NOVEMBER 2019
C at c h i n g
Abreakthrough in concentrated
a t cshoimneg Temperatures over 1500°C will be required to
A smart approach to mirror
control has driven a fundamental
breakthrough in the hugely
promising area of concentrated
solar power.
solar power – and potentially
for industrial green energy –
was claimed last November
from an unexpected source. A
hitherto unknown company,
Heliogen, announced its
arrival with the revelation that it had
been able to concentrate solar energy to
achieve a temperature of over 1000°C, on
a commercial scale, for the fi rst time.
This, the company claimed, meant
it could replace fossil fuels in critical
industrial processes such as cement,
steel or aluminium manufacture, and
potentially for thermally spli ing water
to produce hydrogen.
Concentrated solar power uses
mirrors or lenses to focus sunlight from
a large area on to a receiver. Heliogen
founder and chief executive offi cer
Bill Gross said that by “commercial”
Heliogen meant that the temperature
was combined with suffi cient power –
between 250 and 400kW – for industrial
purposes. The company is aiming to
increase this fi gure to between 1MW and
10 MW.
Gross said such temperatures had
been achieved before, but only at a power
of around 5kW, using a parabolic dish
to focus the sun’s rays. However, using
April 2020 / www.theengineer.co.uk 36
each measuring 1.5m2. With around 33 per cent ground
coverage, they are spread over a total area of 1800m2 .
Gross explained: “In our system we have to align every
mirror slightly diff erently to bisect the angle between the sun
and the tower” – because the angle of incidence of light from the
sun equals the angle of the refl ected ray.
Not only do all the mirrors have to point in a slightly
diff erent direction, he adds, “every mirror is not pointing at
either the sun or the tower but at a point halfway between – in
other words at an invisible point in the sky.”
In previous arrays this has been solved by positioning every
mirror on the ground highly accurately using sophisticated
surveying techniques, and making each mirror very rigid.
They are then pointed in a direction to track the calculated
position of the sun at any time.
This can be done with only limited accuracy, says Gross.
“Most of the beams don’t point accurately at the target, and
this inaccuracy prevents extremely high temperatures being
reached.”
With the mirrors several hundred metres from the target,
being out by a fraction of a degree can result in an error
measured in metres at the target. The result is typically a
spot of around 10m diameter and a maximum temperature of
600oC.
This is hot enough to make steam and drive a turbine to
generate electricity, but this is not competitive with electricity
produced directly from photovoltaic panels.
Gross said: “What we did was fi nd a way to move the mirrors
more precisely” – as well as eliminating the need for them to
be accurately positioned initially. As a result the light can be
focused on an area the size of a basketball hoop – 18in (460mm)
diameter.
COVER FEATURE: ENergy
Dav i d Fowler reports
this method, the focus moves with the
sun. Heliogen’s system concentrates the
power on a stationery target mounted on
a tower. Its breakthrough is based on the
use of computer vision and sophisticated
software to control an array of mirrors.
“With a dish, when you want to
concentrate sunlight, all you have to do is
point it at the sun,” Gross said. Tracking
the sun can be accomplished easily with
a simple light sensor.
By contrast, Heliogen’s installation at
Lancaster, California, has 400 mirrors,
produce green hydrogen
/www.theengineer.co.uk