I'm trying to make a solar hot water heater and before I go through the time and expense I wonder if it can be calculated to work or not.
I'll basically have 55 gallons of water as the thermal mass. I'll be using a 1.625 diameter copper tube that's 34 inches long to transfer heat to. I can figure out the tube will hold .21 gallons
(1.5 ID). If I'm flowing at 2 gpm that gives a dwell time of ~6 seconds. If the thermal mass is at 150 degrees F and the incoming temp is 80 F what will the out flowing temp of the water be? How about at 1 gpm of flow? 125 degrees F?
Aorry, I'm not gonna find the equations right now.
However, I will point out that the 150 degrees isn't gonna stay there long. How long do you need to cool / heat?
If this is for continuous heat transfer, that is a tiny amount of 150 degree water.
Is this for a taking a shower?
In reply to wvumtnbkr :
Yea just a shower. This will be a temp setup for camping at the property. Figure a 10 minute shower.
In reply to EvanB :
Look at Mr. Richie Rich over here. I actually just thought a few minutes ago. I know it wouldn't get reused but could probably sell it afterwards. I need to add up my copper and see how much that will cost me.
EDIT - had to laugh at the description.
Also, 1.32 gallons of hot water can be produced per minute at the flow rate of 1.32 GPM.
EDIT EDIT. Use outdoors only. Protect from rain.
In reply to Tom Suddard :
That one isn't on Amazon anymore but I'll look for cheaper ones. Thanks Evan and Tom.
How are you getting the 55 gallons up to 150 degrees?
Moving that small amount of water that fast through the hot water won't heat it much. It would be more efficient to pass lots of small tubes thru the mass to transfer the heat. into your shower water. (e.g. car radiators. the reverse of your idea. Cooling really hot water with hot air)
If you have 55 gallons of 150 degree water just mix it with cold water at the shower valve and many people can take showers.
In South Florida way back in the 1950s I grew up in a house that had DIY solar hot water. Consisted of enough pipes on the roof to hold about 25 gallons, all painted black. It was one continuous pipe, cold went in one end, scorching water came out the other end. Worked pretty good most the time for evening showers, not so much in the morning. 
SV reX
MegaDork
9/4/23 4:10 p.m.
Nope. No thermodynamic knowledge.
But I use a solar camping water heater bag at my off-grid place in N GA. Works fine. When I want warm and there is no sun, I just heat a bit with propane and put it in the bag. I've used it for 20 years.
I also built some DIY solar water heaters years ago in Costa Rica. Basically, a large 5' diameter coil of 3/4" black tubing mounted on a rack made of rebar facing the sun. Looked ridiculous, but it worked.
In reply to Purple Frog (Forum Supporter) :
The idea was use solar to heat a 55 gallon drum. Problem is getting a food safe not lined drum is expensive so then I thought I could just keep the drum full and then use sealed copper piping to pass potable water thru as a heat exchanger. Final build for the house would be a lot different.
I last took a thermodynamics course in 1972 and got a "C" ,so take this with a grain of salt. Do something kind of like Paul described. Use a 100' coil of black hose and an old insulated hot water tank. The 55 gallon drum will lose heat faster than a short copper pipe can gain it, so you need much more dwell time (length) in the solar heater and insulated storage. You could also insulate the drum as an alternative.
Stampie said:
In reply to Purple Frog (Forum Supporter) :
Problem is getting a food safe not lined drum is expensive
See if there is an industrial container recycler/reseller in your metro area. Denver-area has "Container Reclaimer"; they sell used 55 gal drums among other things. A food grade plastic drum is double the price ($20) of non-food grade ($10).
In reply to DeadSkunk (Warren) :
There are other constraints right now. It all needs to fit on a 12x7 trailer with bathroom, sleep area, and kitchen plus a 48x40x46 water tank. Power will be limited to two 100w solar panels so I'd rather not run a pump all day. I'm thinking the propane heater will do for this situation.
In reply to newrider3 :
I was trying to avoid plastic totally. I was afraid of heating it too high.
It's not clear to me if you want an inline heater or something that heats up the water in the drum and you use it later (like a tanked water heater in your house)
Some basic math for inline heater:
2 gpm is about 7.5 kg/minute or 125 g/second.
1 joule is the energy to raise 1 gram of water by .239 degrees (C). You want to go about 40 degrees C, so that's 21 kJ/second (125 * 40/0.239).
1 watt is 1 joule/second, so you want 21kw of heat.
Solar irradiance at the Earth's orbital distance is 1370 watts/square meter, so the bare minimum would be 15 square meters of heat exchanger. That's assuming 100% efficiency, at the equator at noon, and with zero atmospheric losses, so obviously it's going to require a lot more area in practice.
Let's say instead that you wanted to spend 4 hours heating up the drum full of water. Your 55 gallon drum lasted 27.5 minutes at 2 gpm, so you have about 8 times as long to heat it. That means your minimum heat exchanger size is going to be 2 square meters -- probably double or triple that once you account for the losses above.
In reply to codrus (Forum Supporter) :
Thanks, that gets me close enough to know that wouldn't have worked. Maybe preheating for a hot water tank but not on it's own.
Have you looked at using solar panels to directly power the elements in a hot water heater?
In reply to MrJoshua :
I probably will when I have kW available but just going to have 200W in this temp setup.
Rustoleum Black is your friend, for cheap solar absorption. You can buy hot water panels and they will often have part numbers ending in BC and BP. BC is black chrome, which is a very effective coating for the purpose (absorbs more than it emits). BP is black paint and is generally Rustoleum black. It is a much cheaper 2nd place, but absorption and emission are about equal for the paint.
Skip over kludging a water heater system and just buy water heater panels. You can make them; they are not hard, but you can buy them done for not a lot and I am sure you can buy them hurricane rated. When you build your house, move them to the roof and keep them running. If you do want to make them, just to say you did, follow online plans, as the mistakes were all worked out decades ago.
For a pump, 35 watts will do it for circulation. If you use a 5 gallon water heater on top of your regular water heater you can make a "bubble tank." Water pumps from the "real" hot water heater on the bottom, up through the panel and falls through the 5 gallon tank. The pump is powered by a solar panel ONLY. If it is bright enough to power the pump, it is bright enough for the panel to heat water. When it gets dark, the water in the panel drains into the 5 gallon tank and the bubble fills the panel. Cheap, easy, fool-proof freeze protection and it keeps you from losing heat to the sky at night. Obviously you can seal up buckets and drums and do the same thing, but you will likely want to insulate them and the returns may diminish quickly.
I am assuming you will be mounting whatever on the ground. Face south (obvious for northern hemisphere) and angle up from horizontal equal to your latitude (less obvious). This will maximize the amount you face into the sun for non-active use. The typical "round down" efficiency for a stationary water panel is 1000 W/m^2 when faced in this manner. A typical panel is 4'x8' which is about 3 square meters.
As a 56-year-old DIY problem solver, I have a much greater appreciation for thermo than I did as a 20-year-old college junior retaking thermo.
Man, I have all the books in my personal library to compute this, including solar flux for your latitude and longitude, etc to determine if your expectation of 150 is realistic... I haven't really cracked the thermo book or the principals of solar power book in over a decade now.
I'll see what I can easily look up on the thermo book tomorrow if I have time.
In reply to Apexcarver :
Wait until later ... I have plans.
MrJoshua said:
Have you looked at using solar panels to directly power the elements in a hot water heater?
Energy is energy. If your goal is to heat water, you're not going to improve efficiency by converting the solar energy into electricity and then converting that back into resistive heat.
Google suggests solar water heater panels are about 80% efficient at turning solar energy into heat in the water. PV panels are barely a quarter of that.
In reply to codrus (Forum Supporter) :
Good point. I've been watching my current typical FL concrete block home to plan my electrical needs in the new house. This summer 50% of my electric usage has been AC. I'm hoping to cut that by 6 in the new house. Next I would say hot water heater. By cutting down those I hope to have a much lower electrical overhead and therefore a much cheaper solar install.
