Solar water heater

[Eric Johnson]

Totally off-topic, I know, but I'm thinking about building a solar water heater and was wondering if anyone knows of a forum or board similar to this one devoted to the topic.

I'm getting so used to non-existent gas bills that I'm considering ways to keep sticking it to the gas company this summer.

 

[Eric Johnson]

That's a good idea. I've been thinking about this for years and when you consider what you need for a storage tank (insulation, PRV, appropriate tapping, etc), a hot water heater fits the bill.

I also thought about making a collector out of old 3/4-inch fin & tube baseboard elements painted black. Any thoughts on that?

 

[Sundeep Arole]

How about considering the guys who sell whole "kits." Probably a little more money than making everything from scratch, but should save a bunch of heartache bacause you know the panels and everything else has been tested and works as designed. Maybe you could just get the panels fro them, and put together your own tank, pipes, etc. As I understand the panels are not all that trivial to make, and a best just bought off the shelf.

 

[Eric Johnson]

That would take all the fun out of it, HotFlame.

 

[roac]

I'm sure you have read the article at Mother Earth News about solar hot water heaters. It looks interesting. I'm thinking that a well insulated box with double pane insulated (non low-e) windows along with 3 or 4 black 55 gallon steel drums hooked up in series might be an inexpensive solution that would provide a lot of hot water in the summer time. Make sure that the used drums you get had food stuffs in them previously and that the lids are not removable. Here is the article if nothing else it might give someone some ideas.

(broken link removed to http://tinyurl.com/dy2ty)

 

[Sundeep Arole]

Well, the crudest form of 'panel' would be simply a circuit of pipes, painted black, facing south, perhaps with a mirror behind them. A alternate design would be a curved parabolic cross section, shaped sort of like a half cylinder, with a tightly wound coil of piping (so it has a narrow cross section) placed right at the focus. You could take the first version and enclose the pipes in a box with a glass top, silvered at the bottom. A blackened cover of sheet metal, on top of the pipes, under the glass, would further increase efficiency. If you evacuate the box, that will be even better.

Perhaps it will 'take the fun' out of it to do it with prefab panels, but the process for making these is down so well that commercial panels are of high quality, and mass produced enough that anything you make yourself will likely cost more and not work as well. You still can design the whole system, but get commercial panels. That should still leave a lot of 'fun' in it.

 

[Eric Johnson]

That's probably good advice, HotFlame. I should add that I would be setting it up in a greenhouse that's attached to my house, and I envision an insulated collector box with glass or poly faces. Kind of like a solar collector within a solar collector.

Leaving aside the issue of collector panels for the moment, how would you get the hot water into your hot water heater?

One thought I had would be to circulate the water between the collectors and into an auxiliary tank. At some temp set point, the contents of that tank could begin circulating through the hot water heater.

Alternatively, how about using the collector as a preheater, so that any water going from the domestic water supply into the hot water heater would pass through the collector piping first?

Or some combination of the above?

roac: I read the MEN articles but had trouble getting past all the exclamation points and the end of each sentence! They seem more inclined towards convection and other passive methods, but I think I'd probably want more control, so we're talking aquastats and circulators and maybe even zone valves and timers. Like HotFlame says, that's where much of the fun is, although I do enjoy soldering and working with copper.

 

[webbie]

That's a good idea. I've been thinking about this for years and when you consider what you need for a storage tank (insulation, PRV, appropriate tapping, etc), a hot water heater fits the bill.

I also thought about making a collector out of old 3/4-inch fin & tube baseboard elements painted black. Any thoughts on that?

Use foam board - the foil stuff, as the backer. Aluminum frame if possible.

You can buy special black spray paint that actually soaks up more sun than regular...dig around, it should be available. Anti-freeze system is best for north country, you can use external heat exchanger or new tank with internal coil.

Glazing either glass or best yet a plastic - but make certain the stuff is rated for UV.

The fin and tube would have too much surface area for proper efficiency...I think you are better off with a flat sheet of copper or similar, with 1/4 tubing - perhaps a riser every 4-6 inches and header on bottom and top - wired and soldered (or I think there is heat transfer epoxies, etc.) to the background sheet.

When all is said and done, you might be able to find some scratch and dent prebuilt collectors or something wholesale for a similar price as building it yourself. Top of the line, if they still exist, are what is called "tube in sheet" collectors, which are produced (Olin in the old days) by fusing two sheets of copper together and then actually forming the tubes within the copper. The best collections are also black chrome plated as opposed to painted.

See if tax credits - state or fed are available.

 

[webbie]

I'm sure you have read the article at Mother Earth News about solar hot water heaters. It looks interesting. I'm thinking that a well insulated box with double pane insulated (non low-e) windows along with 3 or 4 black 55 gallon steel drums hooked up in series might be an inexpensive solution that would provide a lot of hot water in the summer time. Make sure that the used drums you get had food stuffs in them previously and that the lids are not removable. Here is the article if nothing else it might give someone some ideas.

(broken link removed to http://tinyurl.com/dy2ty)

Even some of the commercial thermosyphon kits are probably good for many folks for the non-freezing seasons. The best have flat collectors and a tank above them.

 

[webbie]

That's a good idea. I've been thinking about this for years and when you consider what you need for a storage tank (insulation, PRV, appropriate tapping, etc), a hot water heater fits the bill.

I also thought about making a collector out of old 3/4-inch fin & tube baseboard elements painted black. Any thoughts on that?

Good primer and example of DHW solar:
http://tinyurl.com/qnbeu

 

[Eric Johnson]

Thanks Craig.

I might go with glycol, but since I'm putting it into a greenhouse, the threat of unexpected frost is minimized.

Assuming a direct-piping strategy, What's a good basic model for getting the water into my hot water heater?

EDIT: You beat me to it. I'll read the link. That's just what I was looking for.

 

[roac]

That's weird I only counted 2 exclamation points. There were a ton of ellipsis's though, that was annoying ... :shut:

If that is your pleasure then I guess I'm with hotflame, go with a kit. Why re-design a new wheel when the old one works so well? You still get to put it together yourself if that's your pleasure. Also don't negate your savings with an inefficient pump.

You might want to look at getting an on demand natural gas water heater and hook it in-line with your solar set up. They sell a non pilot light system just for set ups like this.

 

[Eric Johnson]

It was a different article, roac. Some guy who built one out of old tin cans or something. Had an exclamation point at the end of every sentence! At least the editors didn't drop the "g"s [i.e., money-savin'] like they used to do back in the '70s.

 

[webbie]

Thanks Craig.

I might go with glycol, but since I'm putting it into a greenhouse, the threat of unexpected frost is minimized.

Assuming a direct-piping strategy, What's a good basic model for getting the water into my hot water heater?
.

Not sure if this is what you are looking for, but the idea of hot water solar collectors is that:

The hotter you want to heat the water, the less efficient it is

What this means is that it is much more efficient to heat pool water in large amounts just 1-3 degrees when passing through the collectors than it is to heat 40 gallons up to 140.

That is why solar tanks have heat exchangers on the bottom of the tank, so you are always heating the coldest water.

I had a great DHW system on my last house - 25+ years and probably still going now. It used a "proportional controller" which changed the pump (antifeeze pump) speed based on the difference between the collector temp and the water at bottom of tank. This made certain that the right amount of water was passed through each time.

In theory I agree with the buying of at least some parts - but that does take some fun out of it. If you keep in mind what you learn from that page and others, you will probably do OK.

BTW, pool heat panel (plastic) are cheap and probably perfect for heating water to temps of 110 or less - as long as it is not to be used for DHW (drinking, etc.)......

 

[Sandor]

I have looked at this long and hard over the last several years. If you want a pre-canned kit, search for "Fireball Collectors"

For a heat exchanger, buy a Ruud 80 gallon solar tank, it already has the exchanger built into the tank. Its availabe with/without a backup 4500 watt element.

Get a Photavolatic recirculator pump, that way, the pump only runs the fluid around when the sun is shining.

I give you credit for your effort.

 

[Eric Johnson]

Knowing me, I'll probably rig up some home-made ghetto arrangement to see how it works before spending a lot of money. You know, maybe just a basic collector convecting into a tank. If I get a tankfull of hot water, I'll be thrilled and ready for the next step.

Years ago I went down in Guadalajara, Mexico to visit my dad and mom, who were volunteering at an orphanage. My dad was trying to rebuild a solar water heater on the roof. Apparently the place was just lousy with solar water heaters at one time, but they must have had a cold snap that froze them all up one night, so now there are a bunch of ruined heaters sitting on roofs all over the city. This one was just 3/8-inch copper tubing soldered directly (no fittings) into holes drilled into 1.25-inch supply and return lines. They were mounted on black corrugated steel roofing and enclosed in sheet metal frames with window-pane glass fronts.

I got back to the States and was all hot to build one of my own, using 1/2-inch copper with 1-inch lines, sandwiched between some black corrugated roofing and enclosed in a wooden box. Never got around to doing it, but I'd still like to try.

But as a practical matter, I should probably noodle out the potential gas savings and invest an appropriate amount in some proven technology, per your excellent suggestions.

 

[Czech]

Has anyone done this, but used a compost pile instead of the sun? I turned mine over this last weekend, it's been in the 30's in the day, teens at night. The inside of this pile was truly cooking, stream and all. Call me nuts, wouldn't be the first time. But what if I made a bin, ran some copper coils through it, and pre-heated the DHW this way? Just something I've been thinking about, that and wrapping the chiminea on the deck with copper too. Heck, what about a few coils around the dog and cats in series? Got to be a few btu's in them considering how much they eat.

B

 

[Eric Johnson]

I think that would work, Gotz, but you'd need a big-ass compost pile and you'd have to turn it on a regular basis and feed it to keep it cooking. As a practical matter, probably too much work on the average backyard compost operation. LOL, maybe I could pre-preheat my DHW in the compost pile before preheating it in the solar water heater.

 

[Czech]

Then a few coils around the outdoor fireplace, couple times around the DHW stack, out to the garage for the loop under the hood of the truck, and don't forget the compressor loop in the central AC (at least when it's running). Oh, and make sure everything is well insulated, too. I've got ideas, oh man do I have ideas. What's 1/2" soft copper going for, I should only need about 2700'. Luck Eric, keep us posted.

B

 

[Sundeep Arole]

Okay, so I will try my usual amateur attempt at analyzing this. The reason a greenhouse traps heat is because the radiation from the sun penetrates the walls of the greenhouse, is thenabsorbed by the objects inside the greenhouse, and then the objects re-radiate that energy at a longer wavelength. This longer wavelength radiation does not have the energy to penetrate the greenhouse walls and escape outside, and so the radiation is trapped inside and warms the greenhouse. If you plan to put a solar collector of the regular kind inside a greenhouse, it will still need to face the sun so it absorbs the radiation coming directly from the sun; it cannot re-trap the longer wavelength radiation already trapped in the greenhouse, as the reason that radiation was trapped in the first place is bacause it did not have enough energy to penetrate the greenhouse walls and go out, and so it is unlikely it will be able to penetrate the walls of the solar collector and heat the cavity inside.

A system designed to extract the heat trapped inside a greenhouse must be engineered fundamentally differently; it will need to be a system which collects heat from its environment both via conduction and radiation; A system designed to do that would be engineered more like a heat pump in reverse, not like a radiant solar collector.

Now you say - how about if I just have pipes or finned baseboard, or for that matter, a series of condenser coils connected in series running inside my greenhouse? Well, that wont work too well either. The cavity inside a radiant solar collector is purposely made thin because that way the heat trapped inside has only a small volume to heat and can raise the temprature of the cavity well above the temprature of the fluid which will extract heat from it. This makes it a efficient heat engine. The volume of a greenhouse is large, and the radiation trapped inside a greenhouse has a lot of volume to heat, hence the temprature of the greenhouse will never reach the kind of tempratures you will see inside the cavity of one of those solar collectors, and hence the maximum efficiency you can get out of such a system is limited.

 

[Eric Johnson]

My plan is to mount the solor collector in the greenhouse facing south, as close to the glass wall as possible. If the right wavelength can penetrate one pane of greenhouse glass, why wouldn't it then penetrate a second pane of glass mounted on the top of the solar collector? It's passing through two panes of glass, which is no different, from the way I see it, from radiant energy passing through a double-pane window. The energy should still be trapped inside the solar collector. The only difference I can see is that the collector will be sitting in an environment that has a much higher ambient temperature than a collector mounted, say, on a roof. As such, it ought to be more efficient.

I don't know why everybody seems to assume that I want to just put some exposed collector elements in the greenhouse and hope for the best. Obviously, if the greenhouse temp doesn't exceed 100 degrees (enough ambient temp to kill plants), the ambient temp is not nearly high enough to heat DHW. The greenhouse is an attractive place to mount the collectors because it's 1.) There; 2.) Protected from the elements; 4.) Facing South; 3.) A much warmer and more stable environment than the outside.

In short, why would sunlight entering a greenhouse be any different from sunlight encumbered by a single pane of greenhouse glazing?

 

[Sundeep Arole]

Whoa Eric, I didn't assume you were going to put it in the greenhouse and hope for the best. The radiation which comes through the greenhouse walls is certainly strong enough that it will go through the collector and get trapped in there. What I was talking about is the radiation which is abrorbed and re-emmitted by the objects in the greenhouse. That radiation is of a longer wavelength, and the reason it cannot escape is it does not have the energy to penetrate the greenhouse glass and go out. My point was the solar collector would not be able to gain advantage from that radiation, the one that keeps your greenhouse temp up, except that it is less likely to freeze.

 

[Sundeep Arole]

And my other point was - IF you decide you want to take advantage of the radiation trapped in the greenhouse as well to heat DHW, then you need a differently engineered system, more like a reverse heat pump, which will extract heat from the environment, but not without some mechanical work being done to extract that heat (like a compressor). That, IMHO, defeats the point of going solar - which is that the input energy cost is zero, save the cost of maintaining the system.

 

[Eric Johnson]

Thanks HotFlame. Actually, after reading your post I took another look at my set-up. Since it's a nice sunny morning, I noticed that I actually get a lot more direct sunlight (earlier) just outside the greenhouse along the 24' outside wall. That might be a better location for many reasons, assuming I go with glycol and a heat exchanger. Since I have hard water, that's probably a better solution anyway. I'll take a pic and post it in a few days so you can see what I'm talking about. The greenhouse is 24' long and since I think 100 square feet of collectors would meet my needs, the collector assembly would only have to be between 2 and 3 feet high along the length of the cinderblock wall. It would probably look better, too.

Do you have any idea of the relative efficiencies involved in direct piping (circulating plain water between the collector and (directly into) the water heater vessel) compared to using a heat exchanger? As it happens, I already have a heat exchanger hooked up to my water heater to take advantage of the heat from the wood-fired boiler. Probably just a matter of a few valves and some piping modifications to switch it over to a solar water heater in the summer.

 

[Sundeep Arole]

One big advantage of glycol/heat exchanger setup is that you can have a separate heat exchanger in a tank with a backup (or auxillary) heat source. Also, you can circulate the glycol several times through the loop, extracting a little bit of heat from it each time. I don't know about the relative efficiencies, there is a section on that in the hydronic heating book I am currently reading, so I will have to read up on what they have to say and get back to you.

FWIW, most of the systems I have seen in developing countries circulate the potable water directly, usually through pvc piping. The plumbing codes in most of those areas arent as stringent, and such a arrangement is allowed. I doubt its widespread adoption has anything to do with efficiency, though; it likely has more to do with the lower cost of the system.