We can’t stop using fossil fuels without renewable energy. We can’t depend on solar or wind without storage. Most storage so far relies on batteries. Hydrogen energy storage has been on the back burner. But if solar electrolysis produces the hydrogen, hydrogen backup generators could displace batteries.

Several technologies can potentially use the sun to split water. Most obviously, excess solar power from existing solar power can run electrolyzers.

President George W. Bush launched massive federal research on green hydrogen. It ran into so many roadblocks that the Obama administration decided to concentrate on other technologies that seemed more practical. It shelved further work on hydrogen.

The Bush effort concentrated on hydrogen fuel cell vehicles. Today, decarbonizing the grid drives the interest in hydrogen.

The current grid lacks the capacity to handle all the energy the sun and wind can generate. It’s necessary to curtail solar and wind farms, that is, cut them off from the grid, during times of excess. Overbuilding solar capacity can help, but, of course, solar farms generate no electricity at night and wind farms generate none when winds are calm.

Renewable energy can’t replace fossil fuels without energy storage. Lithium batteries cannot provide adequate grid storage capacity. Vanadium flow batteries have important advantages, but they are less efficient than lithium and have higher power-related costs.

Many studies today choose not to consider hydrogen energy storage technology because it’s so expensive. But a decade ago, the same mentality rejected solar power as too expensive. And look how rapidly costs have come down! Most hydrogen today comes from fossil fuels and uses electricity from fossil fuels to produce it.

Solar electrolysis can make it both less expensive and greener. Imagine using hydrogen backup generators instead of batteries!

The promise of green hydrogen—and some roadblocks

“Green hydrogen” comes from using excess solar power that the grid must curtail. One can almost say that solar electrolysis uses waste electricity. It goes a long way toward reducing the cost of making hydrogen.

But as things stand now, there aren’t enough electrolyzers to let green hydrogen displace fossil fuels. And, of course, there aren’t enough electrolyzers because manufacturers lack enough customers to mass produce them.

Adding hydrogen to natural gas pipelines

One way to encourage manufacture of more electrolyzers would be to mix green hydrogen with natural gas. Engineers long assumed that elemental hydrogen would leak out of natural gas pipes and cause a safety hazard. It turns out that blending as much as 25% hydrogen with natural gas doesn’t cause seepage or harm pipes.

In 2018, Paris-based energy firm Engie started a pilot program to mix green hydrogen in natural gas pipelines in the village of Cappelle-la-Grande. Extensive testing has led to public acceptance. The blend has reduced the village’s carbon emissions from furnaces, stoves, and hot water heaters by 7%. It therefore provides cleaner air. The added hydrogen enables the natural gas to burn more completely.

In addition, companies in Germany and France are seeking financing and approval to build 100-megawatt electrolyzers. One of them can even use an existing and currently empty pipeline. The largest electrolyzer now in operation is only 10 megawatts.

Still, fear of the potential for hydrogen explosions can slow its acceptance for grid-scale use. Research into enhancing hydrogen safety continues.

This kind of project is not hydrogen energy storage, but it can increase the market for hydrogen enough to make it look more attractive to investors.

Standard fuel cells as hydrogen backup generators

Microsoft has started experimenting with hydrogen fuel cells to replace its diesel backup generators. It has needed to them so seldom that they actually burn more diesel fuel for monthly maintenance than during power outages. The diesel generators and fuel cells have the same generating capacity for about the same price.

Unfortunately, hydrogen costs about three times as much as diesel per unit of electricity generated. That is, if the electrolyzers operate on standard electricity.

Solar electrolysis is inefficient, but for backup, that hardly matters. For one thing, after initial outlay for the panels, the energy is free. And since the generators nearly always sit idle, hydrogen energy storage doesn’t need quick recharge of the fuel cells.

For example, Microsoft’s test hydrogen backup generator, with 250 kW capacity can power ten racks of servers for 48 hours. It takes about 600 kg of hydrogen. Producing that much hydrogen requires 30 MWh of electricity.

But Microsoft’s backup generators sit idle all but about three days a year. They won’t likely run for a continuous 48 hours. About 60 kg of hydrogen is enough to power the racks for a few hours. A one-megawatt solar array can supply the remaining 540 kg in less than a week.

Fuel cells, require 50 kWh of energy to extract one kilogram of hydrogen, which provides only 20 kWh of electricity. That’s less than half the efficiency of batteries.

But batteries have a much larger upfront cost. Increasing capacity of battery systems requires additional expensive batteries. Increasing fuel cell capacity requires only a larger tank. Therefore, despite its inefficiency, solar electrolysis provides more economical backup than batteries.

If enough large corporations such as Microsoft will invest in developing infrastructure for hydrogen energy storage, it will go a long way toward making hydrogen fuel cells economically viable throughout the economy.

Can solar panels back themselves up?

In standard solar panels, photovoltaic (PV) cells convert sunlight to electricity—but only when the sun shines on them. Photoelectrochemical (PEC) cells work differently. Instead of converting sunlight to electricity, they convert it to chemical energy by splitting water molecules. The resulting hydrogen becomes the fuel for hydrogen fuel cells.

Solar water splitting offers the potential for highly efficient conversion of sunlight to hydrogen at low operating temperatures using cost-effective semiconductors. So far, PEC looks great in theory. The technology needs improvements in efficiency, durability, and cost before it will become commercially viable.

But suppose PEC cells that work as successfully as PV cells. And suppose they’re built into the same panel. It would cut the need for building an electrolyzer between a solar electric generator and a hydrogen backup generator.

It works in the lab, but it’s not ready for prime time.

Both PV cells and PEC cells need semiconductor electrodes. Fewer materials work with PEC than PV. Scientists have spent about half a century working with one very promising semiconductor: hematite, more widely known as rust.

Two studies of hematite electrodes show the difficulty of developing viable PEC technology.

Has science overestimated the potential of hematite for solar electrolysis?

Hematite is common, inexpensive, and stable in water. As a catalyst, it speeds formation of hydrogen at the cathode or oxygen at the anode. But after all that work, its photocurrent conversion efficiency remains less than half of its theoretical maximum. Silicon, on the other hand, has an efficiency of about 90% of its theoretical maximum.

A team of scientists from Helmholtz-Zentrum Berlin and two Israeli institutions, Ben Gurion University and Technion, decided to find out why. Team members from each of the institutions measured different aspects of the behavior of light striking thin films of hematite.

Integrating their results, they came to understand the photogeneration yield spectrum. Previous studies of inorganic solar absorbers had not taken that measurement into account.

They found that when hematite absorbs light, most of the energy generates localized excited states. Only part of it does the work of generating mobile charge carriers. In other words, the true theoretical maximum photocurrent conversion efficiency is lower than previously expected. The project concluded that hematite electrodes are as close to their theoretical maximum as they’re likely to get.

The team analyzed titanium oxide and bismuth vanadate with the same method. At present, the latter is the best-performing material for solar electrolysis. Using the same method to analyze other candidates can allow less useful ones to fail quickly. With less time exploring blind alleys, it will hasten the development of successful catalysts.

Has science found a way to boost the potential of hematite for solar electrolysis?

On the other hand, researchers from South Korea’s Ulsan National Institute of Science and Technology and China’s Dalian Institute of Chemical Physics have announced a greatly improved hematite anode. I’ll not try to explain all the steps in detail

Previous strategies for improving hematite’s performance have either enhanced photocurrent generation or reduced current turn-on voltage. This team claims that its nanostructured anode combines high doping, homojunction, and cocatalyst loading. As a result, they improved both performance measures.

So one team has found that hematite electrodes are as good as they’ll get and it’s time to look at other semiconductors. At the same time, another team has found a way to use multiple strategies to improve performance of hematite anodes.

Sooner or later, we’ll have combined PV and PEC solar panels that will generate electricity and serve as their own hydrogen backup generator. One or both or neither of these two studies will bring us closer. Science and technology don’t work in convenient straight lines.

When critics start to complain that solar electrolysis doesn’t work and can’t work, that’s when we’ll know it’s probably starting to work.

Sources:

Hydrogen production: photoelectrochemical water splitting / US Office of Energy Efficiency and Renewable Energy
The pros and cons of hydrogen fuel cells as backup generators / Tom Lombardo, Engineering.com.  August 11, 2020
Solar and wind power could ignite a hydrogen energy comeback / Peter Fairley, Scientific American. February 20, 2020
A step closer to practical solar hydrogen production via elaborately modified hematite photoanode / Asia Research News. November 20, 2020
Study may help develop ideal photoelectrode for solar water splitting