Linear Fresnel technology may be poised to take centre stage, but its performance needs polishing.

By Emma Clarke

Linear Fresnel technology, a latecomer to the CSP market, is about to make its entrance.

It has been a long wait, given the technology was first developed for power generation back in the 1960s.

But technology developers say recent efficiency improvements now present linear Fresnel as a genuine low-cost alternative to its market-leading cousin, the parabolic trough.

Linear Fresnel follows the principles of parabolic trough technology, but replaces the curved mirrors with long parallel lines of flat, or slightly curved, mirrors. These mirrors track the sunlight and concentrate it onto a stationary, single receiver located several metres above the reflectors. The receiver contains an absorber pipe where water is converted to steam driving a turbine to produce electricity.

Low cost and accessible

The main advantages of linear Fresnel are its lower investment and operational costs.  Firstly, the flat mirrors are cheaper and easier to produce than parabolic curved reflectors and so are readily available from manufacturers worldwide.

The structure also has a low profile, with mirrors just one or two metres above ground. This means the plant can operate in strong winds and it can use a lightweight, simple collector structure.

The technology can be sourced locally and installed by a local workforce, says Mauricio Rojas from Solar Power Group, a linear Fresnel technology producer. “You don’t need a PhD to install this,” he says. 

Linear Fresnel collectors also make more efficient use of land, packing more mirrors closer together compared to parabolic troughs. The collectors are also easier to maintain since they have fewer moving parts and they can turn face down during the night for protection from sand and to allow for automated cleaning.

The three leading versions of linear Fresnel technology generate steam directly, which means they do away with the need for expensive and performance-reducing heat exchangers.

Studies carried out by the German Aerospace Centre (DLR) have indicated that linear Fresnel technology costs between 50-60 percent of the costs of a parabolic trough collector per square metre.

Parabolic reflectors still have the edge

But parabolic trough reflector performance is still superior. “If optical efficiency of a parabolic trough collector is in the range of 78-80 percent, a linear Fresnel collector is 70 percent in the best case,” says Markus Eck, from the Institute of Technical Thermodynamics at DLR. Linear Fresnel also has a higher thermal loss.

“To compensate for these energetic losses, you will need to erect a bigger field,” he explains.

Despite these losses in efficiency, linear Fresnel should, in principle, have a lower levelised cost of energy. “The challenge now is to find out how much cheaper it is,” notes Eck.

This requires getting the first large-scale plants into action. Linear Fresnel projects in operation are at the small, test scale. But larger projects are on the horizon.

In Europe, Novatec BioSol is operating a 1.4MW demonstration plant in Murcia, Southern Spain and is arranging financing for a 30MW plant on an adjacent site.

Solar Power Group, working in partnership with industrial services company MAN-Ferrostaal, is planning a 15MW hybrid plant in Libya, North Africa in addition to other projects in Spain and North Africa.

They have been testing a 1MW prototype at the Plataforma Solar de Almería research centre in Spain since 2007 that produces superheated steam at 450 degrees Celcius and 100 bar pressure. “That makes us leaders in terms of efficiency,” says Rojas.

In the United States, Ausra is operating two small demonstration projects using its Compact Linear Fresnel Reflector (CLFR) technology: the 5MW Kimberlina plant in California and a 3MW plant that displaces a portion of coal consumption for the 2,000MW Liddell Power Station in New South Wales, Australia.

The CLFR technology has also been chosen for a 100MW project in Ma'an, Jordan with a back-up fossil boiler (expected to enter operation in 2013) and to produce steam to boost a proposed 750MW coal-fired power station in Queensland, Australia.

US-based, SkyFuel has developed Linear Power Tower (LPT) technology — its trademarked version of Fresnel. The high temperature LPT is the only Fresnel system to use molten salt for heat transfer and direct storage. Commercial deployment is scheduled for 2012.

Each technology has its place

It is still early days, and the unproven linear Fresnel technology continues its struggle to raise finance. But once proven, Eck believes there is space for linear Fresnel in a growing CSP market.

“The market for CSP will be big enough for each technology to have its share,” he says. “Choosing which technology to apply will depend on the project.”

Linear Fresnel, for example, works for projects in countries that demand a higher share of local supply and construction, says Eck.

It’s also ideal for smaller-scale projects, adds Rojas, such as steam augmentation for fossil fuel power plants, or to produce steam for industrial processes such as enhanced oil recovery, desalination or food processing. The steam from Linear Fresnel technology can also be used in cooling or basic heating systems.

Heliodynamics, another spin-off technology that uses the linear Fresnel concept to create a high-temperature collector capable of 24-7 steam generation, is also targeting small-scale operations such as solar air-conditioning systems or process steam for factories.

“Where steam is used and sunlight is available, there is an application for solar steam generators,” says Tom Bartolomei, senior vice president of business development at Ausra.

It has taken almost fifty years to get here, but Barolomei thinks linear Fresnel can now stand its ground in the CSP market. 

“The industry we participate in is very conservative, so it takes a lot of marketing and prudent design work to establish ourselves as a bona fide technology. But we are certainly at that point now,” concludes Barolomei.