Direct Coupling of Water Electrolyser and Battery to PV Towards Enhanced Solar-to- Hydrogen Efficiencies

Uchechi Chibuko^a^,^b, Tsvetelina Merdzhanova^a, Florian Seidler^a, Sergey Shcherbachenko^a, Ursula Wurstbauer^b, Uwe Rau^a^,^c, Oleksandr Astakhov^a

^aIEK5-Photovoltaik, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany

^bDepartment of Physics, University of Münster, 48149 Münster, Germany

^cJülich Aachen Research Alliance (JARA-Energy) and Faculty of Electrical Engineering and Information Technology, RWTH Aachen University, Schinkelstr. 2, 52062 Aachen, Germany

Proceedings of The Future of Hydrogen: Science, Applications and Energy Transition (H2Future)

Ibiza, Spain, 2024 April 17th - 19th

Organizers: Carolina Gimbert Suriñach, Sixto Gimenez Julia and Emilio Palomares

Oral, Uchechi Chibuko, presentation 004
DOI: https://doi.org/10.29363/nanoge.hfuture.2024.004
Publication date: 27th February 2024

Photovoltaic (PV) systems commonly include maximum power point trackers for optimal power delivery to the connected load or storage device. In this work, a simple design with direct coupling of battery and water electrolyser to PV is explored. A key performing metric of directly coupled PV systems is the coupling factor, or coupling efficiency, which is the ratio of the output power of the PV to its power at maximum power point [1]. With the production of green hydrogen from PV being the focus of the study, system components such as the PV device, the electrochemical (EC) cell and the battery (B) needed for this operation are characterized and the performance of the system in direct coupling is investigated.

In our study we compare performance of two systems, two direct-coupled devices: (i) PV directly coupled to EC referred to as PV-EC, and (ii) PV directly coupled to EC and battery referred to as PV-EC-B. Previous theoretical and experimental studies [2, 3] show that the systems with batteries have potential for higher solar-to-hydrogen efficiency (STH). In this work we strive to demonstrate this potential experimentally at high overall efficiency level. The first step towards enhanced STH is aimed at by using high efficiency components. Therefore, we use Gallium arsenide (GaAs) PV modules with PV efficiency of approx. 29 % under 1 sun (1000 W/m²) and efficiency of up to 34.5 % reached at higher solar concentration of 17.3 sun. The PV operation is achieved using emulated current-voltage (I-V) characteristics with an advanced PV emulator. The emulator is based on an algorithm dynamically controlling the output of a source measuring unit (SMU) in such a way that the output reproduces an I-V characteristic of any required PV device with precision and accuracy on par with class A+ solar simulator. A commercially available EC is used with Pt/Ti cathode for hydrogen evolution reaction (HER) and Ru/Ir anode for oxygen evolution reaction (OER) and operated in acidic electrolyte. To simulate real electrolyzer operation conditions, the acidic electrolyte is kept at an elevated temperature of 80^oC with a stable concentration of 0.5 M sulfuric acid. This yields an EC efficiency of about 76% at a current density of 10 mA/cm². Lithium Titanium Oxide (LTO) batteries with a nominal capacity of 6 Ah and nominal voltage of 2.4 V for each battery are used.

For the PV-EC experiments, a high STH value of approx. 21% is achieved as marked by the orange star in Fig. 1 (TOC graphic). An STH value of approx. 25% is achieved for the PV-EC-B system as marked by blue star in Fig. 1 which is higher than results for the PV-EC and confirms the role of adding a battery to the system in terms of STH gain. The addition of the battery to the PV-EC system allows for lower overpotential operation of the system favourable for high STH [2, 3]. Considering different PV efficiencies, we also present the theoretical limit of STH for the PV-EC system using already established methods [4] as shown with red line in Fig. 1. The STH limit of the PV-EC is therefore obtained to be 29.3% pointing towards optimization directions in terms of obtainable STH when compared to an experimental value of 21%.

We will present detailed analysis of both systems in diurnal operations, discuss the energy efficiencies, and show directions to achieve more optimized performance.

References:

[1] Module-level direct coupling in PV-battery power unit under realistic irradiance and load," Solar Energy, vol. 249, pp. 233-241, 2023

[2] O. Astakhov, S. Agbo, K. Welter, V. Smirnov, U. Rau, and T. Merdzhanova, "Storage batteries in photovoltaic–electrochemical device for solar hydrogen production," Journal of power sources, vol. 509, p. 230367, 2021.

[3] L.-C. Kin et al., "Batteries to Keep Solar‐Driven Water Splitting Running at Night: Performance of a Directly Coupled System," Solar RRL, vol. 6, no. 4, p. 2100916, 2022

[4] O. Astakhov, V. Smirnov, U. Rau, and T. Merdzhanova, "Prediction of Limits of Solar‐to‐Hydrogen Efficiency from Polarization Curves of the Electrochemical Cells," Solar RRL, vol. 6, no. 2, p. 2100783, 2022.

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