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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 = 4.1868 cal
1 GJ (gigajoule) = 109 J
1 TJ (terajoule) = 1012 J
1 PJ (petajoule) = 1015 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 m3 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 600 kW, that tells you that the wind turbine will produce
600 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
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