Probability plays a big part in climate change – in determining whether it is caused by anthropogenic greenhouse emissions, or how much we need to reduce these emissions to stop the global warming trigger. Now, the editors of Scientific American have applied the rules of probability to cleantech; weighing the odds that seven unlikely, "radical" energy technology solutions will succeed, and how significantly they could improve energy security and efficiency.

From lasers that coax electricity out of spent nuclear fuel, to alloys that keep rooms cool and food cold, the projects profiled in the Scientific American's May 2011 edition are, as the magazine's editors note, “leading examples of the payoffs that are possible – if, of course, the inventors manage to over­come daunting hurdles to bringing practical, mass-produced and affordable technologies to fruition.” The editors give each of the seven long-shot technologies a "handicap," derived from ratings (on a scale of 1 to 5) of "likelihood of succeeding commercially" and "potential impact on energy supply or use." So, starting with the aforementioned lasers, here's a snapshot of each of the seven radical clean technologies.

Fusion triggered fission: Scientists at California's National Ignition Facility have a new twist on fusion reactions: using fusion to drive fission. How does it work? "Laser pulses produce fusion explosions at the center of a reaction chamber, emitting neutrons that split atoms in a thick blanket of uranium or other fuel lining the chamber’s walls. Energy from these fissioning atoms would multiply the chamber’s power output by a factor of four or more," says SA's Graham Collins. This method eliminates the need to sustain a chain reaction and broadens the menu of possible fuels to enriched uranium, depleted uranium (a waste product of uranium enrichment) and spent fuels. Fusion-fission plants could also achieve "90 per cent burn," thus possibly needing only 5 per cent of the fuel of a typical fission reactor. Can it be done? Collins gives this technology a 1 in 5 likelihood of succeeding – due to various "daunting challenges" such as "an array of unproved technology". But should it succeed, it rates 4 out of 5 for potential impact.

Solar gasoline: The number two long-shot technology is an "intriguing effort" from America's Sandia National Laboratories, which starts with a six-meter-wide dish in the New Mexico desert and – using a complicated process involving mirrors, the sun’s rays, a half-meter long beer keg-shaped machine containing a dozen rotating concentric rings with rusty teeth, extreme heat, steam and CO2 – results in a mixture called synthesis gas, or syngas, the basic building block for fossil fuels, chemicals, and even plastics, says Scientific American's David Biello. "The process could also absorb as much CO2 as is emitted when the fuel is burned," Biello says, and has been described by Arun Majumdar, director of Sandia's advanced research projects agency (energy) as “like killing four birds with one stone," in the solar fuels quest. Practical problems and cost issues see this technology's likelihood of success rating at 3 out of 5, while its potential impact rating is 4 out of 5.

Quantam photovoltaics: In the quest to improve the efficiency of solar cells, research into semiconductor crystals, or quantum dots, has been an exciting development – with the potential to boost maximum efficiency to above 60 percent (currently it's around 25 per cent). University of Texas chemist Xiaoyang Zhu and his team has upped the ante by turning to quantum dots consisting of a few thousand atoms each, in an attempt find a way to catch hot electrons before they cool, thus potentially doubling maximum efficiency. He is now looking for a way to convert as many hot electrons as possible into current so the conductor itself does not absorb them as heat, says Scientific American's JR Minkel. Best of all, the odds on this technology are pretty good – 4 out of 5 for likelihood of success, and 4.5 out of 5 for potential impact.

Heat engines: Scientists at General Motors are working to capture waste energy from vehicles and power plants – and convert it into mechanical energy, and then electricity – using "exotic materials called shape-memory alloys," says Scientific American's Bijal Trivedi. The first goal is to reuse heat from a car’s exhaust to power aircon or the radio. The stars of the show are the thin strands of nickel-titanium alloy that “remember” certain shapes, and "flip back and forth between two states." By contracting at high temperatures and expanding at cooler temps, the nickel-titanium belt pulls itself around the loop, spinning pulleys, which then turn a shaft that drives a generator. But, while the GM design is straightforward, says Trivedi, it's "demonstrating proof of principal rather than actual hardware." Likelihood of success: 3 out of 5. Potential impact: 3 out of 5.

Shock-wave auto engine: Michigan State University is developing a "wave-disk engine" or "shock-wave engine" with the aim of eliminating pistons, and thus potentially allowing future hybrids to go five times further on a litre of gasoline. According to co-inventor, Norbert Müller, the compact engine is the size of a cooking pot and thus requires much fewer moving parts. All put together, it would mean greater fuel efficiency, reduced CO2 emissions and lower manufacturing costs. "Müller and his team are testing a prototype wave-disk generator on a benchtop in their east Lansing lab," says Scientific American's Steven Ashley, with the aim of demonstrating a working, 25kW engine. Will it lead to practical use in cars? Difficult to say. Likelihood of success: 3 out of 5. Potential Impact: 3.5 out of 5.

Magnetic air conditioners: US-based company Astronautics is designing an air conditioner that aims to cool a space of about 1,000 square feet. The secret weapon is a small, flat disk containing porous wedges made of alloys – a mix of of gadolinium, silicon and germanium the company originally hit upon in conjunction with scientists at the US Department of Energy’s Ames Laboratory – that showed huge magnetocaloric effect at room temperature. Fluid inside the system is heated and cooled by the rotating wedges, and the cooled fluid draws heat from the room, says Scientific American's Charles Choi. And while the unit uses water to transfer heat, and the design could be adapted for refrigerators and freezers, a lot of complexities – and high costs – stand in the way of a successful prototype, says Choi. Likelihood of success: 3.5 out of 5. Potential impact: 3 out of 5.

Clean(er) coal: In the quest to strip CO2 from a coal plant's emissions, the University of Notre Dame’s energy center has come up with a "novel material" called ionic liquid which, says Scientific American's Michael Lemonick, is basically a type of salt. It pulls in twice as much CO2 as other similar carbon scrubbers and, in doing so, changes from solid to liquid, releasing heat that is recycled to help drive the carbon out of the liquid to be disposed of. It's a radical idea, not least of all because the captured carbon then has to be stored somewhere. Still, Lemonick rates the likelihood of success at 3.5 out of 5. Potential impact is less promising, at 2 out of 5.