Cool Running 2008

MICHAEL GARRY

With the entire country as its laboratory, Wal-Mart Stores has been conducting step-by-step experiments to create the ultimate “green store.”

The world's largest retailer, based in Bentonville, Ark., began its foray into retail environmentalism in July 2005 with its first experimental store, a 206,000-square-foot supercenter in McKinney, Texas. The store featured around 50 green initiatives, ranging from wind turbines and solar panels to radiant floor heating and a biofuel boiler.

Later in 2005, Wal-Mart opened its second experimental supercenter, in Aurora, Colo., whose foundation includes more than 500 tons of recycled runway from Denver's retired Stapleton Airport. This store pursued many of the same energy-saving projects as the McKinney store, as well as others.

Two government-sponsored labs signed up to monitor the stores over a three-year period: Oak Ridge National Laboratory for the McKinney store and the National Renewable Energy Laboratory for the Aurora store. Wal-Mart has taken one of the technologies in those stores — LED lighting for refrigerated cases — and installed it in more than 500 U.S. stores, said Wal-Mart spokesman Kory Lundberg.

Wal-Mart professes an eagerness to share its energy and environmental findings with others. “I'm here to share all of our learning with the technical world — including our competitors — because the more people who utilize the technology, the larger the market, which drives the price down, or at least keeps it from going up,” said Richard Royal, Wal-Mart's mechanical design manager, prototypical design, in September at the Food Marketing Institute's Energy & Technical Services Conference in Orlando, Fla.

In 2007, Wal-Mart began opening what it calls HE (high-efficiency) stores that incorporate some of the features of the experimental stores plus other technologies. Three first-generation HE.1 supercenters, which are up to 20% more efficient than a conventional supercenter operating in 2005, were unveiled in Kansas City, Mo.; Rockton, Ill.; and Highland Village, Texas. This year, four second-generation HE.2 supercenters, designed to be up to 25% more efficient, debuted in Romeoville, Ill.; Bernalillo, N.M.; Wichita, Kan.; and Garland, Texas. HE.3 stores will be unveiled next year.

The end-game for Wal-Mart, said Lundberg, is to design and build a prototype supercenter over the next few years that will be 25% to 30% more energy-efficient and generate 30% less greenhouse gas emissions than a 2005 supercenter. “The HE stores are helping us to find the technology to do that,” he said. Wal-Mart wants to cut greenhouse gas emissions in existing stores by 20% over the next seven years as well.

But before building that generic prototype, Wal-Mart has taken a detour, creating what it calls its HE.5 prototype, designed for specific climates. The first HE.5 supercenter opened in a desert environment — Las Vegas — in March, and by leveraging the dry Western climate this building is using up to 45% less energy than the baseline, making it Wal-Mart's most energy-efficient U.S. store.

EFFICIENT COOLING

Wal-Mart says that the 195,000-square-foot HE.5 Las Vegas supercenter is believed to be the first U.S. retail store to generate a cool inside temperature by chilling water via evaporation and circulating it underneath the floor. The water is chilled to around 60 degrees Fahrenheit in a series of rooftop cooling towers using a process called indirect evaporative cooling, which lowers the temperature of the water as some of the liquid evaporates.

Other HE Wal-Mart stores employ water that is cooled by means of electricity, but the HE.5 store's evaporative system is a much more energy-efficient cooling process. “We believe this is the most efficient cooling system implemented in a major retail facility,” said Richard Bourne, associate director, The Western Cooling Efficiency Center at the University of California, Davis, which worked with Wal-Mart to develop its heating and cooling system. “This project recognizes the very significant opportunity to integrate advanced natural cooling features in dry climates, thereby reducing the need to build new peak power generating plants.”

The cool water circulates through a mesh of tiny plastic tubes underneath the Las Vegas store, chilling the air directly above what is called a radiant floor, which Royal described as “one of the biggest issues that everybody was excited about.”

The radiant floor, first tested by Wal-Mart at its Aurora store, is much more energy-efficient than a conventional air conditioning system, in part because the radiant system addresses “the area you want to cool — the bottom 10 or 12 feet of the store,” Royal said. The cost of radiant flooring has dropped from $5 per square foot to less than $1.50 per square foot, he noted.

In a more humid climate such as Florida, there is a greater risk of moisture developing on radiant floors, creating slippery conditions. However, because Las Vegas has a very low dew point temperature, Royal explained, “it's very unlikely that the floor is going to get wet.” The store still uses a dehumidification system to keep any moisture from coming in from the outside.

Unlike most Wal-Mart stores, which need up to 40 rooftop units to heat and cool the building, the Las Vegas store uses just 10 air-handling units that bring in fresh air to maintain air quality, reducing noise and maintenance costs. “When you take that many rooftop units off, there's a concern about having proper distribution of air and temperature,” Royal said. “But we did some computational modeling that showed we have very few hot spots in the store.”

The cold water generated by the cooling towers has another purpose — to absorb heat from the store's refrigerated cases (more typically done via air) and its low-temperature condensers. The water is circulated to what is called the water-source format refrigeration system and then to the store's cases and low- and medium-temperature refrigeration systems. “It's easier to reject heat to water than to air,” noted Mark Modera, director, Western Cooling Efficiency Center.

As with most Wal-Mart stores, waste heat from refrigeration is used to heat water for rest rooms and kitchen areas, supplying about 70% of the store's hot water needs.

To refrigerate the medium-temperature cases such as meat, dairy and produce, the Las Vegas store employs a secondary-loop system from Hill Phoenix, Conyers, Ga., which uses R-404A as a refrigerant and propylene glycol as the cooling agent. But unlike other medium-temperature secondary-loop installations, which use five or six compressors in one motor room, this system breaks out the compressors into two rooftop “modular chillers,” which are closer to the cases.

The medium-temperature refrigerant charge in both modular units totals 100 pounds, 90% less than that of a typical Wal-Mart store. “They're able to do the whole store for less than 200 pounds,” a 75% reduction, said Scott Martin, director of sustainable technologies for Hill Phoenix.

The low-temperature refrigeration system is a conventional DX design but is split up into three rooftop distributed units that are located close to the frozen-food cases. The units are connected to the cases via “loop piping” that reduces the amount of piping and refrigerant required; overall, these water-cooled units use 30% less refrigerant.

Splitting up the refrigerated loads into modular units represents “the direction we are going,” said Royal. Modular systems experience a lower leak rate, he said, adding that even if one unit loses its charge, “you can still run the rack” with the other units.

LIGHTING COMPARISON

Among the Las Vegas store's other green attributes, Royal was particularly enthusiastic about LED lighting recently installed in the parking lot. “It reduces our wattage considerably,” he said. “This is one of those technologies that we can scale everywhere; it has no limitations by location.”

Wal-Mart ran a test in the parking lot comparing LED lighting with conventional lighting. “The coverage was much better with LED,” said Royal. “It didn't look like a baseball field at night; it had much lower intensity.”

Wal-Mart also uses LED lighting in freezer and refrigerated cases in its high-efficiency stores and all new supercenters. According to Wal-Mart, LED lights last three to four times longer than fluorescent bulbs, stay cooler and perform better in cold environments; moreover, motion-sensor detectors turn off the lighting when no one is present.

Other green features deployed in Las Vegas and other new Wal-Mart supercenters include daylight harvesting, white roofs and efficient bathroom fixtures. The daylight harvesting system incorporates 210 skylights that refract daylight throughout the store as well as light sensors that monitor the amount of available natural light. During periods of higher natural light, the system dims or turns off the store lights, reducing by up to 75% the electric lighting energy used in the supercenter during daylight hours, an annual savings of 800,000 kilowatt-hours per year.