142 Microturbines

A microturbine (MT) is a Brayton cycle engine using atmospheric air and natural gas fuel to produce shaft power. Figure 1.8 shows the essential components of this device. Although a dual shaft approach is shown in the figure, a single-shaft design is also used in which the power produced in the expander is supplied to both the compressor and the load by a single shaft. The dual shaft design offers better control, but at the cost of another rotating part and two more high speed bearings. Electrical power is produced by a permanent magnet generator attached to the output shaft or by way of a gear reducer driving a synchronous generator.

Figure 1.9 is a photograph of a small MT showing most of the key components except for the recuperator. The recuperator is used in most units because about half of the heat supplied to the working fluid can be transferred from the exhaust gas to the combustion air. Without a recuperator the overall efficiency of a MT is 15 to 17%, whereas with an 85% effective recuperator the efficiency can be as high as 33%. MTs without recuperators are basically burners that produce a small amount of electricity with thermal output to be used for cogeneration.

A handful of MT manufacturers have announced products in the U.S. Sizes range from 25 to 150 kW, with double digit power ratings the most common.

Power out exhaust

FIGURE 1.8

Schematic diagram of dual shaft microturbine design.

Power out exhaust

FIGURE 1.8

Schematic diagram of dual shaft microturbine design.

By early 2000, fewer than 1000 MTs had been shipped to U.S. locations. Attractive MT features include low capital cost, good efficiency (30-33%; LHV basis), modest emissions (<10 PPM NOx quoted by manufacturers), thermal or electrical cogeneration is possible in industry and in buildings, and modularity is excellent (nearly any load can be matched well by multiple units of small to medium capacity).

The key barriers to MT usage include maintenance cost (the exact costs are unknown but are expected to be lower than ICEs due to fewer moving parts) questionable part load efficiency (manufacturer's data vary), limited field experience, use of air bearings is desirable to reduce maintenance but air filtration requirements are stringent, and high frequency noise is produced but is relatively easy to control.

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