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General Category => General Discussion => Topic started by: DEinME on February 26, 2019, 09:24:50 AM

Title: PV hot water
Post by: DEinME 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Looks like the old rule of thumb that PV is too expensive to use for heat may be may be out-of-date.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?
Title: Re: PV hot water
Post by: ClassicCrazy 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

Title: Re: PV hot water
Post by: Westbranch on February 26, 2019, 08:16:40 PM
Larry did you mean this one?

http://techluck.com/    The Solar Hybrid Hot Water Solution
Title: Re: PV hot water
Post by: ClassicCrazy on February 26, 2019, 09:43:47 PM
Quote from: Westbranch on February 26, 2019, 08:16:40 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
Title: Re: PV hot water
Post by: ClassicCrazy 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
Title: Re: PV hot water
Post by: DEinME 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/
Title: Re: PV hot water
Post by: DEinME on February 27, 2019, 07:04:32 AM
Quote from: Westbranch on February 26, 2019, 08:16:40 PM
Larry did you mean this one?

http://techluck.com/    The Solar Hybrid Hot Water Solution
Quote from: ClassicCrazy 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
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.
Title: Re: PV hot water
Post by: ClassicCrazy 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
Title: Re: PV hot water
Post by: DEinME on February 27, 2019, 11:44:59 AM
Quote from: ClassicCrazy 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
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.
Title: Re: PV hot water
Post by: tecnodave 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
Title: Re: PV hot water
Post by: ClassicCrazy on March 31, 2019, 06:10:57 PM
Here is a calculator to make it easy
https://www.rapidtables.com/calc/electric/watt-volt-amp-calculator.html

Larry
Title: Re: PV hot water
Post by: DEinME on April 01, 2019, 03:57:48 PM
Quote from: tecnodave 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
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?
Title: Re: PV hot water
Post by: mike90045 on April 01, 2019, 04:35:54 PM
Quote from: DEinME on April 01, 2019, 03:57:48 PM
Quote from: tecnodave 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
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)
Title: Re: PV hot water
Post by: ClassicCrazy on April 01, 2019, 09:08:32 PM
Quote from: DEinME on April 01, 2019, 03:57:48 PM
Quote from: tecnodave 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
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.
Title: Re: PV hot water
Post by: DEinME on April 02, 2019, 12:45:22 AM
Quote from: mike90045 on April 01, 2019, 04:35:54 PM
Quote from: DEinME on April 01, 2019, 03:57:48 PM
Quote from: tecnodave 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
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.
Title: Re: PV hot water
Post by: DEinME on April 02, 2019, 12:50:20 AM
Quote from: ClassicCrazy on April 01, 2019, 09:08:32 PM
Quote from: DEinME on April 01, 2019, 03:57:48 PM
Quote from: tecnodave 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
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.
People had no trouble reading it for a month.

I suspect that people who read the whole discussion would follow what the discussion was about and not just take part of a sentence out of context and pile on with criticism.
Title: Re: PV hot water
Post by: ClassicCrazy on April 02, 2019, 01:33:12 AM
Oh well - it was an easy mix up - the eyes play tricks on meaning sometimes especially when reading a post fast or if someone is reading on a tiny screened phone.  I had to read it twice after it was pointed out to get it all straight in context.

Larry
Title: Re: PV hot water
Post by: DEinME on April 02, 2019, 02:45:23 AM
Quote from: ClassicCrazy on April 02, 2019, 01:33:12 AM
Oh well - it was an easy mix up - the eyes play tricks on meaning sometimes especially when reading a post fast or if someone is reading on a tiny screened phone.  I had to read it twice after it was pointed out to get it all straight in context.

Larry
At this point people won't need to read any of my posts at all to decide that I'm wrong.

The most recent correction quoted my 4-sentence post that included this sentence: "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 don't think they read it at all.
Title: Re: PV hot water
Post by: RossW on April 02, 2019, 05:24:52 PM
Quote from: DEinME on April 02, 2019, 12:45:22 AM
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.

Might be easier to refer to the coils by their nominal wattage.
A "240V/5.7kW element" will have a resistance of 10 ohms, but if run on 120V will only draw 1.2kW
A "120V/1.2kW element" will ALSO have a resistance of 10 ohms, and will also draw 1.2kW on 120V.

The difference is that a 120V/1.2kW element will be made of much lighter wire, and probably a much lighter general construction than a 240V/5.7kW element. One would expect, therefore, the 5.7kW element would have a far longer service life because you're running it way below it's rated capacity.

A bit like the old "RS" (Rough Service) light globes. I believe they were designed for a higher voltage so they were being run under full load and therefore lasted much longer.
Title: Re: PV hot water
Post by: tecnodave on April 02, 2019, 08:10:12 PM
RossW,

Rough service bulbs are rated at 130 volts, traffic light bulbs too, last longer because they are not run at full brightness.......downside....they are less efficient......less lumens per watt.

david
Title: Re: PV hot water
Post by: ClassicCrazy on April 02, 2019, 08:32:38 PM
Interesting fact for you warm weather climate dwellers . The old inefficient light bulbs in stop and go signals used to melt the snow off from the heat generated.  One city near here replaced with new efficient LED fixtures and we had a very windy sticky snow and it covered all the stop and go lights . The city had to sweep them off because it wasn't melting. Not sure if there have been any design changes since then to help alleviate that winter problem.

Larry 
Title: Re: PV hot water
Post by: tecnodave on April 02, 2019, 08:36:22 PM
Larry,

I don't miss the snow, ...really.  So what is less expensive, LED lights or snow sweepers....lol

David
Title: Re: PV hot water
Post by: bee88man on April 19, 2019, 08:42:17 AM
As for the Titled subject matter... for PV hot water I set up a modified modulating tankless electric water heater and small circulating pump to store up to 200 gallon of hot water stricly from diverted power destined to go to the grid...beautifull sight when meter stands still from early till late day and hot water is all sourced from PV.
Concidering adding a 'heat dump' to avoid the rare event of maxed out storage and dumping kW to the grid...I'm not sharing...