Heating Swimming Pools with Waste Heat from Cooling Towers

Introduction

Swimming pool heating is commonly done with air-to-water heat pumps with a coefficient of performance (COP) of about 3.5. However, in buildings with significant cooling demand, another source of thermal energy may already be freely available: the condenser heat normally rejected through cooling towers. Recovering this heat provides an opportunity to reduce the electrical energy required for pool heating.

This study evaluates an innovative cooling tower heat recovery system installed at an educational facility in Singapore. Its energy performance is compared with the COP of typical heat pumps. Special attention is paid to the pumping power required to bring the waste heat water to the swimming pools, as well as the optimal operational hours of the heat recovery system.

Heat Recovery Concept

Cooling towers uses fan-driven water evaporation to reject condenser heat to the outdoor environment. For this installation, the chiller condenser water enters the cooling tower at about 33°C and leaves the cooling tower at 29.5°C. For the heat recovery concept, about 10% of the low grade condenser heat is intercepted before rejection and transferred through a heat exchanger to a hot water distribution loop. The recovered heat is then transferred to two swimming pools through another set of heat exchangers. Some of the waste heat is also used for other hot-water loads, which is not a focus of this article. This heat recovery concept turns an unavoidable cooling system by-product, condenser heat, into a useful thermal resource, swimming pool heating, reducing the need for a separate heating system. The system serves two swimming pools through heat exchangers, with a separate domestic hot water branch.

Figure 1: Simplified heat recovery system schematic

COP Calculation Method

This analysis uses one-minute operational data recorded from 13 to 17 April 2026. The recovered thermal output was obtained from the condenser water energy meter (BTU meter) on the heat recovery loop. Electrical input was based on the measured electricity of the main 11kW pump (Level 2) and the rated electrical power of the other smaller pumps. Because the thermal and electrical data use the same time interval, the heat recovery system COP can be evaluated minute by minute using this formula:

Equation 1 - System COP for swimming pools

This system level heat recovery COP differs from the conventional COP of a standalone heat pump. In the heat recovery system, the thermal energy is already available as condenser waste heat, and the only electrical consumption is from pumping the waste heat through heat exchangers. The conventional heat pump COP of 3.5 is therefore used here as a performance benchmark rather than as a strictly like-for-like system comparison.

Figure 2: Effect of operating schedule on the heat recovery system efficiency

COP Performance Results

For comparison, a conventional heat-pump system for water heating would typically operate with a COP of about 3.5. The heat-recovery system evaluated here does not rely on a heat pump to generate the recovered heat; instead, only electrical energy is required for pumping the waste heat from the cooling towers to the swimming pools.

However, the benefit of this concept depends strongly on the operating schedule. When the circulation system is considered over a full 24-hour operation, the calculated system COP is 3.0. During night-time periods, useful heat recovery is low because the main chillers are switched off leaving only a baby chiller in operation. However, the circulation pump continues consuming the same amount of electricity. As a result, at night the system COP falls below the typical COP range of a conventional heat pump.

When the heat recovery system operation matches the daytime operation of the main chillers from 07:00 to 16:00, the calculated COP increases to 4.6. This represents an improvement of approximately 55% compared with the 24-hour case and places the heat recovery system efficiency above the typical COP of 3.5 for a conventional heat pump.

Table 1. COP comparison between conventional heat-pump heating and cooling-tower heat recovery

Operational Optimisation

The results show that operating strategy is critical to the performance of the heat recovery system. Under continuous 24-hour operation, unnecessary pumping during low demand periods reduces the effective system COP. Aligning circulation with periods of maximum available waste heat generation from the chillers significantly improves performance to a measured COP of 4.6.

For future similar heat recovery designs, the system COP can be significantly improved, simply by placing the cooling towers closer to where the waste heat is needed, and therefore reducing the pumping energy needed. The current system includes an approximately 150-meter-long pipe-run (300-meter return) between the cooling towers and the swimming pools. Such a long pipe run with several turns along the way contributes to higher pressure losses and pumping demand. Insofar the cooling towers had been placed much nearer to the swimming pools, it is assumed that the pumping power could be reduced to half. This reduced pumping demand (from 11kW down to 5.5kW) would increase the pool heating system COP by 37% to 6.3 for the daytime operating scenario. Likewise, for the 24-hour operation, the COP would increase to 4.3.

Additional Benefit: Cooling Tower Water Savings

In addition to improving the energy performance of pool heating, the heat recovery system provides an additional water saving benefit. In a conventional cooling tower, most of the condenser heat is rejected through water evaporation. By removing part of this heat before it reaches the cooling tower, the amount of heat that must be rejected through evaporation is reduced, thereby lowering cooling-tower water consumption.

Figure 3. Estimated daily cooling-tower water savings during the monitoring period

Based on the monitored data from 13 to 17 April 2026, the estimated water saving ranged from approximately 1,800 to 2,300 liters per day, with an average daily water saving of around 2,000 liters per day. Over the five-day monitoring period, this represents 9,900 liters of avoided cooling tower evaporation. Assuming a similar five-day operating pattern throughout the year, this would correspond to an estimated annual water saving of 515,000 liters, or 515 m³ per year, about a quarter of the volume of an olympic swimming pool. Although water saving is a secondary benefit compared with the energy-performance improvement, it further strengthens the value of recovering condenser heat rather than rejecting it directly through the cooling towers, especially in the water scrace city state of Singapore.

Conclusion

The results show that cooling tower heat recovery can provide an efficient alternative to conventional heat pump pool heating, but its performance depends strongly on how the system is operated. Under constant 24-hour operation, the calculated system COP is 3.0, which is comparable to the efficiency of a typical heat pump (COP 3.5). When the heat recover system is programmed to follow the same operational hours as the main chillers (07:00 - 16:00),  the heat recovery system COP increases to 4.6, an improvement of 55%.

Further energy efficiency improvement could also be achieved through hydraulic optimisation. The current system includes a long pipe run distance of about 150 m between the cooling tower and the swimming pools, which causes a higher pumping demand. By spatially placing the cooling towers much closer to the swimming pools, the electrical pumping demand can be halved, and the heat recover system COP increases to 6.6 for the daytime operation case, almost twice as efficient as a conventional heat pump. This calculation example illustrates the importance of reducing pumping energy for achieving a high energy efficiency for the heat recovery system.

As an added benefit, the heat recovery system also reduces cooling tower evaporation, as some of the heat ejection is diverted into the swimming pools. Based on the monitored five-day period, the prorated annual water saving is 515 m³, or about 25% of the volume of an olympic swimming pool.

Overall, the results show the potential of a well controlled and hydraulically optimised waste-heat recovery from cooling towers to improve pool heating efficiency (COP 6.6) while also reducing cooling tower water consumption (515 m³/year).

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Acknowledgements

• Thank you to Gregers Reimann (IEN Consultants) for his contributions to this article.

• Thank you to Siew Zhi Tao for his early concept calculations of this innovative heat recovery concept during his 2020 internship.