As part of our Design Basics series we are looking at many of the different aspects of the architectural design process. In this post we explore passive solar design.
What is passive solar design?
Passive solar design uses the energy from the sun for the heating and cooling of living spaces, without relying on mechanical devices. The system relies on the natural heat transfer processes of conduction, convection and radiation in order to collect, store and redistribute the solar energy.
Passive solar design takes into account the buildings site, climate and materials to minimise the use of energy and is centred around the movement of the sun. The passive system is considered to be simple, not requiring any substantial use of mechanical devices such as pumps, fans or electrical controls.
Main Principles of Passive Solar Design
The essential elements of the passive solar design are:
A south facing glass through which sunlight will enter the building. The aperture should face within 30 degrees of true south and should permit direct sunlight (no shading by other buildings or trees) between 9am and 3pm each day during the season that requires heating. The sun travels different routes in different seasons, with the winter sun being much lower than the summer sun. If the glass window is facing south, during the winter the sun will be allowed to penetrate into the building.
The area of glazing is determined by the severity of the winter temperatures, building size, and interior thermal mass. A thorough design in imperative to avoid overheating and large fluctuations in temperature.
Some form of absorbing material must be present in order for the solar radiation to be absorbed. These materials include concrete, brick, stone or tile. Other materials such as water and phase change products can be used but masonry is more durable and more commonly seen.
Darker colours are better at absorbing the radiant heat, and therefore a better choice for thermal mass when designing a solar passive home.
The solar heat is circulated from where it is collected and stored to the different areas of the house through the three natural heat transfer modes of convection, conduction and radiation. Convection is heat transfer through a fluid such as air or water. Conduction occurs when heat moves between two objects that are in contact with one another.
Radiation occurs by transferring energy by electromagnetic waves.
The heat gain needs to be controlled in order to avoid overheating during warmer months. This is achieved by designing suitable roof overhang, blinds, shading devices, even trees and landscaping can provide suitable control of summer sun. In turn the winter sun will still be able to penetrate the building as the angle of the sun is lower during the winter months.
Vents and openings are also utilised to allow ventilation.
Types of Passive Solar Heating
Passive solar heating designs allows a building to capture the suns heat and distribute this heat through the building during periods when the sun is absent. This creates a stable and comfortable room temperature in the building.
Every passive solar heating design will have two main key features: the south facing glass and the thermal mass to absorb, store and distribute the heat. The goal can be achieved in a number of ways.
With this type of system, sunlight will enter the house through the south facing glazing and is absorbed into the thermal mass of the masonry floor or walls, which then absorb the heat. The heat is then slowly radiated back out into the room during the evening and night when the temperature outside is cooler. To ensure this method works, it is important that the thermal mass is insulated from the outside temperature, or the heat could dissipate into the ground or outside air.
An indirect gain system places the thermal mass between the south facing windows and the living space. Known as a Trombe wall, a masonry wall situated about 2.5cm from the south facing glass window. The heat from the sun will be absorbed through the window and into the dark coloured masonry wall. The heat travels through the masonry wall and is radiated into the living space when the indoor temperature falls below that of the walls surface.
Vents positioned at the top and bottom of the thermal storage wall will allow the heat to convect between the wall and the glass to the living space.
Isolated Gain (Sunspaces)
The most common isolated gain system is a sunspace. The space will have a floor of high thermal mass and a wall separating the sunspace from the living space which is also high thermal storage. The south facing part of the sunspace will be fully glazed. The space will become heated in the day and the stored heat can be radiated into the living spaces during the evening and night time. If the sun space becomes too hot during the day natural ventilation of operable windows can provide cooling.
Passive Solar Cooling
It is important to plan for summer comfort as well as winter comfort when designing a passive solar home. Considerations must be taken on siting, local climatic conditions, shading and wind conditions.
In their simplest form, passive solar cooling elements can include overhangs or shading devices on any south facing windows, shading trees, thermal mass and cross ventilation strategies.
Shading devices will provide protection to glazed areas to reduce glare and solar heat gain in warmer months, but still allow the sun to penetrate the building during winter months. There are many types of solar shading and their effectiveness is dependent on their form and orientation in relation to the solar altitude and azimuth for the time of day and season of the year.
Due to the low level of the sun during sunrise and sunset, an overhang or shading device will be relatively ineffective on an east or west facing window. A passive solar design would aim to minimise east and west facing windows if cooling is a particular concern.
Thermal mass can be used in passive cooling to absorb heat to moderate internal temperature during warmer months. The thermal mass will absorb the heat during the day, and be cooled during the night using ventilation, ready to absorb the heat again the following day.
In climates where the the indoor temperature is higher than the outdoor temperature, natural ventilation is a suitable cooling technique. Climatic studies will suggest the best approach to a natural ventilation system. Windows need to be positioned in walls facing the prevailing breeze and on the opposite wall in order to create a cross ventilation.
Convective cooling is another option, that brings in cool night air from outside which pushes out the hot interior air. Vent positions are key in order for this strategy to be effective. A high level vent on the leeward side of the building (the side away from the wind) will allow hot air to escape, while a low level vent on the side towards the wind will introduce cool air into the building.
Book Recommendations and references:
A truly vital resource for architecture students, and indeed any book from Francis Ching. Highly recommend.
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Also, check out our other articles from the series – Architectural Design Process:
And from our Design Basics posts: