Direct connection of PV devices to electrochemical (EC) water-splitting cells or batteries (B) is a highly efficient and material-saving solution to stabilize intermittent PV generation on long- and short-time scales. In our study, we address a hybrid device with EC water splitting cell in parallel connection to a battery. The first obvious advantage of this PV-EC-B system is the much more stable operating conditions of the electrolyzer. A sufficiently scaled battery keeps the EC cell running overnight and shaves peak load during the daytime. In our works, we have shown the feasibility of the self-sustained operation of the PV-EC-B system [1, 2]. Besides the foreseeable stabilizing effects, we have found theoretically [1] and confirmed experimentally [2] that batteries can improve solar-to-hydrogen efficiency (STH) in PV-EC-B devices as compared to the PV-EC reference. Batteries can boost STH in PV-EC-B systems because, under periodic irradiance conditions, the battery transfers part of the daytime PV energy to the night, reducing EC cell power and suppressing related overpotential (kinetics) losses. The gain is synergistic – even despite additional potential losses in the battery, the EC-B combination has lower losses than the EC cell operating alone [1, 2]. From the point of view of the system design the reduction of operating power and its stability allows for the use of a smaller electrolyzer at the same nominal PV capacity.

In our report we will present details of the PV-EC-B operation, principles behind the gain in STH attained with battery, and our recent experimental study. An increased average STH efficiency per cycle (11.4 % vs. 10.5 % without the battery) has been observed in the PV-EC-B system of a Si heterojunction PV module, bifunctional NiFeMo electrolyzer, and a commercial Li-ion NMC battery. The results are discussed in the context of the generalized STH limit analysis developed earlier for PV-EC combinations [3].

Applying our analysis to different literature sources using PV-EC without batteries we present an analysis of what could be achieved if a battery is applied to these systems in the figure below. We calculate the STH limit with and without a battery for each experimentally reported system in literature as well as the STH gain with a battery.

Each system reported in the literature is represented by four symbols: filled blue circles show the reported experimental STH of each PV-EC system; open blue circles indicate the STH limit calculated for each system using the reverse analysis [3]; horizontal dashes represent the estimate for the STH limit in this system if the battery is included; finally, the red circles show the estimate of the STH gain for each system.

A separate set of points denoted “this work” represents the experiment on the PV-EC-B system in this work.

For approximately half of the reported systems STH gain of 0.5%_abs – 1%_abs can be expected, several systems can gain 1%_abs – 2%_abs, while several systems can gain 2%_abs – 4%_abs. There is a general trend for higher STH gain at higher PV efficiencies, but the data presented in Figure 4 does not constitute any dependence.