Last year we wrote an article comparing the four main solar thermal technologies: towers, troughs, Stirling dishes and flat plate reflectors.

Those four concepts still lead the industry and it has evolved pretty much as most have predicted. Companies with towers and heliostats such as BrightSource Energy and eSolar lined up a number of deals in the past year although they've had to face questions about environmental impact and financing.

Stirling system advocates have signed fewer deals in the past year, but have continued to make progress. Stirling Solar recently planted 60 of its systems on a 1.5 megawatt power plant. Meanwhile, Ausra, one of the flat plate leaders, experienced a somewhat moribund 2009. Then early this year it got bought recently by Areva.

Since then, a whole host of new ideas for converting the heat from the sun have begun to percolate up and in part it's a result of the way solar thermal functions. Photovoltaic panels are semiconductors: progress thus is controlled by device physics. Solar thermal systems are inherently mechanical. The variety is endless. Most of these are still in the experimental stage, but they bear watching. Like last year, solar thermal will be one of the primary topics at our 2010 Solar Summit taking place March 30 and 31 in Phoenix.

The 1,000 Degree Wind. HelioFocus, which grew out of research at Israel's Weizmann Institute, has created a six-story-high parabolic dish that will concentrate the sun's energy onto an optical receiver at its center. The proprietary receiver in turn converts the light into a stream of hot air that can reach 1,000 degrees Celsius. The hot air then gets funneled through a gas turbine rejiggered for solar power.

"We take a gas generator and solarize it," said Sass Somekh, one of the founders and a partner at Musea Ventures. Musea and Israel Green Corp invested $20 million in the company in 2008.

Pro: The hotter the temperatures, the better. Con: One can imagine there will be questions about material fatigue at those levels.

The Hot Ground. Researchers at the Masdar Institute, the Tokyo Institute of Technology and Cosmo Oil have begun to test ground-based heliostats -- i.e., flat mirrors -- that concentrate heat from the sun and aim it at a tower. So far, that's similar to the heliostat architectures promoted by BrightSource Energy and eSolar. But a tank sits atop the towers from BrightSource and eSolar. In Masdar's system, the tower holds another set of mirrors. These mirrors then take the heat from the heliostats and reflect it to a liquid-filled tank on the ground directly under the tower. Operating temperatures for the prototype come to around 500 Celsius, which is lower than BrightSource, but higher than others. Appropriately, it's called the beam-down project.

The extra set of mirrors reduces the efficiency to around 15 to 19 percent. Classic solar thermal systems are 20 plus percent efficient. But with the extra set of mirrors, fluids don't have to be pumped up into a tower, thereby reducing energy and cost requirements, according to Mateo Chiesa, a professor at the Masdar Institute.

Pro: It does get rid of one part of the process. Con: The gains may be offset by the loss in efficiency.

Flowers and Calcium. Solar Fusion Power wants to build heliostats that are shaped somewhat like flowers. The heat from the collectors is then beamed to a quartz lens that in turn focuses the heat on a mirror right above it. The small mirror then directs heat to a mirrored hemisphere on a tower that beams it toward a collector on the ground. That heat can then be directed to a steam tank.

"The down beam is about a megawatt," said Wayne Bliesner, the inventor.

But wait! There's more. The heat passes into a chamber of liquid calcium simmering at 800 degrees Celsius. Hydrogen is mixed in and a self-sustaining chemical reaction ensues. Heat from the reaction is then used to run a Stirling engine. It can also be stored. The company will try to show off a prototype later this year.

If the system can be proven to work, it could raise the bar for efficiency for solar thermal systems. Most advanced solar thermal systems can convert 20-percent-plus of the energy they harvest into electricity. Solar Fusion claims it can approach the 50-percent mark.

Pro: Interesting. Con: This ornate architecture is about as complex as they come.

The Pragmatic Approach. Raw Solar, a startup created by MIT students, says it can make thermal collectors from easily obtainable, off-the-shelf sorts of parts. The collectors could be deployed individually to provide hot water to homes or could be erected in arrays and linked to a generator for electricity. A target market could be the emerging world and those communities that may not have the spare land for massive solar thermal parks. (Promethean Power, also out of MIT, is working on PV systems that produce heat for the emerging world.).

Pro: It's a big market. Con: The details are still being hammered out.

Aluminum. Alcoa has devised a reflector for parabolic systems out of aluminum instead of glass. SkyFuel has been touting a thin reflective material to replace mirrors that it says could take 35 percent out of the cost of reflectors. While SkyFuel's might be cheaper, aluminum has an advantage in durability. Alcoa has also been around longer than many utilities, which will give banks  sense of security. Skyline Solar makes similar metal reflectors for the PV industry. Skyline stamps them out at an auto body plant in Mexico.

Pro: Not a lot to dislike considering that it works. Con: reflectivity tests will be crucial.

Salt in the Veins. Thermal companies now use large tanks of molten salt to store heat at night but use different liquids to take the heat from the concentrators to create steam during the day. Tyco Flow Control has come up with a way to replace therminol, the oil in the tubes in parabolic solar thermal for transferring heat now, with molten salt.

By deploying salt as the heat transfer mechanism inside the pipes of parabolic solar thermal parks, the efficiency of solar thermal power plants could inch up incrementally, because molten salt retains heat longer than therminol. This approach may also help parabolic solar technology, the reigning but aging standard in solar thermal, better compete against heliostats and some of the other new solar thermal architectures.

Iberdrola is already experimenting with this and Tyco is receiving orders from other developers, says Frank Gilhooly, Director of Global Sales and Marketing for the Power Business Unit of Tyco Flow Control.

Pro: A novel, simple idea that extends trough. Con: Have heliostats and Stirlings already won?

Hot Rock. The German Aerospace Center (DLR) is looking at snaking pipes filled with heated liquid through concrete to store heat for exploitation after sundown. Others, such as Elcal Research, are tinkering with phase change materials, i.e., materials that can go from solid to liquid easily by adding or removing heat.

Pro: Everyone loves new storage ideas. Con: Testing data needed.

Solar Air Conditioning. The concept is tough to wrap your head around, but accept that it works. Heat is collected on a rooftop and injected into a heat exchanger. The heat gets used to boil a refrigerant and the liquid, now chilled with the refrigerant gone, cools buildings. Air conditioning accounts for fifty percent of the demand for power during peak periods in California, according to Peter Le Lievre, founder of Chromasun, one of the leading startups in this space. Also look out for Linum Systems out of Israel.

Pro: It's a great way to knock back peak power. Con: Building owners may not have the money to upgrade. Utilities may have to fund.

Fast Glass. SHEC Energy started out trying to exploit the sun to make hydrogen. Now, it has a reflector dish that can withstand 800 Celsius temperatures. It also has a process that it says is 30 times faster than conventional glass forming technologies.  In other words: reflectors in an instant.   

Pro: It hits all of the pain points, and achieves high temperatures. Con: the company seems to be biting off a lot for a 15-employee outfit. Similar in some regards to Solar Fusion.


Tags: air conditioning, ausra, brightsource, chromasun, elcal, energy storage, esolar, heliofocus, iberdrola, masdar, phase change, photovoltaic, raw solar, solar thermal, solar.