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One of
Focused Sun's new FourFold modules. Image courtesy
of Focused Sun.
for micro-grids around the world by combining elements of CSP
and PV solar technology in a
single module.
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By Jason
Deign
A
US-Chinese team is this month hoping to crowd-fund a novel solar technology
combining CSP with concentrated photovoltaics (CPV) to provide low-cost
micro-grid power.
Developers
from Focused Sun of New Mexico, USA, and the Xiang Yang Institute of Hubei,
China, are working on a system that will incorporate thermal energy storage and
should pay for itself within two years.
Focused
Sun, a former
CPV player, has created a hybrid CPV-CSP module called FourFold to power
the micro-grid. The modules are small enough to build micro-grids in a modular
way from 100 kW to up to 10 MW.
Each
module uses four linear Fresnel mirrors to focus sunlight on an absorber lined
with a two-inch strip of PV cells, which convert about 18 % of incoming energy
into electricity.
At
the same time, mineral oil coolant flows through the inside of the absorber,
keeping the PV cells cool and rising to a temperature of 300 ºC in the process. The
heat transfer fluid can be stored or used to drive a Chinese-built
turbo-generator.
The
combination of CPV and linear Fresnel-based solar thermal power allows the
system to harness a total of up to around 75 % of incoming energy, Focused Sun
claims. The company aims to keep system costs down by employing a range of
low-cost manufacturing techniques.
The
mirrors, for example, are each made of foam sandwiched between two sheets of
galvanised steel. The gear motors controlling the angle of the mirrors,
meanwhile, are attached to the module frame by just two screws and can be
replaced in a matter of minutes.
The
relatively low-tech manufacturing processes being used should allow FourFold
production plants to be set up with relative ease in regions where micro-grids
are needed, including emerging markets.
Local employment
In
fact, Focused Sun is keen to emphasise the local employment opportunities
offered by its technology, and has released guidelines on how to set up sales,
production and installation businesses with between five and 10 staff.
Each
micro-factory would need a USD$150,000 start-up investment and could break even
within about seven months, the company says. Nevertheless, some aspects of the
system design are still being finalised, including the storage technology.
While
batteries could be used to store part of the CPV output, a final decision on
the thermal storage mechanism is not expected until at least March, pending
consultations between Focused Sun and its Chinese partners.
Shawn
Buckley, Focused Sun’s founder, says: “Storage-wise, we're looking at either
sensible heat storage in cement or phase-change heat storage in molten salts.”
A
lithium-nitrate phase-change material (PCM)
storage system could help reduce the volume of storage. However, Buckley says:
“I like the ease of application of poured-in-place concrete that every
construction person knows how to do.
“We
don't need high-temperature concrete since our turbo-generator only needs 300 ºC . Ordinary concrete is
good to this temperature. Still, which way to go is mostly a cost issue.”
Professor
David Gordon Wilson, a Massachusetts Institute of Technology thermal storage
and turbo-machinery expert, is leading the research into PCM storage and
believes it could power generators until the next day, providing
round-the-clock energy.
Also
still under debate is the exact nature of the turbo-generators to be used. “We
think we can bring in a low-cost, mass-produced turbo-generator for under USD$
1,000/kWe, even for small 100kWe units,” says Buckley.
“Mitsubishi
makes them now at USD$ 2200/kWe for their Italian Turboden 100 kWe units, but
those are not made in high volume. We will be exploring various thermodynamic
cycles such as Kalina, Organic Rankine and others.”
Organic Rankine cycle
Buckley
says Chinese companies already make Organic Rankine cycle engines, although the
smaller sizes are only produced in small volumes.
“The
Xiang Yang Institute want to pick a size like 100 kWe or 300 kWe and
standardise on it to make a modular system,” he comments. “Of course, they like
the jobs aspect too since our concentrators can be made locally in small
factories.”
Initial response to the concept has
been mixed.
"It
appears to be a PV system that uses excess heat to produce hot water that could
be used to power a steam turbine,” says Jorge Ignacio Andreotti, an energy consultant and
CSP watcher based in Argentina. “This could increase the performance of the
system.
“At
first sight it looks interesting. These kinds of combinations often produce
good results."
However,
says Madhavan Nampoothiri, founder and director of RESolve Energy Consultants
in Chennai, India: “From a technology perspective, the hybrid system could
work, but in India, the challenges to adaption have always been commercial.
“Operating
and maintaining the systems in rural areas also pose challenges. Even
traditional PV systems have not been able to overcome these challenges, and I
don't see a new product making inroads into the off-grid segment anytime soon.”
Focused
Sun is aiming to fund commercialisation of the system through a USD$ 1 million fundraising
push on the crowd-funding site Indiegogo.
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