Steam turbines are one of the most versatile and oldest prime mover technologies still in commercial production. Power generation using steam turbines has been in use for about 100 years, when they replaced reciprocating steam engines due to their higher efficiencies and lower costs. The capacity of steam turbines can range from 50 kW to several hundred MW for large utility power plants. Steam turbines are widely used for CHP applications.
A steam turbine is a thermodynamic device that converts the energy in high-pressure, high-temperature steam into shaft power that can in turn be used to turn a generator and produce electric power. Unlike a gas turbine where heat is a by product of power generation, steam turbine CHP systems normally generate electricity as a by product of heat (steam) generation. A steam turbine requires a separate heat source and does not directly convert fuel to electric energy. The energy is transferred from the boiler to the turbine through high-pressure steam, which in turn powers the turbine and generator. This separation of functions enables steam turbines to operate with an enormous variety of fuels, from natural gas to solid waste, including all types of coal, wood, wood waste, and agricultural by products (sugar cane bagasse, fruit pits, and rice hulls). In CHP applications, steam at lower pressure is extracted from the steam turbine and used directly or is converted to other forms of thermal energy.
The basic process behind steam power generation is the "Rankine Cycle".
In the thermodynamic cycle liquid water is converted to high-pressure steam in the boiler and fed into the steam turbine. The steam causes the turbine blades to rotate, creating power that is turned into electricity with a generator. A condenser and pump are used to collect the steam exiting the turbine, feeding it into the boiler and completing the cycle. There are several different types of steam turbines:
- A condensing steam turbine is for power-only applications and expands the pressurized steam to low pressure at which point a steam/liquid water mixture is exhausted to a condenser at vacuum conditions;
- Extraction turbines have openings in their casings for extraction of a portion of the steam at some intermediate pressure for process or building heating;
- Back-pressure turbines exhaust the entire flow of steam to the process or facility at the required pressure. Non-condensing steam turbines are also referred to as "back pressure" steam turbines. Here, steam is expanded over a turbine and the exhaust steam is used for to meet a facilities steam needs. The steam is expanded until it reaches a pressure that the facility can use.
The attainable electric annual use efficiency (= annual electricity production / annual fuel input based on its net caloric value) depends on the live steam parameters (temperature, pressure) and on the other hand on the necessary temperature level for the process and/or district heat consumers. Electric annual use efficiencies are usually between 18 and 30 % for biomass CHP plants in the capacity range between 2 and 25 MWel.
Regarding steam turbine technology backpressure turbines and extraction condensing turbines have to be distinguished. If there is a constant heat demand in form of hot water or low pressure steam all over the year backpressure turbines are used. At projects with the need of uncoupling the electricity and heat production extraction condensing turbines are applied, using the steam which is not or only to a low part required for heat supply in the low pressure part of the turbine to increase electricity production.