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miércoles, 8 de abril de 2015

Micro-grid power promise of hybrid CSP-PV concept




One of Focused Sun's new FourFold modules. Image courtesy
 of Focused Sun.
US inventors hope a crowd-funded initiative will provide power
for micro-grids around the world by combining elements of CSP
 and PV solar technology in a single module.
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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