Solar hot water: Cold vs. warm climate

Hackaday occasionally reports on homebrew home automation and the like.

They have recently covered a couple of solar hot water systems, and I thought forum-goers might like the link:

http://hackaday.com/2010/01/11/cold-climate-solar-water-heater/

Paul, Caleb, and Krista talk about hot water in this podcast: hot water podcast

They talk a lot about solar, which Caleb has a lot of experience in.

Welcome Bob, looking forward to reading the conversations on solar water heating. I am wondering if there is a efficient do it yourself model that can work well in cold climates. I have intentions on implementing solar water and air heating. I have solar electricity and passive home design and it works well. We get plenty of sunshine on our south facing slope. Any input greatly appreciated. Thanks

I'm interested in learning more about thermosyphon, glycol, evacuated tube solar hot water system for cold climates. I'd like to hear about the Pros and Cons from anyone using this system in a cold climate.

Dave, it is relatively easy to build your own solar air heating collectors. Just make sure you do not use wood or plastic in them as then they could start a fire. I talk about air collectors in my book. I built my first air collector in 1971. For solar water heating I suggest just purchasing a good flat plate collector. They are not that expensive but are nearly impossible to make yourself, (I've tried).

Rick, I have worked on thermosyphon glycol systems and they can work. The piping installation has to be meticulous with proper slope and no traps. I would not suggest evacuated tube collectors for this application. I suggest using flat plate collectors with a harp- style absorber plate.

Thanks Bob. I just bought your book via your Artha web site. Looks like you have a very nice place in WI. I would like to visit it someday.

Please, SO to explain what is a flat plate collector?

A flat plate collector is the most common solar water heating collector used in the world today. Ask google.

Bob

Ok, here I mainly see the tube ones.
You can see these tubes instead of a glass flat surface.
Is it more adapted to warm climates?
And the FPC to cold climates?

Tube collectors do work best in warm climates and flat plate collectors work great in all climates. My experience with tube collectors has been less than perfect while I have seldom had a problem with flat plate collectors.

How about heating thermal oil (mineral oil) via solar, then circulating the hot oil into the home to heat air and/or water?
Mineral oil should not freeze (break pipes) until it gets -22 F below, so it might work well in cold climates. It can also carry a lot more heat than water.
Also, a rocket stove can be used as a backup to the solar heater. Route the liquid to a heat exchanger in a rocket stove and you have the ability to heat at night or on cloudy days, should you need that.

Back in the 1980's some companies advocated using oil as a heat transfer fluid. The most popular oil was a brand called Bray 888. I imagine that oil is still made.
The problems with using oil are: it can not, in fact, carry a lot more heat than water. Water is actually the best fluid and all other fluids heat carrying capacity is compared to water. The seals and gaskets, including the bladder in expansion tanks are not compatible with oils. When oils are used in solar water heating systems, special seals must be used in all components. Viton seals are typically used.

I thought water boiled into steam at a much lower temperature than oil.
Also, a few solar farms heat oil to over 700 degrees in pipes that run down solar troughs, then use the hot oil to boil water for turbines. If water is better, why not just run water trough the solar trough and skip the oil all together?

I'm pretty sure it takes a lower temperature to freeze oil than it takes to freeze water, allowing oil to remain outside in cold climates without busting pipes.

Or, are you saying that after the water has boiled to steam the steam can carry more heat than oil?

Any links to help me understand this?

Water is the best heat transfer fluid from the respect of efficiency. It is also the best heat storage medium from the respect of heat able to be stored per cubic foot.
If water did not freeze at 32 degrees F., then you would see water as a heat transfer fluid in every residential and most commercial solar water heating systems. So all solar water heating systems that operate all year round in the continental US must be freeze protected. There are 2 types of systems used worldwide for residential and most commercial solar heating/cooling systems, pressurized and drainback. You can read about how those systems work in my book (it is available for purchase and also should be available at your local library). Drainback systems use either pure water or a weak antifreeze solution as the heat transfer fluid and the fluid drains from the collectors every time the system turns off. In a pressurized system the collectors are filled with a non-tozic antifreeze.water mixture at all times.
solar water and space heating and cooling systems do not require temperatures above the boiling point of water, so there is no reason to go there. Because solar energy is low-grade heat, it is difficult to make steam on a reliable basis except in very specific places on earth and with very specialized collectors. These systems are called CSP (concentrating solar power) and are all large facilities that are in high desert regions of the world and are very sophisticated systems that typically operate around 600 degrees F. They in fact often use an oil as their heat transfer fluid. They do make steam and turn a turbine to make electricity. These plants are the cheapest way to make electricity on the planet, with the added benefit of no pollution.

I recommend my book as a resource for solar water heating systems. While there are plenty of good resources on the internet about this subject, I would caution you that there is a lot of mis-information out there too.

Bob
www.arthaonline.com

Thanks for explaining, Bob. Sounds like I've got some more reading to do. I still don't understand how water can be heated to a higher temperature than oil without boiling to steam and increasing the pressure, but I'll look into it.
Solar heating of oil would be backed up with active heating via rocket stove. (I want to combine both active and passive heating into one system) With the heat involved from the stove, I was interested in oil because it doesn't boil into steam and increase the pressure so much when the temp goes over water's boiling point.

I'll get your book soon.

hi, i wanted to say a couple things about cold / warm climate thermal.

the potential to boil is usually a symptom of an atmospheric pressure system. whereas all closed-loop pressure systems will self-regulate via the pressure itself going up and creating a condition in which the boiling point conveniently goes up along with it. therefore, the higher the pressure, the higher the boiling point. the higher the heat, the higher the pressure, the higher the boiling point. you will rarely find liquid from a p.r.v. valve on the solar side of a correctly installed and functional closed loop solar hot water system.

i imagine oil could easily be a replacement in a year-round cold-climate closed-loop system. i don't think the performance would be much better than that of a glycol system. you can study the ratio of performance reduction per percentage of glycol verses water for your specific climate.

you can also build a drainback style system. the advantage in cold climates is that the collector sizing can be greater than that of your ideal storage. in california, 1 sq ft. collector area (high temp copper) to 2 gal of storage. this can be changed to say, 1.5 sq. ft. collector to 2 gal of storage. this can be problematic in a closed-loop system, but not in a drainback situation. during winter, a larger collection of energy is available. during summer a shorter cycle occurs. empty all other hours not requiring freeze protection nor having failures due to hot stagnant liquid problems... i have serviced distilled water drainback systems in freeze territory; they sound funny (literal), look funny, are installed out of plumb and square - but are still there. they work.

additionally, i would be diligent in studying the fitting of a rocket stove to heat water. it seems that the rocket stove is to ramp temperature high enough to cause all particulates to be burned and that the high temp is what causes the efficient fire to work and heat the mass. i would think the introduction of running cool liquid temperatures in an exchanger would cause the stove to have under-performance issues and clogging due to the loss of temperature from doing so? occasional use is probably not much of a problem. or just replace the solid mass with the liquid mass. i would like to experiment more with rocket stoves because of their great efficiency.

Noob (to this 4m) here.  I am nearly 70 and am a retired plumbing contractor with 48 years of on-the-job experience.  I have been building solar water-heating devices for at least 40 years and have, I think, tried every possible scheme including concentrated solar, tracking solar, heating mineral oil, flat-plate, parabolic -- you name it, I've likely done it.  BUT -- the reason I'm posting is, I developed and now use a fool-proof method of doing it, which is summer-heat as well as winter-cold proof.  Here's what I do:

I use PEX (HDPE) pipe, under a plexiglass cover (to shield against wind losses), pipe insulation wherever pipe is exposed to air, a 12-volt in-line impeller pump, which runs off a 12-volt (really 14.2 volt) solar battery maintainer panel and a drain valve.

PEX can handle both heat as well as freeing-cold.  It expands in either case and does not split, like copper can when things freeze.  Now, understand that 'brass Shark-bite fittings' can and will burst if they contain water and the mercury drops down around 31 degrees F.  A trick here is to leave a hot faucet "dripping" and as long as there is flow in the system, it "PROBABLY" WON'T FREEZE.  If you use internal barbed fittings, no worries, but they are wicked expensive and the tool to install them is right up there with engagement rings price wise as well.

So, what I do is build a box out of wood or plastic (or metal, if you've the material and tools), place a piece of Plexiglass sheet plastic on the top and seal it against the container, so moisture and/or wind can't invade things.  Buy a 50' or 100' roll of "BLUE" or "RED" PEX and paint it flat black with spray paint.  Coil it in the box in a "flemish" (looks like a snake coiled in a flat plane).  Insulate pipe entering and leaving the box with foam-core rubber pipe insulation ($1 for 6' at HD), paint the box whatever color you want and mount it facing South at the same angle as is your latitude.  There is a +- 5 to 8 degree deviation between summer and winter solstice, but who's counting?  It'll work fine.

I live in North Texas and it gets blistering hot in the summer and somewhere near absolute zero in the winter.  You can tell I'm originally from Southern California -- cold is a four-letter word!  At any rate, although it's not 100% necessary, mine has a 'Tee" in the system with a valve, so I can drain it when winter rears its ugly head.  If mine were to freeze, likely nothing would happen, but at nearly 70 years old, I've had too many close encounters with "Murphy" to beg any more, so I drain mine in the winter.

Total cost for all the fancy stuff including solar panel, pump, PEX, fittings (I already had the pricey tool), Plexiglass, wooden enclosure, paint and shot pin (nails -- I used a pneumatic pin nailer and wood glue) -- ALL IN -- less than $100; a lousey "C-note".  

Oh, by the way -- You know there's not really any such thing as a hot water heater right?  I mean, really; who would heat hot water?

Thanks Al! Would you have a pic of your installation?
It makes things so more easy to read, especially when you are not an english speaker...
I will anyway look at the materials you use to understand what they are.

@ Xisca:

I haven't figured out the picture thing yet.  When I do, I'll snap a few and post them.  Right now, my PEX solar application is sitting inside my barn until Spring.  I built a new awning and removed it to apply a new metal roof and haven't reinstalled it yet.  As it were, I get FREE electricity at night where I live, so I installed a mammoth switch on my electric, 50-gallon water heater and judiciously turn it on during free-electricity times and off the rest of the day, so I haven't felt the 'urge' to reinstall Mr. solar water heater, but I will eventually get around to it.  I'm nearly 3/4 of a century old; sometimes just doing life takes all my time.  Remember, I build windmills, exotic as well as experimental engines and electric-power generators, meat smokers, radio-controlled toys, planes and lawn mowers, just to mention a few things.  I also dabble in zero-point, so like I said, I'm actually pretty busy, but rest assured, the day will come and the picture will pop up here.  I'm just of late, putting the finishing touches on a windmill, which taps into zero-point energy and charges a bank of capacitors, which in turn .  .  .  I'm beginning to ramble.  Later.

Hiya Al,

I'm mighty interested in your set up as well. Do you happen to recall what diameter pipe you used, and the specs on that pump?

If/when you've got a bit of time, could you also give a description of how the system works, how the pump is controlled/what it controls, etc?

Thanks!

@ Bryan de V:

Here is a link to a pump similar to mine:  https://www.amazon.com/GPM-11-5-LPM-Solar-Threads/dp/B0075J6S3W

My pump is actually smaller, but this one will do.  I think my pump was something like about $35 or so, but then again, that was about 10 years back, so with inflation and all, maybe this one is not over-the-top-expensive after all at $70.

At any rate, I used 1/2" PEX and a can of flat-black stove paint, mostly because I had some kicking around my barn.  Regular flat black paint will work fine, because the surface temperature will never get over 250 degrees and not likely more than 220 or so.

When the small battery-maintainer solar panel (15W?) I got from Harbor Freight reaches its threshold voltage, the pump starts, which is handy, because you wouldn't want it on UNLESS there is ample sunshine to make things work.  The pump is inline with the PEX loop and the PEX loop is inline with my water heater.  I'm a professional plumber, so I take my cold water off the top of the heater, with a check valve to prevent cold surprises in the shower.  The check valve prevents cold water from sucking down into the hot header and dousing my naked fanny with ice water!

I pull cold off at the top of my water heater and tied-in the hot loop from the solar heater at the top as well.  Doing it this way, avoids my  having to dismantle the world, should I have to change out my electric water heater / solar storage tank (same tank).  All lines running to and from the water heater inside my house and outside to the solar collector are INSULATED (very important) with pipe wrap to prevent intermediate cooling due to air currents passing over hot pipes.

I just let it run by itself.  There are no "controls" as it were and I do not use a "tempering valve".  A tempering valve mixes cold water against VERY hOT solar water to create a combined temperature less than 110 degrees F.  Tempering valves are for families with kids and sissies!  Really, who the hell just jumps into a hot shower without testing the temp?  Reminds me of a joke:  Marriage is like taking a bath; once you get your foot in, it's not so hot.

Here's a trick to eliminate the pump:  Mount the solar array LOWER than the receiver tank.  This allows the system to "thermosiphon".  Hot water is less dense than cold and it rises against gravity --------- BUT -------------- this works pretty much ONLY if you use a collector with a top and bottom manifold, with "tees' and stretches of PEX joining the two.  If you do this, I put a piece of shiny galvanized roofing material under the tubes, with each of the connector tubes laying in the trough of an undulation (think sine wave).  This reflects sunlight onto each side of the tube, increasing its exposure to sunlight.  Of course, you have to cover the whole thing with glass (low-iron glass, if you're a perfectionist) or plastic to keep the wind off things.

Franky, I would rather use an "active solar" application using a solar-driven pump, than the mirror-backed header system.  It's WAY easier to build the PEX flemished-roll version than the other and costs WAY less and works every bit as well, if not better.  PLUS -- the smaller unit is easier to anchor against the wind.  I have a lot of wind here, but no worries: I have several windmills also, which make electricity, gather zero-point energy, pump air and so on.  

Here's a tip:  Don't worry about what you don't know. Just do what you know.  It'll get you through just fine.

Hope that helps.  If you have any other questions, fire away.  Of late, I'm building a zero-point energy collector as well as a rocket stove mass heater, which is rather time consuming, so I enjoy taking a break to answer emails and replies on this and other 4ms.

I think that the best way to do Solar hot water today is to use PV panels and a heat-pump water heater.  It's cheaper than most direct solar water heaters, works much better in cold weather (PV output INCREASES when it's cold) and it's easier to store surplus energy for stormy days.

Plus during hot weather many solar water heaters have problems generating too much heat.  With PV you can use the surplus to power other things around the house.

Bob Ramlow wrote:Tube collectors do work best in warm climates and flat plate collectors work great in all climates. My experience with tube collectors has been less than perfect while I have seldom had a problem with flat plate collectors.

Bob, maybe an accident, but you should revise your comment, and edit your book if not!  But i think in the spirit of the text, yes flat plate collectors work well at 45° north latitude and store bought hydronic collectors are way more practical and definitely executed better.

Al Freeman wrote:Noob (to this 4m) here.  I am nearly 70 and am a retired plumbing contractor with 48 years of on-the-job.

Oh, by the way -- You know there's not really any such thing as a hot water heater right?  I mean, really; who would heat hot water?

But the semantics and nomenclature of it all....

Bob Ramlow wrote:Dave, it is relatively easy to build your own solar air heating collectors. Just make sure you do not use wood or plastic in them as then they could start a fire. I talk about air collectors in my book. I built my first air collector in 1971. For solar water heating I suggest just purchasing a good flat plate collector. They are not that expensive but are nearly impossible to make yourself, (I've tried).

Rick, I have worked on thermosyphon glycol systems and they can work. The piping installation has to be meticulous with proper slope and no traps. I would not suggest evacuated tube collectors for this application. I suggest using flat plate collectors with a harp- style absorber plate.

An air to liquid heat exchanger in the air ductway from the solar air heater is a practical solution to diy solar collector set up to heat water.