Capstone Turbine Corporation (www.microturbine.com) is the only producer of a fast, lightweight, high-power, reliable, oil-free device of a type that one might see operating on a space station. Although its exhaust stream is sometimes said be cleaner than the air in Los Angeles, where Capstone has its headquarters, the microturbine is still a combustion device, and its emissions would have to be cleaned up to make the units serviceable in a closed environment. The carbon dioxide in the exhaust could be used to enhanced agricultural production, as is sometimes done with hydroponics at present. Excess carbon dioxide and water vapor could enter a new process stream to make methane and oxygen, for example, using solar energy and a chemical catalyst. The catalyst, defined as a material which assists a chemical process but is not consumed by it, would not have to be replaced. It should not worry the reader too much that the process described here has deficiencies. The journey is the thing, and we have made a start.
Today’s cost of the microturbines themselves is about $1000 per rated kilowatt. Thirty-kilowatt and 65-kilowatt models are currently available. A 200-kilowatt device is in prototype. Units can be ganged together to produce as much power as needed.
In an application that is analogous to what we would need off-world to recycle waste and produce needed heat and power, an anaerobic digester at a Colorado pig farm produces fuel for power and heat from sewage.
The natural gas product is directed to a bank of microturbines which can run continuously for five years without oil -- Capstone microturbines have air bearings -- and without maintenance of any kind except for an annual change of air filter. The microturbines produce power with much higher thermodynamic efficiency than a central power plant (80% vs. 33%), providing the waste heat is used directly for process or environmental control, or indirectly to run an absorption refrigerator or air conditioner.
A similar application at Lopez Canyon landfill, California, uses landfill gas to power fifty 30-kilowatt microturbines. Instead of flaring the gas, the facility feeds the local grid, earning cash for the landfill. Burning this trash gas is a nontrivial challenge. Biogas is low in methane, maybe 67%, sometimes as low as 35% burnable fuel. The rest is water vapor, carbon dioxide, and various pollutants. Treatment to improve the gas quality is necessary.
Even so, microturbines operated with low-quality gas from biological sources have limitations. For example, some folks at Capstone say they see a great market in Asia (lots of biomass, little infrastructure, and increasing amounts of American cash) if their equipment were capable of “black starts” without power from the grid. (The machines come with battery power sufficient for self-starting provided higher-BTU fuel is available.) Ditto for “turndown” capability, the ability to run at lower speeds to adjust power delivery to the demand. Without a grid, there’s no place to dump the excess. Again, a better grade of fuel is required.