Integrating electrolyzers with renewable energy system can present challenges as well as unique benefits. Currently most renewable energy systems produce power and interconnect with thte electrical grid via some form of power electronics (PE). To use electrical grid power, today's commercial electrolyzers also have some type of power electronics interface that can represent a significant portion of the overall system cost.43 The power electronics convert alternating current (AC) from the grid to direct current (DC) power required by the electrolysis cell stack. In addition to the DC requirements of the stack, the system also consumes additional AC power for the balance of plant or ancillary loads. At least one electrolyzer manufacturer offers a version of an electrolyzer that can accommodate a connection to photovoltaic (PV) panels in addition to having the standard AC to DC converter for utility operation.44 The additional power electronics, incorporating maximum power point tracking (MPPT), converts all available DC power from the PV array to run the electrolysis stack. This system appears to be one of the firsts to incorporate dedicated PE to interface with a PV source.
In addition to using PV systems as electricity sources, wind energy can also be used. Today, the majority of wind to hydrogen demonstration projects are focused on installing commercially available electrolyzers and powering them from the AC power from wind turbines.45-47 In these projects the AC from the wind turbines is sent out onto the grid and the electrolyzers tied into the grid achieving a loose coupling of source and load. Scheduling the power to the electrolyzer, based on an output signal from the wind turbines, is relatively straightforward in this case.
Capital costs of electrolysis equipment range from just under $1000 kW-1 for the largest alkaline systems to over $10,000 kw-1 for small proton exchange membrane (PEM) electrolyzers.28,48 Merely taking an off-the-shelf wind turbine with its own PE and commercial electrolyzer with its own PE reduces overall energy transfer from the wind to hydrogen system. The potential exists to characterize electrolyzer performance under varying input power and design a single PE package and intelligent controller to achieve direct coupling between the stack and wind turbine output. This topology would not only eliminate the redundancy of power electronics that exists in the wind turbine and electrolyzer but also achieve gains in system cost and robustness. Characterizing the system demands of renewable energy sources and the requirements of the hydrogen-producing stack appears to have synergistic benefits. Ultimately, the detailed understanding of both systems and design of the directly coupled wind to electrolysis would reduce the cost of renewably generated hydrogen.
In renewable-based energy systems PEM electrolysis seems to have an advantage over alkaline in that the thin membrane and ion transport mechanism can react to nearly instantaneously with the rapidly changing energy output of renewable sources, especially wind. Stacks involving the circulation of a liquid electrolyte have inherently more inertia in the transport of ions in solution than the PEM systems.
On the turbine side, variable-speed wind turbines (which will soon be the norm as a result of enhanced energy capture relative to constant-speed machines) rely on power electronics to convert the variable frequency, variable voltage AC produced at the generator to DC. Small turbines used in battery-charging applications stop here;
however, larger turbines used to connect to the grid must then convert the DC back to AC at grid frequency: 60 Hertz (Hz). It is important to note that because of the economies of scale, it is the large wind turbines that are achieving highly competitive energy costs and will likely be the device of choice in large-scale wind-to-hydrogen operation.
The small wind-to-hydrogen systems (< 20 kW) being studied today are systems incorporating a common DC bus fixed with a battery bank to which the wind turbine and electrolyzer as well as fuel cells and PV panels are connected. Typically, the wind turbine is of the battery-charging type, which requires connection to a constant voltage DC bus (hence, the battery bank) and incorporates power electronics to convert wild AC to DC and to regulate power output. The electrolyzer stack accepts DC power input but the system would also include power electronics to regulate power input and possibly convert DC at one voltage level to another.
There are a number of weaknesses with this configuration, namely a redundancy of power electronics leading to increased cost and potential for failure. The inability to match wind turbine power output to electrolyzer power requirements because of separate power electronic controllers ultimately results in reduced energy capture.
An advanced topology would be the direct coupling of an electrolyzer with a wind turbine. This would allow hydrogen production that is proportional to the available wind energy and reduce electricity storage requirements.. The single point of control will allow the matching of wind turbine and electrolyzer electrical characteristics, thereby increasing the energy capture of the wind turbine. Finally, this solution will eliminate the need for a constant voltage DC bus and provide a true test of electrolyzer operation under fluctuating power-input conditions.
Renewable electrolysis can help overcome one of the key barriers to realizing a hydrogen-based economy by replacing the carbon-intensive one that exists today. There is an excellent opportunity for research in renewable hydrogen production both in terms of understanding the operation of the electrolyzer under variable sources and optimizing, in terms of efficiency, cost, and robustness, the link between a renewable source and electrolyzer stack.
Was this article helpful?
Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.