Immediate lessons on the first day:
The connectors are indeed prone to leaks. Bad.
The return header is nicely warm to the touch. It works!
Above photo shows an extra 12 coils added in 2014 to make a total of 26, together with an overview from Google Earth.
I suspect that it would be better to swap out the rubber grommet connectors for screw-threaded PVC connectors that can be both screwed in directly and cemented into the headers. This may require a choice of 2" headers rather than 1.5" headers that I used here, a minor change. If the screw+cemented connection is as robust as I expect, then a better design would probably have straight runs of agri tubing instead of coils. Straight runs have the advantage of much less waste of space and very low pressure drop, but the disadvantage of having a few hundred connections, instead of the 52 I used. Only catch: I have not managed to find a local supply for screwed connectors, nor a cheap enough source for internet orders. One is looking for a cost of not much more than a US$ each if we use straight runs that call for several hundred connectors.
My objective was to keep the pool at a temperature of 25deg C or higher for afternoon swimming throughout winter. The return valve from the filter to the pool had to be shut slightly, say, at 2/3rds open, to build up a few psi back pressure that is enough to send ample flow to the coils. When the sun shines the return 2" pipe from the coils back to the pool is distinctly warmer to the touch compared to the cool water from the pool and filter. When you put your hand in front of the inlet to the pool, the incoming flow feels distinctly warmer than the general temperature of the pool. Hence the flow is ample enough to collect all the solar heat generated, without being too low that the return pipe feels "hot", leading to possible heat losses. The kids prefer to swim when the house throws shade onto the pool, i.e. in the afternoon. Omanis prefer shade to sun at any time...With the 26 coils, 97.5 sq m collection area and a pool surface of circa 70 sq m, the pool water measured between 24.5 and 26.5 deg C at Noon during December and January (78 deg F). In brief, kids like to swim on warm, sunny days; and the water is warm enough for energetic swimming on those days. Lazier swimmers prefer even warmer water and I would urge others contemplating a similar project to be very aggressive in allocating solar collection area, at LEAST equal to the area of the pool. Why refer to the area of the pool rather than the volume? Because most of the chilling of the pool water comes from evaporation rather than heat losses to the ground. The longer days in February nudged up the water temperature to between 28 and 29 deg at midday (83 deg F).
A few pointers for a new project in a climate similar to Oman:
1. Allocate a solar collection area at LEAST (!!!) equal to the area of the pool, but preferably up to 2x larger. If it is twice the pool area you can expect to heat the pool by as much as 4 degC on a sunny day in January. People who live in a warm climate like Oman like their pool water to be very warm and I have yet to hear anybody actually complain of a pool being too warm. So, if you have the area available for the collectors, go for as much as 3x the pool area!
2. Use straight runs for your agri-tubing collectors, rather than coils; nil dead space between coils, and with the connections threaded and cemented to the headers, there should be a minimum of leaks. I.e. if you can source reasonably-priced threaded connectors. Otherwise use coils like I did.
3. Straight runs imply a massive number of connections and I would stay away from the rubber-grommet squeeze-in type. Use three 2" high pressure PVC pipes as headers for inlet and another three 2" headers for the outlet side. Use drill/thread/screw-in and PVC-cement connections to avoid leaks. You need multiple headers at each end so that you can install an agri-tube every 15mm. Three headers allow you to space the holes in each 2" header at 45mm center to center.
4. Cost as at end 2014? Allow R.O.15/- (=$39) per sq m of solar collector; for locally manufactured agri-tubing, connectors, headers and mild-steel hold-down frames and also labour. During an average January in Oman that square meter will be saving you 85 kWh that would otherwise be consumed by a normal, electric heating element pool heater (say, about R.O.2/500 per month/sq m of collector). Payback in a single season.
5. If space is an issue then do consider laying solar heating beneath the pool surrounds. Dark tiles used on the pool surrounds ought to transmit heat reasonably well to piping laid underneath.
6. If you use the rubber grommet connectors then you should expect minor leaks every now and again, but these are often easily stopped by jiggling the connections or simply replacing the grommets with new ones. You can do that even with the pump running. Consequently do make sure that the connectors are positioned for easy access.
7. Even if you end up installing an electrical pool heater, do install as much solar heating as your available area allows. You will still end up saving a lot of electricity from the boost given by solar heating. The cheaper resistive heaters (those with heating elements) are horrendously expensive to run; so any electrical heating ought to use a heat exchanger (5x the efficiency). You pay much more up front, but longer term you will be better off.
8. If you are building a new house with a pool, it is important to realize that most fancy houses in Oman (and similar climates) have at least one air-conditioner running all year. The crazy thing is that the air-conditioner uses a fan in the outdoor unit to cool (!) the air-conditioning gas-liquid. That is wasted heat that can easily be sent to the pool and allow the outdoor fan(s) to switch off and save electricity. All you need is a couple of Ts on the high-pressure coolant gas lines to send the hot, compressed liquid through a coil immersed in water circulating from the pool. Ergo. Save electricity, make your air-conditioner more efficient and heat the pool a bit more. Put the outdoor unit of the air-conditioner at a location where you can easily route your pool water to. I am surprised that not many people seem to make use of this. Have a look:
http://www.hotspotenergy.com/titanium-pool-heat-exchangers/
I kept the solar heating going on into spring/summer, trying to find out when is the pool water too warm. Well, at 33 or 34 deg C (93 deg F) it was nice and pleasantly warm as you enter the pool, but after some 15 minutes' swimming it does feels rather too warm. Hence I turned off the heating mid April, to be switched back on mid October.
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