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Hydrogen, though the most common element in the universe, is not found in its pure form on earth and must be either electrolyzed from water or stripped out from natural gas, both of which are energy-intensive processes that result in greenhouse gas emissions. Hydrogen is an energy carrier and not a fuel, as is usually wrongly asserted. Hydrogen produced electrolytically from wind or direct solar power sources and used in fuel cell vehicles can provide zero-emission transportation. As for any fuel, appropriate safety procedures must be followed. Although the hazards of hydrogen are different from those of the various hydrocarbon fuels now in use, they are no greater.

The basic question is how to produce hydrogen in a clean, efficient way. Using natural gas, coal, or even nuclear power to produce hydrogen in many ways defeats the purpose of moving toward a future powered by hydrogen. In the first two instances, greenhouse gases are emitted in the process of producing the hydrogen, whereas in the last case, nuclear waste is generated.

As a nearly ideal energy carrier, hydrogen will play a critical role in a new, decentralized energy infrastructure that can provide power to vehicles, homes, and industries. However, the process of making hydrogen with fossil-based power can involve the emission of significant levels of greenhouse gases.

Although the element of hydrogen is the most abundant one in the universe, it must be extracted from biomass, water, or fossil fuels before it can take the form of an energy carrier. A key issue in the future is to promote the generation of electricity from wind, then use that electricity to produce hydrogen.

Extracting hydrogen from water involves a process called electrolysis, defined as splitting elements apart using an electric current. Energy supplied from an external source, such as wind or the burning of a fossil fuel, is needed to drive the electrochemical reaction. An electrolyzer uses direct current to separate water into its component parts, hydrogen and oxygen. Supplementary components in the electrolyzer, such as pumps, valves, and controls, are generally supplied with alternating current from a utility connection. Water is "disassociated" at the anode, and ions are transported through the electrolyte. Hydrogen is collected at the cathode and oxygen at the anode. The process requires pure water.

Despite considerable interest in hydrogen, however, there is a significant downside to producing it by means of fossil fuel-generated electricity due to the emissions related to the electrolysis process. Hydrogen fuel promises little greenhouse gas mitigation if a developing hydrogen economy increases demand for fossil fuel electricity. On the other hand, using cleanly produced hydrogen can fundamentally change our relationship with the natural environment.

Electrolytic hydrogen may be attractive in regions such as Europe, South and East Asia, North Africa, and the southwestern United States, where prospects for biomass-derived fuels are limited because of either high population density or lack of water. Land requirements are small for both wind and direct solar sources, compared to those for biomass fuels. Moreover, as with wind electricity, producing hydrogen from wind would be compatible with the simultaneous use of the land for other purposes such as ranching or farming. Siting in desert regions, where land is cheap and insolation is good, may be favored for photovoltaic-hydrogen systems because little water is needed for electrolysis. The equivalent of 2-3 cm of rain per year on the collectors—representing a small fraction of total precipitation, even for arid regions—would be enough.

Electrolytically produced hydrogen will probably not be cheap. If hydrogen is produced from wind and photovoltaic electricity, the corresponding cost of pressurized electrolytic hydrogen to the consumer would be about twice that for methanol derived from biomass; moreover, a hydrogen fuel cell car would cost more than a methanol fuel cell car because of the added cost for the hydrogen storage system. Despite these extra expenses, the life-cycle cost for a hydrogen fuel cell car would be marginally higher than for a gasoline internal combustion engine car, which is about the same as for a battery-powered electric vehicle.

The transition to an energy economy in which hydrogen plays a major role could be launched with hydrogen derived from biomass. Hydrogen can be produced thermochemically from biomass using the same gasifier technology that would be used for methanol production. Although the downstream gas processing technologies would differ from those used for methanol production, in each case the process technologies are well established. Therefore, from a technological perspective, making hydrogen from biomass is no more difficult than making methanol. Biomass-derived hydrogen delivered to users in the transport sector would typically cost only half as much as hydrogen produced elec-trolytically from wind or photovoltaic sources.

Probably the best way to utilize hydrogen is with a fuel cell. A fuel cell is an electrochemical energy conversion device in which hydrogen is converted into electricity. Generally, fuel cells produce electricity from external supplies of fuel (on the anode side) and oxidant (on the cathode side). These react in the presence of an electrolyte. Generally, the reactants flow in and reaction products flow out while the electrolyte remains in the cell. Fuel cells can operate continuously as long as the necessary flows are maintained. A hydrogen cell uses hydrogen as fuel and oxygen as an oxidant. Fuel cells differ from batteries in that they consume reactants, which must be replenished, whereas batteries store electrical energy chemically in a closed system. Additionally, while the electrodes within a battery react and change as a battery is charged or discharged, a fuel cell's electrodes are catalytic and relatively stable. More details on fuel cells are given in Chapter 7.

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