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post #51 of 66 Old 11-28-2017
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Re: Solar heating element for water heater

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Originally Posted by colemj View Post
I have faith that you will rerun your math and come back to us on this...

Mark
Faith warranted...

I ran my numbers again and came up with 168.
That's 50-80% charge.

Where am I wrong?
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post #52 of 66 Old 11-28-2017
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Re: Solar heating element for water heater

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.

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post #53 of 66 Old 11-28-2017
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Re: Solar heating element for water heater

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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post #54 of 66 Old 11-29-2017
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Re: Solar heating element for water heater

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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post #55 of 66 Old 11-29-2017
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Re: Solar heating element for water heater

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.

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post #56 of 66 Old 11-29-2017
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Re: Solar heating element for water heater

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
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post #57 of 66 Old 11-29-2017
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Re: Solar heating element for water heater

I donít 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 itís 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 Iím not a full time live aboard but Iím am 7-8/12 month one. Only times Iíve had ďspareĒ power to think about doing this is the rare windy and sunny days when I donít have another need or desire. Would look forward to comments from others if they are in the same boat😀.
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post #58 of 66 Old 11-29-2017 Thread Starter
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Re: Solar heating element for water heater

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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Re: Solar heating element for water heater

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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post #60 of 66 Old 11-30-2017
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Re: Solar heating element for water heater

Quote:
Originally Posted by outbound View Post
I donít 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 itís 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 Iím not a full time live aboard but Iím am 7-8/12 month one. Only times Iíve had ďspareĒ power to think about doing this is the rare windy and sunny days when I donít 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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