Gas Turbines
Posted under Energy, Industry, Power Source
The characteristics of combustion gas turbines for electricity generation are somewhat complementary to those of the steam turbine-generators just discussed.
Steam power plants tend to be large, coal-fired units that operate best with fairly fixed loads. They tend to have high capital costs, largely driven by required emission controls, and low operating costs since they so often use low-cost boiler fuels such as coal. Once they have been purchased, they are cheap to operate so they usually are run more or less continuously. In contrast, gas turbines tend to be natural-gas-fired smaller units, which adjust quickly and easily to changing loads.
They have low capital costs and relatively high fuel costs, which means they are most cost-effective as peaking power plants that run only intermittently. Historically, both steam and gas-turbine plants have had similar efficiencies, typically
in the low 30% range.
A basic gas turbine driving a generator is shown below. In it, fresh air is drawn into a compressor where spinning rotor blades compress the air, elevating its temperature and pressure. This hot, compressed air is mixed with fuel, usually natural gas, though LPG, kerosene, landfill gas, or oil are sometimes used, and subsequently burned in the combustion chamber.
The hot exhaust gases are expanded in a turbine and released to the atmosphere. The compressor and turbine share a connecting shaft, so that a portion, typically more than half, of the rotational energy created by the spinning turbine is used to power the compressor.
Gas turbines have long been used in industrial applications and as such were designed strictly for stationary power systems. These industrial gas turbines tend to be large machines made with heavy, thick materials whose high thermal
capacitance and moment of inertia reduces their ability to adjust quickly to changing loads. They are available in a range of sizes from hundreds of kilowatts to hundreds of megawatts. For the smallest units they are only about 20% efficient, but for turbines over about 10 MW they tend to have efficiencies of around 30%.
Another style of gas turbine takes advantage of the billions of dollars of development work that went into designing lightweight, compact engines for jet aircraft. The thin, light, super-alloy materials used in these aero derivative turbines enable fast starts and quick acceleration, so they easily adjust to rapid load changes and numerous start-up/shut-down events. Their small size makes it easy to fabricate the complete unit in the factory and ship it to a site, thereby reducing field installation time and cost.
Aero-derivative turbines are available in sizes ranging from a few kilowatts up to about 50 MW. In their larger sizes, they
achieve efficiencies exceeding 40%.