[ water treatment ]
H 2
O . Since oxygen is 16 times heavier than hydrogen , it is responsible for 89 % of the mass of water , which means that 9 litres of water is needed to produce 1 kg of hydrogen ( Figure 1 ). This ratio can be quite useful to determine the water requirements for a specific production capacity of hydrogen . For instance , production of 100,000 tons of green hydrogen per year will consume 900,000 m 3 ( tons ) of ultrapure water .
water is required per kWh , or 163 – 200 L / h of ultrapure water is required per MW of electrolyser capacity .
In my experience 200 L / h has proven to be an excellent rule of thumb as a first estimate of the requirements for ultrapure water ( Figure 2 ). Thus , a 10 MW plant needs 2 m 3 / h , and a 1 GW plant needs 200 m 3 / h of ultrapure water .
This approach tells us how much water is needed on average , but it will not tell us about the rate of water consumption ( m 3 / h ), which is required for dimensioning the water treatment system . Here , we need to look at the electrolyser power rating ( MW ). The power rating will determine the rate at which hydrogen is produced and thus the rate of water consumption . The amount of ultrapure water needed per MW depends on how much energy the electrolyser needs to convert the 9 L ( kg ) of ultrapure water to 1 kg of hydrogen . Most electrolysers consume 45 – 55 kWh per kg of hydrogen , which means that 0.16 – 0.2 L of ultrapure
Consumption of cooling water
While the consumption of ultrapure water as a feedstock will always be part of a green hydrogen facility , it is more difficult to give a precise evaluation of the consumption of cooling water . Many of the smaller projects that are realised today are based on dry cooling , while the very large electrolyser systems currently in planning may seek to integrate the waste heat into other water-based infrastructure systems such as wastewater treatment plants and district heating systems . Also , offshore-based systems may rely on the use of seawater for cooling .
Fig . 1 . The 1:9 rule for consumption of ultrapure water in green hydrogen production
Hydrogen Tech World | Issue 6 | October 2022 23