Passive solar building design

In passive solar building design, windows, walls, and floors are made to collect, store, reflect, and distribute solar energy in the form of heat in the winter and reject solar heat in the summer. This is called passive solar design because, unlike active solar heating systems, it does not involve the use of mechanical and electrical devices.

The key to design a passive solar building is to best take advantage of the local climate performing an accurate site analysis. Elements to be considered include window placement and size, and glazing type, thermal insulation, thermal mass, and shading. Passive solar design techniques can be applied most easily to new buildings, but existing buildings can be adapted or “retrofitted”.

Passive energy gain
assive solar technologies use sunlight without active mechanical systems (as contrasted to active solar). Such technologies convert sunlight into usable heat (in water, air, and thermal mass), cause air-movement for ventilating, or future use, with little use of other energy sources. A common example is a solarium on the equator-side of a building. Passive cooling is the use of the same design principles to reduce summer cooling requirements.

Some passive systems use a small amount of conventional energy to control dampers, shutters, night insulation, and other devices that enhance solar energy collection, storage, and use, and reduce undesirable heat transfer.

Passive solar technologies include direct and indirect solar gain for space heating, solar water heating systems based on the thermosiphon, use of thermal mass and phase-change materials for slowing indoor air temperature swings, solar cookers, the solar chimney for enhancing natural ventilation, and earth sheltering.

More widely, passive solar technologies include the solar furnace, but this typically requires some external energy for aligning their concentrating mirrors or receivers, and historically have not proven to be practical or cost effective for widespread use. ‘Low-grade’ energy needs, such as space and water heating, have proven over time to be better applications for passive use of solar energy.

As a science
The scientific basis for passive solar building design has been developed from a combination of climatology, thermodynamics (particularly heat transfer: conduction (heat), convection, and electromagnetic radiation), fluid mechanics/natural convection (passive movement of air and water without the use of electricity, fans or pumps), and human thermal comfort based on heat index, psychrometrics and enthalpy control for buildings to be inhabited by humans or animals, sunrooms, solariums, and greenhouses for raising plants.

Specific attention is divided into: the site, location and solar orientation of the building, local sun path, the prevailing level of insolation (latitude/sunshine/clouds/precipitation), design and construction quality/materials, placement/size/type of windows and walls, and incorporation of solar-energy-storing thermal mass with heat capacity.

While these considerations may be directed toward any building, achieving an ideal optimized cost/performance solution requires careful, holistic, system integration engineering of these scientific principles. Modern refinements through computer modeling (such as the comprehensive U.S. Department of Energy “Energy Plus” building energy simulation software), and application of decades of lessons learned (since the 1970s energy crisis) can achieve significant energy savings and reduction of environmental damage, without sacrificing functionality or aesthetics. In fact, passive-solar design features such as a greenhouse/sunroom/solarium can greatly enhance the livability, daylight, views, and value of a home, at a low cost per unit of space.

Much has been learned about passive solar building design since the 1970s energy crisis. Many unscientific, intuition-based expensive construction experiments have attempted and failed to achieve zero energy – the total elimination of heating-and-cooling energy bills.

Passive solar building construction may not be difficult or expensive (using off-the-shelf existing materials and technology), but the scientific passive solar building design is a non-trivial engineering effort that requires significant study of previous counter-intuitive lessons learned, and time to enter, evaluate, and iteratively refine the simulation input and output.

One of the most useful post-construction evaluation tools has been the use of thermography using digital thermal imaging cameras for a formal quantitative scientific energy audit. Thermal imaging can be used to document areas of poor thermal performance such as the negative thermal impact of roof-angled glass or a skylight on a cold winter night or hot summer day.

The scientific lessons learned over the last three decades have been captured in sophisticated comprehensive building energy simulation computer software systems (like U.S. DOE Energy Plus).

Scientific passive solar building design with quantitative cost benefit product optimization is not easy for a novice. The level of complexity has resulted in ongoing bad-architecture, and many intuition-based, unscientific construction experiments that disappoint their designers and waste a significant portion of their construction budget on inappropriate ideas.

The economic motivation for scientific design and engineering is significant. If it had been applied comprehensively to new building construction beginning in 1980 (based on 1970s lessons learned), America could be saving over $250,000,000 per year on expensive energy and related pollution today.

Since 1979, Passive Solar Building Design has been a critical element of achieving zero energy by educational institution experiments, and governments around the world, including the U.S. Department of Energy, and the energy research scientists that they have supported for decades. The cost effective proof of concept was established decades ago, but cultural assimilation into architecture, construction trades, and building-owner decision making has been very slow and difficult to change.

The new terms “Architectural Science” and “Architectural Technology” are being added to some schools of Architecture, with a future goal of teaching the above scientific and energy-engineering principles.

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.

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 although the sun is in the same relative position six weeks before, and six weeks after, the solstice, 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 or winter solstice. Movable shutters, shades, shade screens, 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 heat transfer principles
Personal thermal comfort is a function of personal health factors (medical, psychological, sociological and situational), 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 weatherization / weatherstripping / draft-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 a hot attic, or through nearby 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 and leaf bearing summer shade trees which shed their leaves in the fall. 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, sun path, and insolation (sunshine)
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
Placement of room-types, internal doors and walls, and equipment in the house.
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 roof 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)

Efficiency and economics of passive solar heating
Technically, PSH is highly efficient. Direct-gain systems can utilize (i.e. convert into “useful” heat) 65–70% of the energy of solar radiation that strikes the aperture or collector.

Passive solar fraction (PSF) is the percentage of the required heat load met by PSH and hence represents potential reduction in heating costs. RETScreen International has reported a PSF of 20–50%. Within the field of sustainability, energy conservation even of the order of 15% is considered substantial.

Other sources report the following PSFs:

5–25% for modest systems
40% for “highly optimized” systems
Up to 75% for “very intense” systems
In favorable climates such as the southwest United States, highly optimized systems can exceed 75% PSF.

Landscaping and gardens
Energy-efficient landscaping materials for careful passive solar choices include hardscape building material and “softscape” plants. The use of landscape design principles for selection of trees, hedges, and trellis-pergola features with vines; all can be used to create summer shading. For winter solar gain it is desirable to use deciduous plants that drop their leaves in the autumn gives year round passive solar benefits. Non-deciduous evergreen shrubs and trees can be windbreaks, at variable heights and distances, to create protection and shelter from winter wind chill. Xeriscaping with ‘mature size appropriate’ native species of-and drought tolerant plants, drip irrigation, mulching, and organic gardening practices reduce or eliminate the need for energy-and-water-intensive irrigation, gas powered garden equipment, and reduces the landfill waste footprint. Solar powered landscape lighting and fountain pumps, and covered swimming pools and plunge pools with solar water heaters can reduce the impact of such amenities.

Sustainable gardening
Sustainable landscaping
Sustainable landscape architecture

Other passive solar principles

Passive solar lighting
Passive solar lighting techniques enhance taking advantage of natural illumination for interiors, and so reduce reliance on artificial lighting systems.

This can be achieved by careful building design, orientation, and placement of window sections to collect light. Other creative solutions involve the use of reflecting surfaces to admit daylight into the interior of a building. Window sections should be adequately sized, and to avoid over-illumination can be shielded with a Brise soleil, awnings, well placed trees, glass coatings, and other passive and active devices.

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 oriented 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.

Reflecting elements, from active and passive daylighting collectors, such as light shelves, lighter wall and floor colors, mirrored wall sections, interior walls with upper glass panels, and clear or translucent glassed hinged doors and sliding glass doors take the captured light and passively reflect it further inside. The light can be from passive windows or skylights and solar light tubes or from active daylighting sources. In traditional Japanese architecture the Shōji sliding panel doors, with translucent Washi screens, are an original precedent. International style, Modernist and Mid-century modern architecture were earlier innovators of this passive penetration and reflection in industrial, commercial, and residential applications.

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.

Climate and comfort
Every building is constructed in order to shelter and protect us from the outside environment creating an interior climate. When the conditions of the exterior impede the comfort of the interior space, heating or cooling systems are used.

Among the most effective measures is the saving of energy through the use of thermal insulation. But conserving energy means isolating us from the outside, the passive design seeks to open the building to the exterior in such a way that natural conditioning can be achieved.

Thus the climate where the building is to be located is defined by the temperature, the humidity levels, the speed and direction of the winds and the sunlight of the site. Then the climatic conditions can constitute a disadvantage or an advantage for an adequate energy efficiency of the house. Simple concepts of everyday life are then applied, such as:

If it’s too cold to feel comfortable, then we wrap ourselves = thermal insulation
if it is a windy day and we are cold we look for some object to protect us and return to comfort = wind protection
if it is too hot and we are in the sun, we look for the shade = solar protection
if it’s hot, even in the shade, we look for the breeze to cool us = ventilation
if it’s hot and the air is very dry, look for some shade and cool basement = thermal mass

Mountain house
For a mountain house located in a place where it is very cold and there is a lot of wind, we want the location to be on a sunny slope protected from the wind, incorporate thermal insulation to ceilings, walls and windows; locate the windows towards the midday sun preferably; build in such a way that there is the least amount of slits where cold air penetrates and dissipates the heat inside.

House of the desert
A house in the desert should be protected from solar irradiation. On the other hand, since the temperature variation between day and night is high, due to the lack of humidity in the air, it is advisable to make use of the thermal mass by building thick walls with local materials. It is necessary to take advantage of the low night temperature to cool the mass of the building through strategically located openings that allow ventilation.

The basis of any environmentally conscious design that is intended to be effective is an adequate response to the inconveniences and advantages of the climate of the place. If this is not taken into account we will have to go to mechanical systems of thermal conditioning, with the energy consumption and the resulting greenhouse gas emissions.

Comparison to the Passive House standard in Europe
There is growing momentum in Europe for the approach espoused by the Passive House (Passivhaus in German) Institute in Germany. Rather than relying solely on traditional passive solar design techniques, this approach seeks to make use of all passive sources of heat, minimises energy usage, and emphasises the need for high levels of insulation reinforced by meticulous attention to detail in order to address thermal bridging and cold air infiltration. Most of the buildings built to the Passive House standard also incorporate an active heat recovery ventilation unit with or without a small (typically 1 kW) incorporated heating component.

The energy design of Passive House buildings is developed using a spreadsheet-based modeling tool called the Passive House Planning Package (PHPP) which is updated periodically. The current version is PHPP2007, where 2007 is the year of issue. A building may be certified as a “Passive House” when it can be shown that it meets certain criteria, the most important being that the annual specific heat demand for the house should not exceed 15kWh/m2a.

Passive solar systems
The passive solar systems are used mainly to capture and store the heat from the solar energy. They are called passive since other electromechanical devices are not used to recirculate the heat. This happens because of basic physical principles such as conduction, radiation and heat convection.

Direct gain: it is the simplest system and involves the capture of the sun’s energy by glazed surfaces, which are sized for each orientation and depending on the heat needs of the building or premises to be heated.

Non-ventilated wall of accumulation: also known as trombe wall, which is a wall built with stone, bricks, concrete or even water, painted black or very dark color on the outside. To improve the capture, a property of the glass is used, which is to generate a greenhouse effect, through which visible light enters and when the wall is touched, it heats it, emitting infrared radiation, which can not penetrate the glass. For this reason the temperature of the dark surface and of the air chamber between the wall and the glass rises.

Ventilated wall of accumulation: similar to the previous one but that incorporates orifices in the part superior and inferior to facilitate the heat exchange between the wall and the atmosphere by means of convection.

Attached greenhouse: in this case, the wall at noon incorporates a glazed area, which can be habitable, improving heat capture during the day, reducing heat losses to the outside.

Roof of heat accumulation: in certain latitudes it is possible to use the roof surface to capture and accumulate the energy of the sun. Also known as solar ponds, they require complex mobile devices to prevent heat from escaping at night.

Solar collection and heat accumulation: it is a more complex system and allows to combine the direct gain by windows with solar collectors of air or hot water to accumulate it under the floor. Then, in a similar way to the ventilated accumulator wall, heat is brought into the interior environment. Properly dimensioned allows to accumulate heat for more than seven days.

In almost all cases it can be used as passive cooling systems by inverting the operating sense.

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. GPS-based smartphone applications can now do this inexpensively on a hand held device. These design tools provide the passive solar designer the ability to evaluate local conditions, design elements and orientation prior to construction. Energy performance optimization normally requires an iterative-refinement design-and-evaluate process. There is no such thing as a “one-size-fits-all” universal passive solar building design that would work well in all locations.

Levels of application
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. External shading, or a radiant barrier plus air gap, may be used to reduce undesirable summer solar gain.

An extension of the “passive solar” approach to seasonal solar capture and storage of heat and cooling. These designs attempt to capture warm-season solar heat, and convey it to a seasonal thermal store for use months later during the 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. The approach also can move cooling into the warm season. Examples:

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

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.

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.

Passive solar design on skyscrapers
There has been recent interest in the utilization of the large amounts of surface area on skyscrapers to improve their overall energy efficiency. Because skyscrapers are increasingly ubiquitous in urban environments, yet require large amounts of energy to operate, there is potential for large amounts of energy savings employing passive solar design techniques. One study, which analyzed the proposed 22 Bishopsgate tower in London, found that a 35% energy decrease in demand can theoretically be achieved through indirect solar gains, by rotating the building to achieve optimum ventilation and daylight penetration, usage of high thermal mass flooring material to decrease temperature fluctuation inside the building, and using double or triple glazed low emissivity window glass for direct solar gain. Indirect solar gain techniques included moderating wall heat flow by variations of wall thickness (from 20 to 30 cm), using window glazing on the outdoor space to prevent heat loss, dedicating 15–20% of floor area for thermal storage, and implementing a Trombe wall to absorb heat entering the space. Overhangs are used to block direct sunlight in the summer, and allow it in the winter, and heat reflecting blinds are inserted between the thermal wall and the glazing to limit heat build-up in the summer months.

Another study analyzed double-green skin facade (DGSF) on the outside of high rise buildings in Hong Kong. Such a green facade, or vegetation covering the outer walls, can combat the usage of air conditioning greatly – as much as 80%, as discovered by the researchers.

In more temperate climates, strategies such as glazing, adjustment of window-to-wall ratio, sun shading and roof strategies can offer considerable energy savings, in the 30% to 60% range.

Source from Wikipedia