Solar water heater

[nixiebunny]

I hope that plumbing and electricity are closely enough related hat this thread makes sense in this forum.

I'm modifying my 1977 Starcraft popup trailer for use at Burning Man. So far, I've installed a 12V solar power system with a couple 18AH sealed lead acid batteries, and three pumps to move water from a 10 gallon fresh water tank to two faucets (inside, outside) and through a solar water heater.

I'm currently pondering the water heater itself. I've made similar things in years past that circulate water from a 5 gallon bottle through 50 feet of 1/2" black poly irrigation tubing. I plan to do the same this time, but put the tubing on the roof of the trailer and use the freshwater tank as the hot water storage tank. (We will have separate drinking water!) I also want to have an automatic controller to circulate water during the day until the tank is at a nice 100 degree "hot water" temperature, then hold it there.

I'm an electrical engineer, so the circuitry (old school analog) is the easy part. The tricky part is the control parameters. I used a 555 timer in the past to give me a variable duty cycle pump sequence, so the water would cook in the tubing for 10 minutes, then be cycled out with different water in a half-minute pulse of pumping (The tubing holds 3/4 gallon). But it wouldn't turn itself on and off for the diurnal cycle, so I'd go away and forget it. you know how the playa is.

I have installed an auto engine thermistor on the tank's outlet fitting, and I have some thermistors to sense the temperature of the heater on the roof. I figure that I'll run the pump when the water at the outlet of the heater is some number of degrees above tank temperature. It will also stop the pump when the tank is hot enough. This should make the system automatically shut down at night, or when it's too hot outside.

Am I missing something?

[BBadger]

You could get a solar powered pump so that you don't have to worry about leaving it on all the time, and then just use that thermostat or some other feedback controller for temperature regulation. You could also make a thermosiphon and avoid the pump; just tap at a specific height on the tank for a certain temperature.

[Popeye]

It will work fine but a little overengineered. It would be a lot simpler with two thermostats. One that closes on temp rise on the poly tubing. I would cut the poly and put a short piece of copper in as copper is a better heat conducter. Attach the bulb to thecopper and insulate the thermostat bulb. Second thermostat in the tank, closes on temp fall. Wire the thermostats in series and play with the settings and differential to avoid short cycling.
A timer needs to be adjusted for a sunny day, readjusted for a cloudy day, etc. etc. This way the pump only comes on when the water in the tubing is hot and the tank needs hot water.
If you can come across an old electric hot water heater you could swap the tank out and have an insulated tank with one of the tstats already installed. I realize I don't know your space requirements.

[nixiebunny]

Thanks for the suggestions. I'm already pretty far along in the build and the water tank is part of the trailer, so I don't see a reason to change the basic setup.

I forgot to mention that the purpose of the water heater is to make delightful showers available to my camp mates and myself. I've been doing this since 2007. It's gratifying when a camp mate says "That's the best shower I've had in my entire life". This is why I want the water temperature to be just ever so. I provide a nice shower head with a long hose, and a pushbutton to make water spray from it. Just like home, but with a push-to-talk switch.

I will effectively have two thermostats - each are made of thermistor and a comparator, so they are adjustable. Like I said, that stuff is easy for me. I might build the controller circuit on a solderless breadboard and play with it during the week. One thing is for certain - it WON'T be an Arduino!

[FIGJAM]

You know more about this than I do, so let me give you a uninformed opinion.

It seems to me that 50' of black irrigation hose sitting in the sun is going to make the water too hot simply getting from one end to the other.

[some seeing eye]

I understand what you are proposing and am an engineer. Suggest looking at the user experience elements. What time showers, how many gallons per shower, feedback to showeree on gallons and temperature in F/ even with your push to shower.

Thermal technical: insulated hot water tank and copper or steel tubing if available are something to consider. Sounds like your design has the tubing dry after the water is heated. That may be a problem for black plastic tubing, People are often selling/freeing 1980's solar thermal water collectors with metal tubing on Craigslist.

And of course, suggest sharing experiences with the Alternative Energy Zone village! Hey maybe you camp with them already!

[Captain Goddammit]

Or you could just get a propane hot water heater from an old camper.
Compact, works on cloudy days and at night/morning, and already has a thermostat.
Unless that's too easy and works too well...

[BBadger]

But... but... renewables.

[Elliot]

... I provide a nice shower head with a long hose, and a pushbutton to make water spray from it. Just like home, but with a push-to-talk switch.
...

Can you tell me about this shower head with push-to-talk valve? I think I want one.

[nixiebunny]

The solar heater is nicer than propane mainly because it's much less invasive and licensed-contractor requiring... no gas to worry about exploding the trailer. If my popup was originally plumbed with hot water, my decision may have been different.

My shower sprayer setup is a Oxygenics X-stream low-flow shower head mounted to a handle made of 1/2" PVC pipe. I use a doorbell button mounted to the PVC pipe as a pump actuation switch. A standard little water pump pushes water into the shower pipe when the button is pushed, completing the power circuit to the pump. It's the best way to save water, by not letting it run unless you are pushing a button.

You can do the same thing for a handwashing station, but with a foot pedal to run the pump switch.

I'll post photos once I get the solar heater installed.

[Captain Goddammit]

Licensed contractor requiring? If you're afraid of gas appliances in your trailer you could just toss it on the ground and use flexible water hose to it, and a propane bottle, regulator and hose just like a regular gas barbeque grill has.

Elliot, pretty much every camper/RV has a shower head with a super convenient on/off water switch right on it. The pump stays on, but since RV water pumps have a built-in pressure switch, as soon as you turn the water off the pump stops. No wires, and your temp setting doesn't get messed up.

[Elliot]

OK, I already have a standard RV water pump system installed, so all I'm missing is the RV shower wand. Thanks, both!

Oh... I like the idea of the black hose on the roof. I'm still using a small black barrel, which I carry in and out of the bus.

[krly]

There's always another alternative......http://www.duckworksbbs.com/gear/shower/
krly

[nixiebunny]

Did someone say "overengineered"? Because yes.

Building a deluxe shower is my Burning Man art project.