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An Overview of Passive Solar Heating

10/19/2007 by Andy Walker, PhD PE

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Every building with windows is solar-heated, whether to the benefit or detriment of occupant comfort and utility bills.

In cold climates, the goal may be to capture and store as much solar heat as possible, while in warm climates the objective is to keep heat out. In general, a building must perform both functions, using solar heat in winter and rejecting it in summer.

Strategies to meet a higher percentage of the heating load through architectural design solutions are known as passive solar heating. The word “passive” means that the architectural elements, such as windows, insulation, and mass, operate as a system without the need for power input to mechanical equipment. Passive solar designs are categorized as direct gain, sunspaces, or Trombe walls (named after a French inventor).

Direct Gain

Direct gain spaces admit the solar radiation directly into the occupied space. This strategy is most effective in residences or within atriums and hallways of commercial buildings. Direct gain is generally not recommended for workspaces, or where people view computer screens or televisions, due to excessive glare and local heat gain. In a residence, occupants can move to a chair that is not directly in the sun, but in workspaces, people usually have to remain in place to accomplish a task.

The required window area varies from 10%–20% of floor area for a temperate climate, to 20%–30% for a cold climate.10 The percentage of the heating load that can be met with solar energy in a direct gain application is limited by the need to maintain comfortable conditions. The space cannot be allowed to get too hot, which limits the amount of solar heat that can be stored for nighttime heating. Nor can it get too cold, which means it will require the use of a back-up heater at times.

Sunspaces

A sunspace avoids the limitations of a direct gain space by allowing the temperature to vary beyond comfort conditions. In sunspaces, the mass can overheat and store more energy when sun is available. Sunspaces can also reuse fuel by allowing the spaces to subcool at night or during storms. As a consequence, the sunspace may not be comfortable at all times, and its uses should be programmed accordingly. Appropriate uses for a sunspace include casual dining area, crafts workspace, or an area for indoor plants.

Skylights or sloped glazing in sunspaces are common in practice, but are not recommended, since the high sun is not gladly received in summer, and since the sun hits the horizontal skylight only at an oblique angle in winter. (Skylights are available that address this issue by incorporating shades and louvers to control direct heat gain in summer.) It is also common to see sunspaces that project out from the house wall, another approach that is not recommended. It is better to have the house partially surround the sunspace (except on the south side) to reduce heat loss from both the sunspace and the house. Thus, the sunspace differs from a direct gain space more in terms of temperature control and the use of the space than it does in terms of architecture.

The recommended amount of glazing in a sunspace varies from 30%–90% of fl oor area in temperate climates to 65%–150% of sunspace floor area for cold climates. In most applications, the wall between the sunspace and the building acts as a massive thermal storage wall. In very cold climates or if the sunspace windows are poorly insulated (high U-value), it may be necessary to insulate this wall. Operable windows and doors between the sunspace and the building are opened and closed to provide manual control. Vents and fans are also used to extract heat from the sunspace under automatic control based on the temperature of the sunspace.

Trombe Wall

A Trombe wall is a sunspace without the space. It consists of a thermal storage wall directly behind vertical glazing. This passive solar heating strategy provides privacy and avoids glare and afternoon overheating.

Over the course of the day, the wall heats up, and releases its heat to the space behind the wall over a 24-hour period. The outside surface becomes very hot during the day, but due to the thermal inertia of the mass, the interior surface remains at a rather constant temperature.

Since the wall is not insulated, care must be taken to ensure that the heating cycle by the sun matches the cycle of heat loss to the interior and exterior. Well-insulated glazing can reduce this heat loss, but multiple panes, low-E coatings, and ultraviolet fi lters also reduce the amount of solar heat that gets through the glass, so the trade-offs must be evaluated to optimize cost.

Trombe wall area varies from 25%–55% of floor area in temperate climates, and from 50%–85% of fl oor area in cold climates.12 The wall is covered with a thin foil of blackened nickel called a selective surface, which has a high absorbtivity in the short wavelengths solar spectrum, but a low emissivity in the long wavelength infrared spectrum, thus reducing radiant heat loss off the wall. The heat must conduct into the wall from the selective surface, so proper adhesion to avoid blistering or peeling of the surface from the wall is critical to performance. Rather than hollow block, the wall should be solid to allow the heat to conduct through uniformly. Since the space between the mass wall and the window can exceed 180°F, all materials, including paint and seals, must be able to tolerate high temperatures.

Similar to direct gain spaces and sunspaces, an overhang over the glazed trombe wall reduces unwanted summertime heat gain.

Effective Passive Solar Design

An understanding of solar radiation and of the position of the sun in the sky is essential to effective building design.

In the northern hemisphere, winter sun is at its maximum on the south side of a structure, so this is the façade most affected by passive solar heating design. All passive solar heating features have a southerly orientation. The building floor plan would be laid out to provide sufficient southern solar exposure, with the long axis of the building running from east to west. The extent of this elongation must be optimized for the climate, since it also increases surface area and associated heat loss.

Some eastfacing windows are also recommended in areas with cool mornings.

One strategy to maintain a compact plan while also admitting solar gain into the northern rooms of a building is to use high, south-facing clerestory windows. The fact that the clerestory windows are high up also ensures high-quality daylight, along with passive solar heat gain.

It is important to take into consideration any surrounding objects that might shade the solar features, such as hills, other buildings, and trees.

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