Hydrogen
Hydrogen is a colorless, odorless, light
gas, and it has a very high specific energy (about 3 times higher
than methane). Hydrogen does not exist in nature in a free state,
but it must be extracted from other compounds (typically water and
fossil sources such as methane), even if many biochemical production
processes are being investigated. Hydrogen can be used (in a
combustion engine, in a boiler, in a welding torch) as any other gas
with a good calorific value, but its more useful application – from
the energetic point of view – is the use in fuel cells for DC
electricity generation, with water and heat as its only waste
products. The storage of hydrogen (which is the lightest element in
the periodic table of elements) is a partially overcome challenge,
but at the price of high investment costs resulting from the use of
advanced material or of very heavy tanks. Hydrogen can also be
liquefied and transported in this form, in order to maximize the
amount of storable energy per unit volume; however, the liquefaction
process is expensive because hydrogen liquefaction temperature is
equal to 20 K only.
The multiplicity of technological solutions that can lead to
hydrogen production and to the transformation of hydrogen itself
into other forms of energy (in fact, hydrogen is an energy vector,
not an energy source) makes it extremely interesting to analyze and
develop integrated energy systems based on the use of hydrogen as a
temporary energy storage medium.