Steam Turbines

Size Range:

0.5 MW to 500 MW

Technology

One of the most common power generation technologies today is a Rankine Cycle plant based on steam and turbine driven generators, and it is used by virtually all of the world’s coal, nuclear and other solid fuelled power plants. In addition, many central generation plants utilize boilers burning natural gas or oil in the same cycle.

The heat produced during combustion of the fuel is used to raise steam in a boiler to high pressures and temperatures. Once the steam has reached its designated temperature and pressure, it is passed through the turbine blades at high velocity. The impact of the steam on the blades creates a mechanical rotation that turns the generator. The power produced by the generator depends on the drop in steam pressure through the turbines. The remaining heat in the outlet steam can be processed to meet the onsite requirements.

Steam powered district heating cogeneration plant.
Source: Applied Global Cogeneration 

Steam turbines are applied in two quite distinct approaches – condensing and backpressure.

Back-pressure Turbines Condensing Turbines
Back-pressure turbines expand high-pressure steam through a turbine. The output steam is exhausted at a relatively low pressure suitable for onsite heat requirements. It is possible to release the steam at various points through the turbine allowing access to more than one grade of heat; the extraction of steam from the turbine will result in a decrease in power across the blades. Backpressure steam turbines have traditionally been the most popular generation technology for CHP. In a condensing turbine, steam is exhausted into a condenser, achieving maximum possible pressure drop across the blades. The condensing steam turbine thus generates more electricity from a given quantity of high-pressure steam than a backpressure turbine, but then wastes all of the thermal energy. Central plants, with no economic possibility of transmitting by-product heat to remote thermal users, utilize condensing steam turbines.
Source: WADE 2003
 


Performance and Efficiency

The efficiency of fuel conversion to power is 20% to 38% from condensing turbines and, to energy, is 80% to 90% for backpressure turbines (7% to 20% electrical efficiency). There are no technologies yet invented that convert over 60% of the energy in fuel to power, and the average delivered efficiency for centralised power systems hovers around 33%. The remaining 2/3’s of the energy content of the fuel is typically wasted – vented to atmosphere by central power plants.

Fuel Types

The steam required in Steam Turbines can be produced from a range of fuels, including coal, oils, gas and biomass. The fuel is used to heat water in a boiler and does not come into direct contact with the steam or the turbine, meaning that only the calorific value of the fuel is important - the cleanliness or quality of the fuel does not affect the turbine operation. Therefore fuel versatility os one benefit of Steam Turbines.

Applications

Industrial, District Energy, Commercial

  • Providing high-grade heat for industrial process plants. Condensing turbines are generally used for these applications due to the ability to control the electrical output by varying the mass flow rate of the steam.
  • Providing heat for district heating systems - a suitable application for both backpressure and condensing turbines.
  • Industrial energy recycling. The recycling of waste heat, low grade by-product fuel or gas pressure drop is achieved with Rankine cycle technology.
  • Use of organic fluids such as iso-butane or propane in lieu of water enables turbines to extract more power at lower temperatures, and then either use or reject the remaining heat. This can be useful for industrial applications with specific thermal demands.

Advantages and Disadvantages

Advantages Disadvantages
  • High overall cogeneration efficiencies of up to 80%;
  • A wide range of possible fuels including waste fuel and biomass;
  • An established technology;
  • Production of high temperature/pressure steam.
  • Low electrical efficiencies;
  • Need for expensive high-pressure boilers and other equipment;
  • Slow start up times;
  • Poor part load performance.

Economic Performance

Cost Range for Steam Turbines
Installed Capital Cost ($/kW)400 - 1,500
Operating and Maintenance ($c/kWh) < 0.4
Levelized Cost ($c/kWh) 
8000hrs/year 2.5 - 6.5
4000hrs/year4.0 - 12.0
Source: WADE, 2006

Backpressure steam turbine CHP systems use mature technology, with a long and successful history. The economic performance is well proven in situations where there is demand for both electricity and large quantities of steam. As the table above shows, installed capital costs for steam turbines vary from $400 -$1,500 depending on size, required inlet and exit steam conditions, rotational speed and standardisation of construction.

Installed Costs for Steam Turbines
Source: WADE, 2006

Because of the steam turbine’s maturity, there is little scope for cost reduction or further efficiency gains, except in the fuel handling, storage and preparation systems. Backpressure steam turbines have been deployed in commercial operations with as little as 40 kW of output, but historically backpressure turbines below 1 MW have faced economically prohibitive electric interconnection requirements and been confined largely to powering other rotating equipment such as pumps, fans and air compressors. Where the fuel is natural gas or distillate oil, gas turbines, reciprocating engines and fuel cells often have advantages relative to Rankine-cycle steam turbines. The operational lifetime of steam turbines often exceeds 50 years. Maintenance is minimal, so operating and maintenance costs (O&M) are low - frequently less than $0.1c/kWh. Steam turbines require periodic inspection of auxiliaries such as lubricating-oil pumps, coolers and oil strainers and safety devices.

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