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Old 12-10-2015, 10:30   #1
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Location: Caribbean
Boat: Norman Cross Trimaran, 46'
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Would appreciate some feedback on my design...

Hey all. I've written up a design for a hybrid solar electric thermosiphon to build as a water heater on our trimaran. I've gotten some really good advice here on this forum on other topics and would appreciate anyone willing to give me some feedback on my design. Not trying to promote our site or anything, I'd just like to have the design bugs worked out as much as possible before starting the build / investing the money. Thanks!

DIY: The Quest to Go Green on a Sailboat – aka – WTF Is a Hybrid Solar Electric Thermosiphon? | Oh Sail Yes!

Hybrid Solar Electric Thermosiphon Design – aka – Nerd Alert! | Oh Sail Yes!
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Old 12-10-2015, 18:49   #2
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Re: Would appreciate some feedback on my design...

I've occasionally looked at something similar, and it seems like using evacuated vacuum tubes may be a good way to go.

There's a bunch of diy designs out there, using the same basic concept (small solar driven circulation pump), as well as commercial setups. This one feeds the solar heated water to an existing water heater to "boost" the temp if needed. ( http://www.apricus.com/html/solar_co...m#.VhxRsnnD_qA )

As a side note, we charted for the first time in the bvis last winter, and the swim step fresh water shower was surprisingly warm without any additional heating. Just a thought, assuming you're heading for the tropics.

In any case, do let us know how it works out!
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Old 12-10-2015, 19:26   #3
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Location: Townsville Breakwater Marina Qld Australia
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Re: Would appreciate some feedback on my design...

There's a lot of sunshine in the tropics. A small collector, say a copper plate 30 cmx10 cm with 10mm copper pipe silver soldered on and sprayed flat black would do fine, Put it in an insulated perspex topped box (The plate gets VERRY hot . No need to slope it (pump does the work not convection) using expanding foam for your box container insulation. To prevent tank rupture, get a relief valve from your plumbing supply shop (explain what your doing to the shopkeeper he should be able to advise) Hot water systems with all that copper about usually have an anode incorporated somewhere. (A small 30l HWS from the house wreckers or local dump recyclers would fit he bill and would come with a relief valve and anode)
A friend of mine has built and operated such a system. If you need more info I can send the article that Bob had published in Seabreeze. Email me ted4200@bigpond.com
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Old 13-10-2015, 14:32   #4
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Re: Would appreciate some feedback on my design...

Whit a solar panel that feed the pump direct you can get problem.
(The numbers are taken out of the air but are quite accurate.)

In the middle of the day the panel gives 14 V. That will the pump stand.
2 h before and 2 h after midday the panel will give 12 V. That what the pump want.
4 h before and 4 h after midday the panel will give 8 V. That could be a problem. Maybe will the pump not rotate but it will take current. That current will make the pump hot, maybe so hot that it burns.

If you take the power from a small separate solar panel or from the big panels you already have will not make any difference.
The finest solution (and most complicated) is to measure the temperatures both on the panel and in the tank. As long as the panel is warmer than the tank the pump should operate.
A simpler solution is to assume that the tank have a temperature of 40°C and have a thermostat that starts the pump when the panel gets over 40°C.



If you have a tank with warm water and mount a coil of pipe inside of that to get warm water for the shower, you need a long pipe. To calculate the length is quite straight, the only figure that could be hard to find is how much power you can get over between the water inside the tube and the water in the tank. A realistic figure is 100 W/°C m². If you want so can I make a spreadsheet that helps you to calculate the length. But it will be with metric units.
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Old 13-10-2015, 17:56   #5
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Re: Would appreciate some feedback on my design...

Lars, you make a very good point about the voltage variances that will occur at different times of the day. I'll take your advice and tap into the boat's existing 12V supply instead of a small dedicated panel. I had originally thought the small panel would make a nice easy way to shut the pump on and off automatically when the sun went down. I may still play with this idea a little since I already ordered the panel and the pumps. I'm hoping that the pump will be tolerant of voltage variations. We'll see.

Also really impressed by the thermostat idea, though I think I will only pursue that if I find that I need to regulate temperatures more accurately.

No need for the spreadsheet, but thanks for offering - I can do the calculation myself once I settle on a reservoir size. Thanks!
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Old 13-10-2015, 18:08   #6
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Re: Would appreciate some feedback on my design...

Also, ted4200, thanks for pointing out the galvanic corrosion issue. I'll now also be adding a sacrificial anode to the design.
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