Technology World | September 2007
Can Lasers Help Decrease Our Dependence on Fossil Fuels?
Proposal envisions solar-powered laser as a key element.
It has been estimated that the solar power
impinging on Earth in a single hour could satisfy mankind’s energy
needs for more than a year. Nonetheless, solar energy today provides
less than 0.1 percent of the world’s electric power and, according to
the US Energy Information Administration, that percentage is not
expected to increase significantly in the next quarter century. Figure 1.
A solar-pumped laser drives the magnesium-based cycle, which produces
hydrogen gas as its product. The hydrogen can be used in fuel cells or,
alternatively, can be burned directly to drive a turbine.
reasons are both economic and practical. On the practical side, one
drawback of solar power is that it is available only in daytime. Over
the years, researchers have experimented with many techniques for
storing solar energy, from batteries to molten salts, but none has
proved efficient or economical enough to warrant widespread
Recently, scientists at Tokyo Institute of
Technology and at Osaka University, both in Japan, proposed an approach
using a solar-powered laser to drive a magnesium-based cycle that
generates hydrogen gas as its product, as described in the Dec. 25
issue of Applied Physics Letters
. Either the elemental magnesium that is generated by the cycle or the hydrogen gas itself can be stored until needed.
A laser heated the MgO to 4000 K, sufficient to separate it into its
constituent elements. A steady flow of argon gas directed the
vapor-phase magnesium to a copper plate, where it condensed. Reprinted
with permission of Applied Physics Letters.
acts to concentrate the intensity of sunlight so that it is capable of
temperatures high enough to reduce MgO to its constituent elements
(Figure 1). The magnesium subsequently reacts with water to produce
hydrogen gas, which can be used in fuel cells or burned to produce
The scientists have demonstrated several aspects of
the cycle, including the laser-induced reduction of MgO. They heated
the MgO to temperatures above 4000 K with a laser and allowed the
gaseous Mg to condense on a copper plate (Figure 2). To suppress
reoxidation in atmospheric air, they injected an argon-gas flow to
guide the Mg to the condensation plate.
The solar-powered laser
is a key component of the whole system. One possibility is a
diode-pumped laser whose diodes are powered by solar cells; however,
the scientists chose to demonstrate a directly solar-pumped Nd:YAG
laser. Theirs is not the first to utilize sunlight to optically pump an
Nd:YAG laser, but their laser does manifest several significant
advances over previous ones.
They co-doped their laser with
chromium to increase its absorption of sunlight and used ceramic YAG to
obtain higher doping levels than would be possible with crystalline
YAG. They arranged two YAG rods in an oscillator-amplifier
configuration, each rod pumped by a separate Fresnel lens solar
collector (Figure 3).
Figure 3. A pair of Fresnel lenses (total area ~1.3 m2)
collected sunlight to pump the ceramic Cr:Nd:YAG oscillator amplifier.
The entire assembly rotated to track the sun across the sky. Reprinted
with permission of Applied Physics Letters.
believe that all previous solar-pumped lasers depended on parabolic
mirrors or heliostats to collect sunlight. With the Fresnel lenses,
they obtained 24.4 W of output from the laser, from a solar-collecting
area of 1.3 m2
. This is nearly three times the best collector-to-laser-power ratio achieved with previous lasers.
the investigators believe that much greater efficiencies are possible.
Their Fresnel lenses were designed for image projection, not solar
collection, and focused less than 70 percent of the incident sunlight
onto the focal point. They expect much better efficiency from Fresnel
lenses designed for solar collection, which recently have been
constructed and currently are being integrated into the system.
slope efficiency of the solar-pumped laser was 14 percent, although the
absolute efficiency was only 2.9 percent because the laser was
operating only slightly above its threshold. However, when they
implement an improved Fresnel lens to boost the pump power from ∼900 W
to ∼4 kW, the researchers expect that the laser will be far above
threshold and that its absolute efficiency will approach the 14 percent
For a solar power station, the scientists
envision tens of thousands of kilowatt-class solar-pumped lasers
reducing MgO and generating hydrogen, as illustrated in Figure 1. Of
course, the ultimate question for any solar energy scheme is the
economic one, but the researchers believe that it is far too early to
make a meaningful estimation of the economies of scale that may be
possible with their magnesium-cycling conceptApplied Physics Letters, June 25, 2007, 261120.
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