Question

In \(1903,\) Aegidius Elling of Norway designed and built an 11 -hp gas turbine that used steam injection between the combustion chamber and the turbine to cool the combustion gases to a safe temperature for the materials available at the time. Currently there are several gas-turbine power plants that use steam injection to augment power and improve thermal efficiency. For example, the thermal efficiency of the General Electric LM5000 gas turbine is reported to increase from 35.8 percent in simple-cycle operation to 43 percent when steam injection is used. Explain why steam injection increases the power output and the efficiency of gas turbines. Also, explain how you would obtain the steam.

Short answer

Answer: Steam injection increases the power output and efficiency of gas turbines by increasing the mass flow rate through the turbine, expanding the hot combustion gases, and enhancing kinetic energy recovery. This process also improves the thermal efficiency by reducing the peak combustion temperature and utilizing waste heat from exhaust gases. The steam is typically obtained using a heat recovery steam generator (HRSG), which captures waste heat from the gas turbine's exhaust gases and produces steam for injection.

Step-by-step solution

Understand the basic working principle of gas turbines

Gas turbines convert the chemical energy stored in fuel into mechanical energy by burning the fuel in a continuous flow process. The main components of a gas turbine are the compressor, combustion chamber, and turbine. The compressed air from the compressor mixes with the fuel in the combustion chamber and ignites. The hot combustion gases then expand and drive the turbine, which in turn drives the compressor and generates mechanical power.

Explain the effect of steam injection on power output

Steam injection increases the power output of gas turbines by increasing the mass flow rate through the turbine. Injecting steam into the combustion chamber or between the combustion chamber and the turbine expands the hot combustion gases, which increases the total volume and pressure of the gases. This, in turn, increases the work done by the turbine, hence increasing the power output. Moreover, the higher mass flow rate of gases through the turbine also leads to a higher exhaust gas velocity, which enhances the kinetic energy recovery and further boosts the turbine's power output.

Explain the effect of steam injection on thermal efficiency

The thermal efficiency of a gas turbine is the ratio of the useful output power to the input heat energy from the fuel. Steam injection improves the thermal efficiency of gas turbines in two main ways. First, it reduces the peak combustion temperature, which reduces the energy lost as heat to the surroundings and allows the use of lower-cost materials. Second, the extra energy added through steam injection comes from the waste heat recovered from the exhaust gases, which would otherwise be lost to the environment. This heat recovery and utilization result in better utilization of the input heat energy, thus improving the overall thermal efficiency of the gas turbine.

Explain how to obtain steam

A common method to obtain steam for injection in gas turbines is by using a heat recovery steam generator (HRSG). The HRSG is a heat exchanger that captures the waste heat from the gas turbine's exhaust gases and uses it to produce steam. This steam is then injected into the combustion chamber or turbine, or used to drive a separate steam turbine, in what is known as a combined cycle power plant. This way, the otherwise wasted heat is utilized to produce additional power and improve the overall efficiency of the power generation process.