Solar Heating design brainteaser 1

[cheeryble]

Hi from Chiangmai
I am getting my head around possible solar heating for a swimming pool in an apartment block.
The pool, pumps, etc, are on the 3rd floor.
The solar collector (black irrigation tubing) would be on the roof effectively the 12th floor.

So a loop is needed rising to the collector and running back down. The question is open or closed loop system?
An open system could be pumped straight up from the pool connections and would obviate need for a heat exchanger. Only one problem springs to mind.
Whilst a sealed loop would save energy as there would be weight of water on the fallside as well as the riseside, therefore only circulation pumping required, with an open loop it struck me that above the water equivalent of 30 inches of mercury, about 32 ft of water I believe, there would be a vacuum formed in both sides of the loop.
Then I imagine water being pumped up to the tipping point at the zenith. As it runs over to the fallside one imagines a suction acting on the water rising. On reflection though after 32 ft of rise there is no atmospheric pressure left to form the positive pressure on the rise side which seems like suction on the fallside.

So are my concerns right, that it may need constant pumping against 12 minus 3 floors of head to keep this system running? Or are things more complicated than I think, and is a lighter pumping load possible?

thanx Cheeryble

I am disregarding the energy needed to pump warmer and therefore lighter water downwards to displace heavier water.....that OK?

 

[JJPellin]

You are on the right track. You can only recover 32 feet of head on the return line. If you don't want to draw a vacuum at the high point, you will need to put a back-pressure device at the bottom of the return line. But, in either case, the pump will have to provide the 12 floors minus 32 feet of head, as a minimum. The fact that the water is hotter on the return leg can only make the situation worse. But that affect will probably be very small. The specific gravity of water does not drop drastically with temperature until you get very close to the boiling point. I assume you don’t want you pool to boil.

Johnny Pellin

 

[BigInch]

You might consider buying a pump with 12 floors + a bit of head, unless you can fill the pipes and solar collectors from a high level connection.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[BigInch]

As a matter of fact, I just bumped into one of the best solar thermal hot water system websites I've seen yet. Lota' ideas for you there.

http://www.solar-rating.org/education/og300education.htm

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[cheeryble]

Thankyou for the comments and the reference site.

I am tending toward a sealed loop which could have a simple header tank. The problem then becomes making a budget heat exchanger. (I am on the committee of the apartment block involved.....we are just finishing a rebuild of the pool system with salt water......this idea will only float if it's cheap!) Budget means something simple like running a load of coils of the black PE pipe i use for the roof collector in a tank with a flow and return to the pool system.
As a humble jack of all trades maths/physics-educated (40 years ago) bod I would appreciate how to work out heat flow in the heat exchanger.....ie how many coils necessary. (Not to mention flow rate in the collector system!)

One nice little brainwave I had about simply laying the black PE pipe on the roof as a collector is it doesn't need to be tilted. If there is a gap between the back-and-forth pipe runs the full vertical angle of the sun will be subtended (not horizontal obviously) all the time.

thanx Cheeryble

 

[BigInch]

Buy some kind of UV stabilized pipe. Temperatures can get quite high at times. Up to around 200 F could be possible.

You should probably enclose the pipes in a glass box, or a lot of the heat will be convected and radiated away. That will complicate the heat transfer.

Not tilted towards the optimum sun angle will mean a big loss in efficiency, more pipe loops, more resistance, more pumping. Snow, if any, will not slide off.

Hope you have no snow and lots of sun.

As the salt water evaporates, you'll have to add fresh water to keep it from getting as salty as the Dead Sea.




**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[cheeryble]

<Buy some kind of UV stabilized pipe. Temperatures can get quite high at times>

A friend showed me the same type of pipe which has been on the top of his garden wall, sometimes with water in, sometimes not. After 4 years it feels good as new.

<You should probably enclose the pipes in a glass box,>

I think simplicity will be the key. I would prefer to use loads of piping and simply have it lying methodically on the roof. At 700 baht ($20) per 200m roll it's really cheap.

<Not tilted towards the optimum sun angle will mean a big loss in efficiency, more pipe loops, more resistance, more pumping.>

But as I said if there is a suitable gap between pipes, then circular pipe will subtend the it's full dose of sun no matter what the angle.......won't it?


Snow, if any, will not slide off.

<Hope you have no snow and lots of sun.>

Snow I doubt as this is Northern Thailand! Though the water still gets chilly for about 5 months. This is what I would like to correct.

<As the salt water evaporates, you'll have to add fresh water to keep it from getting as salty as the Dead Sea.>


Perhaps another reason to have an indirect system with fresh water in it.

cheers John

 

[WH74]

If you are thinking of using flexible pipe/tube on the roof and it's under negative pressure from the 12 floors of water below it, the piping above the pump should all be rigid or it will collapse. Schedule 40 black PVC should be fine.

 

[hydrae]

Solar in Pools

you do not need the glass box,

the glass box you regularly see on solar systems allows the temperature to rise above ambient for hot water use, here you are heating the water to just 80° F or so. If you allow the water in the panels or tubes to heat up it will then lose some of that heat to the air around the tubes. you only need the water to rise 1° per trip and you will have a warm pool in just a few sunny days. EG run the water through the tubes as fast as possible to get the heat into the pool

If using the pipe idea verify the roof can handle the weight of the pipes with water. Getting a good structure to line up the pipes so they stay flat is a chalenge

Off the shelf pool solar panel are reasonable in price, are light enough in loading, and easy to install.

How much area do you need, a rule of thumb 50 to 75% of the surface area of the pool in panel area will be normally required for pools at a 45° latitude

I worked on swimming pools including solar systems while at university

Hydrae

 

[jpvancouver]

I have a simplified version of this setup - from an above ground pool to a second story roof. I'd like to minimize costs, which would mean using a typical pool magnetic impeller pump if possible.

Unfortunately the specs are limited for these pumps. They state flow, approximate horsepower as well as electrical power, but nothing else.

Anyway, I hooked up 100 ft of 1" black poly to the existing 1000 gph 1/8 hp and had no flow through the collector. Vertical rise is about 24 ft up. There's no back-flow valve.

Using the water pumping horsepower table on

http://www.engineeringtoolbox.com/pumping-water-horsepower-d_753.html

gives me 0.11 hp for 15 gpm / 30 ft. Calculating with P = 62.4*Q*h/33000/efficiency is about the same.

Even adding 20 ft of pipe head for the 100 ft/1" diameter of pipes (C=150) gives 0.13 hp @ 10 gpm. At 50% efficiency, I should at get about 5 gpm, and at lower flow rates the pipe head should go down to about 2 ft.

I've neglected the collector, which I think is safe because it's based on 1 1/2" pipe between mats, I think relying on internal convection for the heat transfer.

If these estimates are reasonable, then maybe it's my implementation - a leak is letting air in, or not having the back-flow valve.

If not, then what am I missing? Is the pump efficiency likely to be much worse than 50% If pump capacity is the limiting factor, then would the closed loop work better? Reduce flow with 3/4" or even 1/2" pipe? Pump head should double with two identical pumps in series, but is there anything about cheap impeller pool pumps that would prevent this from working?

 

[ccjersey]

Your pump may be malfunctioning, (impeller jammed etc) but most likely it cannot develop the pressure (head) needed to move water up to the top and overcome the friction in the pipeing.

This is a characteristic of the design of the specific pump, you need the curve for it or the one you replace it with.