Ask most electrical contractors about the latest cogenerating technology—microturbines—and you’re likely to get silence, or possibly: “micro-what?” This technology, which has only been on the market a few years (Capstone Turbine’s C30 model was introduced in December 1998), may still be relatively unknown, but is poised to take off in the next decade in commercial, municipal and light industrial use.

These small generators are in use at more than 1,000 public and private facilities in the United States and Japan, largely because of their ability to create energy onsite that is cleaner and cheaper than utility-delivered power.

Microturbines are small combustion turbines that produce between 25kW and 250kW of power. Originally derived from aircraft auxiliary power units, today’s microturbines deliver 25 to 30 percent electrical efficiency and, when the exhaust is used for thermal applications, a total fuel efficiency of 80 percent or more. According to Department of Energy figures, the average fossil-fueled utility power plant is 33 percent efficient, but 5 to 10 points of that is lost just getting it to the end user.

Most, but not all, microturbines are single-stage, radial flow devices with high rotating speeds; 90,000 to 120,000 rpm. As with other gas turbines, fuel is injected, along with air, into a combustion chamber. The fuel-air mixture is ignited, generating gas pressure that spins a turbine extremely fast. The basic design includes a turbine wheel, a compressor wheel and a permanent-magnet generator, all connected to a central shaft. In the most popular models, that shaft is the only moving part in the entire system.

While microturbines can be used at a building for emergency power, their super-low-emission characteristics and design enable them to be operated 8 to 24 hours daily, cutting energy costs no matter what happens on the local power lines.

Capstone was the first to market commercial microturbine power systems. With more than 2,700 Capstone MicroTurbines shipped worldwide, they remain the most prominent maker. Competitors include Ingersoll-Rand (, which has a 70kW microturbine on the market and is working on fielding a 250kW model; and Britain-based Bowman (, which has an 80kW model marketed by Kohler ( in the United States.

These microturbines also produce voluminous amounts exhaust that can reach 600 F—perfect for water heating and other thermal applications. Another nice feature of microturbines is their built-in capability to plug together a dozen or more units for higher capacity arrays. They can also be started, operated and monitored automatically and remotely.

Low maintenance

One advantage of the Capstone MicroTurbines is the absence of any liquid lubrication or coolants. The entire system is air-cooled, and the shaft, which is the only moving part, is supported by air bearings. As a result there is very little required maintenance. In fact, most maintenance involves only annual cleaning/replacement of air and fuel filters and even less frequent cleaning/replacement of minor parts.

Because there is no fluid, concerns about hazardous material handling, storage, replacement, disposal and leakage are eliminated. There is plenty of environmental incentive to use these new generators. For one thing, air pollution emissions of gas turbines are comparatively low. Most models running on propane or natural gas have NOX emissions less than 9 ppm. Carbon monoxide (CO) emissions are similarly low. Other alternative systems such as fuel cells and wind-energy systems have low or even no emissions but are also considerably more expensive than microturbines.

Other than those that use lower-cost microturbine energy to slash their utility bills, some other companies that have an interest in microturbines include businesses that require highly reliable power or dependable standby power, such as manufacturing and healthcare facilities. Numerous other facilities are showing an interest in cogeneration, such as hotels, retirement homes, supermarkets, manufacturing plants and office buildings with absorption cooling or dehumidification systems. Also, some utility companies use microturbines to boost localized generation capacity and on more remote grids. A couple hundred are even turning flare gases at landfills and sewage plants around the nation into near-zero-emission power and heat.

The installation of a microturbine itself is not much different than any gas generator. The variation often lies in the paperwork.

“The installation is rather simple,” said John Fielder, construction consultant for Calpwr ( and a former electrical contractor. “The challenge is integrating the output into the customer’s electrical system.”

Connecting to the utility grid can often mean working around a variety of regulations that vary from state to state. The greatest challenge for the contractor is finding an efficient way to connect the output back into the system within the utility regulations. Some state energy commissions, such as those in California, New York and Texas, have set up guidelines to keep utility regulations regarding microturbine installation uniform. “There’s not a lot of interest with the utilities to standardize requirements,” Fielder said.

“They have a circle-the-wagons mentality,” agreed Calpwr lead engineer Doug Price. He added that installing microturbines requires some experience working with the technology and with the utility companies. “After a couple of projects it goes very cleanly. But you have to learn it.” He added that the Capstone systems Calpwr installs are precertified to the statewide standards and the company offers technical support and can bring in a consultant for difficult installations.

Varying concerns

Regulations are necessary to prevent damage to equipment and ensure safety. But many of the 3,000 different power utilities in North America have their own rules and regulations about interconnection that predate inverter-based microturbines and other modern marvels of distributed generation.

Because of this, they generally insist that each of them be able to test the protective relaying functionality and other safety and operational characteristics. Most microturbines have all this safety and operational flexibility built in, but short of the UL 1741 interconnection standard, there is no existing nationwide set of rules and regulations.

There are various other concerns related to how and where to install microturbine units. Some units have no locks on them; therefore, it is generally recommended that they be enclosed in a secured environment to protect individuals from injury as well as protect the equipment from vandalism.

Some utilities may require a visible disconnect switch (despite redundant internal protective relays) in order to protect linemen and avoid any possibility of backfeed into a line during a grid outage. This would require that utility personnel have access to the microturbine. These are just a few rules that customers need to consider when dealing with the utility regulations. Noise levels, air permitting, gas and electric codes, and whether the unit will be used as a backup source are other factors.

The Kaiser Permanente medical office building in Elk Grove, Calif. recently installed a microturbine. The building now has the lowest total energy costs of any Kaiser facility in the nation. The cogeneration system uses natural gas fueled microturbines to help Kaiser maintain low, stable energy costs and, in the event of a grid outage, to provide emergency electrical back-up for refrigeration, operational and communications needs.

“The rolling blackouts of the 1990s started us looking for an emergency back-up system that could keep critical equipment running,” said Kevin Long, energy manager for Kaiser Permanente in a Calpwr press release. “Cogeneration offered us that, plus an opportunity to take control of our energy future while reducing costs.”

The Kaiser Permanente cogeneration plant is expected to reduce utility grid electricity consumption by at least two-thirds. Heat recovery from the cogeneration plant will provide all of the building’s heating and hot water needs, further reducing its operational costs. Total fuel efficiency of the array exceeds 70 percent.

Calpwr deployed four clean-burning Capstone MicroTurbines at Kaiser to create redundant reliability as well as low emissions and operating flexibility. And cooling capability is another benefit of the microturbine units. Ultimately the cogeneration system will add absorption cooling to produce up to 70 percent of the building’s cooling needs. Absorption cooling systems use exhaust heat energy, rather than electric energy, to create chilled water for air conditioning.

About 30 percent of the project cost will be rebated by the Sacramento Municipal Utility District as part of a statewide program. EC

SWEDBERG is a freelance writer based in western Washington. She can be reached at