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Author Topic: Solar water heaters.  (Read 2012 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.
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old seagull man

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Re: Solar water heaters.
« Reply #11 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.

glort

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Re: Solar water heaters.
« Reply #12 on: March 30, 2019, 01:17:01 AM »

Some rambling long winded thoughts as usual.  Make a cuppa before reading, I love this stuff!  :0)

First thoughts are if you have panels you can put to the HWS, is there a reason you can't connect them up to the mains?  IF you are worried about being " Legal" skip to the next part. I have no such hangups or Preoccupations.  :0)

You can set up an array on a normal Grid tie inverter and plug it into an outlet PROVIDING you don't exceed the power rating of said outlet which here in oz is 2400W on a standard outlet, 3600W on a standard Circuit. As sparky mate pointed out, only difference between 10A and 15A outlet  is the earth pin and he seems to be right.  Been Pumping 3000w+ through a standard outlet for many months now no problems. With the upgrades I did to the orphan array  the other day I saw 3800W yesterday! Not bad from a 3600W inverter!


Being the cautious type though ( ::) ) I did put a load on the outlet right where the inverter connected to pull off some of the excess. Mrs was pleased when she came home and there was a load of clean dry Clothes in the basket.

In my worthless opinion you are better where possible to use a GTI and hook into the household power because the water heater will only run a limited time and the rest of the time you'll be saving power if not getting a Pittance back for it which is ( slightly) better than it going to waste.
If you have spinny meters which you won't if your solar is legit, they will spin backwards. If you have a Smartarse Meter you will reduce if not satisfy the the house hold loads and there will be more going back earning you some FIT.  We are lucky here in oz that there are plenty of cheap, used systems available that can repay pack their investment in under a year.

If you want to go direct, start reading here!   :)

The way to go about this is keep the thermostat as is but instead of having it do the electrical switching, have it just switching a relay so all the work is done there and your thermo is not going to fry.

I have been using Regular AC DPDT ( double pole, double throw) Relays  They are like 2 switches pared but instead of an on/ off switch there are an on/on with 2 sides to them. One side make contact when the relay is off, the other when it is energized.       
The way I have found to use these successfully as a DC switch is not to turn the DC OFF, but rather Divert it. This stops the arcing because it gives the DC somewhere to go as it were. I think of it as having momentum. Once it starts flowing it does not like to stop. IF you give it somewhere to go it won't trying to keep going where it was going before. If it has no where to go it will want to continue and will arc.
   
DEPENDING  on the amount of power you have and the rating of the Relay, ( usualy 10-20A)  you can just have the unit set up so when ON it powers the element and when OFF, it is a direct Short Circuit.

If One is trying to power a 240V element, then the higher the voltage from the panels ( IE, in series) the better.  Feeding half the voltage does not mean half the power due to the resistance and pull down on the panels. You will get like 1/4 or 8th.  Can be calculated on the resistance but I forget how to do it because I'm satisfied it's terribly inefficient so avoid it.   :laugh:
Of course the higher the voltage the lower the amps so if you are using something like 190W panels, the amperage is less than 5a so dead shorting the wiring isn't going to hurt anything.

There are of course The SSR's ( solid state relays and while I have a few in the parts box, haven't got round to trying them yet. Read Mixed reports so I'll have to test one and see what the real story is. They are available in AC and DC versions so you would need the DC in this application. There was a reason  I didn't use them but can't think what that was now.


Another thing I have found with solar DC. on a resistive load, VOLTAGE doesn't matter.  I have run 350V+ into a 250 halogen light and heating elements and they are fine PROVIDING, the total wattage of the solar array does not exceed the wattage of the load being driven.

IE, I was feeding 450W @ 350V into a 500W halogen 240V  lamp and it was fine. When I measured the voltage under load, it sagged down to under 200V so the lamp was not even fully lit.  Trying that on a 150W lamp and it blew like a flash bulb.  The output from panels is variable and will change depending on load. You can series panels as much as you like ( although I don't go over 600V Myself as the arcing can get severe above that at any amperage)  PROVIDING you don't go over the wattage of what you want to drive.

For example, a lot of the panels I have are around 30V under optimum load.  IF you had 20 in series that would give 600V.  if they were 190W panels they would give 3800W.  If you had a 3600W heater element, technically you would be over but in reality it would probably be ideal as you are never going to get 100% efficiency out of panels especially if they are 190's which would now be Minimum 5+ years old. Also if they did hit 100% output, you could over drive a water cooled element that much without any problems.

The other aspect to the panels is you could instead of running 1 20 panel string, you could split the array into 2x 10 panel strings and use them as an array. You would be working with an easier to handle 300V instead of 600  but because it's the WATTAGE that's important not the voltage, You would still be able to get the max output from your element.... for a few hours a day anyhow. In practice I would say that would in many cases be less often that not. In summer the heating will kick in before the sun peak and will most likley have the tank up to temp before the panels can develop peak power. In winter they are never going to get near peak output so irrelevant anyway.

The other thing is they would only produce max output for a short time because they won't do it during the morning and afternoon ramp up, fall off and you would be lucky to get 2 hours where they were pushing full on.

For switching you would leave the relay in place and feed it a from something like a 12V Plug pack and have it just switching the relay. This takes the load off the contacts of the thermo and puts them on the relay. Easier and far cheaper to change out if you do fry it which you shouldn't in my experience.

The other thing with switching DC is you can also use the relay for a Dummy or diversion load. I have fund that a 50% load is sufficient to stop the arcing.
If you have for instance 3000W of DC solar power, you can switch that into a 1500W load and the arcing will quench just fine. I tested lower diversionary loads  and you can run much less however the arc quench is not as reliable.  50% is a good safe number in my tests with plenty of headroom.
I'm as dodgy as they come and basically hate safety but then there is preservation and you don't want Relays catching fire when near your house so on that score, 50% load diversion is a number you'll never have a concern with.

As a diversion load for the water tank, In winter, once the water was up to temp you could divert the solar to a heater in the house. Like the water heater, a bar heater should work fine as it won't care if the power is AC or DC.  ( That has an unfortunate sexual connotation!)
Just don't turn it off and make sure your solar wattage is not more than the heater's rating. May are around 2Kw I have seen. Fan heater won't work on DC, least not the one I tried with the Universal motor..... Doh!

For about a year now I have taken my HWS off the off peak and put it on the standard rate circuit and had it wired into a Voltage monitoring Relay that Bruce put me onto and has been the best thing I discovered in the last 12 months!

I have it set so when my solar pushes up the line voltage ( which is too high here anyway) the water heater klicks in. This has 2 advantages...
1. It lowers the Line voltage allowing the inverters to give max power without tripping out on high line Voltage.
2. Heats my water for free.

On many " Plans" you pay more for off peak than you get FIT.  in this case you are selling power to the grid for around .08C and buying it back at night for around 12-18C.  Ripoff.
I have simply changed my water heating from night time to day.
This has had an added and unforeseen bonus. When I did it the women folk asked if I had made the hot water hotter? I never touched the thermostat but thinking about it, with off peak the water may have been sitting there 20 hours since it was last heated and has been diluted through the day with washing and washing up.  heating through the day it may have been heated as little as 4 hours or less before so obviously the temp will be higher. I wouldn't expect the fall off to be that great especially as the system is only a few years old However they both commented so obviously they did notice a difference.

I am saving about $90 per quarter on off peak bills now and still make more power than I can use. Changing the water heating around is a thing many people on solar could benefit from.

If effect, using the Bottom element would be better for solar feed because the top element only heats a small amount of water and there will be stratification in the tank. Top could be up to temp and the bottom only luke warm. I am told that the thermostat on the bottom which is a double type can be moved to the top. The way I believe they work is they switch elements. Forget how now but one kicks in and then when that is hot it diverts power to the other element.  I think it works top down and there is a thermo at the top same as a normal single element heater.  By switching thermostats which I'm told are interchangeable, you can have the solar doing the main heating and just use the mains power for top off.

 I am looking at a preheater for the main tank. Going to use a a S/H tank and have the cold water feeding to that and that feeding to the tank.
The reduction in water temp from winter to summer inputs would take an extra 4KW of heating just to bring the temp up the same as it is in summer.  I'm looking to get another tank and dedicate that to solar as I really can't push much more solar through the circuits I have anyway and that will take some load off the winter generation.  This will allow the main tank to be always up to full temp as usual but still reduce the load on the mains power I pay for in winter.





Boxelder

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Re: Solar water heaters.
« Reply #13 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 #14 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