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.

Physical basic for utilization of wind energy

where m is mass of the gas and v is speed of the wind . The power of the wind is the instant energy of the wind

where D is density of the air and A is surface through which the air moves .

At speed of the wind with 5m/s we get 2 times more energy than at speed 4m/s

The wind power station is profytably if the average year speed of the wind is larger than 4m/s

Energy Units

         1 GJ (gigajoule) = 10 exp 9 J 
         1 TJ (terajoule) = 10 exp 12 J 
         1 PJ (petajoule) = 10 exp 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