By Jason Deign

Picture a CSP technology that is fully proven in the field, backed by an established engineering giant and cheaper than parabolic trough or power tower. Oh, and with an operating temperature of 500ºC or higher, and molten salt storage. Welcome to the next generation of linear Fresnel.

That’s right. CSP’s third technology looks set for a comeback. Linear Fresnel has historically trailed behind parabolic trough and power towers in terms of popularity. The CSP Today Global Tracker only lists 10 operational plants worldwide, totalling not much more than 50MW.

This is despite the fact that linear Fresnel is the cheapest of the big three CSP technologies. It does not need the kind of fancy engineering required for a power tower, nor the mirror precision that parabolic trough demands.

What it is commonly perceived to have lacked so far, though, is an operating temperature high enough to provide storage.

Plants typically achieve around 370ºC.

This may not have been a problem when these plants were commissioned. But it has become something of a sticking point in a solar industry dominated by low-cost PV.

Cheap as it may be, linear Fresnel does not compete with PV on a cost basis, and without storage it has no added value for grid power generation. That has been the story so far, at least.

But linear Fresnel has friends in high places, and they have not been content to let the technology languish.

Linear Fresnel

Two of the companies offering linear Fresnel plants are Novatec Solar, and the French industrial conglomerate Areva.

Both have been addressing the operating temperature issue. According to Lux Research associate Ed Cahill, this can be achieved mainly by increasing the reflector area. “It’s an engineering challenge,” he says, and not an insurmountable one.

Indeed, Novatec cracked the problem back in 2009 with its Puerto Errado 1 plant, which has an operating temperature of 500ºC. Its later plants work at 270ºC.

Perhaps more importantly, though, Areva has been looking at higher temperature ranges to incorporate storage into its plants.

Working with the US Department of Energy’s Sandia National Laboratories, the company has built a prototype at the National Solar Thermal Test Facility in Albuquerque, New Mexico, incorporating a molten salt storage tank.

Jayesh Goyal, global vice president of sales for Areva Renewables, says: “Let me be clear as to why linear Fresnel was not seen as having a good storage capacity. The issue wasn’t temperature.

“The temperatures right now with linear Fresnel have got to 500ºC, which is way above where parabolic trough is, at 390. The issue is when you use water as the working fluid. It’s much harder to do an efficient heat exchange.”

Exchanging the heat from steam to a heat storage medium such as molten salt is highly innefficient, he explains, so Areva has looked at using molten salt throughout the system.

Molten salt

“If you go to molten salt you have the advantage that you can go to even higher temperatures,” Goyal says.

Areva is even looking to take its molten salt experiment further, by using it simply as a heat transfer fluid with a phase-change material for storage. A pilot plant is already in the pipeline. “That’s the second initiative we are working on,” Goyal says.

In the meantime, though, Areva is looking to use molten salt storage to boost the value of its bids in a number of markets. When could we see a commercial linear Fresnel plant with storage, then?

“The most likely one would probably be the MRNE [Indian Ministry of New and Renewable Energy] demo programme, assuming the RFP [request for proposals] comes out in January as expected,” says Goyal. “That would probably be our first commercial offer.”

Areva is also keeping a watch out for the tender expected from the King Abdullah City for Atomic and Renewable Energy in Saudi Arabia. Storage will be a basic requirement for the bid.

Going forward, “for standalone power plants, I think storage will pretty much always be there,” Goyal believes. “We are now barely seeing any customers who are asking for standalone plants without storage being mandated.”

And if Areva’s molten salt bet pays off, there is no reason why linear Fresnel might not start to pick up speed in the industry. This would not be the first time the technology has been touted as a future winner, of course.

However, Cahill says, with storage: “If they can get to 550ºC, it outdoes parabolic trough. It’s cheaper.”

To respond to this article, please write to Jason Deign

Or contact the editor, Jennifer Muirhead