25mm or 40mm Plastic Tube. Which is Best for Direct Solar Heating?

Are you talking about using the same length of tube, or the same total volume? Same mass flow rate? Gimme more!

My 'prickling fingers' solution is to go for the smaller diameter, but longer (i.e. same contained volume). Can't rigorously justify it, tho' (without a lot more brainstrain).

I think I like the idea of knowing both really, so I will divide the question in two:

1. 25mm or 40mm tube with equivalent overall volume (i.e. longer 25mm)?

2. 25mm or 40mm tube with equivalent overall length (i.e. less volume in 25mm)?

I would go for the smaller pipe in both cases for fastest heating.

The volume increases to the square of the radius. that implies 2.56 time the volume for 1.6 times the facing surface area.

Equal volumes would imply 1.6 times the area and energy.

Hmmm on a fixed length L of pipe, assuming the sun is shining directly on one side (and discounting nasty angles of incidence on the sides) The area exposed is effectively just the diameter 2RxL (I was initially thinking it was 1/2 the circumference...but...) The volume is pi (R)² L

So the ratio of area/volume is 2/pi R (I'm sure someone will tell me if my maths is wrong )

So this implies that the smaller the radius the higher the ratio...
But of course we havn't proved that a high surface area to volume is a good thing.

Anyhow for the same volume then smaller pipe is better (I think)....
For same length then presumably bigger piper is better to maximise the surface area.

It allgets tricky if the volume in the pipe becomes significant compared with the volume in the tank... hmmm...then factor in that heat transfer is at a maximum when there is maximum difference between the water temperature and the pipe....errr.

I've confused my furry brain now... where's Phyz when you need him?

Del

There are many parallels in nature (eg cellular construction) where the issue of the optimum ratio of surface area to volume play a significant part. It is such a complex field for me, Del, that my head is buzzing as well.

Why is the area exposed calculated by 2RxL rather than 1/2 circumference?

Can you let me have the breakdown of how you got the ratio of area/volume to 2/prR? My maths is not great and I have not been able to make this ratio work yet. Hendrick talks about the volume increasing as the square of the radius. How does this relate to your ratio?

It seems obvious from what you and Hendrick have said that the higher the ratio of surface area to volume the quicker the water will heat.

So, let's accept that the greater surface area the better, what would be the optimum diameter of any tube given the properties of H²O, a given m² insolation (let's say 1000w/m²), a given initial temperature (say 35°F) and a target end temperature (let's say, 100°F).

While I leave you to think about this, I am going to do some experiments...

Why is the area exposed calculated by 2RxL rather than 1/2 circumference?

Welllllll, you could call it what you like ...but if you imagine you are the sun (ok it's a bit warm...but imagine you're the sun with a cold beer...now where was I...?) From that distance the Sun will 'see' just the plan view, the Sun's rays will be parallel so they won't impinge on the whole 'half circumference' at 90degrees... so it's a sort of quick approximation...I'm sure some maths guy could come up with loads of stuff about angles of incidence and coeficients of reflection vs absorbtion... but I like the simple first order approximation.

Bottom line is often try it and see....I see maths as a guid to get you in the right ball park, not a definition of an exact result.... I'm sure others will argue, but give me practice over theory anyday.

Del

Put black tubing in back box; may be wood or rocks whatever under glass or not, then proceed

It won't matter, because PVC tubing will deteriorate rapidly in direct sunlight, unless it is UV blocked. Furthermore, if the residence time for the water in the tube is too long, the water could well exceed the temperature limit of the material, resulting in further degradation...

I personally would not use plastic (copper or steel painted black with antifreeze in the water for the winter time), but that is something else!

I like the larger diameter pipe idea best, if possible slightly flattened (oval shape) to increase the area being warmed, as it allows more water to flow easily, therefore a smaller pump can be generally used. The heat from the pump should be added to the water, so the cold return should also cool the pump. This results in an even more efficient system.

I assume also that you are using a heat pump to increase the temperature to a usable level, if so, then the amount of heat "gained" by the water in the pipe is also a function of the "difference" in temperature between the sun warmed exterior and the water in the pipe. The greater the difference, the greater the energy capture.....so having relatively "slow" water could be a failure as the "heat pickup" gets less as the water is warmed up.....

But if you are only using the water to supply the house hot water system with just "pre-warmed water", then slow temperature controlled water needs to circulate.....I personally would not build such a system......

Any sun/solar collector needs a few safety systems for correct usage:-

1) There must be a way to stop the sun warming the water, some sort of automatic blind is needed. You will not want to start faultfinding, or leak fixing on a running system with almost boiling water (on a good hot day) that you cannot stop!!!

2) Use antifreeze in the water that runs through the external system to make sure that in winter, no damage can occur. I would use best quality car antifreeze (for engines with aluminium AND steel parts) as it is far cheaper than the plumbers sort and nobody can tell me that it is not good enough!!!! Plumbers try, but they have a vested interest.

This antifreeze also stops electrolytic corrosion in the system as well between dissimilar metals! A point forgotten by many, till the holes appear!!!

3) Make sure that if a leak does develop, the drop in pressure is noted/leak detection, a warning is given, the pump is stopped and you close off the suns energy to the system automatically as in 1) above, and the fluid leak is gathered/guided into a tank for holding the antifreeze water safely and for further use. Filter before re-using.

4) have a "standby pump" in parallel to the main pump, to allow the system to be used, whilst replacing or cleaning the main pump.

5) Install valves so that each "unit" (assuming several!) can be isolated (and if needed also removed) from the system for repair and maintenance. There are some very good inline "ball valves" at least here in Germany. I am sure where you are something similar can be found. These valves can also be used to "balance" the units in use so that water flows through all of them at about the same speed. This saves having to have a pump on each panel. The shortest "circuit" will otherwise "Hog" the water supply.....and the others may overheat.

6) Do maintenance on your system at least once a year to acquaint yourself with any leaks or aging before it becomes a problem. Working on a roof in deep winter is no fun, do it in spring or autumn on a nice dry day......

7) Build everything to a "Rolls-Royce" quality as a) you want it to work for many years b) you do not want to be repairing each every few weeks. Just good quality solid engineering, no fancy stuff!!

8) Make sure that you have remote temperature sensing for the input and out temperature of each panel, its amazing what this information can do in helping set things up.

I would design a "Unit" and test it at ground level for some months, before finalizing it.

Don'T buy those expensive evacuated tube systems, they are delicate, VERY EXPENSIVE (take years to get your money back!) and are only a few % better than a simple well made home system costing cents on the Dollar (or €) in comparison.

You may need an extra unit on your home based system in comparison, but you might build the whole system for the price of one or two of these expensive units.....

Using old radiators, with all the leaks properly welded shut (by yourself, learn if you cannot weld!) and all the old paint stripped off before re-painting with mat black paint, is a far more interesting way to go.....

I speak as though I have done this, not quite yet, but the design side is proceeding slowly!!! You have here the benefit of most of my ideas......

A nice range of ideas, too! Thanks Andy.

I am going to start experimenting with different pipes and configurations as well as different locations and delivery methods. I am lucky that I live on a mountainside and will take advantage of gravity in the first stages.

The idea of the black radiators is interesting. I think I might get some and try it out.

I love your German approach to engineering - although I am a bit too scatty to always adhere to it when I am motivated to experiement but the ideal parts are a couple of hour's drive away.

I will do a compromise...

Thanks again,

Joe

Keep us up to date when possible.....

Will do.

Currently experimenting with the 25mm black solar coil. It is producing lashings of hot water and, once I have finished another project I am working on, I intend to do another short video of the next stage - linking it up properly to the house.

Best wishes

Joe

There are some plastic piping products that may serve for this purpose- I haven't tried them yet myself, but results with other applications and spec sheets suggest they may be appropriate. I am referring to high-density polyethylene (with which I have most experience in adverse environments) and cross-linked polyethylene. I am not really clear on the differences between the two, but I believe the cross-linked is cheaper...

Hello Joe Bath,

Why not use a semi-flat tubing ie. wider than it is thick this would have more surface area than a round tubing. I would think you would have a higher heat input for the same internal volume as either of the two sizes you mention.

Any one else agree or disagree?

Charles

Agree, but where can one get it (at a reasonable price)?

As Andy has suggested, some old radiators had non-circular water channels...But I am not sure how one would orient these to maximize exposed surface area!

One of the flat sides on most modern "flat" radiators should be orientated towards the Sun.

The ones I saw years ago were mounted "in the middle" of a box that was insulated, covered with aluminum foil inside and sealed up so that warm air could not leak out easily anywhere above the level of the lowest point. Warm air rises, or tries to. The pipes all came in at the bottom, some small air leaks were always there to allow air to enter and leave if needed....warm air could help warm the rear side where the sun does not get to - possibly!

Though it may be better to lay the rad onto styro foam to stop heat loss on the back where the sun cannot get to. This makes for a thinner box.....

I have no idea personally which design is best, sorry.

Andy, did you check out JB's video, linked in #30?

Yes, but I felt that the copper tube was simply not enough "length". I would have used a proper pipe bender and made the smallest bend possible to get more tube "in the box" so to say.

Or even used a thicker tube and two 90° connectors instead of bending, more expensive, but more concentrated. On ebay, you can get job lots of fittings quite cheaply, at least here in Germany!

I would have soldered the completed tube into a sheet of copper and painted everything black.

I still feel though that the old steel radiator method, though not being so efficient as a copper one, is so much cheaper to build that you could probably add 25-35% more area and achieve the same heating effects/amount of energy.....and still pay less than half of what the copper one would cost.....I am of course talking about secondhand radiators, not new.

I do feel that one should have enough area exposed to the sun to cater for a cold day....and be able to "mask off" units from the sun on a very hot day - automatically of course.

Once I am fully retired, I intend to build a single unit for testing and calibration!! In about 2 years time.

"Yes, but I felt that the copper tube was simply not enough "length" ...."

Andy, you're getting me worried here. I just had to re-run the video, to check I wasn't dreaming or going mad (maybe I was a bit to soon changing my tag line??). I couldn't see an inch of copper tube - here's a frame which may reinforce the point:

???

Sorry, I mixed up the videos, I was talking about someone who made a box and hand bent copper tubing for it. My failure......thanks for the Pix.

By the way, the huge plastic coil is rather good, but simply does not have the sheer quantity of water for at least two people to shower effectively. (My wife needs at least 10 minutes under VERY hot water....)

What is needed is that a closed volume of water circulates through it to a holding tank, where a heat pump takes the heat out of it and places that energy in the actual hot water tank, that also has another method of heating in it as well.

The reason being that then if the solar heated water runs out, the other heating system cuts in and warms up the water.....that way they compliment each other and the solar is used up first.....I am sure such a system would save money even in winter, just by taking the chill (or better) off the mains water before heating with other methods....

By the way, I can see a need for a thermostatically controlled mixer tap for the shower, we have had one for years, they are perfect and iron out any pressure changes/temperature changes perfectly so that nobody gets a sudden cold (or hot) shower due to other water users in the house.....

Of course if the hot water just "Peters out", it cannot fix that.....!!

Nice point. Perhaps some form of black plastic bag that when filled with water lays flat and wide. But then we are getting towards a different project - perhaps a better one, but different!

Black PE pipe can be quickly and economically "flattened" by passing it through a set of rolls similar to those used for curving plate or bar. The PE will rebound but not completely, leaving you with an oval shaped tube. Alternatively, if you had an old mangle or clothes "wringer", that would also be suitable for PE up to 4.0mm wall thickness. If it is too difficult, the process is simplified greatly if the tube has been in the sun for a couple of hours.

Pete.

But wouldn't you get to a critical point where the volume was so low that you would need several times more length of pipe.

Hell! I am assuming that my basic geometry is right here...

If you crush, say, a 1m long tube to almost flat, it would have less volume than an uncrushed tube of the same diameter, wouln't it?

Help!

I suspect any flattening would be lost as the material heated up during use, based on my experience with HDPE (I have actually used a heating process to reform HDPE to a cylindrical shape after it had been stored such that it was no longer round). I'm not sure about the cross-linked PE:

Hello Joe,

I take it you want to use the tubing they use for irrigation here in Spain! Lucky for you, I get to play with that stuff quite a lot! I have found that the 63mm dia 10 atm plastic tube for the wells heats water up rapidly in the day but looses that heat quickly at night! Therefore would it be an idea to use the tube to heat the water then get that water to an insulated storage tank? The tube they use is pretty resilient to the UV of the sun so I would not worry too much about the exposure!

Just a quick foot note!

I forgot that you said 25 or 40 mm! Go for the 40mm! More water! Both of them will warm up very quickly here! Rule is, if it's black take care when you touch it because it will be bloody hot! Yesterday, I burnt my hands when I was pulling new underground cables! In the hour they were in the sun, it was enough to be painful to the touch!

Brilliant Mr T.

I like the idea of the insulated storage tank. What level of insulation would you recommend?

Also, do you think it might be a good idea to put a temperature-sensitive valve in front of the tank so that it can automatically open once the water has reached the desired temperature?

I have enough height above the house to stretch around 75 metres of pipe up the mountain in a gentle wiggle shape, so that it could be gravity fed from the cold water tank and neither need a pump nor get any air locks.

Once I have experimented with this method, I can play around with connecting it to the house proper.

That system is similar to what I have described...

It is simpler, but the hot water will only flow into the storage tank when water is drawn off.
My suggested system is a little more complex, but while no water is being drawn off the water is being pumped around the pipe to heat the water in the hot water cylinder (which is insulated of course...these normally have a jacket of 2 or 3" of insulation ..or old duvets! Or if you buy a new one they usually have PU foam on them already.)

Del

As Del said, 2" to 3" would do it! I like to use the KISS system! If it's gravity fed, put a check valve at the top of the poly pipe and one at the bottom! Only close them both if you want to change the pipe! (Maybe a good idea is to put a drain tap at the bottom and an air intake tap after the top tap to aid water removal for maintenance! The water inside the tube will be hot!) Then go straight into the KrisDel^inc solar water heater on the roof to give it a final hit before going into the boiler! Bingo, Bobs ya uncle....The perfect hotwater system for all your hotwater needs! Who needs an electric boiler when you have us lot on your side!

Looks good to me. Workbench Creations is fascinating stuff. Anyone who hasn't looked at this lot, click here.

Cheers.

Hello Joe.Bath,

I would have thought metal tube would be more efficient. You could half bury the plastic pipe in sand, it will absorb radiation from the sun and the sand then.

I would definitely go for the smaller dia pipe.

jfmfit

Certainly black metal would get hotter quicker. It would, though, not fit in to the "use what I have lying around" scenario!

Burying the plastic pipe in sand? Since (as Del the Cat initially pointed out) it is only roughly half of the pipe which is receiving direct heat (angles of incidence apart) so the other half will be losing it. It would, therefore, seem a good idea to insulate the non-receiving half.

Suppose it's a balance between effort and rewards.

One would get enough benefits in really hot climates like here in southern Spain from a really simply and low-effort pipe on the ground, especially if one was to tailor life around the times of day when the water was hot (for showering, clothes and dishes washing etc).

The returns of doing anything else from half-burying, through building a "insulated receiving & storage box", purchasing metal pipe, buying and installing insulated storage cylinder to actually purchasing a professionally-made system off the shelf would all be worth it if the extra benefits were needed.

Another idea, which I could try, is laying the pipe on top of rusty metal sheets that are lying around. That should heat up the non-sun facing sides as well. You have got me thinking now...

especially if one was to tailor life around the times of day when the water was hot (for showering, clothes and dishes washing etc).

No need! ..If the hot water cylinder is big enough you have plenty of hot water left in the morning assuming it was sunny the day before....
(no sure if it's Mony or Tuesy the day before tho' )

Yeh, with the old metal sheet idea you wouldn't need so much pipe...I like yor analysis of effort vs reward ... I shall lie down and think about it .

Del

Just making a short presentation of the creation and first use of the "solar coil".

Will post it in stages.

I think you HAVE to be right about the storage - my first analysis of the results of just 5 minutes of heating at midday sun levels is amazing!

Joe

It would have to be some serious insulation - it's bloody hot here!

Click here for the short video I have made as Stage One of my humble version of "How to Get Hot Water from the Sun" sort of thing...

See what you think.

JB

Hey...brill' video...you are having so much fun...

That's a real neat coil arrangement, with your rusty metal back plate you'll have a real tidy compact panel.

That's what is commonly referred to as a batch heater, as it heats one batch of hot water, It's quick and efficient.

Adding a tank and a pump gives a slower warm up, but a bigger quantity.

(BTW if you do go for a pump at all, an ordinary domestic central heating pump is cheap and effective...although being cast iron, I don't know if corrosion would be a prob...certainly good for a quit try out).

It looks nice out there are you offering solar workshop holidays ?

Del

There'll always be a welcome in the hillside for you, Del.

Yup, you get a ....

... from me, too.

Looking forward to the next thrilling installment!

Fishy praise indeed!