Solar Water Heater

[cvo]

I've a remote cabin in East Tennessee that I sometimes allow small groups of people, 5-15, to use in the Spring/Summer.

Currently I've a on-demand propane water heater but I would like to get into using some type of solar system to heat about 150 gallons of water and get away from using the propane.

Thats 15 people X 10 gallons/person.

Is this feasible? Anything homemade out there? I think all I can do now is use solar to preheat incoming water.

Thank you,

Curtis

[Norm]

   They have these black plastic containers at Walmart, solar showers for camping out.. about 5 containers like that would be sufficient for the shower part very little hot water is needed for any thing else if you are really camping out.

                  ( :>) Norm.

[cvo]

Thanks Norm, I think thats called "Batch Heating" a "passive" solar system.

I like your idea cause its real simple.

What I'm concerned with is the temps can sometimes be below freezing here in April.

I could do your idea with 3 60gallon tanks for example, paint them flat black and enclose them in an super insulated box with a clear roof. Even tilt the tanks for maximum sun exposure.

[Dan M]

If you can pre-heat your water with solar then you can use solar for your primary, you might need a lot of collecting area.

Example:

Panel made from 4x8 plywood.  32 sq ft or about 3.5 sq yards.

Lets say whatever you do is about 50% efficient for the system (we can argue about this all day long, plug in a different number at the end if you like).

At about 750 watts per sq yard, your box will "see" about 2600 watts.  At 50% efficiency you collect about 1300 watts.  At 1 watt = 3.4 BTU/hr this is 520 BTU/hr.

A BTU is the amount of heat energy required to raise 1 lb of water 1 degree F.

You have 150 gallons or about 1200 lbs of water.  If you have 6 hours of good sunlight each day, then the 4x8 collector can raise 1200 lbs of water 2.3 degrees per hour or 14 degrees in 6 hours.  If your incoming water is at 50 degrees and you want to get to 105-110 degrees in 6 hours then you need 4 of the 4x8 panels.  If efficiency is more like 75% then you only need 3.

Someone please check my math,

-Dan M

[tecker]

   Switch to captive solar ( strip a couple of water heaters paint with stove pait and install in a glazed area) and use your on demand as a preheater . Cloudy days and hot water hogs and all those times when your solar can't keep up.Change the thermostatic arrangment and a little plumbing .    

[NickCoons]

[At about 750 watts per sq yard,]

I thought solar energy was closer to 120 watts per square yard?

[At 50% efficiency you collect about 1300 watts.  At 1 watt = 3.4 BTU/hr this is 520 BTU/hr.]

If 1 watt = 3.4 BTU/hr, then 1300 watts = 4420 BTU/hr, no?

[ghurd]

120 watts per sq meter is for solar electric panels output. They are only about 10-14% efficient, lots of variables.

I think 1000w /sq meter is under 100% perfect conditions. Never happens here.

750w is more reasonable for real life good conditions.

G-

[cvo]

DanB,

Thanks for info. Now I've got something to work with.

I was hoping to get away from PV panels though and have a copper tubing system on the roof or ground & if not that the Batch type system.

I havn't found any data on a non PV panlel system that meets this requirement.

I'm sure it would work for me, I just like to see the numbers first!

Thanks again everyone for the imput.

[Dan M]

Thanks Nick,

You're absolutely right.

I have a number pad that's gonna cost me a fortune someday.  The "3" doesn't work.  So the number I had was 1300 x .4 instead of 3.4

I keep forgetting to use the number row along the top instead of the number pad.

I guess if I had taken 5 seconds to proof-read this it would have been obvoius.

The new calcs:

at 4420 BTU / hr

heating roughly 1200 lbs of water 60 degrees F takes about 16 hours (or 2 - 3 4x8 panels to do it in 6 hours).  I had a screw-up in those calcs as well.

ALSO:

Passive solar has to be covered up at night, or devise a system to let the water gravity-drain when your pump is off.

Radiation coefficients work both ways.  If something "absorbs" heat well it will also radiate it very well when it is warmer than everything else.

-Dan M

[Ungrounded Lightning Rod]

I have a number pad that's gonna cost me a fortune someday.  The "3" doesn't work.  So the number I had was 1300 x .4 instead of 3.4

Aren't replacement keyboards under $10 these days?

[Ungrounded Lightning Rod]

Switch to captive solar ( strip a couple of water heaters paint with stove pait and install in a glazed area) and use your on demand as a preheater .

Huh?

Shouldn't you use your on-demand as a POST heater?  That way you burn no fuel if the water is already hot enough.  (Your demand heater has to be able to figure this out, though.)

If you're using it as a preheater why bother with the solar at all?

[cvo]

Thunder Rod, your right on, but the on-demand heater is not a pre-heater. I'm trying to get totally away from the propane.

My on-demand heater can be part of the system as a post heater, but I'm wanting to get away from using propane as much as possible.

I don't know why I should bother cause propane is so cheap here, but I'd like to get into solar.

All I'm wanting to know is:

1. Is there a stand-alone passive/active non-PV system out there that can cook 150 gallons at outside temps of 25F?

Be it active or passive system, does not matter.

My needs are:

I need 150 gallons of warm/hot water by 5 p.m. each day during the 25F degree temps I can get in April.

Warm weather solar systems are easy, but during the cold?

Thanks everyone.

Curtis

[troy]

If there is any possiblity of a hard freeze on your system (and it sounds like there is) then you have three options.

1.  A drainback system.  When you shut down the pump (or open a special valve) the water in the collector drains completely out.  So everything has to be plumbed in so that gravity will clear all the collectors and all the lines.  Not really difficult, but you do have to pay close attention in the design phase.
   
2.  A heat exchanger system.  The fluid in the collectors is high temp rated antifreeze, and there is a liquid/liquid heat exchanger inside the house to transfer the collected solar heat into the domestic hot water system.  Poof, no freezing problems.  The downside is that a heat exchanger reduces overall efficiency and the cost and complexity of the system goes up a lot.
   
3.  If you never got protracted hard freezes, you could build your batch heaters with insulated covers for at night or for a day or two below freezing.  You would also need well insulated/buried lines as well.  Or you could just drain the solar system for the few weeks or months of winter and take your losses on the propane.  
   
   

The "perfect" system is the enemy of the good enough system.  Although perfection is a worthy thing to strive for, it is often so expensive or so complex that it prevents us from even attempting a good enough system that makes a real difference.

Good luck and have fun!

troy

[tecker]

   If using as a preheater you won't need to turn it on unless the water is below the comfort level and the a thermostat is manipulated to add to the accumulated heat.and you could keep the mass above freezing with a small pump when away for a while .    

[nicolai]

I suppose you need the water mostly for shower.I dont know the local conditions,but if your people can shower one at a time during the sunny late hours,you can do a simple solar collector out of a garden hose put inside plastic bottles.I invented this myself and I shower march to october in Eastern Europe with it.In the summer the water gets even too hot for the skin at noon.For a shower (10-15 liters),you need only a 40m hose(pipe-low cost) and about 120 bottles (free).

You could store the warm water(ready in 10 min.)anytime in a bigger tank with a pump,if you want.

It seems to be ridiculous simple,not worhtly trying,but it works great!

Nicolai

[nicolai]

I suppose you need the water mostly for shower.I dont know the local conditions,but if your people can shower one at a time during the sunny late hours,you can do a simple solar collector out of a garden hose put inside plastic bottles.I invented this myself and I shower march to october in Eastern Europe with it.In the summer the water gets even too hot for the skin at noon.For a shower (10-15 liters),you need only a 40m hose(pipe-low cost) and about 120 bottles (free).

You could store the warm water(ready in 10 min.)anytime in a bigger tank with a pump,if you want.

It seems to be ridiculous simple,not worhtly trying,but it works great!

Nicolai