Solar water heating

[cruzbill]

If you're gonna be in one place, why reinvent the wheel, just buy a ready-made SWH

https://www.ebay.com/i/331878211226?chn=ps&dispItem=1

[1bigmess]

Originally Posted by cruzbill

If you're gonna be in one place, why reinvent the wheel, just buy a ready-made SWH

I'm not the OP, but I'm gonna be a little books and respond that based on the price of the item in the link, the reason is cost.

[okmunky]

Originally Posted by cruzbill

If you're gonna be in one place, why reinvent the wheel, just buy a ready-made SWH

https://www.ebay.com/i/331878211226?chn=ps&dispItem=1

Now where's the fun in that? Besides all that replaces is the black tubing. There's still the challenge of tying it into the RV. I can think of better ways to spend $1,950.

[hclarkx]

Originally Posted by oatmeal

How about this?

SOLAR HOT WATER HEATING KITS -

Do you already have solar panels? We have 1,050 watts and moved the water heater to the inverter panel so it could run on the batteries. When the batteries are full from solar, the solar controller diverts the excess solar to the water heater. We've spent weeks at a time boondocking only on solar and had hot water every day.

Oat, can you tell us what solar charger you have and how it controls energy to the water heater? Does it ramp up power to the water heater as the lead-acid battery current drops in the second stage of charging? This seems like an ideal way to heat water and make better use of the solar system.

[hclarkx]

Ideally a solar hot water system would be simple and automatic. For those of us with solar battery charging, I think using left-over solar energy to heat or pre-heat water would be great. And, expanding an existing PV system could be much simpler than building an independent water heating system. PV for water heating along the lines of the Sun Bandit home system would work but having just one system and using left-over solar for water heating (after the batteries are charged or are into the second stage) as Oatmeal's system does would be ideal and least costly especially if it means just adding a panel or two. I asked Oatmeal about his solar controller in another post.

[oatmeal]

Originally Posted by hclarkx

Oat, can you tell us what solar charger you have and how it controls energy to the water heater? Does it ramp up power to the water heater as the lead-acid battery current drops in the second stage of charging? This seems like an ideal way to heat water and make better use of the solar system.

Ha! You called me "Oat." I like it.

First of all, your inverter has to be big enough to handle the water heater. Ours is 2,000 watts and is a modified sine wave. We also have six golf cart batteries, and 1,050 watts of solar.

The basic idea is to rewire the AC power to your water heater through a solid state relay that comes ON when the battery voltage is at float level, and turns OFF when it drops below that, which happens almost instantaneously once the water heater begins to draw current. So the water heater is on for just a fraction of a second, and the battery voltage recovers just as quickly, and then it does the ON/OFF thing again.

This happens dozens, even hundreds, of times a second. The fraction of time ON versus OFF tracks the amount of incoming solar power available constantly, via a process called pulse width modulation ("PWM"). And since the water heater is a simple resistive load, it doesn't mind being rapidly pulsed like that.

We currently have a Midnite Classic controller, which has an AUX output and a high-speed FLOAT-tracking PWM mode that works great for this purpose, by connecting the controller's AUX output to a high-current solid state relay that switches the water heater's AC on or off.

We originally had an Outback MX-60 controller. We did get this scheme to work with that controller, but it was kind of tricky and took some monitoring that's not needed with the Midnight, which is basically "set and forget." However, with the Midnight's "set and forget," it does it only when the batteries are in float mode, and not when they're in absorb, when there's also excess solar that could be used.

So neither one is perfect, but the hardware cost was just the relay, which was something like $15 when we did this over 10 years ago. So the return on investment is quite favorable.

Do you already have a controller? Let me know if it's either of the above and I can provide more specific details about the setup if you'd like.

I should emphasize that of course we don't get hot water in the morning, while a dedicated solar water heater would start "working" as soon as the sun comes up. But we don't use this as a primary source for hot water, but instead as a way to use excess solar production, and there are lots of days that the 10-gallon water heater turns off because it's finished. That's very satisfying.

[hclarkx]

Originally Posted by oatmeal

Ha! You called me "Oat." I like it.

First of all, your inverter has to be big enough to handle the water heater. Ours is 2,000 watts and is a modified sine wave. We also have six golf cart batteries, and 1,050 watts of solar.

The basic idea is to rewire the AC power to your water heater through a solid state relay that comes ON when the battery voltage is at float level, and turns OFF when it drops below that, which happens almost instantaneously once the water heater begins to draw current. So the water heater is on for just a fraction of a second, and the battery voltage recovers just as quickly, and then it does the ON/OFF thing again.

This happens dozens, even hundreds, of times a second. The fraction of time ON versus OFF tracks the amount of incoming solar power available constantly, via a process called pulse width modulation ("PWM"). And since the water heater is a simple resistive load, it doesn't mind being rapidly pulsed like that.

We currently have a Midnite Classic controller, which has an AUX output and a high-speed FLOAT-tracking PWM mode that works great for this purpose, by connecting the controller's AUX output to a high-current solid state relay that switches the water heater's AC on or off.

We originally had an Outback MX-60 controller. We did get this scheme to work with that controller, but it was kind of tricky and took some monitoring that's not needed with the Midnight, which is basically "set and forget." However, with the Midnight's "set and forget," it does it only when the batteries are in float mode, and not when they're in absorb, when there's also excess solar that could be used.

So neither one is perfect, but the hardware cost was just the relay, which was something like $15 when we did this over 10 years ago. So the return on investment is quite favorable.

Do you already have a controller? Let me know if it's either of the above and I can provide more specific details about the setup if you'd like.

I should emphasize that of course we don't get hot water in the morning, while a dedicated solar water heater would start "working" as soon as the sun comes up. But we don't use this as a primary source for hot water, but instead as a way to use excess solar production, and there are lots of days that the 10-gallon water heater turns off because it's finished. That's very satisfying.

Oat, thanks hugely. I'm a retired electric power engineer (and ham and audio aficionado) so followed all 100%. The approach is novel and very interesting. I'm working from a 300 AH LFP battery which does not let voltage change much with load. That might rule out the voltage-based switching or make it tricky. The rapid cycling of the battery might also be questionable even though it's very shallow.

My solar controller is a basic 30A SolarEpic. Solar is 400W (under ideal conditions, of course). Inverter is 900W (covers all but the A/C). It has an unused load terminal good for 20A but nothing else helpful. I don't use the 20A load terminal because separating smaller loads from the inverter would be difficult in my RV. It has a user-set charge profile that I use for the LFP (though it's far less than satisfactory until I get an external state-of-charge monitor to turn it off).

What's really needed is a solar controller that will do the battery charging and modulate power to the water heater to soak up any excess power. It would effectively regulate charge current by adjusting power to the water heater. Seems doable but I've already got too many hobbies.

Thanks for the help.

Than