Is green hydrogen a sustainable solution?

Could increased pressure on the desalination market call into question the environmental credentials of green hydrogen?

New green hydrogen projects are being created at an unprecedented rate: by 2040, the target capacity for green hydrogen is 206GW. For this goal to be realised, we will need 1.7 million cubic metres of fresh water per day, but is this achievable, and do the environmental impacts of producing green hydrogen mean that ‘green hydrogen’ is less sustainable than many might assume?

To create green hydrogen, we need very pure water, from which most of the salts have been removed. We achieve this through a desalination process.

The technology already exists to increase desalinating capacity, so in that sense, meeting the growing need of pure water is not in question.

What is in question, however, is whether this increased pressure on the desalination market will lead to environmental problems which make green hydrogen unsustainable in the long run.

Desalination demands for green hydrogen production

The more impure the feedstock (or water source) is to begin with, the more waste will be produced. Seawater, for example, has to go through an intense desalination process to separate the pure water from the dissolved solids. As the demand for pure water goes up, the issue of what to do with the waste product intensifies. Developers are faced with the challenge of what to do with the reject brine and how to dispose of it responsibly.

Simply directing brine to sewers could be wasteful and impractical. And more concentrated saline brines could be even more problematic. Disposing of hypersaline discharge into a constrained environment, such as hot, shallow seawater could lead to a build-up that would be beyond the tolerance levels of plants and animals living in that environment. Responsible developers will therefore need to think creatively when it comes to disposal.

For green hydrogen to be sustainable, the environment that receives the waste product from the electrolyser, be it land, sea or another watercourse, must be able to accommodate the effluent without adverse effect. That is what developers will need to focus on.

Where there is enough room to do so (such as in large desert areas), effluents can be concentrated to a very high level of dissolved solids and stored in a holding vessel on land — a salt pan, effectively — and left to evaporate. There would still be the residual salt to deal with in time, but this may be preferable to relying on the sea as a sustainable disposal route.

This method of effluent management requires heat, space and sunshine, which are not often in great abundance in many locations in the world and therefore this method of effluent management does not present a widespread solution.

Energy usage for green hydrogen production

There is also the issue of energy usage. When the feedstock is seawater, a great deal of energy is required to perform desalination. The hydrogen production process, including conditioning, drying, compressing and storing, can be power-thirsty, even before the energy required to purify feed water is considered – costing money and raising the question of whether the energy balance makes the undertaking worthwhile. As long as the energy used is from renewable sources, the process and the hydrogen can still be considered “green”, but green does not automatically equate to sustainable.

Sustainability should be a holistic consideration that takes into account all outputs from the system. If the balance is tipped and ‘green’ takes on a murky grey tinge because the process, when considered as a whole, is damaging to the environment, then that is not a behaviour we should encourage.

So, while officially speaking, green hydrogen could arguably still be called ‘green’, if the process is causing aquifer depletion or groundwater pollution as a result of how the effluent is being disposed of, then it does somewhat undermine any efforts to minimise environmental impact. This would be a case of partially solving a problem only to create another, possibly even greater, one.

That is not to say that the sustainability issue of desalination is intractable. Technology of the future may well be able to provide the answer.

There are emerging technologies out there which are showing promise. A combined desalination-electrolysis system is currently being developed in China. The process uses a low-energy method to purify seawater through the power of evaporation.

While innovations such as this are heartening, they do not yet solve the key problem of how to deal with the waste brine. This particular technology is still at the research stage, but it would need to prove scalable and cost-effective to make it an attractive alternative to pure water electrolysis.

Although pressures on the desalination market have already begun to increase as the world demands more green hydrogen, the conversation around the sustainability of the practice has yet to keep pace. There is no doubt, however, that the noise around the responsible disposal of liquid waste effluent from green hydrogen production will gain volume as demand increases still further.

When this happens — in the very near future — my prediction is that awareness will grow, environmental assessment requirements will increase and regulations will tighten, meaning that developers must be extra vigilant to ensure they comply.

This article featured in Power Engineering International in April 2023.