Thermal mass

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Basic definition

Thermal mass is a heat storage material, such as water, concrete, masonry, or earthen construction, used in solar thermal energy systems also known as passive solar heating systems. With passive solar heating, the house itself acts as the solar collector and the storage facility. The essential elements of a passive solar system are:

Thermal mass and insulation

Where thermal mass is used in walls, e.g. stone, strawbale or earth construction, these thick walls also provide significant insulation. See insulating walls.

Types of passive systems

There are three categories that passive systems can fall into.

Direct gain

One of which is a direct gain system. In this system, large south-facing windows are used to admit the sunlight. and thermal mass is placed in the house to absorb the solar radiation. Another system is indirect-gain.

Indirect gain

Indirect gain collects and stores the solar energy in one part of the building and uses natural heat transfer to distribute the heat throughout the building, such as the Trombe wall.


Finally, the third category of these passive solar heating systems is an attached greenhouse on the south side of the house. The greenhouse acts as an expanded thermal storage wall. It will share some of its heat with the adjoining house.

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