Electrolysis, the process of cleaving water into hydrogen and oxygen using electricity, is an energy-intensive but well-proven method of producing hydrogen. It is presently about three to four times more expensive than the production of hydrogen from natural gas reforming, which explains its present small share in global hydrogen production. However, it is potentially the cleanest method of producing hydrogen with respect to greenhouse gas emissions, as long as the electricity needed comes from renewable or nuclear energy sources and not from fossil fuels. One should always bear in mind that hydrogen energy is only as clean and environmentally friendly as the process used to produce it. Commercial electrolysis is a mature technology that has been around for over a century to produce high-purity hydrogen. It has been used, however, to a significant extent only in locations where cheap electricity sources exist, such as hydropower in Canada and Norway. A typical commercial electrolyzer has an efficiency of about 7080%, but a higher efficiency can be obtained with more elevated temperature water or steam electrolysis. Because the efficiency of the electrolysis reaction is independent of the size of the cell or cell-stack, electrolyzers allow both centralized and also decentralized hydrogen production, such as in local service stations. The absence of moving parts requires low maintenance, and electrolyzers are well-suited for use with intermittent and variable power sources, such as wind or solar. Furthermore, any excess electricity generated during off-peak periods could be stored in the form of hydrogen, which then could be used to produce additional power during peak demand.
Basically, any energy source that produces electricity can be used to produce hydrogen by electrolysis. Today, more than 60% of the electricity in the world is still produced by fossil fuel-burning power plants. It would however, be unreasonable to use fossil fuels to generate electricity and then use the electricity to generate hydrogen. As each transformation involves energy loss, the overall efficiency would be lowered and much more CO2 would be emitted than had fossil fuels been used directly or transformed to hydrogen by reforming. In order to be sustainable and environmentally friendly in the long term, electricity for water electrolysis must be derived from renewable or nuclear energy sources which do not emit CO2 and air pollutants, such as SO2 and NOx.
Hydropower, which is by far the largest renewable electricity source today, is clearly well-suited to produce hydrogen, although its availability, as discussed in Chapter 6, is limited.
Considering its enormous potential, wind power, compared to all the other renewable energy sources, has probably the greatest impact for the production of pollution-free hydrogen at a reasonable cost in the foreseeable future. Electricity from wind is already competitive with power from fossil fuels in some areas, and constant developments and improvements in turbine technologies are expected to further reduce significantly its costs. On the other hand, the intermittent nature of wind energy, with capacity factors of only about 30%, is a serious drawback, resulting in coupled electrolyzers for hydrogen production operating at full capacity only for limited periods of time, and requiring considerable hydrogen storage capacity to offset lack of production when the wind is not blowing. With significant optimization of wind-coupled electrolysis and hydrogen storage systems, however, costs for hydrogen generation could fall from current estimations of $6-7 kg-1 to less than $3 kg-1 .
Another source for hydrogen, which could potentially meet all our energy needs into the future, is that of solar energy. Like the wind, solar energy is a non-polluting and plentiful source of energy but, being also an intermittent source of energy it suffers from the same drawbacks as wind energy. Unlike wind energy, however, it is still a very expensive way to generate electricity. With current technology, the production cost of hydrogen generated by photovoltaic systems is estimated at $28 kg-1  - an order of magnitude higher than that based on fossil fuels, and also more expensive than that based on other renewable energy sources. Even with further development, including improved efficiency and the use of thin-film technology instead of crystalline silicon solar cells, the cost is estimated to remain above $5-6 kg-1 hydrogen . Important technological breakthroughs would be needed to make any significant reductions in the costs of solar electricity and thus hydrogen produced by photovoltaic cells. Currently, there are new concepts at the research stage based on conductive organic polymers or nanostruc-tured films, which could possibly be mass produced at lower costs than silicon-based photovoltaic cells. By avoiding the need to couple a photovoltaic device with an electrolyzer, the possibility of producing hydrogen directly from sunlight and water in a so-called photoelectrolysis (PE) device is also under development. Besides photovoltaics, electricity from thermal solar power plants could be used to produce hydrogen, but for the foreseeable future this also is too expensive. Experiments have also been conducted in France, Canada, Israel and other countries, to thermally split water into oxygen and hydrogen at high temperature (20002500 °C) using solar furnaces, although there has been limited success and no practical applications are in sight. Thermochemical water splitting using solar energy is another possibility that must be further explored. If solar energy is to become a low-cost option for the sustainable production of hydrogen in the future, then clearly further research is needed.
Electricity from geothermal energy, could possibly also be used in some geother-mally active areas such as Iceland, The Philippines and Italy, to produce hydrogen. However, even if found economically viable, this process would only be of minor importance on the global scale, as the number of geothermal sources of the quality required for electricity generation is relatively limited.
Resource limitation, a lack of mature technologies, and difficulties of exploitation or environmental concerns also explain the limited development of energy extracted from oceans under various forms: tidal power, wave power or thermal energy, which are thus not expected to play any significant role in hydrogen production.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.