Great River Energy, the fifth largest generation and transmission cooperative in the United States, has a promise to “practice, as well as promote, energy conservation,” and in building their new company headquarters, they did exactly that. They built one of the most energy-efficient and sustainable buildings in the country. It was awarded LEED Platinum certification in October 2008 achieving all possible energy efficiency points.

The headquarters building is a four-story, 180,000 square foot facility located in Maple Grove, Minn., at the northwest corner of the Twin Cities metropolitan area. This new building is a showcase of state-of-the art energy-efficient and sustainable strategies, and Great River Energy proudly shares it with the 8,500 visitors who have toured the building since it opened.

Great River Energy challenged the design team, Perkins + Will Architects, and Dunham, the mechanical and electrical engineers, to “do something with energy efficiency that had never been done before.” The team looked at the opportunities the building site offered for renewable and ground-source energy as well as building systems that would maximize the energy efficiency of the facility. The result of this analysis was the unique creation of a lake-source geothermal system coupled with an HVAC system that provided the highest levels of energy efficiency.

The design team began the process of designing a building that maximizes energy efficiency by looking at the opportunities offered by the site such as the adjacent Arbor Lake.

An HVAC System that Begins in a Lake

The building site was large enough to develop a ground-source energy system. However, a lake source system utilizing nearby Arbor Lake provided a very attractive option.

This 32-foot deep lake was created when a former gravel pit was filled in. As a result, it was not under the jurisdiction of the Department of Natural Resources and had no public boat access, which meant that any piping installed in the lake would not be vulnerable to damage from fishing activities or boat anchors. In that regard, it appeared to have excellent potential as a geothermal source of energy for the building.

Proceeding with the design, Dunham needed to know that the lake had sufficient volume to handle the building load and that the lake would not be adversely affected, so they commissioned a lake study. A computer simulation of the lake as a geothermal source for the building demonstrated negligible changes in temperature compared to its natural state. With that assurance, Dunham began the design of a system that would use the lake as the heat sink for both the heating and cooling energy required for the building.

Enlarge this picture The schematic drawing of the building’s heating and cooling shows the core water exchange of energy to and from Arbor Lake when system return water and lake water temperatures are favorable.

Polyethylene piping was chosen as the heat-exchanger material because of its durability. HVAC calculations indicated that over 30 miles of tubing would be required for the energy exchange between the building core water loop and the lake. The piping was coiled on skids and installed on 12-inch concrete blocks to elevate it slightly above the lake bottom. A total of 39 heat-exchanger bundles were floated out onto the lake, set in place using GPS, and sunk to the bottom by filling the tubes.

The core water loop serves water-to-water heat pumps, generating chilled water, which is distributed to the building’s interior air-handling units. It also directly cools the building’s interior space through core water coils most of the year. Perimeter areas are served by water-to-air heat pumps. The core loop allows for simultaneous exchange of heating and cooling energy within the building.

The building air is delivered using thermal displacement ventilation through an under-floor plenum. This system offers more-effective delivery of fresh air to building occupants than a conventional overhead mixing system and significantly reduces cooling and fan energy. Adjustable round in-floor diffusers maximize individual comfort.

The energy-efficient measures compounded considerably because of the integrated approach that Dunham and Perkins + Will took toward sustainability. By analyzing strategies in terms of how they worked together, a great deal of synergy became evident. One significant example is that the temperature of the lake between October and June provided a free cooling source to the building due to the higher supply temperatures in a displacement ventilation system, creating further energy savings over a conventional system design.

Extensive daylight harvesting and attractive views of the outdoors contribute to a high level of indoor environmental quality (IEQ). Building occupants receive conditioned air from the under-floor thermal displacement ventilation system through individually controlled floor diffusers.

Additional Measures


Daylight Harvesting

Much of the buildings energy savings is a result of the extensive use of daylight coupled with a state-of-the-art lighting control system. In addition to the perimeter windows on the long north and south elevations, the building has an atrium with a clerestory that brings daylight into the center of the building. Photocells sense available daylight and dim the fluorescent lighting accordingly. In addition, occupancy sensors de-energize lighting circuits at unoccupied times, and low-voltage switching allows individual control when necessary. These measures significantly minimize the heat produced by the lighting system, thereby reducing the air-conditioning load. Overall, the lighting system saves up to 40 percent on lighting energy compared to traditional buildings.

On-site renewable energy provides nearly 15 percent of the building’s energy. A 200 kW wind turbine is expected to generate 375,000 kWh per year. Photovoltaic arrays provide an additional 72 kW of power to the building.

On-Site Renewable Energy

Great River Energy includes two on-site renewable energy sources to offset the normal utility power demand of the building loads. A 166-foot tall wind turbine, rated at 200kW, produces 375,000 kWh/year, or approximately 11 percent of the building’s annual energy requirement. A 72kW photovoltaic (PV) array is estimated to produce 2-3 percent of the buildings' annual energy use. Together, the renewable energy sources provide nearly 15 percent of the building’s annual energy consumption.

The building electrical service is configured to accept a normal utility source plus power from the on-site renewable energy sources. Each of the renewable sources feeds into the main switchboard, and when the on-site wind and photovoltaic energy exceeds the current building demand, the excess energy flows back into the utility power grid.

Water Conservation Strategies

Low-flow plumbing fixtures are used throughout the building. Rainwater is harvested from the roof and used for flushing toilets, saving over 265,000 gallons of potable water per year. Overall, potable water consumption in the building is reduced by 89 percent.

This project achieved the goals established by the owner, and in so doing, it set a new standard for sustainable design and energy conservation.

Data Gathering and Analysis

The electrical distribution system was deliberately configured to enable metering by load type for future load analysis. Separate feeders are dedicated to lighting, heat pumps, plug loads, and general mechanical loads. All of the major building systems are monitored by 44 meters, which are located throughout the building, tracking both the power produced and power used on-site. This data is collected and analyzed to study building loading and trends and made available to the public on Great River Energy’s Web site.

As a result of the design innovations, the building will save 45 percent on annual energy costs compared to new code-compliant buildings. Working together with their design partners, Great River Energy helped define a new standard for energy efficiency in building construction.