[ solar coupling ]
Threshold loss In electrolyzer systems, the turndown ratio is primarily constrained by issues related to ion transport and gas bubble management, and gas cross-over 8, 9, 10 at lower current densities. As the power input decreases, gas evolution reduces, but the diffusion rate does not, leading to increased impurities from crossover gases, which can affect overall system efficiency and stability. Typically, safe turndown ratios of 20 – 40 % are reported for large alkaline stacks, depending on operational temperature and pressure.
Considering a generic off-grid solar system connected to an electrolyzer, curtailment and threshold losses are illustrated in Fig. 2. Curtailment losses around noon result from limitations in the sizing of power electronics equipment, while threshold losses occur after sunrise and before sunset, when available power is lower than the electrolyzer’ s turn-down ratio. The losses shown in Fig. 2 are representative of methods 1 and 2.
While threshold losses also apply to method 3, it is important to note that, due to the modularity of the electrolyzer system, the effective turn-down ratio is below 1 %, compared to 20 – 40 % for a conventional megawatt-scale stack.
Fig. 2. Illustration of curtailment and threshold losses based on a normalized PV power profile. Curtailment is mainly set by system sizing, while threshold loss depends on solar intensity and electrolyzer design limits.
Maximum power point tracking
It is well known that the efficient use of a solar field requires a Maximum Power Point Tracking( MPPT) system to ensure optimal utilization of the generated solar power. MPPT is achieved using power electronics in methods 1 and 2, as shown in Fig. 1a.
In a modular system – method 3 in Fig. 1c – consisting of several stacks working in parallel, MPPT is realized without the need for power electronics. Instead, MPPT is accomplished by rapidly and frequently switching the modules on and off, adjusting the nominal power of the system at any given moment to match the spontaneous maximum power output of the solar field. This allows the system to dynamically regulate the spontaneous current drawn from the solar field by adjusting the voltage as solar irradiance fluctuates. Through this innovative approach, the electrolyzer system effectively performs the role of an MPPT system by modulating its current offtake from the PV panels to maximize power extraction. Direct coupling is realized using blocking diodes, as depicted in Fig. 1c.
The working principles of MPPT for different system architectures are illustrated in Fig. 3. In Fig. 3a, a conventional electrolyzer stack is coupled to solar power. The dashed gray line represents the power curve of the electrolyzer, while the blue line shows the spontaneous available solar power as a function of voltage. Since solar power depends on irradiance and ambient temperature, it is a dynamic variable. In principle, the intersection between the solar power curve and the electrolyzer power curve determines the operating voltage of the system and the amount of power delivered from the solar field. However, as shown in this example, the intersection does not coincide with the maximum power point. A DC / DC converter between the electrolyzer and solar panels( MPPT) can regulate the voltage, thereby adjusting the instantaneous
Hydrogen Tech World | Issue 22 | June 2025 17