Wind Energy Reference Manual Part 2: Energy and Power Definitions
Energy
Physicists define the word energy as the amount of work a physical system is capable of performing. Energy, according to the definition of physicists, can neither be created nor consumed or destroyed.
Energy, however may be converted or transferred to different forms: The kinetic energy of moving air molecules may be converted to rotational energy by the rotor of a wind turbine, which in turn may be converted to electrical energy by the wind turbine generator. With each conversion of energy, part of the energy from the source is converted into heat energy.
When we loosely use the expression energy loss (which is impossible by the definition above), we mean that part of the energy from the source cannot be used directly in the next link of the energy conversion system, because it is converted into heat. E.g. rotors, gearboxes or generators are never 100 per cent efficient, because of heat losses due to friction in the bearings, or friction between air molecules.
Most of us have the sensible notion, however, that as we e.g. burn fossil fuels, somehow, loosely speaking, the global potential for future energy conversion becomes smaller. That is absolutely true.
Physicists, however, use a different terminology: They say that the amount of entropy in the universe has increased. By that they mean that our ability to perform useful work converting energy decreases each time we let energy end up as heat which is dissipiated into the universe. Useful work is called exergy by physicists.
Since the vast majority of wind turbines produce electricity, we usually measure their performance in terms of the amount of electrical energy they are able to convert from the kinetic energy of the wind. We usually measure that energy in terms of kilowatt hours (kWh) or megawatt hours MWh during a certain period of time, e.g. an hour or a year.
People who want to show that they are very clever, and show that they understand that energy cannot be created, but only converted into different forms, call wind turbines Wind Energy Converters (WECs). The rest of us may still call them wind turbines.
Note
Energy is not measured in kilowatts, but in kilowatt hours (kWh). Mixing up the two units is a very common mistake, so you might want to read the next section on power to understand the difference.
Energy Units
1 J (joule) = 1 Ws = 0.2388 cal
1 GJ (gigajoule) = 10 9 J
1 TJ (terajoule) = 10 12 J
1 PJ (petajoule) = 10 15 J
1 kWh (kilowatt hour) = 3,600,000 Joule
1 toe (tonne oil equivalent)
= 7.4 barrels of crude oil in primary energy
= 7.8 barrels in total final consumption
= 1270 m 3 of natural gas
= 2.3 metric tonnes of coal
1 Mtoe (million tonne oil equivalent) = 41.868 PJ
Power
Electrical power is usually measured in watt (W), kilowatt (kW), megawatt (MW), etc. Power is energy transfer per unit of time.
Power may be measured at any point in time, whereas energy has to be measured during a certain period, e.g. a second, an hour, or a year. (Read the section on energy , if you have not done so yet).
If a wind turbine has a rated power or nameplate power of 1000 kW, that tells you that the wind turbine will produce 1000 kilowatt hours (kWh) of energy per hour of operation, when running at its maximum performance (i.e. at high winds above, say, 15 metres per second (m/s)).
If a country like Denmark has, say 1000 MW of wind power installed, that does not tell you how much energy the turbines produce. Wind turbines will usually be running, say, 75 per cent of the hours of the year, but they will only be running at rated power during a limited number of hours of the year.
In order to find out how much energy the wind turbines produce you have to know the distribution of wind speeds for each turbine. In Denmark's case, the average wind turbines will return 2,300 hours of full load operation per year. To get total energy production you multiply the 1000 MW of installed power with 2,300 hours of operation = 2,300,000 MWh = 2.3 TWh of energy. (Or 2,300,000,000 kWh).
In other areas, like Wales, Scotland, or Western Ireland you are likely to have something like 3,000 hours of full load operation or more. In Germany the figure is closer to 2,000 hours of full load operation.
The power of automobile engines are often rated in horsepower (HP) rather than kilowatt (kW). The word "horsepower" may give you an intuitive idea that power defines how much "muscle" a generator or motor has, whereas energy tells you how much "work" a generator or motor performs during a certain period of time.
Power Units
1 kW = 1.359 HP
© Copyright 1997-2003 Danish Wind Industry Association
Updated 12 August 2003
http://www.windpower.org/en/stat/unitsene.htm
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