Resilient Design: Passive Solar Heat
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A passive solar home in Halifax, Vermont. High-SHGC, triple-glazed, south-facing windows were used to improve the direct-gain passive solar performance. Click on image to enlarge. As I discussed in last week's blog, a resilient home is extremely wel... 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 dark surfaces of 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 most 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. 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 Energy 10 , EnergyPlus , and REM Design 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. 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). 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. In this resilient design series, I'm covering how to achieve resilient homes and communities, including strategies that help our homes survive natural disasters and function well in the aftermath of any event that results in an extended power outage, interruption in heating fuel, or shortage of water. We'll see that resilient design is a life-safety issue that is critical for the security and wellbeing of families in a future of climate uncertainty and the ever-present risk of terrorism. 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 .