Solar Power – Sunlight powering the desert

In the US state of Nevada, Solargenix Energy is developing a large-scale concentrating solar power plant that will add important capacity to state utilities’ obligation to increase power generation from renewable sources.

The word ‘vast’ is inadequate to describe Nevada’s desert. Its sheer scope, breadth and expanse are daunting. The mind can play tricks when judging distance. What looks to be just down the road can turn into a 30-minute drive.

Of course, this being desert the climate and landscape can be extremely inhospitable. The southern region of Nevada, part of the Mojave Desert, has both mountainous and flat or uneven bush land, where away from the main towns you would be hard pressed to find a living soul. It is also hot, with minimal rainfall and maximum sunshine every year.

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Thirty-two km from the bright lights of Las Vegas is the quieter and more relaxed Boulder City. Originally created in the 1930s as a home to the construction workers that built the nearby Hoover Dam, Boulder is the city with the largest geographical area in Nevada. Curiously, it is also the only city in the state with no gaming, but exploiting the potential for solar power generation is anything but a gamble for the city.

In fact, in these environmentally conscious times using southern Nevada’s ample sunlight to provide electricity for local homes is a sound energy policy that allows utilities to edge closer to legislative requirements for renewables.

Renewables commitment

In 1997, the State of Nevada adopted a Renewable Portfolio Standard that placed a mandate on the state’s two utilities – Nevada Power and Sierra Pacific Power – to use renewable energy to provide a minimum percentage of electricity consumption. A revision to that state law in 2001 meant that this amount was to increase by two per cent every two years. Each phase can be broken down as thus: seven per cent in 2005, nine per cent in 2007, up to 15 per cent by 2013.

A further amendment has since been made as Gilbert Cohen, vice president of Solargenix Energy Inc – a leading energy and environmental engineering company headquartered in North Carolina – outlines, “Nevada has a renewable portfolio requiring the utilities to install 15 per cent of renewable energy by 2010 – five per cent of the 15 per cent is set aside for solar. In general, Nevada authorities are very supportive of solar and renewable development.” The Nevada Solar One project currently under construction will go some way to achieving that ideal.

In addition, the US Department of Energy (DOE) issued a report that identified suitable land and solar resources in Nevada that could produce over 600 000 MW of electricity, using concentrating solar technologies. Currently, Nevada’s power consumption is less than three per cent of this resource capacity. The DOE report also claims that the economic benefits far exceed the cost to develop this clean renewable energy source.

Nevada Solar One

On a 1.62 km2 site in Boulder City’s Eldorado Valley – just off the Route 93 highway linking Las Vegas and Phoenix – is a solar power complex that when complete will be the third largest in the world. Solargenix Energy is developing Nevada Solar One. Earlier this year, the Spanish renewable energy firm Acciona Energàƒ­a SA purchased 55 per cent of Solargenix and said it was investing €220 million ($298 million) in the Boulder City project.

Nevada Solar One employs solar parabolic trough technology
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“Nevada Power was requesting [power production of] 115 000 MWh per year,” explains Cohen. “Our plant will produce about 134 000 MWh; this is sufficient for about 50 000 homes.”

The first stone for Nevada Solar One was laid at a special ceremony in February of this year. This followed a long period of gaining planning permission and securing financing and tax incentives – about $10-15 million – from the Nevada Commission on Economic Development.

“We have worked very closely with the city council, the Mayor, the city manager etc,” says Cohen, “and all of them are very supportive of the project”. “It was requested by Nevada Utilities through a ‘request for proposal’ (RFP) in 2002. Our company, then Duke Solar, responded to the RFP and were selected for negotiations. These negotiations were successful and a long term ‘power purchase agreement’ (PPA) was signed in December 2002.”

In September 2005, Solargenix announced the approval of amendments to its PPAs with Nevada Power and Sierra Pacific Power by the Public Utility Commission of Nevada. The approval allowed it to proceed with the development of its 64 MW solar power plant.

Capturing the power

The plant uses solar parabolic trough technology to reflect the sun’s heat onto oil-filled reception pipes running through the mirror collectors. The synthetic oil is passed to heat exchangers on the power island, which transfer the heat to generate steam for the turbine.

Capturing the sun via this process is known as concentrating solar power (CSP). It uses curved (parabolic) mirrors to concentrate solar radiation on a thermally efficient receiver running the length of the trough. The receiver consists of a specially coated absorber tube embedded in an evacuated glass envelope. Absorbed solar radiation warms up the heat transfer oil flowing through the tube to almost 400 à‚°C. The system also uses a tracker, so that the line of the sun can be followed throughout the day. Schott AG of Germany is the manufacturer of the receiver tube technology, and Nevada Solar One is using 19 300 of these units.

Cycle efficiency improved

For the power island, Solargenix awarded the contract to Siemens Power Gen-eration in August 2005 for a high efficiency, direct reheat SST-PAC-700RH steam turbine generator set. The contract also included stand-alone Simatic S7 control systems and installation, and commissioning support plus staff training. The 72 MW SST-700 turbine was manufactured at Siemens’ facility in Finspong, Sweden, with components also shipped from Germany. The package was then transported by sea to the port at Los Angeles before being trucked to Nevada.

Rotor of the solar-optimized SST-700RH industrial steam turbine
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The steam turbine is a dual-cased geared engine using one high pressure (HP) and one low pressure (LP) module. It was delivered on site in the second week of November, and the plant itself was commissioned in April 2007.

Steam is generated via the heat exchangers at 371 à‚°C/1250 psia. After passing through the HP turbine it is taken back into the steam generator for reheating and bringing LP steam up to 371 à‚°C again. This will further reduce the oil temperature back to the solar collectors and improve the overall cycle efficiency of the plant.

No water discharge

The reheated steam is now admitted into the LP turbine to further generate power; it then enters a vacuum condenser, where the remaining steam is condensed to be pumped back into the steam generator again. This careful use of water is vitally important considering the desert location and value of water sources. It has also been a consideration for the plant’s cooling towers. Solargenix is justifiably proud of the fact that the plant has no water discharge.

Cohen explains, “Solargenix negotiated a long term water supply with Boulder City. Although the project is located in a desert area, water from Lake Mead was made available for energy production. In the future, we are planning to use hybrid and/or dry cooling systems for CSP power projects.”

Supplementing the heat cycle is a small, natural gas fired auxiliary boiler, which will be used at times when the sun’s radiation is not available, or during cleaning of the solar mirrors. The cycling capability of the plant is significant when dealing with likely variables in steam temperature and pressure.

The steam turbine is coupled to a generator, which produces electricity at 13.8 kV (60 Hz). Power is then stepped up by a transformer and distributed to the grid at 230 kV from the huge switchyard. With a separate (and unconnected) combined-cycle power plant adjacent to the solar plant this section of the desert is a curious mixture of bush land and a sea of transmission towers.

An impressive sight

To give some idea of the scale of the plant, from the sites overall area of 1.6 km2, one km2 is taken up by the solar array system. At a distance the plant looks like a small, flat, white mass.

As the site draws closer its enormity is jaw-dropping; hundreds of huge CSP troughs with crystal clear mirrors reflecting the desert with a burst of deep blue and sandy brown colours – impressive!

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