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Scientist (plunky) Solar Question for you

billfred

billfred

New member
I want to make a home made solar heating system for my pool. Not full winter heat but hoping to add a month to each side of my swim season.

I have a guest house with a south facing roof that is the back of the house with an alley. Appearance is not going to be an issue.

I am planning on running either black poly pipe or painted PVC up and down the pitch of the house - as much weight as I can get away with.

Question is, should I just constantly circulate water through my pipe coils or should I set up some sort of on off switch where I leave the water resident until it reaches a certain temp and then vacate to the pool?

Roof to pool is about 50' underground.

Real talk.
 
Man just buy me a case I'll heat it up for you after I have had about 6
 
Nothing more manly then building your own shit...

sounds dangerous tho remember safety first

Sent from my GT-I9300 using EliteFitness
 
I've got zero experience on this one.

Surely somebody has tackled it before tho.
 
Circulate the water.

In fact, although it seems counter intuitive, higher velocity through the tubing is best, since the heat transfer is much improved for turbulent flow.
 
Headholio said:
Circulate the water.

In fact, although it seems counter intuitive, higher velocity through the tubing is best, since the heat transfer is much improved for turbulent flow.
Click to expand...

I am just worried the heat loss from the transfer to the pool will be greater than gained on a continual run.

So the you think the turbulent flow is why they use small hose for the coil on most of these systems? What is the science behind that? Turbulent flow causes more water particles to come in contact with the warmer surface??
 
billfred said:
I am just worried the heat loss from the transfer to the pool will be greater than gained on a continual run.

So the you think the turbulent flow is why they use small hose for the coil on most of these systems? What is the science behind that? Turbulent flow causes more water particles to come in contact with the warmer surface??
Click to expand...

Small tubes are used mainly for greater surface area but also for turbulent flow.
You are basically right about turbulent flow. with slow flow (called laminar flow), the highest velocity is in the center of the pipe and the velocity at the wall is zero. That stagnant layer inhibits heat transfer.

The reason it is counter intuitive is because people think in terms of having the highest exit temp from the panel, but instead should be concerned with the highest heat transfer rate.
The equation is:

Q= UA(T2-T1)

Heat transfer rate = heat transfer coefficient x contact area x the difference in temp from the hot to the cold side (delta T)

To maximize heat transfer:

Increase U by having turbulent flow
increase A by having many small tubes instead of large tubes, and have as big a panel as practical.
Delta T increases when the temp of the water coming out of the panel is lowest!
 
Headholio said:
Small tubes are used mainly for greater surface area but also for turbulent flow.
You are basically right about turbulent flow. with slow flow (called laminar flow), the highest velocity is in the center of the pipe and the velocity at the wall is zero. That stagnant layer inhibits heat transfer.

The reason it is counter intuitive is because people think in terms of having the highest exit temp from the panel, but instead should be concerned with the highest heat transfer rate.
The equation is:

Q= UA(T2-T1)

Heat transfer rate = heat transfer coefficient x contact area x the difference in temp from the hot to the cold side (delta T)

To maximize heat transfer:

Increase U by having turbulent flow
increase A by having many small tubes instead of large tubes, and have as big a panel as practical.
Delta T increases when the temp of the water coming out of the panel is lowest!
Click to expand...

ok.. so I need to get as much small tubing on the roof as possible to keep it turbulent, increase the surface area, and increase the resident time. Wonder how much my roof can support.
 
I have solar panels on my roof heating the pool. It circulates 8-12 hours a day, I can bypass the heater if I just want to run the normal filter. I did need to replace my roof a couple years ago as it was completely fucked under the solar panels.
 
i have 12' x 24 '... at night it will act as a cooling fan on the water, so yes turn it off..

youtube has some crazy gas fired home made furnaces, i'm thinking of doing one that's made in a grill, should be about 100k btu.. issue becomes it will double the gas use amount.. so in the end $1200 for a real gas fired one will work wonders and be cost effective..
 
billfred said:
I am just worried the heat loss from the transfer to the pool will be greater than gained on a continual run.
Click to expand...


Take 2 cases where your panel is 150 degrees:

Fast flow- Water goes in at 60 degrees and exits and 70 degrees
Slow flow - water goes in at 60 and exits at 90

Slow sounds better at first glance but you have transferred less energy overall. In the slow case, energy is transferred to less water and takes more time to transfer. So the result is that even though you are dumping 90 degree water into your pool, it takes longer for the pool to reach your desired temp.

You don't lose heat, you transfer heat to water which is what you are trying to do. You will have this type of scenario
hour 1 - In=60, out = 70
hour 3 - In = 63, out = 72
hour 5 - In = 66, out = 74

Etc. Your temp increase will slow asymptotically an eventually reach steady state where your delta T becomes too small to get any heat transfer greater than your losses.

So you see, fast flow doesn't cause you to lose heat, the heat is transferred to the water and then the warmer water returns to the panel.
 
SpyWizard said:
i have 12' x 24 '... at night it will act as a cooling fan on the water, so yes turn it off..

youtube has some crazy gas fired home made furnaces, i'm thinking of doing one that's made in a grill, should be about 100k btu.. issue becomes it will double the gas use amount.. so in the end $1200 for a real gas fired one will work wonders and be cost effective..
Click to expand...

I have a gas fired furnace. The pool is so big, it takes a week of constant running to get temp up. My jew wife won't spend the money on the gas bill.
 
The harder you want to run the pump, the more energy it's going to consume as well. Especially through smaller diameter tubing, which increases the necessary head pressure.

There should be a theoretical point that optimizes the amount of heat transferred without consuming excessive pump energy.
 
mrplunkey said:
The harder you want to run the pump, the more energy it's going to consume as well. Especially through smaller diameter tubing, which increases the necessary head pressure.

There should be a theoretical point that optimizes the amount of heat transferred without consuming excessive pump energy.
Click to expand...

You are right (not often I say that!) about the pressure drop. But its not so much a concern of energy usage as of loss of flow rate. Centrifugal pumps normally just generate whatever head they can, and the resistance in the pipe determines the flow rate (You can dead head a centrifugal pump and watch the amps on the motor actually go down).

You need 4 one inch pipes in parallel for equivalent flow rate and velocity to a 2 inch pipe, or 16 half inch pipes etc. Even with equivalent velocity you have more pressure drop.

I would definitely put the panel down stream of the filter or you will have trouble pushing through the filter. Or you could T off before the filter and add 2 valves so you can either heat or filter, so you don't have to do both at the same time. That would also allow you to bypass the panel when the pool gets too hot.

But I think filtering would be a much greater line loss than a panel if designed reasonably well. I have a friend with a solar heated pool and I think he can filter and run through his panels at the same time. But his pump was probably sized to do that from the beginning. But if you set up your panel so that resistance isn't too great you will probably be ok. You might have to run the pump longer each day to get equivalent filtering because of the loss of flow rate (so it is an energy issue after all).
 
Headholio said:
You are right (not often I say that!) about the pressure drop. But its not so much a concern of energy usage as of loss of flow rate. Centrifugal pumps normally just generate whatever head they can, and the resistance in the pipe determines the flow rate (You can dead head a centrifugal pump and watch the amps on the motor actually go down).

You need 4 one inch pipes in parallel for equivalent flow rate and velocity to a 2 inch pipe, or 16 half inch pipes etc. Even with equivalent velocity you have more pressure drop.

I would definitely put the panel down stream of the filter or you will have trouble pushing through the filter. Or you could T off before the filter and add 2 valves so you can either heat or filter, so you don't have to do both at the same time. That would also allow you to bypass the panel when the pool gets too hot.

But I think filtering would be a much greater line loss than a panel if designed reasonably well. I have a friend with a solar heated pool and I think he can filter and run through his panels at the same time. But his pump was probably sized to do that from the beginning. But if you set up your panel so that resistance isn't too great you will probably be ok. You might have to run the pump longer each day to get equivalent filtering because of the loss of flow rate (so it is an energy issue after all).
Click to expand...

I have one pump that I can use that does not go through the filter. It is for one side of my hottub jets. I am planning on routing the water from there through the coils.
 
pin said:
Ppfff. The sun isn't real. You guys are so stupid.
Click to expand...

Billfred is a xheap ass bish. Bill if you want your pool heated just jump in I'll throw you a blow dryer

Sent from my SCH-I535 using EliteFitness
 
billfred said:
I have one pump that I can use that does not go through the filter. It is for one side of my hottub jets. I am planning on routing the water from there through the coils.
Click to expand...

Oh, look at me. I can afford hottub jets!

Here at the plunkey household, we have to fart in the bathtub if we want to see bubbles in our bath water.
 
Headholio said:
Take 2 cases where your panel is 150 degrees:

Fast flow- Water goes in at 60 degrees and exits and 70 degrees
Slow flow - water goes in at 60 and exits at 90

Slow sounds better at first glance but you have transferred less energy overall. In the slow case, energy is transferred to less water and takes more time to transfer. So the result is that even though you are dumping 90 degree water into your pool, it takes longer for the pool to reach your desired temp.

You don't lose heat, you transfer heat to water which is what you are trying to do. You will have this type of scenario
hour 1 - In=60, out = 70
hour 3 - In = 63, out = 72
hour 5 - In = 66, out = 74

Etc. Your temp increase will slow asymptotically an eventually reach steady state where your delta T becomes too small to get any heat transfer greater than your losses.

So you see, fast flow doesn't cause you to lose heat, the heat is transferred to the water and then the warmer water returns to the panel.
Click to expand...

Yeah - I will also transfer heat to the ground on the way to the poo. That was the heat loss I was referring to.
 
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