Not all that glistens has to be gold – even in Nevada’s El Dorado valley, where the newest attraction is a shining example of a technology we can expect to see a lot more of. Jackie Jones recently visited the site.Seen from the Las Vegas road, which offers a view across the wide El Dorado Valley, Acciona Solar Power’s Nevada Solar One plant seems tiny – a distant silver-coloured football field amongst a wide expanse of rock and gravel. The open desert space seems to represent the many locations in this, and neighbouring, US states – and those of North Africa and sunbelt regions around the world – where concentrating solar power production can blossom into a truly significant source of electricity.
The signs are good: the early SEGS plants of California’s Mojave Desert have been producing megawatt-hours for more than two decades, proving that this type of technology really can deliver. Now, with new technical advances, and with the right policy drivers and a favourable financial climate in place, it’s time to see whether the growth curve of concentrating solar power can follow that of wind power or solar PV.
The 129 hectare Solar One development in Nevada is the first multi-megawatt parabolic trough system built in the last 15 years ACCIONA ENERGY
Christoph Fark of SCHOTT-Rohrglas GmbH, is convinced of the technology. ‘We are just at the beginning’, he says. Fark, whose company has developed the high-efficiency solar receivers being used at Nevada Solar One, sees CSP as now being at a level of maturity close to that of wind power in the early days.
We know it works, he says – more than 20 years’ operation of the Mojave Desert plants have shown that. Plus, he points out, CSP has an advantage over (some) other renewables, in that the heat generated can be stored (using molten salt, or other storage methods) and released to generate power some hours later when most needed. This feature is more than just useful, as it significantly helps the overall economics by increasing the number of saleable megawatt hours a plant can generate.On reflection
Close up, the small, square of silver in the desert reveals itself as an impressive, 300-acre (129 hectares) collector field, with frames of (largely recycled) aluminium supporting 760 parabolic cylinder concentrators and over 18,000 solar receivers. The parabolic mirrors were angled downward, away from the sun, when I visited. At that stage, although all the collectors were in place, no heat transfer fluid had been pumped into the system, and the receivers would have sustained damage if the sun’s rays had been focused on them.
The initial 64 MW capacity collector area is connected to a 70 MW steam generator, and is contracted to supply 129 million kWh annually, explains Gilbert Cohen, Senior Vice President of developer, Acciona Solar Power (formerly Solargenix Energy). In reality it will probably supply 134 million kWh. Increasing the field size to produce heat for storage, enabling the plant to continue to produce power after the sun has dropped, would enable the Nevada Solar One to produce perhaps another 25 million kWh. It has been reported (but not confirmed by Acciona Solar) that the plant will produce electricity at $0.15-0.17/kWh. With greater efficiencies and economies of scale, costs can be expected to fall in coming years.
Although the desert is a perfect location for CSP, the cooling towers still require a substantial amount of water ACCIONA ENERGY
Nevada Solar One was built directly adjacent to the existing 480 MW El Dorado Energy combined cycle gas power plant (owned by Sempra), which means that transmission lines were already in place. Sometime in April, while this magazine is being printed, solar power should start being fed into the grid. Renewable electricity produced at Nevada Solar One will enable Nevada Power Company and Sierra Pacific Power Company to meet part of their renewable energy portfolio standard requirement. (The RPS requirement is increasing incrementally and will reach 20% by 2015.) A 20-year power purchase agreement is in place with these two companies, both of which are subsidiaries of Sierra Pacific Resources.
The plant has been constructed on schedule, and impressively rapidly – especially when it’s considered that this is the first large-scale CSP plant to be built for many years, and so construction is hardly routine. Groundbreaking took place in February 2006, and just over a year later the job was nearly complete. Meanwhile, according to Gilbert Cohen, 1.5 million working hours were put in by a workforce that averaged 400 and peaked at 850. The cost of the plant is estimated at US$220-250 million.Solar receivers
Approx 62% of the solar receivers used at Nevada Solar One were manufactured in Germany, at Schott’s dedicated (and highly automated) solar thermal receiver production line at Mitterteich, which went into operation early in 2006 (there had previously been a pilot production line at Mitterteich). This plant has the capacity to manufacture 150-200 MW worth of receiver tubes per year. A new production line, being built near Seville, Spain, at a cost of some US$28 million (€22 million) and coming online in 2008, will be ideally placed to serve the growing Spanish market and North Africa. It will bring Schott’s total capacity to around 300-400 MW/year. Firmly committed to this technology, the company anticipates further expansion of capacity in the future. It seems likely that if the market in the US fulfils expectations, the company may well invest in production facilities in the US, in due course.
While Schott, as one of the world’s leading manufacturers of glass, would certainly have the capacity to manufacture and provide the type of mirrors used at Nevada Solar One (which are, surprisingly perhaps, made of curved mirror glass rather than highly polished metal), the company is focusing on providing receivers.
A close up of a parabolic dish system schott solar
Schott’s Christoph Fark explains that making complete absorber tubes is a new departure for the company, which in the past used to supply the glass envelope tubes to another manufacturer (now a main competitor). Its new receiver has an absorption rate of 95%. But how much of an improvement is that over the past? Fark explains that the tubes have been used in test troughs at the Plataforma Solar in Almeria (Spain) and at NREL test sites (US). In each case, they were slotted into existing troughs so the tubes were the only variable. The tests demonstrated that the overall performance of the solar field increased by 2%. While that may not sound much, an extra 2% over a 20-year (or longer) lifetime is impressive in power generation terms. But the longer-term view is most important, says Fark, and certain other design features will ensure a longer working life for the tubes by reducing stresses.
A typical receiver has a steel inner tube that carries heat transfer medium – the temperature of which can reach as high as 400ºC. Surrounding this is a vacuum area, and on the outside another glass tube, which in daytime desert conditions heats up to approximately 40ºC. One of the big issues for R&D is dealing with the stresses caused by the different rates of expansion of these two materials. Schott has dealt with the longitudinal differential by installing bellows that allow some ‘play’. Another key innovation (and where Schott’s glass expertise has proven itself) is the design of a type of glass that follows the radial expansion curve of the steel used in the receivers. The result is that the stress breakages typically experienced in parabolic trough CSP are avoided.
As an additional feature, Schott has developed a highly antireflective glass coating for the outer tube, which allows 96% of the radiation to pass through. This new product, the PTR70, was first manufactured for the 1 MW Saguaro plant, developed by Solargenix Energy in Arizona during 2005. The receiver tubes were manufactured at what was then a pilot plant in Mitterteich, Germany.A market on the move
Christoph Fark is by no means alone in his positive view of the potential growth in solar thermal technology. According to the Greenpeace/ESTIA/SolarPACES scenarios published in 2005, solar thermal technology is destined to move from being a relatively modest renewable energy source to a significant contributor in 2040, alongside current market leaders like hydro and wind power. Total installed capacity will have exceeded 6400 MW by 2015. That is almost 18 times today’s capacity. By 2025, the annual installation rate will be 4600 MW/year. By 2025, total installed capacity around the world could have reached the impressive figure of 36,850 MW.
Spain is currently a hot market in solar thermal power, with encouraging feed-in tariffs, and 500 MW of CSP capacity could be in place by 2010. Two 50 MW solar trough plants (AndaSol 1 and 2) are being promoted jointly by ACS Cobra and Solar Millennium, according to SolarPACES, with another dozen or so 50 MW plants being developed by electric utility companies such as Iberdrola. Acciona/Solargenix, the team behind Nevada Solar One, is working on two 50 MW in Southern Spain and a 15 MW solar trough plant near Pamplona, while an 11 MW power tower was inaugurated on 3 April at Sanlucar la Mayor in south-west Spain – Abengoa is the company behind the PS10, as it is known. Elsewhere, Israel, Egypt, Australia, Algeria and France are all investigating CSP, with South Africa considering a 100 MW power tower.
Within the United States, the Department of Energy anticipates that as much as 20,000 MW of CSP could come online by 2020, given the right investment and technology advances. Nevada, California, New Mexico and Arizona are states with the right climate and policy to stimulate development in CSP, according to the Schott White Paper on solar thermal power plant technology. In addition, Texas is starting to look increasingly seriously at CSP, and developments there are likely in the coming few years. Since 2003, Nevada’s RPS has been in place, obliging the state’s two investor-owned utilities (Nevada Power and Sierra Pacific Power) to generate at least 15% of their electricity retail sales from renewable energy sources by 2013. California’s RPS will require investor-owned utilities to produce 20% of their electricity retail sale from renewable sources by 2017. Out-of-state generators are also eligible if they deliver electricity directly into California. Since July 2003, New Mexico has had an RPS in place, which required investor-owned utilities to generate at least 5% of their retail sales for New Mexico customers from renewables by 2006, with at least 10% required by 2011. Arizona has an Environmental Portfolio Standard (EPS) that increases to 1.1% in 2007. 60% of the EPS should come from solar sources, and may come from out-of-state solar energy provided it can be proven that it reaches customers in Arizona. Renewable energy credit multipliers in Arizona also provide additional incentives for in-state solar power generation.The developers
Nevada Solar One has been developed by Acciona Solar Power, formerly Solargenix Energy. The business that is now Solargenix was originally set up in the late 1990s by Duke Energy, with the name Duke Solar Energy LLC – in fact it was Duke Solar that, in December 2002, signed the original power purchase agreement with Sierra Pacific Resources to supply 50 MW of electricity generated by solar power from a plant in El Dorado Valley. (Nevada Power then contracted for approximately two-thirds of the power and Sierra Pacific Power Company contracted for approximately one-third.)
Just a few months later, at the start of April 2003, the company’s name was changed to Solargenix Energy, LLC. Amendments to the Solar One power purchase agreements were made in September 2005, allowing the project to go ahead.
Solargenix Energy established three divisions, working across the entire solar thermal sector, one focusing on solar for power generation, one on solar hot water, another on solar cooling/air conditioning.
In 2006, the Solargenix Energy power division, representing 55% of Solargenix Energy, was purchased by Acciona Energy for between $20 million and $30 million (reported as €22 million), forming a new entity named Acciona Solar Power. (Acciona Energy, based in Madrid, Spain, is a renewable energy company that owns or operates some 3000 MW wind, biomass, hydro and solar power plants, and is the renewable energy subsidiary of Acciona, a Spanish industrial group.)
In March 2004, Solargenix broke ground on the 1 MW Saguaro CSP plant at Red Rock, Arizona – the first CSP plant to be built in the US since 1988. This was completed around a year later and supplies power to Arizona utility APS. While 1 MW is very small for this technology, that project no doubt provided valuable experience.
Jackie Jones is Editor of Renewable Energy World.
Parabolic troughs – how they work
The technology is suited to the clear skies of the world’s sunbelt regions (within Europe, it can only be used at Mediterranean latitudes). Concentrating solar plants work by using the sun’s heat to produce steam that – as with any form of thermal power generation – turns a steam turbine. Therefore all but the front end of the power generation system is ‘familiar territory’ for the conventional power industry. (It’s worth noting that, in spite of being an ideal technology for desert conditions, creating steam does of course require a water supply.)
The curved ‘trough’ mirrors (there are 760 at Nevada Solar One) track the sun as it moves across the sky, concentrating it onto solar receivers, also known as absorber tubes, located along the focal line. The receivers are specially coated to maximize their absorbency – even a small percentage improvement in absorbency converts into greater power production. Being pumped through the receivers, and around the system, is a heat-resistant transfer fluid that reaches 400ºC. When it reaches the central generating unit, the fluid passes through several heat exchangers, and steam to drive the turbines is produced. (Other CSP technologies are being commercialized, including mirrored dish systems that produce heat to run Stirling engines, and power towers, in which a circle of mirrors concentrate solar radiation onto a single, elevated point.)
Figure 1. Schematic showing the working of a parabolic trough generation plant Source: ACCIONA ENERGY
FPL increases commitment to CSP
At the end of March, CSP pioneer Solel Solar Systems, whose parabolic trough technology is used in California’s Mojave desert SEGS plants, announced that it had signed a new contract with FPL Energy, LLC, for the supply of thousands of its advanced solar thermal receivers systems. FPL Energy is one of the leading providers of electricity in the United States, with a very extensive wind power portfolio. It is also the co-owner and operator of seven of the Mojave desert solar power plants. Eight months earlier, Solel announced an agreement with FPL Energy to upgrade its existing solar plants with 48,000 new thermal solar systems. The new agreement for the additional units will enable FPL Energy to increase its output of clean electricity and reduce the costs associated with operating its solar plants, said Solel.
In November 2006 Solel – headquartered in Beit Shemesh, Israel, signed a frame agreement with Sacyr-Vallehermoso, one of the largest Spanish infrastructure corporations, to jointly build three solar power plants in Spain with a total capacity of 150 MW and a total overall investment of $890 million. Solel’s portion in this project, in which it will serve as the supplier of the technology and provider of energy, is estimated to be $500 million.