Author Topic: Solar water heaters.  (Read 1041 times)

BruceM

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Re: Solar water heaters.
« Reply #30 on: April 10, 2019, 05:55:30 AM »
12v is great stuff for circ pumps, controls, audio, but it's pretty grossly inefficient for real power.  24 is 12V's bigger sister but still, it's crap for much inverter power.  The amps just get out of hand.  48V was a better but still poor choice to allow non-electricians to do PV installs.

I just skipped back to the future and went to 120VDC.  My computer and rear projection workstation are running on it now, with stock power supplies.  This is handy in the US as many small kitchen appliances, as well as soldering irons, work nicely on 120VDC.  I have a coffee grinder, stick mixer, and other goodies that do just fine.  Brushed motors which have some current interruption and inductive kickback allow the original switches to work fine.

Most US Data Centers have gone 350VDC, which is compatible with switching supplies designed for 230VAC.  They chose the highest practical voltage for the coolest running supplies, and largest battery capacity.

BuilditSolar.com has some good articles and comparisons of flat plate vs evacuated tube collectors.  The need for higher water temperatures (I don't need more than 80F to heat the house) might be an issue that forces evacuated tube.  Flat plate is easy and economical to build- my aluminum fin over copper pipe collector is right out of their designs.

mikenash

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Re: Solar water heaters.
« Reply #31 on: April 10, 2019, 09:27:53 AM »
Interesting thoughts, gents.  Two minutes here, two minutes there

I'm a fan of low-tech.  Having the luxury of living largely on my own I can please myself about how I live when I'm not at work.

In summer there's solar hot water.  My electricity needs are lights, fridge, phone charger.  Power tools?  I'll run a generator

In winter there's the wetback that heats water in the 180L cylinder.  Often I'll be away for a week or two and the water in that cylinder is at three degrees when I get there.  There is good sun in the crisp winter days & it seemed to me maybe a couple of panels could push a few 24VDC amps into the element there when they had nothing else to do without doing anything any real harm . . .

The fire in the woodstove pumps out heat a lot better if the water in the wetback is at 30 degrees rather than 3 degrees initially

Thanks for the inputs

veggie

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Re: Solar water heaters.
« Reply #32 on: April 10, 2019, 02:23:03 PM »
I just skipped back to the future and went to 120VDC.  My computer and rear projection workstation are running on it now, with stock power supplies.  This is handy in the US as many small kitchen appliances, as well as soldering irons, work nicely on 120VDC.

BruceM, that's very interesting.

Using 120 vdc really reduces wire size in a big way.
120 vdc would work very well when applied to a 120 volt water heating element.
(Although switching on/off 120 vdc with any substantial amperage may be tricky)

1] Is your 120 vdc derived from a battery bank? or are you pulling 120 volts from a solar array?
If it's an array, how are you regulating the voltage as the array output varies?

2] How do you run your computer on 120vdc?

cheers,
Veggie
- 6/1 GM90 Listeroid - Delco 33si Alternator
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- Kubota Z482 - 4kw
- JiangDong R165 Air cooled - 2 kw

BruceM

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Re: Solar water heaters.
« Reply #33 on: April 10, 2019, 02:30:37 PM »
Sounds like a good plan Mikenash, but your PV to element power may be very low if the PV voltage isn't within the ballpark for the element.  The power curve for your proposed panels will be given by the manufacturer.

PS, a small 12/24v system for a cabin is the exception to the 48v rule.  I still strongly advise keeping batteries and inverter out of the cabin, and PV for same off the cabin roof.  Then at least add a commercial 2 or 3 stage filter on the AC going to the cabin. Better, also put cabin wiring in EMT conduit with compression fittings.  There is growing independent research supporting that our brains are directly affected by radiated emissions, affecting endocrine and other systems. 

Veggie, my 120VDC system has 10- 12v batteries in series, and battery management to the 12v level similar to early EV systems.  Awesome battery life.  I'm not regulating the 120VDC, the switching supplies don't care.  Lighting is incandescent soft whites, 60 or 100watt. They get used mostly at night when voltage is more like 126v. 

mike90045

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Re: Solar water heaters.
« Reply #34 on: April 10, 2019, 06:03:31 PM »
Water heater elements don't care about AC or DC

The Thermostat is a problem.  using DC on a AC thermostat will fry it pretty quickly.

Matching your PV voltage to the heating element voltage helps. Putting 12V into a 120V element will not do much heating, like hooking a 120v bulb to a car battery, you might see it glow a bit, but no usable light

glort

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Re: Solar water heaters.
« Reply #35 on: April 11, 2019, 12:29:45 AM »

Matching your PV voltage to the heating element voltage helps. Putting 12V into a 120V element will not do much heating,

Actually depends how you do it.

-IF- you match  the ohms and parallel and series the panels correctly,  and you can be WAAAY below 120V, one can in fact get useful heating by using lower voltage panels with higher voltage elements.  It's actually more about the resistance than the Volts.

I was testing a 2000w 204V element with 2 and 3  32V panels in series and getting heat output that frankly surprised me. On a couple of sunny days I had over 100L of water  Too warm to take a bath in that's for sure. I'd guess given my threshold with hot water it was about 50oC from a start temp of 16.

Putting a single panel, 12V or otherwise, on a 120V element  would be completely and utterly useless.  Putting multile panels, often and surprisingly in parallel, can in fact be very effective in putting heat into the element.
From there it depends how much water you want to heat.

I did see a few years ago a DIY heater that had a Dozen elements.
The builder first matched it  to panel arrays and then Built an arduino controller that stepped the elements in and out to match the solar input  through a PWM controller.  This was certainly not a cheap effort for heating some water for a holiday shack but the principals are entirely scaleable.

There is also the ability to use a PWM type controller with large Capacitor banks to discharge the power at a compatible output to the element.

BruceM

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Re: Solar water heaters.
« Reply #36 on: April 11, 2019, 02:20:39 AM »
There are solid state relays for switching rather massive amounts of DC, though I make my own since it cheaper and I can have just what I want.  Just make sure they provide the "on" resistance, so you know how much heat sinking. 

It's not a bad thing to learn how to use a MOSFET transistor for switching DC power.  Plenty of tutorials on line.

Not much power required to warm up an insulated tank if you can wait days.  Heating watts are just volts times amps, and the resistance of the element is stated or can be calculated via ohms law.  The power curve of the panels allows you to accurately predict voltage at a given amp load.


mikenash

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Re: Solar water heaters.
« Reply #37 on: April 11, 2019, 07:16:12 AM »
There are solid state relays for switching rather massive amounts of DC, though I make my own since it cheaper and I can have just what I want.  Just make sure they provide the "on" resistance, so you know how much heat sinking. 

It's not a bad thing to learn how to use a MOSFET transistor for switching DC power.  Plenty of tutorials on line.

Not much power required to warm up an insulated tank if you can wait days.  Heating watts are just volts times amps, and the resistance of the element is stated or can be calculated via ohms law.  The power curve of the panels allows you to accurately predict voltage at a given amp load.

Bruce, Yes, thanks

When I fitted the wetback cylinder I took out the element and fitted a low-rated 1500-watt one. 

I have had lots of experience with these fairly modern, relatively well-insulated cylinders.  In my last house, in summer, I'd run the wetback on a Sunday while I was doing outside stuff so the heat of the fire didn't annoy.  Once the cylinder was almost boiling I'd turn it off.  They hold heat well and some kind of thermocline process stops cold & hot mixing as you use hot water (obviously, cold water enters at the very bottom and "pushes" hot water out the top).  Often there would still be acceptably-hot water for a shower on the following Thursday

So when you put heat into the water in the cylinder it holds it quite well

If I'm away for a week and the photovoltaic is connected direct to that 1500w element, it can pop a few amps into it at 24 - 40 VDC all day long, and whatever temperature rise it may manage will still be 90%there the next morning when the sun shines again

(no thermostat - it wouldn't like that)

When I turn up on Friday night the water temp might be 10 degrees, or 20 degrees, or 30 degrees?  Who knows?  It's probably gonna be an improvement on two degrees, anyway.  A "head start" for the wetback, really

If it does bugger-all then I guess I've wasted an hour with wire & screwdriver

Next time I'm up there I'll have a tinker with the meter and see what actual voltages come down the line into the controller.  Who knows?

If you have (a) no money, (b) off-grid, and (c) the luxury of pleasing yourself how you do things - you can do quite a bit with creative use of small amounts of energy (LED lamps might be an example)

I guess we'll see.  There's a stand-alone panel I can have a play with easily.  If it makes, say, 5A at 35VDC, then there might be 10% of the nominal wattage of that 1500W element tricking into the water . . .

dieselspanner

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Re: Solar water heaters.
« Reply #38 on: April 11, 2019, 08:51:04 AM »
Thanks for that guys,

I read up more on RMS and now have a bit of a clue as to what it all means, gotta love the university of Google!

No Money and little power is where I'm -slowly - going with the barn up the hill.

After almost a year I'm getting closer to buying it, the outfit that controls the refurbishment of ancient barns have given me permission for the conversion but the local council won't give me the go ahead 'cos the first lot put a caveat on the permission to the effect that I cannot remove any soil to construct the track.

This is a bit of a sticking point as the last 40 meters goes up at about 1 in 4, and the only way to get a track suitable for vehicles is to put in a hairpin bend. Big digger and loads of soil movement!

So yesterday I went back to our 'county town', Tarbes, and got the name of the boss who, naturally, wasn't there. At least I got to speak to an underling, who was very nice but totally non committal, and came away with an email address to try and get an appointment. Amazingly my French held up pretty well, tho having a partner who is extremely fluent helps......

Although I've shaken hands on the deal I've said I won't finally commit until all the permissions have been granted and refusing to pay an architect 6500 has cost me time rather than money, with a bit of luck another 5 months should do it!.

In the meantime I've been keeping an eye on the stream and it seems to be fairly constant, even in the dry spells, so the mini hydro plant is still looking favorite.

I'm leaning heavily in the direction of driving a 24v truck alternator with a pelton wheel, running it straight into a pair of 120 ah 12v truck batteries and out through a 6000x inverter. The idea being everything is plug and play, relatively cheap and easy to install. It's also very familiar to me as it's essentially the same as the system I had on my Dutch barge.

I don't know if it has any merit, but I'm also thinking of installing a second loop of under floor heating pipe alongside one driven by the wood burner. The - vague - plan is to power it with the output of the alternator in the early morning dark hours, once the batteries are topped up. Nothing complicated, just a timer, circ. pump, 24v immersion element and a temperature controlled switch to cut out the element if the pump packs up and / or it all gets too hot.

I got the idea from Glorts ponderings on 'what to do with excess solar power'

Bit of a long post for me, you won't believe how crap my typing skills are, I'm killing an hour whilst waiting to set off with a mate's truck for the French vehicle control test!!

Cheers
Stef

Tighten 'til it strips, weld nut to chassis, peen stud, adjust with angle grinder.

mikenash

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Re: Solar water heaters.
« Reply #39 on: April 11, 2019, 09:35:00 AM »
Well done, Stef.  Fascinating as always the lives of folks here

oldgoat

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Re: Solar water heaters.
« Reply #40 on: April 11, 2019, 10:52:45 AM »
Something that has to be remembered is that the heating effect of an element depends on the square of the current flowing through it.  If it is run at reduced voltage the output will be alarmingly low.

mikenash

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Re: Solar water heaters.
« Reply #41 on: April 12, 2019, 07:14:55 AM »
Something that has to be remembered is that the heating effect of an element depends on the square of the current flowing through it.  If it is run at reduced voltage the output will be alarmingly low.

Just another thing I didn't know.  Thanks

I think next time I'm up there I might just mock up something to hold an element in a 20L bucket or some such & connect it to a panel & see what happens if it sits there for a few hours and extrapolate from that . . .

Cheers

dieselspanner

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Re: Solar water heaters.
« Reply #42 on: April 12, 2019, 08:54:15 AM »
I Like the bucket plan, Mike,

I was going to just put the pipe in and do it eventually, but seeing as I've almost got all the kit here spending 20 quid or so on the heating element seems like a better plan. we have a pressure reducer on the incoming cold water, down ro around 3 bar, bypassing that should give me enough head to perform a meaningful experiment, just need to  organise some shed time - maybe organise the shed too!

Cheers
Stef 
Tighten 'til it strips, weld nut to chassis, peen stud, adjust with angle grinder.