The selection of a particular technology for power generation is not a straightforward decision. It actually depends on a lot of factors, the most important are; load factor, which is the percentage of hours that the power plant will operate at its maximum capability in a given time period. Another important factor is the capacity required. This depends on the energy demand within certain region or application. Cost, available natural resources, emissions, fuel, fuel availability etc. are other factors that have to be taken into account when deciding which technology is the most appropriate for a specific case. In this article, focus will be given to two very popular power generation technologies; gas turbines and steam turbines.
A steam turbine is a form of heat engine that extracts energy from high-pressure steam to produce electricity. These engines generate approximately the 90% of the electricity in the U.S. Steam cycle power plants operation are based on the Rankine Cycle. The fundamental operation of this cycle is described as follows. Water is pumped at high pressure into a boiler where heat from the combustion process is used to generate steam. The steam then passes through a steam turbine that rotates a shaft that is connected to an electrical generator. The spinning of the electrical generator produces electricity. The low pressure and low-temperature steam coming from the turbine is then passed through a condenser that decreases the temperature of the steam to convert it back to saturated water. The saturated water is then pumped back to the boiler closing the cycle.
- One of the main advantages of steam turbines is the boiler ability to burn multiple types of fuels. The boiler is capable of burning different fuels at each stage of the start-up process. For instance, it can use LPG for ignition, light diesel oil for start-up and then heavy oil for continuous operation as an example.
- Steam turbines have a high power-to-weight ratio which makes them more practical than reciprocating engines. Also, for a given power output, their size is smaller and produce less vibrations than their reciprocating counterparts.
- Power generation is not affected by atmospheric conditions as it operates on a closed cycle.
- Another advantages of steam turbine power plants compared to gas turbines are the high operating efficiency, higher reliability and high operating efficiencies.
- Long start-up time. Start-up of a steam turbine takes a considerable amount of time specially during cold start-ups and requires a long list of steps before it can operate at base load. Therefore, steam turbines cannot be used as peak power plants.
- The operation of a steam turbine power plant requires more components than that of a simple cycle gas turbine. Among this components are huge boiler, cooling towers, feedwater systems and so forth.
- Require to be installed near large water reservoirs.
A gas turbine is a continuous combustion engine that consists of three main components: compressor, combustor and turbine. These three components are connected through a shaft which. Gas turbines are capable of producing power by suctioning atmospheric air, compressing it, burning it and expanding it producing a torque on the shaft which in turns spins an electrical generator that transforms the mechanical energy of the shaft to electrical energy.
- Gas turbines are capable of reaching full load operation in a matter of minutes. These engines are very popular for using as peak power plants, this due to their ability of be taken into service during peak power demand when and as required.
- Furthermore, they have a very attractive initial cost as compared with steam turbines.
- High power-to-weight ratio.
- Require much less water than steam turbines power plants.
- Smaller size and weight compared to steam turbines.
- Less space is needed for installation.
- One of the disadvantages of gas turbines is their limited ability to use different fuels
in comparison with steam turbines. In fact, just a little variation in the fuel specs
used in a gas turbine can lead to efficiency drop, increase maintenance factor, early
- Low part load efficiency.
- Due to high operating temperatures, special and expensive materials are needed.
- About two thirds of the power produces is used to drive the compressor.
- External source of power needed to start self-sustained operation.
The selection of a power generation technology clearly depends on the plant utilization and it is always a compromise between fix and marginal costs. If the power plant is expected to be used in a regular basis efficiency is a main aspect, therefore, the owner must have in mind a high initial investment. In this case a combined cycle would be the right choice.
On the other hand, if the plant is expected to be used just in short periods of time, i.e. for peak power demand, it would be make more sense, form the economical point of view, the installation of an open cycle gas turbine.