Passive Solar Energy use
By Emil Bedi, CANCEE and Hakan Falk, "Energy Saving Now".
Comments by Energy Saving Now.
The following sample house, in article about Passive Solar design, do not comply with our understanding of sustainability or efficiency. It represents popular opinions within the Architectural community and from this point of view it is very interesting. It also promotes an understanding of thermal mass and wider temperature bandwidth, that are essential issues for energy saving and as such very positive. About the sample (not the principles), we have the following to clarify,
- The displayed building is very nice to look at and we find it attractive from a design perspective.
- Many of the principles mentioned are important and complaint with what we are promoting on Energy Saving Now.
- The sample building with large glass areas and the functionality is only valid for very limited geographical areas.
- The savings are expected to be 15% and we assume that this is in comparison to normal buildings in USA and the area where it is built. If it is compared to a building according to the Swedish Building Codes, the energy consumption will be 2 to 3 times higher than the Swedish house. If it is located in a cold or warm climate, the surface temperature on the glassed areas will be very uncomfortable.
- A 15% energy saving can be achieved in many easier ways and even in existing buildings. If you use paint for the building, that are Aluminium based instead of normally Titan, the saving will be around 15% and work in all geographical areas. It would also provide better comfort.
- With the extended allowed temperature variation and operation policies, nearly all buildings will give 15% or better performance.
- Sustainable buildings, based on long experience, was standard until a couple of hundred years ago. They used the location, thermal mass and/or insulation. A very common feature was the small window openings and covering of the windows to regulate the indoor temperatures depending on time of day and seasonal changes.
Article about passive solar design.
Passive solar design, or climate responsive buildings use existing technologies and materials to heat, cool and light buildings. They integrate traditional building elements like insulation, south-facing glass, and massive floors with the climate to achieve sustainable results. These living spaces can be built for no extra cost while increasing affordability through lower energy payments. In many countries they also keep investment in the local building industry rather than transferring them to short term energy imports. Passive solar buildings are better for the environment while contributing to an energy independent, sustainable energy future.
Passive solar system uses the building structure as a collector, storage and transfer mechanical equipment. This definition fits most of the more simple systems where heat is stored in the basic structure: walls, ceiling or floor. There are also systems that have heat storage as a permanent element within the building structure, such as bins of rocks, or water-filled drums or bottles. These are also classified as passive solar energy systems. Passive solar homes are ideal places in which to live. They provide beautiful connections to the outdoors, give plenty of natural light, and save energy throughout the year.
HISTORY
Building design has historically borrowed its inspiration from the local environment and available building materials. More recently, humankind has designed itself out of nature, taking a path of dominance and control which led to one style of building for nearly any situation. In 100 A.D., Pliny the Younger, a historical writer, built a summer home in Northern Italy featuring thin sheets of mica windows on one room. The room got hotter than the others and saved on short supplies of wood. The famous Roman bath houses in the first to fourth centuries A.D. had large south facing windows to let in the sun’s warmth. By the sixth century, sunrooms on houses and public buildings were so common that the Justinian Code initiated “sun rights” to ensure individual access to the sun. Conservatories were very popular in the 1800’s creating spaces for guests to walk through warm greenhouses with lush foliage.
Passive solar buildings in the United States were in such demand by 1947, as a result of scarce energy during the prolonged World War 2, that Libbey-Owens-Ford Glass Company published a book entitled Your Solar House, which profiled forty-nine of the nations greatest solar architects.
In the mid-1950’s, architect Frank Bridgers designed the world’s first commercial office building using solar water heating and passive design. This solar system has been continuously operating since that time and the Bridgers-Paxton Building is now in the National Historic Register as the world’s first solar heated office building.
Low oil prices following World War 2 helped keep attention away from solar designs and efficiency. Beginning in the mid-1990’s, market pressures are driving a movement to redesign our building systems to more in line with nature.
Passive Solar Space Heating
There are few basic architectural modes for the utilisation of passive solar utilisation in architecture. But these modes, as presented below, can be developed into many different scheme, and enrich the design.
The essential elements of a passive solar home are: good siting of the house, many south-facing windows (in Northern Hemisphere) to admit solar energy in winter (and, conversely, few east or west facing windows, to limit the collection of unwanted summer sunshine), sufficient interior mass (thermal mass) to smooth out undesirable temperature swings and to store heat for night time and a well-insulated building envelope.
Siting, insulation, windows orientation and mass must be used together. For least variation of indoor temperature the insulation should be placed on the outside of the mass. However where rapid indoor heating is required some insulation or low heat capacity material should be placed at the inside surface. There will be an optimum design for each micro-climate and indications are that a careful balance between mass and insulation in a structure will result not only in energy savings but in initial material cost saving as well.
Site
Landscaping and Trees
According to the U.S. Department of Energy report, “Landscaping for Energy Efficiency” (DOE/GO-10095-046), careful landscaping can save up to 25% of a household’s energy consumption for heating and cooling. Trees are very effective means of shading in the summer months as well as providing breaks for the cool winter winds. In addition to contributing shade, landscape features combined with a lawn or other ground cover can reduce air temperatures as much as 5 degrees Celsius in the surrounding area when water evaporates from vegetation and cools the surrounding air. Trees are wonderful for natural shading and cooling, but they must be located appropriately so as to provide shade in summer and not block the winter sun. Even deciduous trees that lose their leaves during cold weather block some winter sunlight - a few bare trees can block over 50 percent of the available solar energy.
Windows
Glass permits sun radiation of wavelengths 0.4 to 2.5 microns to pass through it. As this radiant energy collides with opaque objects on the other side of the glass, it’s wavelength increases to 11 microns. Glass acts as an opaque barrier to light of this wavelength thereby trapping the sun’s energy. The amount of light penetrating a glass is dependent on the angle of incidence. The optimum angle of incidence is 90o. When sunlight strikes the glass at 30o or less, the most radiation is reflected. 
All effective passive systems depend on windows. Glass or other translucent materials allow short-wave, solar radiation to enter a building and prohibit the long-wave, heat radiation, from escaping. Windows control the energy flow in two principle ways: they admit solar energy in winter, so warming the house above the otherwise cool to cold internal conditions; and by excluding sun from the windows (by orientation and shading) there exist the opportunity to use ventilation to cool the otherwise warm hot house in summer. If use is to be made of the sun’s heat, then it has to reach buildings when it is useful. Generally, the sun should be able to reach the collection area between 9 a.m. and 3 p.m. in winter with as little obstruction and interference as possible.Trees on the site or the neighbours’ site might shade the vital areas of the building. This need to be checked and the building located to minimise any such interference. It is possible to plan a house to have its main outlook in any direction and still be an efficient low energy building. The building envelope, i.e. the walls, floor and roof are the important elements in design, rather than the location of internal spaces. If a window needs to face west it requires correct shading and its size restricted.
Understanding the Solar Spectrum and Heat Transfer
To make good choices on glazing, it is needed to understand a bit about light and heat. The sunlight that strikes the Earth is comprised of a variety of wavelengths and different glazing will selectively transmit, absorb, and reflect the various components of the solar spectrum. Likewise, reducing glare (via reflection or tinting) is helpful in the workplace by allowing the transmission of visible, or natural, light it is possible to save energy for artificial light. But perhaps the greatest effect on human comfort levels is determined by infrared heat transfer. By specifying the right type of glass, it is possible to trap the infrared heat for warmth, or reflect the infrared heat to prevent warming.
There are three ways that heat moves through a glazing material. The first is conduction. Conductive heat is transferred through the glazing by direct contact. Heat can be felt by touching the glazing material. The second form of heat transfer is radiation; electromagnetic waves carry heat through a glazing. This produces the feeling of heat radiating from the surface of the glazing. The third method of heat transfer is convection. Convection transfers heat by motion, in this case, air flow. The natural flow of warm air toward colder air allows heat to be lost or gained.
The R-value of a glazing - its insulating capabilities or resistance to the flow of heat - is determined by the degree of conduction, radiation, and convection through the glazing material. However, air infiltration will also determine the overall R-value of a glazing system. The amount of heat that travels around a glazing is as important as the heat transfer through a glazing. Air can leak in or out of a building around the glazing via the framing. The quality, workmanship, and the installation of the entire glazing system, including the framing, affects air infiltration.
Advances in glass technology have perhaps been the single largest contributor to building efficiency since the 1970s and they play an important roll in solar design. Some window advances include:
- Double and triple pane windows with much higher insulating values.
- Low emissivity or Low-E glass employing a coating which lets heat in but not out.
- Argon (and other) gas filled windows that increase insulating values above windows with just air.
- Phase-change technologies that can switch from opaque to translucent when a voltage is applied to them.
Basic Glass Types
Glazing materials include glass, acrylics, fibreglass, and other materials. Although different glazing materials have very specific applications, the use of glass has proven the most diverse. The various types of glass allow the passive solar designer to fine-tune a structure to meet client needs. The single pane is the simplest of glass types, and the building block for higher performance glass. Single panes have a high solar transmission, but have poor insulation - the R-value is about 1,0. Single pane glass can be effective when used as storm windows, in warm climate construction (unless air conditioning is being used), for certain solar collectors, and in seasonal greenhouses. Structures using single pane glass will typically experience large temperature swings, drafts, increased condensation, and provide a minimal buffer from the outdoors.
Perhaps the most common glass product used today is the double pane unit. Double pane glass is just that: two panes manufactured into one unit. Isolated glass (thermopane) incorporate a spacer bar (filled with a moisture absorbing material called a desiccant) between the panes and are typically sealed with silicone. The spacer creates a dead air space between the panes. This air space increases the resistance to heat transfer; the R-value for double pane is about 1,8-2,1. Huge air spaces will not drastically increase R-value. In fact, a large air space can actually encourage convective heat transfer within the unit and produce a heat loss. A rule of thumb for air space is between 1 and 2 centimetres. It is also possible to go as large as 10-12 centimetres without creating convective flow, but at that point you are dealing with a very large and awkward unit. The demand for greater energy efficiency in building and retrofitting homes has made insulated glass units the standard. With good solar transmission and fair insulation, such unit is a large improvement over the single pane. Windows, doors, skylights, sunrooms, and many other areas utilize double pane glass.
HIGH PERFORMANCE GLASS
High performance or enhanced glass offers even better R-value and solar energy control. By further improving the insulating capability of glass, it is possible dramatically increase also design options. What were once insulated walls may become sunrooms. Solid roofs and ceilings become windows to the sky. Dark rooms can “wake up” to natural light, solar heat gain, and wonderful views. For a relatively small increase in cost it is possible to improve efficiency, provide better moisture and UV protection, and gain design flexibility. A variety of high performance glass is now available.
What are the advantages of this glass? Low emissivity (Low-E) glass is succeeding double pane glass in energy efficient buildings. Emissivity is the measure of infrared (heat) transfer through a material. The higher the emissivity, the more heat is radiated through the material. Conversely, the lower the emissivity, the more heat is reflected by the material. Low-E coatings will reflect, or re-radiate, the infrared heat back into a room, making the space warmer. This translates into R-values from 2.6 to 3.2. In warmer climates it is possible to reverse the unit and re-radiate infrared heat back to the outside, keeping the space cooler. Low-E glass improves the R-value, UV protection, and moisture control.Gas-filled windows increase R-value. Properly done, gas-filling will increase the overall R-value of a glass unit by about 1,0. The air within an insulated glass unit is displaced with an inert, harmless gas with better insulation properties. Typical gases used are Krypton and Argon.
Window curtains
In addition to decorative functions, curtains can be used to reduce the heat losses that occur during the cold months as well as the heat gains during the warmer months. The plywood box over the curtain top prevents warm ceiling air from moving between the glass and curtain. The curtain should drop at least 30 cm below the window for it to be effective. The optimum condition would be for it to drop to the floor.
Thermal mass
Solar radiation hitting walls, windows, roofs and other surfaces is adsorbed by the building and is stored in thermal mass. This stored heat is then radiated to the interior of the building. Thermal mass in a solar heating system performs the same function as batteries in a solar electric system (see chapter on photovoltaics). Both store solar energy, when available, for later use.
Thermal mass can be incorporated into a passive solar room in many ways, from tile-covered floors to water-filled drums. Thermal mass materials, which include slab floors, masonry walls, and other heavy building materials, absorb and store heat. They are a key element in passive solar homes. Homes with substantial south-facing glass areas and no thermal storage mass do not perform well.
It is important to know that dark surfaces reflect less, therefore, absorb more heat. In case of a dark tiled floor, the floor will be able to absorb heat all day and radiate heat into the room at night. The rate of heat flow is based on the temperature difference between heat source and the object to which the heat flows. As described above heat flows in three ways - conduction (heat transfer through solid materials), convection (heat transfer through the movement of liquids or gasses), and radiation. All surfaces of a building lose heat via these three modes. Good solar design works to minimize heat loss and maximize efficient heat distribution. The need for thermal mass (heat-storage materials) inside a building is very climate-dependent. Heavy buildings of high thermal mass are consistently more comfortable during hot weather in hot-arid and cool-temperate climates, while in hot-humid climates there is little benefit. In cool-temperature climates the thermal mass acts as a cold-weather heat store thus improving overall comfort and reducing the need for auxiliary heating, except on overcast or very cold days. In intermittently heated buildings, however, it tends to increase the heat needed to maintain the chosen conditions.
Providing adequate thermal mass is usually the greatest challenge to the passive solar designer. The amount of mass needed is determined by the area of south-facing glazing and the location of the mass. In order to ensure an effective design it is important to follow these guidelines:
Locate the thermal mass in direct sunlight. Thermal mass installed where the sun can reach it directly is more effective than indirect mass placed where the sun’s rays do not penetrate. Houses that rely on indirect storage need three to four times more thermal mass than those using direct storage. Distribute the thermal mass. Passive solar homes work better if the thermal mass is relatively thin and spread over a wide area. The surface area of the thermal mass should be at least 3 times, and preferably 6 times, greater than the area of the south windows. Slab floors that are 8 to 10 centimetres thick are more cost effective and work better than floors 16 to 20 inches thick. Do not cover the thermal mass. Carpeting virtually eliminates savings from the passive solar elements. Masonry walls can have drywall finishes, but should not be covered by large wall hangings or lightweight panelling. The drywall should be attached directly to the mass wall, not to covers fastened to the wall that create an undesirable insulating airspace between the drywall and the mass. Select an appropriate mass colour. For best performance, finish mass floors with a dark colour. A medium colour can store 70 percent as much solar heat as a dark colour, and may be appropriate in some designs. A matte finish for the floor reduces reflected sunlight, thus increasing the amount of heat captured by the mass and having the additional advantage of reducing glare. The colour of interior mass walls does not significantly affect passive solar performance. Insulate the thermal mass surfaces. There are several techniques for insulating slab floors and masonry exterior walls. These measures should introduced to achieve the energy savings. Unfortunately, problems in some case can arise like with termite infestations in foam insulation for perimeter slabs. This can complicate the issue of whether and how to insulate slab-on-grade floors. Make thermal mass multipurpose. For maximum cost effectiveness, thermal mass elements should serve other purposes as well. Masonry thermal storage walls are one example of a passive solar design that is often cost prohibitive because the mass wall is only needed as thermal mass. On the other hand, tile-covered slab floors store heat, serve as structural elements, and provide a finished floor surface. Masonry interior walls provide structural support, divide rooms, and store heat.
When developing a thermal storage system or simply comparing materials it is useful to look at the storage capacity of the proposed building materials which is referred to as the volumetric heat capacity (J/m3. Deg. Celsius) or more commonly the specific heat and the rate at which the material can take up and store heat. Some examples of common storage materials are given in the following table:
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Thermal insulation
Materials generally available for building purposes can be classified into two generic groups - bulk materials and reflective foil laminates (RFL). The first of these relies on the resistance of air trapped in pockets between the fibres of the blanket type materials (mineral fibre materials) or the cells formed in the foamed structure of board or slab type materials (usually made from plastics such as polystyrene and polyurethane foams). The second reflects radiant energy away from the object or surface being protected. Thermal insulation in the outer fabric of a building is a vital component of an energy-efficient design strategy. The key to successful energy-efficient design is the control of heat flow through the external fabric. All the solar energy gained could be easily lost from an inadequately insulated building before it is able to be of benefit. It will have been noted that some materials have a very much higher thermal resistance per unit thickness than others irrespective of their density. The fact that air is a good insulator especially if it is bounded by a bright foil surface to limit radiation transfer can be very useful as well.
Cooling
In many parts of the world a passive solar building needs cooling as much as heating. One of the best, time proven methods of cooling is thermal coupling with the earth’s constant temperature. Dropping the ground floor at least one meter into the earth provides a more even exterior temperature which aids cooling as well as heating. Adequate structural engineering, drainage, and damp proofing are essential in below ground areas. Thermal isolation is the best and most economical way to temper the building’s environment. Using the earth’s thermal mass keeps the house at a reasonable temperature, and so does good insulation. Shades located outside and inside the windows, ventilation and reflective films on the windows are also very important in order to control temperature inside the building.
External Shades and Shutters
Exterior window shading treatments are effective cooling measures because they block both direct and indirect sunlight outside of the home. Solar shade screens are an excellent exterior shading product with a thick weave that blocks up to 70 percent of all incoming sunlight. The screens absorb sunlight so they should be used on the exterior of the windows. From outside, they look slightly darker than regular screening, but from the inside many people do not detect a difference. Most products also serve as insect screening. They should be removed in winter to allow full sunlight through the windows. A more expensive alternative to the fibreglass product is a thin, metal screen that blocks sunlight, but still allows a view from inside to outside. Hinged decorative exterior shutters which close over the windows are also excellent shading options. However, they obscure the view, block daylight completely, may be expensive and may be difficult for many households to operate on a daily basis.
Interior Shades and Shutters
Shutters and shades located inside the house include curtains, roll-down shades, and Venetian blinds. Interior shutters and shades are generally the least effective shading measures because they try to block sunlight that has already entered the room. However, if passive solar windows do not have exterior shading, interior measures are needed. The most effective interior treatments are solid shades with a reflective surface facing outside. In fact, simple white roller blinds keep the house cooler than more expensive louvered blinds, which do not provide a solid surface and allow trapped heat to migrate between the blinds into the house.
Reflective Films and Tints
Reflective film, which adheres to glass and is found often in commercial buildings, can block up to 85% of incoming sunlight. The film blocks sunlight all year, so it is inappropriate on south windows in passive solar homes. However, it may be practical for unshaded east and west windows. These films are not recommended for windows that experience partial shading because they absorb sunlight and heat the glass unevenly. The uneven heating of windows may break the glass or ruin the seal between double-glazed units.
Ventilation
Ventilation is the changing of air in buildings to control oxygen, heat and contaminants. Ventilation may occur in few forms. Building orientation, form, plan and user actions also alter air flow paths. Natural ventilation consumes no energy and has few if any running costs, but depends on weather conditions and can be difficult to control. Mechanical and air-conditioned ventilation are energy-driven alternatives to natural ventilation, normally dictated by building type, site and function. They can be particularly efficient as supplements to natural ventilation. Mechanical ventilation uses fans and ducts to supply and extract air in localised areas such as a kitchen. Air conditioning both treats and supplies air. It is particularly useful to cool air below ambient temperatures.
SOLAR ARCHITECTURE & ACTIVE SYSTEMS
It is important to design the house with the aim to incorporate active solar systems (see below) like collectors or photovoltaic modules as well. The building should orient these appliances due south. Tilt of the solar collectors should be in Europe and North America more than 50° (from horizontal) to maximize winter heat collection. Solar collectors should be thermally locked with the roof. Non-tracking photovoltaics receive the most yearly insolation (exposure to the sun’s rays) when tilted at an angle, from horizontal, equal to the building’s latitude. Design of the building’s roof should be done to such angles and southern orientation as integral aspects of the building. Hot water collectors and photovoltaic panels should be located as close as possible to their main areas of use. It is important to concentrate these areas of use. For example, putting the bathrooms and kitchen close together economizes on their installation and minimizes energy loss. All appliances should be selected with efficiency as the prime criterion.
SUMMARY
Passive use of sunlight contributes around 15% of space heating needs in typical building. It is important source of energy savings which can be utilised everywhere and almost at no extra cost. There are some principles which can help a designer to harness solar energy through thermally efficient buildings.
SITE
It is important to become familiar with the energy flows of house surroundings. The nature and relationship of the lay of the land, water courses, vegetation, soil types, wind directions, and exposure to the sun should be investigated. A site suitable for solar design should balance and complement these elements. It must have unobstructed exposure to the sun from 9 am to 3 pm during the heating season.
HEATING
In Northern hemisphere orientation due south of the main solar insolating spaces, i.e. greenhouse, and/or main daytime activity areas is important. Glass should be open to the sun patterns during the winter. By facing of the windows to the south, and virtually none to the north maximaze solar gain. Multiple pane glass in all windows is recommended.
THERMAL MASS
Thermal mass including masonry floors, walls and water storage is important to absorb ambient heat during the day and release it at night. Insulation of the building further minimize heat loss through windows, walls and roof.
NATURAL HEAT FLOW
It is useful to design the house with the natural heat flow in mind. Hot air rises, so placing some activity areas on a second floor to draw heat up from a lower collector area and across other areas can save a lot of energy. Buffer areas of the building (unheated rooms, or partially heated spaces such as utility rooms, vestibules and storage areas) should be oriented due to the north to lessen the impact of the winter’s cold. Using a vestibule on doors to the exterior can lead to energy savings. Vestibules cut heat loss and provide a buffer zone between the exterior and the interior.
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