Passive solar: Actively heating local government buildings


When it comes to relying on solar energy to save on heating costs, Doug Stahl has always been a skeptic. He’s a practical, soft-spoken man who has been running school buildings for many years.

But standing on the roof of St. Anthony High School on this overcast October day, next to the solar wall installed almost a year ago, it’s clear Stahl—Building and Grounds Director for the St. Anthony-New Brighton School District—is now a believer. “I really didn’t expect it to work out as well as it has,” he says, gesturing to the wall of dark gray aluminum facing south. “But so far it’s really been great.”

What turned Stahl around turns out to be a surprisingly simple technology. The solar wall—a product patented and manufactured by Conserval Engineering of Toronto, Ontario— is basically a wall of dark, perforated aluminum paneling attached to a south-facing wall of the mechanical penthouse perched atop the school. The panels, heated by the sun, intercept fresh air being drawn into a building’s heating system, trapping it just long enough to preheat it. The panels warm outside air that is, say, -10° F in January to something much closer to “room temperature,” relieving the demand on the high school’s heating system. Stahl says when outdoor air temperature drops below about 55° F, a computerized system kicks in; above that level, the air is taken in through bypass vents that do not preheat the air.

Stahl standing next to solar wall. The wall was installed on the south side of the mechanical penthouse on top of the school. Photo by Paul Purman.

The bottom line? The district and its contractor, McKinstry Company, will sit down next month and crunch the numbers from the past year; early indications are the wall may deliver as much as $5,000 in energy savings annually directly resulting from the SolarWall, estimates Tom Laufenberg, a project manager with McKinstry. The payback period on the wall installation is estimated at 15 years, according to Laufenberg. Related upgrades such as new high efficiency water heating boilers push estimated annual savings up to over $48,000. St. Anthony-New Brighton superintendent Rod Thompson, too, is enthusiastic. “Every penny saved [with the wall] can stay in the classroom,” says Thompson.

Passive solar benefits for public entities

St. Anthony-New Brighton’s solar wall is an example of a passive solar technology, as opposed to “active” solar. The difference is that passive solar captures the sun’s heat and light “without the use of mechanical or electrical equipment,” while active solar projects typically use such equipment to convert sunlight into electricity or heated water. That’s according to a report on reducing energy costs for local government issued in July by Minnesota State Auditor’s office.

The same report highlights the Minneapolis’ Third Police Precinct building’s solar wall project as a case study of passive solar energy use. The building, at East Lake Street and Minnehaha Avenue, was extensively renovated and expanded between 2003 and 2005. The renovation project was underway before the city’s official Sustainability Initiative began, but project staff were mindful of the potential for passive solar. “We did what we could with the money we had,” says Paul Miller, a project manager with the city’s Public Works Department.

Street level view of solar wall at Third Precinct. Top view in photo at top of this article. Photo by Paul Purman.

Miller shows off “green” aspects of the building on the way to the roof to see the solar wall. Much of the flooring material was recycled from other locations, and light filters down into the main hall of the building from a massive skylight on the roof. The solar wall was one of several solar options the city considered. Working with contractors and Xcel Energy, city staff determined the building’s large blank south facing wall was a natural fit for the solar wall product.

Additional Resources

The Minnesota State Auditor’s Office issued Best Practices Review:
Reducing Energy Costs in Local Government in July, 2008. Access the full report online. The report has little that would be of use to homeowners or small businesses interested in incorporating passive solar into new construction or remodeling. Two sites that do provide relevant information to homeowners are the state’s Energy Info at the Department of Commerce and the private nonprofit Minnesota Renewable Energy Society.

Grants and assistance

The State Auditor’s report lists many state resources for local government interested in reducing energy costs, including where to look for grants or rebates. Neither the New Brighton school nor the Minneapolis police building benefited from grants to local governments.

Says the school district’s superintendent Thompson “What we did was consider the payback benefits of the solar wall, and decided it was worth it to come right out of our budget.” The district’s contractor was confident enough of the energy savings to guarantee the district a minimum savings amount.

In Minneapolis, Miller says, “no grants, but we were able to tap into a design assistance program through Xcel Energy that saved the precinct money on equipment purchases.”

Independent verification of benefits?

So far the solar wall’s actual energy benefits at the Third Precinct have not been objectively measured or verified, but that’s about to change.

Patrick Tebbe and two colleagues from the Mechanical and Civil Engineering Department at Mankato State University are setting up a monitoring program on behalf of the Minnesota Department of Commerce. Tebbe, an associate professor at Mankato, says they’re interested in tracking and comparing solar walls at several different locations in the Twin Cities so that reliable independent data is available for others interested in installing a solar wall.

“These walls are all hooked up to their buildings a little different,” Tebbe says, examining the solar wall setup on the roof of the Third Precinct Building. “Over about a twelve-month period we’ll track temperature, humidity and other conditions, and look at the heating performance of the buildings to see what the effect is of the wall.”

Paul Miller welcomes the proposed outside monitoring program. Like Stahl, he’s a believer. “The wall has worked so well that at first we had to turn down our heating controls—so it’s worked better than it was predicted to.”

Other locations in the Twin Cities that have installed a solar wall include the Breck School, and InterDistrict Downtown School in Minneapolis, and the Aveda Headquarters in Blaine.

Paul Purman lives and writes from St. Paul’s Mounds Park neighborhood. Reach him at