Energy and power

        Physicists define the 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. The expression energy loss, which is impossible by the definition above, means 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% 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 e.g. fossil fuels are burned, somehow 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 the ability to perform useful work converting energy decreases each time energy ends up as heat and dissipates into the universe. Useful work is called exergy by physicists.
        Since the vast majority of wind turbines produce electricity, usually their performance is measured in terms of the amount of electrical energy they are able to convert from the kinetic energy of the wind. That energy is measured in terms of kilowatt hours (kWh) or megawatt hours (MWh) during a certain period of time, e.g. an hour or a year.
        It should be also noted that exactly because energy cannot be created, but only converted into different forms, wind turbines are also called Wind Energy Converters (WECs).
        In the next table the energy content of some fuels is shown.

Energy content of fuels
Fuel Energy [GJ per tonne]
North sea crude oil 42.7
LPG (Liquefied petroleum gas: Propane, Butane) 46.0
Petrol (gasoline) 43.8
Jet aircraft fuel (JP1) 43.5
Diesel/ Light Fuel oil 42.7
Heavy fuel oil 40.4
Orimulsion 28.0
Natural gas 39.3 per 1000 Nm3
Steam coal 24.5
Other coal 26.5
Straw 14.5
Wood chips 14.7
Household waste 1995 10.0
Household waste 1996 9.4

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 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. Electrical power is usually measured in watt (W), kilowatt (kW), megawatt (MW) etc.
        If a wind turbine has a rated power or nameplate power of 600 kW, that means that the wind turbine will produce 600 kWh of energy per hour of operation, when running at its maximum performance, i.e. at high winds above, say, 15 m/s. If a country like Denmark has, say 1000 MW of wind power installed, that does not tells how much energy the turbines produce. Wind turbines will usually be running, say, 75% 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 and thus it comes 2.3 TWh of energy. In other areas, like Wales or Scotland 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.

Energy Units

1 kW = 1.359 HP