Since there is a lot of confusion about diversion loads, we will discuss what they are and why they are used.
A dump load is nothing more than a resistor (heating element) that is used for dumping electricity when it is not needed or to prevent your battery bank from being overcharged.
It could be a water heater or air heater or some other resistive load. Inductive loads (like a pump or television) aren’t usually the best idea for a dump load but they have been used successfully on occasion. Some AC heating elements make great dump loads.
When the battery (battery bank) is full and the water turbine/wind turbine/solar module is still making power, a dump load will give the electricity somewhere to go. Click here for examples of affordable dump loads.
The extra electricity is simply dumped or diverted into the diversion aka dump load.
There are three reasons why you might want to add a dump load to your off grid power system.
1. You have a wind turbine
2. You have a water turbine
3. You make more electricity than you consume
Do I need a diversion load for my system?
The short answer is maybe. Solar modules are a unique piece of equipment as they are very difficult to damage.
You can short circuit a solar module and nothing happens. No current flows.
You can open circuit a solar module and nothing happens. No current flows.
When your batteries are full and the solar modules are still producing power you have three good options:
- Disconnect the solar modules from your battery bank
- Short circuit the output of the solar modules
- Dump the excess electricity into an air heater or water heater
Since solar electricity is expensive is does not make good sense to just waste the extra power as would happen if you disconnect or short circuit the modules. In this case a dump load (diversion load) makes much more sense. It is not necessary but likely a good idea.
A wind turbine must always be connected to a load (that is what the batteries are). Unlike solar modules, a wind turbine will be damaged if allowed to open circuit. A wind turbine produces voltage according to the speed of the alternator. The faster the alternator spins, the higher the voltage. This is true of any alternator that doesn’t have a voltage regulator. If you remove the voltage regulator from a car alternator, it can easily produce 120 volts (10 times normal) or more.
In an off grid system, the battery bank will limit the voltage the wind turbine produces. When the turbine is connected to a 12 volt battery bank it will produce 12 volts (more like 13 or 14 volts) as it slowly raises the voltage of the battery bank until it is charged.
If the battery bank is full and the turbine is still producing power, that power must have a place to go.
As we have already discussed, a solar module can simply be disconnected, but a wind turbine cannot.
A wind turbine cannot be disconnected as the voltage will spike and damage components inside the turbine such as windings and rectifiers. Most wind turbines cannot be short circuited either as in a strong wind they will keep making power, heat up and possibly destroy the windings and/or diodes.
There are a few exceptions to this rule such as the Skystream by Southwest Windpower. The Skystream has a sine wave, grid tie inverter built in that allows the Skystream to connect with the grid without purchasing any controls. It is designed to check for grid power constantly and in the event of a power outage the turbine will apply an electronic brake. You can open circuit the turbine and it will simply shut down. Keep in mind the turbine still has a load which is the inverter. This turbine was built for the on grid folks but it works great as a battery charger as well. Check out this page to see how to use the Skystream to charge batteries.
Like wind turbines, water turbines must always be connected to a load (or battery bank). A water turbine’s voltage is generally a little more stable than a wind turbine (as the water pressure doesn’t vary wildly like the wind) but can still spike to 3 or 4 times it’s usual output when allowed to open circuit. If your turbine is producing upwards of 60 volts in a 48 volt system, it could spike to 240 if allowed to open circuit. This voltage will likely not be en0ugh to damage the winding but can certainly destroy the rectifier or external charge controller.
AC coupling is becoming more common as folks who have their solar systems connected to the grid have become upset that their power goes out when the utility power goes out.
One solution to this issue is AC coupling. It is the act of adding a battery backup and battery inverter system to your grid tied system. When the power goes out, the solar array goes into the grid tie inverter, out of the grid tie inverter as DC and is fed to the output of the battery based inverter. After this electricity goes into the battery based inverter, it is converted to DC and charges the battery bank.
The danger of this system is there is nothing to stop the battery bank from overcharging. That is where the dump load becomes a necessity.
Whether you are using a dump load for your water turbine, wind turbine, off grid solar array or grid tie solar array go to dump loads to learn how to set up a dump properly.