Oct/Nov 2005
Sustainable solar success
Fuel Cell Review
Jonathan Wills
Sunlight plus methane plus water is the key to sustainable
hydrogen generation, says a Canadian R&D company. Time will
tell.
A new take on solar-hydrogen generation in which sunlight is used
to convert natural gas and water into hydrogen is being pioneered by
Solar Hydrogen Energy corporation (SHEC), an R&D company based in
Saskatoon, Canada. SHEC's hybrid technique, catchily dubbed
"dry-fuel-reformation solar-hydrogen technology", results in a net
stored-energy gain of more than 14%, since the energy consumed in the
process is harvested directly from sunlight and is therefore
effectively free from any energy penalty.
The problem today is that almost all industrial hydrogen (around
95% of the 42 million tonnes produced annually) is generated from
reforming fossil fuels, a process that results in a 30-35% loss in
stored energy. The remainder is produced by the electrolysis of water,
though even this approach usually employs electricity generated by
fossil fuels. Tomorrow, if the transition from hydrocarbon economy to
a hydrogen economy is to move beyond the demonstration phase, things
are going to have to be different.
In theory, the Sun holds the key to sustainable hydrogen production.
Yet while the total amount of sunlight incident on the outer reaches of
the Earth's atmosphere is equivalent to about 1.4 kW/m²,
harnessing this renewable source of power for large-scale conversion
to hydrogen is far from straightforward. "We've been working on
hydrogen production from water and hydrocarbon sources since 1996,"
Thomas Beck, president and chief executive officer of SHEC, told
The Fuel Cell Review.
In contrast to traditional approaches to solar-hydrogen generation
— in which sunlight is first converted into electricity (by
photovoltaic cells) and then into hydrogen via electrolysis of water
— SHEC produces hydrogen from water using a thermochemical
process. The technique uses sunlight to drive the catalytic reformation
of methane into hydrogen. More precisely, heat from the Sun drives the
endothermic reaction (equation 1) between methane and carbon dioxide
to produce hydrogen and carbon monoxide.
Equation 1
CH4 + CO2 → 2H2 + 2CO
ΔH = 917 kJ/mole
Equation 2
CO + H2O → H2 + CO2
ΔH = 40.6 kJ/mole
The second stage of the process is the exothermic regeneration of
CO2 and splitting of water through the well known
water-gas-shift reaction (equation 2).
Beck explained SHEC's thinking: "Our short- and long-term plan is
to use methane from biomass sources, from coal fields, landfills and
gas wells, from sources that would normally leach methane into the
atmosphere. Every major city has a refuse site." That's good news for
another reason: methane in the atmosphere is a powerful greenhouse gas,
21 times more powerful than carbon dioxide, so taking it out of
circulation is a smart move. Working with its partner, Clean 16
Environmental Technologies, Toronto, SHEC claims to have the
collection and purification technology to make this work.
"We have a pilot plant that we're currently designing," added Beck.
"The scale of it is a production capacity of 1.2 million kg of hydrogen
a year. With that we will have 30 concentrating arrays, each measuring
13 x 13 m and each with its own reactor." The pilot plant is scheduled
to go live in 2008 and, if successful, SHEC plans to roll out even
larger plants. "Our first plant is going to cost us C$16 m [US$13.5 m]
to build," said Beck, adding that SHEC is "looking at less than 20% of
the revenue to run and service the plant".
Elsewhere, other researchers have been registering notable advances
on the solar generation of hydrogen (The Fuel Cell Review
Oct/Nov 2004 p19). Two developmental processes, in particular, have
been the focus of attention. First up, there's photoelectrochemical
hydrogen production, in which sunlight incident on a semiconductor
electrode generates electron-hole pairs. The holes and electrons react
with oxygen and hydrogen ions respectively to generate gaseous
hydrogen and oxygen. Second, there's photochemical hydrogen production,
in which a system of chemical reactants and sunlight come together to
split water (a process sometimes referred to as artificial
photosynthesis).
One of the companies first out of the blocks is Hydrogen Solar of
Guildford, UK. That startup specializes in hydrogen production and is
exploiting nanotechnology to enhance the efficiency of its
photoelectrochemical cell. Called the Tandem Cell, the system can now
convert more than 8% of the energy from sunlight directly into pure
hydrogen — closing in on the 10% solar-to-hydrogen conversion
level quoted as the benchmark for commercially viable hydrogen
production.
Meanwhile, a new approach to photochemical hydrogen generation is
being pursued by researchers in the chemistry department at Virginia
Tech in Blacksburg, Virginia, US. Here, a team is employing
supramolecular complexes to catalyse the release of hydrogen from
water using just the energy from solar radiation. Supramolecular
complexes comprise a number of discrete molecular components, each of
which has its own discrete chemical properties.
