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Passive solar building design

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. Passive solar building design is one part of green building design, and does not include active systems such as mechanical ventilation or photovoltaics.

As a science

The scientific basis for passive solar building design has been developed from a combination of climatology, thermodynamics (particularly heat transfer), and human thermal comfort (for buildings to be inhabited by humans). 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. While 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.

The solar path in passive design

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

The converse is observed in the Southern Hemisphere, but the sun rises to the east and sets toward the west regardless of which hemisphere you are in.

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.

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.

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.. 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.

Passive solar thermodynamic principles

Personal thermal comfort is a function of ambient air temperature, mean radiant temperature, air movement (wind chill, turbulence) and relative humidity (affecting human evaporative cooling). Heat transfer in buildings occurs through convection, conduction, and thermal radiation through roof, walls, floor and windows.

Convective heat transfer

Convective heat transfer can be beneficial or detrimental. Uncontrolled air infiltration from poor weatherisation / weatherstripping / draught-proofing can contribute up to 40% of heat loss during winter, however strategic placement of operable windows or vents can enhance convection, cross-ventilation, and summer cooling when the outside air is of a comfortable temperature and relative humidity. Filtered energy recovery ventilation systems may be useful to eliminate undesirable humidity, dust, pollen, and microorganisms in unfiltered ventilation air.

Natural convection causing rising warm air and falling cooler air can result in an uneven stratification of heat. This may cause uncomfortable variations in temperature in the upper and lower conditioned space, serve as a method of venting hot air, or be designed in as a natural-convection air-flow loop for passive solar heat distribution and temperature equalization. Natural human cooling by perspiration and evaporation may be facilitated through natural or forced convective air movement by fans, but ceiling fans can disturb the stratified insulating air layers at the top of a room, and accelerate heat transfer from and hot attic, or through near by windows. In addition, high relative humidity inhibits evaporative cooling by humans.

Radiative heat transfer

The main source of heat transfer is radiant energy, and the primary source is the sun. Solar radiation occurs predominantly through the roof and windows (but also through walls). Thermal radiation moves from a warmer surface to a cooler one. Roofs receive the majority of the solar radiation delivered to a house. A cool roof, or green roof in addition to a radiant barrier can help prevent your attic from becoming hotter than the peak summer outdoor air temperature (see albedo, absorptivity, emissivity, and reflectivity).

Windows are a ready and predictable site for thermal radiation. Energy from radiation can move into a window in the day time, and out of the same window at night. Radiation uses photons to transmit electromagnetic waves through a vacuum, or translucent medium. Solar heat gain can be significant even on cold clear days. Solar heat gain through windows can be reduced by insulated glazing, shading, and orientation. Windows are particularly difficult to insulate compared to roof and walls. Convective heat transfer through and around window coverings also degrade its insulation properties. When shading windows, external shading is more effective at reducing heat gain than internal window coverings.

Western and eastern sun can provide warmth and lighting, but are vulnerable to overheating in summer if not shaded. In contrast, the low midday sun readily admits light and warmth during the winter, but can be easily shaded with appropriate length overhangs or angled louvres during summer. The amount of radiant heat received is related to the location latitude, altitude, cloud cover, and seasonal / hourly angle of incidence (see Sun path and Lambert's cosine law).

Another passive solar design principle is that thermal energy can be stored in certain building materials and released again when heat gain eases to stabilize diurnal (day/night) temperature variations. The complex interaction of thermodynamic principles can be counterintuitive for first-time designers. Precise computer modeling can help avoid costly construction experiments.

Site specific considerations during design

Latitude and sun path
Seasonal variations in solar gain e.g. cooling or heating degree days, solar insolation, humidity
Diurnal variations in temperature
Micro-climate details related to breezes, humidity, vegetation and land contour
Obstructions / Over-shadowing - to solar gain or local cross-winds

Design elements for residential buildings in temperate climates

Orienting the building to face the equator (or a few degrees to the East to capture the morning sun)
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
Erecting correctly-sized, latitude-specific overhangs, or shading elements (shrubbery, trees, trellises, fences, shutters, etc.)
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)

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 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
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:

direct solar gain
indirect solar gain
isolated solar gain
passive cooling

Direct solar gain

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

The cost effectiveness of these configurations are currently being investigated in great detail and are demonstrating promising results.

Indirect solar gain

Indirect gain attempts to control solar radiation reaching an area adjacent but not part of the living space. Heat enters the building through windows and is captured and stored in thermal mass (e.g water tank, masonry wall) and slowly transmitted indirectly to the building through conduction and convection. Efficiency can suffer from slow response (thermal lag) and heat losses at night. Other issues include the cost of insulated glazing and developing effective systems to redistribute heat throughout the living area.

Examples:

Trombe walls
Water walls
Roof ponds

Isolated solar gain

Isolated gain involves utilizing solar energy to passively move heat from or to the living space using a fluid, such as water or air by natural convection or forced convection. Heat gain can occur through a sunspace, solarium or solar closet. These areas may also be employed usefully as a greenhouse or drying cabinet. An equator-side sun room may 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. Glass placement and overhangs prevent solar gain during the summer. Earth cooling tubes or other passive cooling techniques can keep a solarium cool in the summer.

Measures should be taken to reduce heat loss at night e.g. window coverings or movable window insulation

Examples:

Other considerations

Insulation

Thermal insulation or superinsulation (type, placement and amount) assists in significantly reducing unwanted heat transfer.

Special glazing systems and window coverings

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.).

Generally, Equator-facing windows should not employ glazing coatings that inhibit solar gain.

There is extensive use of super-insulated windows in 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.

Glazing selection

Equator-facing glass

The requirement for vertical equator-facing glass is different than for the other three sides of a building. Reflective window coatings and multiple panes of glass can reduce useful solar gain. However, direct-gain systems are more dependent on double or triple glazing to reduce heat loss. Indirect-gain and isolated-gain configurations may still be able to function effectively with only single-pane glazing. Nevertheless, the optimal cost-effective solution is both location and system dependent.

Roof-angle glass / Skylights

Sloping roof-angled glass is difficult to shade and insulate without sophisticated movable systems. In hot climates with significant degree day cooling requirements, it can create a summer solar furnace (from the Ancient Greek / Roman term "heliocaminus).

Roof-angled glass or skylights are not optimally placed to receive low-angled winter sun. At the same time, they are the site of heat loss during winter from the buoyant warm air that rises. As a result, they will increase heating and cooling energy requirements, which exceeds the benefit of daylight energy consumption reduction compared to more energy-efficient lighting systems such as light tubes.

Transparent glass and plastic have little structural strength. Vertically, they bear their own weight because only the thickness is subject to gravity. As the angle tilts from vertical, an increased area (the sloped cross-section) must resist gravity. Glass is brittle - It does not flex much before breaking - To counteract this, you must increase thickness, or structural supports - Both increase overall cost, and reduce solar gain potential. Sloped glazing is exposed to the weather, leaks, hail, ice-and-snow load, wind, and material failure. Excess solar gain, harsh lighting, and undesirable heat transfer thru sloped glass are difficult to control. “Therefore, vertical glazing is the overall best option for sunspaces.”

Angle of incident radiation

The amount of solar gain transmitted through glass is also affected by the angle of the incident solar radiation. Sunlight striking glass within 20 degrees of perpendicular is mostly transmitted through the glass, whereas sunlight at more than 35 degrees from perpendicular is mostly reflected

All of these factors can be modeled more precisely with a photographic light meter and a heliodon or optical bench, which can quantify the ratio of reflectivity to transmissivity, based on angle of incidence.

Alternatively, passive solar computer software can determine the impact of sun path, and cooling-and-heating degree days on energy performance. Regional climatic conditions are often available from local weather services.

Operable shading and insulation 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.

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. Heat storage on the Earth's surface causes "thermal lag." Variable cloud cover influences solar gain potential. This means that latitude-specific fixed window overhangs, while important, are not a complete seasonal solar gain control solution.

Control mechanisms (such as manual-or-motorized interior insulated drapes, shutters, exterior roll-down shade screens, or retractable awnings) can compensate for differences caused by thermal lag or cloud cover, and help control daily / hourly solar gain requirement variations. Home automation systems that monitor temperature, sunlight, time of day, and room occupancy can precisely control motorized window-shading-and-insulation devices.

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 the need for energy-and-water-intensive irrigation.

Other passive solar principles

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.

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. 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

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.

It is possible to have active solar hot water which is also capable of being "off grid" and qualifies as sustainable. This is done by the use of a photovoltaic cell which uses energy from the sun to power the pumps.

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. 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 evaluate design elements and orientation prior to building works commencing. Energy performance optimization normally requires an iterative-refinement design-and-evaluate process.

Levels of application

Pragmatic

Many detached suburban houses can achieve reductions in heating expense without obvious changes to their appearance, comfort or usability This is done using good siting and window positioning, small amounts of thermal mass, with good-but-conventional insulation, weatherization, and an occasional supplementary heat source, such as a central radiator connected to a (solar) water heater. Sunrays may fall on a wall during the daytime and raise the temperature of its thermal mass. This will then radiate heat into the building in the evening. This can be a problem in the summer, especially on western walls in areas with high degree day cooling requirements. External shading, or a radiant barrier plus air gap, may be used to reduce undesirable summer solar gain.

Annualised

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 ("annualised 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:

Passive Annual Heat Storage (PAHS) - by John Hait
Annualized Geothermal Solar (AGS) heating - by Don Stephen
Earthed-roof

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, computers, and other task-specific appliances (such as those for cooking, entertainment, etc.), showering, people and pets. The use of natural convection air currents (rather than mechanical devices such as fans) to circulate air is related, though not strictly solar design.

Passive solar building design sometimes uses limited electrical and mechanical controls to operate dampers, insulating shutters, shades, awnings, or reflectors. Some systems enlist small fans or solar-heated chimneys to 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 fan to more-evenly distribute 10 kW of solar heat through an entire house would have a COP of 300.

Zero Energy Building

Passive solar building design is often a foundational element of a cost-effective zero energy building. 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.