A cautionary note on the use of relays.
Inductive loads are difficult to switch, due largely to the wind turbine generator wanting current to continue to flow, even as a contact is being broken. Stored energy within the inductor will induce arcing across the relay contacts and an arc-suppression scheme of some sort is often employed. Relay’s need to be derated even if a 'snubber' circuit is used. Industry practice is to derate relay contacts to 40% of their resistive load rating.
Hence 40A automobile relays require derating to 16amps (40a x 40% derating factor = 16amps) when used with inductive loads which are typically associated with wind turbine generators. For further information use Google and search for "relay derating". Relays are mechanical devices and have a finite life (operating cycles).
Direct Heating without using a battery.
Heating takes lots of power, so you would require a turbine rated at around 1000W or more for it be effective.
Placing a heater directly onto the turbine output places a permanent load onto the turbine and consequently the turbine will have difficulty in starting to rotate, due to the presence of the load.
If load remains disconnected until the turbine is up to speed then the rotor will have much more freedom to rotate in light winds.
This can be achieved with a voltage sensitive switch which is monitoring the turbine voltage output.
If the voltage is 'low' the load is disconnected. As soon as the turbine is up to speed and producing sufficient volts then it connects the heating load.
This connection may cause the turbine to slow and the voltage switch to 'disconnect'.
With such a simple circuit, this on/off oscillation is not uncommon during light winds. Relays will tend to "chatter".
There is a better solution and that is to utilise a PWM controller in series with the turbine and the load (no batteries involved).
With no winds there is no power and there is no load so the rotor is free to rotate.
During light winds the rotor is turning and the PWM controller is being powered but the PWM controller is sending 0% of the power to the heater.
As the winds increase the PWM controller attempts to maintain a constant voltage to the heater by increasing the duty cycle (0% to 100%). The ramping is proportional to the power being extracted from the wind. As the increase in power is continuously variable (between 0% and 100%) there is no sudden step change in the loading on the turbine, consequently there is no on/off oscillation.
The turbine power level at which the PWM controller starts to ramp from 0% and the turbine voltage at which it reaches 100% may be user selectable, so that it can be optimised for the particular turbine configuration and wind conditions.
Direct Water Heating without using a battery.
If you were heating water (as opposed to under-floor heating or air heating), then you may have an additional requirement. Specifically if the water is approaching 100°C then you may require that the generated power is diverted to avoid boiling the water in the tank.
As the water approaches (say) 95°C a thermostatic switch can divert the power away from the water heating element (and slow the turbine, if required).
To raise the temperature of 1 litre of water by 1° Celsius, you will need 0.001164 kWh of heating energy.
If you have a standard domestic hot water cylinder of 115L capacity and you wish to raies the temperature from 20°C to 100°C you will require (0.001164 x 115L x 80°C) = 10.71 kWh of heating energy. If you have a 500w turbine, it's going to take 21hrs at full turbine output to boil the water in the tank (assuming that you are not using the hot water, in the meantime).
Grid Tie Inverters
A grid-tie inverter, or a (GTI) is an electrical device that allows turbine or solar panels to complement their grid power with renewable power. It works by regulating the amount of voltage and current that is received from the turbine or solar panel and converting this into alternating current. The main difference between a standard electrical inverter and a grid-tie inverter is that the latter also ensures that the power supplied will be in phase with the grid power. This allows individuals with surplus power (wind, solar, etc) to sell the power back to the utility. This is sometimes called "spinning the meter backwards" as that is what literally happens.
On the AC side, these inverters must supply electricity in sinusoidal form, synchronized to the grid frequency, and limit feed in voltage to no higher than the grid voltage including disconnecting from the grid if the grid voltage is turned off. A major advantage of Grid Tie Inverters is that the requrement for batteries is eliminated.
Grid-tie inverters have a maximum permitted input voltage. As windspeeds increase, this limit may be exceeded. In these circumstances the grid-tie inverter will automatically disconnect the turbine from delivering power to the mains. At this point the rotor is no longer loaded and it will rapidly increase to a dangerous speed. High voltages are still being applied to the input of the "off-line" Grid-tie inverter, which will destroy it. Then the rotor may also be destroyed by high speed vibration and centripetal forces. An additional problem is mains failure as this will also remove the loading on the rotor, with the same consequences. A frequency switch can be used to apply a diversion load and brake the turbine down to a safe speed, during either of these two conditions.
Grid-tie inverters also have a minimum input voltage which needs to be maintained if you wish to remain connected to the grid. Falling outside of this min-max window will result in the GTI disconnecting from the grid. To reach the minimum voltage you need to improve your ability to capture the wind. Tower height, rotor size, number of blades, blade design, etc.
Uninterruptible Power Supply.
A UPS can be used to store generated power within the internal batteries. They can also invert the stored battery power into sinusoidal mains power (AC).
The internal batteries can be complemented with additional external batteries to increase battery capacity. Second hand UPS are frequently sold on eBay and provide a cheap means of storing and producing clean AC power. They are available in many different power capacities. They are particularly useful in locations where grid power is not available.
If you have a water turbine and an AC induction motor producing 230v AC, it will be impossible to achieve the frequency stability of the national grid. Many UPS units are incompatible with such domestic generators as they have been designed to utilize the grid. The UPS designers have written the microprocessor code to where they need exactly 50.0Hz (or 60.0Hz) power. If it is not, they don't "see" the incoming power at all.
UPS manufacturers that are known to have this design problem include APC, TrippLite, and Best (as shipped from the factory). The Best "Fortress" line (and maybe others) can have its tolerance window expanded.