question

moz avatar image

Can I use excess power to heat water off grid?

I'm turning a truck into a campervan/housetruck, and since I have the roof space I would like to use excess solar to run a small hot water system. I have 12x200W (40V@5A) panels, and I'm thinking 24V to match the truck. The loads will total less than 3000VA because I'm willing to use one cooking appliance at a time (I have been doing this already as a test). I'd rather ask dumb questions of online forums than people trying to make a living selling solar setups, so here I am :)

I think I want something like this: either MPPT 150/100 (3s4p, 120V strings) or MPPT 250/100 (6s2p, 240V strings) feeding a Multiplus 24/3000/70 with a Color Control GX and BMV-702 battery monitor. In Australia a Sentry 24V/150AH is $4000 ($1.05/Wh, charge at 60A) while the Victron 24/200 is $6695 ($1.35/wh, charge at 100A) and I'm not keen on spending the extra $3000 if I don't have to. I don't need the extra storage, just the extra charge current acceptance.

Hot water can be either 240V AC or 24V DC, element changes are easy. And the thermostat will cut power when the tank is hot enough.


The question is how I get the power diversion to work. I'm told it can be done but I have been wandering round the Victron website feeling lost. Please advise.


Second question: if I want to limit charge current to 60A, can I use a MultiPlus 24/3000/70 rather than needing a 24/5000/100 and program it to limit the charge current?


thanks for any help

offgridunused power
2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

8 Answers
bluepowershop avatar image
bluepowershop answered ·

Hi Moz,

If the battery is fully charged you can switch on the heater, a Victron BMV can assist you with this.
The BMV is equipped with a relay which can switch on and off on state of charge.
If you program for example to switch on the heater at 100% SOC and switch it off at 95% it uses the excess solar power for most of the heating.
Switching must be done thru a appropriate 230V relay, of beefy 24V relay in case of a 24V heater.

If 60A of charge current is needed for your installation it can be done with the Multiplus 24/3000/70.
Bear in mind that the inverter of this unit is way smaller than the 5000VA unit of the 24/5000/120.

Kind regards

Mark

2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

fcriado avatar image
fcriado answered ·

Hi!

In my case I am starting to work on the same topic.

The idea is to use a RPi to retrieve data via ModbusTCP from the CCGX, and then use the RPi to control a triac which will provide power to a water heater.

I got the idea as well from the Fronius Ohmpilot, but I use a regular DC coupled Victron system.

I am still learning about the different protocols, so in the next months I will start with the tests.

Hope this helps!

Fernando

2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

tilo avatar image
tilo answered ·

Hi Moz,

You don´t need to buy a big battery (for its current absorption capacity) just to be able to use a big solar array. Since Victron introduced the DVCC feature in their Venus devices, you can limit the net current that flows to the battery. The system will take into account the DC consumption of the inverter, and regulate the MPPT output current so that the configured maximum charge current to the battery is not exceeded. The same limit then applies to the Multiplus/Quattro charge current in case you charge with AC power, or when both the Multi and the MPPT can charge. The battery will not receive a higher charge current than what was configured as the limit. Read here: https://www.victronenergy.com/live/ccgx:start#dvcc_-_distributed_voltage_and_current_control

We have 3 kWp solar with a 250/85 MPPT, Multi 48/3000 and just 10A charge current to the battery with DVCC, because we want to consume mainly during the day and need little battery backup capacity.

About controlling the heater, read also the discussion in the link below, especially the answer from user @valos, he uses the relay of the Multiplus based on battery voltage, on at 54V and off at 52V or load bigger than 2000W.

https://community.victronenergy.com/questions/2089/mppt-dump-load.html


2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Spyros Tziakouris avatar image
Spyros Tziakouris answered ·

Does Victron plan to manufacture an PWM load controller (f.e. triacs for up to 3-5 kW element) to control heating elements? This will be the best way to utilize opportunity loads for off-grid and DC-coupled self consumption projects. I imagine color control GX can be the brain in this situation and send data to the load controller based on the MPPT charger's available energy, without charging and discharging the battery.


Fronius offers a solution for grid-tied inverters for self consumption projects in order to have zero feed-in.

2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

bluepowershop avatar image
bluepowershop answered ·

Hello Moz,
If your solar is providing enough energy the battery level will be on 100% so the heater can function, as soon as the power from the sun is not enough the inverter will take power from your battery and the SOC begins to drop which will will switch of the heater as soon as it reaches 95% (in the example)
If the solar power is increasing again your battery will be charged to 100% and so on.
Switching of the element itself is done by a simple relay with a 24V coil, bare in mind that the contact on the BMV can switch a maximum of 1A at max 60V!

If a 3000VA inverter and a 70A charger serves your needs there is no need to invest in a larger unit.
The only thing you have to adapt is the charge current setting.
The factory setting for this unit is 52A (75% of max power)
This can be done with the VE.Configure software.

Regards, Mark

2 comments
2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Hello Blue,
Isn't the BMV able to switch the relay on/off based on current going in or out of the batteries ?

Current flow out of the battery => Open relay
Batterie Fully charged => Close relay.

This way the heater will never take current from the batteries.

An other question, is it possible to use the charger to directly power the heating element ?
If batteries full => Divert charging current to Heater.

For what i understand, the current after the MPPT should be "cut" by priorities :

1 - DC for in house DC circuit (pumps, lights.. etc)
2 - DC to feed the inverter : in house AC circuit (appliances)
3 - DC to charge batteries
4 - DC for the heater

Is there any tool that is capable of sensing the need and distribute it accordingly ?

jomugu avatar image
jomugu answered ·

For all of you. I developed a system a few years ago to use the energy excess. The system has been tested with really nice results with AC-Coupled Victron Fronius, 3kVA, 4,5 KWp, 24V-445Ah flooded batteries.

This system calculates in a simple and really cheap way the PV available power and calculate the excess when the Fronius intruder is throttled.

With that system, I'm running two AC, a water treatment plant and a water heater. The system can handle up to 6 loads and also has its own App to monitor, set parameters and switch on/off the different loads.

I want to start developing a version for DC coupled systems, although for excess energy usage I would recommend Ac coupling.

2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

jomugu avatar image
jomugu answered ·

I've been working for the last years in an off-grid nergy managment. I use a AC coupling Fronius Victron. The system can manage up to 6 loads that switch on and off depending on the energy excess. The prototype is now running two AC, in summer and Winter. Now I'm planning to add a 7th load, variable one, with a TRIAC, but not sure how this load could affect to the whole instalation. My concern is if the methods (Voltage Crossover Firing or Phase Angle Firing) Will affect to the installation creating a continuos micro charge/discharge clycles to the battery, accelerating aiging. Any idea about that concern? Has anyone tested a Voltage Crossover Firing or Phase Angle Firing power regulator in an off-grid PV installation?

2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

moz avatar image
moz answered ·

Thanks for the quick answer, Mark. Yeah, the smaller inverter is fine, I've been running those numbers for a while. It's the battery size and charge stuff that I want reassurance on.

If I can't easily split excess power out before the inverter/charger I'll probably go with 240V water heating just because switching 5A or 10A AC is so much easier.

2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.