DIY Solar Water Heater

[slowbutsure]

I live in the Philippines with wall to wall sunshine. I have a PV system so household electric is covered. However, I want to make a Solar hot water system. I’ll give the general idea here, appreciate comments for improvement and if I’ve got it right.

On the roof I will have a 500L water tank, 2ft high x 3ft wide. Attached to it will be a coil of black water pipe. One end of the pipe will connect to the top of the tank and one to the bottom.

My theory is I would need a pump to circulate it as the cold water pressure at the bottom and the hot water will naturally rise, giving some kind of circulation.

Does that seem reasonable?

Also inlet and outlet of course.

Thanks.

 

[sunshine_eggo]

1 square meter receives 1000W

Water Heating Time Calculator

Calculator that tells you how long it takes to heat water from start to end temperature with a given heating power.

gettopics.com

So a 1m^2 collector harvesting 50% of the energy received by the sun would need 46.5 hours to heat 500L from 10°C to 50°C.

Here's an example using coiled irrigation pipe:

 

[crossy]

You may not need a pump if you have the tank and collector correctly positioned, check out thermosyphon operation.

Thermosiphon - Wikipedia

en.wikipedia.org

 

[slowbutsure]

1 square meter receives 1000W

Water Heating Time Calculator

Calculator that tells you how long it takes to heat water from start to end temperature with a given heating power.

gettopics.com

View attachment 189638
So a 1m^2 collector harvesting 50% of the energy received by the sun would need 46.5 hours to heat 500L from 10°C to 50°C.

Here's an example using coiled irrigation pipe:

That’s really helpful, thanks!

 

[slowbutsure]

You may not need a pump if you have the tank and collector correctly positioned, check out thermosyphon operation.

Thermosiphon - Wikipedia

en.wikipedia.org

Does this process work with coiled pipe?

 

[DIYrich]

Does this process work with coiled pipe?

I don't see why a coil, like a vertical spring would not work. Backside doesn't add much, but helps with bending radius.

 

[slowbutsure]

1 square meter receives 1000W

Water Heating Time Calculator

Calculator that tells you how long it takes to heat water from start to end temperature with a given heating power.

gettopics.com

View attachment 189638
So a 1m^2 collector harvesting 50% of the energy received by the sun would need 46.5 hours to heat 500L from 10°C to 50°C.

Here's an example using coiled irrigation pipe:

After watching videos the coil seems more difficult to do than straight pipes with bends at the top and bottom. Does this second method work ok?

 

[sunshine_eggo]

After watching videos the coil seems more difficult to do than straight pipes with bends at the top and bottom. Does this second method work ok?

It's about surface area and flow. More surface area for heating/transfer and the right amount of flow to absorb as much heat as possible.

 

[slowbutsure]

It's about surface area and flow. More surface area for heating/transfer and the right amount of flow to absorb as much heat as possible.

Smaller pipes for slower flow is better?

 

[sparrowhawk]

1 square meter receives 1000W

Water Heating Time Calculator

Calculator that tells you how long it takes to heat water from start to end temperature with a given heating power.

gettopics.com

View attachment 189638
So a 1m^2 collector harvesting 50% of the energy received by the sun would need 46.5 hours to heat 500L from 10°C to 50°C.

Here's an example using coiled irrigation pipe:

I built a solar water heater for my swimming pool like that with irrigation pipes and it failed the first time water wasn't flowing through it when the sun was on it. The water boiled out and the pipe slightly melted. I didn't watch the video so I'm not sure if he used different pipe. I also don't know if he used glass on top of the boxes like I did either.

 

[slowbutsure]

I built a solar water heater for my swimming pool like that with irrigation pipes and it failed the first time water wasn't flowing through it when the sun was on it. The water boiled out and the pipe slightly melted. I didn't watch the video so I'm not sure if he used different pipe. I also don't know if he used glass on top of the boxes like I did either.

hmm, so maybe using a pump to circulate would be safer. Helpful info, thanks.

 

[slowbutsure]

Thanks for replies. I can’t easily get copper tube here Philippines) and am concerned about plastic pipe bursting. The solar collector will be on the roof, not easily accessible so want it as maintenance free as possible. Some questions.

1. Would you put the pump on the roof with the collector or at ground level?

2. What kind of pump? I’m guessing it needs a slow circulation and robust enough to work all day.

3. Best way of it automatically on during the day and off at night? Make it Solar?

4. How could you know, without checking all the time, that the pump has failed, which could result in a burst pipe?

Thanks.

 

[sparrowhawk]

Thanks for replies. I can’t easily get copper tube here Philippines) and am concerned about plastic pipe bursting. The solar collector will be on the roof, not easily accessible so want it as maintenance free as possible. Some questions.

1. Would you put the pump on the roof with the collector or at ground level?

2. What kind of pump? I’m guessing it needs a slow circulation and robust enough to work all day.

3. Best way of it automatically on during the day and off at night? Make it Solar?

4. How could you know, without checking all the time, that the pump has failed, which could result in a burst pipe?

Thanks.

If your pump is going to be exposed to hot water it needs to be designed to tolerate the heat. When I was seriously considering installing a solar electric system I kicked around many ideas for heating water. But in the end I decided to scale up my system design to include using my standard electric water heater to heat my water.
The beauty of it is the simplicity. Now mind you my system at 6000 watts is to small to run a water heater with the standard 4500 watt elements along with my other loads so I replaced them with 2500 watt ones. I also use a timer and only heat water at around midday so I have solar to also charging my batteries. Of course that also means I don't have an unlimited supply of hot water but it's enough for 2 showers in the evening.

 

[slowbutsure]

If your pump is going to be exposed to hot water it needs to be designed to tolerate the heat. When I was seriously considering installing a solar electric system I kicked around many ideas for heating water. But in the end I decided to scale up my system design to include using my standard electric water heater to heat my water.
The beauty of it is the simplicity. Now mind you my system at 6000 watts is to small to run a water heater with the standard 4500 watt elements along with my other loads so I replaced them with 2500 watt ones. I also use a timer and only heat water at around midday so I have solar to also charging my batteries. Of course that also means I don't have an unlimited supply of hot water but it's enough for 2 showers in the evening.

Thanks. I have thought about this and it has appeal. I have a 10kw solar system. What I can’t find here is an insulated water tank. I have found an immersion heater but no tank.

 

[sparrowhawk]

Thanks. I have thought about this and it has appeal. I have a 10kw solar system. What I can’t find here is an insulated water tank. I have found an immersion heater but no tank.

Actually I had forgotten about the solar water that I built about 25 tears ago. In this case I took a an electric water heater removed the insulation, painted it black and put it in wooden box with a glass top and pointed it towards the sun. I worked pretty good but didn't hold the heat very well overnight especially in the winter. In the Philippines losing heat at night wouldn't be much of an issue.
If uninsulated hot water tanks are available there then that may be the way to go.

 

[Steve777]

I am guessing the OP is in a climate where it never goes below freezing. If so, all the freeze protections stuff we do in colder climates are not applicable. But you may need to consider water expansion, unless this will be an open system with a vent to air at some point. Closed/pressurized solar DHW systems typically include an expansion tank to absorb the increased volume as the water is heated. Much the same as conventional tank water heaters (should) have one. If they don't then the water pressure will increase as the water heats putting stress on the pipes and fittings.

Thermo siphon systems can and do work, just not as well as systems with a pump to circulate the water. You could run a pump off a dedicated PV panel(s). Whether it is worth the extra parts and complication really depends on how much sun you get and how much room for collectors you have and if your hot water needs can be met with a passive system.

And as @sparrowhawk mentioned, just a black tank in a glass covered insulated box works too, again just not as efficiently as system with larger collectors and perhaps active water moving. If accessible, such a system can have an insulated lid that you flip over the glass at night.

I assume you've checked that your roof can structurally handle the extra weight of tank, 500l of water plus whatever collectors. The tank and water are a considerable additional weight in a small area. Just something to consider.

 

[slowbutsure]

I am guessing the OP is in a climate where it never goes below freezing. If so, all the freeze protections stuff we do in colder climates are not applicable. But you may need to consider water expansion, unless this will be an open system with a vent to air at some point. Closed/pressurized solar DHW systems typically include an expansion tank to absorb the increased volume as the water is heated. Much the same as conventional tank water heaters (should) have one. If they don't then the water pressure will increase as the water heats putting stress on the pipes and fittings.

Thermo siphon systems can and do work, just not as well as systems with a pump to circulate the water. You could run a pump off a dedicated PV panel(s). Whether it is worth the extra parts and complication really depends on how much sun you get and how much room for collectors you have and if your hot water needs can be met with a passive system.

And as @sparrowhawk mentioned, just a black tank in a glass covered insulated box works too, again just not as efficiently as system with larger collectors and perhaps active water moving. If accessible, such a system can have an insulated lid that you flip over the glass at night.

I assume you've checked that your roof can structurally handle the extra weight of tank, 500l of water plus whatever collectors. The tank and water are a considerable additional weight in a small area. Just something to consider.

Thanks. Yes, building the house now, roof goes on in about 2 weeks so building with this in mind. Though thinking we could reduce it to 300L, that would do us as a family I think.

 

[slowbutsure]

This might be a dumb question. When you are drawing hot water from the tank, the ball float is replacing it with cold water at the same time. So how does it not significantly decrease in temperature as you draw it?

 

[Bluedog225]

Depending on the construction, pipe can get hot enough to create steam and pressure in a closed system when water is introduced. Something to be mindful of.

 

[slowbutsure]

Depending on the construction, pipe can get hot enough to create steam and pressure in a closed system when water is introduced. Something to be mindful of.

Pressure relief valve needed to then I’m guessing.