by stevendamron

Passive solar design seeks to optimize the comfort of your home using the energy of the sun. This means taking advantage of the sun’s power to heat your home in the winter and preventing over-heating in the summer. Strict passive solar design aims to achieve this without using any supplemental electricity or gas to heat or cool the home. Three basic principles of thermodynamics govern how the heat transfer occurs in the built environment: convection, conduction and thermal radiation. Passive solar design combines these underlying concepts with local conditions to optimize heat gain (heating) and heat loss (cooling).

Key Thermodynamic Concepts

Heat transfer occurs in three fundamental ways: conduction, convection and thermal radiation.

• Conduction is the heat transfer between matter due to a difference in temperature – so when something (gas, liquid or solid) cold touches something hot, heat is transferred from the hot thing to the cold thing until the temperatures equalize. The most important form of conduction that occurs in your home is through the windows. For example, when it is cold outside and warm inside, heat loss occurs through the windows as the temperatures try to equalize.

• Convection is heat transfer that occurs only in gases and liquids due to diffusion or currents. In the context of passive solar design, convection refers to how air moves both within the house and between the house and the outside.

• Thermal radiation is electromagnetic radiation emitted by all bodies in the form of heat. This heat from the sun transfers to your house primarily through the roof and windows. Radiation also occurs from a warm house to a cold outside environment leading to heat loss. Windows with low-E coatings acts like mirrors that keep the interior heat in and the exterior heat out. These basic principles of heat transfer are the main building blocks for climate control through passive solar design.

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Local Conditions

Understanding and capitalizing on the particularities of the building site is a central part of effective passive solar design.

• Climate: Detailed local climate data plays a key role in passive solar design. Heating-degree days and cooling-degree days are key metrics that help passive designers model the heating and cooling requirements based on local climate data. These are measurements designed to reflect the energy needed to heat or cool a building based on the outside temperature. Understanding the local climate conditions in this way allows the designer to determine how much solar heat gain you need to heat your home.

• Solar Path: The trajectory that the sun follows in the sky each day varies throughout the year because of the tilt of the Earth’s axis in relation to its orbit around the sun. The magnitude of these variations depends upon latitude: places close to the equator have minimal variation and places near the North or South Pole have the most extreme variation. While the sun rises in the East and sets in the West regardless of where we are on earth, in the Northern hemisphere the angle at which the sun rises becomes more southerly as winter solstice approaches. What this means in our practical experience is that in the winter the sun is “lower” in the sky and nearer to the southern horizon.

Common Design Elements

One overall design goals for passive solar homes in North American heating-driven climates, is to allow sunlight in during the winter and keep it out during the summer. South-facing windows that have sun exposure in the daytime during the winter are key. These windows will have at least an R-value of 5 and be tuned with custom Solar Heat Gain Coefficients (SHGC) based up on the number of heating degree days of the local climate. High R-values are important to limit conductance, and a high SHGC will provide more passive heating than a low SHGC.

To prevent overheating in summer, carefully designed overhangs may be installed over windows. These will expose the windows to the low, winter sun and shield them from the higher summer sun. Other measures may include window coverings, vents, or deciduous plants with foliage that covers windows in summer but leaves them bare in summer allowing light to pass through.

A well-insulated, airtight building envelope also plays a big part in a passive solar home. This reduces air infiltration, which will heat the home in summer and cool it in winter, causing higher energy bills for the owner. Most passive solar design will incorporate “thermal mass” – a material that can absorb and store heat during the day and release it at night to minimize temperature fluctuations. “Thermal masses” may include a cistern, a rooftop pond or a Trombe wall.

The circulation of air within the well-sealed space also poses a challenge to passive solar design. While convection (warm air rising) can contribute greatly to the circulation of air, many design chose to install fans or a Heat Recovery Ventilation (HRV) system. HRVs can efficiently expel stale air and draw in fresh air from the outside while capturing the heat energy in the old air and transferring it to the new air. While not strictly passive, HRVs use a minimum amount of active energy in an efficient way to achieve excellent indoor air quality.

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