Passive solar building design

Passive solar building design involves the modeling, selection and use of appropriate passive solar technologies to maintain the building environment at a desired temperature and humidity range (usually based around human thermal comfort) throughout the sun's daily and annual cycles. The goal is to reduce the requirement for active heating and cooling systems, the consumption of active solar, renewable energy and especially fossil fuel technologies.

Passive solar building design is only one part of thermally efficient building design, which in turn is only one part of sustainable design, although the terms are often used erroneously as synonyms (passive solar design does not relate to factors such as ventilation, evaporative cooling, or life cycle analysis unless these operate solely by the sun).

Passive solar building design is often a foundational portion of an effective Zero energy building.

Key concepts

There are four basic passive solar energy strategies, including: (1) direct solar gain, (2) indirect solar gain, (3) isolated solar gain, and (4) passive cooling. In the winter, solar gain is desirable. In the summer, avoiding solar radiation is critical.

Although some of these methods can be scientifically complex, they all share the same principles of Physics and heat transfer. Constructing a passive solar building is not difficult, but it is very different than conventional building design and construction.

During the cool season: to correctly control the admission and storage of 'free' solar heat energy while minimizing heat losses through the building envelope.

During the warm season: to significantly slow heat gain, and exhaust undesired heat.

During the moderate temperate season: to provide controls that accomodate daily weather differences.

In most passive solar buildings, additional thought must be given to controlling the direction of heat transfer from one section of the building to another. For example, excess heat in an equator-facing solarium used to warm the opposite (cold) side of the building.

The ability to achieve these conflicting goals effectively is fundamentally dependent on the seasonal altitude-and-azmuth path of the sun between the summer and winter solstice (where there is a 47-degree seasonal altitude difference, and sunrise / sunset azmuth changes greatly). In the Northern hemisphere the winter sun rises in the southest, peaks out at a low angle, and then sets in the southwest. In the summer, the sun rises in the northeast, peaks out nearly straight overhead, and then sets in the northwest. These latitude (and hemisphere) specifc differences are critical to effective passive solar building design. Solar architects must know the precise angles for each location they design for. (In the U.S., the numbers are available from NOAA.)

Direct solar gain

File:Illust passive solar d1 319pxW.gif

Elements of passive solar design, shown in a direct gain application

Direct gain involves using the positioning of windows, shutters, exterior shading and interior movable window insulation to control the amount of direct solar radiation reaching the interior spaces themselves, and to warm the air and surfaces within the building.

Basic passive solar building design elements include:

Orient the building to face the equator (or a few degrees to the East to capture the morning sun) - Longer in the East / West dimension.
Maximise windows to face the midday sun (latitude-and-climate-specific window area sizing)
Minimise windows on other sides, especially western windows
For thermal reasons, never use skylights or roof-angled glass in any location. In the summer, it creates a solar furnace when the sun is nearly straight overhead. In the winter, most of the low-angled sunlight reflects off of the glass to the sky. Warm air rises like a hot-air ballon, touches the glass and then conducts and radiates heat to the sky. roof-angled glass is easier to break, harder to clean, and very difficult to provide shade or light control.
Erect correctly-angled shading to reduce solar heat gain during the hot season. West side shade must go down to the ground to block the hot afternoon setting sun.
Use the appropriate amount and type of insulation in the form of radiant barriers or bulk insulation to minimise seasonal excessive heat gain or loss
Consider the use of thermal mass to store excess solar energy during the day (which is then re-radiated during the night)^[1] But, do not allow the thermal mass to block solar gain into the inner portion of the building.

The use of sun-facing windows and a high-mass floor is a short-cycle example of this. John Hait's "Passive Annual Heat Storage" (PAHS) method is an example of an annualized solar approach primarily using this path.

Not infrequent errors in design leading to poor performance include:

Major deviations from ideal orientation
Incorrect proportions of glazing at each aspect - especially West-facing and roof-angled glass
Excessive glazing ('over-glazing') resulting in overheating, glare and fading of soft furnishings
Lack of adequate shading during summer and daily temperature variations in the moderate seasons
Lack of understanding of the characterisitics of different types and amounts of insulation
Inadequate or incorrectly located thermal mass in relation to glazing area
Interior open staircases that allow warm air to rise, making upper rooms to hot and lower rooms too cold year round
Too many exterior envelope corners that increase the heat transfer surface area, and air infiltration leak opportunities
An attic that gets hotter than the peak outside air temperature in the summer (increasing cooling requirement)

The amount of equator-facing glass and thermal mass depends on latitude, altitude, climatic conditions, and heating/cooling degree days.

The effectiveness of Direct solar gain systems are significantly enhanced by insulative (e.g. double glazing), spectrally-selective glazing (low-e), or movable window insulation (window quilts, bifold interior insulation shutters, etc. This has been exemplified in Europe where super-insulated windows have been developed and are widely used to help meet the German Passive House standard. Selection of spectrally-selective window coatings depend on the ratio of heating versus cooling degree days.

A design that uses too much equator-facing glass can result in excessive heating, and uncomfortably-bright living spaces at certain times of the year. Control mechanisms like exterior roll-down shade screeens, or interdrapes can reduce this design error, and help control dail solar gain variations.

Indirect solar gain

Indirect gain is a building design method by which solar radiation is captured by a part of the building envelope designed with an appropriate thermal mass (such as a water tank or a solid concrete or masonry wall behind glass). The heat is then transmitted indirectly to the building through conduction and convection. Examples of this are Trombe walls, water walls and roof ponds. The Australian deep-cover earthed-roof, innovated by the Baggs family of architects, is an annualized example of this path.

In practice, indirect solar gain systems have suffered from being difficult to control, and from the lack of reasonably priced transparent thermally insulating materials. Obsolete systems like the Trombe wall block low winter sun from penetrating into the interior portion of rooms, which significantly slows solar gain in the morning, when it is needed the most.

Isolated solar gain

Isolated gain involves capturing solar heat and then moving it passively into-or-out-of the building using a fluid like water or air. For example, a natural convection (warm fluid rising) thermosiphon solar space heating system) or perhaps using a solar chimney), either directly or using a thermal store.

Sun-spaces, solariums, greenhouses, and "solar closets" are alternative ways of capturing isolated heat gain from which warmed air can be taken. In practice, it has been found that some owners use these structures as sunny living spaces when the sun is shining. They should not be heated or cooled with conventional fuels, since it would significantly increase, rather than reduce, utility bills and the environmental impact of the building.

Don Stephens' "Annualized Geo-Solar" (AGS) heating is an annualized example of this option, which offers the advantages of preventing over-heating when living spaces are already deemed warm enough, and of extending time-delays until such heat will be desired.

Other passive solar design techniques

Building position - Based on the local climate and the sun's positioning (determined using a heliodon), the entire building can be positioned and angled to be oriented towards or away from the sun (according whether heating or cooling is the primary concern), overshadowing from other structures or natural features can be avoided or used, and the building can be set into the ground using earth sheltering techniques.

Building properties - The shape (and consequently the surface area) of the building can be controlled to reduce the heating or cooling requirement, and the use of materials properties to reflect, absorb, or transmit energy (for example using visible colour) is also a consideration.

External environment - Energy-efficient landscaping materials, including the use of trees and plants can be selected to reflect or absorb heat, create summer shading (particularly in the case of deciduous plants), and create shelter from the wind.

Although not classified as a passive solar technology, the use of thermal insulation or superinsulation can be employed to reduce heat loss or unwanted heat gain.

Solar house development

Passive solar building design dates back into antiquity and has remained a traditional part of vernacular architecture in many countries.^[2] In the developed world, these techniques were continued by some rural populations and enthusiasts, though were largely ignored by the construction industry until the beginning of the 21st century.

Despite this lack of general enthusiasm by builders, passive solar technologies were refined and developed during the 20th century, boosted by the 1973 oil crisis, and aided by the development of computer modelling techniques, resulting in a number of pioneering passive solar buildings.

At the start of the 21st century the subject has been receiving greater interest due to concerns over global warming and resource depletion.

Although earlier experimental solar houses were constructed using a mixture of active and passive solar techniques, the first engineered passive solar houses of the modern era were built in Germany after World War I, when the Allies occupied the Ruhr area, including most of Germany's coal mines. These designs were studied in the United States, but had little influence on builders.

The first consciously passive solar house in the US^[3] was designed in 1940 by George F. Keck for a Chicago area real estate developer named Howard Sloan in Glenview, Illinois. Keck had designed an all-glass house for the 1933 Century of Progress Exposition in Chicago and was surprised to find that it was warm inside on sunny winter days, even though the furnace hadn't been installed yet. Keck was not aware of the research being done elsewhere on solar architecture, but he gradually started incorporating more south-facing windows into his designs for other clients, and by 1940 he had learned enough to design a passive solar house for Sloan.

Sloan built a number of passive solar houses in the 1940s, and his publicity efforts influenced a number of other builders during the postwar housing boom (Sloan is also credited with popularizing the term "solar" to describe his houses). But some builders of that era didn't realize that the houses were designed to face south, and many were built facing other directions, which hurt their reputation. Critics also pointed out that windows and doors weren't always properly sealed. Public interest declined by 1950 due to cheap oil and general prosperity, until it was revived after the 1973 oil crisis.

Edward Mazria's book 'The Passive Solar Energy Book' published in 1980, was an important milestone from which interest in this field developed.

Levels

Pragmatic: Many detached suburban houses can achieve reductions in heating expense without obvious changes to their appearance, comfort or usability [1]. This is done using good siting and window positioning, small amounts of thermal mass, with good but conventional insulation and occasional supplementary heat from a central radiator connected to a water heater. Sunrays may fall onto a wall during the daytime, which will radiate heat in the evening.

Annualized: Most "passive solar" approaches have depended on near-daily solar capture and storage, only expected to maintain temperatures through a few days and nights. These are now termed "short-cycle passive solar". More recent research has developed techniques to capture warm-season solar heat, convey it to a seasonal thermal store for use months later during the cool or cold season. This is referred to as "annualized passive solar." This requires large amounts of thermal mass. One technique buries water-proof insulation in 7-metre skirts around the foundation, and buries loops of plastic pipe or ducts under the foundations and slab. The "skirts" of insulation prevent heat leaks from weather or water.^{[citation needed]}

Minimum machinery: A "purely passive" solar-heated house would have no mechanical furnace unit, relying instead on energy captured from sunshine, only supplemented by "incidental" heat energy given off by lights, candles, other task-specific appliances (such as those for cooking, entertainment, etc.), showering, people and pets. The use of natural air currents (rather than mechanical devices such as fans) to circulate air is related, though not strictly solar design.

Systems sometimes use limited electrical and mechanical controls to operate dampers, insulating shutters, shades or reflectors. Some systems enlist small fans or solar-heated chimneys to start or improve convective air-flow. A reasonable way to analyse these systems is by measuring their coefficient of performance. A heat pump might use 1 J for every 4 J it delivers giving a COP of 4, a system that only uses a 30 W ceiling fan to heat an entire house with 10 kW of solar heat would have a COP of 300.

References

^ http://www.greenhouse.gov.au/yourhome/technical/fs14.htm
^ The lost skills of sustainable design
^ Ken Butti and John Perlin. A Golden Thread: 2500 Years of Solar Architecture and Technology=1980. ISBN 0917352076.

External links

Canadian Solar Buildings Research Network
www.greenbuilder.com - Passive Solar Design
www.eere.energy.gov - US Department of Energy guidelines
Residential Green Building Checklist
Commercial Green Building Checklist
Passive Solar Design Guidelines
http://www.solaroof.org/wiki
Calculation of insolation (houses, garden, roof, appartment...)

[1] ttp://www.greenhouse.gov.au/yourhome/technical/fs14.htm

[2] The lost skills of sustainable design

[3] Ken Butti and John Perlin. A Golden Thread: 2500 Years of Solar Architecture and Technology=1980. ISBN 0917352076.

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