Solar pool heating systems require no separate storage tank, because the pool itself serves as storage. In most cases, the pool's filtration pump is used to circulate the water through solar panels or plastic pipes. For daylong operation, no automatic controls are required, because the pool usually operates when the sun is shining. If such controls are employed, they are used to direct the flow of filtered water to the collectors only when solar heat is available. This can also be achieved by a simple manually operated valve. Normally, these kinds of solar systems are designed to drain down into the pool when the pump is switched off; thus the collectors are inherently freeze protected (ASHRAE, 2007).
The primary type of collector design used for heating swimming pools is the rigid black plastic panels made from polypropylene (see Chapter 3, Section 3.1.1). Additionally, plastic pipes or tube-on-sheet panels can be used. In all cases, however, a large area is required and the roof of a nearby building can be used for this purpose.
Recommendations for the design, installation, and commissioning of solar heating systems for swimming pools, using direct circulation of pool water to the solar collectors, are given in the technical report ISO/TR 12596:1995 (1995a). The report does not deal with the pool filtration system to which a solar heating system is often connected. The material presented in the report is applicable to all sizes of pools, both domestic and public, that are heated by solar energy, either alone or in conjunction with a conventional system. Additionally, the report includes details of heating load calculations. The pool heating load is the total heat loss less any heat gains from incident radiation.
The total heat loss is the sum of losses due to evaporation, radiation, and convection. This calculation requires knowledge of the air temperature, wind speed, and relative humidity or partial vapor pressure. Other causes of heat losses, which have a much smaller effect, are turbulence caused by swimmers, conduction to the ground (usually neglected), and rainfall, which at substantial quantities can lower the pool temperature. The addition of make-up water should be considered if the temperature differs considerably from the pool operating temperature. Pools usually operate in a narrow temperature range of 24-32°C. Since the pool has a large mass, its temperature does not change quickly.
The use of pool cover reduces heat losses, particularly evaporative losses; however, when designing a solar pool-heating system, it is often not possible to know with certainty the times during which a cover will be in place. In addition, the cover may not have a perfect fit. Hence, a conservative approach should be taken when allowing for the effect of a cover (ISO/TR, 1995a).
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