencies and catalyst footprint for sub-freezing ambient temperatures of -20C. These conditions are found for a significant fraction of the year in many world regions. Using a combination of electrochemical experiments and modelling, we demonstrate that thermal coupling broadens the operating window and significantly reduces the required catalyst loading when compared to electrolysis decoupled from photovoltaics. Efficiency benefits differ qualitatively for double- and triple junction solar absorbers, which has implications for the general design of outdoor-located photoelectochemical devices. Similar to high-efficiency photovoltaics that reached technological maturity in space, application cases in polar or alpine climates could support the scale-up of solar hydrogen at the global scale.

The annual-hydrogen-yield-climatic-response ratio: evaluating the real-life performance of integrated solar water splitting devices

“…regions. 19 Hourly resolved data for location 1 was obtained from NREL's spectral on-demand database [39][40][41] for the year 2019. It inter alia contains the solar spectrum, the ambient temperature, as well as the solar zenith and azimuth angle.…”

“…Note that the modelling in this work neglects potential device operating failures due to extreme cold temperatures. However, it was shown that the right choice of an electrolyte that sufficiently depresses the freezing point of water enables solar water splitting far below 0 • C. 19 The hourly resolved total spectral irradiance impinging on the device for both locations is shown in Fig. 5d and e.…”

“…In this work, however, this effect is neglected since the area for heat dissipation is rather high in these devices and the operating temperature is mostly defined by the ambient temperature and irradiance. 19 Fig. 6a and b show the hourly resolved annual coupling efficiency and the corresponding 2D histogram with regard to the total spectral irradiance, respectively.…”

“…Recent studies from the solar water splitting community highlight the efficiency gains caused by thermal coupling of the solar cells and the electrochemical component. 5,19,25,29 To the best of our knowledge, however, these efficiency gains were not evaluated for an outdoor-operating device exposed to changing weather conditions yet. To get a first impression, the AHYCR ratios for both locations were therefore also determined for a thermally decoupled device configuration.…”

“…Moreover, higher integration is accompanied by the possibility to use the heat loss in the absorber as an efficiency boost via simultaneously enhancing the catalytic performance of the water splitting reactions, decreasing the Ohmic losses, and cooling the absorber. 19,25,29 Whereas the installation and commissioning of electrolyzers in the multi-MW range is already ongoing in several countries, more integrated approaches still suffer from a low technology readiness level (TRL). While lab-scale PEC devices based on III-/V dual-junction absorbers are reaching solar-to-hydrogen (STH) efficiencies of up to 19.3%, 7 the high material costs, non-trivial upscaling and long-term stability currently hinder practical applications.…”