Demystifying Combined Heat and Power buzzwords

by / Monday, 10 April 2017 / Published in CHP, COGEN, Technical Expert

CHP, or combined heat and power, is an innovative way of reducing the operating costs of your building by generating a portion of your buildings on-site electrical demand and supplementing heat to your primary heating source and domestic hot water systems. Natural gas is burned in an engine to produce electricity and useful heat.

CHP Generators come in a variety of sizes and features, but this article focuses on the various common, and not so common terms one is likely to encounter with respect to these systems.

Alternator: An Alternator, or Synchronous Generator, is a device traditionally used to generate electricity for a variety of applications.  A movable rotor is housed within a fixed stator; when the rotor is turned by an engine, steam turbine, or some other means, an electric current is created at the stator windings. This type of generator usually permits blackstart as the stator does not rely on external power to be energized.

Blackstart: When a power station or generator, if off, is able to start up and supply electric power without relying on an external power source, normally the grids transmission network. A key distinction should be made that a blackstart-capable CHP unit may not necessarily be suitable to operate as a standby or emergency generator due to the strict performance and code requirements mandated for such generators, i.e. UL2200, CSAC282.

Bootstrapping: The process of returning an electric power grid to operation in the event of a large or system wide failure affecting multiple generating stations. It is typical for power stations to rely on the grid it is connected to for power to start-up. In the event of a large, or system wide power outage, power stations that normally rely on external grid to energize are unable to do so. Key power stations with blackstart capability must first turn on and begin energizing the utility grid (i.e. bootstraping the grid) so the remaining generating stations can return to normal operation.

Connection Impact Assessment: An analysis performed by the Local Distribution Companies of how the installation of a CHP facility will affect the surrounding electric distribution grid, local feeder and sub-station. This is required for each candidate site.

Distributed Generation: Used to describe the generation of electricity close to where it is consumed, reducing reliance on larger central power stations and a less efficient distribution grid.

Dry (or Fluid) Cooler: A device used to reject excess heat generated by a CHP plant to the atmosphere. Heat generated by a CHP plant is removed by a fluid. This heated fluid is pumped through a series of heat exchangers installed in the building to reject the heat where it can be used to offset other thermal loads. If the building has insufficient heat (thermal) load, excess heat must be rejected to the atmosphere to prevent throttling or overheating. If the CHP plant does not have an internal dry cooler, an external one must be installed.

Induction Generator: a.k.a. an electric motor, but operated in reverse. Constructed similar to an Alternator with a movable rotor and fixed stator. Because the induction generator’s stator relies on the grid power to remain energized, an Induction Generator must be connected to an outside power source in order to generate electricity and cannot be used when blackouts occur.

Islanding: When a Distributed Generator continues to generate power when the power grid it is connected too ceases to operate. Islanding can create dangerous conditions for utility workers who may be working on the electrical infrastructure nearby.

Inverter: An inverter is a device used to convert Direct Current to Alternating Current. Inverters are used commonly in Solar panel arrays to efficiently convert the DC power generated by the panels to AC that can be used by the grid. CHP Units that employ Inverter based technologies use an engine to turn a DC Generator, versus an AC Induction or Synchronous Generator. Unlike Alternator or Induction based CHP Generators that exclusively supply three phase power, Inverter based devices are capable of supplying power to single/split phase systems.

 

References:

https://en.wikipedia.org/wiki/Bootstrapping#Electric_power_grid

https://en.wikipedia.org/wiki/Black_start

https://en.wikipedia.org/wiki/Distributed_generation

https://en.wikipedia.org/wiki/Islanding

Sheldon Petrie, BACc.

Sheldon is a Mechanical Designer at Efficiency Engineering with a degree in Mechanical Engineering from the University of Waterloo. He been designing HVAC systems for more than 8 years. His experience includes, equipment selection and sizing, design heating and hot water plant retrofits and Combined Heat and Power (COGEN) retrofits.

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