Thorium: Next-generation generator?

Sept. 12, 2011
A thorium-powered generator set could be the next cheap and compact means of generating ac power.

At least one potential competitor was not allowed to compete in the recently concluded 100 mpg Automotive X-Prize competition. Charles Stevens, an inventor and entrepreneur, was ready to enter a car powered by thorium. After all, a single gram of thorium has as much energy as 7,500 gallons of gasoline. But Stevens says X-Prize officials figured there were several issues associated with a thorium-powered car that would be problematic. Among other things, "They insisted on providing fuel for all competitors to head off any possibility of putting additives in gasoline or diesel fuel, but they weren't sure how they would provide our thorium," he says.

Stevens' Massachusetts-based R&D firm, Laser Power Systems (LPS), is developing an electric generator powered by a thorium-based laser. The thorium laser is used as a heat scorce rather than light, heating water to generate steam for a turbine that drives an electric generator. Stevens' sees transportation applications as the "ultimate expression" of his thorium-based generator, but the first commercial use for the concept will probably be as power sources that hook up to the grid. Stevens' says a 2.5-MW thorium generator for powering data centers, hotels, and similar facilities would be about the size of a large refrigerator. A 500-kW unit for smaller uses would have a footprint comparable to that of a residential A/C condensing unit.

The idea of a thorium-powered generator has several factors going for it. Thorium is only mildly reactive and is as abundant as lead. It is non fissionable and can not sustain a chain reaction, so run-away melt-down scenarios for thorium reactors are highly unlikely. Several entities -- including TerraPower LLC funded by Microsoft billionaire Paul Allen --have expressed interest in fielding nuclear reactors based on thorium.

But Stevens' design, as he explains it, generates heat through a reaction characterized by alpha-beta particle decay rather than neutron decay as happens in nuclear plants. Ordinary aluminum foil can serve as a shielding media. Key to the device's' operation is a method of accelerating the alpha-beta decay which is proprietary.

The use of radioactive materials in lasers is not new, however. One of the first demonstrations of lasers in 1960 used a flashlamp to excite a crystal of uranium-doped calcium fluoride. The concept didn't catch on partly because the laser-gain medium got hot enough to require a liquid-helium cooling system.

Stevens' thorium-based heat amplifier system is integrated with a steam turbine inspired by the Tesla turbine. In a simple Tesla turbine, nozzles apply a moving gas to the edge of a set of smooth disks, which in turn move by means of viscosity and the adhesion of the surface layer of the gas. But Stevens says his turbine design goes far beyond Tesla's concept.

"Tesla's turbine had horrible low-speed torque. We've found a way to vary the turbine geometry that lets us not only get good low-end torque, but also have torque curves that are highly parallel to the horsepower curve. So unlike gas engines that peak out at high speeds, we can get almost as much torque as we get horsepower at any given rpm. This makes the design ideal for running high-speed generators," he says.

Laser Power Systems has built a prototype turbine producing 250 hp which occupies a 12x12x16-in footprint. The rotor in the device has a 5-in diameter. Another device now on the drawing board has a 6-in rotor and should produce about twice as much horsepower.

Interestingly, the toughest part of the power source design is turning out to be associated with the generator itself. It must work well at the turbine’s high rotational speeds and Stevens hasn’t found an off-the-shelf generator that fits the bill. He is now designing a radial flux switched-reluctance generator that runs at the same speed as the turbine. Among its design features are NASA-designed gas foil bearings on the rotor that don’t wear once rotational speed hits about 1,500 rpm.

Stevens says he has investment partners that are manufacturing parts for the turbine and lasers. He also says he is negotiating with investors to build a production plant for the device in Georgia. Several countries have expressed interest in the device, Stevens says.

GE did an online feature on Stevens' design:

Laser Power Systems:


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