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A LT E R N AT I V E ENERGY

The US is poised to buildthe world’s largest solarenergy project. Want tocatch some rays?

Some like it hot

Solar sunflowers: each 25kW system is built on a steel frame with themirrors laminated to a honeycomb aluminium structure

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A500MW, 4500-acre solar project in

the Mojave Desert, California, isset to become the world’s largest solarfacility, promising to power morethan a quarter of a million homes ayear at a price to rival conventionalgeneration.

The plant will comprise 20,000solar generators sited in the desert,northeast of Los Angeles, andaccording to developers StirlingEnergy Systems (SES), will be able toproduce more electricity than allother US solar projects combined.

SES beat bids from 45 rivalrenewable energy businesses and hassigned a 20-year power purchaseagreement with US utility SouthernCalifornia Edison (SCE). Awaitingthe go-ahead from the CaliforniaPublic Utilities Commission, theproject also includes an option toexpand capacity to 850MW.

MAKING LIGHT WORKWhile most solar projects rely onphotovoltaics (PVs), this installationwill use SES’s so-called solargenerator to convert thermal energyto electricity. Each unit consists ofan 11m diameter solar dish –comprising 82 mirrors – that focusessunlight onto a Stirling enginegenerator to produce electricity (see‘Hot stuff ’, below).

SES chief executive BruceOsborne claims the solar dish

systems hold the world record forefficiently converting sunlight to gridquality electricity. The peak efficiencyof their solar generator is 29.4% –delivering grid-ready AC electricity –compared to around 15% for PVs.

SES’s solar generators will alsoproduce power in line with consumerdemand in southern California;peaking during the middle of thedays when summer load is highest.This removes the need for costlystorage technologies, and, as Osbornesays, “We have plenty of base-loadpower in the US, but we’re short onpeak power during the day. And we’rein tune with our effect on the grid andare making sure we don’t cause anyproblems.”

If plans go ahead, SES will build a1MW test facility using 40 dishassemblies. The full-scale plant

would follow, taking around fouryears to build. Power is expected tocost ‘significantly less’ than the USpeak power rate of 11.33¢/kWh

The project is the first majorapplication of SES’s solar dishtechnology for commercial electricitygeneration, which Osborne attributesto the sheer cost of manufacturing asolar generator. This, he says, makesthe units more suited for use in theirthousands as large scale powerplants, rather than domestic use.

“[Our first dishes] were like hand-built Maseratis,” he laughs. Butwhile he would not be drawn on themanufacturing cost of a singlegenerator, US research partnerSandia National Laboratories, says aprototype costs around $150,000.Volume production, however, couldcut costs to $50,000.

Hot stuffThe Stirling dish generatorconverts thermal energy toelectricity by using a mirrorarray to focus the sun’s rayson the receiver end of a closedcycle regenerative Stirlingengine. The internal side of thereceiver heats hydrogen gas,which expands to drive a piston,

crank shaft and driveshaft assembly that turns agenerator.

Prototypes have operated for26,000 hours and a six-dishproject is on trial at the SandiaNational Labs, New Mexico.Each unit operates automati-cally; the generator starts upeach morning, operates all day,tracking the sun, before shuttingdown at sunset.

Three control systems areused to track the sun, controlthe engine, and monitor andcontrol the plant. Researchersare ensuring each unit workswith the same level ofautomation and are confidentthe software will scale to muchlarger facilities.

Stirling power

conversion unit

IEE Power Engineer | October/November 2005 19