Declared net capacity
Encyclopedia
Declared net capacity is a measure of the contribution that a power station
makes to the overall capacity of a distribution grid. It is measured in megawatts (MW), or in megawatts electrical (MWe
) for a thermal power station.
DNC is sometimes expanded as developed net capacity in British English
; The two expansions have exactly the same meaning.
In a conventional power station, the DNC rating is simply the maximum rated output minus the power consumed onsite. It is sometimes termed the switchyard output, and takes no account of transmission losses in the grid, which may be considerable in the case of a remote hydro station for example. Most but not all quoted power station ratings are DNC ratings rather than the simple capacity of the alternator
s.
In the case of a wind power
station, the situation is more complex. The alternator of a wind turbine
is normally specified to match the strongest wind in which the turbine is designed to operate. This is because most of the cost of a wind turbine is in the rotor and the tower and bearings that support it, rather than in the alternator. It makes no economic sense to restrict the size of the alternator to anything less than the maximum that the rotor will deliver. However, this means that, unlike a conventional power station, a wind turbine rarely achieves its maximum rated output while operating.
While for conventional power stations, the station is only regarded as available if the full power output is achievable, for wind power stations no power at all may be available depending on the wind strength, and even if a turbine is operating it may be producing as little as a tenth of its maximum rated capacity. A typical average figure is between one-third and one-half of the maximum rated capacity.
There are several suggested methods of allowing for this when quoting a DNC figure for a wind farm
, but none has gained general acceptance, and the capacity quoted for a wind farm is normally a simple total of the maximum rated capacities of the turbines, sometimes termed the peak capacity. Many wind schemes now also quote their expected or actual annual output in GWh, to allow more meaningful comparisons with other forms of generation than is possible just by considering this total rated output.
Power station
A power station is an industrial facility for the generation of electric energy....
makes to the overall capacity of a distribution grid. It is measured in megawatts (MW), or in megawatts electrical (MWe
MWE
MWE may refer to:*Manufacturer's Weight Empty*McDermott Will & Emery*Midwest Express, an airline*Merowe Airport - IATA code*Multiword expressionMWe may refer to:*Megawatt electrical...
) for a thermal power station.
DNC is sometimes expanded as developed net capacity in British English
British English
British English, or English , is the broad term used to distinguish the forms of the English language used in the United Kingdom from forms used elsewhere...
; The two expansions have exactly the same meaning.
In a conventional power station, the DNC rating is simply the maximum rated output minus the power consumed onsite. It is sometimes termed the switchyard output, and takes no account of transmission losses in the grid, which may be considerable in the case of a remote hydro station for example. Most but not all quoted power station ratings are DNC ratings rather than the simple capacity of the alternator
Alternator
An alternator is an electromechanical device that converts mechanical energy to electrical energy in the form of alternating current.Most alternators use a rotating magnetic field but linear alternators are occasionally used...
s.
In the case of a wind power
Wind power
Wind power is the conversion of wind energy into a useful form of energy, such as using wind turbines to make electricity, windmills for mechanical power, windpumps for water pumping or drainage, or sails to propel ships....
station, the situation is more complex. The alternator of a wind turbine
Wind turbine
A wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...
is normally specified to match the strongest wind in which the turbine is designed to operate. This is because most of the cost of a wind turbine is in the rotor and the tower and bearings that support it, rather than in the alternator. It makes no economic sense to restrict the size of the alternator to anything less than the maximum that the rotor will deliver. However, this means that, unlike a conventional power station, a wind turbine rarely achieves its maximum rated output while operating.
While for conventional power stations, the station is only regarded as available if the full power output is achievable, for wind power stations no power at all may be available depending on the wind strength, and even if a turbine is operating it may be producing as little as a tenth of its maximum rated capacity. A typical average figure is between one-third and one-half of the maximum rated capacity.
There are several suggested methods of allowing for this when quoting a DNC figure for a wind farm
Wind farm
A wind farm is a group of wind turbines in the same location used to produce electric power. A large wind farm may consist of several hundred individual wind turbines, and cover an extended area of hundreds of square miles, but the land between the turbines may be used for agricultural or other...
, but none has gained general acceptance, and the capacity quoted for a wind farm is normally a simple total of the maximum rated capacities of the turbines, sometimes termed the peak capacity. Many wind schemes now also quote their expected or actual annual output in GWh, to allow more meaningful comparisons with other forms of generation than is possible just by considering this total rated output.
External links
- Wind Farm Performance article at the UK Department of Trade and Industry site.
- Another article at the DTI site, using DNC for developed net capacity.