Water heating with solar

[martinwvogt]

Hello, I currently have excess solar capacity (3.2kw) and I am wondering if anyone has experience using a dedicated water heater powered directly by dedicated solar panels. My option would be to use a 24 or 48v heating element and connect directly to a single inverter. Maybe a bad idea, but I wanted to check ,.. I have the ability to use a propane on demand heater as a backup. Any thoughts are appreciated ... Thx

 

[MaikaiLife]

I've certainly not seen everything, but typically if you want to heat water with panels you use a Solar Panel which is different than a PV Panel which converts sunlight to electricity.

It's unlikely that a device that can consume your excess solar could run exclusively off the output from your PV panels. Electric water heaters have large inrush and running current needs that typically a PV panel and even a configuration of parallel PV panels cannot support without an inverter, even if you found a water heater like this, it would most likely have an integrated inverter.

What is it that you're trying to achieve, where does the extra capacity currently go, why do you want to heat water with only excess?

 

[martinwvogt]

Thanks, I have an extra array that's not needed (long story). I'm trying to make some use of that array to lower my dependance on propane. I have checked into water heaters which allow low-voltage heating elements ... so at least in the summer it should be doable.

 

[MaikaiLife]

How many panels are in your array and what are they currently hooked up to?

 

[Bluedog225]

Search here. There are many threads on this with good info.

I’ve seen them in passing but not dug in.

 

[martinwvogt]

thanks, I have close to 12kw ... more than enough,

 

[Jbird2000]

Some inverters have a feature ( or an add-on) allowing you to dump excess AC into a tank of water

 

[martinwvogt]

yes, that is correct. That’s definitely another option and would leave existing batteries and inverters out. In my case I’m assuming the charge controller would take the excess and feed a water heater when the battery is full

 

[OzSolar]

yes, that is correct. That’s definitely another option and would leave existing batteries and inverters out. In my case I’m assuming the charge controller would take the excess and feed a water heater when the battery is full

I'd tie it into my current system using using a standard AC water heater that is ran by your current inverter(s). Install it upstream so it acts a preheater to your current propane water heater. Not perfect but quite simple and reliable.

What is the AC voltage of your current system? 120 or 240?

There's many choices of water heating element wattages. I can suggest one based on your answer above for one that doesn't tax your inverters but still gives you a place to send extra energy.

Dedicating PV modules water heating takes them away from ever being able to charge your batteries. Even if you don't need them it still takes away from the flexibility of having redundancy.

The FlexMax charge controllers have a programmable "diversion load relay that you could use to to control a water heater via a standard relay.

Diversion (AUX Mode)
When external DC sources (wind, hydroelectric) are directly connected to a battery bank, any excess power should be sent to a diversion load, such as a heating element, using a mechanical relay or solid-state relay

https://www.outbackpower.com/downloads/documents/charge_controllers/flexmax_6080/owner_manual.pdf

 

[Daddy Tanuki]

I agree with OZ Solar, unless you are heavily retarded like I am you should follow his advice...

I on the other hand being mentally challenged, am building a dump load water heater with a 1600 liter tank that will heat in the spring-fall with excess solar.

it will also provide heat in the winter via a waste oil burner boiler that will heat the water in the tank if the tanks temperature gets to low.

solar water heater/waste oil boiler

Been talking about this for over a year now, its time to get started, So originally this idea was formulated as a way to use up excess production as my panels and batteries are all specc'd for winter time with low solar output, and then it morphed into why not heat the house, or at least the...

diysolarforum.com

 

[efficientPV]

It is no different than charging a battery. You can connect up a solar array directly to a battery if you are careful. Don't pay attention and disaster will soon follow. With 3200W of dedicated panels there is no need to be concerned about efficiency. 1KW is more than enough for domestic use when converter efficiently. Don't heat more than you need. Everybody wants to use 24 and 48V heating elements. That is their comfort level. Heating with low voltage is insane, the high currents and wire losses make a bad design. Higher voltage is your friend and it allows you to use standard heating elements. Element resistance has to be matched to the array and there are videos of how to calculate that. They are technically correct but in practice are very wrong. Use a resistance of twice that ideal for best overall performance. With just small drops in current, wattage drops dramatically and full current almost never happens. Power is a function of the square of the current. It is best to design for more typical currents like half the array rating.

The hard part of this is turning it on and off with temperature. Heating elements with a built in thermostats that can run on DC is just a lie. These really don't work for long and when connected to a solar array the open circuit voltage is far too high for them and the contacts burn. In electronics, switching currents and efficient matching of panel to the heating element is stupid simple. But that is far beyond most everyone in solar. I have no experience in low tech switching methods and will leave that to others. With that much power you shouldn't have much of a problem whatever you do.

 

[Daddy Tanuki]

It is no different than charging a battery. You can connect up a solar array directly to a battery if you are careful. Don't pay attention and disaster will soon follow. With 3200W of dedicated panels there is no need to be concerned about efficiency. 1KW is more than enough for domestic use when converter efficiently. Don't heat more than you need. Everybody wants to use 24 and 48V heating elements. That is their comfort level. Heating with low voltage is insane, the high currents and wire losses make a bad design. Higher voltage is your friend and it allows you to use standard heating elements. Element resistance has to be matched to the array and there are videos of how to calculate that. They are technically correct but in practice are very wrong. Use a resistance of twice that ideal for best overall performance. With just small drops in current, wattage drops dramatically and full current almost never happens. Power is a function of the square of the current. It is best to design for more typical currents like half the array rating.

The hard part of this is turning it on and off with temperature. Heating elements with a built in thermostats that can run on DC is just a lie. These really don't work for long and when connected to a solar array the open circuit voltage is far too high for them and the contacts burn. In electronics, switching currents and efficient matching of panel to the heating element is stupid simple. But that is far beyond most everyone in solar. I have no experience in low tech switching methods and will leave that to others. With that much power you shouldn't have much of a problem whatever you do.

much easier to buy a sigineer all in one and have it power the water heater. put it on a timer so that the inverter is only on from xx to xx time. or better yet use a voltage sensing relay to turn the inverter off and on. batteries full at 54.4? turn on the inverter. below xx.xx turn off.

no need for extra batteries, solar controllers and the wiring is for the most part 200-240 single phase so not overly expensive.

this is my plan anyway. here is a link to the EU standard inverter which is under 1k, but puts out 230 VAC single phase, so not useable for much of anything else in the US... but since it is 230 volts and does not sell well in the US its price is right. at 5000 watts it is just the right size to drive an electric water heater.

5000W Off Grid European Solar Inverter 100A MPPT Solar Charger DC 48V to 220V 230V 240V Support Batteryless Operation – Sigineer Power

sigineer.com

 

[SeaGal]

I am wondering if anyone has experience using a dedicated water heater powered directly by dedicated solar panels.

Check out @Steve T - IoW UK's Loadmaster Open source Arduino PV Hot Water Project.

See his posting here..

Direct PV water heater MPPT controller

Hi All This is my first post here, after I've spent countless hours of reading. I am the happy owner of 2 grid-tied systems, one 6.6 kWp the other 4.4 kWp. But now I've got hand on some second hand panels I would like to use for water heating as it's not possible to add them to the existing...

diysolarforum.com

and project here.

LoadMaster XP - A Smart PV MPPT Solar Hot Water Controller

A solid-state, low-cost, reliable solution for solar hot water from PV. Zero maintenance & CO2, connected, smart combi pre-heat control.

projecthub.arduino.cc

 

[martinwvogt]

much easier to buy a sigineer all in one and have it power the water heater. put it on a timer so that the inverter is only on from xx to xx time. or better yet use a voltage sensing relay to turn the inverter off and on. batteries full at 54.4? turn on the inverter. below xx.xx turn off.

no need for extra batteries, solar controllers and the wiring is for the most part 200-240 single phase so not overly expensive.

this is my plan anyway. here is a link to the EU standard inverter which is under 1k, but puts out 230 VAC single phase, so not useable for much of anything else in the US... but since it is 230 volts and does not sell well in the US its price is right. at 5000 watts it is just the right size to drive an electric water heater.

5000W Off Grid European Solar Inverter 100A MPPT Solar Charger DC 48V to 220V 230V 240V Support Batteryless Operation – Sigineer Power

sigineer.com

The unit is very reasonable. I'm about 600ft away from the panels. I'm assuming I'm better off running wires to the inverter over that distance at higher voltage (series) and put the inverter close to the water heater.

 

[rhino]

For some reason there is a lot more direct from PV water heater controllers in countries outside the USA such as Australia. Not sure why there still isn't a simple plug and play system for PV water heaters in USA. I gave up and currently hooked it up as a regular load to EG4 18kPV which provides me with enough power even when water heater kicks on (4500W/240V AC). This was simplest solution for someone to understand since everything is exactly the same as water heater that is grid powered.

 

[martinwvogt]

Looking at the specs it's outputting 20 amps - is this correct? a 240v water heater would need 30 amps, I think.

 

[Daddy Tanuki]

Looking at the specs it's outputting 20 amps - is this correct? a 240v water heater would need 30 amps, I think.

quick google search= "Most electric water heaters use 18.8 amps with 240 volts and a 4,500 watt heating element. A gas water heater uses fewer amps—about 12 Amps with 115 volts. The average residential container heater has a 40-gallon tank and usually requires a 30-amp breaker."

the 30 amp breaker is for the wiring to prevent the anticipated wiring from catching on fire in the walls.

If really worried replace the elements with a lower wattage element and continue on. they are available from a low of 1000 watts on up to 12kW in steps, and that was with a quick 5 second browse of ebay... did not even look at amazon, alibaba or any of the myriad of other shopping sites.

make it about solving the issue...

 

[OzSolar]

Looking at the specs it's outputting 20 amps - is this correct? a 240v water heater would need 30 amps, I think.

Per NEC it needs a #10 wire and a 30 Amp breaker

4500 watts / 240 v = 18.75 amps
5500 / 240 v = 23 amps

If you run them at 120 v you'll get 1/4 the amps.

It will take 4X longer to heat water in a 120v water heater than at 240v.

When you start playing around with theses sorts of things it really helps to understand power (kW) vs energy (kWh).

If really worried replace the elements with a lower wattage element and continue on. they are available from a low of 1000 watts on up to 12kW in steps, and that was with a quick 5 second browse of ebay... did not even look at amazon, alibaba or any of the myriad of other shopping sites.

If it was me I'd use one standard water heater but replace the stock two elements with two different ones. EG: 2,000 and 3,000 watt (not saying those are the right ones, just an example)

I'd rewire the wire water such that each element was on its own circuit. I'd then be able to choose either one or both giving me 2,000 or 3,000 or 5,000 watts when running both.

I'm grid tied and I take a pretty hefty credit into winter. I'm doing something similar, sorta

Last fall I installed a new 50G water with two 5,500 elements for my radiant in-floor heating system. I rewired it and brought in a 2nd 30 amp circuit allowing me 11kW of heating (37,500 BTU's). It works wonderfully, it nothing to use 150 kWh in one day but I was going to lose it at the 12 month true up so why not?

Grainger has large selection.

https://www.grainger.com/category/plumbing/water-heaters/replacement-parts-accessories-for-water-heaters/heating-elements-for-electric-water-heaters

 

[efficientPV]

much easier to buy a sigineer all in one and have it power the water heater. put it on a timer so that the inverter is only on from xx to xx time. or better yet use a voltage sensing relay to turn the inverter off and on. batteries full at 54.4? turn on the inverter. below xx.xx turn off.

no need for extra batteries, solar controllers and the wiring is for the most part 200-240 single phase so not overly expensive.

this is my plan anyway. here is a link to the EU standard inverter which is under 1k, but puts out 230 VAC single phase, so not useable for much of anything else in the US... but since it is 230 volts and does not sell well in the US its price is right. at 5000 watts it is just the right size to drive an electric water heater.

5000W Off Grid European Solar Inverter 100A MPPT Solar Charger DC 48V to 220V 230V 240V Support Batteryless Operation – Sigineer Power

sigineer.com

I wish you would defend that. With 3,200W of PV and a 2,000W element it works. What happens when the array can only produce 1500W? Does it just shut off and produce nothing? That solution is even worse than direct connect.

Heating water is a true test of ones understanding of PV solar and electricity. Nothing in solar tests your knowledge more. There aren't products for heating water because they don't know enough to know they need them. There isn't even buy a good solid state DC brick module for direct connect panels.

 

[Daddy Tanuki]

I wish you would defend that. With 3,200W of PV and a 2,000W element it works. What happens when the array can only produce 1500W? Does it just shut off and produce nothing? That solution is even worse than direct connect.

Heating water is a true test of ones understanding of PV solar and electricity. Nothing in solar tests your knowledge more. There aren't products for heating water because they don't know enough to know they need them. There isn't even buy a good solid state DC brick module for direct connect panels.

in my case it will work. that ends the defense. I have 70kw of battery and with a simple control routine I can turn on the inverter only when the batteries are at sufficient charge, and only at a desired time frame. so desired time frame is when the sun is up and documented evidence has shown that my batteries are normally entering float. a simple voltage controlled relay that is adjustable.... battery bank is at 3.4xx or higher (per cell) so we are in float, and the panels can sustain the voltage and output.

Oh some clouds come over... no big deal, the batteries are still at 3.3xx (per cell) or higher and for the 10 , 15, 20 minutes the battereis can maintain the draw without dropping below the desired set point.

see how that works?

It is not my lack of understanding, as you intimate, it is your lack of understanding that not everybody arrives at the same conclusion via the same methods. mine works well as a dump load without guessing about impedance, without worry if my panels are making enough, without worry that I have the correct cabling from the panels to the dump load.... as its all AC power it works. the system turns on, and off as many times as needed to use the excess power created by the panels when they are at or above the setpoints that I have determined work on my system.

Have a nice day kind sir.

 

[martinwvogt]

Thank you, awesome info. I'm going to take another look with all your input,

 

[Daddy Tanuki]

Thank you, awesome info. I'm going to take another look with all your input,

So I have purchased one of these inverters, I have also purchased a voltage sensing relay and tested this in my shop.

the inverter itself has its on timer for output, you can set it to only putout electricity during set time frames that you set. this prevents any power usage during non producing hours. with enough documentation on your own system you can even have a reasonably good guess on when your system is entering float.

that portion is all in the inverter itself, no need for other items of any sort. the voltage sensing relay's are about 20-30 USD on ebay, amazon etc. set it at your batteries float voltage and the relay turns on only when your batteries are at xx voltage.

the following are numbers pulled from my calcs for my system, you will need to supply the numbers that coincide with your system:

my batteries full at 54.4 volts with a 30 minute absorb setting. usually by 1030~1100 in the winter, or 0930 in the summer. (heavily oversized for guaranteed winter output.)

so I set the inverter to turn on only at 1030 and set it to turn off (after some experimentation for when the solar is too low to work) at 1700. at 1030 regardless of anything else the inverter will come out of search and apply power to the circuit to your water heater. take the voltage sensing relay and set it at your setpoint, (for me it is 3.375/cell or 54 volts for the packs) as long as the voltage is at or above 54 volts the relay will close and you can use it to control a larger magnetic relay that is mounted inline with the inverter.

• time is right, but batteries are low? inverter on but the voltage sensing relay will not close, so no power to the water heater. tare loss is inverter and relay.
• batteries are high but the time is not right (think late evening as sun goes down). inverter will not supply power, tare loss none.
• time is right?, batteries are right? electricity flows to the water heater. clouds come out! battery voltage drops as the inverter draws power and the voltage sensing relay opens, no power to the water heater. This can cycle off and on as often as it wants its only heating elements...

lastly, the sigineer has a simple analog off and on button for power. in my final install my plan is to not use the magnetic relay, just the voltage sensing relay. the draw across the on/off switch for the inverter is low enough that i can control it with just the voltage sensing relay negating the tare loss of the higher amperage magnetic relay.

do not get me wrong, his way will work as well, but there are many ways to skin the proverbial cat and they can all be solved via a simple set of controls that do not need to be digital, or controlled by an andurino or any other high tech control. simple relays, breakers and push buttons took us to the moon.... and this is not rocket science.

 

[Daddy Tanuki]

The unit is very reasonable. I'm about 600ft away from the panels. I'm assuming I'm better off running wires to the inverter over that distance at higher voltage (series) and put the inverter close to the water heater.

sorry never noticed this, I would run high voltage D/C to your battery bank via your SCC like normal and then AC form the inverter to the water heater. if you plan to use the inverters SCC then most of what I was discussing would need to be rethought. not saying it won't work as with enough study and preparation anything can work... look at porsche for example one fo the best examples of engineers engineering their way out of a bad design with over complicated design answers....

 

[efficientPV]

The OP wanted to run without batteries and a lot of expense. I'll admit that I thought Ukiah was some third world country where sourcing components would be difficult. It still seems goofy to me.

 

[rhino]

The OP wanted to run without batteries and a lot of expense. I'll admit that I thought Ukiah was some third world country where sourcing components would be difficult. It still seems goofy to me.

Real Goods Solar opened their 2nd store in Ukiah, CA.