Wind/Solar Hookup Basics and Beyond.

Page 4

 

 

Diversion Loads. What is a diversion Load, and do I need one.

 

Diversion controllers work by diverting excess energy from the wind turbine to a diversion or "dummy load". This diversion allows the turbine to remain under a load at all times. A solar panel may be safely disconnected from the batteries, but an active wind turbine should never be disconnected from its load (battery/diversion load). When a wind turbine is not loaded, it can easily speed out of control in high wind events, which can lead to catastrophic failure of the turbine as well as the possibility of damage and injury to other property and people. It is very important that your turbine has a very reliable load at all times.

If you have a solar only system (or a system that does include a wind turbine) you do not need a diversion load.

 

About load diversion: The basic operating philosophy of a diversion controller is quite simple. Monitor the battery voltage, and if it should rise to a predetermined level, connect a diversion load of sufficient size, to the battery or energy source to prevent the battery voltage from increasing any further. The amount of time the diversion load is connected is generally only 10 to 30 seconds. In this amount of time, the battery voltage will have dropped enough to be back in the normal region. The controller will continue to engage and disengage the relays as often as necessary to prevent battery overcharge. This is the normal mode of operation. The microprocessor (in the Coleman Air Controllers) uses several advanced algorithms to prevent rapid relay cycle, yet it is common for the relays to be engaged and disengaged a few times a minute. This constant attention keeps the batteries very close to (or just below) the trip point you have set.

Several schools of thought on the subject.

  1. The source of power (wind turbine, solar panels etc.) -- should remain connected to the batteries while the dump load controller is actively dumping the excess voltage.
  2. The source should be diverted to the load directly and disconnected from the batteries.

We happen to believe that is far better to leave the wind turbine connected to the batteries at all times. Why? When you remove the battery level voltage from a wind turbine and send it's power directly to a load, then unless the diversion load offers an exact amount of resistance as did the battery, the turbine will immediately see either a reduction or increase in load, causing it to speed up or slow down. At the same time, the battery now has no charge source at all, and it's battery voltage will quickly lower (especially if the batteries are running other loads of some sort, like an inverter.) -- This will cause the controller, to immediately see a large drop in battery voltage and re-engage the turbine. The turbine will be constantly forced to work with a changing load, which can cause heat build up in the stator as the load increases, before the blades have time to slow down.

When you allow the turbine to see the batteries along with the load at all times, the turbine remains more within its design realm -- always a good thing.

 

A diversion load needs to be larger (by at least 20%), than the sum total of all your wind/hydro charge sources combined. When the diversion load is too small, battery voltage may continue to rise, even when the dump is active.

It is also very important to use a load that is not likely to fail. Light bulbs and similar such loads are not good diversion (dummy) loads, since they will fail and you may be left with no method to dump the excess energy from your batteries.

A common dummy load is thought to be a standard 120vac, 2000 watt heating element readily available from your local hardware store, but it will generally take quite a few of these to actually provide much load. The reason is that these elements are designed for 120 volts, not low voltage D/C. When you put them in a lower voltage system, the actual amount of power they can dissipate is reduced by a factor of four.

Since watts equal volts times amps, we can easily determine that the amps required to run this element at 120 volts is 16.66 amps (2000 watts / 120 volts = 16.66 amps). Then per ohms law we can determine this element must have an internal resistance of 7.2 ohms (120 volts / 16.66 amps = 7.2 ohms.)

Lets see if our 7.2 ohms is correct by starting from this point and moving back towards the stated wattage of the element..

Ohms Law states: Volts = Amps x Resistance. We have determined that the electrical resistance of this element is 7.2 ohms and 16.66 amps.

Let's just double check this.

So far so good. Now that we know the resistance of this element, we can determine how much wattage it will dissipate at any voltage. We simply divide the voltage by the resistance to get the amperage. Then multiple the amperage times the voltage to get the watts -- Same two formulas.

 

Using standard 120 volt A/C domestic water heating elements

Nominal Battery Voltage Default dump voltage Amps & Watts dissipated at dump Elements needed for a 1000 watt turbine
12 volts 14.4 volts 2 Amps, 28.8 Watts 35 Elements!
24 volts 28.8 volts 4 Amps, 115 Watts 7 Elements!
48 volts 57.6 volts 8 Amps, 460 Watts 3 Elements.

As you can see, for low voltage systems, the 2000 watt, 120VAC domestic hot water element is really not very practical. These elements must also be in water at all times, and the water must allow for continuous heating without boiling.

If you wish to heat water, then you should consider heating elements designed to run at the voltage level of your battery bank.

 

A more much acceptable dummy load is to use power resistors. These can be obtained from our website at www.ColemanAir.us

Using Power 100 Watt Resistors

Nominal Battery Voltage Default dump voltage Resistance Value Amps & Watts dissipated at dump Resistors needed for a 500 watt turbine Resistors needed for a 1000 watt turbine
12 volts 14.4 volts 2 Ohms 7.20 Amps, 103 Watts 5ea, 100 watt, 2 ohm resistors. 10ea, 100 watt, 2 ohm resistors.
24 volts 28.8 volts 10 Ohms 2.88 Amps, 115 Watts 5ea, 100 watt, 10 ohm resistors. 10ea, 100 watt, 10 ohm resistors.
48 volts 57.6 volts 25 Ohms 2.3 Amps, 132 Watts 4ea, 100 watt, 25 ohm resistors. 8ea, 100 watt, 25 ohm resistors.

 

The resistors in the chart above are an example only; it is perfectly acceptable to use higher wattage power resistors if they can be obtained economically. Place multiple resistors in parallel for a higher wattage load. When you place same value resistors in parallel, you double the wattage rating, and the resistance. This is a safe method of doubling the wattage/amperage handling capability of your diversion load.

Important! You cannot simply use a lower value resistance without also increasing the wattage rating of your resistor. For instance, attempting to use a single 500 watt power resistor of 2 ohms on a 48 volt battery system (60v dump), will result in the dissipation of 1800 watts, however the resistor is only rated at 500 watts, and may be destroyed.

 

Placing resistors in parallel.

 

 

 

Thinking about using other devices such as grid tie inverters, please see our Frequently asked Questions (FAQ's) on this subject.

 

Some of the Coleman Air Controllers have a mode called EDM -- What is this?

Extended Diversion Mode -- As stated above, the basic operating philosophy of a diversion controller is quite simple. Monitor the battery voltage, and if it should rise to a predetermined level, connect a diversion load of sufficient size, to the battery or energy source to prevent the battery voltage from increasing any further. The amount of time the diversion load is connected is generally only 10 to 30 seconds.

There are however, situations where you would really like the controller to engage the relays for a longer period of time once the batteries get to a "Full" state. This is what we call Extended Diversion Mode. When you enable this mode (with the Coleman Air controllers that have this feature -- see jumper settings in the manuals), and the batteries reach the trip point you have set (the same trip point as the normal mode), the controller will engage the relays for approximately five minutes or until our batteries are depleted by 15%, which ever comes first.

The EDM mode is very useful for running such items as water pumps or small grid tie inverters that you do not want turning on and off every few seconds. When you enable the EDM mode, the wiring remains the same; the difference is that the load you connect will be engaged for a longer period of time.

It is very important that the load you choose is 100% dependable if this controller is being used to prevent battery overcharge. If the load is not present, then your batteries will overcharge. Grid-tie inverters are not a load if the grid fails (power outage due to thunderstorm etc.). Such a loss of load can also cause damage to your wind turbine if it depends on this load.

If you will be using the EDM mode with a load that may not be present at all times, then it is important that you have another controller in parallel that is also monitoring the system with a slightly higher trip point. This second, failsafe controller will then divert the excess energy to a diversion load that is 100% dependable should the 1st controller's load not be present or capable of disbursing all of the excess energy.

As in the case with the normal mode, the load you connect cannot exceed the capacity of the relays. Do not attempt to hookup highly inductive loads, as the relays will be damaged due to high currents during the motor start.

 

 

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