Among the most widely used and most efficient prime movers are reciprocating engines. Its technical features make reciprocating engines an economic CHP option in many applications.
Several types of reciprocating engines are commercially available with wide proven operational experiences. In addition to CHP applications, diesel engines are widely used to provide island, standby or emergency power to habitants, hospitals, and commercial and industrial facilities for critical power requirements. Large modern diesel engines can attain electric efficiencies near 50% and operate on a variety of fuels including natural and landfill or bio- gas, diesel fuel, heavy fuel oil or crude oil.
The features that have made reciprocating engines a leading prime mover for CHP include:
- Economical size range: Reciprocating engines are available in sizes 50kW-20MW.
- Fast start-up: Fast start-up allows timely resumption of the system following a maintenance procedure. In peaking or emergency power applications, reciprocating engines can quickly supply electricity on demand.
- Black-start capability: In the event of a electric utility outage, reciprocating engines can be started with minimal auxiliary power requirements, generally only batteries are required.
- Excellent availability: Reciprocating engines have typically demonstrated availability in excess of 95%.
- Good part load operation: In electric load following applications, the high part load efficiency of reciprocating engines maintain economical operation.
- Reliable and long life: Reciprocating engines, particularly diesel and industrial block engines have provided many years of satisfactory service given proper maintenance.
Steam turbines are one of the most versatile and oldest prime mover technologies used to drive a generator or mechanical machinery. A steam turbine is captive to a separate heat source and does not directly convert a fuel source to electric energy.
Steam turbines require a source of high pressure steam that is produced in a boiler or heat recovery steam generator (HRSG). Boiler fuels can include fossil fuels such as coal, oil and natural gas or renewable fuels like wood or municipal waste.
Steam turbines used for CHP can be classified into two main types:
- The non-condensing turbine (also referred to as a back-pressure turbine) exhausts steam at a pressure suitable for a downstream process requirement.
- The extraction turbine has opening(s) in its casing for extraction of steam either for process or feed water heating.
Over the last three decades, the gas turbine has seen tremendous development and market expansion. Gas turbines have been long used by utilities for peaking capacity, however, with changes in the power industry and increased efficiency, the gas turbine is now being used for base load power. Much of this growth can be accredited to large (>50 MW) combined cycle plants that exhibit low capital cost and high thermal efficiency. Manufacturers are offering new and larger capacity machines that operate at higher efficiencies.
Aero-derivative gas turbines for stationary power are adapted from their jet engine counterpart. These turbines are light weight and thermally efficient, however, are limited in capacity. The largest aero-derivatives are approximately 50 MW in capacity today. With advanced system developments, aero-derivatives are approaching 45% simple cycle efficiencies.
Industrial or frame gas turbines are available between 1 MW to 350 MW. They are more rugged, can operate longer between overhauls, and are more suited for continuous base load operation. However, they are less efficient and much heavier than the aero-derivative. Industrial gas turbines are approaching simple cycle efficiencies of approximately 40%.
Gas turbines for CHP can be in either a simple cycle or a combined cycle configuration. Simple cycle applications are most prevalent in smaller installations typically less than 25 MW. Waste heat is recovered in a HRSG to generate high or low pressure steam or hot water. The thermal product can be used directly or converted to chilled water with single or double effect absorption chillers.
Design features of gas turbines for CHP applications:
- Economical size range: Gas turbines are available in sizes from 30 kW (micro-turbines) to 350 MW (industrial frames).
- Quality thermal output: Gas turbines produce a high quality thermal output suitable for most CHP applications.
- Cost effectiveness: Gas turbines are among the lowest cost power generation technologies, especially in combined cycle.
- Fuel flexibility: Gas turbines operate on natural gas, synthetic gas and fuel oils. Plants are often designed to operate on gaseous fuel with a stored liquid fuel for backup.
- Reliable and long life: Modern gas turbines have proven to be reliable power generation devices, given proper maintenance.
Combined Cycle Power Plants
The recent trend in power plant design is the combined cycle which incorporates a steam turbine in a bottoming cycle with a gas turbine. Steam generated in the heat recovery steam generator (HRSG) of the gas turbine is used to drive a steam turbine to yield additional electricity and improve cycle efficiency. The steam turbine is usually a controlled extraction condensing type and can be designed for CHP applications.
Combined cycles become economical for larger gas turbine installations, achieving >400MW CCGT unit capacity and approximately 60% electric generation efficiencies using the most advanced utility-class gas turbines. Since gas turbine exhaust is oxygen rich, it can support additional combustion through supplementary firing. A duct burner is usually fitted within the HRSG to increase the exhaust gas temperature at efficiencies of 90% and greater.