Passive solar building design

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Passive solar buildings aim to maintain interior thermal comfort throughout the sun's daily and annual cycles whilst reducing the requirement for active heating and cooling systems.[1]

Passive solar building design revolves around a set of core physical environmental-and-scientific principles. Specific attention is directed to the site and location of the dwelling, the prevailing climate, design and construction, solar orientation, placement of glazing-and-shading elements, and incorporation of thermal mass. Whilst these considerations may be directed to any building, achieving an ideal solution requires careful integration of these principles. Modern refinements through computer modeling and application of other technology can achieve significant energy savings without necessarily sacrificing functionality or creative aesthetics.[2][3]

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 as synonyms, passive solar design does not include important factors such as ventilation, evaporative cooling, or life cycle analysis, unless these operate solely by the sun without mechanical systems.

Passive solar building design is often a foundational element of a cost-effective zero energy building.[4][5][6] Although a ZEB uses multiple passive solar building design concepts, a ZEB is usually not purely passive, having active mechanical renewable energy generation systems such as: wind turbine, photovoltaics, micro hydro, geothermal, and other emerging alternative energy sources.

Passive solar building design goals

  • During the cool season: Control the admission or storage of 'free' solar thermal energy (heat) while minimizing heat losses through the building envelope.
  • During the warm season: Slow heat gain, and exhaust undesired heat by cross-ventilation and convection currents
  • During the moderately temperate Spring and Fall seasons: Provide controls that accommodate daily weather variations of sunshine, temperature and humidity.[1]

Heat distribution between sections of the building may be controlled actively, or passively by taking advantage of the natural flow of heat(e.g. convection flow loops). For example, venting heat from an equator-facing solarium to warm the opposite (cold) side of the building in the winter.

Solar path fundamentals

Main article: Sun path

The ability to achieve these goals simultaneously is fundamentally dependent on the seasonal variations in the sun's path throughout the day

This occurs as a result of the inclination of the earth's axis of rotation in relation to its orbit. The sun path is unique for any given latitude. Generally the sun will appear to rise in the east and set in the west.

In Northern Hemisphere non-tropical latitudes farther than 23.5 degrees from the equator:

  • The sun will reach its highest point toward the South (in the direction of the equator)
  • As winter solstice approaches, the angle at which the sun rises and sets progressively moves further toward the South and the daylight hours will become shorter
  • The opposite is noted in summer where the sun will rise and set further toward the North and the daylight hours will lengthen[7]

The converse is observed in the Southern Hemisphere, but the sun rises to the east and sets toward the west everywhere.

In equatorial regions at less than 23.5 degrees, the position of the sun at solar noon will oscillate from north to south and back again during the year.[8]

In regions closer than 23.5 degrees from either north-or-south pole, during summer the sun will trace a complete circle in the sky without setting whilst it will never appear above the horizon six months later, during the height of winter.[9]

The 47-degree difference in the altitude of the sun at solar noon between winter and summer forms the basis of passive solar design. This information is combined with local climatic data (degree day) heating and cooling requirements to determine at what time of the year solar gain will be beneficial for thermal comfort, and when it should be blocked with shading. By strategic placement of items such as glazing and shading devices, the percent of solar gain entering a building can be controlled throughout the year.

One passive solar sun path design problem is that the sun is in the same relative position six weeks before, and six weeks after, the solstice, BUT due to "thermal lag" from the thermal mass of the Earth, the temperature and solar gain requirements are quite different before-and-after the summer-and-winter solstice. Movable shutters, shades, shade screen, or window quilts can accommodate day-to-day and hour-to-hour solar gain and insulation requirements.

Careful arrangement of rooms completes the passive solar design. A common recommendation for residential dwellings is to place living areas facing solar noon and sleeping quarters on the opposite side.[10]. A heliodon is a traditional movable light device used by architects and designers to help model sun path effects. In modern times, 3D computer graphics can visually simulate this data, and calculate performance predictions.[2]

Passive solar thermodynamic principles

Main articles: Building insulation and Insulated glazing
  • Personal thermal comfort is a function of ambient air temperature, mean radiant temperature, air movement and humidity
  • Heat transfer in buildings occurs through convection, conduction, and thermal radiation through roof, walls, floor and windows[11]
  • Convective heat movement greatly accelerates heat loss (Uncontrolled air infiltration from poor weatherisation/weatherstripping/draught-proofing can contribute up to 40% of heat loss during winter[12]. Strategic placement of openable windows/vents can enhance cross-ventilation, convective movement and cooling in summer.)[13]
  • Natural convection causing rising warm air and falling cooler air inevitably results in an uneven distribution of heat. This may cause variations in temperature throughout the conditioned space or serve as a method of venting hot air outside of it.
  • Natural human cooling can be facilitated by evaporation through convective air movement.
  • High humidity inhibits evaporative losses.
  • The main source of heat gain is radiant energy from the sun which occur predominantly through the roof (but also through walls and windows)
  • A cool roof, or green roof plus radiant barrier can help prevent your attic from becoming hotter that the peak summer outdoor air temperature.[14]
  • Glazing is particularly difficult to insulate compared to roof and walls (see insulated glazing)
  • Windows are a ready and predictable site for radiant energy transmission.[15]
  • External shading of glazing is more effective at reducing heat gain than internal window coverings.[15]
  • Convective heat flow through and around window coverings degrade its insulation properties.[15]
  • Western and Eastern sun can provide good warmth and lighting but are vulnerable to overheating in summer if not shaded. Like all glazing they are sites of heat loss at night.
  • The non-solar aspect can provide diffused light but little direct radiant warmth. These act as sites of heat loss.
  • The midday sun readily admits light and warmth during the winter but can be easily shaded with appropriate length overhangs or angles louvres during summer.
  • The amount of radiant heat received is related both to its intensity and its angle of incidence. ( seeLambert's cosine law)
  • Thermal energy can be stored in certain building materials and released again when heat gain eases

Site specific considerations during design

Basic passive solar building design elements

  • Orientating the building to face the equator (or a few degrees to the East to capture the morning sun)[16]
  • Extending the building dimension along the east/west axis
  • Adequately-sizing windows to face the midday sun in the winter, and be shaded in the summer.
  • Minimising windows on other sides, especially western windows[15]
  • Erecting correctly-sized, latitude-specific overhangs, or shading elements (shrubbery, trees, trellises, fences, shutters, etc.)[17]
  • Using the appropriate amount and type of insulation including radiant barriers and bulk insulation to minimise seasonal excessive heat gain or loss
  • Using thermal mass to store excess solar energy during the winter day (which is then re-radiated during the night)[18]

The precise amount of equator-facing glass and thermal mass should be based on careful consideration of latitude, altitude, climatic conditions, and heating/cooling degree day requirements.

Factors that that can degrade thermal performance:

  • Deviation from ideal orientation and north/south/east/west aspect ratio
  • Excessive glass area ('over-glazing') resulting in overheating (also resulting in glare and fading of soft furnishings) and heat loss when ambient air temperatures fall
  • Installing glazing where solar gain during the day and thermal losses during the night cannot be controlled easily e.g. West-facing, angled glazing, skylights [19]
  • Thermal losses through non-insulated or unprotected glazing
  • Lack of adequate shading during seasonal periods of high solar gain (especially on the West wall)
  • Incorrect application of thermal mass to modulate daily temperature variations
  • Open staircases leading to unequal distribution of warm air between upper and lower floors as warm air rises
  • High building surface area to volume - Too many corners
  • Inadequate weatherization leading to high air infiltration
  • Lack of, or incorrectly-installed, radiant barriers during the hot season. (See also cool roof and green roof)
  • Insulation materials that are not matched to the main mode of heat transfer (e.g. undesirable convective/conductive/radiant heat transfer)

Key passive solar building design concepts

There are four primary passive solar energy configurations:[20]

Direct solar gain

File:Illust passive solar d1 319pxW.gif
Elements of passive solar design, shown in a direct gain application

Direct gain attempts to control the amount of direct solar radiation reaching the living space.

Indirect solar gain

Indirect gain attempts to control solar radiation reaching an area adjacent but not part of the living space. Heat is stored in thermal mass (e.g water tank, masonry wall) and slowly transmitted indirectly to the building through conduction and convection.

Examples:

In practice, indirect solar gain systems suffer from slow response (thermal lag), being difficult to control hourly, daily, and seasonally, and from the lack of reasonably-priced transparent thermally insulating materials. Obsolete systems such as the 1881-patented 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, such as water or air.

A natural convection (warm fluid rising) thermosiphon solar space heating system), South-to-North natural-convection flow loop (as in the double-shell passive solar thermal buffer zone design) or perhaps a solar chimney), either directly or using a thermal store.

Sunspaces, solariums, greenhouses, and 'solar closets' are alternative ways of capturing isolated heat gain from which warmed air can be used for space heating, growing tropical plants, clothes drying, etc.

The temperature of a sun room is allowed to go through a diurnal (day/night) temperature swing, where it is designed to act as a Thermal Buffer Zone between comfortable interior temperature, and exterior temperature extremes.

In many moderate climates, very-inexpensive single pane (tempered patio door) glass can be used on the equator-facing side to maximize solar gain at a minimum construction cost. In extremely-cold climates, movable window insulation can help retain heat in a solarium or greenhouse at night. Equator-facing glass should not use spectrally-selective coatings that reduce daytime solar gain. Each layer of ordinary window glass reduces solar gain by about 15% (depending on the material, iron content, etc.).[citation needed]

When the winter sun is low, and there is snow on the ground, the sun reflects off of the snow and you get solar gain on the solarium ceiling. This can increase total BTUs of free winter heat by more than one half. When the outside temperature is well below freezing, sun room temperatures of 88 degrees F (31 degrees C) are not uncommon in many locations where the majority of humans live. (This is very location and design specific.) When a cold front passes through, the moisture in the atmosphere precipitates out (snow or ice), and the skies become very clear, for excellent solar gain in most cold winter climates.[citation needed]

Cloudy skies imply warmer nights, since clouds reflect radiant heat from the Earth back downward. Less solar gain is needed on most cloudy days. If you go outside and you can tell that it is daytime, then you are usually receiving at least 30% of maximum solar gain potential.[citation needed]

Isolated solar gain sun rooms act as a Thermal Buffer Zone[21] with a moderating temperature between interior living space and exterior extremes, except during winter days, when they can be warmer than the interior living space. Earth cooling tubes or other passive cooling techniques can keep a solarium cool in the summer.

An equator-side sun room should have its exterior windows higher than the windows between the sun room and the interior living space, to allow the low winter sun to penetrate to the cold side of adjacent rooms. When the sun is high in the summer, this cannot happen. A sun room cross section drawing illustrates the latitude-specific requirement of glass placement, and overhangs.

Other considerations

Insulation

Main article: Building insulation

Thermal insulation or superinsulation (type, placement and amount) assists in significantly reduce unwanted heat transfer.[22]

Special glazing systems and window coverings

Main articles: Insulated glazing and Window covering

The effectiveness of direct solar gain systems is significantly enhanced by insulative (e.g. double glazing), spectrally-selective glazing (low-e), or movable window insulation (window quilts, bifold interior insulation shutters, shades, etc.).[23] This has been exemplified in Europe where superior-insulated windows have been developed and are widely used to help meet the German Passive House standard. Selection of different spectrally-selective window coating depends on the ratio of heating versus cooling degree days for the design location.

Window Orientation

The considerations for vertical equator-facing glass are different than for the other three sides of a building. To increase solar gain, it should not have a reflective window coating. Each pane of glass reduces solar gain by at least 15 percent. There is a location-specific trade-off calculation necessary between single-pane winter daytime solar gain maximization, and annual heat transfer reduction with multiple-pane glass. Direct-gain systems usually require insulated glazing with lower solar gain potential, whereas indirect-solar-gain systems with a thermal buffer zone (TBZ) may be able to use lower-cost, more-effective single-pane equator-facing vertical glazing. The optimal cost-effective solution is both location and system dependent.

Western glass and roof-angled glass should be minimized or eliminated in hot climates with significant degree day cooling requirements, to avoid creating an undesirable summer solar furnace effect. If they are used, more-expensive movable high-insulation-and-shading systems are almost always required.

On cold winter nights, less-dense warm air rises (like a hot air balloon), touches roof-angled glass (skylights), and looses more heat through conduction, convection, and radiation than it can gain during winter days, when the sun is low on the equator-side horizon, and most of the solar radiation reflects off of roof-angled glass. Thus, roof-angled glass normally results in an overall increase in heating and colling energy requirements, which exceeds the benefit of daylight energy consumption reduction (with modern, high-efficiency lighting systems).

The key to understanding the good-versus-bad aspects of glazing orientation and solar radiation is that sunlight striking glass within 20 degrees of perpendicular is mostly transmitted through the glass, whereas sunlight at more than 35 degrees off of perpendicular mostly reflects off of the glass.[24]

For science teachers, this fundamental principle of optical physics is easily demonstrated inexpensively with a piece of window glass, and a bright movable light source (flashlight, spot light) in a dark room. The reflection-versus-transmission is very easy to see as the angle of incidence changes. Slowly move the light, or rotate the glass from perpendicular to nearly parallel - Watch the transmitted and reflected light brightness ratios change significantly as the angle changes.

For more-precise experimental measurements, a simple photographic light meter and a home-made heliodon (solar angle simulation light source), or an optical bench that precisely rotates the glass, can easily quantify the reflection/transmission percentage change. Comparing ordinary window glass to spectrally-selective insulated glass is enlightening. This is an interesting lab experiment that teenaged students can quickly understand. They can take their experimental data and deduce a simple predictive model based on high-school trigonometry. We need to inspire more bright, inquisitive students with the scientific simplicity of basic solar energy design fundamentals necessary for emerging green-collar workers, which most of their parents were never trained in. When the angle of incidence of sunlight approaches parallel, even rough black surfaces become highly reflective, as in a mirage on a long flat asphalt highway.

The second critical element of passive solar glazing design requires a precise understanding of your location-specific sun path, and the degree day heating and cooling requirements for your seasonal climatic conditions (available from your local weather service, such as NOAA in the U.S.). 3D passive-solar modeling software can do all of these things for you, for a particular specific design. Unconventional creativity is required to optimize the cost/benefits of passive solar building design, which common current modeling software does not do for you. All good passive-and-active solar designers must holistically integrate the above optical physics principles into their energy efficient solar building designs, to eliminate all unnecessary utility bills.

Operable shading devices

A design with too much equator-facing glass can result in excessive winter, spring, or fall day heating, uncomfortably-bright living spaces at certain times of the year, and excessive heat transfer on winter nights and summer days. Control mechanisms (such as manual-or-motorized interior insulated drapes, shutters, exterior roll-down shade screens, or retractable awnings) can reduce this design error, and help control daily solar gain variations.

The summer-and-winter solstice altitude vary by 47 degrees, but the hottest and coldest days and nights lag more than a month behind the solstice. Although the sun is at the same altitude 6-weeks before and after the solstice, the heating and cooling requirements before and after the solstice are significantly different. Some type of solar-gain control mechanism is needed to avoid expensive conventional active heating and cooling systems. Interior temperature (and humidity) will vary to uncomfortable levels IF some type of control mechanism is not included in a computer-model-based holistic systems engineering approach. Although not purely "passive", sensors such as temperature, sunlight, time of day, and room occupancy can be used to automate operation of motorized shading and insulation devices, with minimal energy expenditure on a daily / annual basis.

Exterior finishes

Materials and colors can be chosen to reflect or absorb solar thermal energy. See "Cool Colors" by Lawrence Berkeley National Laboratory and Oak Ridge National Laboratory

Landscaping

Energy-efficient landscaping materials, including the use of trees, plants, hedges, or a trellis (agriculture), can be used to selectively create summer shading (particularly in the case of deciduous plants that give up their leaves in the winter), and also to create winter wind chill shelter. Xeriscaping is used to reduce or eliminate eliminate the need for energy-and-water-intensive irrigation.

Other passive solar principles

Passive solar lighting

Main article: Passive solar lighting

Passive solar lighting techniques attempt to take advantage of natural illumination and reduce reliance on artificial lighting systems.

This can be achieved by careful building design and placement of window sections. Other creative solutions involve the use of reflecting surfaces to admit daylight into the interior of a building such as a solar light tube, or light shelf. Window sections should be adequately sized without resulting in over-illumination.[25]

Another major issue for many window systems is that they can be potentially vulnerable sites of excessive thermal gain or heat loss. Whilst high mounted clerestory window and traditional skylights can introduce daylight in poorly-orientated sections of a building, unwanted heat transfer may be hard to control. [26][27] Thus, energy that is saved by reducing artificial lighting is often more than offset by the energy required for operating HVAC systems to maintain thermal comfort.

Various methods can be employed to address this including but not limited to window coverings, insulated glazing and novel materials such as aerogel semi-transparent insulation, optical fiber embedded in walls or roof, or hybrid solar lighting at Oak Ridge National Laboratory.

Passive solar water heating

Main article: Solar hot water

There are many ways to use solar thermal energy to heat water for domestic use. Different active-and-passive solar hot water technologies have different location-specific economic cost benefit analysis implications.

Fundamental passive solar hot water heating involves no pumps or anything electrical. It is very cost effective in climates that do not have lengthy sub-freezing, or very-cloudy, weather conditions. Other active solar water heating technologies, etc. may be more appropriate for some locations.

Design tools

Traditionally a heliodon was used to simulate the altitude and azimuth of the sun shining on a model building at any time of any day of the year.[1] In modern times, computer programs can model this phenomenon and integrate local climate data (including site impacts such as overshadowing and physical obstructions) to predict the solar gain potential for a particular building design over the course of a year. This provides the designer the ability to optimize design elements and orientation prior to building works commencing.

Levels of application

Pragmatic: Many detached suburban houses can achieve reductions in heating expense without obvious changes to their appearance, comfort or usability [2]. 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: An extension of the "passive solar" approach to diurnal solar capture and storage ("short-cycle passive solar"). Other experimental designs attempt to capture warm-season solar heat, convey it to a seasonal thermal store for use months later during the cool or cold season ("annualized passive solar.") Increased storage is achieved by employing large amounts of thermal mass or earth-coupling. Anecdotal reports suggest they can be effective but no formal study has been conducted to demonstrate their superiority.

Examples:

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.

See also

Passive solar design concepts

Energy Efficiency in buildings

Energy Rating systems

Solar-designers

References

  1. ^ a b "Passive design - Introduction". Retrieved 2008-01-14.
  2. ^ a b "Rating tools". Retrieved 2008-01-14.
  3. ^ http://www.greenhouse.gov.au/yourhome/technical/fs71.htm
  4. ^ http://www.nrel.gov/docs/fy06osti/39678.pdf
  5. ^ http://www.toolbase.org/PDF/CaseStudies/ZEHPrimer.pdf
  6. ^ http://www.eere.energy.gov/buildings/info/documents/pdfs/35317.pdf
  7. ^ http://www.srrb.noaa.gov/highlights/sunrise/fig5_40n.gif
  8. ^ http://www.srrb.noaa.gov/highlights/sunrise/fig5_0n.gif
  9. ^ http://www.srrb.noaa.gov/highlights/sunrise/fig5_90n.gif
  10. ^ http://www.greenhouse.gov.au/yourhome/technical/fs13.htm
  11. ^ http://www.greenhouse.gov.au/yourhome/technical/fs16a.htm
  12. ^ http://www.ornl.gov/sci/roofs+walls/whole_wall/airtight.html
  13. ^ http://www.greenhouse.gov.au/yourhome/technical/fs15.htm
  14. ^ http://www.eere.energy.gov/consumer/your_home/insulation_airsealing/index.cfm/mytopic=11680 EERE Radiant Barriers
  15. ^ a b c d "Glazing - Overview". Retrieved 2008-01-14.
  16. ^ http://www.greenhouse.gov.au/yourhome/technical/fs13.htm
  17. ^ http://www.greenhouse.gov.au/yourhome/technical/fs19.htm
  18. ^ http://www.greenhouse.gov.au/yourhome/technical/fs14.htm
  19. ^ "Introductory Passive Solar Energy Technology Overview". U.S. DOE - ORNL Passive Solar Workshop. Retrieved 2007-12-23.
  20. ^ Chiras, D. The Solar House: Passive Heating and Cooling. Chelsea Green Publishing Company; 2002.
  21. ^ "Two Small Delta Ts Are Better Than One Large Delta T". Zero Energy Design. Retrieved 2007-12-23.
  22. ^ http://www.greenhouse.gov.au/yourhome/technical/fs16a.htm
  23. ^ William A. Shurcliff. Thermal Shutters & Shades - Over 100 Schemes for Reducing Heat Loss through Windows 1980. ISBN 0-931790-14-X.
  24. ^ http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd039_e.html Solar Heat Gain Through Glass
  25. ^ Chiras, D. The Solar House: Passive Heating and Cooling. Chelsea Green Publishing Company; 2002.
  26. ^ http://www.direct.gov.uk/en/Environmentandgreenerliving/Greenerhome/DG_064374
  27. ^ http://www.allwoodwork.com/article/homeimprovement/reduce_your_heating_bills.html

External links

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