Fundamentals of Resilient Design #5: Passive Solar Heating

Posted by on Aug 30, 2012

This 2,600 ft2 home in St. Peter, Minnesota, designed by Sarah Nettleton Architects, features passive solar design with concrete floors for thermal storage. Sliding shade screens can be seen on the south-facing fenestration. Photo Don Wong.

 

When combined with a highly insulated building envelope, passive solar is the best way to ensure that a home will maintain livable conditions in the event of loss of power or heating fuel.

As I discussed in my previous blog, a resilient home is extremely well-insulated, so that it can be kept warm with very little supplemental heat—and if power or heating fuel is lost, for some reason, there won’t be risk of homeowners getting dangerously cold or their pipes freezing. If we design and orient the house in such a way that natural heating from the sun can occur, we add to that resilience and further reduce the risk of the house getting too cold in the winter.

Passive solar heating

I had the good fortune of working in Santa Fe, New Mexico for a solar energy organization in the late-1970s, when the passive solar energy movement was just emerging. Northern New Mexico was the epicenter of research into passive solar—the effort, ironically, being led by Los Alamos National Laboratory, which, a generation earlier, had brought us the nuclear age.

It was an exciting time. The relationship between solar gain and thermal storage was becoming understood. It was discovered that very simple south-facing windows and high-mass walls and floors were not only far simpler than the very complex active solar heating systems that emerged (briefly) in the early 70s, but they also worked better.

Direct-gain passive solar

The most common passive solar heating system is known as direct-gain. South-facing windows transmit sunlight that is absorbed by relatively high-mass materials in the house. In a sense, the house itself becomes the solar collector and heat storage system, with different components serving multiple functions. Those windows also provide views to the outdoors and bring in natural daylighting, while the thermal mass consists of the walls or floors that serve structural functions. We need those elements anyway, but by optimizing their area, placement, and configuration, they can become the primary heating system.

The challenge with direct-gain passive solar heating is to provide the right amount of glass in the proper orientations and incorporate the proper amount of thermal mass to minimize temperature cycling and prevent overheating. (Back in New Mexico in the late-1970s, there were a lot of poorly designed passive solar homes that overheated horribly.)

As window glazings have improved in the three decades since my days in New Mexico and as we have recognized the primary importance of highly insulated buildings (see last week’s blog), the opportunities for passive solar heating have improved—but so has the complexity. With better glazings and reduced heat flow out of homes, one has to be more careful to prevent overheating or unacceptable temperature cycling. And we have to choose glazings more carefully, because the best-insulating low-e glazings block too much of the solar gain. For passive solar, we want glazings with high solar heat gain coefficient (SHGC) ratings—values over 0.6 are great, but 0.5 should be considered a minimum when passive solar heating is important.

Trombe walls

Direct-gain is the most common passive solar energy system, but it isn’t the only one. With indirect-gain passive solar, the collection is only indirectly connected to the living space. The most common such system is a Trombe wall—a south-facing high-mass masonry wall with glass or plastic glazing held away from the wall in a frame. Sunlight shines through the glazing and heats the dark surface of the masonry wall. Heat moves into the wall where it is stored and gradually conducts through to the interior, where it radiates heat to the living space.

Some experts question whether it’s better to simply add more insulation to that south wall and skip the indirect solar gain, while others argue that the solar is very important—especially relative to resilience. If other energy inputs to the house become unavailable for some reason, delivering heat with a Trombe wall could be very beneficial.

Sunspaces

Finally, there are isolated-gain passive solar systems in which solar heat is collected in one place and brought into the house only when desired. A south-facing attached sunspace is the most common isolated-gain system. The sunspace heats up during the day and windows or vents connecting the house and sunspace can be opened to deliver heat into the house, or kept closed to keep that heat out. An insulated wall between the house and sunspace ensures that as the sunspace cools off at night (due to heat loss through the large amount of glass), it won’t cool the house down. The sunspace serves as a heating system for the house, even as it also serves as a supplemental daytime living area and a place to grow plants (especially plants that can accept significant temperature cycling).

Energy modeling of passive solar buildings

Fortunately, as the complexity has increased, the computer software tools for modeling energy performance of homes with significant solar gain have also improved. Such programs as EnergyPlus, REM Design, eQUEST, and SUNREL (a somewhat buggy update of SUNCODE from the 1980s, which was a favorite of many) all do a good job at modeling energy performance and passive solar contributions to heating.

With any such software, the designer inputs a location close to where the house is located to load the relevant solar gain and other climate data. Note that even with state-of-the-art software, hiring a designer with experience in passive solar design is key to achieving good performance.

Passive solar and resilience

No matter which type of passive solar heating system is employed, it plays a key role in making a house resilient to power interruptions and loss of heating fuel. If there is no solar gain, even a highly insulated house will gradually cool off. The more insulation, the slower the temperature in the house will drop, but drop it will. With a reasonable amount of passive solar gain and a really well-insulated building envelope, enough heat will enter the house to compensate for most of that heat loss in all but the cloudiest weather.

Builder Bruce Brownell, of Adirondack Alternate Energy in Edinburg, New York, has been very-well-insulated building passive solar homes since the 1970s. He tells me that through extensive modeling he has found that his homes will never drop below 46° or 47°F, even in a very cold, 8,500 degree-day climate. I’ll describe more about Bruce’s houses in a future blog.

More on passive solar design

My July, 2012 feature article in Environmental Building News, “Passive Solar Design,” provides a lot more detail on the topic. A log-in is required.

Along with founding the Resilient Design Institute in 2012, Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. To keep up with his latest articles and musings, you can sign up for his Twitter feed.

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