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Passive Solar Heating

Passive solar heating refers to techniques used to heat a given living space without the use of mechanical or electrical means to control the flow of heat. Gain is an important concept in passive solar heating and is covered in the article on Passive Solar Design.

Direct Gain

In direct gain, the living space acts as the solar collector and distribution system. South facing windows are an example of using direct gain because the sunlight directly penetrates the living space and heats it. Direct gain systems can make use of up to 70% of the sun’s energy that is entering the windows.

Thermal mass floors and walls are used to absorb and retain direct gain. In some construction applications, water containers are used to store heat because water has a relatively high thermal inertia. Thermal mass serves to temper heat gain in the day so that a space does not overheat and then releases heat at night. The best way to think of using thermal mass in direct gain is as moderator, it helps to ensure that extremes of temperature do not occur.

Rules of thumb for direct gain vary by location, so there are few that universally apply. In general, the thickness of thermal mass used in building will increase as the ambient temperature decreases. Thus, in colder climates, thermal mass floors and walls are thicker.

It is roughly true that for every square foot of glass there should be 150 pounds of masonry or 4 gallons of water to meet the thermal mass needs of a moderate environment. It is also true that thermal mass should be as distributed as possible throughout the living space rather than concentrated in a single location.

Indirect Gain

In indirect gain, the thermal mass stands between the sun and the living space. In direct gain, the order of heating is sun to living space to thermal mass. In indirect gain the order of heating is sun to thermal mass to living space.

An indirect gain system is less efficient that is direct gain. It can use up to 45% of the sun’s energy. Two common types of indirect gain are thermal storage walls and roof ponds. Thermal storage walls face south and work with the aid of vents and a panel of glass. In the day, the sun warms the space between the glass and the wall. The vents in the wall are open to allow heat to move by convection from the space, through the wall, and into the living space. Cool air moves in the same cycle, into the space between the wall and the glass where it is heated. At night, the vents are closed. The area between the glass and the wall is not insulated from the house and the wall itself remains warm.  The heat from the wall radiates into the living space.

Roof ponds refer to the use of several inches of water on a flat roof to maintain cooling or heating. Water is stored in containers that are heated by sun in the day. They have a two-fold purpose. They insulate the roof from heat enter in the day and prevent heat from leaving through the roof at night. Roof ponds seem to work better in low humidity climates.

Isolated Gain

Isolated gain is a little more difficult to understand and can make use of up to 25% of the heat from the sun. The principle is to collect heat in one area and then use it in another. A good way to think of isolated gain is as taking indirect gain to the next level. Thus, an entire sunroom, rather than just a thermal mass wall with minimal space, would act as the heat sink and collect heat in the day. The heat in the sunroom would then be used to maintain heat in the rest of the building during the night.

Isolated gain that uses rooms employs the same ventilation and convection principles seen in indirect gain. The only real difference between the two is the size of the space being used to collect the sun’s energy. The advantage to isolated gain is that the space, such as the sunroom, provides additional usable space that may act as a greenhouse or drying room.

Solar collectors that use air/water and a convection current are an example of passive isolated gain as well. In these systems, a concentrator (mirrors or magnification) is used to collect solar energy to warm either water or air, which is then moves into a take that is above it (warm substances rise). The walls of the tank are heated by the fluid, which falls back into the collector thus completing the loop. The heat form the walls of the tank is then used to heat a living space. These systems are somewhat more complicated and can be active as well as passive if fans and pumps are added. The diagram helps to clarify the design.

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