A steam turbine is a machine that exploits the thermal energy of steam under pressure, converting it into useful mechanical energy through a thermodynamic expansion transformation. Specifically, the steam turbine converts the internal energy of steam into rotational kinetic energy.
The steam turbine, thanks to the higher thermal efficiency and better power-to-weight ratio, has completely replaced the steam engine, which was an alternative engine invented by Thomas Newcomen and later significantly improved by James Watt.
From a thermodynamic point of view, the maximum efficiency occurs when the expansion of steam is an ideal process (reversible transformation) in which the pressure of the steam decreases, turning into mechanical work, in an infinite number of stages. Watt's alternate machine was single stage, and later refinements were used on most of the two or three stages (double and triple expansion). In contrast, modern steam turbines achieve high thermal efficiency thanks to the presence of a greater number of stages in series.
Steam turbines are expensive and require advanced manufacturing processes and high-quality materials. In addition, they are highly efficient when operating at speeds of thousands of rpm, so if the load rotates at lower speeds, a gearbox is needed. However, if the installed power is high, the high investment costs are offset by the fact that the steam turbine consumes less fuel, requires less maintenance, and is smaller in size compared to a reciprocating heat engine of equal power.
Steam Turbine Applications
Steam turbines can be used to produce electricity, coupled to generators, often without the need for gearboxes. In this case, they operate at ideal regimes, since the generators have to rotate at a constant speed (3000 rpm for 50 Hz networks and 3600 rpm for 60 Hz networks; in some cases, especially in nuclear plants, they use generators of 4 poles rotating at half speed).
Furthermore, the steam turbine, being a rotary machine, is advantageous as the motor of an electric generator, since it does not require any mechanical member that transforms the reciprocating movement into a rotary one.
Another typical field of application for steam turbines is in plants such as refineries, paper mills, desalination plants and other plants where high levels of process steam are required. The plant can be designed in such a way that it makes use of the steam turbine to obtain a synergy between the production of steam and that of electrical energy or mechanical work.
Steam turbines are also used as marine engines on ships, where the limited dimensions are an advantage. Propulsion steam turbine locomotives were also built, but their diffusion was very limited.
Electricity Generation with Steam Turbines
Power plants based on the production of high temperature steam (thermal power plants, nuclear power plants, geothermal power plants, some solar thermal plants, etc.) use steam turbines connected to electric generators to produce about 80% of the electricity of the planet.
Turbines for electrical production are usually coupled directly to your generator.
As generators must rotate at synchronous speeds with respect to the frequency of the electrical system, the most common rotation speeds are 3,000 rpm for 50 Hz systems and 3,600 rpm for 60 Hz systems.
As nuclear reactors operate at lower temperatures and with less gas pressure than thermal power plants, they usually operate at half the rotation speed, but with 4-pole generators.
Marine Propulsion Based on Steam Turbines
In ships, steam turbine-based propulsion has multiple advantages over internal combustion engines: smaller size and weight for the same power, less maintenance and fewer vibrations. However, a steam turbine is only efficient at high rotational speeds (on the order of thousands of revolutions per minute), whereas most propellers are designed to operate at less than 100 rpm. This requires precise and complex transmissions (and at a high cost).
An alternative is the use of turbo-electric propulsion, in which the turbines generate electrical energy as in a power station and this is used to power electric motors that drive the propellers. Although the manufacturing cost is higher, this is profitable, since the energy consumption and maintenance costs are lower than that of a combustion engine of equivalent power.
It goes without saying that diesel engines are capable of greater efficiency: steam turbines still do not reach 50% efficiency, while diesel cycle engines often exceed this level (especially in marine propulsion systems).