Solar pool heater data for comparison

[JohnRoss]

It would be very helpful to me if some of you good folk who use solar heating for the pool could post your heater performance figures. i.e. flow rate and temperature rise. I need the data to see if my humble efforts have been worth doing or otherwise.

A heater goodness factor (HGF) could be obtained by multiplying the flow rate through the solar heater in litres per minute by the difference in temperature between the pool and the water returning to the pool having passed through the heater.

For my own DIY pool heater I get the following figures at the moment. Flow rate through the solar heated pipes 54 litres/minute 5 degrees rise (Celsius) on the return to the pool HGF 270, 36 litres/minute 7 degrees rise HGF 252, 21 litres/minute 9 degree rise HGF 189. Pool is 75 000 litres and solar heated pipes are some 260 metres of 100mm plastic pipe painted black with non-reflective blackboard paint................JR.

[Thebiga]

HI, I'm afraid I don't have any data for you but I have been following your diy efforts for sometime and you seem to have got a very good return from what you have done. Are the pipes on the ground or are they raised. Do you have any reflective backing behind the piping.

I was wondering about doing something along the same lines but maybe in 50mm pipe as this is the same size as the piping for the pumps and the fittings would all then be a lot easier to do. I also think that with a smaller pipe that would also get a higher temperature as the water flowing through would be more compacted with the smaller pipe I know in theory that the water would be travelling faster (I think) but if the flow was lowered maybe . What do you think? How do you adjust the flow rate on yours? Do you have one of the large red shut off valves and you just turn it a certain amount to lower the flow rate and open it right out for full rate?

Thanks 

       

[JohnRoss]

Hi thanks for coming back as I was feeling lonely! I have asked this question on other sites but nobody seems to know what their flow rate and temperature rise figures are and without that data how do you know how well or otherwise your system is doing? However to answer your questions, and I will keep it as short as possible but will amplify on my thinking if asked and do bear in mind that I am just an enthusiastic amateur, I would not use 50mm pipe for the heat receiving system for several reasons.

In no particular order:

 1/ It is almost as expensive as 100mm pipe and has less heat pickup area compared to a larger diameter. I use 100mm for the heat collecting pipes as it is about the cheapest per square metre of heat collection surface area. I run my pipes in a zig zag in 20 metre runs parallel to each other and about broadside on to the sun at the solar peak i.e. East West. I guess that about one third or slightly less of the pipes surface, say the same as the diameter 100mm, picks up useful heat. One long run, to, and then another back again, fro, would be better as fewer expensive 90 degrees elbows would be used but my field is not long enough in terms of unshaded area. At the moment I run the system between say 11. 30 am and 5.30pm local time, 9.30 to 15.30 GMT or longer if the pool is in use and the total useful area of exposed pipe surface is about 50% of the pool surface area. If I had used 50mm for the whole run then I would have needed much more than the 260 metres of 100mm pipe that I have at the moment to get the same pick up area! 

2/ I do not want to increase the load on the pump by using a smaller diameter, more side wall drag on a long run, although only part of the pump output passes through the pipe. I have a second output on the Desjoyaux system onto which I have fixed a T junction, it takes a 40 mm fitting, and the other end of the T has a screw fitting, tampon de visite, which takes 40mm screw blanking caps of which I have several with either no hole or different diameter holes so I can regulate the flow through the solar heated pipes. This is a by-pass system if you like for the solar pipes and reduces rather than increases the load on the pump. The body of the 40mm T converts to 50mm pipe which runs, wrapped in polystyrene sheet a short distance under the lawn and emerges in the field where it converts to 100mm, same thing in reverse on the way back but returning to the pool in 50mm. The main part of the pump output is unaffected and returns straight back to the pool. There is no sand filter and all is contained in a block mounted on the side of the pool including pump, filter bag, electrolytic cell, skimmer and steerable output nozzle under the surface which gives some back pressure to help the flow in the solar pipes.

3/ My figures and what I have read indicates to me that a high flow rate in a large diameter pipe and a small temperature rise is better than a low flow rate and large temperature rise for pool heating, the reverse for house hot water heating. A small temperature rise must reduce re-radiation from the pipes so glazing is not needed. What you are trying to do is raise the temperature of a large amount of water by a small amount and not the reverse! I cannot stress enough the importance of making sure there is no air trapped in the system. I have put air bleed valves at the highest points on my pipe run, easy to make with a clip on tampon de visite, and check the pipe by touching to see if there are any hot spots after filling which would indicate air pockets which would impede the flow and reduce efficiency. The pipes are on bricks every two metres, or maybe a bit less, to stop them touching the ground and sinking in and losing heat when the ground is wet, easier to spray the weeds as well. If the ground under the pipes were painted white this would help by reflecting heat but I have not done this yet. I should say that you chuck away a lot of water when draining the pipes for winter but it is only once a year. A parallel fed grid might be better but more expensive as one or two manifolds would be needed and Ts and reducers all put up the cost. I will stop now as don't want to write a book here but do come back if you have questions or pm me if you wish. Hope this helps you....................JR

PS One thing I should have mentioned is that I don't know how long the pipes and the joints would last if you did not have water flowing when it is sunny. Ok for a few hours as there is a large thermal mass with all that water but I have found that it gets almost too hot to put your hand in it, say 45 degrees Celsius plus, if I have been out for more than a few hours with the pump off. Not a problem for us as we need to run the pump to produce chlorine, salt water pool and electrolytic cell, but if you went on holiday you would have to cover them with white sheets to prevent over heating! At the moment I have the flow through the pipes on minimum as the pool is at 30 degrees already, despite air temperature being as low as 8 degrees last night, and although the water is returning at near 40 degrees this is less heating than if I used a higher flow rate with a lower temperature rise. i.e. a product of 189 instead of 270 which I would get with my maximum flow.

[Thebiga]

Do you cover the pool at night as due to the shape of ours we cannot cover it and would lose heat at night and wondered whether the pipes would be able to put in more than what it would lose? .

[Dave&Olive]

hi ok

           you could always heat it   " THE RED NECK WAY " 

             http://www.youtube.com/watch?v=qvTq92a8A3E&feature=related

               Dave

[JohnRoss]

Yes we do and I doubt if the pipes could compensate for the loss at night if we did not cover it. Of late with clear skies it has gone down as low as 6.8 at night and I suspect the loss would be greater than the input during the day. We keep the Summer cover on all the time the pool is not in use and it keeps out some of the leaves and bird poo etc.........................JR 

[Poolguy]

Hi John Ross

I'm certainly interested in your experiment even though, as a group of products I know very well the performance of the heating system you are trying to construct.

Your question of comparative analysis although a good one, is very difficult to answer as pool heating depends on so many variables. Firstly no 2 pool are the same even if they are built by the same person in the same way; location and things like exposure are very important, as 60% of the heat losses from a pool are from transpiration from wind chill and evaporation from radiation. So from a pool based result there is little I can offer you.

However as an internal system performance based analysis, there are a few numbers which are easier to extract; such as heat exchanger. Now in your system, the whole array is a heat exchanger as you are running pool water directly though it. The fact that its built out of Polypropylene (or another plastic) is its primary problem as this material is extremely poor at exchanging heat, hence you need a considerable area of 'mat' to get any significant temperature exchange, between 60% and 100% of the pool area in fact. Improvements to that system are to be found in the choice of material rather than flow rates or other design.

Modern metal heat exchangers are not operating on the same idea but rather bringing together 2 separate circuits to undertake the exchange in a metal cylinder (I am sure that you know this). There are various forms ranging form the stainless steel versions, which can boast only moderate performance of about 6-8°C, exchange over its length up to the copper/nickel composites which can double that at up to 15°C over their length. However, recently released is a version that can improve that yet again as an exchanger inside a pressure vessel, promising performance approaching 25°C exchange or better.

None of the technologies above can match your frugal economy however,  but I'm supposing that you want to compare performance and price in some sort of cost: benefit analysis. From my position, unless it’s a pleasure to tinker, I'm thinking that most pool owners want pool heating as efficient as possible and trouble free as possible. Certainly that's the messages I get from the market place. 'Make my pool warm enough to swim when I want - that's all I want;'

 As you know, I am sure that none of the above is possible without an effective insulating pool cover. Minimum R rating should be 0.075 (kw) there are available up to 0.13 (kw) at present. Be advised that the majority of the ‘bubble’ summer covers available in France do not reach even the basic standard for insulation. This means that gains made throughout the day cannot be maintained and so more added the next day doesn't increase the result (cool pool)– with an effective thermal cover the contrary is true.

Andrew

 

[JohnRoss]

Yes and all very interesting and no doubt very true but all I am looking for at the moment is flow rate through what ever type of heater used and temperature rise between the input and output of the heater and I would have thought that would be as good a measure of heater performance as anything else. How the heat is retained or otherwise in the pool is beside the point as are details of the type of heater used  though no doubt very important. I find it easy to obtain these figures. All you need is a bucket of known capacity, a second hand on your watch, a thermometer and access to the return pipe from the heater to the pool. I can only assume that it is this last item that may be preventing folk from making this otherwise simple assessment. Do you have the figures I seek Andrew, anyone?....................................................JR

[Poolguy]

John Ross

Flowrates

Akvaterm heat exchanger - 35lt/min

Bowman heat exchanger - 48lt/min

Navitron Plate Exchanger - 54lt/min

Hope that is what you wanted

Andrew

 

[JohnRoss]

Thanks Andrew and yes helpful if you have and can give typical temperature rise input to output for each of those flow rates under clear sky sunny conditions and with a stated area of  panel attached and operational on the primary side of the heat exchanger..............JR

[Poolguy]

John Ross

 

No I can't give you what you want unless you can tell what is 'typical'.

I can however give you 'theoretical' energy tranfer rates at specific temperatures, - I'm not sure that is what you want.

Akvaterm - 35lt/min  90kw  @ 50°c  25°C exchange over 750mm

Bowman - 48lt/min 33kw @ 50°C    14°C exchange over 572mm

Navitron  - 54lt/min 19kw @ 50°C    8°C exchange over 440mm

Note that the last two exchange flow rates refer to the solar circuit/ whereas pool flow is nominally at 190lt/min

 

I hope that this helps more (although I don't yet see how it will help you)

Andrew

 

 

[JohnRoss]

Hi Pool Guy and I appreciate you trying to help but I am having trouble understanding this data. Does it mean that, for example , in the case of the Navitron when the input to the exchanger of length 440mm is 50 degrees Celsius the heat exchanger increases the temperature of the water coming from and returning to the pool by 8 degrees Celsius at a flow of 190 litres/minute. This would give a heater goodness factor of 1520, can this be right? Surely not!....................JR

PS I am not trying to wind you up here, I really want to know and thanks for taking the time to come back on this.

[Poolguy]

John Ross

Sorry I don't have time to explain the physics or debate the efficacy of manufacurers theoretical performance figure 's.

Here is the an extract from the Navitron website explaing the calculation to heat exchanger performace figures  (good luck)

If you wish to explore this further, the following equations may be of help in calculating the heat exchanger performance:   The rate of heat transfer Q = U x A x dTmean = kW U (kW/m2 K) = 1/Rt  and Rt  = total thermal resistance (m2 K/kW) A = surface area (m2) dTmean = True mean temperature difference (K) The temperature of both the primary (Th) and the secondary (Tc) fluids will generally vary as they pass through the heat exchanger and you need to know the true mean dT which is not the same as the dT between the boiler/solar flow and the pool water.  The heat transfer will also vary depending on whether it is a parallel or counter-flow heat exchanger and you may need to calculate the log mean temperature difference (LMTD) depending on the resulting dTmax and dTmin. For a parallel flow heat exchanger dTmax= Th1-Tc1 and dTmin= Th2-Tc2 For a counter flow heat exchanger dTmax= Th1-Tc2 and dTmin= Th2-Tc1 Th1 / Th2 = Primary (boiler/solar side) inlet/outlet temp. Tc1 and Tc2 = Secondary (pool side) inlet/outlet temp. LMTD dTmean= (dTmax - dTmin) / In(dTmax / dTmin) If dTmax=dTmin, then LMTD dTmean = O, so therefore the arithmetic mean temperature difference is used

(ed Navitron.org.uk)

 

Andrew

[JohnRoss]

Ok Andrew I will ask them, surely they would have the figures I seek!...........................JR

PS Thanks for your valuable time

[JohnRoss]

I have emailed over a dozen suppliers of this sort of pool equipment i.e. solar mats and only received one reply that really attempts to answer my questions. A few have replied by reproducing manufacturers data which does not answer the question so I guess they don't know which I find suprising to say the least. If I get more info I will pass it on if anyone is interested. 

 

From what they say 75 000 litres passing through panels of 25 square metres in 8 hours gives a flow rate of 156.25 litres/minute and a product at 1.2 degrees Celsius rise or heater goodness factor of 187.5. It is this last figure I wanted and on these figures my system seems to be doing quite well. They have also suggested that with 35 square metres a temperature rise of 1.7 degrees Celsius, input to output,  would be obtained which, if the benefit is proportional, would give a goodness factor of 262.5. I still need several more examples to come to any firm conclusions. I suspect the figures they have given are for Southern UK and an average for temperature rise over an 8 hour period so some correction factor needs to be applied to allow for the fact that we are further South and that my figures were taken at the solar peak.....................JR  

 

[JohnRoss]

Having looked at American pool sites I have done the following calculation.

My problem is that all I have is my experimental results obtained simply by measuring the flow rate with a bucket of known capacity and a stop watch and a thermometer. I have guessed that about a little less than a third of the circumference of my 4 inch (100mm) diameter pipe picks up significant energy from the sun and I have taken readings at the solar peak when the pipe is broadside on to the sun which is when, I assume, the energy picked up should be at a maximum. From this I deduce that the area equivalent of my DIY system is about 280 sq feet (26 square meters), i.e. a tad more than half the surface area of the pool.

American flat plate manufactures quote performance figures typically in term of BTUs/square foot/day and whilst it might be possible to convert my figures to such I must confess I am at a bit of a loss to know how to do that, just getting old and silly I guess, so I am finding it difficult to make comparisons. Any suggestions?.................................JR

PS If I take 1000 BTUs/Sq foot/day as a typical flat plate collector figure and assuming a day when there is a reasonable amount of sunshine is 8 hours then this equates to 2.08 BTUs/sq foot/minute. Taking my own reading of 14.04 gallons/minute or 112.32 pints/minute and a temperature rise of 9 degrees F gives me 1010 BTUs/minute over 280 square feet gives me 3.61 BTUs/sq foot/minute. However my figures were taken at the solar peak and near the beginning and end of an 8 hour period the temperature rise obtained would be less so an average BTU input would be less than the 3.61 BTUs calculated. So my system will not be as good as it appears to be at first sight. A comparison therefore is still not really possible or have I got my maths wrong which would not surprise me in the least?