Unit 10: Energy Challenges // Section 12: Hydrogen Power
As economies develop and mature, they tend to follow an energy path that moves from high carbon/low hydrogen fuels at early stages to fuels with higher hydrogen and lower carbon contents. Typically, nations use wood as their main primary fuel at a pre-industrial stage, shift to coal during industrialization, and then transition to oil and natural gas as their economies mature. This progression takes place because each new fuel is cleaner-burning and easier to distribute and store (once an infrastructure has been built to handle it) than its predecessor. The United States and western Europe are beginning to plan for perhaps the next stage on the decarbonization path—hydrogen—but this transition will require several decades to design and deploy systems for producing, transporting, and using hydrogen fuel.
Although it is sometimes called a "fuel of the future," hydrogen is more accurately described an energy carrier. Like electricity, pure hydrogen does not occur naturally in quantities worth harnessing to meet human energy needs: the main naturally occurring stocks of hydrogen are tied up in chemical compounds, most importantly water molecules (H2O) and hydrocarbons such as coal (approximately CH), oil (approximately CH2), and methane (CH4). Stripping hydrogen from hydrocarbon fuels or obtaining it by splitting water using electricity or heat is not technically difficult, but in any of these approaches, more electricity or primary-fuel energy is used than the resulting hydrogen contains.
The benefit of paying this energy price to get hydrogen comes in the form of hydrogen's portability, storability, amenability to high-efficiency application not only in combustion engines but in fuel cells (discussed below), and low emissions. One application currently under research is the use of hydrogen fuel cells to power cars (Fig. 20).
Figure 20. View under the hood of a fuel cell car
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Source: © National Renewable Energy Lab.
Today, the oil and chemical industries worldwide use about 50 million tons of hydrogen each year, most of it extracted from natural gas and coal. Deriving hydrogen from fossil fuels emits CO2, so scaling the process up would increase greenhouse gas emissions unless the associated carbon were captured and stored (for more details on carbon capture and sequestration, see Unit 13, "Looking Forward: Our Global Experiment").
Hydrogen can be burned directly to generate energy or used in devices such as fuel cells that combine hydrogen and oxygen to produce electricity, with water as a byproduct (Fig. 21).
Figure 21. Hydrogen fuel cell
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Source: © 2001. Rocky Mountain Institute/www.rmi.org.
Here is how the basic process works:
- Hydrogen and oxygen flow into opposite sides of the cell, separated by a barrier that only allows positively charged ions to pass through.
- An anode (negatively charged pole) strips electrons from the hydrogen atoms, converting them to positively-charged ions that pass through the barrier. The negatively charged electrons flow around the outside of the cell toward the cathode (positively-charged pole), creating an electrical current. Catalysts speed the reactions at each electrode.
- Oxygen enters the cell near the cathode and combines with the hydrogen ions and electrons to form water, which is removed through an exhaust system.
Existing fuel cell technologies can convert as much as 70 percent of hydrogen's energy content to electricity. None of the basic designs in use today are cheap and technically simple enough yet for mass production, although they have been used for applications such as producing power on manned space missions.
Over the past several years, politicians and scientists have endorsed the idea of converting to a hydrogen economy. This transition poses many challenges. In addition to producing hydrogen economically and commercializing fuel cells, it takes seven times as much hydrogen on a volume basis to produce the same amount of energy in a gallon of gasoline. Therefore, adopting hydrogen as a fuel will mean building new energy storage and distribution systems nationwide. The devices that convert hydrogen to energy services—cars, heating systems, and consumer goods—will also have to be converted. Most expert assessments of the timing for a hydrogen economy project that such systems will not start to be deployed on a large scale until 2020 or later, and that making a full transition from fossil fuels to hydrogen in the United States would take until approximately 2050 or later.