The city of San Antonio has been serious about saving energy. By 2012, it had begun energy-efficiency and sustainability efforts across many of its properties as it strove both to be a good citizen and reduce annual utilities costs, which had averaged $33 million. San Antonio’s Office of Sustainability had been created in 2011 to help the city become a national leader in this subject. The department had performed lighting retrofits, HVAC system replacements and control improvements, and installation of solar window film.
With these energy-saving measures in place, officials looked to the city’s pools, wanting to learn how they could become more energy efficient, and whether variable-speed technology was appropriate.
What followed was a nearly three-year process, leading to publication of a report that will be presented at the 2015 World Energy Engineering Congress in Orlando, Fla., this fall.
“This was a first ever,” says Steve Barnes, director of science and compliance for AquaStar Pool Products in Ventura, Calif. “That’s why it took so long for us just to get our heads around the scope of the project. ... It wasn’t just a variable-speed technology retrofit. That’s been happening for years.”
The study not only confirmed beliefs about the energy-saving potential of variable-speed technology, but also its viability in commercial installations.
“This is the largest detailed third-party study of this magnitude to date,” says Jeff Farlow, program manager of energy initiatives for Pentair Aquatic Systems in Sanford, N.C. “It took collaboration with public health and safely regulatory agencies ... to overcome the common misbelief that water quality and sanitation will suffer when energy is saved by reducing flow.”
The city began its efforts with $6 million in seed funding from the American Recovery and Reinvestment Act. It also established a revolving Energy Efficiency Fund in which all money saved through energy efficiency is captured and used for future improvements.
Officials thought San Antonio’s 26 public pools, ranging in volume from 66,000 to 805,000 gallons, presented another savings opportunity.
“The City of San Antonio was doing its research and found a document produced by the Department of Energy about how to swap single-speed pumps for energy savings, but the document was really focused on residential applications,” Farlow says.
City staff reached out to the author of that paper, consultant Steve Easley of Danville, Calif.-based Steve Easley and Associates. He contacted Pentair and Barnes to see if similar information was available regarding commercial installations.
The manufacturers saw an opportunity to fill an information gap on this subject, and use the findings as a precedent to assuage health officials who resist variable-speed technology pumps in public pools because of their higher bather load.
“Typical health departments are leery of anything that impacts water quality or sanitation,” Farlow says. “There’s a common misconception that when reducing flow rates on a commercial pool, you will negatively impact water quality.”
While that is a somewhat logical concern, he says, “In reality, commercial pools are typically way over pumped.”
This was true for the San Antonio pools, many of whose flow rates could surpass the mandated turnover rates. This meant circulation flow rates could be reduced while still meeting turnover goals.
A team formed, including Barnes, key Pentair personnel such as Farlow and Easley, as well as city and state staffers and Pflugerville, Texas-based firm Commercial Swim Management.
The team set out to develop a retrofit plan by which variable-speed technology would be implemented to maintain flow rates that were high enough to satisfy turnover requirements and maintain safe water, but low enough to result in energy savings. They also sought to estimate the potential costs savings.
When setting up the study, the group confronted a snag: Commercial pools in Texas are to be outfitted with NSF-approved equipment, but there were no commercial-grade pumps with such approval. But the standard was silent about adding variable-frequency drives to existing single-speed pumps, despite addressing variable-speed pumps with onboard controls.
The group considered adding external variable-speed control drives to the existing pumps. While city officials believed that likely would nullify the NSF certification on those pumps, the concern was addressed by working with NSF staff to write Annex P, Variable-Speed Pumps Recommendation for Installation and Operation. In addition, the experts decided to retrofit pumps with variable-speed pump controls tied to paddle wheel digital flow meters to monitor and regulate flow rates. This was meant to ensure mandated turnover and optimize chemical dispersion and water clarity regardless of increases to total dynamic head (TDH) resulting from filter loading and other factors.
Certain conditions needed to be in place for any individual pool to be considered for a retrofit. Most importantly, the circulation system’s ability to move water had to exceed flow rates needed for proper turnover. Otherwise, there would be no wiggle room for reducing pump speeds. This didn’t rely solely on the pump’s power, however. Surplus flow could be gained by improving hydraulics through smaller measures that would reduce TDH, or resistance, within the system.
To indicate which pools would qualify, the team assessed more than four years’ worth of data on each unit’s age, dimensions, flow rates, pump and motor information, filter types and energy consumption. This would show trends over multiple swim seasons and help staff project the energy-savings potential.
The players also conducted energy and hydraulics audits on each pool, recording such factors as power used, total dynamic head and pressure on the suction and pressure sides of the system.
During this process, the team could pinpoint hydraulic conditions hampering turnover flow rates. Two primary factors were observed. First, they evaluated the effectiveness of filter backwashing practices, in terms of frequency and how well the system was set up. They looked for problems that could undermine the needed backward flow rates, such as undersized pumps, inoperable or leaking backwash valves, and backwash waste lines incapable of handling the minimum flow rate. Secondly, they checked for factors that would restrict flow, such as undersized pipes and filters, air vacuum leaks that could cause pump cavitation, and improperly adjusted or missing bypass valves on heaters.
The team determined that 22 of the 26 pools could be retrofitted. City staff would be trained on how to properly operate the equipment during the study period.
After installing variable-speed technology and making hydraulic tweaks, they conducted another audit and inspection to project energy savings and water quality.
Performance was audited with a clean filter, and under simulated dirty-filter conditions, achieved by throttling a valve to cause a 10- to 15-psi rise in pressure, in an effort to understand when the pump performance did not match the pump curve.
While the pools were being studied, city staff temporarily changed how they monitored the water. Rather than checking quality three times a day as had been the norm, they began checking it hourly.
“So as soon as the filter had a problem or a chlorinator wasn’t feeding, they were finding that out quickly,” Barnes says.
The final audit revealed that the retrofits exceeded expectations. The San Antonio pools saw a total annual savings of 707,517 kWh per year, trimming $62,847 off utility costs, based on current rates with an average blended rate of approximately $.096/kWh. Perhaps most important to utilities, demand during peak times, called kilowatt demand, was reduced by 151 kW.
Considering that the equipment retrofits cost about $136,675, the expected investment payback was projected for just over 2.1 years, sans utility rebates. With the nearly $87,000 in rebates the city enjoyed, the average payback period was reduced to just under 10 months.
“In addition to the money this project is saving ... the project also has tremendous positive environmental impacts,” the study says. “This project will reduce the City of San Antonio’s greenhouse gas emissions by 1,075,567 pounds per year, every year going forward.”
Not only that, but after the post-retrofit audit, there was no detectable difference in water quality found in most pools.
“When the filters got dirty, the variable-speed drive received feedback from an external flowmeter that said, ‘We’re having increased resistance in the system due to the filter loading, so speed up to maintain the required flow rate,’” Farlow says. “So at no point were we ever violating the health code requirements for turnover.”
The team determined water quality before and after the retrofits by observing photographic documentation showing clarity — specifically the ability to see main drains — and through daily water testing by Parks and Recreations Department staff, as well as bather load records.
“In fact, retrofits potentially afford better filtration in many circumstances due to reduced flow rates across sand media filters,” the report read. “High flow rates create more pressure, which forces water through the sand medium at a faster rate, thus trapping fewer particulates.”
Where water quality did drop, it was found to be caused by hydraulic or filter media issues that were corrected, at which point water quality came back up to par.
“One thing [the study] demonstrates clearly is that 8-hour turnover flow rates with properly maintained chemistry under extreme bather load conditions is not a problem,” Barnes says.
But the study showed that filter backwashing may play a more important role in helping achieve this goal than experts had realized. The most common practice currently is to allow a filter to continue loading until it shows a pressure change of 10- to 15 pounds per square inch. But that may not be the best approach for those who want to optimize the savings potential of variable-speed technology, Barnes says.
“By waiting until the pressure builds up 10 psi, it doubles the electric cost,” Barnes says. “As the back pressure builds up, the flow rate goes down, and the flow meter tells the pump to speed up. Then the pump speeds up to maintain [programmed flow].” The higher speed consumes more energy.
The motors in the study ran more efficiently with little to no filter loading. On one example, the pump used 3.17 kW with a clean filter, compared to 6.05kW with a simulated 10-psi increase, and 12.5kW of usage with a fully loaded filter.
When employing variable-speed technology, Barnes says, pool professionals and operators may want to adjust their approach to backwashing for commercial pools, with their heavy bather loads and increased premium on energy savings.
In addition to information about variable-speed pump performance, the study affirms that the industry has the ability to accurately predict how much savings can be enjoyed on commercial pools with a thorough pre-retrofit analysis, Farlow says.
But it doesn’t happen quickly.
“It took us almost 2½ weeks to [audit] 26 city-owned pools,” Farlow says. “We took detailed electrical measurements, hydraulic measurements, and we even drilled and tapped pressure sensors in the pipe so that we could obtain very accurate readings on … total dynamic head conditions. So due to that extensive initial analysis, we were able to very accurately predict the savings that were achievable.”
Additionally, he says, the paper presents another vehicle not only for convincing health officials that variable-speed technology can work in commercial settings, but also to show them how.
“I can see a best-practices-type document coming out of this,” he says.