Author Topic: Solar water heaters.  (Read 312 times)

glort

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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

glort

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Re: Solar water heaters.
« Reply #2 on: August 16, 2018, 11:14:53 AM »

Thanks Bob.

Now you mention it I have seen that. A mate has a solar water heater and I have been trying to get to see him and ask him about it.  He had a flat plate and went to tubes so would be interesting to see what he things of the 2.  I have seen and heard the little pump running and have one up the back. I think it's 20-30W. Very small and I remember trying to use it for something and it had no lift or head being only for closed systems.

The young guy today mentioned they had to use the electric element a lot over winter and was pondering if he'd be better with off peak rather than trying to heat his water at full tilt rates.  I said even if the thing gets your water to 25, you have still saved that much power over the temp the water comes out the tap. 

BruceM

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Re: Solar water heaters.
« Reply #3 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 #4 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 #5 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 #6 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 #7 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

glort

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Re: Solar water heaters.
« Reply #8 on: August 17, 2018, 11:21:41 AM »

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

That would be my ideal soloution.
My Aunt and Uncle that live not far from Bob have an Aga stove that does all that.  Heats the house, the water and cooks the food.  they have a solar water heater but it has no booster.  If the water is a bit cook, they fire the stove up and in an hour it's hot enough.

The stove gives a lovely deep soaking warmth to the place and my Aunt does a lot of Delicious baking in it. Nothing like a wood fired Oven.
The Mrs wants a wood fired heater but I'm not keen. Too much work and too expensive to buy wood.  If we did go down that road I would have a stove rather than just a heater.  Mrs isn't at all keen on the idea but I reckon one of these things would be cool as all get out.
They are not cheap ( or lightweight) but well worth it I would say.       

I have emailed Aga here twice now about their oil burner which fits into the stove's but got no reply. I always forget to ring them at an appropriate time.... Like when they are open.  If I could retro fit one of these stoves to burn Veg, definitely going to be investing in one.       

Might be able to catch up with my mate tomorrow to see what he thinks of his Evacuated Tube heater in winter.
I'm still biased towards PV for ease of install, purchase cost and ability to do more than heat water but if the tubes will give me hot water in the same area PV won't, then it may well be worth it.                           

glort

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Re: Solar water heaters.
« Reply #9 on: August 26, 2018, 08:20:48 AM »

Caught up with my Eco aware mate Yesterday and sat round enjoying wood Fire Pizza for the first time in ages.

I asked him about his Tube hot water system and he said he had taken it down. I asked what and he said according to his calcs, the thing was using about as much power in winter to drive the pump  as he was saving on water heating.
He said he took down the water heater to put up more panels and said from what he can see so far, he is in front with the panels.  He said as all the Kids have moved out temporarily at least being in their own places or off living in different parts of the world, there is only him and the GF there now and at least they get credit on the power they don't use where the water was still costing them money to heat a tank which was a lot more than they needed.

So far I can't see where these things do a lot of good in winter which is the main need I have. In summer they seem overkill But like anything solar, it's at it's best when you need it the least.

Think I'll Just stick to PV,  So much easier to install and you can do a lot more with any excess than you can with hot water.

Hugh Conway

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Re: Solar water heaters.
« Reply #10 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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