A Trombe Wall Solution for Passive-Solar Storage
An indirect solar-heat-gain strategy addresses the challenge of orienting a passive-solar house's views northward.
Synopsis: Orienting windows to face the north—the location of the views for this Michigan home—typically limits a home’s ability to reap any potential benefits from solar gain. Here, a passive-solar heating strategy called a Trombe wall collects heat from the sunlight on the southern side of the house and stores it in the wall to distribute into the conditioned spaces. The article includes details of the Trombe wall installation and a section drawing of the assembly, in addition to descriptions of other strategies that contributed to the home’s energy efficiency.
The owners of this certified Passive House in Ann Arbor, Mich., cherish their nearly five-acre property, which backs up to wildlife-rich woods and fields. Seeing that natural beauty from the house was a top priority. They wanted most of the glazing on the north side in order to optimize their views of nature in that direction and to ensure privacy on the other sides of the house, which are subject to potential eyesores from future development. Orienting windows to face north typically limits a home’s ability to reap any potential benefits from solar gain, but they wanted to use that free energy to help heat their Passive House in colder months. The solution was to incorporate a passive-solar storage and delivery system in the form of three Trombe walls.
Passive solar in a Passive House
There is a distinction to be made between Passive House, the building standard, and passive-solar design. The former emphasizes an exhaustive list of requirements to reduce energy consumption, while the latter is focused on non-technological means of harvesting, storing, distributing, and controlling solar energy. While a debate continues around the viability of using passive solar in super-insulated, high-performance homes, there are five basic fundamentals of passive-solar design: 1. At least one side of the building needs an unobstructed view of the sun and should be oriented toward it (due south in the Northern hemisphere); 2. Windows are required on the sun-facing exposure to allow in direct sunlight during the heating season; 3. A medium is needed to absorb heat from the sunlight in the heating season; 4. The design should allow for the stored heat to be distributed where it’s needed through natural conduction, convection, and/or radiation; and 5. There has to be a way to stop or limit solar gain during the cooling season, when it could overheat the building.
The most common passive-solar designs take a direct-gain approach, using the interior space itself as the medium to absorb and distribute heat. In these designs, the long axis of the house is oriented in an east-west direction and most of the windows are arranged on the sun-facing exposure to allow in sunlight. Extra interior thermal mass, usually in the floors and/or walls opposite the sun-facing windows, absorbs heat from the sunlight throughout the day and releases it at night. The rooms people occupy most get arranged on the sunny side of the house to take advantage of the warmth and daylight, and the utility rooms, bathrooms, closets, stairways, and hallways get pushed to the shaded side, where glazing is kept to a minimum to minimize heat loss. Roof overhangs are usually designed to shade the sun-facing windows during the summer months and allow low winter sun to beam straight in.
Given the homeowners’ stipulations for views to the north, this project couldn’t use the direct-gain strategy. Instead, Michael Klement, principal of Architectural Resource, took an indirect-gain approach, using a space-heating passive-solar wall system popularized by French inventors Jacques Michel and Felix Trombe in the mid-1900s: Trombe walls. A Trombe wall consists of a sun-facing masonry wall behind glass, with an air space in between them. It blocks sunlight from entering the living space directly, but it doesn’t waste it. The masonry is ideally coated with a dark, heat-absorbing material that collects heat from sunlight and stores it in the wall. There are various approaches to distributing this stored energy. One is to use vents to move the heat to the interior using natural convection; another is to utilize conduction to move heat to the wall’s interior surface, where it’s radiated into the space; another tactic is to combine the two techniques.
From Fine Homebuilding #299
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