Reversible systems principle
Instead of merely producing heat from fossil fuels or electrical energy, it is much more profitable to use the heat already available in ambient air for the heating of the pools and slides.
The awesome efficiency of reversible systems comes from their capacity to extract the heat from ambient air, even at low temperatures. The theory behind these appliances is known since 1824, so it is not a brand new principle. Even if the original Carnot cycle has been improved with the headway of knowledge and manufacturing techniques, this proven concept is still at the base of the exceptional performances of the reversible systems.
In the nature, heat always passes from a warm body to a cold body. As an example, transferring energy from a warm hand to a cold hand, or yet the heat transfer from the hot coffee to the stirring spoon. These heat transfers occur spontaneously with no external work!
Heat pumps allows us to benefit from this principle by heating a pool from the energy contained in the air. As their name suggests, these systems can "pump" heat from one place to another. This result is achieved by maintaining a part of the system much colder than the ambient air and another part warmer than the water that has to be heated.
The energy needed to maintain this temperature spread is much smaller than the energy that can be transmitted through it.