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Author Topic: Solar water heaters.  (Read 8314 times)

guest22972

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Solar water heaters.
« on: August 16, 2018, 09:56:38 AM »

I have been looking into direct solar water heaters but am getting confused.
Some people say they work well enough in winter, some say they are utterly useless. Seems the type, Flat plate or Tube has equal confusion.

A young bloke came to see me today about WVO Conversions.  He's also into solar, wants to run a genny and go off grid etc so thought he hit the jackpot when he landed here and saw all the crap I have.
He has flat plate water heating and said the water is barely warm in winter. He lives in the next main town so not far away.
He did say that the tank he got was made OS and apparently don't hold the heat well due to poor insulation but also said that the water is never really hot anyway. Summer of course is no problem.

I was wondering if those that have these water heaters installed could give some feedback on their experiences.
At this point I am just curious about the things as I reckon it will always be easier for me to just put up panels than plumbing but I am always open to changing things for the better.  There are a few of these heaters going SH atm and they may be worth using just as a pre heater for the main electric HWS.  The water out the tap was getting down to 4oC here where as in summer it can be pushing 28 or more.  That 20o Difference makes a big impact on the power required to heat the water especially when the solar yield is down so much to start with.

Bit of practical experience and feedback from people that own the things rather than trying to sell them would be interesting!

ajaffa1

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Re: Solar water heaters.
« Reply #1 on: August 16, 2018, 10:33:21 AM »
Hey Buddy, those solar water heaters can be great however they usually have a small circulation pump to drive solar heated hot water through a heat ex-changer coil in the storage tank. Generally the heating coil in the tank separates the heating system from the domestic hot water, allowing it to contain anti-freeze without poisoning anyone.

If that pump has failed no heating water will circulate to heat the water in the tank. Normally they have some sort of timer, thermostat or sunshine sensor to tell the pump when to cut in and out. Continuing to run the pump with no sun will pump the heat out just as fast as pumping it in. They also usually contain an electric immersion heater for when the sun don`t shine. If the pump is running, while the electric element is heating the tank, all you are doing is heating the atmosphere.

Bob

BruceM

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Re: Solar water heaters.
« Reply #2 on: August 16, 2018, 05:40:47 PM »
The vast majority of new designs today are drain back systems, to avoid antifreeze, and the stagnation related breakdown and corrosion problems it makes.  Drain back systems are inherently freeze proof, and typically last twice as long.  My system is the simplest of the drainback systems- I have a single 800 gallon, insulated drain back tank, mostly below grade. One pump runs/starts the system and if it fails, the water drains back into the tank and that's it.  I use that water directly for my in floor heat, and domestic water is heated via copper coils in the tank.  The tank is EPDM pond liner with 3" foam board, cement board, dirt. A 12" tall framed wall above and insulated, EPDM covered lid let me have the pump sit on the floor next to the tank (in my shop building).

Flat panels are much cheaper to build, but your maximum temperatures will be much lower.  My flat panel collectors did have 20F higher temperatures in winter when they were double glazed with greenhouse polycarbonate panels, but the panels failed from mechanical stress- the difference in temperature of the inside and outside polycarbonate (very, very thin) was too great, causing a great deal of bending and buckling, especially during stagnation (tank maxed out so panel left sitting in the sun with no water flow).  I replaced them after a few years with a single layer of 1/16th inch thick polycarbonate... which has performed marvelously.  I do notice on the rare very cold, windy days that production is down perhaps 30%.

If you need higher water temperatures you have no choice but to use evacuated tubes.  The flat panels just can't give you water temps in winter much over 110F, and often 100F. That's fine for my in floor system, which can heat the house just fine with 90F water.

The other option for larger hot water capacity at higher temperatures needed for radiators and such is parabolic trough collectors with the central tube in evacuated glass tube.

To recap: Flat panels are fabulously easy to build (see BuildItSolar.com for the copper pipe/aluminum fin designs like mine).  But they are not designed to make high temperature water in winter.  They work fabulously well with a well done in-floor heat system, which will work with low temperatures. 


LowGear

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Re: Solar water heaters.
« Reply #3 on: August 16, 2018, 08:10:12 PM »
Solar Hart, an Australian product, is popular here in Hawaii.  Passive system that sets up on the roof with an electrical element back up.  I'm looking for one of their thermostats.  The jerk distributor here won't sell them unless they do the service call.
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BruceM

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Re: Solar water heaters.
« Reply #4 on: August 16, 2018, 08:29:22 PM »
I had a Solahart unit on my home in Gilbert AZ (near Phoenix). Good for a place with only very rare freezing temperatures.   

Batch (direct glazed tank heating) heaters are also viable for mild winter climates.

As a preheat, an inexpensive flat panel approach could work.  For just domestic hot water alone, you have to be careful about up front cost and maintenance cost.  That's what's driving many to PV-electric hot water for just domestic hot water. 

 I got my money's worth for my solar hot water system here, since the solar water system does winter space heating as well.  The endless hot water the rest of the year is pleasant luxury.

Even in winter I don't skimp on hot water for laundry or showers as domestic hot water is a drop in the bucket compared to space heating. 

mike90045

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Re: Solar water heaters.
« Reply #5 on: August 17, 2018, 04:30:53 AM »
First, does the proposed site have no frost, light frost, or heavy freeze.  That will greatly influence the type of system to be installed.

As to what I have, in moderate frost climate (winter night time 5F )

 I have a flat plate collector with elevated insulated storage tank and thermosiphon glycol loop.
Pic attached.

Summer, I hit 140F easily, winter, is negligible because of poor roof/sun angle, but my masonry heater preheats the incoming cold water to about 90F before it goes to the tankless heater for finishing.


mikenash

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Re: Solar water heaters.
« Reply #6 on: August 17, 2018, 09:25:00 AM »
There are many answers to this question.  IMHO I believe the simple solution (my process in four different houses over 40 years) is simply to combine a wood-stove with a wetback (runs all through winter, and there is so much hot water it gets wasted 'cos it's "free") with solar panels (hot water through summer).  And you can always fire up the woodstove for a few hours in summer if it's overcast for days on end

My last house had $55-60 monthly power bills all year round, really

Also IMHO there are few things more satisfying than a wood-stove that heats the house, cooks the dinner and heats the hot water as well

Hugh Conway

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Re: Solar water heaters.
« Reply #7 on: August 26, 2018, 08:39:36 PM »
Off grid here.
Using an inexpensive evacuated tube with integral 300lt collector tank on top. At the end of the day, I manually switch on a  pump that circulates between my original electric water heater (now just a storage tank) and the outside collector tank. I run the pump for about 1/2 hour during dinner, after the sun is off the collector.. Works fine from April to end of September. The sun goes below the treeline in early autumn. We drain the system for winter.
Then:
In winter, we heat with wood and have a loop inside of our woodstove. Thermosiphon to a nice 30 gallon brass tank in the kitchen gives us plenty of hot in the heating season.
We are at 50*N and winters are overcast most of the time. Solar panels don't charge the Batteries then, so it is Listeroid and 3kw PMG for a 2 hour daily charge.
Cheers,
Hugh
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old seagull man

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Re: Solar water heaters.
« Reply #8 on: March 29, 2019, 01:14:57 PM »
On the subject of hot water heaters, but on a very different tack.

There must be a was to run your electric hot water heater of solar panels, Glort and I have lots of them.

This is the thought:

My water heater is a twin element job. and the element, don't care what electricity it get to make its heat. 500watts AC makes the same energy as 500 watts DC
So i should be able to connect my 240 volts dc from say 500 watts of modules, and power the element in my heater.

The problem is that the mechanical thermostat, wouldn't survive that kind of dc voltage. I would probably weld it self together or just melt.
And i don't want to change it all to electronic, control. I just want to feed some power into the booster element, when i have it, so i can get some cheaper hot water.
I new get almost nothing for my excess power that returns to the grid.
Or just connect the boost element to the inverter and leave the main element  on the off Peak as it is at the moment.
If it was a bigger heater i would just connect it to the inverter.

 Somewhere somebody mentioned that off grid users were using solar panels and mppt controllers and converting the output to ac at  a few hundred hertz, so it was basically AC as far as the thermostat was concerned, so no arcing out.

So any ideas the madder the better, through them in.

Boxelder

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Re: Solar water heaters.
« Reply #9 on: March 30, 2019, 06:18:38 PM »
We (my wife and I) started a business about a decade ago with the express intent of importing evacuated tube solar water heaters to the U.S. and selling them here.  It was an interesting experiment which resulted in excellent tax write-offs but was the wrong business for us to be in.  First problem - neither my wife nor I enjoy selling things to people.  In fact, we usually avoid interacting with the general public as much as possible.  This turned out to be the stake in the heart, and in retrospect we knew it was probably doomed from the start but we were so gung-ho that we went for it anyways.  The second problem is that Americans are notoriously fussy about their home aesthetics, and we heard "It looks too weird" more times than I could count when we took the samples to home shows.  The third problem was that we couldn't get the FSEC/SRCC certification as they had a 3-year backlog of systems to test to make solar water heaters eligible for the tax breaks available at the time.

Anywho, the Chinese really have their stuff together with the solar water heating.  At the time (2009) the payback period on an integrated system was about two years, and then you'd have free hot water for as long as the system lasted - typically 8 to 10 years.  I have several test systems we imported at the time still chugging along like champs.

We went to Jiaxing, China to see the solar eclipse (er - I mean "meet the manufacturer") and were stunned by how many buildings had the evacuated tube collectors on them.  Basically, if you have hot water in China, it's solar.  They even lay out the new construction east-west, presumably because it's most efficient for solar collection.  Proof - go to Google Maps and zoom in on pretty much any area in China with recent development, then tell me they're not thinking solar efficiency.

If you have excess PV like Glort does, it's a no-brainer to dump it into the water heater if you have no other use for the energy.

I've been playing around with my diesel toys lately, having acquired a 1935 6/1, two LT1A's, and a Changfa ZS1115.  They're generator projects waiting to be built, and free time is limited with a two year old running around.  Nice to have them on the burner, even if it is the back one.  Gives me a sense of future potential.

Must be nice to have all the sunshine in the world down there in Oz.  Dependable as the day is long, eh?

Wrong sub but what the hell:  What's the American equivalent of the Lister and the Changfa?  I've seen Witte diesels and all kinds of hit-and-miss gasoline engines, but is the answer "Briggs and Stratton"?  There's no "chuff-chuff" with those.  Much less satisfying.

quinnbrian

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Re: Solar water heaters.
« Reply #10 on: March 31, 2019, 08:34:36 PM »
I , too am interested in solar hot water heating...but I live in Canada...so we get a little snow and -20 to -30 temps a couple of months a year. Say from about Nov-April LOL. Anyways , the setup I've looked at , would be solar water panel on the roof , with the 12 volt pump , all is control by a thermostat , when the panel get warm...I think 180ish, the pump will turn on and deliver the hot water to a insulated holding tank ( 500 gallon or ??) ...in the ground...basement etc. If the panels never get warm (like in the winter months) then the pump will never turn on...= no hot water from the roof system, but you could have a dual purpose with oil or propane back up. If it was propane you could uses a mil switch for the temperature control , so no power would be needed to run the propane side. And with minimal power needed on the solar side of the system.
I'm still thinking about it....
Cheers
Brian

quinnbrian

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Re: Solar water heaters.
« Reply #11 on: April 04, 2019, 03:02:48 AM »
The large Hot Water holding tank is for "staying power" if you have a couple of day where the sun doesn't shine , you will still have heat for the house, water , etc.
The big trick is getting it up to a good temper and maintaining it, get everything just right, and your golden...

buickanddeere

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Re: Solar water heaters.
« Reply #12 on: April 08, 2019, 06:47:20 PM »
Here in the land of ice and snow . We are getting away from plate collectors and pumping water . Instead PV panels are used to power resistance heaters in an “ordinary” electric water heater .
  No risk of freezing , less to leak, no pumping and higher water temperatures.

BruceM

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Re: Solar water heaters.
« Reply #13 on: April 08, 2019, 09:58:13 PM »
Big 4'x32' plate collector and 800 gallon insulated storage tank work great here for home and domestic water heating...but since I changed to 1/16 single glaze polycarbonate on the panel my winter peak temps are a lower, about 110F.  In Canada that might let the high temps too low from glazing losses. 

DO NOT use greenhouse double wall polycarbonate for flat plate collectors.  It will fail from heat stress/distortion; too much temperature difference between inside and outside, and the poly is too thin to take that much stress.  I've been very happy with the 1/16 single layer polyc.  I replaced it with.

Direct water heating gives you about 85% efficient energy collection, so for space heating, it's worth it.


mikenash

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Re: Solar water heaters.
« Reply #14 on: April 09, 2019, 09:18:10 AM »
A question from my electrically-challenged self, so excuse me . . .

Am I right in thinking that the element in my hot water cylinder is simply a resistance-becomes-heat device and that it won't give a hit whether electricity fed to it is AC or DC or what voltage it is?

(I was just reading the components of this and related threads about finding ways to "store" excess solar . . .)

Cheers