Reducing water consumption with green hydrogen

Over the last century, global water consumption has rapidly increased at over twice the rate of population growth. Rather than being directly consumed by end users, the majority of water use takes place within supply chains that serve each sector of the economy. Due to their dependency on hydrocarbon fuels and/or electricity generated by thermal power plants, most existing industrial and transport activities have large water footprints. But, by utilising renewable power and water, an electrolyser based on proton exchange membrane (PEM) technology can have a significant positive impact on water use. 

Regardless of the source, the input water to an electrolyser stack to create green hydrogen must first be cleaned and deionised (removing the ions or ionic constituents). The reverse osmosis purification process – where contaminants are removed by pushing pressured water through a semi-permeable membrane – is commonly used prior to deionisation to ensure the electrolyser receives water of a suitably low electrical conductivity. A proportion of the water withdrawn from the supply is therefore rejected – for example, fresh water has a withdrawal rate range of about 0.3 tonnes per megawatt hour (MWh), while it rises to approximately 1.5 tonnes per MWh for river water. At present, a typical value for a commercially available PEM electrolyser connected to the mains water supply is 0.51 tonnes per MWh.

Thermal power plants account for 41% of all freshwater withdrawals in the USA, according to McKinsey & Company. By comparison, wind and solar power generation have zero or minimal water footprints, so their increased adoption will yield large water savings. 

Water withdrawal rates by power stations vary widely and are a strong function of the cooling technique employed, but the rates of water consumption relate mainly to the efficiency of generation. Hence the water footprint of electrolytic hydrogen is heavily influenced by the power source. For example, an electrolyser of 70% efficiency will produce hydrogen with a water footprint of about 4.1 tonnes per MWh if powered by nuclear electricity versus 0.51 tonnes per MWh if powered by wind electricity.

The water savings achieved by adopting green hydrogen depend on the fuel that is being displaced and whether a combustion device, engine or fuel cell is being used. The volume of water used in the extraction of crude oil, for example, usually amounts to 6-8 times that of the oil produced – or up to 12 times if enhanced oil recovery techniques are applied (such as injecting water, steam or CO2 into the well). Because of this and the need to use water (as steam) for refining crude oil, the overall water footprint of petrol is estimated to lie in the range 0.3-1.3 tonnes per MWh.

Accessible fresh water and wastewater can enable green hydrogen production, but electrolysers will need to use desalinated seawater in arid regions and at offshore wind/solar farms. Fortunately, the seawater resource on Earth is approximately 39 times greater than the fresh water resource. Together, these respective amounts frame the water resources available for satisfying both established uses and the new demand associated with green hydrogen production.

At present, global consumption of molecular energy in the form of oil, natural gas and coal amounts to approximately 162,000 terawatt-hour (TWh) a year. Predictions of future requirements for green hydrogen vary but, put simply, if all this fossil fuel consumption were replaced with green hydrogen, the annual water use for electrolysis would be 8.3 x 1,013kg (or approximately 28kg per person per day). This is equivalent to using 0.000006% of the seawater resource, or 0.09% of the accessible fresh water resource. It amounts to 1.8% of current global water consumption. 

Alternatively, it may be expressed as roughly one quarter of our current annual rate of wastewater production, or of the fresh water added to the ocean due to glacier ice melt. This outlines the potential scale of water consumption in a future ‘net zero’ scenario involving a multi-terawatt deployment of electrolysers. 

Water use due to electrolysis should, however, not be viewed as gradually using up the water resource, because when green hydrogen is oxidised (by combustion or via a fuel cell) it yields the same amount of water as was originally electrolysed. This may enter the atmosphere as water vapour, or be condensed at the point of use and recovered as liquid water. Moreover, the production of green hydrogen simultaneously produces oxygen in the exact amount required to oxidise the hydrogen: this is an important characteristic, because atmospheric oxygen depletion is contributing to global warming. Consequently, the widespread production and use of green hydrogen is expected to have a comparatively neutral effect upon Earth’s water and oxygen resources, and the increased adoption of renewable energy (as electricity and hydrogen) will serve to reduce global water consumption.

The use of oxygen as well as hydrogen improves the commercial viability of electrolysis. When a guaranteed demand for oxygen exists, some of the costs of hydrogen production can be offset. The water industry is particularly well placed to exploit this, because it has scope to:

• Provide the necessary water for electrolysis;
• Process the water stream rejected by the electrolyser system;
• Utilise green oxygen for wastewater treatment to improve process efficiency;
• Recover heat from the electrolyser to improve process efficiency;
• Use waste-to-energy or renewable power sources on site to provide electricity;
• Export green hydrogen.

Wastewater treatment results in the emission of three global warming gases (nitrous oxide (N2O), methane (CH4) and CO2). Cleaning wastewater is essential for ensuring water of sufficient quality is produced for returning to rivers and the ocean, or for use as drinking water. Aeration of active sludge is the most critical part of the energy-intensive wastewater treatment process, where microorganisms and oxygen work together to break down the organic matter. 

The water industry has the potential to use electrolysers to meet its own oxygen requirements, whilst simultaneously producing green hydrogen for other applications including vehicle refuelling and/or injection into nearby gas distribution networks or industrial processes. Because wastewater treatment plants tend to be located relatively close to towns, they offer numerous possibilities for creating decentralised hydrogen hubs and driving the deployment of electrolysers. 

It has been estimated by IRENA that approximately 2.4 x 1,013kg of desalinated water is produced annually and that demand is increasing at more than 9% per year. There are now more than 21,000 desalination plants in at least 174 countries, which are used mainly for supplying drinking water, irrigating crops and oil and gas extraction. They operate thermally by distillation or mechanically via reverse osmosis, with the latter being more common. Reverse osmosis has a relatively low electricity requirement for desalination, so an electrolyser system will incur an overhead on the electrolyser of only 0.1% relative to producing green hydrogen from fresh water.

In general, the implementation of green hydrogen production from seawater affords an interesting set of secondary opportunities, including:

• The integration of an electrolyser and its desalination plant with a renewable power source as an engineering product;
• The provision of a rainfall-independent drinking water supply by oversizing the desalination plant relative to the water requirement of the electrolyser;
• Use of green oxygen for oxygenating desalination effluents and hypoxic zones in estuaries and coastal areas;
• Extraction of minerals from the desalination effluent;
• Off-grid production of hydrogen, oxygen and drinking water in regions where the electricity grid is weak or non-existent.

In developing countries, the high demand placed on traditional water sources is causing communities to increasingly use rainwater, stormwater, brackish water and seawater. By implementing green hydrogen production, a new impetus could be established for resolving water supply problems. By oversizing the water purification plant required by an electrolyser facility, a contribution can be made to supplying clean drinking water. In general, an integrated approach to providing green hydrogen and water could assist public health – as well as decarbonisation – objectives in many regions of the world.

In developed countries, the existing water consumption per household far exceeds that needed to produce green hydrogen for fuelling a fuel cell car or heating a house. For example, for an average UK household with a water consumption of 349kg per day, a water overhead of 27kg per day would be sufficient to produce enough green hydrogen for space heating, hot water and travelling 50km per day in a fuel cell car. Household water use and heat demand vary across the building stock, but in general the amount of water required for switching heat and mobility to green hydrogen equates to only a small percentage of existing use. 

If all existing fossil fuel use were switched to green hydrogen, the water requirement for electrolysis would amount to 1.8% of current global water consumption. This new demand would be counterbalanced by water savings achieved by not having to produce fuels from petroleum or biomass and by reducing the use of conventional thermal power plant. Electrolysis should therefore play a more central role in future policies concerning energy and water: achieving a multi-terawatt electrolyser capacity by mid-century would yield massive positive benefits.

Based on a paper published in the December issue of ‘Fuel Cells Bulletin’ by Dr Graham Cooley and Marcus Newborough, CEO and Government Relations at ITM Power respectively. Download the paper here.