In 1766, Henry Cavendish discovered a lightweight gas which, when burned in air, turned into water. In 1787, Antoine Lavoisier named this new gas “hydrogen”, a combination of the roots hydro and genes–quite literally “water-former”. 1 Not long after, scientists discovered that by adding electricity to water, hydrogen can be produced by the reverse reaction. Today, hydrogen is used as a feedstock for chemical synthesis, but other applications have become realities including energy storage and transportation fuels. If hydrogen is generated from renewable electricity, it releases no greenhouse gas emissions, meaning that it can be a key player in the battle against climate change.
To develop a “green hydrogen economy” where emissions-free hydrogen is widely used in daily life, innovators are using electrochemical water electrolysis to generate hydrogen from two simple ingredients: electricity and water. As renewable electricity prices drop and improvements in electrolyzer efficiency are achieved, some critics have begun to ask a question about the second ingredient: is there enough water to support a hydrogen economy? Some argue that the answer is no, due to perceived significant water demand throughout the entire production process, including the use of water as a feedstock and a cooling agent for thermoelectric methods of producing hydrogen such as steam methane reforming (SMR).2 Yet the objective of the green hydrogen economy is to derive energy completely from renewable sources which do not use water for cooling or fossil fuel processes. Therefore, it is critical to include only water that is directly used for water electrolysis when considering the impact of hydrogen production on global water reserves. In the analysis that follows, we find that by isolating the water used for the electrolysis step, there is a negligible impact of the amount of water consumed for hydrogen production compared to the amount of water available.
1) Water Requirement of Electrolysis. In our previous work, we considered the total future need for hydrogen in all applicable sectors including chemical synthesis, transportation, buildings and heating, and energy storage. The calculated hydrogen demand in the distant renewable future is 2.3 Gt per year.3 In our vision, hydrogen will be produced by water electrolysis powered by renewable energy. Such a vision can reduce the carbon emissions from the energy sector by up to 10.2 Gt annually from future emissions projections compared to the International Panel on Climate Change’s worst-case RCP8.5 scenario.3
Before moving to further develop the hydrogen economy, it is important to determine the feasibility of the amount of water 2.3 Gt of hydrogen will require each year. Several authors2,4 have expressed their concerns about water for hydrogen, stating that obtaining water for the economy will be too expensive or demanding on the water and energy requirements. Here, we calculate the amount of water needed for the predicted hydrogen economy, including total water that is withdrawn and consumed for electrolysis. Withdrawals are water that is directly returned to the body of water from which it was extracted. Any water that is converted into another unusable form or is not returned to the original body of water will be considered to be consumed.
Based on the reaction stoichiometry, for every kg of hydrogen produced, 9 kg of water must be consumed. Therefore, 2.3 Gt of hydrogen requires 20.5 Gt, or 20.5 billion m3 , per year of freshwater, which accounts for only 1.5 ppm of Earth’s available freshwater. Most applications for hydrogen require it to be combusted or pumped through a fuel cell, which converts hydrogen gas into electricity and water, but while most water can be recovered, it is not generally returned to the original body of water and will be treated as consumed. The only sector in which the use of hydrogen does not regenerate the entirety of the water feedstock by fuel cell or combustion is chemical synthesis, which will account for 540 Mt of hydrogen, using at most 4.8 billion m3 or 0.3 ppm of global freshwater annually.3
When compared to other projections for the water demand of hydrogen production,2 the freshwater requirements above are quite low. This is due to our assumption that all hydrogen in the future will be produced using renewable energy sources such as wind and solar, which have little to no water consumption. When fossil fuels are used for primary energy production and power generation, the water requirement is quite significant. In 2014, 251 billion m3 of freshwater were withdrawn for power generation and energy production from fossil fuels such as coal, oil, and natural gas, and 31 billion m3 were consumed as the water was used for cooling, mining, hydraulic fracturing, and refining.5 In comparison, even though the 20.5 billion m3 of water withdrawn for hydrogen production by electrolysis must be consumed, this is still 33% less than the current fossil fuel energy-related uses. Furthermore, electrolysis will render fossil fuel energy sources obsolete as the energy sector is able to move more toward renewable technologies, saving 10 billion m3 of fresh water that would have been consumed by energy-related uses of fossil fuels. Thus, it is apparent that using hydrogen as a method to reach a renewable energy society will lead to drastic water savings, not expenditures.
The water consumption of electrolysis is especially minimal relative to other sectors such as the irrigated agricultural sector, which is responsible for 70% of the world’s total freshwater withdrawals, or over 2700 billion m3 annually.5 Of this, around 1100 billion m3 of water are consumed each year5 over 50 times as much water as a future hydrogen economy would require. Nonetheless, even with adjacent sectors using far more water than even the most ambitious prediction for hydrogen, concerns about freshwater scarcity6,7 call for reductions in water extractions at all available angles. Therefore, proposing a solution that allows for hydrogen to tap into Earth’s extensive saltwater resources can further decrease hydrogen’s water footprint.
2) Desalination of Saltwater. Accessible freshwater makes up just less than 1% of the planet’s water,8,9 and it is best to avoid creating any additional burden on freshwater usage, especially in areas where drinking water is difficult to attain. However, almost all the remaining 99%, or about 1.4 billion km3, is seawater, which can be purified through desalination processes before being used as an electrolysis feedstock. The leading desalination technology today is reverse osmosis (RO), which uses an applied pressure and a semipermeable membrane to reject ions present in the water, consuming less energy than other desalination methods such as distillation.10
However, some water that is fed to the RO process cannot be utilized, and the recovery defines the percentage of usable clean water that is produced by the process out of the total amount of feedwater. Current state-of-the-art RO plants, such as the Ashkelon plant in Israel, can achieve recoveries of up to 50%,8 meaning that twice the amount of water desired at the outlet must be fed into the process, for a total of 41 billion m3 of seawater withdrawn annually for hydrogen production. This is around 30 ppb of the world’s available supply of seawater each year, a negligible amount compared to the resources available, and the water that cannot be recovered is returned to the same body of water, so that it is not consumed.
It is important to note that adding a desalination process increases the energy requirement of the life cycle of electrolytic hydrogen production, but this too is negligible in comparison to powering the electrolyzer itself. Overall, RO requires 3.5−5 kWh of energy for each cubic meter of clean water produced.10 For a global hydrogen demand of 2.3 Gt, this yields an additional 0.26−0.37 EJ of annual energy required to perform RO for water electrolysis, i.e., 0.06−0.13% of the minimum energy required to produce the hydrogen by electrochemical water splitting. From an economic viewpoint, desalination by RO would add an energy cost of $0.53−1.50 per m3 of clean water produced,8 which would add no more than $0.01 to the cost of hydrogen production per kg. This is in agreement with an analysis by Khan et al. which found that desalination would comprise 0.1% of the energy requirement of electrolysis and add $0.02 to the cost of hydrogen per kg.11 Therefore, even if desalination processes were integrated into hydrogen production, DOE targets12 to produce hydrogen for less than $2.00 per kg would still be within reach.
Green hydrogen production will consume 1.5 ppm of Earth’s freshwater or 30 ppb of saltwater each year, an amount smaller than what is currently consumed by fossil fuel-based energy production and power generation. If desalination by RO is utilized, the additional energy requirement would be less than 0.2% of the minimum energy required to produce the hydrogen by electrolysis, and the energy cost would add approximately $0.01 to the price of hydrogen per kg. These numbers suggest that water supply will not be the limitation for electrolyzers, and we should instead continue to focus on technological improvements for the energy efficiency of electrolyzers, which is currently the limiting factor and has the potential for significant advancements. While the concern about the “water problem” is much more prevalent in the journalistic community than among scientists, journalism can have a significant influence on the acceptance of hydrogen as a growing market. It is therefore essential that we rigorously characterize the true water requirement of electrolysis technology, without the influence of water needs for the current non-renewable infrastructure, to get a clear idea of the impact hydrogen can have on a renewable energy future.
REFERENCES
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