For millions of people worldwide, outdoor swimming pools offer easy and affordable access to entertainment, recreation, and exercise. However, to ensure that they stay comfortable year-round, their water needs to be maintained at a highly consistent temperature: neither too hot nor too cold for their users. To achieve this, all while minimising the use of water and energy resources, the thermal energy balance in outdoor pools needs to be managed extremely carefully.

According to international guidelines, the temperature of a 50-metre Olympic-sized pool should be kept between 25 and 28 degrees Celsius all year round, with water flowing in and out of the pool at a rate of 220 to 250 cubic metres per hour. On top of these requirements, there is also a growing demand for outdoor pools to reduce their greenhouse gas emissions, often by heating their water using renewable energy sources.

Achieving this balance requires precise predictions about how a pool’s temperature will be affected by conditions in the surrounding environment. Today, engineers often rely on empirical models which can predict how temperatures will be affected day-to-day by varying weather conditions.

However, these conditions can vary widely between different regional climates, as well as at different times of year. As a result, these predictions are often extremely difficult to make – especially during colder periods. This often leads to inaccurate temperature estimates which can cause models to either over- or underestimate pool temperatures.

In these cases, engineers will often inadvertently choose heating systems with sizes poorly suited to their requirements, driving up both operational costs and carbon emissions. For this situation to improve, these models will need to predict temperature fluctuations far more accurately.

In a new study, Chua’s team developed a more advanced model that accounts for numerous factors which affect the transfer of heat to and from pool water. By accurately predicting this balance of thermal energy, they hope to make the operation of outdoor pools more sustainable in the future.

When choosing their heating systems, a key factor for engineers to consider is the amount of heat lost from the pool to its surrounding environment. This can happen through a variety of mechanisms, all of which are influenced by local weather conditions.

Firstly, radiative cooling occurs as water emits electromagnetic radiation, causing it to lose heat passively to its surroundings. This effect can be mitigated by rainfall since it effectively increases the sky temperature to close to ambient, reducing the amount of heat radiated away.

Secondly, through evaporation, heat in the pool is used to convert liquid water into vapour, which then escapes into the environment. This loss occurs at a higher rate in dry weather, or when the surrounding air has a low relative humidity. Evaporation can occur in two possible ways. The first is through ‘free’ convection, where the rate of heat transfer is only influenced by the natural buoyancy of air in contact with the pool water. The second is ‘forced’ convection, where the movement of heat is influenced by external forces, such as wind blowing across the water’s surface. Since winds can change direction quickly and unpredictably in some climates, they play a major role in this process.

Finally, convection heat loss occurs when warm water loses heat to the cooler air around the water. This is particularly pronounced in colder weather, when the air is much cooler than the water. Just as in evaporation, convection heat loss can occur through both ‘free’ convection and ‘forced’ convection.

In their study, Chua’s team integrated all of these heat transfer mechanisms into their model, allowing it to more accurately estimate how the thermal energy balance in pool water is affected by local climates and weather conditions. To validate their model, they compared its predictions with real data collected from an Olympic-sized swimming pool in Perth.

Like many others in Western Australia, this particular pool is heated geothermally using warm groundwater, sourced from aquifers about 1 kilometre underground. After exchanging heat with the pool water, the groundwater is returned to the ground, ensuring that the process does not have any net impact on the region’s water resources.

This type of geothermal heating is more cost- and space-effective than other sustainable methods, such as solar-powered heating. However, it often causes empirical models to underestimate outdoor pool temperatures, especially in winter. When this happens, geothermal systems may need to be supplemented with more traditional, fossil fuel-burning heating plants, negating their environmental benefits.

When Chua’s team applied their new model, they found that its predictions aligned far more closely with real data than existing models. This was especially true when the Perth pool was exposed to the clear skies and strong winds that are typical in Australia.

Altogether, these results strongly suggest that the team’s model could significantly improve the accuracy of temperature predictions for outdoor pools across a diverse range of climates, weather conditions, and times of year.

Based on the success of their research, Chua’s team now hopes that their model could be used to predict the pool temperatures of outdoor pools in many different locations around the world.

By using this model, engineers could better predict their heating requirements, leading to more efficient and sustainable heating systems. Ultimately, this could help ensure that outdoor pools continue to offer easy, affordable access to health and recreation, all while minimising their impact on the environment.