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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 |
