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Passive solar design
|See also the Passive solar category.|
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Passive solar design is the harnessing or directing of solar energy through non-mechanical, non-electrical means. It is a key principle of green building, often applied in designing buildings for maximum solar heating during cold winter months and maximum protection from the sun's heat during hot summer months.
Passive solar is distinguished from active solar by the lack of operable devices.
Passive solar vs Passive House
Passive solar design is about making best use of available natural light and heat. Heat from the sun is collected (if heating is desired, i.e. in a cool climate). A major part of passive solar design is careful orientation with respect to the sun.
Passive House design is about managing heat loss and heat gain (not just from the sun but from all sources). The intended result is to have a structure that requires minimal energy to heat and cool.
As such, some state that passive house design has greater applicability than solar design since orientation with respect to the sun is not absolutely necessary. Passive House design does not necessarily require a structure which is elongated along the east-west axis with extensive glazing on the equator facing side. Indeed, the ideal form of a passive house is a cube (lower surface area to volume ratio). Passive solar houses are also said to be more complicated to design, and the internal temperature may fluctuate uncomfortably if they are designed poorly. Proponents of superinsulation state that compared to solar houses, greater savings can be obtained by building air tight with lots of insulation in walls and high R windows.
So, while some consider that the debate between passive solar and superinsulation resulted in the wider acceptance of superinsulation, a more sensible approach would be to utilise both passive solar and passive house design elements, which has been termed "Solar Passivhaus". The remainder of this article therefore deals with passive solar in the modern sense of the term which is overlapping that of passive house design (rather than the original passive solar concepts in the 70s and 80s).
There are 6 main types of passive solar design. These can be considered in 3 generic categories in terms of the relationship between the thermal collector (or dissipator)and the interior space of the structure. There are 2 distinct approaches of each generic category.
Direct gain passive solar systems are where the thermal transfer occurs within the building interior. Thermal mass may either be distrubeted throughout the building (e.g. the floors and the pole facing walls), or mass may be concentrated.
Key design techniques
Key building design techniques include:
- Planting deciduous trees or trellises covered in deciduous or annual vines. This may be as a fence or as a shade roof, and provides shade in summer and light and warmth in winter.
- Designing and building components with consideration for the seasonal change in the sun's position in the sky, and the influence this will have on the angle and intensity of light hitting the structure. Examples of this include:
- Broad eaves, which block sunlight from entering windows during the summer, but allows it to enter during the winter when the sun is lower in the sky.
- Shade walls (less common)
- A wall or fence beyond the limits of the house, to provide shade. The wall runs east-west, on the east and/or west side of the house, and is on the north side of the house in the northern hemisphere, and on the south side in the southern hemisphere (i.e., away from the equator). The angle and placement are calculated to shield the house from early morning or late afternoon sun in summer but not in winter. However, this requires significant resources and, unless there are other reasons for a wall, it is generally better to use other methods, such as deciduous trees, or an awning, especially where there are large windows.
- Thermal mass Thermal mass is a critical component of any passive solar design. Its purpose is to absorb and re-radiate heat energy. This has the effect of averaging out the daily extremes of high and low temperature. Thermal mass generally falls into two categories, passive and active. Passive thermal mass heat-storage systems include:
- Thick masonry walls and floors
- Phase-change materials
- These materials change phase (typically from solid to liquid) when heated, storing a great deal of heat energy without a corresponding change in temperature. Examples include various high-tech concrete additives and also natural resins, such as those found in Southern Yellow Pine.
- Large liquid storage tanks inside the space to be heated (or cooled)
- Pipes to transfer heat between air and the thermal mass (e.g., in the ground under the house, either with natural convection or with a quiet low-power fan)
- White roofs
- Dark roofs absorb large amounts of solar energy, which transfers into the building. This increase in absorbed heat requires increased electrical energy to remove and cool buildings, homes, and workplaces and maintain a comfortable environment. Dark colors also radiate heat more easily in cold conditions, meaning greater heat loss from buildings and higher heating costs.
- White surfaces have an increased albedo or "whiteness," which is the reflectivity of a surface. If a roof is more reflective less energy will be absorbed and transferred to the building.
- Solar closets
- A shallow glass-fronted closet, lined with dark material, becomes very hot when exposed to the sun. One-way valves (which might be as simple as plastic sheet over wire mesh) allow cool air to enter at the bottom and hot air to exit at the top. They can be used for heating hot water, as well as for space heating.
- When these measures are appropriately utilized and combined the energy efficiency of a building can be much higher than with more conventional design measures.
- Passive Solar Architecture Pocket Reference. D Thorpe. Earthscan from Routledge, 2018
- Solar Versus Superinsulation: A 30-Year-Old Debate. Martin Holladay. Green Building Advisor, 2010
- Passive Solar Architecture Pocket Reference. Ken Haggard, David A. Bainbridge, Rachel Aljilani. International Solar Energy Society / Routledge, 2016
- Passive Solar Architecture: Heating, Cooling, Ventilation, Daylighting and More Using Natural Flows. D Bainbridge, K Haggard. Chelsea Green Publishing, 2011