Efficient conversion of solar energy into molecules such as hydrogen is one of the most important challenges for sustainable energy systems of the future. Photovoltaic (PV) driven electrochemical (EC) water splitting particularly in direct-coupled design offers highest solar-to-hydrogen efficiency. In most practical cases, PV and EC devices are developed separately and then tested in PV-EC combination at a later stage. While state-of-the-art PV devices are represented by a range of standardized commercial modules, the electrolyzers are much less established. Large number of materials, approaches, and designs are constantly tested for water electrolyzers. Enormous efforts of many groups produce increasing number of new catalysts every year. In this situation, it is desirable to predict performance of a specific electrolyzer in combination with PV devices of various technologies and sizes. It is useful to evaluate potential efficiency of a PV-EC system from the electrolyzer point of view without actually building and optimizing an experimental device. To address this issue, we developed method for fast assessment of the maximal solar-to-hydrogen efficiency (STH limit) attainable by the specific electrolyzer via the “reverse analysis” of its polarization curve [1]. We show that despite the complexity of the parameter space, there is a surprisingly simple way to estimate the limit of efficiency in any PV-assisted electrolyzer system.

Principle of the reverse analysis is presented in Figure 1. The maximum STH for a specific electrolyzer is achieved when the current-voltage (IV) characteristics of a PV device intersects polarization curve of an EC device at maximum power point (MPP). Conversely, each point on the EC polarization curve can be considered as the MPP of the PV device optimally coupled to the EC device, Figure 1 (a). Therefore, at each point on the polarization curve, the minimum PV efficiency and maximum EC efficiency can be calculated for a specific irradiance, Figure 1 (b). Product of both efficiencies generates the STH limit value attainable at that specific point on the polarization curve, Figure 1 (b). By plotting the STH limit versus PV efficiency, the resulting dependence is achieved, Figure 1 (c). It is the STH limit attained with PV device of any efficiency at the target irradiance and size. The solution is not analytic. It is a shortcut avoiding solving transcendental equations of PV and EC IVs. Therefore, finding STH limit at specific PV efficiency requires numerical interpolation of the dependence in Figure 1 (c). In return this "reverse analysis" is performed with elementary conversion of the EC polarization curve and does not involve any modeling or analysis of PV IV characteristics. It provides quick and simple way to quantify the potential of the electrolyzer as for the STH efficiency limit in combination with any PV device.

In our paper we present the principle of the reverse analysis, show how to identify, and quantify losses in experimental PV-EC combinations. We present an extended set of results of reverse analysis applied to a variety of PV-EC combinations described in the literature. We discuss how the method can be used to analyze mutual scaling of PV and EC devices to address design of practical systems in the field. Finally, we show how the analysis can be used to analyze systems with indirect coupling, involving DC-DC converters or other power management electronics between PV and EC parts.