A very simple system is possible.
If WT output is AC rectify to DC.
WT output must inclde a "blocking diode" to stop "backfeed" from mains power supply.
This diode is usually part of the rectifier.
Smooth WT DC enough that it will operate LEDs OK by itself without flicker etc when there is enough energy available.
Wind turbine makes power at some nominal DC voltage = V_WT_Out.
Assume this is ~= 12V below but any other voltage is OK.
Idfeally this voltage is the maximum power point voltage for the WT - but this will seldom match VLED_load. A MPPT controller can be used to maximise energy provision but that is a subsequent step.
This voltage V_WT_Out is in the allowable LED operating voltage range.
WT actual Vout will rise above this voltage if load is very light and will fall below this voltage when WT is too heavily loaded.
Independent of this solution a "controller" stops WT Voltage to LEDs from rising above max allowed value when load is light. This controller can be as simple as a voltage controlled switch that sends excess power to eg a light bulb or water heater.
Mains power supply is provided which outputs a steady DC voltage via a "blocking voltage". The power supply voltage on the output side of the blocking voltage is named Vpsu.
Crucial requirement: Vpsu is stable so that it varies very little between no load and full load. This is easily achieved electronically.
Vpsu is inside the LED allowable operating range.
Vpsu < V_WT_out by a small amount.
System "just works".
When WT can supply load OK then WT Vout will be above V_WT_Out and diode from mains supply will "block".
When WT can supply only part load WT Vout will be pulled down by excess load untiL voltage reaches Vpsu, when Vpsu will supply any excess.
This is an update answer based on the folowing now known parameters as at December 2nd 2011:
- Save money by using wind turbine to power LED lighting as much as possible.
500 Watt, 12 VDC LED lighting load.
Wind turbine with output typically in 100 - 500 Watt range.
Grid electricity always available.
Nominal 12VDC wind turbine output.
Higher under no load.
Drops below 12V if loaded excessively.
The WT output may be used relatively "raw" - eg providing 3 phase rectified DC output in a 12V DC system, OR may have use advanced controllers such as MPPT (Maximum Power Point Tracking) equipment.
Regardless, the term eg 'WT output' will be used for simplicity and any extra assumptions and equipment wll be discussed when relevant.
WT = Wind Turbine.
(1) The easiest and cheapest system is to use diodes to combine power from the two sources.
The grid power system is a psu (power supply unit) which contains a "blocking diode" so that a voltage higher than it can be applied to its output without causing it damage and without causing current to flow back into the supply.
The output of the grid power psu is set to V_LED_min (ie a bit less than 12 V in this example).
The WT output is fed to the LEDs via a "blocking diode". This prevents grid power psu curent flowing backi into the WT system.
At any stage that there is energy available from the WT (wind turbine) it will cause the 12V "rail" to rise above V_LED_min and automatically supply whatever energy it is able to. When wind energy is adequate this will automatically provide all power from the wind.
As there is no battery this is almost a complete answer.
This is also, perhaps surprisingly, also an essentially complete answer for systems in which WT energy never exceeds the current LED load. Only when WT_energy > Current_LED_load do you need a battery or fancier control systems.
While the above method of using diode feed from WT and grid supply would also work with a battery, better controllers can produce better rsults