A charge controller or charge regulator limits the current being delivered by the power source to the battery.
To be useful, "12 volt" wind generators need to be capable of delivering 16 to 20 volts in moderate winds (at say 250-400rpm).
The number of cells in a solar panel is large enough so that a useful level of charging current is provided even when the light level is low and the battery voltage is high. Typically solar panels (which are intended for use with 12v batteries) are designed to deliver 17v in sunlight.
If there is no regulation the storage batteries would soon be damaged, either by the wind generator or by solar panel overcharging. Most 12v batteries need around 14 to 14.5 volts to get fully charged.
Wind and water turbines need to be protected from 'overspeed' which could occur if a load was suddenly removed or switched 'off'. Overspeed protection is normally achieved by maintaining a constant electrical load on the turbine. As well as providing voltage regulation the charge controller also ensures that this electrical loading is present at all times. The electrical load is either provided by charging the battery, or if the battery is fully charged then the excess power is normally diverted to a dump load/braking resistor (which could be used for air, water or under floor heating).
Many solar charge controllers are designed to disconnect (or open circuit) the solar panel when the battery becomes charged and re-connect the solar panel when the battery needs recharging. While this is acceptable for solar panels, these series regulators are unsuitable as wind and water turbine charge controllers as they would cause the turbine to overspeed and damage would result from excessive centrifugal force or excessive vibration.
Have the following characteristics…
- The wind generator is not regulated or controlled and continuously delivers the available power to the regulator and battery.
- The regulator constantly monitors the battery voltage and switches between two states determined by the battery voltage.
- If the battery voltage falls below a "low" set limit the controller disconnects the dump load and allows the battery to charge.
- If the voltage rises above a "high" set limit the controller turns on a dump load and isolates the battery from further charging.
In normal operation the wind controller will cycle between these two binary operating states (Charging and Charged), thus achieving the battery voltage regulation between the controllers low and high voltage set points* (*Note: see hysteresis below).
Two modes of operation.
There are two possible ways in which the simple shunt regulator can be incorporated into a wind generation system. Firstly as a "dump load controller" (sometimes called a "simple battery shunt" or "shunt mode") and secondly as a "turbine brake controller" (sometimes called "back EMF braking" or "diversion mode"). The difference is that in “Diversion Mode” the regulator only diverts the instantaneous generated power to the dump load and only when the battery is charged. Note: Stored battery power is never dumped by a regulator in Diversion Mode (this is prevented by the presence of a blocking diode).
In “Shunt Mode” the regulator operates as a simple battery shunt and has to dump the generators full rated power capacity each time it turns on (whatever the prevailing conditions) consequently the dumping of battery power is a feature of this mode of operation.
DC Controller configured in "shunt mode" DC Controller configured in "diversion mode" (turbine brake controller).
AC Controller configured in "shunt mode" AC Controller configured in "diversion mode" (turbine brake controller).
In the "shunt mode" configuration, and in windy conditions, once the battery is fully charged the rotor speed will not change significantly when the controller switches between the Charging and the Dumping states.
In the “turbine brake controller" configuration, once the battery is fully charged (and the controller has entered into the “charged/dumping” state) the rotor speed will be determined by the braking resistor impedance. If the braking resistor is a low impedance, then the rotor will be observed to slow down. As the controller switches back into the “charging” state then the rotor will speed up again.
Some shunt regulators are designed to operate in one mode only, some can be configured in either of the two modes during installation, some can be dynamically switched during operation. Shunt regulators can’t operate in both modes at the same time.
Pulse Width Modulation Regulators
- PWM charge controller regulates the power being sent to the battery.
The PWM regulator is a proportional controller which is capable of varying the charge duty cycle between 0 and 100%. The controller constantly checks the state of the battery to determine how fast to send pulses, and how long (wide) the pulses will be. In a fully charged battery with no load, it may just "tick" every few seconds and send a short pulse to the battery. In a discharged battery, the pulses would be very long and almost continuous, or the controller may go into "full on" mode. The controller checks the state of charge on the battery between pulses and adjusts itself each time.
- A PWM dump load controller regulates the 'excess' power which needs to be dumped.
This is an alternative way in which a PWM regulator can be configured. Instead of regulating the power being sent to the battery (see above) it regulates the excess power that needs to be dumped into a braking resistor. With a discharged battery, pulses would never be sent to the braking resistor. When the battery is fully charged and excess power is still being generated then the PWM dump load controller sends pulses or may go into "full on" mode if the generated power is high. Perhaps I should have written this paragraph like this...Dynamic braking systems use a "braking resistor". When slowing down is desired, the braking resistor is connected in varying duty cycles depending on how much slowing is desired. Such brakes kick in when the power produced is greater than the power needed or consumed by the ordinary load?
With the development of PWM charge controllers came a new and improved way of charging batteries using bulk, absorption, float and equalization charges. These are a great improvement over shunt charge controllers as they are able to keep the battery voltage much more stable. Further information on the benefits of PWM can be found at "ablesolar.co.nz/images/products/regulators/Morninstar Controllers/Why PWM.pdf".
Wind turbine systems normally require a PWM regulator with a dump load.
(to maintain the load on the generator/turbine and to dissipate energy when the battery becomes charged). Such regulators allow the wind turbine to deliver all of the available power to the regulator and battery. Examples of PWM shunt regulators which support an external dump load include the Xantrex C40 and Morningstar Tristar-45 family of regulators.
Shorting the generator output?
The output from a DC generator should never be ‘shorted’ while it is rotating since the commutator and brushes will quickly burn out. Some small machines with more internal resistance and servomotors may survive limited abuse but shorting the DC generator output as a means of continuous regulation should be avoided.
AC wind generators have lots of kinetic energy stored within the rotating components and shorting the generator output induces very large currents flowing within the coils. This may cause excessive heat build-up and premature failure of the windings (particularly if the alternator windings are potted within resin, as air cooling is severely constrained).
Shorting the windings of an AC generator should only be considered as a maintenance function. If the turbine/generator does not stop within 10-15 seconds then the braking effect is insufficient to overcome the wind strength. If the generator is allowed to continue to rotate with the generator output shorted then permanent damage could occur. Shorting the AC generator output as a means of continuous regulation should be avoided.
Wind compatible “Solar style” charge controllers?
There are an increasing number of “solar style” charge controllers which utilize the shunt/diversion mode architecture without a dump load. When the battery becomes charged the “solar style” charge controller applies a short to the power source, which works perfectly well with solar panels, but care needs to be taken when considering their use for wind generator applications. These “solar style” charge controllers include the JUTE CMP24 family (20A, 30A and 45A), Hybrid controller CQ1210 and Seca’s Solarix; Alpha, Gamma, Sigma and Omega family (with the ATONIC® chip architecture).
Modern wind turbines can be designed to take advantage of “solar style” charge controllers (they are cheaper than conventional PWM controllers which require a dump load). However they need to be designed from first principals for use with “solar style” charge controllers. Two wind turbine systems that are compatible with “solar style” charge controllers (which do not have a dump load) include the Wren Micro-turbine which is compatible with the Samrey 30A Shunt Charge Controller (a rebadge Seca Solarix Omega) and the Macro-Wind small wind turbines (MW-200 and MW-400) which are compatible with the solar style charge controller supplied by Macro-Wind. Additional protection has been embedded within the wind turbine manufactured by Macro-Wind to ensure compatibility with a solar style charge controller which has no provision for a dump load.
You should not assume that a new solar style charge controller which has no provision for a dump load will be compatible with your existing wind generator. You need to check for compatibility with your generator and the solar style charge controller suppliers. Apart from the damage referred to above caused by the application of frequent shorts to the generator output there will be the additional problem that the turbine would be beset with frequent stops. If the winds are light then frequent stops means that you will loose the ability to generate power in low winds.
Solar style charge controllers with no provision for a dump load are effectively regulating the generator (not the battery) and consequently they do not allow the wind turbine to deliver all of the available power to the user. These problems can be avoided by using a conventional charge controller with a dump load.
Maximum Power Point Tracking.
MPPT charge controllers can be used in conjunction with uniform solar arrays consisting of multiple, identical solar panels. The MPPT controller is designed to maximise the quantity of power obtained by performing a periodic sweep of the solar power curve to determine the ideal voltage at which the maximum power can be extracted. The timing of the sweep has been optimized to take account of solar events like "passing clouds" (typically the sweep occurs every 7 minutes).
The power output from fixed pitch wind generators have significant short term fluctuations, as the speed is constantly changing with the variable wind conditions. MPPT systems are not fast enough to keep up with the changing condition of the turbine. Consequently the MPPT sweep algorithm will produce erroneous data with each gust of wind. Hence MPPT controllers are not generally used for fixed pitch wind turbine generators.
The power output from variable pitch wind generators and from water turbines can remain constant over the longer term. This makes them more suitable for use with MPPT power controllers. Eoltec reccomends the Aurora® MPPT controller with their variable pitch Scirocco see "power-one.com/renewable-energy/index.php".
Hysteresis is an integral characteristic with shunt regulators (but not with PWM regulators).
The regulator is either 'off' or 'on', with nothing in between.
The regulator is a system; its input is the battery voltage, and its output is the 'Charging' or 'Charged/Dumping' binary state.
If we wish to maintain a battery voltage of 12.5v, then the regulator may be designed to turn the dump load 'on' when the battery voltage rises above the 12.6v set limit, and turn it 'off' when the battery voltage falls below the 12.4v set limit.
The controllers "low and high voltage set points" and a "lock out" time constant within the controller define the characteristic hysteresis properties of the controller.
Domestic central heating thermostats also exhibit hysteresis. Further information on hysteresis can be found on Wikipedia.
Ametek and treadmill motors.
The best DC (Permanent Magnet) motor for use as a generator is the one that has the highest rated voltage at the lowest RPM figure (this applies to Ametek and treadmill motors).
There is some useful information on different Ametek motors which can be found at "tlgwindpower.com/ametek.htm". As treadmill motors are supplied by a multitude of vendors there is no one source of similar information.
You should be able to calculate the number of RPM required to generate one volt. This figure is useful for comparing DC generators. Simply divide the stated RPM on the rating plate by the voltage on the rating plate. DC motors are linear devices, voltage and speed form a straight line graph. Lets assume that one volt is produced for every 20RPM. As the controller requires about 17v to fully charge a 12v battery (see Introduction above) then the DC motor will have to turn at (17 x 20) = 340RPM. This is frequently called the "cut in speed" which is the lowest speed at which the generator will charge the battery. At speeds above 340RPM the quantity of current generated and delivered to the battery will increase. DC generators require a "blocking diode" to prevent the controller/battery from powering the generator as a motor.
You stand a much better chance of charging a 12v battery than a 24v one as the "cut in speed" is half for 12v compared with 24v systems.
3 Phase AC generators.
One of the advantages of using 3 Phase AC generators is that the cable losses are reduced considerably. DC gererators generally require thick copper cables and a short run to the controller and battery (to minimise voltage drop). Three phase AC generators are capable of transmitting the generated power further as higher voltages are generated and lower currents are used. Consequently the cable losses (I²R) are lower.
In most small-scale designs, the rotor is connected directly to the shaft of a permanent magnet alternator, which creates wild, three-phase AC. Wild, three-phase electricity means that the voltage and frequency vary continuously with the wind speed. They are not fixed like the 50 Hz, 240 VAC electricity coming out of household outlets. The wild output is normally rectified to DC. A three phase rectifier is often located close to, or as part of, the charge controller. However there are exceptions. The Aerogen 4 is a 3 phase AC generator. It has the 3 phase rectifier within the turbine head and delivers DC on two wires.
Lead-acid batteries fall into two categories. 1. Shallow cycle - these are the type used to start your car. They are designed to deliver a large amount of current over a short period of time. This type is unsuitable for a home power battery bank. They cannot withstand being deeply discharged, to do so shortens their life. 2. Deep cycle - Designed to be discharged by as much as 80% of their capacity, this is the type of choice for home power systems. The life of deep cycle batteries will be extended if the discharge cycle is limited to 50% of the battery capacity and if they are fully recharged after each cycle (this avoids positive plate sulphating). The quickest way to ruin lead-acid batteries is to discharge them deeply and leave them standing "dead" for an extended period of time. When they discharge, there is a chemical change in the positive plates of the battery. Batteries that are deeply discharged, and then charged partially on a regular basis can fail in less than one year.
Second hand batteries from computer UPS and GSM base-station installations frequently come onto the market. These batteries are normally removed from service when the battery backup time (i.e. the battery capacity) has fallen below acceptable operational limits. Batteries always have a manufacturers date code on them (for warranty purposes), make sure you know what it is before you purchase. Second hand traction batteries (milk float, fork lift and submarine) are ideal but difficult to value. However the price will never fall below the scrap value for lead. Storage batteries need adequate ventilation.
State of Charge (approx.) 12 Volt Battery Volts per Cell
100% 12.70 2.12
90% 12.50 2.08
80% 12.42 2.07
70% 12.32 2.05
60% 12.20 2.03
50% 12.06 2.01
40% 11.90 1.98
30% 11.75 1.96
20% 11.58 1.93
10% 11.31 1.89
0% 10.50 1.75
Dump Loads (as used in 'battery shunt' configuration)
Typically 0.5 to 2.0 ohms. (for example: a 12volt 200watt dump load would consume 16.6amps and have a resistance of 0.72ohms).
The dump load should be dimensioned to dissipate the generators maximum output power. You can use a "car ceramic heater" or a regular 12/24/48v immersion heater. If you need a higher capacity dump load you can use a cheap DC-AC inverter to generate 240volts and a domestic oil filled radiator.
Car headlight bulbs may be used for experimentation, but are not suitable as a permanent fixture since they will burn out during high winds and without the dump load the controller will either "boil" the battery or fail to load the generator which will then overspeed (depending upon the controller design and failure mode). Incandescent bulbs also have a low impedance when cold and induce very high switching currents. Dump loads can be controlled by MOSFET's or by relays.
Braking Resistor (as used in 'turbine brake controller' configuration)
Typically 1 to 5 ohms.
To determine your optimum braking resistor value you may need to experiment with different power resistors during various wind conditions. A very low impedance braking resistor would cause the turbine to slow instantaneously to a low speed, which could place unnecessary stresses on the turbine. The benefit of the “turbine brake controller" configuration, which slows the rotor down, is less wear and tear on the rotating components while the battery remains in its fully charged state. A Rheostat is useful in determining the ideal brake resistor value when configured in the "turbine brake controller" configuration. The braking resistor should be dimensioned to dissipate the generators maximum output power.
Wind and Solar systems.
When designing a hybrid system, select the wind turbine first and use the flexibility of solar panel sizing to round out the system. If additional energy is needed in the future, a few more solar panels can easily be added to the system. Increasing the size of a wind system is not as simple. Usually the tower will be too light for a larger turbine and the guy anchors and power cable will need to be replaced. Avoid making the wind system too small as it is costly to increase its size, even by a small amount.
Wind power is proportional to the wind speed cubed.
(e.g. 2 times the wind speed provides 8 times the power)
Increasing the wind speed from 5 mph to 10 mph increases the power 8 times. (2³)
Increasing the wind speed from 5 mph to 15 mph increases the power 27 times. (3³)
Increasing the wind speed from 5 mph to 20 mph increases the power 64 times. (4³)
Increasing the wind speed from 5 mph to 25 mph increases the power 125 times. (5³)
Increasing the wind speed from 5 mph to 30 mph increases the power 216 times. (6³)
Increasing the wind speed from 5 mph to 35 mph increases the power 343 times. (7³)
Increasing the wind speed from 5 mph to 40 mph increases the power 512 times. (8³)
Increasing the wind speed from 5 mph to 45 mph increases the power 729 times. (9³)
Increasing the wind speed from 5 mph to 50 mph increases the power 1000 times. (10³)
Consequently wind regulators/charge controllers have to cope with a large dynamic range of input power (voltage and current) from your generator. Note: There is very little energy in low wind speeds.
Doubling the tower height increases the wind speed by a minimum of 10%.
Due to the cubic relationship with speed, the power increase is 33% (1.1 x 1.1 x 1.1 = 1.33).
You need to avoid turbulent air flow (caused by adjacent buildings and trees) ideally you need to site your turbine in an undisturbed laminar air flow. You can try researching your local air flow conditions at various heights by flying a kite. Tie several 10' streamers to the kite line at various locations below the kite. If the streamers lie parallel and horizontal then they are in laminar air. If the streamers are all over the place they are in turbulent air.
Centripetal force within the spinning rotors increases as the square of the rotation speed, which makes this structure sensitive to overspeed. Search YouTube for "Windmill/Wind Turbine Explosion".
Overspeed protection in high winds.
Wind turbines with a blade diameter greater than 1 meter are often equipped with a furling mechanism to limit the power and to protect the turbine from overspeed. Furling is a passive form of control in which the rotor yaws and/or tilts out of the wind to limit the aerodynamic torque and thrust loading. It is somewhat difficult to describe in text. Fortunately you can see one such mechanism in action. Search YouTube for "Wind generator Furling in High winds".
Furling control is impractical in large utility-scale wind turbines because of the enormous gyroscopic loads that would ensue. Instead, large wind turbines usually employ active controls to regulate power, limit loads and improve stability. These include active control of the blade pitch, generator torque and nacelle yaw. The "soft-stall" method is found to offer several advantages: increased energy production at high wind speeds, energy production which tracks the maximum power coefficient at low to medium wind speeds, reducing furling noise, and reduced thrust. These means of control are unreasonable for small wind turbines due to the large costs involved.
Fixed Pitch and Variable Pitch
There is an excellent comparison between fixed pitch and variable pitch wind turbines at "vpturbines.com/html/faq.html" Further information on pitch and stall control can be found at "windpower.org/en/tour/wtrb/powerreg.htm" Suppliers of variable pitch wind turbines include Proven with their patented Flexible Blade System®, ALTERNATE POWER TECHNOLOGIES Inc., Eoltec with their Scirocco, Superwind GmbH with their Superwind 350 and Jacobs Wind Systems. Variable Pitch is either...
"Active Pitch Control" where the wind speed is measured and a computer controls the pitch of the blades, and
"Passive Pitch Control" where the centripetal force of the rotating blades activates a mechanical system that changes the pitch.
Pitch control. Yaw control. Out of control.
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