Page 284 - Alternative Energy Systems in Building Design

258 FUEL CELL TECHNOLOGY

HTE processes are ideally suited for deployment in nuclear power plants because a
nuclear heat source is a nonchemical form of high-temperature heat and is much more
consistent than heat produced by solar thermal concentrators. An added advantage of
HTE and a nuclear reactor combination is that the process could produce both elec-
tricity and hydrogen simultaneously.

THERMOCHEMICAL PRODUCTION
Thermochemical hydrogen production is a process that makes use of the S-I cycle,
which produces hydrogen, oxygen, and significant amounts of heat from water without
the use of electrical energy. The thermochemical process is somewhat more efficient
than HTE (discussed previously). Like HTE, the thermochemical hydrogen-production
process has been demonstrated only in a laboratory environment.

REACTIVE PRODUCTION
Other chemical reactive processes that produce hydrogen involve a number of chemical
reactions between metals, such as sodium, and water that results in sodium hydroxide
and hydrogen. Similarly, aluminum-gallium alloy, when reacted with water, produces
aluminum oxide and hydrogen.

HYDROGEN STORAGE CHALLENGES

Even though molecular hydrogen has a relatively high energy density on a mass basis,
owing to its low molecular weight, as a gas at ambient temperatures it has a very low
energy density by volume. In order for hydrogen to be used as a viable fuel for vehicles,
the gas must be pressurized or liquefied so that it can provide sufficient driving range. To
increase the energy density by volume, hydrogen must be liquefied under extremely
low temperatures and pressure. Subjecting hydrogen gas to high pressures requires
significant external energy. Liquid hydrogen is cryogenic, which means that it lique-
fies at extremely low temperatures and boils at 20.268 K (–252.882°C or –423.188°F).
Low-temperature storage reduces hydrogen’s mass, but the process of liquefaction
requires substantial amounts of energy. The liquefaction process involves pressurizing
and cooling steps, both of which are energy-intensive. Owing to the low density of lique-
fied hydrogen, it has a lower energy density by volume than gasoline by a factor of
4. Gasoline, which is a composition of hydrogen and carbon, effectively has 116 g of
hydrogen in a liter, whereas a liter of pure liquid hydrogen has a mass of only 71 g.

METALLIC HYDROGEN COMPARTMENT

Specific chemical characteristics of hydrogen are such that, in its liquid form, it must
have a storage compartment that is well insulated to prevent boiling. If poorly insulated,
in freezing temperatures, ice accumulation around the tank can corrode the metal com-
partment. In order to withstand the intense pressure required to maintain liquefaction,
the storage tank must have a significant mass, yet it must be light enough to reduce the
vehicle’s weight.