Of all the pool heating systems available, only solar energy and heat pumps are legal in Spain. Oil and gas heating are illegal for swimming pools and would be very expensive to run. Electric heaters are illegal and very few houses would have the supply to run them.
Solar energy and heat pumps are not very expensive to install. The disadvantages of solar are that you need a very large surface area for the panels and that they will only extend the swimming season by 3 months total on a good year.
Heat pumps are the only way to legally and effectively heat your pool all year round. They are relatively inexpensive and very straightforward to install and also not expensive to run.
CALOREX have developed the New Pro-Pac range of pool heat pumps. They have sound-proofed compressors, titanium heat exchangers and intelligent automatic defrost. They are by far the best pool heat pumps on the market. ACAIR are the agents for Calorex for Andalucia.
See more at http://www.acair.com/pool-heating/