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Would note my House has solar hot water and solar for electricity. The solar hot water just runs our domestic hot water through a black panel which is sufficient to bring it up to 110f even on 20 degree short winter days. On cloudy or rainy days it at least boosts the tank temp up so less outside power is needed. We also sell power back to the grid year round so if you have enough panels short winter days aren’t the issue it’s made out to be.
I note this to point out solar hot water is much more efficient when the sun directly heats the water rather than making electricity then heating an element.
I would further note in the tropics we get equal power from wind as solar given the trades and short winter days. Summer in New England is 90% solar only 10% wind.
If his goal is hot water then get a panel for that purpose. We have 2 fairly large ones on our roof but I think they make smaller ones. Ours are very low tech and flexible. Easy to mount. Only control is thermostats to turn onor off that loop.
If his goal is a heat sink get a heat sink. If his goal is to not waste electricity put it a second abet smaller house bank and charge that.
 

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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.


There you go, IF it was viable, and not to much of a drain on batteries, charging systems, then every cruiser I know, Including myself would buy, install, use and sing it's praises. We didn't just fall off the turnip truck out here...
 
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That's funny! 128 gallons!
I guess my point wasn't clear. The reference to 128gal was that in order to boil your 6gal tank dry, you would have to boil all of the water on the boat, because as the heater tank boiled off, it would be replenished with new cold water from your water tank.

The reason you aren't hearing about 12Vdc heater elements is that there are only 6 or so people on this thread replying, and most of them aren't cruising or do not have excess solar. Doesn't require a fall from a turnip truck. I know many people using them - they are very common on boats with windgens as load dumps. People with solar use them less so because most don't have excess solar, and solar can be regulated by just shutting off the controller output - unlike wind.

As I mentioned, they aren't terribly efficient, particularly for excess solar which is usually late in the day. However, they are viable, and people are using them. Many don't have AC power on board, so replace their tank elements with DC ones and take whatever excess DC comes their way to supplement their engine heat.

I read your post as asking about the possibility of putting some excess solar energy to use heating water and tried to focus answers on that. But it seems like the consensus is that you should have a solar shower, or a solar heat panel, or a heat sink, or a new battery bank. And that all of the cruisers I know using DC heating elements are gullible newbies being taken for a ride by the big corporations. It would seem that none of these responders actually used physics to come to their conclusions.

In a final attempt to actually be helpful, here is how to determine how much energy it takes to heat a gallon of water:

Q=mc∆T, where m=mass of water, c=specific heat of water, ∆T=change in temperature in Celsius. The resulting units will be in Joules.

Since the mass and specific heat is constant for water at sea level, this simplifies to Q(J)=(15,818)∆T. Converting to Wh for solar:

Q(Wh)=4.39∆T. Put another way: ∆T=(Wh of excess solar)÷4.39

Now, if you know how much extra solar power you have, you can easily determine how hot it will make your water.

For instance, if you have 300W of solar, and 3hrs/day to dedicate it to heating water, and the water is at 27C (I've moved you down to Mexico), then those 900Whr of solar will heat your 6gal of water to 135F. Of course, that is at nominal panel specs for the solar, so this should be adjusted to ~40% to accommodate late day sun and imperfect conversion. Given this, you can expect those 900Whr of solar to heat 6 gal of 80F water to 105F.

So the question is how much solar do you have, and how much excess do you expect?

The question is not which turnip truck you fell off.

Mark
 

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A couple of thoughts.

A thermal solar panel is the more efficient way to heat water, at least compared to solar electric. One problem is you would need to run hoses between the water tank and the panel. Probably a difficult retrofit.

Another option no one has mentioned is a heat pump. That means using a refrigeration unit to do the heating rather than a resistance coil. Think of rejecting the heat from your refrigerator to the hot water tank rather than the air or the sea.

All this complexity is only of use if you are not running your engine, so at anchor etc. If you are motoring for any period, the water tank is already hot. Not sure it's worth the effort.
 

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Here's something for the OP to by and test. Let us know how it works. If your still having that crap wx that I experienced on the 16th on Novemeber at SFO, then it's going to be a long winter...

https://www.amazon.com/Watt-Submersible-Water-Heater-Element/dp/B00KLKGJ1I

Once you get further south, hot water and all that will be a distant memory...we had to put t shirts over the solar showers in Mexico..way to hot..

PS-I found that a far better use for the excess power in my system is to make ICE! All afternoon making cold drinks colder...ice cold beer, huge ice cubes for gin and tonics..I think this will become more apparent as you move to warmer climes..Nobody ever asked me for a cup of hot water..
 

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You know that would be a pretty cool idea.
IF your PV system completed charging your house bank & there were still a couple hours of good sunlight left, this would work providing your PV system had the nugget to drive a 300 watt element.

The 300 watt DC element is capable of raising the temperature in a 6 gal tank 20 degrees in an hour.
One watt equals 3.413 BTU's in my world. A 300 watt element would equal 1023 Btu's per hour.
One BTU raises the temperature of a pound of water one degree. Water weighs 8.34 pounds per gallon.
6 gallons X 8.34 X 20 degree rise= 1000 Btu's

So if you were some place that the ambient water temperature of your heater was 70 degrees, hour later it's at 90. Not a bad deal in my book.

It would also be pretty easy to make a small open loop thermal solar panel & just let it work by convection. You'd have to valve it off so at night it didn't operate the opposite way. All the above sounds like a good science experiment to me:)
 

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You know that would be a pretty cool idea.
IF your PV system completed charging your house bank & there were still a couple hours of good sunlight left, this would work providing your PV system had the nugget to drive a 300 watt element.

The 300 watt DC element is capable of raising the temperature in a 6 gal tank 20 degrees in an hour.
One watt equals 3.413 BTU's in my world. A 300 watt element would equal 1023 Btu's per hour.
One BTU raises the temperature of a pound of water one degree. Water weighs 8.34 pounds per gallon.
6 gallons X 8.34 X 20 degree rise= 1000 Btu's

So if you were some place that the ambient water temperature of your heater was 70 degrees, hour later it's at 90. Not a bad deal in my book.

It would also be pretty easy to make a small open loop thermal solar panel & just let it work by convection. You'd have to valve it off so at night it didn't operate the opposite way. All the above sounds like a good science experiment to me:)
It might be sound science in a lab, but not so much on a sailboat somewhere between 36 to 42 feet, especially in the Frisco area, as in the OP's case. You are talking about approximately 25 amps of 12 volts to get your 300 watts, sometime after peak operating hours, at which time we can assume the batteries are fully charged. With a fixed system that is at best 65% efficient in the tropics over the daylight hours, just how many panels and how much surface area are you expecting to get on a boat somewhere between 36 to 42 feet?
 

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You know that would be a pretty cool idea.
IF your PV system completed charging your house bank & there were still a couple hours of good sunlight left, this would work providing your PV system had the nugget to drive a 300 watt element.

The 300 watt DC element is capable of raising the temperature in a 6 gal tank 20 degrees in an hour.
One watt equals 3.413 BTU's in my world. A 300 watt element would equal 1023 Btu's per hour.
One BTU raises the temperature of a pound of water one degree. Water weighs 8.34 pounds per gallon.
6 gallons X 8.34 X 20 degree rise= 1000 Btu's

So if you were some place that the ambient water temperature of your heater was 70 degrees, hour later it's at 90. Not a bad deal in my book.

It would also be pretty easy to make a small open loop thermal solar panel & just let it work by convection. You'd have to valve it off so at night it didn't operate the opposite way. All the above sounds like a good science experiment to me:)
This is the same analysis I did above, only in imperial units all the way, where I only brought it down to them at the end.

If anyone is interested in different imperial units, raising that water 20F would take 110 mile-pounds :grin

However, your assumption of 100% solar efficiency is off. Assuming late daylight hours and non-optimal incident angle, one can really only expect ~40% efficiency from the solar. So a 300W array would only put out 120W.

Mark
 

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You're putting major juice to the batts with the engine when charging the fridge - not the 170w solar. Dunno what your alternator and regulator system is...
560 worth of regular or agm batts are probably worth 170ish amps usable (not going below 50%) unless you are running the engine overtime to get them past 20% long wall of charging.

The only way you will have 'overage' solar with 240w solar will be because of engine charging.

Just a thought....

Why do you think your batts are full....what tells you that?
 

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Maybe I misunderstood. You said 560Ah of batteries that would be drawn down 50%. That would be 280Ah, not 170.

Oops, I just saw that you mean 50-80% - 30% of 560Ah. Then yes, your number is correct. Your first post didn't make that clear, but was explicit in your second post.

Mark
 

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Solar direct heating in a bag or box on deck makes an easy installation but last time I looked at the science ,hot water goes up to tank (convection) and down only if you pump it.or dump it. So much for enclosed passive loops .There are some great little 12v circulation pumps available so it can be solved . Now we get concerned with pressure system leaks and air locks and venting. Advanced power use math is impressive but common sense can be useful.
 

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Modern marine hot water tanks for heating water via 110 v or engine loops have thermostats and pressure relief valves too...Not going to be circulating that hot water to your fresh water tank. That common sense thing..well not to common..the faux engineers struggle here.
 
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The hot water tank is fed by the fresh water tank, not circulated through it. When water is taken out of the hot water tank, it is automatically refilled with cold water from the fresh water tank. Otherwise, it wouldn't work. If the hot water tank springs a leak, or is continually vented through the relief valve, then the freshwater tank will be emptied by it.

Mark
 

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Ya know I did qualify my statement at the beginning of the post stating if your PV system had the nugget to drive a 300 watt element... Putting the output of a PV system aside I thought it was cool being able to heat water with a DC element, still do.

I do have 40+ years of contracting/engineering experience with hydronics & thermal solar. I can assure you 100% if a small panel was set up & piped properly one could heat water on a boat with a solar panel by convection alone. Once I get my PV system on the boat shaken out this year this is something I will play with.

Seeing how I have an electric boat I need every amp hour I can squeeze :laugher
 

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I don’t know. Have spent several seasons in tropics. Seems we use a lot of hot water. Use it to wash greasy dishes. When showering in the morning or evening like water temperature the same as body temperature. Tank water is colder. After diving get cold so a hot shower is nice.
Also think never have enough electricity because always can use extra fresh water. If tanks are full it’s time to turn on the TV or computer or charge all the devices.
Guess for me and I think most this whole discussion seems off target.
Know I’m not a full time live aboard but I’m am 7-8/12 month one. Only times I’ve had “spare” power to think about doing this is the rare windy and sunny days when I don’t have another need or desire. Would look forward to comments from others if they are in the same boat😀.
 

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Discussion Starter #58
Hmmmmmmm.....seen onion trucks, tomato trucks...don't think I've ever seen a turnip truck let alone fell off of one.

If I had it to do over again I would have titled this thread "anyone out there heat their water heater via solar?".

I did start out thinking excess pv but now see that would be very inefficient and expensive. But passive.... Directly recirculating water from the water heater through a passive panel on the dodger is an interesting idea. Controller could sense temperature on the panel and compare it to temp in the water heater and apply a max temp. That's how our old swimming pool solar worked. Simple. Small solar pump could trickle the water.

Not sure I'll ever do it but interesting thought experiment. Hope you the discussion!
 

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Or Just put the sun shower on deck and lead the hose down thru your galley window to the sink to clean those greasy dishes..

BTW-If you have plans on bounding across the main, a spare PRV for the hot water heater is a good idea..most of the rest of the word uses BSP threads... while your stuck with NPT...

This I learned falling off the turnip truck.
 

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I don’t know. Have spent several seasons in tropics. Seems we use a lot of hot water. Use it to wash greasy dishes. When showering in the morning or evening like water temperature the same as body temperature. Tank water is colder. After diving get cold so a hot shower is nice.
Also think never have enough electricity because always can use extra fresh water. If tanks are full it’s time to turn on the TV or computer or charge all the devices.
Guess for me and I think most this whole discussion seems off target.
Know I’m not a full time live aboard but I’m am 7-8/12 month one. Only times I’ve had “spare” power to think about doing this is the rare windy and sunny days when I don’t have another need or desire. Would look forward to comments from others if they are in the same boat😀.
We spend all of our time in the tropics and can relate our experience for comparison. We have 700W of solar lying flat on the hardtop. We used to have FLA batteries, but installed lithium batteries 1.5yrs ago. We consume ~150Ah/day at anchor and ~250Ah/day on passage. With the FLA batteries, the solar got them back to 100% SOC every day at anchor if it was greater than 50% sunny. In accomplishing this, only ~150-200W of solar was available as excess during the afternoon absorption tail-off charging.

December and January are the tough months if above ~18N because of shortened daylight. Closer to the equator, it is 12hrs of daylight year round. In the Bahamas, with full sun every day in Dec/Jan, we would struggle to get the FLA bank to 100% SOC, with no solar to spare. In the summer above 18N, we had lots of excess solar and could easily heat water with it through a tank element. At the equator (where heat works against solar), it averaged as above - 150W or so to spare. Not much.

Lithium batteries completely changed everything. First, they never need to be fully charged, and are more happy operating in partial discharge states, so there is no pressing need to get them back to full charge regularly. One can use electricity for other things instead of charging any time one wants.

Second, in charging by solar, they act like an additional panel was added. Two things responsible for this - they take all the charge the panels give with no absorption tail-down, so the efficiency is almost 100%, and second, they charge a full volt lower than FLA, which allows an MPPT controller to turn that spare voltage into amperage - another 10-15% jump in output.

700W of solar has now gone from just meeting our needs to more than we need. We now run our AC powered watermaker off the inverter/batteries, as well as our 1500W water heater. The AC power on the boat is left on all of the time, with the device chargers always sucking from it. The generator hardly ever gets used anymore - only if several contiguous full rainy days.

BTW, Outbound, consider using an electric kettle instead of your water heater tank for hot dish water. We recently got one ($20 Walmart). It heats 1.5L of water to boiling in ~2 minutes, and only consumes 5Ah of battery (through the inverter) in doing so.

Getting back to the thread title, it is possible to regularly have hot water with minimal excess solar, if the system is managed well. If the hot water tank is well-insulated, and heated to temp by motoring or otherwise, then all one needs is to replace the smaller amount of heat that is lost through the insulation, as well as the smaller amount of new water that is brought in during use. Not the entire 6 gallons from cold every day. Just a bit of excess solar makes this possible, assuming one does not use the full 6 gal tank every day.

Mark
 
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