Researchers at the Australian National University (ANU) in Canberra, have proven that solar generated power is no longer a pipe dream and could pave the way to supplying the world with clean energy.
A Solar Thermal Group led by Associate Professor Keith Lovegrove, has constructed the world’s largest solar collecting dish, and have been fine-tuning its ability to automatically track the sun and maximise the energy collected.
The reflecting parabolic dish – measuring 25 metres in diameter – has 500 square metres of highly efficient purpose-built mirrors, which together collect enough energy to power a four cylinder steam engine driven purely by reflecting the sun’s rays onto a collector coil to convert water to steam.
Building fewer large dishes is more economic than building lots of smaller ones.
Plans are already afoot to build a pilot solar generating plant which will prove out the concept already demonstrated by the existing solar generator dish located on the ANU university campus in Acton, ACT.
At the same time, the team behind ANU’s Solar Generator 4 or ‘SG4’ (so named as it is the fourth design of solar generating system they have constructed – each larger than the one before it) are realistic about their ambitions and see solar energy as complementing, rather than replacing, existing power generation technologies.
Efficient design
Lead researcher, Greg Burgess, also sees other potential applications for super-heated steam produced by reflected solar energy. Such is the intensity of energy generated by the dish’s efficient design that when concentrated it can melt through solid aluminium, stainless steel plate and even the hardest ceramic known to man.
According to Burgess, the aim of the solar project was to demonstrate that solar generated electricity is viable on a commercial scale. To prove this, the researchers needed a dish design that was optimised for cost effective mass production on a large scale.
Their thinking is that building fewer, large dishes which can be replicated in the field, is more economic than building lots of smaller ones.
The genesis of the project started in the early 70s, with a team lead by Stephen Kaneff and Peter Carden, both since retired. Their early work led to the construction of the White Cliffs solar power station in remote Northwest NSW, comprising 14 small 20 square metre diameter dishes.
Convinced the idea had merit, the team at ANU proceeded to scale up the solar generator, with the first "Big Dish" built in 1994 using commercially available spaceframe technology.
The steam drives a small steam engine and generator located in a small enclosure near the base of the dish.
The latest dish, SG4 was completed in June 2009 and since then a great deal of further effort and research by Burgess and others has gone into fine tuning the solar tracking, sourcing computer and communication components, testing optical performance and installing and testing the first steam generating receiver.
Despite a shoestring budget over many years and continued uncertainties over government-sponsored solar programs, SG4 is now a reality and may ultimately leapfrog existing ‘solar trough’ technologies based on its superior efficiency, especially given any favourable moves in public policy to encourage greater uptake of alternate energy technologies.
For example, the special low-iron curved mirrors are 94% reflective, optical and thermal efficiencies are also in the high 90s, which means that overall more than 80% of the collected sunlight is transferred to the superheated steam leaving the receiver.
Purlin style frame
The newest parabolic dish at ANU has also been made from freely available building materials and some custom made rolled sheet steel purlin style frame sections. The mirrors are also flexible enabling them to be bent to fit into the structure of the dish.
Costs have been kept down through clever engineering, including using the mirror panels as an integral part of the structure to provide stiffness with less framework and thereby reducing costs and unnecessary weight. Also, each of the 380 individual mirrors is identical in shape, making them completely interchangeable.
"The cost of everything has to be commercially viable, so we have designed the system to be mass-produced, which will further reduce costs as soon as it is put into mass production," said Lovegrove (pictured alongside, image courtesy ANU).
"For example, we designed and constructed a very accurate dish assembly jig which actually costs as much as a dish itself, so that the framework for hundreds of future dishes can be produced more affordably, making the prospect of solar energy generation more viable."
Just how accurate, is demonstrated by the fact that a 25 metre wide dish built using the special jig matches a perfect parabola to within a millimetre. This means many other identical dishes could be built and installed around the country with predictable results.
Perfect tracking
The large parabolic dish is supported on a two axis tracking system which is continually pointed at they sun. The excellent optics and precise tracking control system means it achieves optimum efficiency at any time of the day and any time of the year.
The entire system is controlled by a PLC SCADA system supplied by Yokogawa which computes precise sun position information to ensure millimetre perfect tracking of the sun.
Simplified piping and instrumentation diagram of the ANU Big Dish Steam system.
This accuracy has been built-into the FA-M3 PLC SCADA control system which has been provided to the university by Yokogawa Australia – a specialist in process control systems used in power stations, oil platforms, solar fields and processing plants.
A water line runs up to the ‘target’ at the top of the collector and a coiled tube within superheats the water to steam. The steam travels back down a steam line and in turn drives a small steam engine and generator located in a small enclosure near the base of the dish; three-phase power is then put back into the grid.
A second FA-M3 is used for the steam turbine control. Yokogawa’s FastTools software is also used for data logging and has the capability to handle additional automation such as putting the dish in a ‘parked’ position at night and to prevent damage in times of high winds.
However, ANU now plans to transition from the FA-M3 to the new Yokogawa HSX10 for even greater efficiency.
Future units could be cost-effectively controlled with the new HSX10, which is designed to automatically track the sun to ensure maximum efficiency of the solar reflecting dish.
Energy storage
Unlike wind and photovoltaic solar systems – which only produce energy when the sun shines or wind blows – overseas experience shows concentrating solar thermal power systems have the potential for energy storage using molten potassium nitrate or similar salt ‘thermal batteries’.
This would add to solar’s current ability to provide peak demand during daylight hours.
Greg Burgess: The aim of the solar project is to demonstrate that solar generated electricity is viable on a commercial scale.
"Even now the steam produced from the solar generator can drive the steam turbine for half an hour after the dish goes ‘off-sun’, just from the steam in the pipes. Adding storage would add to the flexibility of solar as an energy source," Lovegrove said.
Best of all, the technology can be easily integrated with existing coal-fired power stations as it produces steam – and could provide a ‘solar boost’ to the existing water-steam stream of the electricity generation cycle, according to Managing Director of Yokogawa Australia and New Zealand, John Hewitt.
"The change to the power station would be largely limited to the front-end, which makes the prospect of ramping up with solar-supplemented power more plausible in the real world.
Best of all solar power is produced during the bulk of peak demand – during daylight hours when business and commerce is operating and home air conditioners are switched on," Hewitt said.
Paying for itself
Lovegrove explained that although commercial scale concentrating solar plants would represent a significant up-front capital investment, the technology pays for itself over a lifetime of use, unlike conventional technologies, which had high operating costs over their lifetime, such as coal extraction, haulage and boiler maintenance costs.
And because the ANU prototype is far more efficient than existing solar arrays in operation in parts of Europe (notably Spain) and the US, ANU’s design could help speed up the adoption of solar as a viable energy supplement to fossil fuelled plants.
According to Lovegrove, typical ‘trough’ type solar fields produce about 50 times the concentration of normal sunlight onto a water tube to produce steam. "Our dish produces 2,000 times the concentration of the incident solar rays which gives you some idea of its efficiency."
"People don’t realise how ‘real’ the technology is, and haven’t latched onto the idea of concentrated power yet as they are more used to the more established photovoltaic ‘solar’ power which has been around for a long time.
"As an example, in a large system with an array of dishes all feeding steam to a single efficient large steam turbine, each dish of this size would contribute the production of around 100 kW when operating at full efficiency in full sunlight.
Overseas experience has shown this is enough energy to power 50 homes and Lovegrove believes that in the future up to 100 advanced energy-efficient homes could be powered by every dish. A solar field the size of the ACT could power the entire country," Lovegrove said.
Smaller plants
In addition to feeding into grid power, Lovegrove sees an opportunity for smaller concentrating solar plants in remote regions to power mines and provide energy to minerals processing plants and communities. For some years, a small scale solar dish field operated in White Cliffs to provide power to the small opal mining town.
"As a clean energy source, solar energy is a natural complement to wind and based on our climate is more available and provides most of the energy when you need it.
"And from a practical perspective it could be easily married to coal-fired power stations and provide raw energy to other emerging technologies such as gasification.
"We see concentrated solar technology as the ideal transition from smaller diesel-fired power plants, with a comparatively small 100-dish solar field providing around 10 MW of power.
"The medium term goal for the concentrating solar technologies is to make electricity for about the same cost as wind, which is currently around 10-12 cents a kilowatt/hour.
While that’s around double the cost of conventional coal-fired power stations, it may prove to be cheaper than adding carbon capture and storage to existing coal-fired power stations," Lovegrove said.
Of course, the ultimate energy economics will be influenced by ruling government policy, with any moves to tax carbon impacting on fossil fuel power stations and tipping the economics more in favour of alternate energy sources in the future.