Page 281 - Alternative Energy Systems in Building Design

FUEL CELL APPLICATION 255

2H O(l) → 2H (g) + O (g)
2
2
2
The simplest form of the electrolysis of water is achieved by passing dc current from
a battery or other dc power supply through a container of water and small amounts of
salt, which increase the reaction’s intensity. Using platinum electrodes, hydrogen gas
cumulates and bubbles up at the cathode, as does oxygen at the anode. In general, var-
ious metals are used as anodes and cathodes, such as iron and platinum. In the case of
iron, oxygen can react with the anode and prevent gas accumulation at the probe. For
instance, if iron electrodes are used in a sodium chloride solution, iron oxide will be
produced at the anode, which will react with iron to form hydroxide. It should be noted
that one of the least expensive methods of water electrolysis is the use of electricity pro-
duced by wind and solar power.
During the electrolysis process, a significant portion of electrical energy is converted
to heat, which is considered wasted energy and translates into the heating value of the
hydrogen gas. In general, the energy-conversion efficiency of water electrolysis is at
best 70 percent. The lower heating value of hydrogen is the thermal energy released
when hydrogen is combusted.
It should be noted that only 4 percent of hydrogen gas produced worldwide is the
result of water electrolysis. Aside from use in fuel cell applications, hydrogen is used
extensively for the commercial production of ammonia for fertilizer and for the conver-
sion of heavy-petroleum carbon chains into lighter products through a process called
hydrocracking.

HYDROGEN ENERGY RESEARCH
Thermolysis Water splitting refers to a chemical reaction in which water is split
into two separate molecules: hydrogen and oxygen. Unlike electrolysis, where the
bonded elements are chemically separated by passing dc current in an electrolytic
environment, this process deploys thermal decomposition, also called thermolysis,
whereby chemical substances are broken down into two or more components under
extreme temperatures that exceed 2000°C. The efficiency of water electrolysis is
measured in terms of the percentage of electrical energy used.
An example of this is a process referred to as the sulfur-iodine (S-I) cycle, in which
hydrogen is generated. The S-I cycle consists of three chemical reactions in which
water is used as the reactant. Owing to the excessive amount of heat required in this
process, it is not economical unless steam is harvested as a by-product from geothermal,
steam-driven electrical, or nuclear power plants. Figure 7.4 is a diagram of a Hoffman
voltmeter used in electrolyzing water.

COMMERCIAL USE OF HYDROGEN FOR ENERGY PRODUCTION

According to the U.S. Department of Energy (DOE), at present, electricity is produced
in the United States from the following sources:

■ 49.7 percent from coal
■ 19.3 percent from nuclear