A microturbine is a sort of jet-airplane engine in which compressed gas is injected into a high-pressure air stream in order to produce electricity instead of thrust. It's an emerging method of what's called cogeneration. "By producing both electric and thermal energy at the same time, cogeneration technology produces more energy from a single fuel source," says Bruce Beckwith, director of sales and projects for RSP Systems, a distributor of microturbines.

Such cogeneration, by emitting low levels of smog-producing nitrogen oxide and by pproviding two things at once with the same fuel, can help protect the environment. It can also put a dent in your building's energy costs — particularly with the help of state subsidies: Over the past eight years, the New York State Energy Research and Development Authority (NYSERDA) has offered incentives for about 100 cogeneration projects. About two dozen involve microturbines, though only a handful of those are at residential buildings.

Killo-what?

Microturbines get a bad rap from some who argue that conventional engines are more efficient, converting about 35 to 40 percent of the fuel into electricity as compared to 20 percent with turbines. (In both cases, the remainder gets released as heat.) Also, because of Con Ed rules, microturbines can't feed power back into Con Ed's electrical grid, so any power you create but don't is essentially wasted.

Because of that, where microturbines are concerned, smaller is sometimes better. Dana Levy, NYSERDA's program manager for industrial research, cites a hypothetical 300-unit building that uses 300 kilowatts (kW) of power during a peak-use period and 100 kW during a low-demand one. Levy says in such a case, he would suggest a 100 kW microturbine system that could "chug along at a constant speed day and night." Steve Stone, president of DSM Engineering Associates, adds that a 100 or 150 kW system in such a property would generate about 30 to 50 percent of the building's electrical needs and potentially all of its heat for hot water.

Buying and installing of a microturbine can cost between $3,000 and $4,000 per kW. In the 300-unit building, that would mean a $300,000 to $400,000 price tag. Beckwith estimates a three-and-a-half-year return on such an investment; Stone guesses about five years.

And there are subsidies. NYSERDA's Levy says that his agency aims to distribute about $15 million for microturbines and other cogeneration projects in the next few years. NYSERDA's cash incentives for microturbine projects in Con Edison territory start at 40 percent of the project's cost, and bumps up to 50 percent if the system is in a building that can serve as a safe haven to others in a disaster, or if it is installed in a way that it can supply power to the building without interruption if the electrical grid goes down.

Another positive factor: New York City recently changed its regulations governing the use of the equipment. The country's first set of standards for microturbines, according to Mayor Michael R. Bloomberg, they govern the size and placement of the systems, the types of materials used, and the types of permits and inspections needed.

"Through the new rule, we have set a standard for the safe use and installation of microturbine systems in residential and commercial buildings where no standard existed before," says DOB press secretary Kate Lindquist. Moreover, since they require that microturbines with external gas compressors have "materials and equipment acceptance numbers" from the city, this means that such equipment has been pre-approved, which streamlines the applications process with the Department of Buildings (DOB). In all, Stone adds, the DOB "has not been a problem in getting approvals."

Image above: Capstone's model C30 MicroTurbine provides up to to 30 kW of power and 85 kW of heat.