Public Power Magazine

Optimizing Rooftop Air Conditioning

From the March-April 2011 issue (Vol. 69, No. 2) of Public Power

Originally published March-April 2011

By Alice Clamp
March-April 2011

Rooftop air conditioners are ubiquitous. Although widely used by commercial and industrial utility customers, these rooftop units—or RTUs—have several drawbacks. In addition to their high energy demand and consumption, these units tend to suffer from unregulated humidity control, fluctuating room temperature, compressor short cycling and noise.

So when Omaha Public Power District in Nebraska began exploring new ways for its commercial and industrial customers to reduce demand and save energy, its gaze fell on RTUs.

“We wanted to find a technology that would ensure the more efficient operation of these units,” said Bill Lenagh, OPPD’s division manager for customer sales and service. After extensive research, the trail led to Bes-Tech and DHL Controls, which design and manufacture—among other products—RTU optimizers.

The optimizer technology uses a variable frequency drive (VFD) to modulate both the fan and the compressor speed of the RTU. A traditional RTU switches on and off—going through what is known as a “hunting” cycle—instead of operating continuously. By modulating fan and compressor speeds, the optimizer eliminates this hunting cycle. The result is lower demand, reduced energy use and a longer life for the RTU.

OPPD believed that the digital optimizer technology—when retrofitted to RTUs—had the potential to save money for its customers and benefit the utility, too. That potential dovetailed nicely with OPPD’s demand-side management program, which has a goal of shaving 50 MW off demand by 2014. The utility already had a lighting incentive program in place for its commercial and industrial customers, but realized that more efficient lighting wouldn’t get it to its goal. Something more was needed.

Based on estimates from Bes-Tech and DTL Controls, OPPD projected that the RTU optimizers could lead to a significant reduction in demand, moving it toward that 50 MW goal. But the utility needed real numbers, which meant a pilot program. OPPD turned to the American Public Power Association for help, applying for—and receiving—a $50,000 grant from the Dem-onstration of Energy-Efficient Developments (DEED) program.

Before launching a test of the RTU optimizer, OPPD wanted to test the technology itself on a small scale. In 2009 the district initiated two pilot projects involving a heat pump optimizer. In both projects, demand was reduced by up to 50 percent and compressor cycling was cut by up to 70 percent. Satisfied that the technology was viable, OPPD began selecting customers for a pilot test of the technology—dubbed the Digi-RTU Optimizer—in rooftop air conditioning units.

To identify program participants, the utility turned to its account executives with commercial and industrial customers as well as electric service designers, whom Lenagh describes as OPPD’s “eyes and ears” when it comes to working with customers.

The utility developed a set of criteria for customers The rooftop unit had to be well maintained, it couldn’t be older than 20 years and it couldn’t be under manufacturer’s warranty. The unit also had to be operating within its design parameters. “If a unit wasn’t operating correctly, we were willing to work with the customer or the agency responsible for maintenance to resolve the issue and allow the unit to be part of the pilot program,” said Lenagh.

OPPD chose 40 rooftop units installed on 18 buildings that represented a variety of commercial and industrial businesses. The units were divided into two groups: one for 3- to 12.5-ton units with one- or two-stage compressors, and one for units from 15 to 30 tons, with three- or four-stage compressors.

Prior to the installation of the Digi-RTU Optimizers, OPPD set loggers at each unit’s location to collect “before” data on electrical demand and energy consumption. Later in July and during August—when some of the optimizers had been installed—the utility began collecting “after” data. OPPD compared pre-installation data on one day with post-installation data on a day with similar average and maximum temperatures.

Early results indicate a significant reduction in both demand and energy use as well as compressor cycling. Peak demand has been reduced by up to 60 percent and compressor hunting—the on and off cycle—has been cut by up to 70 percent.

Two examples—a manufacturing facility and a restaurant—serve to illustrate the drop in demand and energy use. In both cases, the readings were taken when the average temperature was at least 80 degrees F and the maximum was in the low 90s. OPPD also reported that building comfort level was maintained and humidity control was improved.

For the manufacturing facility, which has a 20-ton unit, OPPD recorded a reduction in demand of 64 percent, from 16.8 kW on July 22 to 6.1 kW on Aug. 9. For the same two days, energy use fell by 58 percent, from 278.34 kWh to 116.55 kWh.

For the restaurant, which has a 12.5-ton unit, OPPD recorded a drop in demand of 45 percent, from 16.1 kW on July 22 to 8.8 kW on Aug. 9. For those two days, energy use fell by 41 percent, from 209.1 kWh to 123.8 kWh.

“We are encouraged by the initial results of the pilot,” said Lenagh. “We would like to turn this into an incentive program, similar to the one we offer for lighting.”

OPPD has promised preliminary results by the first quarter of 2011 to all the customers participating in the pilot. “We plan to leave the optimizers in place through [this] year’s cooling season so we can get more data,” says Lenagh.

The Digi-RTU Optimizer benefits OPPD’s customers in several ways. “The reduction in energy consumption translates directly to a lower bill,” said Lenagh, “as does reduced demand.” He added that the reduction in compressor short cycles or “hunting” could extend operating life and reduce maintenance costs.

OPPD benefits, said Lenagh, by postponing the construction of a new power plant.

OPPD has learned several valuable lessons as a result of the pilot program, said Lenagh. They concern the selection and installation of the variable frequency drives as well as the need for regular maintenance of rooftop air conditioning units by customers.

“The success of a large-scale rollout of a Digi-RTU Optimizer incentive program depends on the resolution of these issues,” he said. The issues:

VFD selection. OPPD found that three variable frequency drives (VFDs) were undersized for some of the rooftop units. Although OPPD followed the specifications in matching the VFD to the motor size, in some cases the result was an overestimation of the unit’s performance. For other units with the same capacity, however, the same size VFD worked well. OPPD concluded that to accurately size VFDs, it must develop a detailed specification manual on VFD selection and sizing that considers a range of application scenarios.

Installer training. OPPD also found that the installation of a Digi-RTU Optimizer is not a “plug and play” process. The utility said it underestimated the level of training required. To provide such training for contractors, OPPD developed and implemented procedures for the majority of installations.  Once trained, contractors will be well equipped for a variety of RTU situations and configurations, according to the utility. To ensure quality control, OPPD recommends working with existing trade allies, if possible, or training a small group of contractors.

Need for regular maintenance. The rooftop units used by many businesses are not regularly maintained, according to OPPD. The utility identified eight units with mechanical/electrical issues after the Digi-RTU Optimizers were installed. As a result, the optimizers were bypassed until the building owners resolved the issues. This “reactive” process requires extra time and effort on the part of the building owner and the installation contractor, said OPPD. This experience highlighted the need for a pre-inspection of the rooftop unit to determine the maintenance required. OPPD has developed inspection procedures that it will use for the rest of the pilot program.  

Cost issues. OPPD’s Lenagh said the price of the Digi-RTU Optimizers used in the pilot program ranged from $2,000 to $9,000 for units from 3 to 20 tons. Installation costs ranged from $300 to $2,000. Given the Digi-RTU Optimizer’s price point at the time of the pilot, the utility believes an incentive would be needed to promote rapid market penetration.

“We’ve adopted a long-term approach to our relationship with our customers.” said Lenagh. “We are conducting this pilot program to help make them more energy efficient and competitive in the marketplace.”


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May-April 2011
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