Having already developed dozens of projects for other companies with an eye on sustainable development, Opus Northwest is going green in its own back yard with an expansion of its headquarters in Minnetonka.

The Minnetonka-based developer has set its sights on having the project certified by the nonprofit U.S. Green Building Council's Leadership in Energy and Environmental Design (LEED) building program. Established in 2000, the LEED system rates buildings in a variety of areas, including water and energy conservation, use of recycled or environmentally friendly materials and indoor quality.

The number of LEED-certfied buildings nationwide has jumped from 38 in 2002 to more than 1,500 so far this year, according to the council, based in Washington, D.C.

Minnesota has 14 certified commercial projects, up from just four a year ago, while the number of developments seeking certification has grown to more than 170.

In Minnesota Opus is pursuing LEED certification for the Excelsior Crossings office complex in Hopkins and new office buildings in Minnetonka for Syngenta Seeds and UnitedHealth Group.

"Virtually all our new speculative projects are geared toward sustainable development," said Opus Senior Vice President Tim Murnane. "We decided that with our own building it was important for us to walk the walk."

Work on the 148,000-square-foot building, made in part with recycled construction materials, has been going on for several months. Other environmentally friendly features include the use of low-vapor paints, adhesives and sealants, and high-performance glazing and glass to maximize energy efficiency.

But work on perhaps the most ambitious energy-saving feature of the building just began. Earlier this month two rigs began the task of drilling 299 wells, each about 185 feet deep, on the land surrounding the new building. The wells are part of a geothermal system that will use the relatively constant temperature of the ground to heat and cool the building.

Each of the wells will contain a pipe that runs to the bottom of the hole and back to the top. The pipes will be filled with a fluid that allows for the exchange of heat from the ground to the building. The pipes will be connected to the building through a complex series of circuits, first to a vault outside the building and then to a mechanical room inside. The fluid in the pipes will be circulated through the building to heat pumps located in the ceiling space.

Opus expects a 50 percent energy saving over a conventional heating and cooling system that uses a boiler and a cooling tower, according to Hollis Linehan, manager of mechanical/ electrical technology.

Opus declined to disclose the total cost of the expansion but said the investment in geothermal and other features to qualify for LEED will add 3 to 5 percent to the project's cost. It expects to recoup the added costs in about eight years from energy savings.

Linehan said the cost of drilling the well field should run to about $700,000. Three more rigs are expected to join the two now at the site, with drilling work expected to take about four months, he said. The drilling work doesn't mean the project will take longer to complete, although it requires an extra level of planning to coordinate drilling with other construction activities, Linehan said.

This is Opus Northwest's second commercial project with a geothermal system. Five years ago it built one with 88 wells, each 300 feet deep, for Luther College's Center for the Arts in Decorah, Iowa. It recently did some soil testing for the University of St. Thomas in St. Paul to determine the feasibility of a geothermal system in future campus development. Soils have to be tested because those with more moisture tend to be better for geothermal systems than dry, sandy soils, he said.

Geothermal systems have been around for several years and can be found in homes as well as some commercial and government buildings. Judith Webb, a spokesperson the U.S. Green Building Council, said an increasing number of commercial projects seeking LEED certification include geothermal heating and cooling technology. In Minnesota, they include a system buried under Maple Grove's Arbor Lake for the headquarters of Great River Energy, which was completed last year.

The technology was available several years ago when Opus built its first headquarters building, but the economic benefit wasn't as great then.

"The motivations are different today," Linehan said. "Remember that we all used to go driving around, too, just for fun."

Susan Feyder • 612-673-1723