Technology

The concentrated solar power sector kicked off 2O12 by bringing several game-changing innovations online. David Craik gives an overview of the latest breakthrough concepts, and finds out what is in store for the remainder of the year.

In January, Torresol Energy began commercial operations of two 50 MW solar thermal power plants in Cadiz, Spain.

Valle 1 and 2 are parabolic trough plants equipped with thermal storage allowing them to continue producing energy for 7.5 hours at full power capacity without sunlight. Valle 2 is perhaps the most interesting plant as it boasts a SENER single tank molten salt storage system and its innovative trough collector SENERtrough2.

According to Sener, running a single instead of a two tank storage system of cold salts and hot salts, with an insulation barrier will lead to a 25% reduction in storage costs because of a reduction in thermal losses.

The tank is permanently full with a similar capacity to that of a dual-tank system. The pumps are less expensive as they require short shafts which are much more common than the alternative.

US firm Bell Independent Power is also working on a single-tank system for Tucson Electric Power. Expect to hear something from them soon.

The concept and the potential results from the single tank sound impressive but there is always a drawback and this one is that a thermocline storage system must maintain its thermocline zone in the tank. If not, according to the National Renewable Energy Laboratory, it will expand to occupy the entire tank.

According to Sun Lab it is still an unproven concept and there could be safety issues regarding fuel (thermoline) next to an oxidizer – nitrate salt.

Naturally inspired

More innovation is being promised by the Massachusetts Institute of  Technology. Researchers there, in collaboration with RWTH Aachen University in Germany, have come up with a CSP plant design that reduces the amount of building land required whilst increasing the amount of sunlight its mirrors collect.

The template for analysis was the PS10 plant near Seville with 600 mirrors arranged around the central tower in concentric circles. According to MIT the spacing between the PS10 mirrors is “similar to the seats in a movie theatre, staggered so that every other row is aligned”. This, it claims, results in higher than necessary shadowing and blocking throughout the day, reducing the reflection of light from mirrors to the tower.

The answer according to Alexander Mitsos, Rockwell International Assistant Professor of Mechanical Engineering is indebted to the simple sunflower with the mirrors arranged in a similar pattern to the spirals on the face of the plant, with each one being angled at 137 degrees to its neighbour.

MIT says the innovation reduces the footprint of the plant by 20 per cent, minimises mirror shading and blocking and ramps up production. The company says: “If we’re talking about going to 100 per cent or even 10 per cent renewable, we will need huge areas, so we better use them efficiently.”

Mitsos adds: “You will get these results in any plant not just PS10. We would love this to be tested out operationally and confirmed.”

MIT, which has filed for patent protection, is currently talking to CSP plant operators with the possibility of a trial being announced sometime later this year.

Direct steam generation

Direct steam generation in parabolic troughs is another game changer being led by Germany’s Solarlite.

Solarlite chalked up an industry first when its direct steam generating TSE1 5 MW plant in Thailand came online this year. TSE1 is the first of 15 plants to be built with joint-venture partner Thai Solar Energy and it started producing power on January 25.

The plant uses solar radiation to heat water directly in an absorber tube, which produces the steam required to run the turbine. There is no need for the more commonly used thermal oil as a heat transfer fluid or heat exchangers.

The biggest advantage of this, according to Solarlite, is that operating temperatures of up to 500 degrees Celsius can be achieved, compared to the 395 degrees Celsius possible with thermal oil. This give the plant the potential to increase efficiency by as much as 25%.

“You are using cheaper equipment and fuel. Those are some of the cost advantages that have been realised so far,” says CFO Moritz von Plate. “Another plus is the environmental friendliness of water compared to oil. It is not hazardous like oil and not flammable. In countries such as Thailand you can’t build these oil plants because of fire risks.”

Fresh thinking needed

Eduardo Zarza Moya of CIEMAT has long railed about the ‘conservative’ plant designs and ‘little degree of innovation’ in CSP.

“Most CSP plants have a configuration very similar to that of the last SEGS plants installed in California in 1990-1991,” he states. “The solar collectors have the same parabola width, focal distance, receiver tube size and working fluid. The only major difference is the two-tank molten-salt thermal storage system. So the promoters are not taking any significant technical risk. There is much room for innovation and improvements.”

His call for a more significant R&D effort is clear. The four areas of importance, he outlines, are cost reduction, improved efficiency, better dispatchability and environmental sustainability.

His highest R&D ‘priorities’ in the near future are new cooling systems with lower water consumption, Stirling engines with better reliability and lower maintenance costs, new cheaper and more durable receivers for parabolic trough and tower plants and the development of new thermal storage systems.

It’s been an impressively productive start to the year but more needs to be done to continue the commercial deployment of CSP.

To respond to this article, please write to the Editor:

Rikki Stancich: rstancich@csptoday.com