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post #31 of 66 Old 11-27-2017 Thread Starter
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Re: Solar heating element for water heater

With my planned 240w dubious that I could get enough excess solar electricity....but if adding a little more would do it I might be motivated. We are at 38N but planning to go south and west....but then north maybe to 50+N for a while. I'm assuming solar won't do much that far up and we'll be running the engine....we don't have a gen set or have room for one. Btw, I do assume my link 10 battery monitor provides a good reading and with all leds we really dont use many AH with the instruments off. We've heated water on the stove with propane. Have not gotten a solar shower although though about it And probably will. Still...the little water heater is a nicely contained and well insulated container plumbed into the boat.

besides electrical solar I have wondered about passive solar...i.e. A black water panel with a small pump circulating through the water heater. Lived in the great northwest around 46n and heated our 30,000 gallon swimming pool 6 months out of the year. Seems like to get 6 gallons of mildly hot water wouldn't be too hard.

From all the responses, while most helpful and interesting, I haven't heard anyone actually using, first hand, solar (in any way) to warm the tank. Sure seems like a product opportunity.
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post #32 of 66 Old 11-27-2017
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Re: Solar heating element for water heater

If you're talking PV electric, you've been given many explanations for why there is very little practical reason for such a product.

You could sell them to ignorant suckers, sure, just like all the overpriced batteries in a box sold as "solar generators".

But the laws of physics prevent it from being practical, and certainly not cost-effective.
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post #33 of 66 Old 11-27-2017
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Re: Solar heating element for water heater

Quote:
Originally Posted by dadio917 View Post
I do assume my link 10 battery monitor provides a good reading.
Never assume 8-)

http://www.pbase.com/mainecruising/p...attery_monitor
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post #34 of 66 Old 11-27-2017
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Re: Solar heating element for water heater

shore based off grid systems often dump exccess to tank heater element . Wood heated in winter and huge array to be useful. A deck mounted black bag or tank can heat shower water. it could fill from boat supply and directly dump thru deck fitting into shower.
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post #35 of 66 Old 11-27-2017
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Re: Solar heating element for water heater

Minne-
"Above my pay grade to know who to hook it up so that one circuit was not backfeeding the other."
Tell me, have you ever seen a salon or a living room or a bedroom where there are at least TWO lights? One in the ceiling, the other on a floor stand?
And the incredible thing is, they are smart enough to work totally separated in the same room. Each will make light, and never backfeed the other. Honest, try it some time.(G)
Two heating elements in one hot water tank will work the same way. Each should be on a thermostat, and each thermostat only cares about how hot the tank is--not why it got that way.
OTOH if you hook up the 12v element without any thermostat...it might try to boil water until the tank vented out, went bone dry, and slagged down. You know, what could possibly go wrong.(G)

Heating element, thermostat, safety fusing, voltage sense with hysteresis...And then the only other issue I see, is that it is possible there can be a fault from the AC element through the water to the DC element, possibly just a volt or two, unlikely but possibly lethal?
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post #36 of 66 Old 11-27-2017
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Re: Solar heating element for water heater

The dual light circuit isn't a valid analogy in any way, so don't get sidetracked on that.

The tank already has a 120V element in it, so any leakage with just one element is going to transfer to the casing and plumbing. Throw in the engine heat exchanger and it could easily find its way all over the boat. But this is pretty much unheard of outside of fear mongering. I've never heard of an example, and will bet a donut an example has never existed.

AC voltage doesn't kill - current does. Any current leakage will be a fault short and the unit will shut down. Of course, if the tin foil hat is off, one could conjure that failing also.

The good thing about solar is that the current from harvesting it ends every night - sure as the sun sets. It would take a lot of sun juice to boil a tank, and going dry would only occur once all of the water on the boat had been heated past boiling. It takes 2,727 watt hours to boil a gallon of water, so a 6gal water heater during a 10hr day would require 1,636 watts of solar for all 10hrs to boil.

On a Valient 39, there is up to 128gal of water to heat to boiling. That is a load of sun juice needed within 10hrs - 34,901 watts of solar working at spec.

I think the bigger worry for the OP than boiling his heater tank dry is where he is going to install 34,000 watts of solar to do so...

For the OP, and any one else interested, it takes 220 watt hours to heat a gallon of water to 150F (assuming you start at 70F). For a 6 gallon tank, that is 1,320 watt hours. In a 10hr day, this requires 132W of nominal solar doing nothing but heating water. Given that solar is 60-70% efficient in most applications on a boat, this requires ~180W of solar dedicated to heating water for a full 10hrs to get 6 gallons to 150F. 330W of solar for an 11gal tank.

So heating water by solar panel output isn't quick for most of us with modest solar.

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post #37 of 66 Old 11-27-2017
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Re: Solar heating element for water heater

Quote:
Originally Posted by hellosailor View Post
Minne-
"Above my pay grade to know who to hook it up so that one circuit was not backfeeding the other."
Tell me, have you ever seen a salon or a living room or a bedroom where there are at least TWO lights? One in the ceiling, the other on a floor stand?
And the incredible thing is, they are smart enough to work totally separated in the same room. Each will make light, and never backfeed the other. Honest, try it some time.(G)
Two heating elements in one hot water tank will work the same way. Each should be on a thermostat, and each thermostat only cares about how hot the tank is--not why it got that way.
OTOH if you hook up the 12v element without any thermostat...it might try to boil water until the tank vented out, went bone dry, and slagged down. You know, what could possibly go wrong.(G)

Heating element, thermostat, safety fusing, voltage sense with hysteresis...And then the only other issue I see, is that it is possible there can be a fault from the AC element through the water to the DC element, possibly just a volt or two, unlikely but possibly lethal?
You do not need to hook up the 12 volts without the thermostat as most thermostats are nothing more than temperature controlled mechanical switches and care nothing about the voltage they are controlling. The wattage difference going from 120 VAC to 12VDC or (14.5 when dumping) is approximately 1/64 that of the AC, so your AC is more likely to boil the water than the DC.


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post #38 of 66 Old 11-27-2017
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Re: Solar heating element for water heater

I would think the two lights in a room corresponds only to

two separate elements, each optimized for the V/A being fed by the two sources.

But only relevant for contexts like land off grid installs where massive ove- panelage makes the effort worthwhile.

Or wind which **needs** a load dump.
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post #39 of 66 Old 11-27-2017 Thread Starter
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Re: Solar heating element for water heater

That's funny! 128 gallons! I think it would be nice to get 6 hot enough to do a load of dishes. Our home water heater is set to 125. 90-100 would be lovely on the boat at anchor to do the dishes after the evening meal. I would not expect full function. Maybe half.

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post #40 of 66 Old 11-27-2017
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Re: Solar heating element for water heater

Well if we're actually going to be peractical, and beyond a showerbag or other passive solar,

I guess on a boat diesel HWS?

Propane instant one on land.
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