Introduction to the Hydrogen Economy

Overview

The hydrogen economy is based on three elements: production, storage, and utilization of hydrogen as an energy carrier. Currently most hydrogen is produced by steam reforming of petroleum. There are still fundamental problems that need to be solved before any sort of hydrogen economy becomes a reality. The U.S. government has pledged $1.7 Billion towards producing commercially available hydrogen powered cars by 2020, and the European commission has similarly created an €2.8 Billion public-private partnership to develop fuel cells [2].

Sources and Production

Though hydrogen is the most abundant element, it is never found as a diatomic gas molecule in nature. Historically, hydrogen was generated from adding a metal such as magnesium or zinc to an acid such as hydrochloric acid to produce a metal oxide and hydrogen (H2). This is neither practical nor economical for any large scale process, and currently any large scale hydrogen production is done via steam reformation of natural gas (primarily methane, CH4). The problem here is that natural gas is a non-renewable, carbon-based resource, so this method does not really help atmospheric conditions. Coal gasification, another potential method, faces the same pitfall despite large domestic reserves [3].  Electrolysis

There currently is no sustainable method for making hydrogen in any large capacity [4]. If excess electricity can be generated, electrolysis of water can be performed, however this is rarely efficient (20-25% overall efficiency) and is hard to do on any kind of commercial scale economically [1]. Thermochemical cycles have been theoretically shown to be up to 52% efficient in producing hydrogen, and electrolysis would require an electricity generation efficiency of 74% efficient to overcome this figure [4]. Furthermore, even if the electricity used for electrolysis is from renewable sources such as wind or solar panels, this is little more than an energy storage strategy; it would be more efficient to just use this electrical energy that has been generated rather than sacrifice some of the energy in a hydrogen conversion process. Due to the need for load-leveling with intermittent and inconsistent power sources (such as wind and photovoltaic cells), intermediate hydrogen production as energy storage mechanism may in fact be a worthwhile strategy in the future [5]. New innovative methods such as photobiological hydrogen production are being developed. Such technology uses natural processes of living organisms to produce hydrogen. These processes may be able to use saltwater feedstocks, as opposed to pure water used in thermochemical or electrochemical processes, which may be vital in areas where clean water is scarce [3]. 

Utilization

Hydrogen is ideally used in a fuel cell due to their high efficiency; hydrogen fuel cell vehicles have efficiencies that are roughly 2.5 times that of gasoline-powered vehicles with internal combustion engines [6]. Fuel cells are not heat engines, and therefore theoretically are not subject to the Second Law of Thermodynamics, which states that heat cannot be completely converted into work (since universal entropy tends to increase for any process) [7]. Fuel cells work by reducing oxygen to O2- at the cathode and oxidizing hydrogen to H+ at the anode. The electrons flow from the anode to the cathode, and the H+ and O2- species combine to form water, the only product of hydrogen fuel cells [8]. However while fuel cells are being developed so that they be sold at a lower cost, hydrogen may actually see significant use as a natural gas additive or substitute. Hydrogen will see use in all facets of energy usage, be it transportation, power generation, heating/cooling, and small applications such as cell phones and laptops. In addition to fuel cells, hydrogen can be used in internal combustion engines (ICEs). Though not as efficient, a hybrid-ICE vehicle powered by hydrogen may be 80% as a fuel cell vehicle, which is a considerable improvement over conventional gasoline ICEs [9]. Another potential use is in aviation, where jets would use liquid hydrogen. Hydrogen is a more efficient fuel per unit mass, yet is 4 times less energy dense per unit volume when compared to conventional jet fuel. Due to the inherent safety concerns of air transportation, hydrogen fuel for commercial airlines may be decades away from a reality. Jets will need to be re-engineered to account for larger fuel tanks that can accommodate cryogenic temperatures [10].

 

H2 uses

References

[1] Abanades, S., Charvin, P., Flamant, G., and Neveu, P. Screening of water-splitting thermochemical cycles potentially attractive for hydrogen production by concentrated solar energy. Energy, 2006, 31 2805-2822.
[2] Service, R.F. “The Hydrogen Backlash”, Science, 2004, 305, 958-961.
[3] Turner, A. “Sustainable Hydrogen Production”, Science. 2004, 305, no. 5836, 972-974.
[4] Brown, L. C., J. F. Funk, and Showalter, S.K. “INITIAL SCREENING OF THERMOCHEMICAL WATER-SPLITTING CYCLES FOR HIGH EFFICIENCY GENERATION OF HYDROGEN FUELS USING NUCLEAR POWER”, 2000.
[5] Bernal-Agustın, J. and Dufo-Lopez, R. “Hourly energy management for grid-connectedwind–hydrogen systems”, Intl. Jour. Hydrogen Energy, 2008, 33, 6401 – 6413.
[6] Dresselhaus, M. et al., “Basic Research Needs for the Hydrogen Economy”, 2003, http://www.sc.doe.gov/bes/reports/files/NHE_rpt.pdf
[7] Grant, P. M., “Hydrogen lifts off — with a heavy load”, Nature, 2000, 424, 129-130.
[8] Kendall, K. “Hopes for a flame-free future”, Nature, 2000, 404, 233-235.
[9] Scrope. M. “Which Way to Energy Utopia”, Nature, 2001, 14, 682-684.
[10] Contreras, A., et al., “Hydrogen as aviation fuel: A comparison with hydrocarbon fuels.” Intl. Jour. Hydrogen Energy, 1997, 22, no. 10/11, 1053-1060.


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