MPPT + PWM diversion controller?

[Justoine Knight]

So, I'm building an RV and want to put solar panels on the roof. When the battery is fully charged I'd like to use the 'spare' power to heat water.

I've managed to track down the Blue Sky SB3024iL DUO which appears to do everything I want except it only works on 24V. Unfortunately, I want to run a 48V charger/inverter so I need to look further.

Obvious solutions are:
1) wire the panels direct and just use a diversion controller - it pains me to be wasting all that energy.

2) use separate MPPT and diversion controllers - could be conflict between the two

3) just use an MPPT and manually switch on/off the heater - what a pain!

Is there an integrated solution?

[Vanhook2]

A thermostat on the tank of the water heater.
I suggest that you install a manual cutoff in case of tank failure / lack of water in the RV, so the element does not burn.
Since you don't want to waste the power, what happens to the power when the battiers are full & the water is hot?

Just some thoughts off the top of my head.

[birdhouse]

justoine-

the morningstar ts-45 and ts-60 will work for 12, 24 or 48 volts.  they will work in pwm or diversion mode.  i believe they also make a mppt version, but do not know if it will run in diversion mode and do mppt at the same time.  i doubt it.  like mentioned above, make sure your water heating element is always submerged in water or you can burn it out.  once the tank is hot, you could make a dump load out of nicrome wire and use it to heat your rv, or have the extra juice run to a second battery bank with another controller to use as a back-up.         

adam       

[hydrosun]

The outback FM-60 handles 48 volts, will do mppt, and the aux  can be programed to turn on a 12 volt power relay to turn on your water heater. When the thermostat shuts off the water heater the voltage will rise to the bulk  voltage of the FM-60 and it will start  letting the solar voltage rise to limit the input to the batteries. I've got that setup on my system.
 I think the Xantrex mppt also has a 12 volt aux signal too, but I'm not sure.
The morningstar products use the main transistors for diversion so you can't do both at the same time.
Chris

[Justoine Knight]

Thanks guys.

Yes, although some adverts say the Morningstars will do diversion I couldn't find any reference to this in the manual. It looks like you have to add the relay pack to make this work.

The Outback website is far too clever and slow for me to find a manual there. At least it looks like it's possible.

It still worries me that this is a bang-bang solution. I suspect that the heater would just keep cycling on and off depending how fast the controller reacted to the change in voltage.

Strikes me I might be better off with a simple bit of analogue electronics and an off-the-shelf voltage-to-PWM controller that monitors the PV voltage. This could gradually switch on the load as the MPPT controller starts letting the input voltage rise.

[defed]

the xantrex mppt does have the aux feature.  haven't gotten mine hooked up yet, but close.

i found the manuals for the morningstar, outback, and xantrex mppt's online in pdf easily.  just do a search for "<the model #> manual".  many online solar retailers include the link to the pdf...almost sure sunelec.com does when you click on each controller's details.

[ghurd]

It still worries me that this is a bang-bang solution. I suspect that the heater would just keep cycling on and off depending how fast the controller reacted to the change in voltage.

Strikes me I might be better off with a simple bit of analogue electronics and an off-the-shelf voltage-to-PWM controller that monitors the PV voltage. This could gradually switch on the load as the MPPT controller starts letting the input voltage rise.

The guys at Outback are good at what they do.
The aux function will be doing what it should and when it should.

I don't think it would be a good idea to put anything on the PV side of any MPPT controller.

Plus it seems like the PV voltage will depend on quite a few factors, like which stage it is in, light conditions, etc.  Choosing a fixed PV voltage to trigger the water heater would not be accurate a very high percentage of the time.
G-

[hydrosun]

The Outback FM-60 aux output has a solid state mode that does a pwm to keep the voltage rock steady if you put the voltage changed low.
Chris