There’s no point ramping up hydrogen if other resource constraints are going to bring it to a halt. Here, Herib Blanco at IRENA summarises their research into how much water will be needed in the production of hydrogen through electrolysis (i.e. from water) and the costs involved. A wide range of analyses have been reviewed to calculate the amount of water used during the hydrogen production, and by the energy source used to power it (renewables or gas). Assuming the world is using over 70EJ of electrolytic hydrogen by 2050, the water consumption will be about 25 bcm. That is relatively small compared with the global figure of 2,800 bcm for agriculture (the largest consumer), 800 bcm for industrial uses, and 470 bcm for municipal uses. It would be equivalent to a developed country with 62 million inhabitants (400 m3/capita). Even in the most conservative case, where water desalination is used, the water cost (treatment, transport) would be less than 2% of the total hydrogen production cost and the energy consumption for water desalination would be only about 1% of the total energy needed for the hydrogen production. Blanco notes that the water footprint is very location-specific and depends on the local water availability, consumption, degradation, and pollution. The impact on the ecosystem will have to be considered too. But the numbers suggests that water consumption shouldn’t be a major barrier for scaling up renewable hydrogen, says Blanco.
Water consumption for green hydrogen: is it a constraint for scaling up?
Hydrogen is an essential component of a net zero emissions energy system. It complements electricity by providing an energy carrier with high energy density that is more easily transported and stored than electricity. Hence, it enables the decarbonisation of applications that are difficult to electrify.
Similar to electricity, hydrogen can be produced from multiple energy sources. The electrolytic route allows splitting water into oxygen and hydrogen by using of electricity. This allows taking advantage of the continuously declining renewable electricity costs, providing flexibility to the power sector and enabling an integrated energy system rather than separate sectors.
One common concern for electrolysis is the water consumption that is used as input and if it could pose a limitation to the large-scale production of hydrogen. This article aims to answer some of the key questions related to this issue.
How much water does electrolysis consume?
…Step 1: hydrogen production
Water consumption comes from two steps: hydrogen production and the production of the upstream energy carrier. Looking at hydrogen production, the minimum water electrolysis can consume is about 9 kg of water per kg of hydrogen. However, taking into account the process of water de-mineralisation, the ratio can range between 18 kg and 24 kg of water per kg of hydrogen or even up to 25.7-30.2 according to .
For the incumbent production process (steam reforming of methane), the minimum water consumption is 4.5 kgH2O/kgH2 (needed for the reaction), which increases to 6.4-32.2 kgH2O/kgH2 when considering the water for the process and cooling [1, 2].
…Step 2: the energy source (renewable electricity or natural gas)
The other component is the water consumption for the production of renewable electricity and natural gas. Water consumption for PV can vary between 50-400 l per MWh (2.4-19 kgH2O/kgH2) and between 5-45 l per MWh for wind (0.2-2.1 kgH2O/kgH2) . Similarly, natural gas production can be 1.14 kgH2O/kgH2 increasing to 4.9 kgH2O/kgH2 for shale gas (based on US data) .
In sum, the total water consumption for hydrogen from PV and wind can be, on average, around 32 and 22 kgH2O/kgH2 respectively (see Figure 1 below). Uncertainties arise from the solar radiation, lifetime and silicon content . This water consumption is in the same order of magnitude as hydrogen production from natural gas (7.6-37 kgH2O/kgH2 with an average of 22 kgH2O/kgH2) [1, 2].
Total water footprint: lower when using renewable energy
Similar to the CO2 emissions, a pre-condition for the electrolytic route to have a low water footprint is the use of renewable energy. If just a fraction of fossil-based generation is used the water consumption associated to the electricity is much higher than the actual water consumed in the electrolysis process.
For instance, water consumption for electricity production from natural gas can be up to 2,500 l per MWh . This is also using the best case for fossil fuels (from natural gas). If coal gasification is considered, it can consume 31-31.8 kgH2O/kgH2 for hydrogen production  with another 14.7 kgH2O/kgH2 from the coal production . Water consumption for PV and wind is also expected to decrease over time as the manufacturing process becomes more efficient and the energy output per unit of installed capacity also improves.
Total water consumption in 2050
Future global hydrogen use is expected to be many times larger than today. For example, the World Energy Transitions Outlook from IRENA  estimates the 2050 hydrogen demand will be about 74 EJ, of which about two thirds will be from renewable hydrogen. This compares to 8.4 EJ today (for pure hydrogen).
Even if the entire 2050 hydrogen demand would be satisfied with electrolytic hydrogen, the water consumption would be about 25 bcm. Figure 2 (below) puts this number in perspective with other anthropogenic water consumption flows. Water consumption for agricultural use is the largest one of the order of 2,800 bcm, water for industrial uses is close to 800 bcm and 470 bcm for municipal uses [7-9]. Current hydrogen production from natural gas reforming and coal gasification has a water consumption of about 1.5 bcm.
So, even though a large growth of water consumption is expected due to the change to the electrolytical pathway and to the demand growth, water consumption for hydrogen production will still be much smaller than other flows for human use. Another point of reference is that the water withdrawals per capita is between 75 (Luxembourg) and 1,200 (US) m3 per year . Taking an average value of 400 m3/(capita*year), the total 2050 hydrogen production would be equivalent to a country with 62 million inhabitants.
What is the cost and energy consumption of the water supply?
The electrolysers need high-quality water which requires water treatment. A low-quality water can lead to faster degradation and shorter lifetime. Many elements, including the diaphragm and catalysts for alkaline and the membrane and porous transport layer for PEM, can be adversely affected by water impurities such as iron, chromium, copper, among others. Water with conductivity of less than 1 μS/cm and total organic carbon of less than 50 μg/L is required.
Water is a relatively small share of both energy consumption and cost. The worst case for both parameters is the use of water desalination. The dominant technology for desalination is reverse osmosis with almost 70% of the global capacity and the main technology installed since early 2000s . This technology has a cost of USD 1,900-2,000/(m3/d) with a learning curve rate of 15% . With such an investment cost, the treatment cost is about USD 1/m3 and potentially lower in regions with low-cost electricity.
Furthermore, transport cost can roughly add another USD 1-2/m3 . Even in such a case, the water treatment cost would be around USD 0.05/kgH2. To put this in perspective, renewable hydrogen cost can be USD 2-3/kg H2 today with good renewable resources and USD 4-5/kgH2 with average resources.
Thus, the water cost would represent less than 2% of the total cost in this conservative case (see Figure 3, left). Using seawater would increase the water intake by 2.5-5 times (based on the recovery ratio) .
Looking at energy consumption for water desalination, this is also very small compared to the electricity required as input to the electrolyser. Reverse osmosis plants in operation today consume around 3.0 kWhel/m3. In contrast, thermal desalination plants have a much higher energy consumption, ranging from 40 to 80 kWhth/m3, with additional electrical energy requirements of 2.5 to 5 kWhel/m3, depending on the desalination technology . Taking the conservative (i.e. higher energy demand) case of thermal plants, the energy demand would translate to about 0.7 kWh/kg of hydrogen assuming the use of heat pumps. To put this in perspective (see Figure 3, right), the electricity demand for the electrolyser is about 50-55 kWh/kg, so even in the worst case, the energy demand from desalination is in the order of 1% of the total energy input of the system.
One challenge with seawater desalination is the disposal of the brine, which could have a local impact on the marine ecosystem. The brine could be further treated to reduce its environmental impact increasing the cost of the water by another USD 0.6-2.4/m3 . Additionally, a stricter water quality for electrolysis compared to drinking water could lead to higher treatment cost but this is still expected to be small compared to the electricity input.Water footprint is a very location-specific parameter that depends on the local water availability, consumption, degradation, and pollution. Balance with the ecosystem and impact from long-term climate trends should be considered. However, it is not foreseen that water consumption will be a major barrier for scaling up renewable hydrogen.