Hot water from PV?

[goeasy123]

I hear tell that you can run a 220VAC water heater from the inverter by stepping down the voltage between the inverter and the heater so that the PV can top up the batteries at a reasonable rate. I believe it's call 'load diversion'.


It needs some kinda handy-dandy controller that only allows the water heater to get power when the battery voltage is between two high limits. I.e. when the suns a-shining bright.


Anyone know exactly how to do it?

[Rucksta]

Voltage sensitive relay aka one way battery 2 battery charger

[LeaseOnLife]

We use a 12V to 48V DC/DC converter and feed that 48V into the 750W/120V heating element (19.2 Ohm).

At 48V this results in a current of 2.5A (120W), at 12V it is 10A plus any in-efficiencies of the DC/DC converter. 3h of heating (~30Ah@12V) give us a very comfortable temperature for showering.

We have a rather small water heater (15L) and if my calculations are correct, the water temp is heated by 20deg C in those 3h.

[goeasy123]

Originally Posted by LeaseOnLife

We use a 12V to 48V DC/DC converter and feed that 48V into the 750W/120V heating element (19.2 Ohm).

At 48V this results in a current of 2.5A (120W), at 12V it is 10A plus any in-efficiencies of the DC/DC converter. 3h of heating (~30Ah@12V) give us a very comfortable temperature for showering.

We have a rather small water heater (15L) and if my calculations are correct, the water temp is heated by 20deg C in those 3h.

Our heater is similar size and wattage, but 220VAC.


How do you control it? I.e. make sure the heater is only drawing current when the batteries are topped up and you don't kill them?


What DC/DC converter do you use?

[mako]

You want to power your water heater without drawing down the batteries.

I’ve thought about this before also and the best solution that I came up with was a dedicated cheapo MSW inverter from Harbor Freight wired to the water heater. Say a 1500w element for example.

A signal from your battery charger when it enters float would switch a relay, thus opening a connection from the MPPT to the inverter.

A diode between the MPPT and the batteries would eliminate any back current.

This would make it automatic, although you could just flip a manual switch also.

[BjarneK]

You might find this thread useful: https://www.cruisersforum.com/forums...ls-179819.html

[LeaseOnLife]

Originally Posted by goeasy123

Our heater is similar size and wattage, but 220VAC.


How do you control it? I.e. make sure the heater is only drawing current when the batteries are topped up and you don't kill them?


What DC/DC converter do you use?

I got this type of DC/DC converter:

https://www.amazon.com/SMAKN%C2%AE-W...dp/B017TPRLLY/
It gets slightly warm, but not hot to touch.


I use a simple timer relay to turn it on at noon and off at 15:00. With LiFePo4 batteries I don't have to charge to 'full' everyday. If, after several days with clouds/rain, I am in a deficit, I turn off the timer. Same if we aren't on board for a few days.

[goeasy123]

There's a thread on YBW. A guy's using a cheap 240v to 110v yard transformer and one of these to control the on/off.... https://www.bimblesolar.com/digital-voltage-relay.


The transformers are common in Europe as trademen prefer to use 110v power tools.

[zstine]

Many MPPT controllers have a feature to do this. They either can dump supplied power to a load resister (ie, heating element) when SOC reaches near 100% or they have a relay output that switches when 100% SOC is reached to allow you to wire a relay and divert power to your heating element.



You don't say what controller you are using. you may be able to do this already with what you have. Depending on your budget, you may want to purchase a controller that has this feature. A voltage sensing relay will pull voltage before you get to 'float' since you will be charging at higher voltages in absorption. This will slow your charge down and could prevent full charge.

[goeasy123]

Originally Posted by zstine

Many MPPT controllers have a feature to do this. They either can dump supplied power to a load resister (ie, heating element) when SOC reaches near 100% or they have a relay output that switches when 100% SOC is reached to allow you to wire a relay and divert power to your heating element.



You don't say what controller you are using. you may be able to do this already with what you have. Depending on your budget, you may want to purchase a controller that has this feature. A voltage sensing relay will pull voltage before you get to 'float' since you will be charging at higher voltages in absorption. This will slow your charge down and could prevent full charge.

Good info. Thanks. I haven't bought the solar yet and budget isn't too much of a issue (value is).


I can only fit somewhere between 300 and 400w on the side rails. I have 420Ah of AGM. I'm only running a fridge. LED lights and the usual navigation stuff. I'm thinking Victron MPPT, but it's not clear from threads on their community forum they can do it.

[Simi 60]

I'd be more wanting something that makes sure the circuit is disconnected.

Our HWS if left on, starts again when temp gets to 53c and thats at a time when there is no sun around so battery's get raped.

We manually turn on at 10am and turn off when done
Have a sign that goes up to remind me to turn it off.

Would love to have something that does it automatically and, shuts down if there is cloud cover and starts up again when gone

[goeasy123]

Originally Posted by Simi 60

I'd be more wanting something that makes sure the circuit is disconnected.

Our HWS if left on, starts again when temp gets to 53c and thats at a time when there is no sun around so battery's get raped.

We manually turn on at 10am and turn off when done
Have a sign that goes up to remind me to turn it off.

Would love to have something that does it automatically and, shuts down if there is cloud cover and starts up again when gone

I think this does it.... https://www.bimblesolar.com/digital-voltage-relay.


Also look on Aliexpress. I'm sure there's something there.

[svfinlandia]

I use a diversion controller between my wind generator and my batteries. When the batteries are low the wind generator charges them until they are fully charged. Once the batteries are fully charged the diversion controller routes the current to a 12 V water heater element. This should also work with your solar panels.
I did a web search but could not find the exact version control me that I use but here is one similar and they also have the 12 V/24V water heater element.
Don’t know if this will work in your situation but check this website: https://www.backwoodssolar.com/products/diversion-loads
Since the water heater element is dual 120 or 220 VAC and 12 V. there is no need for an inverter or other young device.

Al, S/V Finlandia

[Simi 60]

Originally Posted by goeasy123

I think this does it.... https://www.bimblesolar.com/digital-voltage-relay.


Also look on Aliexpress. I'm sure there's something there.

Thanks but perhaps not as our water heater is a household rheem 240v unit

Add: but maybe yes at its based on battery bank SOC not 240v availability

[Mike Banks]

I have a household solar system which uses a priority switch. As soon as the sun comes up it turns on the water heater--and that takes only about one thousand watts, until the thermostat in the water heater itself switches it on or off as required.

The same thing could be set up on a boat, if you use an electric hot water system wholly or in part.

For my money, I would use an electric heater or direct solar collector. The electric is simpler, the solar collector is about the same in final cost--but the solar collectors get so hot they can be dangerous, so a thermal controlled mixing system is necessary to stop people scalding themselves. Either system will requite about six to ten ten gallons or so of storage for the hot water if more than two people are taking showers, or taking showers more than once a day.

Or, you can use on-demand propane fueled gas hot water. Bosch make a really good one--but they tell you it is not designed for caravans or boats. I have had no problems with it and it passed inspection no worries.

I used it on my boat for years--turn on the hot tap--ten seconds later hot water. A little turbine spins as the water enters, turns an alternator and a spark generator ignites the gas. Never failed to light ONCE. It also had its own in-built temperature regulator--set it to your outlet temp-- and they are not really heavy on gas unless you spend long periods in there. You do need a flue though--which means they need to be under a dodger or in a dog house. Mine was on the side of the dog house under a mushroom vent.