Abu Dhabi's Shams 1, an innovative hybrid 100 MW solar thermal power station is in its startup phase and is undergoing standard tests and modifications<br>Credit: Shams Power Company
Abu Dhabi’s Shams 1, an innovative hybrid 100 MW solar thermal power station is in its startup phase and is undergoing standard tests and modifications
Credit: Shams Power Company

At present, the solar market is dominated by parabolic trough technology, but major investment in countries with plentiful strong sun is leading to the increased development of others, such as linear Fresnel plants, parabolic dishes and central tower plants, says Penny Hitchin.

Good day sunshine. In the US and Spain, solar thermal power stations have been generating electricity for years. Solar power stations are expensive to build, and the cost of the electricity they generate is not always competitive. However, buoyed by renewable quotas and other incentives, the low-carbon technology has proved a reliable performer and is attracting foreign investment.

Concentrating solar power (CSP) technology uses arrays of mirrors or lenses to concentrate heat from the sun into a small area. The heat is passed via a liquid to a conventional power plant, where it is turned into steam to drive a turbine. Unlike photo voltaic (PV) solar installations, CSP plant can incorporate storage systems, and can be hybridised with other fuel, enabling generation to continue when there is no sun,

Solar thermal power generation was kick-started when the 1970s’ oil crisis led to US tax and investment breaks for alternative energy. Between 1985 and 1991, 354 megawatts of parabolic trough CSP was installed in California, with tax credits making a hefty contribution to the capital costs. The world’s largest CSP plant, the 354 MW Solar Energy Generating Systems (SEGS) plant has been operating in the Mojave Desert since the 1980s, and continues to meet the original engineering performance predictions, producing low-cost electricity.

Ten years ago, Spain’s government introduced renewable energy feed-in tariffs, which triggered the installation of 1,800 MW of CSP, making the country a world leader. Europe’s first utility-scale CSP plant was the 50MW parabolic trough project at Andasol in Grenada, where three units now operate with a total capacity of 150 MW.

In 2011, 450 MW of new CSP was installed, and in 2012, 811.5 MW was added, bringing global capacity to over 2,500 MW. Currently, interest from countries including Australia, Chile, China, Egypt, India, Iran, Israel, Jordan, Morocco, Saudi Arabia and UAE sees CSP expanding into new areas.

New tower power to compete with parabolic troughs?

There are four main basic configurations for CSP reflectors and receivers. The market is dominated by the established parabolic trough technology, but other systems coming into the picture include solar towers, linear Fresnel plants and parabolic dishes.

Parabolic trough collector plants are made up of horizontal arrays of u-shaped reflectors. The angle of the mirrors can be tilted to follow the sun. The mirrors concentrate the light on to pipes containing heat transfer liquid (usually synthetic oil). The liquid, at temperatures as high as 4000°C, vaporises water to drive a generator, producing electrical energy for the grid. Parabolic trough plants have been in operation since the 1980s, and comprise the bulk of the global CSP fleet of power stations. The largest is 300 MW.

Central tower CSP plants have an array of ground-mounted dual tracking reflectors (heliostats) surrounding a tall central tower. The central receiver at the top the tower is inundated with sunlight from the reflectors. The heat-transfer fluid (usually molten salt) in the receiver heats up and the thermal energy generates steam to run the turbine. Molten salt can heat up to 5600°C, and retains heat efficiently, so it can be stored. Power tower technology has the potential to offer higher efficiency and better energy-storage capability than trough systems,

The first large-scale tower plant is being built in California’s Mojave Desert at BrightSource Utility’s 392 MW Ivanpah facility, and will consist of three 123 MW tower units. The plant has been three years in construction, and the first unit is due to start operation later this year.

Alexis Gazzo from Ernst & Young Cleantech and Sustainability Services believes that the construction of the Ivanpah power plant marks a change in the CSP landscape, as a number of tenders now include tower technology requirements. “Recent tenders within Algeria, Morocco and South Africa have all included tower technology element, as investors become more familiar with it.”

Fresnel reflectors are made of flat mirror strips that concentrate sunlight on to tubes through which working fluid is pumped. Flat mirrors have more reflective surface in the same amount of space as a parabolic reflector, so can capture more of the sunlight, and are cheaper than parabolic reflectors. Only a few commercial Fresnel CSP plants are in operation, the largest being Novatec Solar’s 30 MW Puerto Errado 2 plant in Spain.

Gus Schellekens, Global COO Sustainability and Climate Change Network at PwC, says recent studies show that for large (50 MW) trough plants, investment costs range between $3800/kW and more than $8000/kW, depending on the solar field size, the size of the storage facility and the labour and land costs. Capital costs and the amount of direct solar irradiation largely determine resulting levellised costs of energy (LCOEs). These range between $130/MWh and $300/MWh.

The LCOE of towers with large storage appears to be lower than that of trough plants, and can be below $150/MWh in markets such as India and the US. Stand-alone Fresnel plants without storage still need to demonstrate their claimed lower costs.

Parabolic-dish systems provide the highest solar-to-electric efficiency of CSP technologies. However they are smaller scale, with one parabolic dish of mirrors concentrating sunlight on to a central engine that produces electricity.

Where to get the best results?

CSP plants use fields of solar collectors that power a conventional generating plant via a solar receiver. The best locations are hot, dry regions with no cloud cover. The stronger the sun, the greater the potential fuel supply.

The minimum solar resource threshold for solar thermal development is around 1800 kWh/m2/year Direct Nominal Irradiation (DNI). The DNI figure is critical in CSP economics, as a high DNI means fewer solar collectors are needed than for a plant of equivalent output in an area of lower DNI.

Solar tower schematic<br>Credit: BrightSource Energy
Solar tower schematic
Credit: BrightSource Energy

In Spain, DNI levels are up to 2100kWh/m2/year. Higher DNI values are found elsewhere: for example, in the abundant solar resources of the Middle East and North Africa (MENA) region. Levels vary locally, so careful site selection is required. Irradiation data from satellites can be unreliable, as dust, water vapour or smoke cut the amount of sunlight reaching the ground. Field readings taken over a period of at least 12 months are needed to ascertain essential environmental data, including wind speed, humidity, DNI levels and solar availability.

Ivanpah will produce clean power to more than 140,000 homes<br>Credit: BrightSource Energy
Ivanpah will produce clean power to more than 140,000 homes
Credit: BrightSource Energy

CSP power plants need large tracts of fairly level open land, preferably without vegetation. For a site to be suitable for development it needs road access and proximity to transmission infrastructure availability. CSP developers generally avoid urban centres and sensitive areas such as national parks. Deserts are favourite locations, but securing permissions to use the land may come at a price: BrightSource spent $53 million to move a colony of desert tortoises living in the Ivanpah project zone.

CSP case study: Shams 1

The parched and windswept desert of Western Abu Dhabi is home to Shams 1, an innovative hybrid 100 MW solar thermal power station coming on stream this year. The $600 million project was designed and developed by Shams Power Company, a joint venture between Masdar (60 per cent), Total (20 per cent) and Abengoa Solar (20 per cent).

The new plant is fuelled by parallel rows of big curved mirrors designed to catch the solar radiation. More than a quarter of a million mirrors are mounted on tracking parabolic trough collectors, angled to follow the sun. The mirrors concentrate heat from sunlight on to oil-filled absorber tubes, which connect to a conventional power plant. A gas-fired booster heater super-heats the steam in the turbine, and the condenser is air-cooled. It is a hybrid plant designed to be powered by solar power, augmented by gas from the local field.

The plant is in its start-up phase and is undergoing the tests and modifications that are standard in any conventional power plant. Process engineer Abdulaziz Al Obaidli talked to PEi about the operational challenges of establishing a stable solar power plant.

“The behaviour of the solar field makes the plant harder to stabilise than a conventional power plant,” he says. The solar field at any time depends on the sun, cloud and wind, which makes it harder to operate than a conventional plant.

“Every day is different. As a new operator we are learning how to react in the correct way to the different scenarios. Operators, mechanics, engineers, maintenance staff, we are all learning,” added Al Obaidli. By the end of this year, he believes that the operational team will have acquired experience of operating the plant in the range of conditions that the harsh environment will throw at them.

Al Obaidli says: “This is the first plant in the desert, and we need to get information about the behaviour of power plants in desert conditions.” The wind is stronger in the spring. The plant is designed to operate in wind speeds up to (average) 36 km per hour, but dust storms are common, and the windbreak wall has been built to reduce the impact of windblown sand on the mirrors. Tons of sand accumulated around the walls will be shifted by truck twice a year. The desert conditions dictate that the mirrors should be cleaned twice weekly to maintain reflectance.

New plants worldwide

Proposed sites for future CSP plants include Chile’s Atacama Desert, the Tibetan plateau, South Africa, Australia and India.

Morocco has ambitious plans for CSP: in 2009 the Government set a goal of installing 2,000 MW of solar power capacity by 2020 through five concentrated solar power projects, including a 500 MW plant at Ouarzazate in the Atlas Mountain plateau on the fringe of the Sahara Desert. Ouarzazate I is the initial phase of a regional investment plan to drive costs down and develop capabilities for CSP through deployment and learning. It is financed by the Clean Technology Fund ($197 million), International Finance Institutions ($1 billion+), and the Moroccan government ($883 million over the life of the plant, including a 25-year power purchase agreement), and will be developed through a consortium of private developers and the Moroccan Agency for Solar Energy (MASEN).

A consortium led by Saudi Arabia’s ACWA Power started construction of the 160 MW project in May 2013. The parabolic trough project will incorporate three hours of molten salt thermal energy storage capacity. The second unit will be a 100 MW central tower, while the third is intended to generate 200 MW from parabolic trough technology.

The Climate Investment Fund’s Clean Technology Fund plans support CSP in the MENA countries and has earmarked $218 million for the 300 MW Ouarzazate 2; $123 million for the 100 MW Kom Ombo in Egypt, $62 million for the 50-100 MW Akarit in Tunisia, and $50 million for up to 100 MW in Jordan. The plan, implemented by the African Development Bank and the World Bank, has been delayed by the Arab Spring and the economic difficulties in Europe.

In November 2012 Abengoa began constructing South Africa’s first CSP plants in the Northern Cape Province: Khi Solar One, a 50 MW solar power tower at Upington, and KaXu Solar One a 100 MW parabolic trough plant at Pofadder. South Africa has other CSP projects in the development or planning stages, including a 100 MW Fresnel site and a 50 MW parabolic trough project at Bokpoort.

India wants CSP in its energy mix in the future. Reliance Power has raised US $300 million to build the country’s largest concentrated solar power (CSP) project at Dhursar in Rajasthan. The 100 MW project developers intend to use Areva’s Fresnel reflector technology.

Future prospects for CSP

It is not all rosy in the world of CSP, as projects are being postponed or cancelled due to economic uncertainty or regulatory restraints.

Spain became the first country to introduce a feed-in tariff (FiT) for solar thermal in 2002. A premium was added for the first 200 MW of solar thermal power installed, with the tariff guaranteed for 25 years indexed to average electricity price increases. But 2012 changes to Spain’s FiT and tax regimes will reduce potential revenue from future plant by nearly a third, which is halting development.

However, Spanish expertise will be welcome elsewhere: Abengoa has installed 743 MW of installed solar capacity around the world and, with another 910 MW under construction, is the world leader.

Despite ambitious plans and projects appearing in various countries’ development pipelines, the financing, building and operating of CSP plants is fairly novel outside Spain and the USA. Schellekens says it is important for the Middle East and North Africa that the UAE’s Shams 1 and Morocco’s Ouarzazate I projects are successful. If they can be delivered on time and budget, it will send messages to investors and developers about the ability to develop utility scale projects involving CSP technology in the region, and provide feedback for governments about its viability.

Looking more widely, Schellekens says that the immediate prospects for CSP capacity coming on-line are modest, with maybe another 2-3 GW being connected over the next couple of years if some of the larger 250-500 MW projects happen. “The main challenge in many countries is the price of electricity. Where this is subsidised, most new technologies will struggle to be successful unless they look to address peak periods of demand,” he says.

As CSP generation is more expensive than its competitors, lowering costs is vital. A 2011 study estimated that CSP costs have gone down by 15 per cent with each doubling of cumulative deployment in the past 20 years.

Alexis Gazzo of Ernst & Young Cleantech and Sustainability Services says studies suggest that a reduction figure aorund $10c per kWh is needed by 2020. He believes that this not impossible but likely to be the result of larger-scale market rollout rather than pure technological improvements.

roadmap

The IEA sees the potential for CSP to play a massive role in the future global generation mix. Its roadmap projections to 2050 say that with appropriate support, CSP could provide 11.3 per cent of global electricity, with 9.6 per cent from solar power and 1.7 per cent from backup fuels. This ambitious scenario would require concerted investment in plant and cross-border transmission infrastructure (to carry power from MENA countries to Europe) without delay. In the current economic climate, this is unlikely.

Penny Hitchin is a freelance writer.

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