Control Radiation Gain/Loss
In order to minimize heating or cooling load, the last aspect is to control radiation gain/loss. It means we should try to:
- Minimize radiation gain in summer - reducing cooling load
- Maximize radiation gain in winter - reducing heating load
- Minimize radiation loss at night in winter - reducing heating load
How to accomplish above goals? Through something called passive solar design.
Passive Solar Design
Also called climatic design, a design approach that uses structural elements of a building to heat and cool a building without the use of mechanical equipment. Every passive solar building includes five distinct design elements:
- An aperture or collector – the large glass area through which sunlight enters the building.
- An absorber – the dark surface of the storage element that absorbs the solar heat.
- A thermal mass – the material that stores the absorbed heat. This can be masonry materials such as concrete, stone, and brick; or a water tank.
- A distribution method – the natural tendency of heat to move from warmer materials to cooler ones (through conduction, convection, and radiation) until there is no longer a temperature difference between the two. In some buildings, this strictly passive distribution method is augmented with fans, ducts, and blowers to circulate the heat.
- A control mechanism – to regulate the amount of sunlight entering the aperture. This can be as simple as roof overhang designed to allow more sunlight to enter in the winter, less in the summer (see picture below).
There are three basic passive solar designs for heat regulation, each of which incorporates these five elements in different ways.
Direct gain -This the simplest passive design technique. In direct gain, sunlight enters a building through an opening – usually south-facing windows. It then strikes the building's thermal mass – usually dark-colored masonry floors and/or walls in the interior space that absorb and store the solar heat. At night, as the building cools, heat stored in the floors and walls warms the rooms.
- Trombe wall (indirect gain) - In the trombe wall design, a dark-colored wall is placed between a building's south-facing windows and its living or working space. The wall absorbs solar heat through radiation, stores it, and then releases it into the building when the indoor temperature falls below that of the wall's surface.
- Sunspace (isolated gain) - This design uses a separate solar room (solarium) to store solar heat. A sunspace can be built as part of a new building or as an addition to an existing one. Sunspaces also require a thermal mass to store heat. This stored heat is distributed throughout the building via ceiling and floor-level vents, windows, and doors, sometimes with the addition of fans. Daylighting requires windows that are placed on or near a building's roof, such as skylights. High-level windows and skylights can allow sunlight to reach throughout a building – including north-facing rooms and upper levels. This typically requires an open floor plan – one that incorporates very few full walls. Effective use of daylighting reduces the need for electric light.
Proper landscape features are also important in passive solar design. As shown in picture below, evergreens are planted on the north side of the houses blocking cold north wind in the winter. Deciduous trees are planted on the south side of the house so that in the summer these trees in full leaves shade home but all radiation heat to come through in the winter that they have lost all their leaves. Note most windows are located in the south side of the house to collect solar energy in the summer while cold north wind to strike the side of the house with few windows, so you don't lose as much heat.
Passive solar design can be used in most parts of the world. If designed by an experienced passive solar architect, buildings using passive solar design principles need not cost more up front than conventionally designed buildings. And when they do, the savings in energy bills quickly pay for themselves.
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