Heating houses is a major energy suck. Behind passenger cars and trucking, it’s the largest energy draw in the United States.
But it turns out homeowners looking to stay warm don’t need to rely just on a pile of electrons or a pipeful of methane.
Research out of the University of Oregon shows that, even on the cloudy western sides of Oregon and Washington, a large chunk of our heating needs could be met with a few well-positioned (and well-managed) skylights.
“I’ve been interested in passive solar heating for quite a number of years, but we live in a cloudy climate. And there has been a lot of skepticism about how well passive solar heating can work when you have a winter that’s dominated by cloud cover and rain,” said study lead author Alexandra Rempel, a building scientist in the Environmental Studies Program at the University of Oregon.
Passive solar heating is the warmth you feel when you stand near a sunny window. It’s heat captured indoors directly from the sun, immediately warming the air and heating surfaces that will radiate that heat over time.
Home heating is a significant contributor of carbon pollution. It contributes approximately 8% of all climate change-causing carbon dioxide emissions in industrialized nations — including the United States and Canada — that are part of the International Energy Agency.
Passive solar heating is one way to reduce reliance on fossil fuels like natural gas and heating oil to warm homes. Rempel set out to quantify just how much of this solar energy is available for use in locations across the country.
“Just what is the size of this resource we’re talking about? Is it worth going after or not?” she asked.
And according to the analysis, it is.
The researchers found that there is enough solar radiation available to provide about one-third of residential heating needs nationwide. The calculations were based on homes having 100 square feet of skylights oriented towards the south.
To understand the potential, the researchers used home energy consumption data and paired it with satellite observations of solar radiation.
“We can calculate that on a surface of any given area, at any given tilt, in any given location across the United States,” said University of Oregon earth scientist Alan Rempel, who provided the mathematical analysis. “What we’re interested in is not just the amount of energy that’s coming, but the amount of energy that’s coming at a time when we need it.”
The passive solar energy collected couldn’t take the place of a heating system in most climates, but it could cut heating costs — especially in the fall and spring when temperatures aren’t so cold and the days aren’t so short.
In Western Oregon and Washington there’s enough solar radiation to provide about three megawatt hours of annual heating energy relief for homes with 10 square meters of well-positioned, unobscured skylights. This is enough to power the average U.S. home for more than three months in 2020.
On the east side of the two states, where there’s less cloud cover in the winter, there can be more than 5 megawatt hours of passive solar energy captured — but heating costs are higher there, as well.
A small number of organizations and builders currently promote passive solar as a way to cut energy costs.
The University of Oregon research quantifies how much energy is available via skylights and other forms of passive solar, but Kosmecki says designers and builders have to look beyond that.
“The issue has [been], and is still, how to effectively and affordably capture and control the [solar energy],” he said.
These challenges might have hampered the integration of passive solar systems into building design in the past.
In some places promoting passive heating, the focus has largely been on windows and roof overhangs designed to maximize heat collection in the winter and sun blocking in the summer. Skylights have been more of an afterthought where the problems outweigh the benefits.
A U.S. Department of Energy publication on passive solar design states, “Sloped or horizontal glass (e.g., skylights) admit light but are often problematic because of unwanted seasonal overheating, radiant heat loss, and assorted other problems.”
But the researchers found that skylights are ideal for capturing solar energy because of their orientation to the sun. It’s the angle of these skylights that allow this type of heat collection to be possible — especially in cloudy climates.
The cloudier the climate, the closer to horizontal the skylights should be in order to catch the diffuse solar radiation. In the Pacific Northwest, that angle ranges between 45-60 degrees above horizontal, depending on location.
“They’re broad optimums. If you’re off by 10 degrees plus or minus, that’s not so bad. You’re still getting 90% or some large fraction of the available energy,” Alan Rempel said.
Practically, achieving these optimal angles while retrofitting existing structures with skylights (and the insulation needed to manage them effectively) could be cost-prohibitive. It would be easier to do in new construction.
And Alexandra Rempel says that the amount of energy that could be saved might not be enough to trigger action on the individual level. Even so, the analysis revealed real potential for local climate policy.
“The city of Portland might say, ‘Oh yes, on the city scale, we could actually be gaining something like 400,000 megawatt hours. And now it’s worth our while to offer some kind of an incentive,’” she said.
Rempel says not only would that contribute to decarbonizing space heating, which is a difficult thing to do, that kind of policy could have benefits for the grid. Using more passive solar heating could free up capacity on the grid as the Northwest transitions away from fossil fuels by electrifying transportation and buildings.
The research was published in the Journal Renewable & Sustainable Energy Reviews.