Author Topic: PV hot water  (Read 784 times)

DEinME

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PV hot water
« on: February 26, 2019, 09:24:50 AM »
Posted for general discussion of PV hot water.

PV hot water has been gaining acceptance for the past 5 years or so. I just never heard of it until recently.

Wind guru Hugh Piggott mentioned PV hot water in a letter in Home Power issue 179. Reader Christopher Jessop responded in HP 184 with a longer list of advantages plus mention of a "Willis immersion heater" for getting some very hot water quickly.

So far I've found:

Two videos by a guy rigging up a comparison between PV and a trickle-down drain-back water heater. The second video is shorter and has the results.
https://www.youtube.com/watch?v=kkoesuG72xQ
https://www.youtube.com/watch?v=UULYjx8eFtI
He started with a single PV panel connected to the heating element and then added a second PV. No specs on those panels.
For equal collector area the water heater made water hotter but for less money the 2 PV panels made the water hotter.

PV hot water has advanced to the point that manufacturers produce MPPT hot water controllers:
https://www.commodoreaustralia.com.au/product/solar-hot-water-booster/

Article in Green Tech Advocates
http://greentechadvocates.com/2012/10/22/heating-hot-water-with-solar-pv/

Article in CleanTechnica
https://cleantechnica.com/2013/09/30/pv-better-thermal-solar-water-heating/

.

I currently get hot water for about 3/4 of the year from 2 used flat-plate collectors, antifreeze, heat exchanger, and a 42 gallon galvanized tank. It works. I can see the appeal of never having to check the coolant SG. A PV array large enough for winter hot water will never boil over in summer. Losses to winter cold temperatures are also not a problem. Conduit for PV is much easier to route than insulated copper pipe. PV just sits there and works.

Looks like the old rule of thumb that PV is too expensive to use for heat may be may be out-of-date. Am I wrong?
4800W PV in 2 circuits of 96V & 50A each after the Midnight combiner. Two Midnight Classic 200. MagnaSine 4448 split phase inverter on a Midnight e-panel. 700Ah or 900Ah FLA @ 48V. Birdhouse,WBjr, etc. Replaces 16-year-old off-grid full-time 408W PV & 300W Morningstar inverter.

ClassicCrazy

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Re: PV hot water
« Reply #1 on: February 26, 2019, 12:09:04 PM »
If you also consider you don't need all the piping , pumps, and glycol the the simplicity of having just some wire running inside really makes it look a lot simpler.

Somewhere I saw a company - may have been from Sweden who had a product that retrofitted a standard hot water heater - you installed it right on the water heater and hooked up the PV - it had all the controls built in it . It may have come with a different heating coil - I don't remember now but looked like they had all engineering and controls issues thought out.

Larry

system one
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system two
Classic 150 ,5s 135 watt Kyocero , 16s Calb 100AH LiFePO4 , Outback VFX 3648 inverter
system three
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Westbranch

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Re: PV hot water
« Reply #2 on: February 26, 2019, 08:16:40 PM »
Larry did you mean this one?

http://techluck.com/    The Solar Hybrid Hot Water Solution
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ClassicCrazy

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Re: PV hot water
« Reply #3 on: February 26, 2019, 09:43:47 PM »
Larry did you mean this one?

http://techluck.com/    The Solar Hybrid Hot Water Solution

No there was a real slick Swedish one  - I will have to try and find the link for it.

Larry
system one
Classic 150 , 5s4p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system two
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system three
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ClassicCrazy

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Re: PV hot water
« Reply #4 on: February 26, 2019, 09:49:43 PM »
Found it !
I guess it is German made
Look at their other products too
https://my-pv.com/en/products/elwa

Larry
« Last Edit: February 26, 2019, 09:52:12 PM by ClassicCrazy »
system one
Classic 150 , 5s4p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system two
Classic 150 ,5s 135 watt Kyocero , 16s Calb 100AH LiFePO4 , Outback VFX 3648 inverter
system three
Midnite KID MPPT 24 DC in to  12 volt AGM batterie

DEinME

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Re: PV hot water
« Reply #5 on: February 27, 2019, 06:53:18 AM »
More info here on the "Economizer" or Willis immersion heater:

http://www.elemex.ie/external-immersion-heater/
4800W PV in 2 circuits of 96V & 50A each after the Midnight combiner. Two Midnight Classic 200. MagnaSine 4448 split phase inverter on a Midnight e-panel. 700Ah or 900Ah FLA @ 48V. Birdhouse,WBjr, etc. Replaces 16-year-old off-grid full-time 408W PV & 300W Morningstar inverter.

DEinME

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Re: PV hot water
« Reply #6 on: February 27, 2019, 07:04:32 AM »
Larry did you mean this one?

http://techluck.com/    The Solar Hybrid Hot Water Solution
Found it !
I guess it is German made
Look at their other products too
https://my-pv.com/en/products/elwa

Larry
All these options make me wonder about the optimal arrangement of 1) dedicated PV, 2) diverted PV, and 3) backup inverter power for an off-grid installation that really needs hotter water after a few cloudy days..

The elwa looks good with the ability to connect to the grid (or inverter) as needed. And the site says to promote stratification by using 2 of them.

The techluck says put the PV to the bottom element and grid to the top one. So normal daily heating will be whole-tank heating.

A heat-pump water heater would make hot water more efficiently than resistance as long as it was installed in a warm-enough place. I don't think you could add extra insulation on top as easily to have next-day hot water without adding more electricity.

In my experience, stratification really helps on partly-cloudy days and for next-day hot water.

I'm strongly considering adding another array for hot water. If it isn't efficient just connecting 100VDC to the heating elements I can always add a third Classic and use 48VDC heating elements.
4800W PV in 2 circuits of 96V & 50A each after the Midnight combiner. Two Midnight Classic 200. MagnaSine 4448 split phase inverter on a Midnight e-panel. 700Ah or 900Ah FLA @ 48V. Birdhouse,WBjr, etc. Replaces 16-year-old off-grid full-time 408W PV & 300W Morningstar inverter.

ClassicCrazy

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Re: PV hot water
« Reply #7 on: February 27, 2019, 10:11:34 AM »
There is no efficiency difference using heating elements other than their resistance and how much power the use ( how many watts /btu's  put in the water ) .
For some reason I wasn't using my thinking cap when I changed out a 240v heat coil with 110v one.  I burned the 110v one out cause water got too low once - but then I just put in the 240v one ( using 110v)  and got the same heating value as 110v one - it was half of 240v heating one - or close - you get what I mean.

Larry
system one
Classic 150 , 5s4p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system two
Classic 150 ,5s 135 watt Kyocero , 16s Calb 100AH LiFePO4 , Outback VFX 3648 inverter
system three
Midnite KID MPPT 24 DC in to  12 volt AGM batterie

DEinME

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Re: PV hot water
« Reply #8 on: February 27, 2019, 11:44:59 AM »
There is no efficiency difference using heating elements other than their resistance and how much power the use ( how many watts /btu's  put in the water ) .
For some reason I wasn't using my thinking cap when I changed out a 240v heat coil with 110v one.  I burned the 110v one out cause water got too low once - but then I just put in the 240v one ( using 110v)  and got the same heating value as 110v one - it was half of 240v heating one - or close - you get what I mean.

Larry
They should work the same as long as the resistance is the same. When they're both running on 120V, A 240V 10 ohm coil will put out the same heat as a 120V 10 ohm coil. I'd expect the 240V 10 ohm coil to be more robust.  (Different wire?).

A string of solar panels running without charge control or inverter will match some resistances better than others.

48V water heating elements often have very low resistances to bring the watts up to a useful value.

My current diversion is using a 240V air heating coil running on 120V. It was easily available.

edit . 48V water heating elements have low resistance to keep the current higher to keep the heat dissipated higher. I'm not saying this well although I'm sure you all know Ohm's law inside and out.
« Last Edit: February 27, 2019, 11:49:53 AM by DEinME »
4800W PV in 2 circuits of 96V & 50A each after the Midnight combiner. Two Midnight Classic 200. MagnaSine 4448 split phase inverter on a Midnight e-panel. 700Ah or 900Ah FLA @ 48V. Birdhouse,WBjr, etc. Replaces 16-year-old off-grid full-time 408W PV & 300W Morningstar inverter.

tecnodave

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Re: PV hot water
« Reply #9 on: March 31, 2019, 05:49:00 PM »
DEinME,

NOT!

10 ohms at 120 volts is not the same current as 10 ohms at 240 volts!  Total power will be 4 times as much at 240 volts.......simple math......at a given resistance twice the current will flow at twice the voltage and twice the current at twice the voltage is four times the power!   Basic ohm's law!

David
« Last Edit: March 31, 2019, 06:06:35 PM by tecnodave »
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#3 Kid/WBjr 4/6 Sanyo 200 watt multilayer 4/6 P
#4 Kid/WBjr 4/6 Sanyo 200 watt multilayer 2S 2/3 P

ClassicCrazy

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Re: PV hot water
« Reply #10 on: March 31, 2019, 06:10:57 PM »
system one
Classic 150 , 5s4p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system two
Classic 150 ,5s 135 watt Kyocero , 16s Calb 100AH LiFePO4 , Outback VFX 3648 inverter
system three
Midnite KID MPPT 24 DC in to  12 volt AGM batterie

DEinME

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Re: PV hot water
« Reply #11 on: April 01, 2019, 03:57:48 PM »
DEinME,

NOT!

10 ohms at 120 volts is not the same current as 10 ohms at 240 volts!  Total power will be 4 times as much at 240 volts.......simple math......at a given resistance twice the current will flow at twice the voltage and twice the current at twice the voltage is four times the power!   Basic ohm's law!

David
My post said "When they're both running on 120V . . . "

What I meant was that a 10 ohm coil will put out the same heat at 120V no matter if that 10-ohm coil is made for 120V or 240V. I just wrote a different version of that in the earlier post, but my post was correct.

So why not get a 240V coil?
« Last Edit: April 01, 2019, 04:04:05 PM by DEinME »
4800W PV in 2 circuits of 96V & 50A each after the Midnight combiner. Two Midnight Classic 200. MagnaSine 4448 split phase inverter on a Midnight e-panel. 700Ah or 900Ah FLA @ 48V. Birdhouse,WBjr, etc. Replaces 16-year-old off-grid full-time 408W PV & 300W Morningstar inverter.

mike90045

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Re: PV hot water
« Reply #12 on: April 01, 2019, 04:35:54 PM »
DEinME,

NOT!

10 ohms at 120 volts is not the same current as 10 ohms at 240 volts!  Total power will be 4 times as much at 240 volts.......simple math......at a given resistance twice the current will flow at twice the voltage and twice the current at twice the voltage is four times the power!   Basic ohm's law!

David
My post said "When they're both running on 120V . . . "

What I meant was that a 10 ohm coil will put out the same heat at 120V no matter if that 10-ohm coil is made for 120V or 240V. I just wrote a different version of that in the earlier post, but my post was correct.

So why not get a 240V coil?
NO

because you really need to look at wattage, not ohms.   

put a 10 ohm resistor on 120V , you get 12amps  (1,440 watts)
put a 10 ohm resistor on 240V  , you get 24amps  (5,760 watts)
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ClassicCrazy

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Re: PV hot water
« Reply #13 on: April 01, 2019, 09:08:32 PM »
DEinME,

NOT!

10 ohms at 120 volts is not the same current as 10 ohms at 240 volts!  Total power will be 4 times as much at 240 volts.......simple math......at a given resistance twice the current will flow at twice the voltage and twice the current at twice the voltage is four times the power!   Basic ohm's law!

David
My post said "When they're both running on 120V . . . "

What I meant was that a 10 ohm coil will put out the same heat at 120V no matter if that 10-ohm coil is made for 120V or 240V. I just wrote a different version of that in the earlier post, but my post was correct.

So why not get a 240V coil?

Yeah I reread your post - you do say when they are both running on 120v but that is easy to mix up with the rest of the statement.
I see your point was that the wire may different in a 120v manufactured  heating coil compared to a 240v heating coil.
system one
Classic 150 , 5s4p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system two
Classic 150 ,5s 135 watt Kyocero , 16s Calb 100AH LiFePO4 , Outback VFX 3648 inverter
system three
Midnite KID MPPT 24 DC in to  12 volt AGM batterie

DEinME

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Re: PV hot water
« Reply #14 on: April 02, 2019, 12:45:22 AM »
DEinME,

NOT!

10 ohms at 120 volts is not the same current as 10 ohms at 240 volts!  Total power will be 4 times as much at 240 volts.......simple math......at a given resistance twice the current will flow at twice the voltage and twice the current at twice the voltage is four times the power!   Basic ohm's law!

David
My post said "When they're both running on 120V . . . "

What I meant was that a 10 ohm coil will put out the same heat at 120V no matter if that 10-ohm coil is made for 120V or 240V. I just wrote a different version of that in the earlier post, but my post was correct.

So why not get a 240V coil?
NO

because you really need to look at wattage, not ohms.   

put a 10 ohm resistor on 120V , you get 12amps  (1,440 watts)
put a 10 ohm resistor on 240V  , you get 24amps  (5,760 watts)
I keep repeating that we are always using 120V here.

Please read it again.

I keep repeating that we are always using 120V here. 

What changes is the voltage that the coil is made for. Just stipulating that it's a 10-ohm coil for 120V or a 10-ohm coil for 240V BOTH RUNNING ON 120V.

I keep repeating that we are always using 120V here.

A 240V coil provides more options if I change the system configuration in the future.

I keep repeating that we are always using 120V here.
4800W PV in 2 circuits of 96V & 50A each after the Midnight combiner. Two Midnight Classic 200. MagnaSine 4448 split phase inverter on a Midnight e-panel. 700Ah or 900Ah FLA @ 48V. Birdhouse,WBjr, etc. Replaces 16-year-old off-grid full-time 408W PV & 300W Morningstar inverter.