Running a pool is costly, of up to £1000 or more per year to heat. Have solar will reduce the cost by 50%.
It will still be beneficial to have solar, but you would require fewer collectors as the pool will already get 10% gain from the sun rays. When sizing the systems it is always sized on the surface area of the pool, and this depends on required pool temperatures. Rule of thumb - 60% for outdoor pools and 70% for indoor pools, but remember and indoor pool temperature has to be 2° less than the ambient air temperature of the building. You can juggle this by using the solar to also heat the ambient air temperature via a themal store or radiator system.
If you do not use your pool in winter you can just turn off the solar as the expansion tank is capable of taking up the fluid expansion. As we always use a contactor if there is solar gain in the winter, the contactor will activate the swimming pool pump to switch on to releave the heat away from the swimming pool heat exchanger. Even during the summer this should always be operational, in conjunction with the solar pump, or otherwise the plastic pipe work could melt.
We would always install flat plate collectors to help reduce the cost, as these are cheaper than an evacuated tube, unless the customer requests otherwise. Installation costs for a 6 x 10 outdoor pool costs around £7000 installed.