Connecting solar panels to hot water heater

alexdavies · July 2018

So, I'm sure I'll have some folks shaking their heads at this one...

I don't know what I'm doing here, and I know just enough about electrical and solar to be dangerous.

But, I had this idea the other day...
I bought a bunch of solar panels a while back for cheap which I planned to hook up to our off grid cabin. They're old, morphus 42 Watt panels that were bought and never used for a project. Upon receiving them, I found out that they put out around 100 V.

I never ended up investing in a charge controller that can handle this higher voltage, and kept our super small, and simple 12V, 200W solar array for our LED lights etc...

I've been trying to figure out the best option for hot water at the cabin... Tossing around the idea of propane on demand, or maybe solar hot water panels etc... THen, I got to thinking that maybe I could try running these old panels of mine directly into an electric hot water heater, and when it's sunny, maybe I'll be able to slowly heat up my water enough for a warm shower at the end of the day.

So, I did a little experiment with a hot water heater element that I had kicking around. I hooked up to 5 of these 42W panels - direct wired, no breaker, no switch into the element. Just a note; This heating element is a conventional 3000W, 240V element... But... Sure enough, when I connected the panels to the element, it immediately got hot. I dipped the heating end into a 5 Gal pail of water for an hour, and sure enough, the water heated up significantly.

So, here I start thinking that I've come up with some brilliant idea, and I'm dreaming of a warm shower. I go and buy a hot water tank, install 8 panels on the roof, fill the tank up and plumb it all in. I decided to install a regular 120V 15amp on/off switch on the negative wire before the heater, then wire the panels direct into the red and black wires of the hot water heater (240V, 3000W conventional heater). I'm a little nervous at the warning signs about only connecting 240v ac etc... but I'm thinking that it's only a heating element and a thermostat which to me seems like a glorified switch. nothing electronic etc, so I think that 100V dc isn't going to hurt anything... especially in Low watt (only 336W out of these 8 panels)

I turn the switch on, and all is good. I then turn the switch OFF, and the switch burns out (crackles, turns green for a second).
Curse a bit... Then disconnect panels from the water heater, and try wiring panels into my test element that I tried the day before in the 5 gal pail with a new switch. Turn the switch ON - element heats up... Turn it OFF switch burns out same as before.
Switch of course also says 120V AC only... But 336W of these panels at 100V is only pushing 8 or so amp. So switch should handle that sort of power...

So, that's where I'm at now... I don't want to burn my cabin down, and need to seek some advice before moving forward. Not sure if I've burned out the brand new thermostat on the hot water heater...

Maybe I need some sort of a controller... Or a DC 24V water heating element (but then of course my panels are too high voltage).
I noticed there's a lad selling dc water heating elements with built in thermostats online... Maybe I plug this into the water heater tank and bypass the ac thermostat and heater...

HELp?!

Estragon · July 2018

Switching DC at 100v is not the same as switching AC at 120v. AC averages out to 120v, but crosses zero volts as it cycles, making it harder to sustain an arc. DC has level voltage, so arcs much more easily. Any switches/relays, including the thermostat should be made for DC, and rated for the kind of voltage you're using.

The element itself doesn't care about AC or DC.

NANOcontrol · July 2018

It is very difficult to switch DC over about 35V as you get those results. Your contacts were probably brass which is about the worst to use. That said, I heat water with PV and love it. I do it with about 60V because I use grid tie panels in series. I also operate the panels at power point for maximum efficiency. I do this switching electronically, so it really isn't a disconnect. There are switches and relays that can handle this DC voltage. If I advised you I would get a lot of crap, but any switching should be fast and have a wide space. There are videos doing this on youtube. Take that for what it is worth.

alexdavies · July 2018

Right...
So, If I just hardwire direct to my heater element, bypass thermostat and don't install a switch, then I'm maybe in the clear...? I can just experiment with adding or dropping panels on a really hot sunny day so that the water doesn't get so hot that it blows the pressure relief valve...

In this configuration, there's no possibility of short circuit?

Thanks

MrM1 · July 2018

Direct from solar panel to AC waterheater conversion ideas

http://waterheatertimer.org/Convert-AC-water-heater-to-DC-water-heater.html

http://waterheatertimer.org/convert-AC-water-heater-to-DC-using-low-voltage.html

This may or may not help you

MrM1 · July 2018

DC water heater thermostat

http://mwands.com/store/water-heater-thermostat

MrM1 · July 2018

I do water heating with solar panels but opted to use my Inverter as an AC source and down sized my element to 1000W 240v as to not overload the inverter or battery.

jonr · July 2018

You can see here for a pre-built product that I think can hold the panel approximately at Vmp (using fast PWM and some filtering).

https://www.ebay.com/itm/Solar-Water-Heater-EZ-DIY-Save-No-Pipe-Changes-Hot-Water-System-PV-MPPT

alexdavies · July 2018

jonr said:

You can see here for a pre-built product that I think can hold the panel approximately at Vmp (using fast PWM and some filtering).

https://www.ebay.com/itm/Solar-Water-Heater-EZ-DIY-Save-No-Pipe-Changes-Hot-Water-System-PV-MPPT

Hi Jonr, This page wasn't available for me to load... Maybe it's because I'm in Canada?

westbranch · July 2018

It IS still there, just moved

https://www.ebay.ca/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=Solar-Water-Heater-EZ-DIY-Save-No-Pipe-Changes-Hot-Water-System-PV-MPPT&_sacat=0

mike95490 · July 2018

> I turn the switch on, and all is good. I then turn the switch OFF, and the switch burns out (crackles, turns green for a second).

You can use a DC rated breaker from Midnight Solar. They are pretty cheap $20 150VDC switch usage rated..
http://www.midnitesolar.com/productPhoto.php?product_ID=197&productCatName=Breakers&productCat_ID=16&sortOrder=13&act=p

Estragon · July 2018

> @alexdavies said:
> Right...
> In this configuration, there's no possibility of short circuit?
>
> Thanks

Any circuit can short... a breaker like the one @mike95490 suggested would be a good idea.

oil pan 4 · July 2018

A hot water heater element is a short circuit.
You may also try a dpsp switch and wire the contacts in serries.
It might break the dc arc.

mcgivor · July 2018

Use a DC rated switch or breaker, there is a reason they're made, to properly extinguish the arc and prevent welding of the contacts or worse causing a fire. An alternative would be to use a DC relay see attached pdf, and use the thermostat and switch to control the relay coil.

MarkP · August 2018

I'm working on this now. I scored a huge 119 gallon 4500 watt electric water heater, basically new, for $250. I regularly burn gas in a generator during the winter when my panels don't give me enough juice so I am going to get more panels. I will burn less gas on cloudy days and have a lot more excess power on sunny days so I will burn less gas in the generator and still some propane in the water heater on cloudy days. Less propane but still some. Trying to find the sweet spot. It is still not competitive to buy panels and hook them up only to the water heater and have them sit idle the rest of the time compared to a thermal solar system but as described above that's not what I plan to do. Convenience is worth some money as is overall simplicity. Maintenance on the generator is more and costlier than maintenance on the hot water heater. When I burn fuel in the water heater a warm breeze wafts up the chimney. When I run the generator a hot blast gets fired off into space. Trading the generator use for the water heater use has got to be a win.

Anyway I have been running the electric resistance water heater on 120vac off of the inverter. At that voltage it draws around 1,000 watts and uses 1/4 to 1/3 of the capacity of the inverter. That power goes through the charge controller with its losses, the inverter with its losses, and sometimes the round trip to the battery with its losses, on top of hogging the capacity of the inverter. There are times when my 60 amp charge controller is maxed out so adding more panels would mean I have to buy another charge controller.

I use the aux feature on my Conext MPPT 150/60 to control a 2-pole contacter rated for regular A/C power. I had been controlling it off of battery voltage but setting the trigger at float voltage means trying to run the water heater during absorption since the Conext MPPT is not too smart and doesn't take into account what stage the charger is in. If only there was a way to trigger based on whether the charge controller was wide open or throttling the power from the array because that's all we really care about. Well with my 3-panel strings when the CC starts throttling back the power from the array the array voltage climbs from the 80s into the 90s and 100s so I am now triggering the aux on at 95 and off at 90. That works pretty good although the water heater load does sometimes suck from the batteries depending on light levels and how long a delay before disconnecting I have put in.

I plan to feed the 12.75 ohm resistance elements in the water heater with nominal 85vdc to 105vdc directly from the array without going through CC or inverter or batteries (the batteries sort of shallowly "breath" whenever the load pulles the voltage below float then recharges as the load is cut off). I purchased some SSRs rated to switch 400vdc on eBay. Each element will draw around 800 watts at 8 amps at 100vdc. I would like to figure out how to turn on both elements at once rather than how the water heater is presently wired where only one comes on at a time. I want to control them by load so both come on when needed but only one when that's all the system can supply. Still working on that.

Finally that still leaves the thermostats on the water heater. Realistically, will they burn up or fuse on? I run a 48 to 60 vdc water pump on a SquareD pressure switch meant for 220vac and that thing has been fat dumb and happy for years. I did wire it so that power runs through both contacts which must help kill the arc.

Has anybody actually experienced failure of water heater thermostats due to running on DC? I once had someone tell me that you couldn't collect water from the roof of an A-frame house because the energy cost to pump water the extra few feet up from the lower level gutters would be too much. I mean technically the difference would probably be measurable if you really looked but in real life nobody would even notice. On the internet factoids take on a life of their own that bears no resemblance to reality.

Has anybody ever actually seen it happen? Was it after long use or right away? One benefit of piping juice straight from the array is that if the thermostat stuck on it could only rob power from the array, not run the batteries down overnight, burning your house down in the process.

mcgivor · August 2018

MarkP said:

I'm working on this now. I scored a huge 119 gallon 4500 watt electric water heater, basically new, for $250. I regularly burn gas in a generator during the winter when my panels don't give me enough juice so I am going to get more panels. I will burn less gas on cloudy days and have a lot more excess power on sunny days so I will burn less gas in the generator and still some propane in the water heater on cloudy days. Less propane but still some. Trying to find the sweet spot. It is still not competitive to buy panels and hook them up only to the water heater and have them sit idle the rest of the time compared to a thermal solar system but as described above that's not what I plan to do. Convenience is worth some money as is overall simplicity. Maintenance on the generator is more and costlier than maintenance on the hot water heater. When I burn fuel in the water heater a warm breeze wafts up the chimney. When I run the generator a hot blast gets fired off into space. Trading the generator use for the water heater use has got to be a win.

Anyway I have been running the electric resistance water heater on 120vac off of the inverter. At that voltage it draws around 1,000 watts and uses 1/4 to 1/3 of the capacity of the inverter. That power goes through the charge controller with its losses, the inverter with its losses, and sometimes the round trip to the battery with its losses, on top of hogging the capacity of the inverter. There are times when my 60 amp charge controller is maxed out so adding more panels would mean I have to buy another charge controller.

I use the aux feature on my Conext MPPT 150/60 to control a 2-pole contacter rated for regular A/C power. I had been controlling it off of battery voltage but setting the trigger at float voltage means trying to run the water heater during absorption since the Conext MPPT is not too smart and doesn't take into account what stage the charger is in. If only there was a way to trigger based on whether the charge controller was wide open or throttling the power from the array because that's all we really care about. Well with my 3-panel strings when the CC starts throttling back the power from the array the array voltage climbs from the 80s into the 90s and 100s so I am now triggering the aux on at 95 and off at 90. That works pretty good although the water heater load does sometimes suck from the batteries depending on light levels and how long a delay before disconnecting I have put in.

I plan to feed the 12.75 ohm resistance elements in the water heater with nominal 85vdc to 105vdc directly from the array without going through CC or inverter or batteries (the batteries sort of shallowly "breath" whenever the load pulles the voltage below float then recharges as the load is cut off). I purchased some SSRs rated to switch 400vdc on eBay. Each element will draw around 800 watts at 8 amps at 100vdc. I would like to figure out how to turn on both elements at once rather than how the water heater is presently wired where only one comes on at a time. I want to control them by load so both come on when needed but only one when that's all the system can supply. Still working on that.

Finally that still leaves the thermostats on the water heater. Realistically, will they burn up or fuse on? I run a 48 to 60 vdc water pump on a SquareD pressure switch meant for 220vac and that thing has been fat dumb and happy for years. I did wire it so that power runs through both contacts which must help kill the arc.

Has anybody actually experienced failure of water heater thermostats due to running on DC? I once had someone tell me that you couldn't collect water from the roof of an A-frame house because the energy cost to pump water the extra few feet up from the lower level gutters would be too much. I mean technically the difference would probably be measurable if you really looked but in real life nobody would even notice. On the internet factoids take on a life of their own that bears no resemblance to reality.

Has anybody ever actually seen it happen? Was it after long use or right away? One benefit of piping juice straight from the array is that if the thermostat stuck on it could only rob power from the array, not run the batteries down overnight, burning your house down in the process.

That's quite a mouthful do digest, one way to add panels without wasting potential energy when the batteries are fully charged would be to use a diversion controller such as Morningstar, for the Schneider 60-150, adjustable current sensors could be used to divert energy whenever loads are light, anything is possible but much thought has to go into design to ensure ballance.

As far as DC is concerned with AC rated contacts, this is not good practice, at very least use the AC thermostat to drive a coil of a DC rated relay/contactor, this way the current is kept to a minimum. SSR's are nice, but when dealing with high current they generate heat, or losses, which need to be compensated for with heat sinks or fans.

Suggestion, start a new discussion for a single query, this will focus on one issue rether then going off in all directions, which leads to confusion. Just a friendly suggestion, hope you appreciate.

NANOcontrol · August 2018

I don't see why it isn't commonplace. This system here is trick. Above is a MakeSkyBlue charge controller. Personally, I think there is some hokum to how this CC works. The bottom grey box is connected to the same panels and a water heater. This maintains the power point voltage and dumps whatever the charge controller isn't using to heat. I've seen it dump as little as 6W into the tank. Every little bit counts. And it does use the tanks temperature controller contacts on 60V DC without an arc thru special control circuitry. Free hot water is nice. Those really bad days are far and few. Panels are enough to keep everything else running, so you don't get hot water. It is a great camp setup where a little compromise is expected.

Image: https://us.v-cdn.net/6024911/uploads/editor/xx/06b2dofy4c28.jpg

MarkP · August 2018

I can see rigging up a magic box that chops the DC or inverts it in some simple fashion such that when the thermostat breaks the circuit it stays broke instead of arcing. Chopping the DC with PWM would also allow very close control of the power output. I am purposely programming in delays so as to avoid short-cycling my relays but PWM is inherently based on thousands of cycles per second. Would chopping the voltage for 1/1,000th of a second be enough to quench an arc at 100volts I wonder? I have been using an electro-mechanical relay but have ordered a couple of SSRs. With them I can reduce the delays however I want although I am not sure what benefits there will be in doing so.

BB. · August 2018

Low frequency "chopping" may help quench the arc (50-60 Hz).

High frequency chopping may make the arc worse (over 60 Hz? 200 Hz seems to be a thing for HiFreq)... Not exactly sure here, but there are "high frequency" welder units that are used to make for easier arc starting. Not sure if the "high freq" is in combination with an inductor of some sort that actually increases output voltage too or not.

http://arcpig.com/faq.html (some Q&A for High Freq welding).

-Bill

oil pan 4 · August 2018

How about a Hot water heat pump?
Use half the power for the same amount of hot water.

NANOcontrol · August 2018

If you reverse voltage as in a MSW inverter, that dead time can be almost zero. Otherwise a dead time of about 1.2ms is needed. On time also should be kept short <20ms. Dead time does limit maximum power with higher resistance elements. This system is about 160Hz if I remember and no dead time is needed. I have a heat pump at home, but they strain to get over 120F. Added solar gets it hotter to last thru the night and can lower daytime running. Heat pumps run almost constantly but certainly at lower power. I just checked my system and I've dumped 800W into it by noon. That may not seem a lot and it is because my fridge does not run at night. Solar use is very heavy in the morning. I'm dumping 400W into it now, the maximum because it only has a 1500W 120V element at around 60V.

jonr · August 2018

With a DC-SSR/mosfet, you can use a high frequency and/or avoid arcing/contact wear. Add some capacitance and you can limit current flow - to hold a panel at Vmp.

The existing thermostat can be used with a low (like 3.3V) signaling voltage.

Agreed, panels operating above Vmp is a good indication that the charge controller isn't using all of the available energy (and some could be used for water heating).

mike95490 · August 2018

oil pan 4 said:

How about a Hot water heat pump?
Use half the power for the same amount of hot water.

Those units are starting to exhibit field failures and they are non-repairable.

NANOcontrol · August 2018

I would highly caution against using a DC-SSR/mosfet. These tend to be very slow speed devices. Trying to do so will put them in a linear region and will overheat them till they destroy themselves. If they say they need to have a heatsink, that is a bad sign. You can buy a couple high current FET and put them in parallel and have no heating. Current ratings are illusive. Using a 110A FET at 20A is asking for trouble.

jonr · August 2018

True, many SSRs have slow opto-isolators on the gate - check the specs for your timing requirements (or use a bare mosfet and sufficient gate drive).

MarkP · August 2018

I'm out of my depth as far as SSRs go but I had overlooked that the thermostats are separate from the elements. The elements are dumb resistors and AFAIK it is meaningless to designate them DC vs AC. The thermostats are in line before them and can be cut out of the power loop. Enough light gauge wire and I can virtually relocate them into the 12vdc xx milliamp control loop, controlling the SSRs at whatever combination of on/off voltage and delay gives the sweet spot in terms of performance and longevity. BTW do the SSRs generate continuously or per switching event? Do they get hotter at higher frequency?

MarkP · August 2018

Do the SSRs generate heat continuously or per switching event?

BB. · August 2018

Probably sort of both.

When the SSR is "on", you have p=i^2*r...

R is at its lowest value when on. Note that heating goes up with the square of the current, but only 1x change in resistance.

2x current = 4x power (heating). 2x resistance only 2x heating.

And as the devices switch from off to on, the parts are in the linear mode. P=VxI. So as the device switches, you have VxI loses.

If this happens 1x second, usually not a concern. If this happens 1,000x per second, then it can be s big concern (more time in Lynett linear mode, more VxI heating. Big power FETs tend to be slower, so more linear heating.

Also big power FETs tend to have more capacitance, high frequency means more energy charging and discharging the gate capacitance. There will be some maximum practical switching frequency (slower devices can only switch every xxx mseconds). Should be in the source specifications somewhere.

I am not a power engineer, but this is roughly how I understand the issues.

Bill

MarkP · August 2018

Sounds like I should have them switching on and off no more than once per minute or so. In fact I could program in on/off delays of several minutes and in the grand scheme of things the overall energy harvested would probably be about the same. I wonder how the MPPT charge controller will react to the array voltage changing abruptly every minute or two? How often does the MPPT update?

westbranch · August 2018

How often does the MPPT update?

That depends on the make of CC.... the better ones have more options.

BB. · August 2018

Older (US) MPPT controller designs may "scan for Vmp/Imp" every 5 minutes. The newer ones can do this "continuously". Some controllers have several MPPT modes to select from if you have different needs.

-Bill

Connecting solar panels to hot water heater

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