The charge that officials at Great River Energy, Maple Grove, Minn., delivered to the architecture firm of Perkins + Will, Minneapolis, was both simple and complicated -- simple in its directness, but complicated in its execution.

“Our president and CEO, David Saggau, speaking on behalf of the 28 distribution cooperatives that comprise Great River Energy, clearly stated what it was we were looking for in our new corporate headquarters,” said Gary Connett, director with Great River Energy. “The goal from the outset was to develop the most efficient building possible -- and that’s a LEED Platinum building.”

The goal was to design a structure that contributes to the advancement of sustainable green building and serves as an educational tool for others to see how easily -- and relatively inexpensively -- these technologies can be applied. “After all,” Connett said, “we can’t ask our customers to use less energy unless we show them that it can be done, how well it can be done and how it can be done comfortably without sacrificing beauty, functionality or practicality.”

And so began a two-year journey in innovative design that moved the not-for-profit electric cooperative from its 30-year-old headquarters in Elk River, Minn., to a new four-story, 166,000-square-foot structure in nearby Maple Grove.

Great River Energy grew out of the 1999 merger of two cooperatives -- United Power and Cooperative Power Associates. Today the cooperative provides wholesale electricity to more than 1.7 million people through 28 member distribution cooperatives in Minnesota and Wisconsin. With more than $2 billion in assets, Great River Energy is the second largest utility in the state based on generating capacity and the fifth largest generation and transmission cooperative in the United States. Its stated mission is to provide members with reliable energy at competitive rates in harmony with a sustainable environment.

Tremendous growth in the size of its customer base, the cooperative’s sales, and the additional staffing required to support that growth forced Great River Energy to relocate its headquarters to a new, larger facility, according to Connett. So, in 2006, the cooperative sought the help of design firm Perkins+Will and general contractor McGough Construction, Minneapolis, to assess possible sites for their green building potential.

The 12.5-acre site in Maple Grove proved to be the best location for a number of reasons. “First,” Connett said, “the site is located just two blocks from Maple Grove Transit, providing bus access to our employees and reducing the number of parking spaces we required at the new facility.” Great River Energy worked with Maple Grove Transit to establish a bus route between the former Elk River headquarters and Maple Grove that is subsidized by the cooperative, enabling employees to park at the old office building and ride to and from work at no charge. It also qualified the project for up to two LEED points in the Sustainable Sites category (Credit 4.1 and 4.2 for alternative transportation).

Daylight harvesting represents another important energy-efficient solution Great River Energy employs and is immediately apparent to visitors as they tour the facility. “It’s a very open building with windows everywhere,” Connett said.

“In fact,” added Perkins + Will architect Russell Philstrom, “over 90 percent of the building’s occupants have a view to the outside, and 85-90 percent have enough daylight that they do not need to turn lights on in their work spaces.”

The building is designed with a long axis running east-west, according to Philstrom, allowing most of the glass to face north and south reducing unwanted solar heat gain. Narrow floor plates are designed to harvest maximum daylight reducing the need for artificial lighting, minimizing heat gain from lights, and reducing the need for air conditioning to cool the building. In addition, daylight atriums between floor plates in the room allow natural light into the center of the building.

Dimming ballasts, daylight sensors and motion sensors also help reduce artificial lighting needs and provide consistent lighting inside regardless of daylight levels. Overhead lighting fixtures run parallel with exterior walls so the lights closest to the windows and internal atriums dim to reduce energy use on sunny days. All of the windows also have high-performance coatings, which limit heat gain from the sun while allowing daylight to pass through.

In addition, workstations are designed with lower walls to allow more daylight into desk areas, and all offices have a windowed wall to allow the passage of natural light. Where artificial lighting is required, the building uses high-efficiency ultra-low mercury fluorescent lights. “As a result of daylight harvesting and the related technologies installed in the building, we use approximately 40 percent less energy for lighting than similarly sized buildings that use standard technology,” Connett said.

Optimizing daylighting worked with another important strategy to help the new building capture all ten of the LEED points in the optimized energy performance category. That unique and possibly first-of-its kind strategy combined lake-source geothermal technology and underfloor displacement ventilation in a high-efficiency heating, ventilation and air-conditioning (HVAC) system.

Geothermal heating and cooling systems transfer heat from the ground or, in this case, water into buildings in winter and reverse the process in summer. A nontoxic, biodegradable fluid called propylene glycol is mixed with water and pumped through 36 miles of plastic piping coiled at the bottom of nearby Arbor Lake to exchange heat with the lake.

“The geothermal system at Great River Energy takes advantage of the proximity of Arbor Lake, and extracts heat from the building during the summer and absorbs heat from the lake in the winter, relying on the relatively constant 39- to 60-degree lake bottom temperature” explained Dale Holland, executive vice president and chief technical officer for Dunham Associates Inc., Minneapolis. “When the building needs air conditioning, that water runs through a coil in one of the displacement ventilation fan coil units to make 65-degree air, which cools the core of the building. We don’t have to run a compressor, and we’re using much less energy. What’s more, the system does not require a chiller or a boiler.”

The efficiency of the geothermal system is compounded by raised floors that allow air to be delivered by in-floor diffusers located throughout the building rather than through a traditional overhead forced-air system, which requires significantly more energy to operate. “We recommended underfloor displacement because it’s a very efficient way to get clean, cool air to people,” Philstrom said. “It has huge energy savings because you’re not pulling air down to the user from an overhead system.”

“Displacement ventilation mixes the supply air under the floor with room air about six inches above the floor,” Holland said. “Then, the low-velocity, cool air rises over warm bodies and is displaced to air vents at ceiling level. In effect, natural convection instead of fan power drives the airflow and conserves energy in the process.”

Holland and his firm actually created a model of how the underfloor displacement ventilation system would behave using a 3-D airflow and energy modeling process called computational fluid dynamics -- the same modeling process used in the design of airplanes, rockets and cars. “This modeling process allows you to see the stratification that occurs within a room and forecast temperatures that will occur under the floor,” Holland said. “We use it to predict and design the air to the underfloor plenum using stub ducts and registers to blend the air so that it will be uniform in temperature under the floor and comfortable above.”

The selection of diffusers and Tate for access floors was also important to the success of the underfloor displacement ventilation system. according to Holland. “The diffusers we used throw air horizontally across the floor surface in a flat spin,” Holland said. “As a result, air does not blow directly on people and comfort levels are maintained.

“We chose the Tate system, because Tate makes a good quality system. The floors that were designed for this building included all kinds of different surfaces, from carpet to tile to recycled glass, and the Tate system offered the flexibility to accept each of them.”

“The use of large format terrazzo tiles on the Tate raised floor system was a unique application,” Philstrom added. “So, extra coordination was needed to be sure the height of the raised floor was the right height to accept both the substrate and the tile. We did a mock-up of the system, and everything worked out beautifully thanks, in large part, to the flexibility of the Tate system.”

However, Philstrom and Holland would agree that sealing the plenum and keeping it sealed was probably the single most important contributor to the success of the underfloor displacement ventilation system. “Unlike a traditional raised floor system that is just there for conduits and cabling,” Philstrom explained, “because we were running the air system through the floor, adequately sealing the plenum space was a high priority.

“As a result, extra care was taken to make sure details were drawn and there was coordination between architectural and mechanical drawings in terms of where diffusers were located as well as walls that extended below the floor surface. The contractor worked hard to be sure everyone knew that whenever they put a hole through the floor or a wall, that hole needed to be sealed.”

“We talked to the Tate suppliers and found that they had some really innovative details of how to seal off the wall penetrations,” Holland said. “The details were very important, and I think we followed them, and we have a successful project because of that.”

Intermediate inspections performed by the design and construction team ensured those seals were made. Later, a third-party commissioning agent checked to be certain all mechanical systems were sealed properly, diffusers were working correctly and air velocity was accurate.

The benefits of this unique HVAC system begin with energy savings directly attributed to the way the system operates. “An underfloor displacement ventilation system is very energy efficient because it requires very little energy to move air under the floor,” Holland said. “Far less fan energy is required.

“And if we use displacement ventilation, we’re really only air-conditioning the occupied zone, which is the lower six feet of this high atrium building. So, we’re saving energy by not air-conditioning a 40-foot high space. You combine that with the efficiency you gain with geothermal technology and you begin to see how all of these strategies work together to achieve super energy savings.”

Holland is quick to point to other benefits the underfloor displacement ventilation system offered, including occupant comfort. An underfloor displacement ventilation system delivers low-velocity clean air to the building through a series of small, circular vents located in the floor. “Each employee work station contains its own adjustable vent, giving employees the freedom to control the airflow in their work spaces,” he said, “improving their comfort, health and, ultimately, their productivity.”

Underfloor systems also provide flexibility allowing spaces to be repurposed easily. “And they have an aesthetic appeal,” Holland added. “Because there are not ceiling ducts for air distribution, we were able to significantly raise ceiling heights or even remove ceilings to improve daylighting.” According to Philstrom, “The result is 10-foot-high ceilings without increasing the slab-to-slab height and a floor-to-ceiling curtain wall system that deliver more light to the building core, minimize lighting requirements and reduce cooling demands.”

The site is also important in terms of renewable energy. “The Maple Grove site enabled us to take advantage of a southern exposure, saving money due to daylighting,” Connett explained, “and it provides good wind resources for the wind turbine we erected on the site.”

The proximity to Arbor Lake provided another site advantage because lake-source geothermal heating and cooling offered a major energy saver for the building. “Finally,” Connett said, “the Maple Grove site includes a number of amenities for our employees -- things like shops, access to daycare and medical clinics, a walking path and other conveniences our employees value.”

The structure erected on the Maple Grove site features an array of energy-efficient solutions. As visitors approach the site, the most obvious of these features is the 166-foot-tall wind turbine. “We experienced some concerns from people in the community about having such a large structure on site. But, we did a lot of work to educate people and make sure everyone understood the benefits of wind energy, and we reassured them that the turbine would not be noisy or disruptive. As a result, we were granted a five-year interim use permit for a single wind turbine, and today, Maple Grove is known as ‘the town that has the wind turbine.’ It’s attracting attention to the community and distinguishes Maple Grove, in a positive way, from other suburban areas of the Twin Cities.”

The turbine, manufactured in Denmark, is a 200 kilowatt NEG Micon M700. It was first installed and put in service in the Netherlands. Energy Maintenance Services (EMS) refurbished the gears in the gearbox, and the generator was rewound to change the machine from a two-speed to a one-speed wind turbine to increase efficiency.

Less obvious than the wind turbine is the array of photovoltaic panels mounted on the roof of the building and at ground level. Together, the panels produce 72 kW of energy at full capacity. This means that between the wind turbine and the photovoltaic panels, the building receives up to 15 percent of its energy from on-site renewable resources. The remainder is purchased from off-site wind generation facilities so that 100 percent of the building’s electricity comes from renewable sources.

The new headquarters for Great River Energy includes a number of additional features that contribute to sustainability and set new standards for building design and construction:

• A rainwater harvesting system collects water on the roof and transfers it to an underground cistern that holds approximately 20,000 gallons of water. The water is filtered, circulated through an eco-friendly water treatment system and used in Great River Energy’s toilets and urinals, limiting the use of potable water.
• The site landscaping uses a significant amount of native and adapted plantings that will rarely require additional water beyond that provided by natural rain events once established. High-efficiency permanent irrigation was installed to assist in getting plantlets established and to provide occasional supplemental watering.
• Approximately 18 percent of the materials used in the building contained post-consumer or pre-consumer recycled content. For example, the facility’s concrete structural frame contains more than 45 percent fly ash, the product created when coal is burned to generate electricity. Fly ash from Great River Energy’s Coal Creek Station was used in both the structure as well as a replacement for cement and in the carpet backing, decreasing the amount of waste sent to landfills and reducing energy used to produce cement.
• Energy-efficient elevators use 60 percent less energy and require less space. The elevators use a counterbalance mechanism and high-efficiency motors.
• More than 95 percent, or 4.3 million pounds, of construction waste was recycled, exceeding the LEED requirement of 75 percent.
• The building’s thermal polyolefin (TPO) white roof, which has a minimum solar reflectance index of 78, reflects much of the sun’s energy back into the sky rather than allowing it to build up as heat on the roof surface. The roof also helps harvest sunlight within workspaces by reflecting sunlight into the daylighting atriums. The smooth surface of the TPO roofing is good for non-potable rainwater collection and is an easy surface for installing photovoltaic racks.
• Approximately 87 percent of the wood used throughout the building is Forest Stewardship Council (FSC) certified, which means it was harvested in an environmentally, socially and economically responsible way.

Great River Energy headquarters was evaluated by the U.S. Green Building Council based on the LEED for new construction version 2.2 (LEED-NC V2.2) rating system, which tallies a cumulative score based on a 69-point scale. On Sept. 26, 2008, the structure was awarded Platinum certification earning 56 points (four more than required for Platinum certification) out of the 63 possible points that were pursued for this project, becoming the first building to achieve such recognition in Minnesota.

In addition to Platinum LEED certification, the building has collected a variety of awards that recognize its sustainable features and unique, groundbreaking design. When compared to similarly sized traditional office buildings, the statistics it generates are noteworthy. The building:

• consumes 50 percent less energy than Minnesota code requires,
• uses 40 percent less electricity for lighting,
• uses 90 percent less water, and
• saves nearly $90,000 in annual energy costs, with an anticipated payback of seven years.

What’s more, Great River Energy accomplished all of this for an incremental cost of less than 10 percent more than a conventional corporate headquarters of the same size.

“This building demonstrates that the construction of an energy-efficient building is not cost prohibitive,” Connett said. “Since it opened a year ago, thousands of people, ranging from architects and builders to curious area residents, have toured the facility, learning firsthand how to grow and build responsibly.”

Interactive screens in the lobby show real-time data on energy savings throughout the building and production from the wind turbine and photovoltaic panels. Other applications include system animations to illustrate how the building’s features work and recommendations for environmental savings in people’s own homes.

“And in the end, that is what we wanted to do -- build a state-of-the-art facility that displays the benefits of energy efficiency, sustainability and conservation and changes the way people think about sustainable building design,” Connett said.

According to Philstrom, “Designing and constructing a LEED Platinum building is not an easy task. Above all else, it requires an integrated, collaborative approach where the goal to produce a sustainable and energy-efficient structure influences every decision. We achieved tremendous success with Great River Energy paving the way for the day when we won’t be talking about LEED Platinum -- when the approach we took to this building and the technologies we employed will be part of every building we design.”

Connett agreed. “This building is progressive, forward-thinking and accurately reflects the open, collaborative culture that is Great River Energy. It shows what is possible today and what will sustain us tomorrow. In short, it’s a blueprint for the future.”