Solar dishes occupy the middle ground of concentrating solar thermal power, sandwiched between lower-cost, lower-efficiency parabolic troughs and Fresnel reflectors – and higher-cost, higher-efficiency solar towers.

Whether this middle ground ends up as a commercial cul-de-sac or a sweet spot, no one knows.

How do they work?

Solar dish systems consist of a dish-shaped concentrator (like a satellite dish) that reflects solar radiation onto a receiver mounted at the focal point. They consist of clusters of small mirrors set into modular, circular arrays to pinpoint solar energy onto a receiver situated above each dish. The receiver may be a Stirling engine and generator (dish/engine systems) or it may be a type of solar PV panel that has been designed to withstand high temperatures. The Stirling engine uses heat to vary the pressure inside a hydrogen-filled sealed chamber. This drives pistons to produce mechanical power.

Dish systems can often achieve higher efficiencies than parabolic trough systems partly because of the higher level of solar concentration at the focal point. Dish systems are sometimes said to be more suitable for stand-alone small power systems due to their modularity but there is no reason why they should not be installed in large numbers in desert regions where they could generate large amounts of electricity.

Compared with ordinary PV panels, CPV has the advantage that smaller areas of PV are needed and, since PV is still relatively expensive, this can mean useful savings in costs.

Unlike thermal CSP systems (parabolic trough, Fresnel mirror, or power tower), dish/engine systems and CPV systems do not lend themselves very well to the storage of solar energy in the form of heat, and they are not well suited to hybridisation with gas firing. This means they are less able to provide dispatchable power – unless or until there are methods for storing electricity that can compete in price with the relative cheapness of storing solar heat.

Are they commercially viable? – and who are the players?

Two major deployments are now underway in solar thermal dishes. One represents the world's biggest financial bet to date on very large scale concentrating solar thermal power. The other is an attempt to become the first CSP plant to provide 24-hour power through energy storage using ammonia.

In California, Arizona-based Stirling Energy Systems Inc. plans to install an initial capacity of 20,000, 11.5 m solar dishes totalling 500 MW, in the Imperial Valley east of San Diego. In subsequent phases, Stirling could ramp the project up to 1750 MW. If it does, it would involve up to 70,000, 12 m diameter, 90 m² solar dishes.

This would put in train huge economies of scale, potentially causing the price of each dish to fall 80% from its current price of US$225,000 – to as little as US$50,000. Such economies of scale in manufacturing, coupled with larger overall solar project sizes, ongoing research and development and increased experience in deployment and operation, represent the holy grail in solar thermal. Why? Because together, they create a circle of interlocking downward price synergies that could speed the technology's arrival at ‘grid parity’ – or a price equal to coal – in a few years.

Or so the proponents say.

At present, Stirling Energy Systems’ massive California plant project holds bragging rights as the world's largest planned CSP plant. It's also the most expensive, estimated at US$1 billion. The company's credibility is underpinned by a 20-year power purchase agreement (PPA) with regional utility San Diego Gas & Electric. However, the company has yet to file official permitting papers with the state of California, even though it has stated it plans to begin construction in 2009.

Nonetheless, the company has also been racking up impressive scientific milestones. In March 2008, Stirling, in conjunction with Sandia National Laboratories, achieved a new world record of solar-to-grid system conversion efficiency of 31.25%, significantly beating the previous record set in 1984 – 29.4%. And Neither Stirling, Sandia nor anyone else believes this will be the end of this upward efficiency climb for solar.

Big money has taken notice. In April, Ireland-based renewable energy project developer NTR plc sank US$100 million into Stirling, in exchange for a majority stake, making Stirling a player to watch.

The other main player to have emerged to date in solar dishes is Australia-based Wizard Power, which is commercialising solar thermal dish technology under development at the Australian National University for 20 years. Wizard is definitely thinking big, with dishes more than five times th e size of Stirling's.

In early June 2008, Wizard laid concrete foundations for a 71 m diameter, 500 m² dish, outside the rust belt Outback town of Whyalla, South Australia. Combining traditional solar thermal power with an innovative, ammonia based thermal storage system, Wizard hopes to be able to demonstrate the ability to produce power 24 hours a day as early as the end of 2009. Everything about this particular project is big. The dish itself is roughly the same height as a two story house, and Wizard claims its big dish can concentrate the sun nearly 1500 times to create temperatures as high as 1200ºC.

A third major player in the solar dish field is Washington-state based newcomer Infinia Solar Systems, which recently raised US$50 million from investors, including venture capitalists such as Khosla Ventures and Idealab. Infinia's aim is to generate electricity through smaller, 8 m diameter dishes. It isn’t nearly as far advanced as Wizard Power or Stirling Energy Systems, and has announced no specific project deployments as of yet, but says it is involved in talks with potential backers and customers for plants ranging in size from 10 MW-150 MW. Like Stirling, Infinia has been testing prototypes at Sandia National Laboratories.

The European Union has dipped its toe into solar dish research, with dish technology trialled at the EU's major solar thermal research site at Almeria, Spain – alongside troughs and towers. But at present dishes appear to be a technology without a commercial dance partner in the EU, as companies like Abengoa move headlong into developing both parabolic trough and solar tower projects, and companies such as Solar Millennium move ahead in developing parabolic trough projects.

Even so, with three commercial players in the market stretched across two continents and with two of them well advanced towards developing real projects, the market should get some indication of the technical staying power of this technology as early as 2011. If Stirling can execute on its huge California solar farm, or Wizard Power can prove its big solar dish can deliver power (and the ammonia can store it), this middle position technology may yet prove a dark horse.

Two other things make solar dishes worth watching, and both involve the flexible, modular nature of the technology compared to troughs or towers.

The first intriguing element is the theoretical possibility that backyard or rooftop units no larger than, say, a satellite television dish, could come on the market for decentralised applications such as providing electricity to individual homes. Small scale, self contained units would also be easier and faster to deploy than long-lead time coal-fired power or natural gas plants, or even trough or tower plants.

With assembly-line units that can be installed within hours, solar dishes hold out the possibility of immense flexibility in meeting the needs of evolving grids at much shorter notice than big infrastructure projects like natural gas, coal or oil.

Taken further, solar dishes have often been mentioned as an ideal offgrid source of power for small communities, although energy storage still presents a hurdle. And lastly, solar dishes don’t require the spaciously-contiguous, absolutely-level ground that are prerequisites for parabolic trough or tower constructions. That makes them more attractive for areas like hillsides or where there is uneven terrain – which may have few other uses.

The second intriguing element has even greater potential. Boosting the efficiency of solar PV. While traditional solar PV panels have been on an inexorable rise upwards in efficiency, solar dishes can help raise efficiency in solar PV panels by another means; increasing the number of “suns” pointed at them.

By using dishes to focus more sunlight onto traditional solar PV panels, more electricity can be yielded from each solar panel. There are hurdles, however. There's a limit to how large a solar panel concentrating sunlight can be directed at. And traditional solar PV panels already suffer from overheating under strong sun. Multiplying that number of suns tenfold would require very robust solar PV cells.

Among companies to watch in the CPV area are Australia's Solar Systems and smaller companies such as SUNRGI.

For those seeking to handicap solar dishes, therefore, the first to watch is Stirling Energy System's progress toward building out its California plant. The second thing to watch is Australia's Wizard Power, with its potentially ground-breaking “big dish” and ammonia energy storage. The third thing to watch is industry newcomer Infinia, as well as any other companies still in stealth mode who decide to break cover in the coming months and emerge into the sunshine, so to speak.

Lastly, keep an eye on CPV. There's a long way to run in that market.