22
Appendix A: Solar Pool Heating Fundamentals
The performance of the solar pool heating system is determined by how efficiently the Sun’s energy is transferred
into the pool water. The total amount of energy delivered to the pool is the product of the amount of water flowing
through the system’s collector, multiplied by the water’s temperature rise as it travels through the system and returns
to the pool.
The fundamentals by which the Sun’s energy is transferred to the pool water are:
• All natural systems seek to achieve a balance, heat always moves from hot (more energy) to cool (less energy)
• The greater the difference in temperature between two points of an object or between two objects
(Temperature Differential), the quicker the heat is transferred from the hotter to the cooler point. This applies
whether the heat transfer method is radiation, conduction or convection
• The Sun’s energy heats the incident surface of the collector via radiation. The energy of the Sun’s
electromagnetic waves, primarily in the invisible infrared spectrum, is converted to heat energy at the exposed
surface of the collector
• Heat is transferred through the wall of the collector via conduction, transferring heat energy from the outer
surface of the collector to the inner surface in contact with the water flowing through it. The water draws heat
away from the wall of the collector, maintaining a temperature differential across the collector wall – from the
outer surface to the inner surface
• The difference between the surface temperature of the collector and the water flowing through it, the
thickness of the collector wall, and the material that the collector is made from all determine how quickly the
water “absorbs” the Sun’s heat through the collector wall
• As the water flows through the collector it absorbs energy and heats up. As the water heats up the
Temperature Differential between the outside of the collector and the water flowing through it gets smaller, and
therefore the rate at which the water continues to heat up becomes slower. This is the reason why if the water
path from the inlet to the outlet points of the solar collector is too long, the system loses efficiency
• Water flowing through a pipe tends to flow in annular rings – like the rings in an onion that has been cut in half.
At low flow rates, this flow pattern is very dominant, with the outer most “ring” in contact with the pipe wall
being stationary due to friction between the pipe wall and the water. The water flows fastest at the centre of
the pipe through the inner most “ring”. At low water flow rates, there is very little mixing of the water between
each “ring”, resulting in very poor transfer of heat between the collector tube wall and the bulk of the water
flowing through it. At higher flow rates there is good transfer of heat throughout the water as the result of
turbulence (Convective Heat Transfer)
• Low flow rates tend to result in low system efficiencies, while high flow rates tend to result in higher system
efficiencies. However, above a certain flow rate there is little improvement in convective heat transfer, but the
pressure drop over the system continues to rise. This requires an increase in the horsepower of the circulating
pump, and therefore the cost to run the system
• For a solar pool heating system to operate correctly and efficiently, the system requires the pool water to be
pumped evenly throughout the entire system at flow rates between 1.8 to 4.8 litres per minute per square
metre of collector
