Solar MPPT excess power to inverter?

[pyrojames]

I seem to have excess power from the solar panels when I am not on board.

I am pondering how to use this excess as a way to get some hot water on board. If I take a supply from the "load" on the MPPT, and use it to power a 110 VAC inverter and have this connected to the element in the calorifier, it seems that I should get around 200-250 W heating.

The calorifier has a 240 VAC, 1000W element, but if I connect the load to a 240 VAC inverter, the solar wont have enough power for a 1000W element. If I use a 110v inverter, I think I should get roughly a 4x drop in power from the 1000w element which matches the solar quite well.

Am I on the right track?

 

[TernVI]

Won't you need to switch the 'load' circuit, or it will be powered by the batteries when the sun goes in?
The heater element will probably have lower resistance at lower power, so the power might be more than you expect.

 

[pyrojames]

Won't you need to switch the 'load' circuit, or it will be powered by the batteries when the sun goes in?
The heater element will probably have lower resistance at lower power, so the power might be more than you expect.

My understanding of the MPPT controller is that the "load" connections are only live when there is excess solar power. I don't follow the lower resistance at lower power for the element. That would suggest that measuring the resistance with a meter, when the element is not under load, would be inaccurate or useless.

 

[VicS]

Won't you need to switch the 'load' circuit, or it will be powered by the batteries when the sun goes in?
The heater element will probably have lower resistance at lower power, so the power might be more than you expect.

My understanding of the MPPT controller is that the "load" connections are only live when there is excess solar power. I don't follow the lower resistance at lower power for the element. That would suggest that measuring the resistance with a meter, when the element is not under load, would be inaccurate or useless.

ITYWF that it depends on the MPPT controller. Inexpensive ones are likely to be "street light" controllers which switch the load on when there is no solar input. "Top of the range" controllers may be fully programmable. All stations in between as well.

You could get some estimate of the element resistance by measuring it cold and calculating the resistance when operating normally. If under-run it will be somewhere between the two.

 

[[2574]]

The load terminal on the Victron MPPT that I have is voltage switched. So for example if a fridge were the load it would receive a power supply when the measured voltage exceeded a programmed parameter. When the voltage drops below a set parameter the supply is disconnected.

 

[TLouth7]

A dedicated MPPT inverter would be more effective in that it would adjust the AC voltage (and therefore power into the heater) to match the available power from the solar panels.

That said, a standard 'dumb' inverter and some diodes to ensure power cannot flow from the battery to the inverter sounds like a reasonable solution. Worst case you try it and find that all the power has been going to the heater and the batteries haven't been charged at all.

As you say, halving the AC voltage will mean the heater demands 1/4 the power, assuming fixed resistance.

 

[pyrojames]

MPPT is a Victron 75/15. I'll have to go and see if I need to program the load cut in voltage.

 

[ZigZaging]

Hi
Have you considered this option, its a bit pricey but appears a neat solution.


12 Volt/240 Volt Twin Immersion 2 1/4" BSP. 300 watt 12 volt side - 750 watt 240 volt side.
Marine Immersion heaters

 

[pyrojames]

That's quite a neat solution, and no I hadn't seen them. Not an easy fit though, as I would have to pull the calorifier out to fit them. Not significantly more expensive that the 110 inverter and associate changeover 240/110 relays that would be required. More food for thought!

 

[vas]

wouldn't another option be to use a bog standard 220-110V dumb transformer like the following hooked up to the "normal" 230V inverter with a switch operating on the BMV configurable output signal?
Redirect Notice

of course above assumes you do have a decent sized inverter (but iirc you do have one for your watermaker) and a way to configure when this contraption starts pumping current to the 1k heating element.

 

[geem]

We use an el cheapo (£35) 110v inverter to feed the 1200w immersion heater. The 110v inverter only heats water at 300w when connected to the 220v/ 1200w inverter. The inverter is wired in to the AC immersion heater supply via a rotary switch and time switch. You simply turn the rotary switch to select the 110v inverter supply, turn on the inverter and set the mechanical timeclock to 1hr. I usually need a couple of hours on the 110v supply to get enough hot water for two showers.

 

[RupertW]

We use an el cheapo (£35) 110v inverter to feed the 1200w immersion heater. The 110v inverter only heats water at 300w when connected to the 220v/ 1200w inverter. The inverter is wired in to the AC immersion heater supply via a rotary switch and time switch. You simply turn the rotary switch to select the 110v inverter supply, turn on the inverter and set the mechanical timeclock to 1hr. I usually need a couple of hours on the 110v supply to get enough hot water for two showers.

We also have the 240-110v transformer after the inverter. As our Victron 100/50 has no load circuit I put in a programmable VSR to mimic that so the whole thing is automatic and heats the water every day with no intervention.

 

[vas]

out of curiosity, how big are your hot water tanks guys? I see 1000-1200W, shall I guess they are circa 40lt?
and if so, how high would the temp reach after a couple of hours at 1/4 W operation?
Mine is 60lt (iirc- still haven't fitted it and 1.6kW element)

cheers

V.

 

[RupertW]

out of curiosity, how big are your hot water tanks guys? I see 1000-1200W, shall I guess they are circa 40lt?
and if so, how high would the temp reach after a couple of hours at 1/4 W operation?
Mine is 60lt (iirc- still haven't fitted it and 1.6kW element)

cheers

V.

Mine is 60l with a 220v 1500w element so 375w after transformer. It takes about 25 minus to heat water from cabin temp so 25-30C to 65C with 220v so about 4 times that. The reason I don’t know precisely is because I have set it up to switch on and off over the afternoon depending on battery voltage with time to keep absorbtion going - I just know it’s always reached full temp before darkness, often hours before.

EDIT: Just did the calcs and I can’t be right in what I said above. I know that water is hot enough for a shower after 25 mins on 220v and I know the tank reaches full temp at 375w before dark each day. But there is no way either the 220v or the 110v can heat 60l of water by 40C in the times that I have said. So I must have some real world figure wrong - a smaller tank, a lower thermostat setting or higher initial water temp, or some combination. So my setup works in practice but not in theory.

 

[pyrojames]

Mine is a 20 or poss 25l tank. My calcs give around one hour to heat up (1000W), which tallies with my experience. The engine or diesel fire heater heats it faster, about 20-30 minutes. That's working on a 60 degree rise.