CogenerationCogeneration, also known as on-site power generation, Combined Heat and Power (CHP), Distributed Generation (DG) and others, is the simultaneous production of electricity and useful 'waste' heat. Any facility that has significant thermal load requirements could be a technical fit for cogeneration. The economic fit will depend on the cost of electricity, how close the thermal demand matches the thermal production, and the complexity of the installation (first cost). This Section is organized by: Prime Mover Overview
Prime Mover Overview
Reciprocating EngineHistory
Reciprocating engines are machines in which pistons move back and forth in cylinders. Reciprocating engines are a subset of internal combustion engines, which also include rotary engines. Small to medium sized engines are primarily designed for transportation applications and are converted to power generation units with little modification. Larger engines are, in general, designed for power generation, mechanical drive, or marine propulsion. Reciprocating engines are currently available from many manufacturers in all DG size ranges. OperationAlmost all engines used for power generation are four-stroke and operate in four cycles (intake, compression, combustion, and exhaust). The process begins with fuel and air being mixed. Some engines are turbocharged or supercharged to increase engine output, meaning that the intake air is compressed by a small compressor in the intake system. The fuel/air mixture is introduced into the combustion cylinder, then compressed as the piston moves toward the top of the cylinder. In diesel units, the air and fuel are introduced separately with fuel injected after the air is compressed by the piston in the engine. As the piston nears the top of its movement, a spark is produced that ignites the mixture (in most diesel engines, the mixture is ignited by the compression alone). Dual fuel engines use a small amount of diesel pilot fuel in lieu of a spark to initiate combustion of the primarily natural gas fuel. The pressure of the hot, combusted gases drives the piston down the cylinder. Energy in the moving piston is translated to rotational energy by a crankshaft. As the piston reaches the bottom of its stroke the exhaust valve opens and the exhaust is expelled from the cylinder by the rising piston. Reciprocating engines can be used in three different types of industrial cogeneration applications:
For more information about
Reciprocating Engines
Combustion TurbineHistory
OperationHistorically, industrial turbines have been developed as aero derivatives using jet propulsion engines as a design base. Some, however, have been designed specifically for stationary power generation or for compression applications in the oil and gas industries. A combustion turbine is a device in which air is compressed and a gaseous or liquid fuel is ignited. The combustion products expand directly through the blades in a turbine to drive an electric generator. The compressor and turbine usually have multiple stages and axial blading. This differentiates them from smaller microturbines that have radial blades and are single staged. Industrial turbines can be used in two types of industrial cogeneration applications:
For more information on Combustion Turbines
Steam TurbineHistory
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). Steam turbines offer a wide
array of designs and complexity to match the desired application and/or
performance specifications. In utility applications, maximizing efficiency
of the power plant is crucial for economic reasons. Steam turbines for
utility service may have several pressure casings and elaborate design
features. For industrial applications, steam turbines are generally
of single casing design, single or multi-staged and less complicated
for reliability and cost reasons. OperationThe thermodynamic cycle for the steam turbine is the Rankine cycle. The cycle is the basis for conventional power generating stations and consists of a heat source (boiler) that converts water to high pressure steam. The steam flows through the turbine to produce power. The steam exiting the turbine is condensed and returned to the boiler to repeat the process. A steam turbine consists of a stationary set of blades (called nozzles) and a moving set of adjacent blades (called buckets or rotor blades) installed within a casing. The two sets of blades work together such that the steam turns the shaft of the turbine and the connected load. A steam turbine converts pressure energy into velocity energy as it passes through the blades. The primary type of turbine used for central power generation is the condensing turbine. Steam exhausts from the turbine at sub-atmospheric pressures, maximizing the heat extracted from the steam to produce useful work. Steam turbines used for CHP can be classified into two main types:
MicroturbineHistory
OperationMicroturbines consist of a compressor, combustor, turbine, and generator. The compressors and turbines are typically radial-flow designs, and resemble automotive engine turbochargers. Most designs are single-shaft and use a high-speed permanent magnet generator producing variable voltage, variable frequency alternating current (AC) power. An inverter is employed to produce 60 Hz AC power. Most microturbine units are currently designed for continuous-duty operation and are recuperated to obtain higher electric efficiencies. Non-recuperated engines have lower electric efficiencies, but higher exhaust temperatures, which make them better for some industrial cogeneration applications. Microturbines can be used in three different types of industrial cogeneration applications:
A list of Manufacturers and Vendors of Microturbines is located within the Applications Guide, Manufacturers Section.
Fuel CellHistory
Operation
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Reciprocating engines, developed more than 100 years ago, were
the first of the fossil fuel-driven DG technologies. Both Otto (spark
ignition) and Diesel cycle (compression ignition) engines have gained
widespread acceptance in almost every sector of the economy and are
in applications ranging from fractional horsepower units powering small
hand-held tools to 60 MW baseload electric power plants.
Combustion turbines have been used for power generation for decades
and range in size from units starting at about 1 MW to over a 100 MW.
Units from 1-15 MW are generally referred to as industrial turbines,
a term which differentiates them from larger utility grade turbines
and smaller microturbines. Combustion turbines have relatively low installation
costs, low emissions, high heat recovery, infrequent maintenance requirements,
but low electric efficiency. With these traits, combustion turbines
are typically used for cogeneration, as peakers, and in combined cycle
configurations.
Steam turbines
are one of the most versatile and oldest prime mover technologies used
to drive a generator or mechanical machinery. Most of the electricity
in the United States is generated by conventional steam turbine power
plants. The capacity of steam turbines can range from a fractional horsepower
to more than 1,300 MW for large utility power plants.
The technology
used in microturbines is derived from aircraft auxiliary power systems,
diesel engine turbochargers, and automotive designs. A number of companies
are currently field testing demonstration units for small-scale distributed
power generation in the 30-400 kW size range. Several units are available
commercially, and more are slated to enter the market in 2001 and 2002.
Although the
first fuel cell was developed in 1839 by Sir William Grove, the technology
was not put to practical use until the 1960’s when NASA installed fuel
cells to generate electricity on Gemini and Apollo spacecraft. There
are many types of fuel cells currently under development, including
phosphoric acid, proton exchange membrane, molten carbonate, solid oxide,
alkaline, and direct methanol. However, fuel cells are not generally
commercially available, except for a 200 kW phosphoric acid unit made
by International Fuel Cells.
The combined cycle plant is an integration of two types of prime
mover the gas turbine and the steam turbine combining many of the advantages
of the both. It can provide large amounts of power on short notice with
its quick start-up time. It has a short delivery time, a low heat rate,
and its capital cost is an effective compromise between pure gas and steam
turbines.