> What is Passive Solar?

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What is Passive Solar?
Last updated 7:07 am, Thursday 20th April 2017

Passive Solar Design in buildings

Passive Solar design is an aspect of building design in which the solar cycle is exploited in Winter to provide passive building heating for free. In essence the heat of the Sun is 'captured' in Winter to provide building heat - so known as designing for solar gain.

The Passive part of passive solar design comes from the fact nothing 'active' is done to achieve this, i.e. no machinery or complex technology is employed, just the way the building is constructed that does the work.

The big win for the home owner is that the cost of keeping the house comfortable is greatly reduced; no 'active' air cons etc are required. This also protects the owner from rising fuel costs in the future. Also a Passive Solar designed house often has a better air quality and general 'atmosphere' than a traditional house as more light enters the property.

Passive solar design, how it works...

There are five separate principals that when combined provide a complete passive solar building design, as follows:

1. Aperture (Collector)
A large glass (window) area through which sunlight enters the building. Typically, the aperture(s) should face within 30 degrees of true South (or North if in in the Southern hemisphere) and should not be shaded by other buildings or trees from 9 a.m. to 3 p.m. each day during the heating season.

2. Absorber
    The hard, darkened surface of the storage element. This surface; which could be that of a masonry wall, floor, or partition (phase change material), or that of a water container; sits in the direct path of sunlight. Sunlight hits the surface and is absorbed as heat.

3. Thermal mass
    The materials that retain or store the heat produced by sunlight. The difference between the absorber and the thermal mass, although often the same wall or floor, is that the absorber is an exposed surface whereas thermal mass is the material below or behind that surface.

4. Distribution
    The method by which solar heat circulates from the collection and storage points to different areas of the house. A strictly passive design will use the three natural heat transfer modes (conduction, convection, and radiation) exclusively. In some applications fans, ducts, and blowers may help with the distribution of heat through out the house.

5. Control
    Roof overhangs can be used to shade the aperture area during Summer months. Other elements that control under and/or overheating include: electronic sensing devices, such as a differential thermostat that signals a fan to turn on; operable vents and dampers that allow or restrict heat flow; low-emissivity blinds; and awnings.

A practical example of using Passive Solar
A typical passive solar house design would often have a large glass surface area on the South (or North if Southern hemisphere) building side in relation to the other sides (note: this implies that the orientation of the building to the Sun needs to be correct for this to work, see the Related Articles below for an article on this subject). In those rooms with these large windows you will often find tiling up to to the window and extending back into the room by at least the height of the window. This tiling is often directly mounted onto a concrete sub floor (i.e. slab), so providing the thermal mass for distribution. You might also find an uncovered brick feature wall perpendicular to the window to catch sunlight heat at the end or beginning of the day (when the sun is low in the sky). Control is often just simply roof eves of sufficient depth to shade the window in summer (as the sun is higher in the sky).

Working out the required eave depth

The eave depth you need depends upon two things:
  • Your latitude, and
  • The height of the window to the eave itself.
With this information and knowledge of the position of the Sun in the sky during Winter and Summer, you can work out the exact length of Eave required to get best benefit over the year. The maths are somewhat complex, so we have provided a passive solar eaves calculator to work this out for you.

You can also combine all these features into one system, such as a trombe wall, see below.

The 3 types of passive solar system:

1. Direct Gain
The actual living space is a solar collector, heat absorber and distribution system. South (or North if Southern hemisphere) facing glass admits solar energy into the house where it strikes masonry floors and walls, which absorb and store the solar heat, which is radiated back out into the room at night. These thermal mass materials are typically dark in color to absorb as much heat as possible. The thermal mass also tempers the intensity of the heat during the day by absorbing energy. Water containers inside the living space can be used to store heat, but unlike masonry water requires carefully designed structural support, and thus it is more difficult to integrate into the design of the house. The direct gain system utilizes 60-75% of the sun’s energy striking the windows.

2. Indirect Gain
Thermal mass is located between the sun and the living space. The thermal mass absorbs the sunlight that strikes it and transfers it to the living space by conduction. The indirect gain system will utilize 30-45% of the sun’s energy striking the glass adjoining the thermal mass.

There are two types of indirect gain systems: thermal storage wall systems (Trombe Walls) and  roof pond systems

Trombe Walls
Trombe walls are the most common indirect gain approach. The thermal mass, a 6-18 inch thick masonry wall, is located immediately behind South (or North) facing glass of single or double layer, which is mounted about 1 inch or less in front of the wall’s surface. Solar heat is absorbed by the wall’s dark-colored outside surface and stored in the wall’s mass, where it radiates into the living space. Solar heat migrates through the wall, reaching its rear surface in the late afternoon or early evening. When the indoor temperature falls below that of the wall’s surface, heat is radiated into the room.

Operable vents at the top and bottom of a thermal storage wall permit heat to convect between the wall and the glass into the living space. When the vents are closed at night, radiant heat from the wall heats the living space.

Important guidelines to remember:

    * The space between the thermal mass wall and the glass should be a minimum of 4 inches;
    * Vents used in a thermal mass wall must be closed at night;
    * Thermal wall thickness should be about 10-14 inches for brick, 12-18 for concrete, 8-12” for adobe or other earth material and at least 6 inches for water.

Roof Pond Systems
This system can provide both heating and cooling. 6-12 inches of water are contained on a flat roof, usually stored in large plastic or fiberglass containers covered by glazing. During the cooling season, an insulated cover is removed at night to expose the water to cool night air. The water absorbs heat from below during the day, and radiates it out at night. During the heating season, the insulated cover is removed during the day. The water absorbs heat from the sun, and radiates it in to the building below. In cold climates an attic pond beneath pitched glazing is more effective than a flat roof pond.

Roof ponds require somewhat elaborate drainage systems, movable insulation to cover and uncover the water at appropriate times, and a structural system to support up to 65lbs/sq ft dead load.

3. Isolated Gain
The integral parts of an isolated solar heating system are separate from the main living area of a house. The most common isolated-gain passive solar home design is a sun space or sun room. The isolated gain system will utilize 15-30% of the sunlight striking the glazing toward heating the adjoining living areas. Solar energy is also retained in the sun room itself. Sun rooms may experience high heat gain and high heat loss through their abundance of glazing. The temperature variations caused by the heat losses and gains can be moderated by thermal mass and low-emissivity windows. Heat is distributed to the house by means of conduction through a shared mass wall in the rear of the sun room, or by using ceiling and floor level vents, windows, doors, or fans that permit the air between the sun room and living space to be exchanged by convection.

Passive collectors use a south (or North) facing air collector to naturally convect air into a storage area. Convective air collectors are located lower than the storage area so that the heated air generated in the collector naturally rises into the storage area and is replaced by return air from the lower cooler section of the storage area.

The sun room has advantages in that it can provide additional usable space to the house and plants can be grown in it effectively.

Some points on Thermal Mass and Passive Solar

The thermal mass you have in your building must be able to interact with its environment, what this means is if it is the floor, you cannot carpet it or put rugs all over it. If it is the wall you cannot cover it in gyprock!

Also make sure your thermal mass is actually made out of material with a high thermal density. Concrete or bricks are ideal. Autoclaved aerated concrete (AAC), sandwiched insulation panels, or ICFs on the other hand have very low thermal density (hence why they insulate so well), so are very bad as the thermal mass. Also straw bales are not usable as thermal mass, in fact, we recommend that straw bales are not used within the internal construction of a property as they can breakdown and cause other problems and effect your resale value.

What this usually boils down to is that concrete floors are employed with tiles on them, and thermal mass internal walls are made from brick.

Note: Of course insulation has its place, you need to use it to help isolate the internal environment from the external environment and get maximum benefit from the thermal mass you put in the property (i.e. help it regulate the temperature by having to deal with less external effects).

What must also be understood is that we are looking for sufficient thermal mass in a building to be able to regulate the temperature and allow you to open windows to refresh the air. Opening windows is something you should do in a property on a regular basis anyways, otherwise humidity and 'bad air' can accumulate and you end up with a sick building with sick people in it. If a thermal mass property is designed right there should be no need for air heat exchangers and other fancy heat recovery systems to ensure you have fresh air - the natural daily solar cycle will be sufficient to help keep your property at a comfortable temperature for the majority of the time.

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Related Tags: passive solar, cooling, heating, thermal mass, energy, home, green architecture, enviromental architecture

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