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by Chuck Tanner
Applications Marketing
Capstone Turbine Corporation

(originally published in the Cogeneration and Competitive Power Journal. For subscription information, call (770) 925-9388)


When a new 2 MW steam turbine was installed 1900 in Hartford, it represented a step function change. It was four times bigger than any existing steam turbine.

From then on economy of scale meant bigger and bigger. By the end of the 1970s and largely driven by nuclear power plants, steam turbines exceeded 1000 MW. The electric efficiency of steam turbine power plants eventually reached 34%.

That trend was broken in the 1980s. More efficient gas turbines combined with steam turbines could produce electric power with efficiencies up to 55%. This new technology, combined cycle power plants, was the technology of choice for independent power producers. It was now possible to build competitive power plants down to the range of 100-200 MW. One may say that new technology (combined cycle power plants) together with regulatory changes (the PURPA Act) jointly drove this paradigm shift.

This trend to commercially viable smaller power plants has continued. Technical development as well as the advantage of economy of scale (mass production) for established technologies, in particular reciprocating engines, are increasingly replacing the old paradigm of economy of size. In the power generation industry 500 $/kW is generally very competitive, while in the automotive industry an engine must be below 50 $/kW to be competitive!


Distributed generation is generally used for power generation less than I MW. Some stretch the definition up to 5 MW. In any case distributed generation is not only a matter of power generation. It brings transmission and distribution (T&D) into the equation.

The costs for T&D are significant. For a traditional vertically integrated electric utility they could represent 400-500 $/kW. During the transmission and distribution from a large central power plant up to 7% of the power is lost. Consequently if distributed generation can offset all or parts of the T&D costs we talk about some serious/significant money. Distributed generation is another way to distribute power, rather than just a smaller scale of generating power.

Distributed generation is already very significant. It has grown steadily during the 1990s and may now represent up to 20% of all new installed power. According to E-Source roughly 10 GW is in the size range of 1-10 MW units. Eighty percent of these units are reciprocating engines. In addition we have even more reciprocating engines used for stand-by power.

Caterpillar has captured a big part of this growth and is now a major manufacturer of power equipment along with GE, Siemens and ABB-Alstohm. Far more Caterpillar engines are used for power generation than to propel construction vehicles!

Reciprocating engines have a huge advantage of the economy of scale and the maturity of the industry/product. First price is low and spare parts and service are available most everywhere in the world. There are two disadvantages with reciprocating engines—emissions and maintenance. Even though there is a continuous effort to improve reciprocating engines, new technologies such as microturbines are better in regards to both emissions and maintenance.

Renewable energy sources such as photovoltaics and wind turbines are gaining acceptance and presence thanks to being "green." However, they have their disadvantages in addition to a higher cost. Both technologies require large physical space and without their primary energy sources, sun and wind respectively, there is no power! They are better but not perfect.

Fuel cells hold many promises of becoming a clean and relatively efficient energy source. However there is still a long way to go. Commercial units have a cost at 4000 $/kW. The challenge of establishing a methane or even more a hydrogen infrastructure may be an even bigger challenge.


When Capstone introduced its Model 330 MicroTurbine it represented the commercial introduction of a new technology, microturbines.

There is no scientifically correct definition of microturbines, but the term is generally used for high speed gas turbines in the size range of 15-300 kW.

Microturbine technology has emerged from four different technologies: small gas turbines, auxiliary power units, automotive development gas turbines and turbochargers.

The core of the microturbine is the high speed compressor-turbine section, which rotates very fast—96,000 rpm in the Capstone Model 330. On the same shaft is generally a high speed generator using permanent magnets. A key element for the best designs are air bearings (or more correctly gas bearings). Air bearings enable the high speed with only air cooling and a long life almost maintenance free.

The high speed generator delivers a high frequency power, in Capstone’s case 1600 Hz. To "gear it down" to useful 50/60 Hz power electronics is the way to go.

Microturbines in general offer two big advantages: low emissions and low maintenance. As illustrated below the Capstone MicroTurbine has one of the best emission performance of any fossil fuel combustion system.

Comparing Technologies



(ppm) (ppm) (ppm)

Reciprocating Engines (500 kW) 2,100 340 150

Gas Turbines (4.5 MW) 25 50 10

Coal Fired Steam (500 MW) 200 n/a n/a

MicroTurbine <9 <25 <9

Source: Cambridge Energy Research Associates

Regarding maintenance there are some very strong indications that the required maintenance is radically less. One example is three units at Williams Energy in Tulsa: more than 20,000 hours and the only maintenance has been air filter changes.

Microturbines are also smaller, lighter, and operate with no vibration and less noise. All of those features help. make on-site installations possible without compromising the environmental aspects.


Microturbines are facing some tough challenges—robustness, interconnection with the grid and costs.

Regarding robustness there has been steady improvement. An endurance test of a Capstone Model 330 has now logged more than 4000 hours and has had more than 99% availability. Still more can be done and will be done. This is probably the least difficult challenge.

The interconnection challenge is shared with other distributed generation technologies. The challenges are both technical and tactical. The latter are nothing but a barrier to entry. However, there is a lot of progress on both the federal and state level for much more simple interconnection requirements. Having said that, one should not neglect or underestimate the technical aspects. E.g., for safety reasons the grid operator cannot accept uncontrolled power being fed into the grid, especially in case of an outage for maintenance. Fortunately power electronics and microprocessors have opened up new approaches. Thus in Capstone’s case we have among others included all protection relay functionality in our controller.

Partly related to the interconnection issue are the communication challenges. Low-cost "mass communication" with the units is a prerequisite for large-scale use of distributed generation. Thanks to the rapid development of all communication technologies, not least wireless and internet, solutions are now available for the virtual power plant concept.

The biggest challenge is probably the cost. For large scale acceptance the cost must eventually be in the range of the reciprocating engines, i.e., 400-600 $/kW. It does not help that microturbines in quantities of single units are already at 1100 $/kW and less, much lower in costs than photovoltaics, wind turbines and fuel cells. With its inherent simplicity with fewer parts and electronics instead of mechanical devices, the economy of scale is faster for microturbines. At annual volumes of 100,000 units, microturbines should have costs equal to or better than those of reciprocating engines.

The only problem is how to get to those volumes. The answer is to sell microturbines initially for applications where their unique features bring extra value, or for applications difficult or even impossible for other technologies.

Microturbines combined with energy storage devices, e.g., batteries or flywheels, will enable a new set of solutions for improved power reliability and quality. The internet infrastructure as well as the "everywhere" use of fast but sensitive microprocessors has created another growth dimension for electric power. Power is not only a matter of kWh but is increasingly a matter of reliability and quality. Most interruptions occur in the distribution side of the system. The best solution in many cases is distributed generation, or more correctly, distributed resources.

With very low emissions and very low maintenance microturbines hold promise to enable small scale cogeneration. The exhaust heat can be used for hot water heating, absorption cooling, dehumidification, etc. It should be possible to reach efficiencies of 70-80%. Thanks to the clean exhaust with no risk of any oil film (due to the air bearings) it should be possible to use the exhaust gas directly in some industrial processes.

T&D deferral is a great potential application. Why tear up streets for additional cables in the case of an established infrastructure that cannot support additional load? Installing microturbines may be a better alternative. In our own case at our "Capstone West facility" reciprocating engines would have been impossible due to the Los Angeles air quality requirements.

One application of great interest is hybrid electric vehicles (HEV). Using the microturbine as a clean and low maintenance onboard battery charger makes it possible to run e.g., a bus for a whole day without any stops for recharging of batteries or swap of batteries. The CARTA 714 HEV bus in Chattanooga built by AVS and using a Capstone MicroTurbine is a real success story. In fact we see it as one of our first major commercial applications.

Another very interesting field of applications is the resource recovery market. It covers oil and gas fields, where the flare gas can be used as energy instead of just being a pollutant waste. Also landfill and other digester gases are of great interest for microturbine applications.

We believe also the combined peak-shaving and standby application holds a great potential. In a perfectly deregulated electricity market one may expect more price volatility as well as more price differentiation for time of use. Microturbines should be very suitable for mitigating such risks.


Distributed generation, including microturbines, will replace the old model of large centralized power plants. However, the new model will not mean just islands of power and no electric grid. On the contrary the new model will take advantage of the grid. Power will be transmitted "both ways." It will be a network connecting large scale power plants with midsize power plants as well as power generating devices all the way down to residential level. The analogy with the computer network is close. The large scale introduction of PCs did not mean the death of mainframes. They are still there and without a huge quantity of servers and an ever increasing bandwidth there would not be the explosive growth of the internet.

Charles D. (Chuck) Tanner is with the Applications Marketing Team at Capstone Turbine Corporation. He has held several positions in both engineering and marketing. Previously, he was with Mobil Oil Corporation, U.S. Marketing and Refining, where he worked in sales and marketing.

Mr. Tanner has a BS in mechanical engineering, Penn State University, and an MS in mechanical engineering, University of California, Berkeley.

Portions of his article are based on documents prepared by Ake Almgren, the president and CEO of Capstone Turbine Corporation. Capstone Turbine Corporation, 6430 Independence Ave., Woodland Hills, CA 91367;

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