By Jason Deign

Gas is a tricky issue for CSP.

In most parabolic trough, linear Fresnel and power tower plants natural gas is indispensable for processes such as maintaining heat-transfer fluid (HTF) temperatures and starting up turbines, which somewhat knocks the idea that CSP is 100% renewable.

Nevertheless, the amount gas used in this way is fairly small. And in many cases, CSP developers have looked to gas as a way to boost a plant’s capacity to provide base-load power, by using it as a backup when the solar power input wanes.

In fact, in regions such as North Africa this hybrid concept, in the form of integrated solar combined cycle (ISCC) plants, has helped provide a bridgehead for the development of CSP.

A classic example is the Algerian Hassi-R’Mel ISCC power plant built by developers Abener and Teyma and operated by New Energy Algeria, where a CSP field was essentially tacked onto a traditional power plant.

At Hassi-R’Mel, the solar field helps the plant produce lower-emissions power during daylight hours while conventional fuels help reduce the total cost of energy and maximise the use of the steam turbine.

Compared to relying on thermal energy storage for base-load energy delivery, CSP-gas hybridisation is cheaper and less technologically risky.

And while it has traditionally been seen as still more expensive and risky than building combined-cycle gas plants alone, a June 2011 study by the US National Renewable Energy Laboratory (‘Gas Turbine/Solar Parabolic Trough Hybrid Designs) pointed to a solution.

Aero-derivative gas turbines

“In the preliminary analysis provided here, it is shown that a single 40MW aero-derivative gas turbine mated with a 100MW parabolic trough plant can be more efficient than two separate power plants,” say the authors.

But just as it begins to look like gas and CSP could have a bright future together, the concept has taken a hit in CSP’s most established market, Spain.

There, as reported in CSP Today, lawmakers are mulling a change in the law that will penalise the use of natural gas as a backup fuel, potentially axing plant revenues by up to 15%.

The crux of the matter, forgetting momentarily that CSP in general is a renewable energy source which it makes sense to foster in a sunny, energy-importing country such as Spain, is that natural gas is a fossil fuel and should be penalised accordingly.

Which then begs the question: could Spain’s CSP plants switch to biogas instead? After all, there is already not insignificant interest in hybridising CSP with biomass.

The TRESERT project developed by Solarlite at the Thai School of Renewable Energy Technology, Naresuan University, Thailand, for example, combines parabolic troughs for steam generation with a biomass boiler delivering 500kW of thermal and 50kW of electrical power.

And SkyFuel, the US parabolic trough maker, is known to be involved in a project to create a combined CSP and biomass power plant with Braxenergy in Brazil.

“Our project with Braxenergy is moving forward and is planned to start construction next year,” confirms Kelly Beninga, SkyFuel’s chief commercial officer. Alas, though, it seems like simply replacing natural gas with biogas might not be an easy option for CSP in Spain.

Biogas definitions

For a start, the definition of ‘biogas’ for feed-in tariff purposes is pretty specific.

According to Spain’s Royal Decree (RD) 661/2007, it has to come from landfill or be generated in anaerobic digesters from industrial residues, urban or industrial water purifier silts, solid urban residues and agricultural or livestock waste.

“The RD 661/2007 establishes that you can hybridise with biomass or biogas, up to 50%,” says Jorge Alcauza of the communication and press department at Protermosolar, the Spanish CSP industry body.

This would mean that a hybrid plant would indeed be eligible for a feed-in tariff for all energy generation, and indeed an Abantia and Comsa Emte project combining biomass and CSP at Borges, Lleida, Northern Spain, should benefit from this arrangement.

However, notes Alcauza, even the Borges plant uses natural gas for certain processes, such as operating the turbine. “You have to bear in mind that natural gas is not used to generate electricity directly, but to maintain the HTF temperature and operate turbines,” he says.

“You would also need to check whether the plants would have to apply for new administrative authorisations, or even a new environmental permit, to account for significant modifications. And obviously you would have to look at the technical viability of doing it in each plant.”

As a result, he says: “In principle I don’t think anyone is considering it as a short- or medium-term measure.”

That does not mean there is no future for CSP and biogas, though. “Combined biogas and solar thermal can definitely be interesting provided that you have the right conditions,” asserts Salvador Escobedo, founding partner of Solam Group, a Latin American solar project developer. 

Maybe it is just time someone tried it out.

To respond to this article, please write to Jason Deign

Or contact the editor, Jennifer Muirhead