Can I use a Victron MPPT to regulate charge a DC water heating element without batteries?

DIY
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1

Can I use a Victron MPPT to regulate charge a DC water heating element without batteries?

I’m not charging batteries, solely using a resistive load (DC water heating element), freestanding- not connected to the grid.

Presumably I need an MPPT designed for a direct DC load control?

As I understand it an MPPT will be more efficient as opposed to charging the element direct from the panels.

I have;

2 x JA Solar JAM66S30

Values for these 505W panels:
• Vmp (voltage at max power): ~38.5V
• Voc (open-circuit voltage): ~45.8V
• Imp (current at max power): ~13.1A

I’ll probably buy a 3rd panel.

I directly wired the two 505W panels to a 48V 1000W heating element.
I tried series and parallel, read some advice somewhere and went for parallel, which may well have been another mistake.

The element has an integrated thermostat.

It turned out to be a mismatch. In parallel the element only ran at
36.78V
14.26A
524W

Naïvely I was expecting 1000W which would have been happy days, off we go. After about a weeks use the element went pop and gave up. Now that did surprise me as I thought it was under stressed. It was rated at 2.5 ohm’s but by my calcs it was running at about 2.9 ohms. Anyway it’s toast , which is a good thing because it wouldn’t have enough grunt to heat 200ltrs. So up with the volts, down with the amps, I’ve ordered a 96v 1000W and a 96v 1500W with thermostats. ( with delivery and special offer it was better to get 2 in case I burn out another).

The plan is;

3 Panels in Series
• Vmp total: 38.5V × 3 = 115.5V
• Voc total: 45.8V × 3 = 137.4V
• Power: ~1515W

What could possibly go wrong?

Would I be better off with an MPPT ( a 150/35 or 45 or even 60)?
Would I need a DC load relay?

Thanks for any help.

Once this is sorted the plan is to get a separate system with batteries because they’ve suddenly taken to cutting the power for the whole country here in Spain. One thing at a time though.

3

It’s not recommended to use an MPPT without battery.
If you use that MPPT only for the heating element, use some small and cheap (AGM) battery to have it as a buffer.
Something like 4 x 12V/5A, like the ones in UPSes, connected in series.
Of course, set proper bulk/absorption/float voltages in order to prolong as much as possible the batteries life.
A DC (solid state, if possible) relay for the heating element is always a good thing, because you can control much better the system, especially when you are out of sun, in order to not let the batteries discharge excessively… :wink:
But the battery must be there uninterrupted on the output of the MPPT.

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Connect the panels directly to the heating Element, nothing can happen if the element can take more power then the panels can supply.

5

That too, but only if the panels power is less than heating element power and panel voltage is the same (or less) as the heating element.
In other words, match the panel power AND voltage with the heating element’s. Otherwise a lot of power will be wasted.
If he plans to expand the panel power, it’s always good to have a constant in the system and that’s the MPPT output voltage and therefore the heating element voltage.

6

Yeah, but he ordered a 96v element…

7

Thank you.

8

I’ll wait for the placement elements to arrive, test them to find out what Ohms resistance they are and try running them just below that.

9

Disconnecting a 96V system at 1000W is going to become increasingly difficult due to the significant arc produced during disconnection. You’ll need a proper DC-rated 2-pole solar breaker to safely interrupt that level of current.

A 24V system might be a better choice. It strikes a good balance between simplicity and performance. You’d only need two batteries, one balancer, and an MPPT controller. A Victron 100/50 would handle up to 1400W at 24V, or you could opt for a larger model if needed.

For batteries, you can go with two 50Ah lithium batteries with a 100A BMS, or a pair of AGMs. Alternatively, a single 24V lithium battery would simplify things and eliminate the need for a balancer.

Consider using a battery protect 100A, between the battery and the heating element. It includes a remote switch for manual control and will automatically disconnect the load when the battery voltage drops too low.

I have heard the Dernord brand is fairly robust and reliable.

10

Thanks for your input. I ordered Dernord elements. I was not aware of the arcing problem, maybe that’s what led to the failure of the first setup. On testing it turns out that the 48v 1000w 2.5-2.7 Ohm resistance element is still working and it was the thermostat that failed. Possibly due to arcing?

The 96v 1000W and 1500W elements have arrived. According to the calculations that I have run, the 1500W element with 3 x panels will get me nearer to matching the element with the panel performance. My concern regarding the use of a 24V system is the ampage would leap requiring a much heavier grade of cable. Presently I’m using 6mm cable.
I have a DC breaker but it’s the thermostat that’s going to be doing the breaking.

11

Yeah, sounds like the problem. I had a quick look but cant find anything high voltage or high current. I found a link below to a 24V 30A, which will be good 600w.

You may need to look at 24V and 600W. 6mm cable should still be fine if it can do 30A rating. You would be better to install a larger 10mm or 16mm cable. If you are going to setup a battery and use a battery protect it will be changing the battery in the morning until you get to the voltage where it connects. All the power will be going into the battery and the excess will be going into water heater. If you size the battery correctly you will still get 100% of the solar from a 600w unit.

If you can find a thermostat that will switch 92V then you can try it.

I will just add a note this is for 24V so a battery voltage of 28V might be too high for this unit. Unless you build a 5S battery with a lower voltage. Hopefully there is somthing that will go to 30V plus at least.

12

Thanks for your input. I’ll pause on this until I can find a good solution. It’s a shame as I thought the 96 volt 1500W element would match 3 panels in series.

Maybe it’s safer to get some batteries and an MPPT. If so, which one?

13

You will only get the max out of the panels with an MPPT you can see how much more you get in the graph below. I get about 70W for every 100W quoted from my panels, and I live in north QLD Australia, where irradiance is high. I only ever get the full panel power when a cloud comes over and cools it, then moves on till it gets hot again.

image
image598×424 12.3 KB

If you want to add an MPPT and battery, from what I can see control-wise wise a 24V system seems popular. The MPPT size will depend on the current of the panels and the voltage. On average, I get 0.5kwh of energy for every 100w of solar I have. I have 800w on a 100/50 MPPT and get a maximum of 4 kWh a day.

If you want to heat 100 litres of water from 25°C to 60°C

The temperature change is 35°C (60 - 25).

Water has a specific heat capacity of 4186 joules per kilogram per degree Celsius.

100 litres of water weighs 100 kilograms.

So, the energy required in joules is
100 kg × 4186 J/kg°C × 35°C = 14651000 joules

To convert joules to kilowatt-hours (kWh), divide by 3,600,000
14651000 ÷ 3600000 ≈ 4.07 kWh

It takes about 4.07 kWh to heat 100 litres of water from 25°C to 60°C.

So I would need 800w of solar for every 100l I want to heat.