Editor’s note: This article first appeared in the May 2016 issue of Business Energy.
An older Walgreens store in Evanston, IL, had been selected for replacement in 2012 when executives suddenly recognized that it would be the ideal location for the net zero project they’d been contemplating. Located just 15 miles from corporate headquarters, the outdated store offered ease of access for engineers and a convenient location for tours. Not a mere showcase, it would become a lab of sorts for trying out all the latest energy-efficient technology. They hoped that it would become the nation’s first net zero energy retail store, anticipated to produce energy equal to or greater than it consumes.
The project posed several challenges, primary of which was time. “This was a quick turnaround project,” recalls Benjamin Skelton, president of Cyclone Energy, the energy efficiency consultant on the project that advised the design and analyzed performance. “We had to tear down, design, and build a new store in 13 months.” They began in September 2012 and had to be ready in October 2013.
Another challenge was Chicago’s weather extremes, but, as Skelton remarks, if they could achieve net zero in that climate, they could achieve it anywhere. “There is limited solar in Chicago from October through April, which is also the highest energy-consuming period,” notes Skelton.
Often, pilot projects such as this occur in milder climates along the West Coast or in the southern US. By proving this technology in the harsher Chicago environment, Walgreens could meet their goal of creating a design that incorporates new and emerging technology to benefit other stores in the chain without changing their operational characteristics.
“One of the fundamentals of the design was that we were not allowed to modify the layout,” recalls Skelton. “We had to work with the 4,000-square-foot prototypical design.” In order to successfully replicate lessons learned from the project in other stores, the building’s footprint and shelving layout could not be substantially changed. Operational aspects of the building standard, including the amount of refrigeration cabinets, lighting levels, natural light, and views had to meet minimum Walgreens standards.
While they may have had to work within physical boundaries, they did not have to work within budgetary ones. “The design team was told not to factor in cost, but rather to think innovatively,” explains Skelton. “Walgreens wanted to evaluate emerging and new technology and apply lessons learned to other locations.”
Here Comes the Sun
The project was “driven by solar,” continues Skelton. The design was based on generating renewable energy within the confines of the building’s footprint, instead of solar and wind generation spread throughout the site. Thus, according to Walgreens’ plan, “the store energy signature would be defined by energy generation capacity solely from photovoltaic panels” on the roof: 840 panels, to be exact. Skelton calls it an innovative design.
Because the site is nine degrees off the north/south axis to accommodate parking and the drive-through pharmacy, project architects designed a tiered roof that was rotated to a true north-south orientation to maximize solar exposure. This resulted in a cantilevered roof over the west façade. A canopy roof extending over the pharmacy drive-through provides additional solar collection.
To increase generation capacity and maximize space for the photovoltaic (PV) panels, the roof was pitched to maximize the quantity of solar panels, which were attached directly to the metal pan roof at the same pitch. A roof tilt of approximately three degrees was chosen instead of 7.5 degrees in order to keep the peak building height under 50 feet, which would have substantially increased the cost of the building.
Battery storage systems require large areas within the building and ventilation, and are expensive to maintain. Because the electrical equipment supporting PV production would have taken up too much space in the building and created a hazard,
Walgreens decided to store renewable energy on the grid.
Use of micro-inverters to convert direct current power to alternating current at the panel eliminated the need for a large storage room. The micro-inverter design also provides real-time visibility of each panel’s performance, which assists in identifying maintenance issues.
Maintenance and safety were facilitated by leaving “walk aisles” between the solar panels on the roof—a requirement by the fire department to enable emergency access.
Let There Be Light
Direct and indirect lighting are incorporated into the store’s design. Clerestory glass on the south side, combined with a light-reflecting film by 3M on the western high glass allows daylight to reach farther into the store’s interior. “It keeps the heat up near the ceiling,” adds Skelton, thereby reducing energy usage. By redirecting 90% of visible light onto the ceiling, the film also eliminates glare.
All lighting in the store was LED, but changes were integrated that allowed Walgreens to reduce usage and become even more energy efficient. “The LED lighting package with daylight dimming in eight zones and a motor-operated shading system that tracks the sun and monitors light levels allowed us to reduce usage as well as glare,” explains Skelton.
South-facing clerestory glass was integrated between the roof tiers to let in more natural light. Extensive use of glass for solar power can sometimes offset heat savings, so a compromise must be reached. Adhering to Walgreens standards, the building is well insulated but not “hyper-insulated,” explains Skelton, in order to accommodate retail customers. The roof and walls were designed to achieve R-30 and R-20, respectively.
One advantage of installing windows is being able to open them. “We installed operable windows for natural ventilation,” points out Skelton. “They work better than predicted.” Control actuators allow the windows to open automatically when the weather station mounted on the store indicates that conditions (temperature, humidity, precipitation, and wind speed) are appropriate. When the windows are open, the HVAC systems in the store are turned off, saving energy.
A curtain wall glass design was incorporated on the west—front—side of the building, but the system under-performed. “It’s one of many things that didn’t go right,” confesses Skelton. “We kept it as is, but changed the shade control system from automatic to manual, and added an insulation barrier.”
Another energy-saving plan was to use overhead linear LED fixtures with customized optics and “directionalized” lighting patterns to achieve the required foot-candle levels on store shelves. “The fixture puts out a halo so we could eliminate four kilowatts of undershelf lighting,” elaborates Skelton.
Because the store has 54 linear feet of cooler and freezer cabinets that run constantly, an innovative approach was chosen to address energy usage. “In order not to contribute further to global warming,” begins Skelton, “we opted for an all-natural refrigerant. We decided to capture the heat that comes off the coolers through geo-exchange tubes.”
The geo-exchange system replaces the usual halocarbon-based refrigeration system with an air-cooled gas cooler. Eight tubes were placed vertically under the parking lot, 550 feet deep, to store energy. A heat pump maintains the evaporators, and produces chilled and heating hot water. “It’s simple to operate,” says Skelton, “and requires no maintenance.”
The heat pump compressors send “waste heat” through the geo-exchange system and store it in the ground like a battery. When heat is needed by the system, it extracts it from the system.
Walgreens requested natural refrigerant. Skelton said it was a difficult search for a manufacturer with a CO2-based system that could quickly create the requested system. “An all-natural CO2 heat pump coupled with geothermal had never been used in the world before,” he explains. “This was the first time.”
He says the lead time was “concerning,” but a manufacturer was found in Sweden. The heat pump manufacturer engineered a custom-made transcritical CO2-based heat pump system that simultaneously maintains a –20°F freezer and a 25°F cooler recovering heat from the freezers and coolers to be used to heat the building or water, or sent to the geo-exchange system. The system includes an air-cooled gas cooler to allow for temperature optimization of the geo-exchange system.
The compressor COPs operate in a typical range between 4 and 6. The chilled and heating water is then circulated through variable speed pumps to single-zone, four-pipe, variable-air-volume, air-handling units throughout the space. A dedicated VAV outdoor air-handling unit provides ventilation to the store with demand control ventilation.
“They use the heat pump for all heating and cooling, water heating, coolers, and freezers,” lists Skelton. “It accounts for most of the energy in the building.”
An Energy Diet
The Evansville store is approximately 50% more energy-efficient than other Walgreens stores, according to a granular metering system to measure loads. “We monitor anything that uses electricity. Compared with our baseline store, all the technology combined provides about 50% more efficiency,” says Skelton.
The average Walgreens store in the Chicago area consumes 425,000 kWh per year. Because the PV solar panels generate only 256,000 kWh per year, the design team had to find ways to permanently reduce the amount of energy consumed without changing the operational characteristics of the store. They found opportunities in the envelope, lighting, HVAC, and refrigeration systems to reduce annual energy consumption by at least 57%.
One such opportunity was found at the store’s entrance. The typical Walgreens store has an airlock vestibule with automatic doors. However, energy modeling indicated that a revolving door could reduce infiltration and decrease estimated energy consumption by as much as 5%—the equivalent of 43 fewer solar panels.
The revolving door was ultimately incorporated into the final design, and a handicap-accessible door was added to the side with an air curtain. Unfortunately, the revolving door didn’t pass envelope commissioning tests because it allowed an extraordinary amount of infiltration.
Additionally, so many customers used the handicap door, which has a 20 second open-to-close cycle that Walgreens had to add a sign to alert customers that overuse negatively impacts their net zero goal.
From April to August 2014, the building was net zero, but it was an impossible goal to maintain during the frigid winter of 2014–15, with temperatures as low as -24°F, and the mean temperature almost 10°F below average.
The store’s design allows for reasonable weather fluctuations, but severe cold from January through March resulted in an increase in heating and cooling energy of 36% and decreased solar generation by 4% over the design model.
Factors limiting the opportunity to achieve net zero include the short timeframe for design and construction and the urban location. Two vertical wind turbines were included in the project, but were not factored into the net zero performance analysis. “The wind turbines weren’t successful,” admits Skelton. “They should generate five kilowatts of power, but vertical turbines just aren’t efficient.”
Other issues contributed to the failure to reach net zero that year, including problems with the utility net-metering. The end use metering was determined invalid in March.
To address some of these issues, the team addressed operational strategies established for the heat pump. Water temperatures were initially adjusted to maximize efficiency, but excessive cycling led to replacement of compressors that better matched the load than the original ones that were oversized to ensure adequate capacity. The difference resulted in a 10% annual reduction from the heat pump system.
Practices that help reduce the amount of energy used include motion sensors on refrigerated cases that turn on lights only when activity is detected. All cases are enclosed, preserving temperatures and conserving energy.
“We removed all open cases because they’re an unnecessary electric expense,” elaborates Skelton.
January and February 2015 were once again harsh winter months that saw heavy energy consumption, but by March the store was nearly net zero. “They were net zero for many months [last year],” says Skelton. He expects the store to achieve net zero “eight to nine months a year, but not always in the winter months.”
But even if the store doesn’t reach net zero year-round, the technology used on the project drove annual operating costs down. Its annual operating cost per year equals what the previous Evansville store’s cost was every two months.
Achieving net zero is the goal, but Skelton says that “energy savings was not the driving factor” of this project. “We wanted to learn the technology.” He says the sustainability manager justified the project by using empirical data. For example, they determined that the savings they can achieve by switching to LED lights are worthwhile for new stores, and they also want to retrofit existing stores.
They evaluated various sustainable features used in the Evansville store, with the notion of using them at other locations. Recycled content brick was used on the building’s façade—an approach that may be feasible only for remodeled stores.
Additional benefits of the project for both Cyclone Energy and Walgreens come from branding their image. For Walgreens, which, Skelton says, used marketing funds to cover construction, the results “significantly out-performed expectations in sales.”
One idea that can be retrofitted for almost any store is a charging station for electric vehicles to encourage customers to consider energy efficiency. Another way Walgreens tries to encourage customers to address their energy usage is by alerting them of the store’s current energy usage through a dashboard.
Cyclone gained momentum in building its reputation. “We’ve published papers [about this project] and used information on other projects on a comparison basis. It has added to the data set. We’re now writing language for RFPs.”
Sharing the findings, research, analysis, and performance of the project with the architecture, engineering, and sustainable communities at-large was always an important element of the project’s mission. “This is a cutting-edge project with lots of unknowns,” states Skelton. “That is helpful in expanding our knowledge base.”
Education is ongoing. “We will follow Walgreens as long as they allow us,” indicates Skelton. “We are still learning: pushing boundaries, finding limits…. Last winter we changed the temperature set points. It’s a dynamic process, with live updates.”
He’s referring to the extensive building automation and energy management system that provides detailed end use measurement and remote monitoring and control of systems. This metering is being used to compare actual building performance with the expected performance of the detailed study model. By doing so, the team has been able to identify the parts of the design that are not performing as expected, look for solutions for the problems, and improve the building’s overall energy performance.
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