Solar water heater

[s/v Jedi]

Originally Posted by coopec43

I've noticed how hot the water is in the garden hose when I first turn the tap on.

QUESTION

If I have a black hose attached to (say) the stanchions and recirculate water through the hose/storage tank with a very small solar powered pump (2L/min) how hot would the water get?

Exactly. I can fit collectors under panels and have space to fit those 24”x24” collectors too but I’m thinking about the most convenient locations and plumbing run to the water heater. I have disconnected it’s heat exchanger from the diesels and now just use an electric element powered by solar electric so of course solar heat is next on the list.

Let’s come up with ideas. Soldering copper pipe is easy but I’m hesitant to put it under glass as that isn’t too conductive and fragile too. A black silicone coating? Attach as a “hinge-up” on the lifeline or from under a solar panel to be deployed when at anchor?
Up high for clear view of the sun is far away from the water for the plumbing...

[Bill O]

I'm going to throw in a slightly different idea for a solar water heater that I saw recently that would not involve adding plumbing and pumps to get hot water.
Similar to how Nick has Jedi set up w/a large solar farm and lithium batteries, this RV guy

this RV guy

used his dump load to make a point of use solar water heater. The Rheem 2.5 gal heater in the vid can be found in the US at large hardware store chains.

The silicone heating mat (this is one example) was a clever way to have both a 12V DC to heat the water and still keep the AC element in the water heater. You could change out the AC element for a DC element and make the heating more efficient.

Just another idea to achieve the same end point.

[MikeHoncho]

Why would you set up a system to radiant heat water then place it in the shade, under a panel? What is your primary objective? Kind of cool the panel or kind of heat the water through ambient air temp? I guess I’m missing something.

[Captain Bill]

Originally Posted by RaymondR

Since heat flows from hot to cold the water being circulated will always cool the panels unless you turn the heaters on or run the engine and put heat in via the calorifier.

Not so! The water being returned from the tank will be at tank temperature so that now becomes your starting point. Each pass of the coolant through the panel will raise the temperature by a certain amount depending on a number of factors, such as insolation, surface area of the collector, transfer efficiency, etc. Lets assume for a moment that it's a 30C day with good insolation and that on every pass through the collector the coolant goes up by 10C. and the ratio of coolant in the collector to tank volume is 100/1. Assuming that the transfer efficiency of the heat exchanger is 100% (it's not but it makes the example a bit easier) then for every 100 passes through the exchanger the temperature of the tank is raised by 10C. Lets also assume that the tank temperature started at 20C and the circulating pump does 100 passes through the collector per hour. At the end of three hours the tank temperature is now 50C and that is now the temperature entering the collector and the temperature leaving the collector is now 60C. An hour later and the temperature leaving the panel is now 70C. On a 30C day in plain air the solar PV panel might reach thermal equalibrium at 50C, but now you are losing a lot of efficiency on your PV. If you turn off the circulation pump, you suddenly make it much worse as the insulation that now exists between the back of the PV panel and the air, the only way it can shed heat is by re-radiating it through the front panel which is made of glass which is a very poor radiator of heat so the panel gets extremely hot. I have used solar water heaters and during a summer day have seen storage tank temperatures of 85C by the end of the day if no hot water had been used for a couple of days. So lets say for some reason you don't use any hot water for a couple of days and your tank cools to 40C overnight. You are now starting your day at 40C and in four hours your collector outlet temperature is near 90C. While using the back of your PV panel to heat water sounds like a good idea on the surface, it is not.

[PirateGuy]

Originally Posted by MikeHoncho

Why would you set up a system to radiant heat water then place it in the shade, under a panel? What is your primary objective? Kind of cool the panel or kind of heat the water through ambient air temp? I guess I’m missing something.

Electric solar panels get hot in the sun. The heat transfers from the front (sun side) of the panel to the back (shaded side).
The idea is to harvest this heat for domestic water use. Ideally this would also cool the panel, increasing its electrical efficiency, though Captain Bill has a different opinion about that aspect.

[RaymondR]

There will be periods in the morning and afternoon when the delta T gradient between panel and water might favor heat flow from water back to panel. The solution is to not circulate the water during these periods.

The operating principal of the heat sinks one often sees on electronics is that the metal in contact with the electronics transfers the heat into the fins which provide a greater surface area for radiation of the heat into the surrounding atmosphere. In the subject case the copper tube soldered to the copper sheet in contact with the glass underside of the panel would provide the extra surface area.

I also pondered on a heat pipe idea with alcohol as the heat transfer mechanism and a peristaltic transfer pump to take the liquid alcohol back up to the panels from the HE coil in the hot water storage tank but it all gets too complicated so when I built the support frame for the solar farm on my new boat I made it large enough to install the solar water heating pipes down the sides of the panels. It's the next project on my list after the fridge rebuild.

[Pete7]

Originally Posted by s/v Jedi

Let’s come up with ideas. Soldering copper pipe is easy but I’m hesitant to put it under glass as that isn’t too conductive and fragile too. A black silicone coating? Attach as a “hinge-up” on the lifeline or from under a solar panel to be deployed when at anchor?
Up high for clear view of the sun is far away from the water for the plumbing...

Nick, do you have one of those temperature guns on board? if so how hot are your panels?

To keep it simple because I like simple things, oh and cheap, how about this idea. However, there is another alternative, and that is pool heaters, sort of black mats that lay flat with a tube moulded into them.

https://www.youtube.com/watch?v=dG2H...=imaginetMedia

[PirateGuy]

Originally Posted by Pete7

Nick, do you have one of those temperature guns on board? if so how hot are your panels?

To keep it simple because I like simple things, oh and cheap, how about this idea. However, there is another alternative, and that is pool heaters, sort of black mats that lay flat with a tube moulded into them

I like the pool mat idea, if you can get them in the right size. They are pretty cheap. A downside is that they pierce rather easily, as I found out on an above ground pool I had some years ago.

How about a fridge aluminum evaporator plate?

[noelex 77]

The shower would be a little too cold for my liking if we used water from our solar panels at the moment .

[PirateGuy]

Originally Posted by noelex 77

The shower would be a little too cold for my liking if we used water from our solar panels at the moment .

I’ve never seen that sort of white stuff on the palms and cacti down here. Is it conducive to bikini wearing?

[Captain Bill]

My description of why putting a heat collection system on the back of PV panel is a bad idea is fact not opinion. While it will collect heat it will do so only at a significant loss of PV performance. There is nothing difficult about building a stand alone solar thermal collector. Basically a box with a bit of insulation with a any number of types of collector on top of it, and a transparent cover will serve as a solar thermal collector. The collector can be metal or a coil of plastic tubing painted black. Sure using IR selective coatings and other techniques can increase the efficiency but they aren't necessary to get adequate performance. The glazing does not even have to be perfectly clear. When I was building solar water heaters 40 years ago we used a fiberglass glazing product that was 90% transparent because it was more resistant to damage than glass and because we were in Florida a loss of 10% efficiency didn't matter much. In more northern climates it would have, especially in winter. One can use a circulating pump hooked to a small PV panel. Because ideally the flow rate is proportional to the insolation, connecting the pump directly to the PV, early in the day when and late in the day the flow rate is low and high at noon, and automatically off at dark.



These things are not rocket science, but they take a little bit of science to understand.

[poiu]

Originally Posted by s/v Jedi

I have not yet read anything about modules to be stuck against the underside of solar panels that circulate coolant in a loop with the heat exchanger of the water heater?

Did anyone experiment with this? Cooling the solar panel should also increase it’s output for the win?

I’ve thought exactly the same. It should work well, even better that the cooling of the panel will improve the PV panel’s efficiency.

How hard can it be?

I know, not that easy or I would have pulled my lazy finger out and done it��.

[MikeHoncho]

Originally Posted by PirateGuy

Electric solar panels get hot in the sun. The heat transfers from the front (sun side) of the panel to the back (shaded side).
The idea is to harvest this heat for domestic water use. Ideally this would also cool the panel, increasing its electrical efficiency, though Captain Bill has a different opinion about that aspect.

Radiant heat is much more efficient than ambient heat. I don’t agree .

[RaymondR]

Since it's fresh water being circulated you could just tun the water directly against the glass using aluminium sheet and that silicon stuff they glue fish tanks with. A 1/8" thick layer of water is probably enough.

[PirateGuy]

Originally Posted by MikeHoncho

Radiant heat is much more efficient than ambient heat. I don’t agree .

I think you are missing the point of a combo electric/hydronic panel. Most monihulls are severely limited in real estate to mount any sort of panel energy collector. Electrical generation takes a far, far higher priority IMO for the little usable space available. The hot water just is icing on the cake.