Can Water From Burned Hydrogen Be Turned Back Into Hydrogen Again
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 past the reverse reaction. Today, hydrogen is used as a feedstock for chemical synthesis, only other applications have become realities including energy storage and transportation fuels. If the hydrogen is generated from renewable electricity, information technology releases no greenhouse gas emissions, significant that information technology can be a key histrion in the battle confronting climate modify.
To develop a "green hydrogen economic system" where emissions-free hydrogen is widely used in daily life, innovators are using electrochemical water electrolysis to generate hydrogen from ii simple ingredients: electricity and water. Every bit renewable electricity prices drop and improvements in electrolyzer efficiency are achieved, some critics have begun to ask a question about the 2d ingredient: is at that place plenty h2o to support a hydrogen economy? Some fence that the respond is no, due to perceived meaning h2o need throughout the entire production process, including the employ of h2o as a feedstock and a cooling agent for thermoelectric methods of producing hydrogen such every bit steam methane reforming (SMR). (2) Yet the objective of the dark-green hydrogen economy is to derive energy completely from renewable sources which practice non employ h2o for cooling or fossil fuel processes. Therefore, it is critical to include merely 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 notice that past isolating the h2o 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 futurity need for hydrogen in all applicative sectors including chemical synthesis, transportation, buildings and heating, and free energy storage. The calculated hydrogen demand in the distant renewable time to come is two.3 Gt per year. (3) In our vision, hydrogen will be produced past water electrolysis powered past renewable energy. Such a vision can reduce the carbon emissions from the energy sector by upwardly to 10.2 Gt annually from hereafter emissions projections compared to the International Panel on Climate change's worst-case RCP8.5 scenario. (iii)
Before moving to further develop the hydrogen economy, it is important to determine the feasibility of the amount of water ii.3 Gt of hydrogen will require each year. Several authors (2,4) accept expressed their concerns well-nigh water for hydrogen, stating that obtaining water for the economic system will be besides expensive or demanding on the water and energy requirements. Here, we summate the corporeality 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 bounding main from which information technology was extracted. Any h2o that is converted into another unusable grade or is not returned to the original sea will be considered to exist consumed.
Based on the reaction stoichiometry, for every kg of hydrogen produced, 9 kg of water must exist consumed. Therefore, 2.3 Gt of hydrogen requires twenty.five Gt, or twenty.5 billion m3, per yr of freshwater, which accounts for only one.v 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 nearly h2o can exist recovered, it is not generally returned to the original trunk of water and will exist treated as consumed. The merely sector in which the use of hydrogen does not regenerate the entirety of the water feedstock by fuel cell or combustion is chemic synthesis, which will account for 540 Mt of hydrogen, using at nigh 4.viii billion m3 or 0.3 ppm of global freshwater annually. (three)
When compared to other projections for the water need of hydrogen production, (2) the freshwater requirements above are quite low. This is due to our supposition that all hydrogen in the future volition exist produced using renewable energy sources such as current of air and solar, which have little to no water consumption. When fossil fuels are used for master energy production and ability generation, the h2o requirement is quite significant. In 2014, 251 billion g3 of freshwater were withdrawn for power generation and energy product from fossil fuels such as coal, oil, and natural gas, and 31 billion thou3 were consumed as the water was used for cooling, mining, hydraulic fracturing, and refining. (v) In comparing, even though the 20.v billion grand3 of h2o withdrawn for hydrogen production by electrolysis must be consumed, this is withal 33% less than the current fossil fuel free energy-related uses (Figure 1). Furthermore, electrolysis will render fossil fuel energy sources obsolete as the energy sector is able to motility more toward renewable technologies, saving 10 billion mthree of freshwater that would have been consumed by free energy-related uses of fossil fuels. Thus, it is apparent that using hydrogen as a method to reach a renewable energy social club will lead to drastic water savings, non expenditures.
Effigy 1
Effigy one. Comparison of the global freshwater withdrawal and consumption of three different sectors: fossil fuel energy product and ability generation, agriculture, and the implementation of a global hydrogen economy. Note that the bar chart is on a log calibration. (three,5)
The water consumption of electrolysis is peculiarly minimal relative to other sectors such every bit the irrigated agricultural sector, which is responsible for 70% of the globe'southward full freshwater withdrawals, or over 2700 billion kiii annually. (5) Of this, effectually 1100 billion m3 of water are consumed each year (5)—over 50 times as much water every bit a futurity hydrogen economy would crave (Figure 1). Nonetheless, fifty-fifty with next sectors using far more than water than even the nigh aggressive prediction for hydrogen, concerns most freshwater scarcity (6,7) telephone call for reductions in water extractions at all bachelor angles. Therefore, proposing a solution which allows for hydrogen to tap into World'due south extensive saltwater resources can farther decrease hydrogen's water footprint.
2. Desalination of Saltwater
Accessible freshwater makes upwardly just less than 1% of the planet'due south water, (8,9) and it is best to avoid creating any boosted burden on freshwater usage, especially in areas where drinking h2o is hard to attain. However, almost all the remaining 99%, or almost 1.four billion km3, is seawater, which can be purified through desalination processes before beingness used equally an electrolysis feedstock. The leading desalination technology today is reverse osmosis (RO), which uses an applied force per unit area 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 procedure cannot be utilized, and the recovery defines the percentage of usable make clean h2o that is produced past the procedure out of the total amount of feedwater. Current state-of-the-art RO plants, such as the Ashkelon plant in Israel, tin can achieve recoveries of up to fifty%, (8) meaning that twice the corporeality of water desired at the outlet must be fed into the procedure, for a total of 41 billion 10003 of seawater withdrawn annually for hydrogen production. This is around 30 ppb of the world'due south available supply of seawater each year, a negligible corporeality compared to the resources available, and the water that cannot be recovered is returned to the same body of h2o, so that it is non consumed.
It is important to annotation that calculation a desalination process increases the energy requirement of the life bike of electrolytic hydrogen production, but this likewise is negligible in comparing to powering the electrolyzer itself. Overall, RO requires 3.5–five kWh of energy for each cubic meter of clean h2o produced. (10) For a global hydrogen demand of ii.three 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 past electrochemical water splitting. From an economic viewpoint, desalination by RO would add together an energy cost of $0.53–1.50 per m3 of clean water produced, (viii) which would add no more $0.01 to the cost of hydrogen product per kg. This is in agreement with an analysis by Khan et al. which constitute that desalination would incorporate 0.ane% of the free energy requirement of electrolysis and add $0.02 to the toll of hydrogen per kg. (xi) Therefore, even if desalination processes were integrated into hydrogen production, DOE targets (12) to produce hydrogen for less than $2.00 per kg would even so be within reach.
Green hydrogen product will consume 1.five ppm of Earth's freshwater or 30 ppb of saltwater each year, an amount smaller than what is currently consumed by fossil fuel-based free energy production and power generation. If desalination past 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 price would add together approximately $0.01 to the price of hydrogen per kg. These numbers suggest that water supply volition not exist the limitation for electrolyzers, and nosotros should instead go on to focus on technological improvements for the free energy efficiency of electrolyzers, which is currently the limiting gene and has the potential for significant advancements. While the concern about the "h2o trouble" is much more prevalent in the journalistic customs than among scientists, journalism can have a meaning influence on the credence of hydrogen as a growing market. It is therefore essential that we rigorously characterize the true water requirement of electrolysis applied science, without the influence of water needs for the current non-renewable infrastructure, to get a clear thought of the impact hydrogen can have on a renewable energy future.
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Source: https://pubs.acs.org/doi/10.1021/acsenergylett.1c01375
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