October 1, 2010
By Scott K. Petty
With the advent of various legislation aimed at improving the energy-efficiency of appliances and similar products, energy-efficient pumps have become a popular topic within the pool and spa industry. Every pool and spa circulation system requires at least one pump; many have multiple units. As a result, pumps represent the largest aggregate consumption of energy on the equipment pad. Some studies have estimated pool pumps to be the second-largest energy consumer within an entire household, following only heating, ventilating and air conditioning (HVAC) systems.
Variable-speed pumps provide a great opportunity to reduce this energy consumption while also lowering operating costs for the homeowner. While energy efficient single- and two-speed filtration pumps have been available for many years, variable-speed pumps are a relatively new addition to the equipment pad. These pumps often represent the latest in motor technology, with advanced features specifically designed to reduce energy consumption while still providing the flow needed to operate sanitization, heating and other equipment necessary to maintain and enjoy pools and spas.
A variable-speed pump has three primary components: the motor, the hydraulic wet end and the variable-speed drive. As with common single- and two-speed pumps, the motor’s primary function is to convert electric energy into mechanical energy, in order to spin the shaft connected to the impeller. However, there are two important distinctions with respect to motor construction typically used in variable-speed pumps:
Compared to the standard induction motors used in most single- and two-speed pumps, permanent magnet motors are more energy efficient, especially at reduced loads that occur at lower speeds. The totally enclosed design prevents debris and water from accessing the windings, which improves the reliability and service life of the motor and, by extension, of the pump itself.
The wet end of the pump converts the mechanical energy from the motor into hydraulic energy, providing the flow and pressure needed to operate and maintain a pool or spa. While much emphasis is often given to the permanent magnet and totally enclosed motor technologies, the wet end design should not be ignored. After all, it is the combination of electric and hydraulic efficiency that drives the pump’s overall energy efficiency.
As hydraulic efficiency improves, the pump can move more water for the same amount of electrical work. When combined with a variable-speed drive, the pump can achieve the necessary flow at a lower operating speed. This energy savings is a result of the Pump Affinity Law. With the impeller diameter as a constant, this law states the following three relationships:
The most common example given when reviewing the Affinity Law is describing the change in flow, pressure and power when motor speed is reduced by 50 per cent.
Take, for example, a 3,450-rpm pump (the speed of a single-speed pump, or high speed of a two-speed pump, operating at 60 Hz). When the speed is reduced by half (1,725 rpm), the flow is also reduced by roughly 50 per cent. In turn, the pressure is reduced by the square (0.5 x 0.5 = 0.25); power is reduced by the cube: 0.5 x 0.5 x 0.5 = 0.125 (actual savings may be less since the impeller is typically optimized for a particular speed). Even if the pump is operated twice as long in order to maintain the same turnover rate, the energy consumption is up to 75 per cent less (0.125 x 2 = 0.25 energy consumption or 75 per cent savings) than when operated at high speed.
To put this example in financial terms, using an average cost of electricity of 15 cents per kilowatt-hour (kWh), a single-speed pump operating at 3,450 rpm and 2,000 watts would consume 16 kWh over an eight-hour period, resulting in a total cost of $2.40 per day. If the speed was reduced to 1,725 rpm, power consumption would drop to approximately 250 watts and total energy used would be 4 kWh over a 16-hour period (run time doubled to achieve the same turnover) at a total cost of $0.60 per day. This would translate to savings of $54 over the course of a month and as much as $648 per year based on a 12-month season. While actual savings will vary based on a comparison pump model, hours of operation, electrical rate and other hydraulic factors (e.g. plumbing size and length), the savings can be quite significant.
The key benefit of variable-speed pumps is that the speed can be reduced beyond half speed (1,725 rpm), which, according to the Affinity Law, will result in even greater energy savings. When combined with an energy-efficient permanent magnet motor and a hydraulically efficient wet end, variable-speed pumps have been proven to save as much as 90 per cent compared to ordinary single-speed pumps, often translating into savings of more than $1,000 per year.
Certainly the most attractive feature of variable-speed pumps, from a consumer’s point of view, is energy savings. While variable-speed models can often pay for themselves in as few as two years, it can still be a challenge to convince homeowners to purchase a variable-speed pump. For clients who may still be closely watching their out-of-pocket expenses, trying to sell the savings based on the Affinity Law, permanent magnet motors and other completely foreign terms, may simply be a waste of time.
An effective way to convey the savings potential of variable-speed pumps is to explain the adjustability of pump speed by using light dimmer switches as a comparable. Instead of using technical jargon, explain to consumers that pump flow can be ‘dialed in’ to match the pool’s needs throughout the day, rather than operating at full power 100 per cent of the time. For example, basic filtration typically requires minimal flow, often just enough for water to spill over the skimmer weir. This is the mode a pump would likely operate throughout the day in order to maintain proper sanitization. Other events—such as running an automatic pool cleaner or operating spa jets—would require greater flow, but usually for much shorter periods of time. This concept is often easier for consumers to grasp, as opposed to explaining the details of the Affinity Law.
Providing savings estimates can also be a challenge. For example, simply stating variable-speed pumps can offer up to 90 per cent savings, while impressive, may not mean much to a consumer, especially if they don’t know how much it costs to operate a standard single-speed pump. Given the variables noted above (e.g. comparison pump model, hours of operation, electrical rate, etc.), determining even approximate savings may be difficult. To alleviate this challenge, most variable-speed pump manufacturers have developed energy savings calculators that, while still based on some basic underlying assumptions, provide reasonable annual savings estimates.
In addition to reduced energy costs, variable-speed pumps can provide other benefits a customer can instantly appreciate. For example, reducing pump flow often improves overall filtration. The filter media—be it sand, cartridge or diatomaceous earth (DE)—is no longer ‘blasted’ with high-velocity water and, as a result, more particulate is usually removed. As with energy savings, improved filtration may vary based on specific installations.
‘Dialing in’ the flow is also particularly useful for water features and spa jets, where individual and situational preferences may dictate varying flow requirements. For instance, a homeowner may want a waterfall to peacefully trickle during quiet nights by the pool, but would prefer a roaring display while entertaining guests at a neighbourhood pool party. Variable-speed pumps also allow for customized spa jet action for each member of the family, often with the simple push of a button.
Perhaps the most distinct benefit customers can recognize is the pump’s relatively quiet operation at lower flows. Most customers cannot readily determine a pump’s power consumption, but they can often hear it operating, especially when at or near the equipment pad. A pump operating at low flows is remarkably quieter than one running at full speed. Many times, customers are only convinced a variable-speed pump is running when they physically touch the pump, since the sound is so much lower than expected.
Reduced energy costs, improved filtration and quiet operation are all a result of operation at reduced speed. The variable-speed drive, which allows the pump to operate at lower speeds, often has additional built-in ‘smart’ features that provide additional value.
Some variable-speed pumps include integrated timer functions that allow the pump to be programmed to operate at varying flows throughout the day. They typically run at a lower speed for the majority of the time (to provide basic filtration), with periods of increased flow needed to operate automatic pool cleaners, heaters, etc.
Variable-speed pumps can also usually be adjusted ‘on the fly’ to correspond to specific demands that may arise outside the programmed setup. For enhanced pool management, variable-speed pumps can be managed by pool and spa controllers to fully automate the entire pad and provide additional conveniences, such as remote-controlled pump speed adjustments.
Other features—such as protection from voltage fluctuations from the power utility (including potential brownout situations), excessive temperatures and loss of prime—help ensure seamless operation and prolonged service life. On-board diagnostics provide immediate feedback that can be used to more easily troubleshoot issues in the field. In addition, some models have incorporated safety vacuum release system (SVRS) technology, which provides added protection from suction entrapment as required by the Virginia Graeme Baker Pool and Spa Safety Act, combining energy efficiency with added piece of mind.
Variable-speed pumps represent a new option for both the industry and the homeowner. However, they still represent a relatively small percentage of overall filtration pump sales compared to typical single-speed pumps. This is most likely due to customers who are hesitant to spend additional money without fully realizing the advantages. Once their benefits are understood and explained, the sales potential for variable-speed pumps is tremendous. It truly represents a ‘win-win’ scenario for all those involved. While legislation has made energy-efficient pumps a requirement in some jurisdictions, the benefits of variable-speed pumps are enough to entice any prospective customer.
Scott Petty is the product manager for pumps and above-ground equipment for Hayward Pool Products. Previously, he managed Hayward’s test facilities and agency compliance activities. Over the course of seven years in the pool industry, Petty, who holds a B.Sc. in mechanical engineering from North Carolina State University and an MBA from Wake Forest University, has developed an expertise in energy efficiency and hydraulics. Petty can be reached via e-mail at firstname.lastname@example.org.
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