There is another type of gas turbine called a micro-turbine that can be utilized for small cogeneration applications. Micro-turbines are tiny gas turbines that operate at very high speeds. Capacities for these machines vary from as small as 25 kW to perhaps 250 kW. There are larger units, between 250 kW and 500 kW, that are sometimes called mini-turbines, but they are best considered with traditional gas turbines and are not included in the following discussion.
Operating speed of micro-turbines can be as high as 120,000 rpm. The bearings are often air lubricated to reduce wear and most micro-turbines incorporate a generator on the same shaft to make the package as compact as possible. Units can burn gasoline, diesel, and alcohol, but for most applications they will burn natural gas.
The high operating speed means that a micro-turbine generator cannot inter- face directly with the grid, and most units are equipped with solid-state inter- faces that convert the high-frequency output to grid frequency at 50 Hz or 60 Hz. Efficiency is relatively low for electricity generation at around 15% to 30%. This low efficiency is not generally a problem because micro-turbines are generally designed for CHP applications with waste heat-recovery systems capable of providing hot water or, in some cases, low-pressure steam.
Micro-turbines are generally supplied packaged so that all they require is a gas supply, an electrical connection, and a connection for their hot water supply. In this form they can be used in a variety of situations such as small commercial environments or office blocks. The units have low emissions so they can be deployed in urban settings without any problem. Noise generation is low too. Larger micro-turbines have been installed in schools and hospitals and they can be used in some small industrial situations.
The most recent development of micro-turbines is for domestic use. Units are packaged with an electrical output as low as 3 kW, suitable for many single homes where they will also supply hot water for heating and other uses. These domestic units are still relatively expensive but large-scale mass production could realize significant economies of scale.