Diversion load question

zeppole

Please bear with me since I am a Newbie to this Forum.  I have an array of 310W panels set up as 4 strings of 4 panels each for a total of 16 panels.  The array feeds a Magnum Energy PT-100 charge controller.  At NOCT the array should deliver roughly 30 amps of current at roughly 120 volts DC.

I want to use the auxiliary relay on the PT-100 to control  an Omron 600VDC 100AMP DC contactor to divert the array output to a water heater when the battery bank reaches float.

Does anyone see any issues with this??

Thanks in advance.

BB.

Welcome to the forum Zeppole.

Some questions I would have:

• How will the Charge Controller behave when the array voltage/current is "hijacked". Will the controller, for example, properly change from float to charge and/or additional current to the battery bank when a well pump turns on (will the array "steal" current from powering your other DC Bus loads).
• There is an Arc Fault detection system on this controller... I don't expect that your contactor+water heater loads to trip it--But if tripped, will the controller shut down until there is a manual reset.
• The MPPT charge control does "optimize" the Current and Voltage from the solar array to obtain Pmp=Vmp*Imp. When you connect your heating element directly to the array, you will probably not experience optimum energy transfer from the array to the heating element. You may find out that even with all of the losses (0.95 charger * 0.85 AC inverter ~ 0.81 AC conversion efficiency) vs the array to heating element coupling. You may get, overall (time, temperature, weather, etc.) energy transfer to the heater by connecting to your AC inverter (possibly even a second "cheap inverter" to power the heating element--Use the float function to control the second inverter).
• Your DC solid state contactor probably needs a decent heat-sink (large metal plate) to keep cool. Do not put in a "box" with poor air circulation.
• As always with electronics (Omron and EBay (and other gray market sources), watch for counterfeit products.

I am not too concerned about the behavior of the MPPT Charge Controller--But I would certainly monitor its operation very closely the first few days to make sure the system does not get "stuck" in some weird state or requiring a manual reset.

I would figure out (measure voltage and current from the array into the heater vs different times of day/weather conditions) and see if direct to array or though MPPT->AC inverter->heater ends up giving you a better harvest.

-Bill

zeppole

Bill-

Thanks so much for your comments, they are very helpful.  To clarify one point, I would be using a mechanical contactor as opposed to an SSR.  I've had some bad experiences with SSR's in the past and avoid using them.

I am tring to avoid an additional inverter for cost and complexity reasons, but appreciate your comments regarding the loss of efficiency since the MPPT controller would be bypassed in the situation I posed.

If I use 48V DC elements in the water heater then could I use the output of the charge controller when the battery bank reaches float?  I have read where some folks have used a second charge controller downstream of the MPPT as a diversion load controller but I haven't been able to follw the logic.  From what I have read, something like a Morningstar 60 amp PWM (non MPPT) charge controller will work for this purpose.

Thanks again!

-Gary

Horsefly

I know it isn't what you were asking about, but...  I'm assuming your 310W panels are 72 cell, probably with a Voc of close to 45V each. 4 in series means a combined Voc at 25 deg C of 180V. I know the PT-100 can handle a higher voltage, but are you risking any damage at low temps? 

mike95490

A quality SSR from Grainger or Newark will not give any problems when heat sunk and snubbers added.
A downstream diversion controller won't really know when to turn on, because it can't sense the change from Absorb to Float. (voltage drop)  Maybe a Midnight Classic, with a WhizBang current sensor, can be programmed to add load when the Absorb current sags halfway through Absorb. Not sure if it's logic can work, but it will sense going to Float and trigger something,

zeppole

Horsefly--
I ran the panel specs through the Midnite String Calculator and get a VOC at -40 degrees C of 221.6 volts whic is still within the limits of the PT-100 which is good to 240VDC with a 48V battery bank.  The PT-100 is hard to beat given it's relatively high voltage input and up to 100 Amp output.

-Gary

BB.

The Midnite Classic has a 12 volt (open collector?) output that can be programmed to run external dump loads (water heating and such)--There are several programmable options (delays, cycle times, etc.). Look for the "waste not" mode (as I recall).

The "typical" configuration with a second "dump load" controller would be to set the main controller to absorb+float at say 59 volts, and the dump load controller to turn on at 55.2 volts (float voltage). If the dump load controller has the option, you would want it to have an absorb voltage (of say 59 volts) and a float voltage after X hours of absorb.

Of course, this does run into issues--What if you want the dump load off (water heater is hot). Then you need the main controller to go to "true float" of 55.2 volts so you don't over stress the battery bank.

Perhaps, if you programmed the main controller to float at 55.2 volts after 3 hours, and the dump controller to turn on after 2 hours of float--this could cause the other controller to stay in absorb because the absorb voltage has fallen due to the water heater load.

Anyway--You see how complex the issue gets with dump loads (typically use for wind and water turbines) get when mixed with a series type MPPT/PWM solar charge controller.

-Bill

jonr

It would be interesting to see a dump load controller that actually diverted all excess energy.   For example, use a current controller to limit the current into a water heater.  Starting at absorb and gradually ramping up as charging current drops.   Panel voltage above Vmp is an indication that more energy is available than is being used.