Torresol Energy's Gemasolar tower plant represents one of the most important milestones in the history of CSP, the audience of a recent CSP Today webinar has heard.

Santiago Arias remembers the reaction he got when people found out his company was looking into CSP storage at a SolarPACES conference six years ago.

"Most of our competitors said we were mad to do something nobody was asking for," says the chief operations manager at Torresol Energy Investments.

How times change, though. In an audience poll carried out during a recent CSP Today Spanish webinar entitled ‘Gemasolar: marking a before and an after in solar thermal development', 86% of some 200 industry insiders said storage would be essential to CSP in the future.

Most of the remainder said it would depend on the market and only 3% felt it would not be essential. And it is likely the opening of the 120MWt, 19.9 MWe Gemasolar plant earlier this year has had a lot to do with this change in perception.

By any standards, Gemasolar, the first commercial-scale plant in the world using molten salt storage with a central tower receiver, represents an impressive piece of engineering.

During the 29 months it took to build the plant, engineers had to break new ground in designing the receptor, molten salt pumps and vaporiser, among other elements.

Getting the plant's 2,650 heliostats, which cover 300,000 square metres, to all point at exactly the same spot all the time was another "small problem," Arias says. However, the end result could indicate the shape of things to come for CSP.

Gemasolar's design makes it possible to focus sunlight to around 1,000 times the intensity on the ground and heat molten salt from an initial 270° to 300°C to up to 550° to 565°C.

Heated salt

At this point, Arias says, the operation of the Gemasolar is essentially dependent on the amount of heated salt rather than sunlight, so it functions in a way comparable to a traditional power plant.

The tower contains enough salt to drive Gemasolar's 20 MW turbine at full power for up to 15 hours without sunlight, giving the plant an expected net annual electrical production of 110 GWh and around 6,500 hours of operation a year, sufficient to power 25,000 homes.

There are other advantages, too.

Since the turbine does not have to be shut down frequently its lifespan is likely to be longer, Arias posits. Output can be regulated to suit demand rather than having to match solar variation. Energy conversion is more efficient because of the higher temperatures.

In addition, not having tubes or heat exchange fluids spread out across the solar field could greatly reduce the chance of breaks or leakages, bringing down operations and maintenance costs.

So far, Arias relates, Gemasolar has already been able to achieve up to 80% of its theoretical maximum output in practice even though it only started operating flat-out in September, after a two-month period of fine-tuning and paperwork delays following plant acceptance in July.

"The plant is performing wonderfully," he says.

As a consequence the plant's developer, Spain's SENER Grupo de Ingeniería, which owns 60% of Torresol Energy versus the 40% controlled by the Abu Dhabi renewable energy company MASDAR, is looking at more tower-and-storage plants.

Parabolic troughs

Arias says SENER currently has a pipeline of 21 CSP projects in Spain and two in the US. The vast majority of these will use parabolic trough technology; SENER has already produced 1,550 km of parabolic troughs, enough to pave a six-metre route from Madrid to Paris.

However, the company is also considering a tower-and-storage design double the size of Gemasolar, with around 5,000 heliostats delivering 240 MWt. This might result in economies of scale that could hasten CSP storage's journey towards grid parity.

But how much bigger could tower-and-storage go in practice?

Manuel Silva, associate professor at the University of Seville and Arias's co-presenter in the CSP Today webinar, says: "There are studies for all tastes but I do not think it would be reasonable to multiply [existing plants] by 10, for instance."

Arias agrees that the upper limit for tower plants might be set by the size of the heliostat field, since the power of light reflected from several kilometres away would be lessened by dust and pollution.

What does seem clear, however, is that storage is rapidly becoming indispensible for CSP in the wake of Gemasolar. A separate vote during the CSP Today webinar showed overwhelming support for molten salt as the storage medium of choice.

"I can see potential in other media," Arias rejoins. "But what I can't see are plants without storage, because then they would be competing with photovoltaics (PV), which long-term I understand will be more economic than solar thermal."

No matter how far panel prices fall, however, Arias and other CSP advocates can at least now rest assured that it will be some time before PV can emulate Gemasolar and start delivering continuous energy night and day.

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