Hot water from PV?

[Jef & Marin, Netherlands]

On our previous boat I had the same issue. I just bought a cheap low-power 12 to 110V inverter, only connected to the heater element. It worked fine.

On 110V your 750W heater element will consume below 200Watt.
This might be the simplest solution.
In 220V countries there will not be much supply of 220V inverters; you might order one from Alibaba or so.

[Sailing Ohm]

Originally Posted by noelex 77

If you have 300-400w of solar (and no option to fit more), water heating via these panels is not likely to be very productive. Water heating takes a lot of energy and after running normal house loads (even if you are frugal) there will not be enough energy left over to make much of an impact on water temperature.

I second this opinion. 3-400w of solar (especially if side rail mounted) will probably not have much energy left over after charging your batteries, especially to heat water (which uses a lot of power). Even if you have a small fridge and no freezer it would likely be pushing it.
If you can’t fit more solar panels maybe look into wind power which is frequently used with diversion loads to heat water (in fact it’s usually a safety feature).

In the tropics I’ve seen people mount Large diameter PVC tubes painted black around their Bimini/solar panel mount. This was plumbed directly to the hot water lines via its own pump.

[SailMoonShadow]

Originally Posted by 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?

Check this out...
https://www.green-yachting.online/solarwaterheating

p.s. if you're a member of the Cruising Association you'll get a 10% discount ;-)

[Pete7]

Originally Posted by Talbot

An alternative approach, that I have seen is to construct a direct solar heating system for your water. - relatively straight forward if you already have a calorifier. Just tap into the feeds and divert through a thermostatically driven pump to copper coils on the deck. Paint the copper black and you have a free heat source.

Bored in lock down I was looking at these last week. First up was a simple solar water panel for £158, so far so good, until I saw they weigh 32kgs.

https://www.bimblesolar.com/more/the...lar-thermal-2m

Then found a complete kit which is quite clever in that any existing calorifier (hot water tank) isn't changed, a pump and valve operate on the calorifier cold feed connection. Only problem? it's a £1051 solution looking for a problem.

https://www.bimblesolar.com/more/the...ar-thermal-kit

Back to running the engine or boiling the kettle.

Pete

[W5PAD]

I've had a 12V dc heating element for 5 years and heat water from PV most everyday using a voltage sensitive relay. If I need to make hot water and there's not enough sun, then I manually turn it on an run the generator. Same thing if I'm one shore power, the charger supplies the power. The 12V heating element can be changed to 24V if need be.



https://www.amazon.com.au/Dernord-60.../dp/B0182BETBE

[fxykty]

Is it bad to just rely on manually turning on the water heater when batteries have sufficient charge?

[W5PAD]

Originally Posted by fxykty

Is it bad to just rely on manually turning on the water heater when batteries have sufficient charge?

I don't understand your question.

However, if the batteries (LiFePo4 BTW) are charged than there isn't a need to turn on the generator. It will take about 50-100 Ah to heat the 11 gallon water tank, which is only 10-20% of battery capacity. So running the generator depends if I can live without the 10-20% until the sun starts shinning again.


I originally used the 120V AC element and turned on the inverter to heat hot water. But, no matter how you convert 12V to any other voltage you're loosing energy in the process. I went with the 12V DC element to eliminate all the loses I could.

[mako]

The largest 24V heater element I've seen on the net is 900 watts. Not the same as a 240v 4.5kW, but still not bad. A nice slow heat while dumping excess solar.

So I could see that if using a domestic water heater, which typically has 2 elements, to keep the 240vac lower element but replace the upper element with the 24vdc. Flexibility!

[Pete7]

[QUOTE=W5PAD;3276629]I've had a 12V dc heating element for 5 years and heat water from PV most everyday using a voltage sensitive relay. If I need to make hot water and there's not enough sun, then I manually turn it on an run the generator. Same thing if I'm one shore power, the charger supplies the power. The 12V heating element can be changed to 24V if need be.

Presumably you have a large solar array?

Pete

[fxykty]

Does a 12V or 24V heating element warm the water hot enough to prevent the potential problems of water sitting at luke warm temperatures for extended periods? Most of the DC heating elements I’ve seen have 300W or at most 600W of power. That seems like it would produce a very slow warming that wouldn’t get the water hot enough without many hours of spare solar power?

Does anyone measure the water temperature in their hot water tank?

[zstine]

Originally Posted by s/v Jedi

You can not rely on bleach for preventing Legionnaires disease for hot water systems because heat evaporate chlorine before it can do it’s work. See https://www.cdc.gov/legionella/about/prevention.html for more info.

"The U.S. Department of Energy recommends setting a tank-based hot water heater to 120 degrees Fahrenheit ."
The Aussie 80C (175F) recommendation is super hot! Nothing in the link you posted says pre-treating water with bleach is not effective. If you bleach the water going into the heater, where does the Legionnaires disease come from? There has to be living bacteria in the water to grow, so if you kill it with bleach before warming... no issue.
I acknowledge your point that there's risk of disease, but hundreds of millions of USA home heaters are set between 120 and 140 and we don't have issues with Legionnaires outbreaks.. Covid though..

[zstine]

Originally Posted by fxykty

Does a 12V or 24V heating element warm the water hot enough to prevent the potential problems of water sitting at luke warm temperatures for extended periods? Most of the DC heating elements I’ve seen have 300W or at most 600W of power. That seems like it would produce a very slow warming that wouldn’t get the water hot enough without many hours of spare solar power?

Does anyone measure the water temperature in their hot water tank?

My home tank is set to 130F. My boat's tank is much hotter, which allows for longer usage out of a small tank as you will temper it with more cold/less hot for usable 110F temps....

Voltage is not relevant, power (watts) is. Wattage alone doesn't determine temperature, the time the wattage is applied does. You can boil water with 300W, but it will take twice as long as 600W.

Example, with a 5gal tank starting at room temp. In 2 hours a 300W heater (any voltage) will warm the water tank to 120F. It only takes 1 hour if you have a 600W heater.
see https://bloglocation.com/art/water-h...e-energy-power

[s/v Jedi]

Originally Posted by zstine

"The U.S. Department of Energy recommends setting a tank-based hot water heater to 120 degrees Fahrenheit ."
The Aussie 80C (175F) recommendation is super hot! Nothing in the link you posted says pre-treating water with bleach is not effective. If you bleach the water going into the heater, where does the Legionnaires disease come from? There has to be living bacteria in the water to grow, so if you kill it with bleach before warming... no issue.
I acknowledge your point that there's risk of disease, but hundreds of millions of USA home heaters are set between 120 and 140 and we don't have issues with Legionnaires outbreaks.. Covid though..

I do not agree. The worry about 80 deg. C is risk of scalding. This is why the output of the water heater has a thermostatic mixing valve, which mixes the hot water with cold water. This is the safe and correct way to do it

[jbinbi]

Victron mppt controllers have a 'load' output that can be enabled when the battery voltage is at V (V is whatever you put in). So if you put V at 13.8, the load will be enabled when the battery is charged and you can then driver a dc/dc or small inverter, etc.

[Culwatty]

Just a thought

Have a look at ElectroDacus he built a system to heat thermal ,ass from pv (think big concrete block) but might still work!