Dump and Diversion Loads
A Dump Load, also known as a diversion load or dummy load, is commonly used in wind and small or microhydro systems to “divert” (hence its name) excess power when the batteries are full in an off-grid system as any excess electrical power generated has no other place to go.
The function of any solar charge controller is to regulate and control the charging of a battery, or batteries, in order to prevent them from overcharging and becoming damaged. The charge controller should disconnect the charging current flow coming from a solar, wind, or hydro power generating device and divert any excess energy to an externally connected secondary connected load, such as a resistance or water heating element. Then basically, a dump load is where the extra unwanted power is sent.
Why is a Dump Load Necessary
Well, with a photovoltaic solar system, when the storage batteries are fully charged, the charge controller can simply disconnect the PV panels preventing any further charging current from damaging the batteries. Another reason to take note of a photovoltaic panels open-circuit voltage, VOC value.
However, wind turbine generators (WTG), either vertical or horizontal, are designed to operate under certain load conditions. So for a stand-alone off-grid wind generator (or a hydro generator), there needs to be some method of controlling the speed of the electrical machine if the batteries are full and the wind continues to blow. As an electrically connected load keeps the wind turbine generator within its designed operating range.
If a wind turbine generator, (WTG) is allowed to rotate in the wind with the batteries disconnected, it will start spinning at very high speeds because it is operating without any connected load to act as an electrical brake. This overspeed condition can cause mechanical damage to the turbine as it could potentially self destruct.
Also, if the charge controller then decides to reconnect the wind turbine to the batteries once again while it is rotating at high speed, a mechanical shock to the wind turbine can occur due to the sudden decrease in rotational speed. Thus wind turbines are designed to operate under load conditions.
There are three common methods for controlling the rotational speed of a wind turbine generator. (1), mechanically spilling wind from the blades by changing their pitch angle. (2), use a mechanical brake to stop the turbines rotation at high speeds. Or (3), use some kind of electrical load in the form of a dump load to act as an electrical brake. Clearly, No3 is the cheaper option.
For a small off-grid stand-alone generating system, the generated power is subject to the availability of the source, either wind or water. Thus, any variation in power demand by the user or batteries must be controlled by the charge controller using a resistive load. Then a dump load or diversion load is nothing more than an electrical resistive element that is correctly sized to handle the full generating capacity of a wind (or hydro) energy system.
How Does A Dump Load Work
As stated previously, in a wind energy system, turbine generators are commonly used to charge batteries or feed the electrical energy back into the utility grid. A dump load charge controller is basically a solid-state voltage sensing device which constantly monitors the terminal voltage of a battery or connected battery bank to determine its state of charge level.
For a single 12.7 volt deep cycle battery, when its terminal voltage reaches approximately 14.4 volts, it is considered “fully charged” so the charge controller senses this voltage level and disconnects the wind turbine preventing overcharging and damage. At the same time, the charge controller switches the wind turbine’s output power to the dump load connected to it which keeps the wind turbine generator rotating at a constant rotational speed.
Now once the battery bank’s voltage drops a below 12 volts (about 50% capacity), the charge controller senses this also and reconnects the output supply from the wind turbine generator back to charging the battery. This connecting, disconnecting cycle of both the battery and dump load is repeated as required preventing the battery from overcharging and maintaining the turbine generator is always operating under load.
Note that batteryless grid-tied systems (wind or hydro) will use the connected grid as their dump load, sending all the excess energy back into the utility’s grid. For off-grid hydro or micro-hydro turbines, they will still require the means to dump excess energy when the batteries are full or the household loads are reduced.
Calculating The Resistive Value
Now that we know that a resistive dump or diversion load is used to convert excess electrical energy into a more useful form of energy, commonly heat. Let’s now look at calculating the resistive value required for a dump load resistance, or an equivalent water heating element.
Let’s assume for our simple example that we are interested in buying the YeaMarine Wind Turbine Generator from Amazon to charge a number of batteries connected together to create a 12 volt battery bank. What value dump load will we need for this turbine.
Ohm’s Law tells us that electrical power is equal to voltage times amperage. That is: P = V x I. The specifications of the selected wind turbine from YeaMarine tells us that it has a 12 volt, 400 watt generator. So this would be ideal to charge our 12 volt connected battery bank at 14.4 volts.
The dump load must be capable of dumping the maximum power available from the wind turbine. The maximum voltage of our battery bank is roughly the voltage of a fully charged 12 volt battery which is 14.4 volts. The amps is the electric current produced by the YeaMarine Turbine at full speed which is:
Amps = Power/Volts = P/V = 400/14.4 = 27.8 Amperes
Then the resistive dump load needs to be large enough to carry the full output current of 27.8 amperes from the turbine, but not so large as to exceed the current carrying capacity of the turbine generator or charge controller.
So for this example, we will use an uxcell Fixed Type 100W Watt 2 Ohm 2R Tubular Wire Wound Resistor, again from Amazon as our selected dump load resistor. This fixed value resistor has a power rating of 100 watts and a resistance rating of 2.0 ohms. Using Ohm’s Law again, we can calculate how much current this resistor will safely pass.
Amps = Volts/Resistance = V/R = 14.4/2 = 7.2 Amperes
Therefore, 7.2 amperes will flow through one of these dump load resistors. However, our dump load system needs to be capable of passing 27.8 amperes, so we need to wire or connect these 100 Watt dump load resistors together in parallel so that the dump load current is cumulative.
Then the total current our parallel connected dump load resistors needs to consume is equal to: 27.8/7.2 = 3.9 resistors. Therefore, we need to wire four of these Uxcell 2.0 Ohm wirewound resistors in parallel to give us the required 400 Watts of dump load capacity.
Dump Load Circuit
Just to confirm, four 2.0 Ohm resistors in parallel will give a combined resistance of: 1/(2-1 + 2-1 + 2-1 + 2-1) = 0.5 Ohms. As electrical power is equal to I2*R, then 27.82 x 0.5 = 386 watts, and within our 400 watt limit. Then the same process of calculating the number of resistive elements required can be used for any wind turbine and battery bank system.
Dump or diversion loads are a convenient way to divert or shunt excess electrical energy that could otherwise damage a renewable energy system once the batteries are full. Then anything that has a resistive element is great as a dump load as they can take a lot of electric power. But as well as large wattage resistances, immersion elements in hot water tanks also make useful dump loads.
Water Heating Element Diversion Load
Resistive dump loads are simple and easy to design but are also somewhat wasteful as they take the excess electricity and convert it into heat which is then dissipated into the surrounding air. Another more useful option is to use the excess electricity to heat water.
Using an ordinary electric water tank, the regular AC powered heating element can be replaced with a special DC (direct current) heating element. Excess power diverted by the charge controller warms the water in the tank, thereby reducing the AC grid power you use.
Water Heating Element
DC water heating is a relatively new idea as an off-grid way to heat water using hydro or wind power. Insulated water tanks and old electric storage heaters can be used as pre-heating tanks with 12, 24 or 48 volt DC elements as dump loads up to several kilowatts of power.
But any electrical resistance water heater used as a dump load must be electrically sized to handle the full generating capacity of the wind, or microhydro turbine to keep the turbine fully loaded all the time and keep the combined load consistent.
After all, the fewer electrical loads that are turned on in the home, the more the dump load will be switched “ON” as any variation in power demand is controlled by the resistive dump load element dispersing it as heat. After all, who doesn’t need hot water sometimes.
Dump Load Tutorial Summary
We have seen here in this tutorial that dump or diversion loads provide a place for excess electrical power to be dumped or diverted away once the batteries are fully charged in an off-grid system. Dump loads such as power resistances, electric water heaters or hot air heaters dissipate any excess power generated by wind or hydro generators otherwise it has no place to go.
In a batteryless grid connected or grid-tied system, the load is the utility grid so is always connected. But for a battery-based solar system charge controllers are required to regulate the charging of the storage batteries. When a photovoltaic panel or array is disconnected from a battery bank or load, it just sits there in the sun not generating power. However, for wind or microhydro turbines their rotational speed may increase to the point that they self-destruct or create overvoltage conditions.
Dump or diversion loads can prevent a wind or hydro generator from spinning too fast once the batteries are fully charged due to the charge controller disconnecting the electrical generator from the attached load. Dump or diversion load control by the charge controller always keeps the generating turbine electrically loaded, which in turn controls the turbine’s rotational speed.
To learn more about how Dump Loads are used with wind turbine generators or ever wondered how wind turbines work and why they look like they do. Or are you interested in adding wind power to your existing off-grid electric system. Or maybe you just want to explore the advantages and disadvantages of wind energy and wind power and how you could use it to power your home, then Click Here to get your copy of one of the top “Hands-On Guide to Harnessing Wind Power” direct from Amazon today.