How to improve energy efficiency in a pool system

Pipe sizing

The size of a pool’s plumbing lines will be the ultimate roadblock for how much water a system can move efficiently. With new residential-grade pumps on the market that have the ability to move 606 lpm (160 gpm) of water, it is easy to see how 38-mm (1.5-in.) pipe leaves much to be desired. To realize anywhere near the full potential of a high-volume pump, like many variable-speed pumps, the plumbing system should be 63.5 mm (2.5 in.) or larger on suction lines and 51 mm (2 in.) on return lines (if not larger). The current industry standard is 51 mm (2 in.) at most and this still severely limits the efficiency at which any system is able to operate.

When a powerful pump is installed on a system with 38 mm (1.5 in.) plumbing lines, or a system with multiple flow restrictions such as ball valves, sharp-bend elbows, or multi-port valves, much of the potential for energy efficiency is lost due to energy loss to friction at higher speeds. Designing the pool system to meet the turnover rate at a lower water velocity can actually maximize its energy efficiency.

Turnover rates

A turnover is the cycle of the entire water volume in a pool passing through the filtration system. Since the water returning from the filter mixes back in with the yet-unfiltered water in the pool, there is a calculation for determining how much of the pool water goes through the filter during each turnover: 63 per cent on the first pass, 86 per cent on the second, and 95 per cent on the third.

Theoretically, after the third turnover, the water in the pool has passed through the filter at least once. As a result, the ideal guideline for a residential pool is one turnover every eight hours or three turnovers every 24 hours. For example, for a typical 75,708-L (20,000-gal) pool, this would require 227,124 L (60,000 gal) to pass through the filter every 24 hours, which is 9464 litres per hour (lph) (2500 gallons per hour [gph]) and 158 lpm (42 gpm). Therefore, in this example, the minimum flow rate on this system would be 158 lpm (42 gpm).

Pump sizing

To determine pump sizing, the system’s minimum flow rate must be known—in this case, 158 lpm (42 gpm)—as well as the maximum flow rate to determine the system’s pipe size. Assuming the system uses a pipe size of 51 mm (2 in.) and has a maximum velocity of 1.8 m/s (6 fps) 235 lpm (62 gpm), the ideal flow rate for this system can be calculated by averaging the minimum flow rate and the maximum water velocity. In this example it would be 197 lpm (52 gpm). Assuming a total dynamic head of 15 m (50 ft), a pump capable of producing 197 lpm (52 gpm) at 15 m (50 ft) of total dynamic head pressure would be required.

Using the same size pool, except for this example it is using 38-mm (1.5-in.) pipe, would result in a maximum of 144 lpm (38 gpm) at 1.8 m/s (6 fps), while a minimum turnover of 159 lpm (42 gpm) is required.

In the field, if the pump has enough power, it will exceed the soft limit of 144 lpm (38 gpm); however, there will be energy lost in the increased resistance to flow in the lines. As the water velocity increases, the greater the friction loss experienced in the system. Therefore, to meet the turnover rate for this pool, the pump will not be operating as efficiently as it would if the plumbing was larger.