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Gas turbine engines can be the best cogeneration system design when engine
waste heat can be fully used, and where the engine can be base-loaded to
operate a maximum number of hours a year. In these situations, operating and
maintenance costs are typically $0.008 - $0.010 per kWh of power generation
per hour of operation. These engines also generally have a relatively low
cost in very large power generating situations, especially in the 100+ MW
situations.
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Cogeneration Designs - Gas
Turbines - Inlet Air Quality |
The engine inlet must be free of dirt, dust,
particulates, or chemicals that could coat blades or adversely react in the
combustor or hot end of the machine. Even the thinnest of films forming on
gas turbine blades can significantly impact power generating performance.
Road salt is especially troublesome. Also be very careful not to locate a
gas turbine air inlet anywhere near a cooling tower
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Cogeneration Designs - Gas
Turbines - Weather Dependence
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As this chart indicates, gas turbine power generating levels are extremely
weather dependent, producing the least power during the hottest days of the
year. Since the site's power use is often highest and most valuable at these
times, this performance must be carefully factored into economic
evaluations. In some cases, it may be worthwhile to use chilled water to
cool engine inlet air to partially mitigate this characteristic.
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Cogeneration Designs -
Gas Turbines - Unique Maintenance |
Reciprocating engines or steam turbines are relatively simple machines to
maintain. A gas turbine is not, and requires a rather unique maintenance
regimen. Visits from factory service technicians can be expensive, although
local service technicians can be trained for routine maintenance. In either
case, service can put the unit out of service undesirable long periods of
time. Therefore, gas turbines should generally not be specified in
situations where the local maintenance staff will not receive adequate
training, or where gas turbines are not typically used for cogeneration
service.
Gas turbines are typically not a good fit
in cogeneration applications requiring frequent starts and stops, as
maintenance costs rise rapidly.
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Cogeneration Designs - Gas
Turbines - Air Emission Levels |
Where local air quality requirements are more
stringent, a Selective Catalytic Reduction (SCR), water injection, or other
NOx abatement system may be required to hold the gas turbine engine's
emission levels to acceptable levels. Water injection can also be used to
lower first cost for abatement, but water use (typically 0.4 of a pound per
pound of fuel) can be a significant cost since it must be demineralized to
avoid the solids buildup in the hot end of the engine. Again, check with
local environmental agencies for specific guidance.
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