In order to heat your pool, four systems are basically available:
The Heating Pump is increasingly used. In fact, it is a valid system to heat your pool, little hulking, completely programmable and very effective.
Plate heat exchanger or tube heat exchanger
The Heat exchanger is an effective heating system when a certain quantity of warm water is already available. For example a sanitary storage.
This is, with no doubt, the most expensive system in terms of energy consumption. In fact, consumption ranges from 3 to 12 kw of power installed.