DIY swimming pool experimental solar heating part 2

[Jackie]

Further to my last post on this subject I now find that the pipe system still produces an input of plus 4 to 5 degrees Celsius at 36 litres/min at the solar peak. Sadly this is only just keeping pace with the loss at night so unless we get a really sunny 4 or 5 days I don't think there will be any overall gain. Night time temperatures have been very much lower of late, as little as 5 degrees which does not help with the  loss during the night. With the sun now lower in the sky the orientation of the pipes seems more critical producing a maximum temperature rise when the pipes are at 90 degrees to the sun. Ideally running East West though in fact they are more North West and South East  facing South West and therefore are at 90 degrees to the sun in early afternoon.

Metal fins glued onto some of the pipes with thermally conductive epoxy resin get quite hot which is a bit disappointing as the pipe they are fastened to runs cool and therefore one must assume not much heat is being sucked out of the fin into the pipe. A larger area of contact helps but given the difficulty of fastening a flat object to a curved surface and the cost of the resin I think funds would be better invested in a longer run of pipe. I will consider extending the pipe run with pipe running East West in the Spring. Meantime it will be soon time to close down the pool for winter. I write this only to keep anyone following my humble experiments informed....................John

[Maricopa]

Hi Jackie

I think the fins on the pipe will COOL the pipe, not add to the heat,  Heat sinks on electronic components consist of a series of cooling fins, they increase the surface area and disipate the heat.[:(]

Keep us posted, I find this subject very interesting[:D].

 

[Jackie]

Hi and yes they do in the case of electronic components like semiconductor power devices which are warmer than the heatsink. But in this case the pipe is cooler than the fin so the direction of heat flow will be from the object at the higher temperature, the fin, to the object at the lower temperature, the pipe. The problem is with a small area of contact and the pipe being plastic and not too good a thermal conductor heat is not flowing fast enough from the fin to the pipe. The one fin I glued on with a greater area of contact does not get as hot as the others. This used rather a lot of the thermal epoxy resin to do it however...........................................J 

[Albert the InfoGipsy]

I can't help thinking that a big sheet of metal painted black on both sides and just laid directly on the pipes might help. You would get a little bit of conduction at the point of contact but also the air under the metal would probably heat up more. Filling the space under the sheet with water might improve matters too.

All of this is a bit of a bodge because, essentially, every time I think about what would improve your system's performance I come back to a fairly conventional flat plate design with the pipes replaced by old radiators and sheets of glass on top.

By the way, you'd be better off continuing your original thread because I (and most other readers probably) can't face going off to look for it when I need a bit of background to what you're doing.

[Jackie]

Ok and a ref for the thread is http://www.completefrance.com/cs/forums/1337731/ShowPost.aspx

Rightly or wrongly these are my thoughts. What we want in a pool heating system is a large quantity of water heated by a small amount whereas in a house water heating system we want a small amount of water heated by a large amount. Glazed solar collectors are most suitable for the latter as the glazing reduces re-radiation losses from the surface of the collector which is at a relatively high temperature. Where the collector is at a relatively low temperature re-radiation loss is minimal. Further a system using old radiators or metal pipes secured to a metal plate cannot have the pool water passing through it as the chemicals in the pool water, in our case salt and chlorine, would attack the metal. Therefore a heat exchanger and secondary circulating pump would be needed.

Placing a metal plate on top of the plastic pipes I am using would prevent direct radiation reaching the black surface of the pipes and one would be relying on re-radiation from the under surface of the metal plate and somewhat iffy contact between the plate and the pipe, also sheet metal seems quite expensive when bought in small quantities.

I have already tried the pipe buried in black concrete technique which did not work as well as the open trough I described in my earlier post. Immersing the pipes in water might give the potential for a large collector area but there is still the inefficiency of the heat exchange as well as the practical problem of stagnant water and the muck there in like dead leaves, worms etc. I don't think increasing the thermal mass of the system further than it is and in my opinion may already be almost too much, 184 metres of water filled 100mm pipe currently, would make for a responsive system. The commercial black mats with their small bore pipes must be better for that reason.

What I am trying to do is develop a system that anyone could build with ease from components available cheaply at any brico depot. I am not trying to compete with commercial kit which is great if you can spend the cash.

However I must say that it is helpful to have comments like yours if only to make me re-examine my own theories so thanks...........J

 

[Albert the InfoGipsy]

The thing about the volume of water v the temperature rise required is actually a bit of a red herring, in my view. If you want to heat 1,000 litres by 50 degrees or 10,000 by 5 degrees you still need the same total heat input. I think you should also look at the maths behind Newton's law of cooling and experimental results for environments with and without airflow (wind in this case).

I applaud your attempts to find other ways of making your heating system more efficient, because without experiments we'll never move on. However, I'd say that glass is a component readily available at a brico and it is of known value in this application. It certainly makes more sense to me than using fiddly and expensive methods like epoxy gluing metal fins to plastic pipe.

You only need to get in a car that's been sitting in the sun to realise how much difference glazing makes. Just keeping the wind off the surface of the collector makes a big difference and when you add in the greenhouse effect it becomes even better.As an experiment try putting a 10m loop of pipe into your car and measuring its input and output temperatures. Then do the same thing with the loop outside on the ground.

 

[Jackie]

I would agree that glazing will protect the solar collector, pipe in my case, from heat loss due to wind but unglazed solar collectors have potentially high efficiency by comparison to any other type when each is used in its intended operating temperature range i.e. only a few degrees above the temperature of the pool. The elimination of glazing and case reduces the losses of incident energy otherwise caused by absorption in and reflection by the glazing, and partial shading by the case. The question is however academic in that glazing 184 metres of 100mm (4 inch) pipe is just not on.

The pipe array could be fitted into a smaller space but only at extra cost as 90 degree bends are expensive and you would need a lot of them. Smaller bore pipe would give a smaller surface area to collect radiation, hence the plastic web between the small bore pipes used in commercial mat type pool heaters. A small bore system would need a separate pump to give a reasonable flow rate which is not possible on our system, with less than 50mm pipe, using a part of the pool pump output saving extra electricity running costs........................................J 

[chem geek]

Jackie,

Your analysis is excellent, but note that the efficiency of unglazed flat black plastic mat solar collectors at around 80% and glazed flat panel collectors at around 74% aren't that much different and that this is only under the ideal conditions of no temperature difference between average water temperature in the panel and air temperature (or a small temperature difference, but no wind).  It's when you get to the evacuated tube collectors with a gross area efficiency of 41% where these are much more appropriate for solar domestic hot water heating due to the much higher temperature differences and, as you point out, smaller water volume to be heated so base efficiency isn't as critical.

I also want to point out that your pool itself is a solar collector (assuming it is exposed to direct sunlight and skylight).  A white plaster pool will absorb around 60% of the sun's energy (a darker pool absorbs even more).  Nearly 25% is absorbed in the first inch near the surface while 40% is absorbed in the first foot.  This is because about half of solar energy is infrared and water strongly absorbs infrared.  This also explains why the surface of pool water is much warmer, especially if the circulation is poor as with many above-ground pools without floor drains.  About 56% of heat loss is due to evaporation, around 26% by radiation, and around 18% by convection so use of a clear pool cover that is transparent to infrared and visible light will significantly help heat a pool and if it's opaque to UV it will prevent chlorine breakdown from sunlight.  If it is a bubble-type cover that is well insulating (low thermal conductivity), it will keep most of the heat in the pool at night.  It is, by far, the most cost effective way of (partially) heating a pool or at least limiting its heat loss.

The following table is an expanded version from another thread and shows how the efficiency of different panel types drops with increases in the temperature difference and at different amounts of sunlight.  This table was constructed from the manufacturers' own efficiency data over gross panel area (which is the actual space it takes up).  Also note that the flat black data is for relatively low wind of 3.9 km/hr.  [EDIT] All designs assume water directly heated in the panel; if a heat exchanger is used then the efficiencies are lower by a factor. [END-EDIT]

                  MOSTLY CLEAR AND SUNNY (800 W/m^2)               CLOUDY OR OVERCAST (300 W/m^2)

deltaT(ºC)    flat black    Gobi glazed    evacuated tube              flat black    Gobi glazed    evacuated tube

    0                  80%            74%               41%                           80%            74%               41%

    5                  68%            71%               40%                           46%            66%               39%

  10                  55%            68%               40%                           13%            59%               37%

  15                  43%            65%               39%                           N/A             51%               35%

  20                  30%            63%               38%                           N/A             44%               33%

  25                  18%            60%               37%                           N/A             36%               30%

  30                    5%            57%               36%                           N/A             28%               28%

  40                  N/A             51%               35%                           N/A             13%               24%

  50                  N/A             46%               33%                           N/A             N/A                19%

  60                  N/A             40%               31%                           N/A             N/A                14%                

  70                  N/A             34%               29%                           N/A             N/A                  9%

[EDIT] A graph ("Collector Efficiency") showing the above (for the sunny case) is shown in [url=http://www.builditsolar.com/References/Measurements/CollectorPerformance.htm]this link[/url]. [END-EDIT]

From the above table you can see why the evacuated tube makes a lot of sense for domestic hot water heating in most climates because the requirement for water that is far hotter than the ambient air temperature means that the deltaT is large (especially in the winter).  The Gobi-style flat glazed panel is best for heating swimming pools in most situations.  The flat black plastic mat is only best in very warm climates (or during clear, sunny warm days).  Of course, the Gobi-style flat glazed panel costs around 3 times as much for the same area as the flat black plastic mat and the evacuated tube costs almost twice as much as that.

As for your DIY situation, a clear bubble-type pool cover is most important.  As for the solar heating, achieving some sort of greenhouse effect to insulate your absorber (black surface) from convection (wind) would be very helpful.  Just try and use a clear material that transmits most visible and infrared light so that you maximize efficiency (I believe this is why glass is generally used rather than plastic).  I understand that you say that glazing is not practical with your setup, but I wonder if even some sort of wind shield (on the sides) would be helpful.

As was mentioned in an earlier post, the idea that a slower flow rate producing a higher temperature does not mean that this produces more heat.  In fact, greater efficiency occurs at faster flow rates since this makes the deltaT lower so trade that off vs. the electricity costs for your pump speed.

Richard

[Jackie]

A most interesting reply, thank you Richard. Yes we have noted that the pool water rises in temperature much more quickly with the floating cover removed. I note your comments about a clear bubble cover and will consider one when the current one, black on the underside and dark blue on top, wears out.

I don't think we are going to get anymore use out of the pool this year sadly so no more experiments for a bit. Air temperatures quite low here at night now. Wind screening on the pipes could be achieved by letting the grass grow in the field around the array of pipes so I may be more selective when mowing.  Lots to think about however....cheers...........J

 

[Jonzjob]

Please tell me that this is a wind up?[8-)][8-)][:D]

[AnOther]

I had a flash of inspiration yesterday.

I have 110,000 BTU paraffin/diesel fired space heater and it ocurred to me that if I constructed some sort of steel heat chamber to pass water through whilst directing those 110,000 btu's at it it would likely prove quite fast and efficient at heating my pool.

OK it's not a zero cost option but if it makes the pool useable then I can live with the cost of a few litres of paraffin, or petrole as I believe it's known here.

Definately going to do some experimenting when I get back from my weekend jaunt.

[Jackie]

If you are serious Ernie it might work. I have an old log burning stove, having had it replaced recently, and a large number of fir trees that need shortening so plenty of wood for next year should I use the stove to heat the pool water. Problem is the metal chamber proposed might not like the pool water with, in our case, salt and chlorine so a circulating pump and heat exchanger would be needed. What to use as a heat exchanger, a length of acier inox? Would the pool chemicals attack it I wonder?..........................................J 

PS Current price of 20 litre paraffin bidons at Bricomarché just over 26 euros so not cheap! 

PPS It would only be a wind up Jonzjob if I suggested using my atomic egg to heat the water! Come to think of it that ain't a bad idea!!

[cookehouse]

UNBELIEVABLE someone else tryng to make a DIY solar panel for a pool! Its criminal to use anything other than free, green oh and did I mention free energy.

This year, I used 'solar rings' (blow up giant polo mints but I didn't think they were very effective.)

I then made a timber box, layered insulation, a sheet of corrugated iron painted mat black with black painted copper tubes in the 'valley' of the corrugation. To get a good contact between the tubing and the iron I drilled small holes and threaded copper wire through pulling and twisting to hold the pipe tight to the sheet. I then covered the box with perspex.

The water supply was a partial bipass of the circulation return - so no extra leccy.

The results were encouraging so mark 2 is now on the drawing board.

As with you, experimentation has now stopped - cold stopped play - I will however be preparing for the spring by laying down some insulated pipes to the 'laboratory' (bottom of the garden) as one of my problems was losing the heat from the returning pipework.

I have no doubt it will work all I have to do is make it effective - not necessarilly efficient oh and of course it must be a work of art - I paid enough for the pool I dont want it look like Steptoes back yard.

Good luck with your future trials

[Jonzjob]

Don't forget that the pool water will corrode the copper pipes Cookie! That's why I paid for a stainless heat exchanger to go with my evacuated tubes...

[chem geek]

I have an update to the efficiency comparison of different panel types.  Actual panel efficiencies were measured for black flat mat panels for pool applications [url=http://www.fsec.ucf.edu/en/industry/testing/STcollectors/pool_ratings/index.htm]here[/url] and for flat glass-enclosed and evacuated tube panels for hot water [url=http://www.fsec.ucf.edu/en/industry/testing/STcollectors/hot_water_ratings/index.htm]here[/url] where the best black flat mat panels had around 1000-1060 BTU/sq.ft. while the best flat glass-enclosed panels were around 850-960 BTU/sq.ft. while the best evacuated tube panels were 650-740 BTU/sq.ft.  This makes the best case relative

efficiency to black plastic mat panels: 100% to 91% to 70%. My original

table at the start of this thread has implied relative efficiencies of

100% to 93% to 51%.  This difference is probably because the best evacuated tubes are better than the Navitron I used for the table.  A high efficiency tube from Sunda Solar with 739 BTU/sq.ft. is described [url=http://www.sundasolar.com/product_seido1%20series%20collector.html]here[/url] with a gross area efficiency of around 54% (compared to 41% for the Navitron) for a relative efficiency of 68% which is much more consistent.  The above links to BTU/sq.ft. data are with actual field measurements in strong Florida sun.

Using the more efficient Sunda Solar as the evacuated tube example would change my table values in the 800 W/m2 case going from 54% to 42% while in the 300 W/m2 case it would go from 54% to 21%, so basically if you just add 13% to all values in the evacuated tube columns in my table, then you will get appropriate comparisons when using one of the best evacuated tube panels.  The main points in my charts that show how evacuated tubes get better when the temperature difference is larger, as with hot water heating and cold days, and also are better on cloudier days still applies as does the relative performance of the three panels.

One should also keep in mind that practical considerations may make a situation where the evacuated tubes should do better than flat glass-enclosed actually turn out to be worse as described in [url=http://www.solarh2ot.com/images/Performance%20-%20Flat%20Plate%20vs%20Vacuum%20Tube.pdf]this report[/url].  The evacuated tube panels in this case are actually "too good" so they don't melt the frost/snow on the panels as quickly thus preventing as much heating as the flat glass-enclosed panel.

Richard

[AnOther]

But can you swim in it yet [6]

[chem geek]

I just have those relatively inexpensive black flat mat panels so haven't even turned on the solar yet (though will do so soon) so my pool is only 52F (11C) [:(].  Our swim season doesn't usually start until around May, though I'll fire up the gas heat (plus solar heat first) to get the pool usable for April.  My wife uses the pool for swim therapy so it needs to be warmer at 86-88F (30-31C).

[Théière]

Hi Chem geek,

So " The above links to BTU/sq.ft. data are with actual field measurements in strong Florida sun" heating a pool in summer when we don't really need it [:D]

What we really need is the data for the above at the early and late season so we can work out if the swim season can be extended and at what cost.

 

 

[Dog]

Crumbs all a bit technical - I just got 4 long strong hosepipes painted them black and coiled them up on some pallets plumbed into pool pump.

Been swimming every day since February.

[chem geek]

[quote user="teapot"]

Hi Chem geek,

So " The above links to BTU/sq.ft. data are with actual field measurements in strong Florida sun" heating a pool in summer when we don't really need it [:D]

What we really need is the data for the above at the early and late season so we can work out if the swim season can be extended and at what cost.[/quote]

The problem is that you can't easily "control" cloudy weather in a field test so that's

why they use the full sun tests in the summer.  However, they DO additionaly measure the

efficiency in simulated sun and shade conditions (with carefully measured light intensities) which is where they

get the efficiency numbers in the spec sheets and you can use those to

create the tables I gave and that does tell you, via efficiency, how

much heat you can expect to get in different conditions.  It's a bit

tricky to figure out, but if you look up your solar insolation typical

for your area for the time of year you are interested in and apply the

efficiency numbers, you can get a heat output.

However, this won't tell you the heat loss from the pool and you need

to know that to know if you can "keep up" with such loss.  So the above

efficiency numbers are useful to tell you if the panels will be better

than nothing and by how much, but it won't tell you what temperature

you will be able to achieve in your pool, which of course depends on

factors such as whether you use a pool cover.  [url=http://eosweb.larc.nasa.gov/sse/RETScreen/]This link[/url] will tell you the solar insolation anywhere in the world for anytime of year, but it's for a sunny day so is lower in the winter due to the sun being lower in the sky (passing through more air).

The only reason I posted the real-world numbers is that sometimes

people believe the laboratory efficiency numbers are just theoretical,

but in reality they do translate to what happens in the field -- at

least for amount of output for a given input (sunny vs. cloudy) and

temperature difference (air vs. water).  It also had me figure out that the Navitron I used in my table as a basis for comparison was not the best choice (most efficient) of the evacuated tube technology.

Richard