Definition

The term ‘microturbine’ is generally used for high speed gas turbines in the size range of 15-300 kW. They have been developed by mixing 4 different technologies: small gas turbines, auxiliary power units, automotive gas turbines and turbochargers.

Just as we saw with the bigger turbines, the key component of the microturbine is the high speed compressor-turbine section. This section can get the shaft rotating very quickly – up to 96,000 rpm. On the same shaft is generally a high speed generator using permanent magnets. The shaft is usually seated on a series of ‘air bearings’ (most commonly these are gas bearings), which permit very high speeds with only air cooling - resulting in a long, low-maintenance life. The high speed generator delivers a high frequency power of up to 1600 Hz. Power electronics are often used to lower the voltage down into the useful 50/60 Hz range.

Main Advantages: low emissions and low maintenance
Microturbines in general offer two big advantages: low emissions and low maintenance. As illustrated below a microturbine can have one of the best emission performance of any fossil fuel combustion system.

Comparing Technologies

Maintenance
The microturbine is a relatively new entrant in the power generation market. As such, it has no developed a lengthy track record, however there are some very strong indications that the required maintenance is radically less.

For example, the three units at Williams Energy in Tulsa have operated for more than 20,000 hours and the only maintenance so far has been air filter changes.

Other Advantages
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 environmental aspects.

Disadvantage: Robustness
There has been a steady improvement with the robustness of these turbines. 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.

Disadvantage: Interconnection with the Grid
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.  

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 the Internet, solutions are now available for the virtual power plant concept.

Disadvantage: High Cost
The biggest challenge is probably the cost. For large scale acceptance the cost must eventually get down into the cost range of reciprocating engines (i.e., 400-600 $/kW). Microturbines in quantities of single units are already at 1100 $/kW and less - much lower in cost than photovoltaics, wind turbines and fuel cells. At annual volumes of 100,000 units, microturbines should have costs equal to, or better than, those of reciprocating engines.

Application: Uninterrupted Power Supplies (UPS)
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.

Application: Small scale cogeneration
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.

Application: Transmission and Distribution deferral
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.

Application: 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 an encouraging prototype.

Application: Flare Gas
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.

Application: Peak Shaving and Standby Power
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 suitable for mitigating such risks.

In conclusion, these turbines are an interesting application of the turbine technology, and may become an integral part of our lives in the future.  For now, they fall into the category of ‘technologies to watch’ along with fuel cells and wind power.

Now that we’ve spent some time looking at gas turbines, let’s take a look at a Steam Turbine.