Passive solar systems. Thermal mass (heat storage). Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Alternative energy sources

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Solar radiation falling on walls, windows, roofs and other surfaces is absorbed by the building and stored as thermal mass. This heat is then radiated into the building.

Thermal mass in a solar heating system serves the same function as batteries in a solar photovoltaic system: storing solar energy for later use.

Thermal mass can be integrated into a passive solar system in a variety of ways, from tiled floors to water-filled containers. Materials that absorb and retain heat include: concrete floor slabs, stone walls, and other heavy building materials. They are the main element in houses that use passive solar energy. Even if most of the windows of the building are facing south, but there is no thermal mass reserve, then such a house will not be energy efficient. You need to know that a dark surface reflects less and absorbs more heat. If the floor is covered with dark tiles, then it will absorb heat during the day and radiate at night. The magnitude of the heat flux depends on the temperature difference between the heat source and the object to which it is directed.

As described above, heat moves in three ways: conduction (the transfer of heat by solid materials), convection (the movement of heat through the movement of liquids or gases), and radiation. The surface of the house also loses heat through these three ways. Good passive solar building design helps minimize heat loss and maximize efficient heat distribution. The amount of thermal mass (heat storage materials) required is highly dependent on climate. Heavy buildings with a lot of thermal mass are more comfortable in hot, dry or cold climates, but they are ineffective in hot, humid climates. In cool climates, thermal mass acts as a thermal reserve for cold weather, thereby improving comfort and reducing the need for auxiliary heating except on cloudy or very cold days.

Ensuring adequate thermal mass is usually the most difficult task for a passive solar designer. The need for thermal mass is determined by the total area of ​​windows facing south and the location of the building. To ensure effective design, the following principles must be followed:

• Locate the thermal mass in places where sunlight hits. Thermal mass located in areas of direct sunlight is more effective compared to mass placed in a place difficult to reach the sun. Buildings designed to absorb solar rays indirectly require 34 times more thermal mass than those designed to absorb solar rays directly.
• Distribute thermal mass. Homes using passive solar design are more efficient when the thermal mass is relatively thin and distributed over a large area. The surface area of ​​the thermal mass should be at least 3 times, and preferably even 6 times, greater than the total surface area of ​​the south-facing windows. Floor slabs with a thickness of 8 - 10 cm are more effective than a floor with a thickness of 40 - 50 cm.
• Do not cover thermal mass. Carpets and walkways impede the transfer of energy to and from passive solar cells. Stone walls may be dry-finished, but should not be covered with large wall tapestries or wood paneling. The dry finish must be applied directly to the wall and not to coatings attached to the wall that create an unwanted insulating air space between the finish and the thermal mass.
• Insulate the inner surfaces of the thermal mass. There are several methods for insulating floor slabs and exterior stone walls. Such measures are necessary to save energy. Unfortunately, sometimes problems can arise, such as termites in foam insulation for floor panels.
• The thermal mass must be multipurpose. To justify financial costs, thermal mass must serve not only as a heat accumulator, but also for other purposes. For example, heat-storing masonry walls, while an element of passive solar design, are prohibitively expensive if needed only as thermal mass. A tiled floor retains heat, serves as a structural element and is a beautiful design element. Interior stone walls are a structural element, separating rooms and storing heat.

When designing a passive solar system, the process of choosing building materials must pay attention to their ability to retain heat. This value is called the volumetric heat capacity (J / m3-oC) or, in other words, this is the amount of heat that the material is able to absorb and store.

Volumetric heat capacity for some commonly used building materials:

In the past, passive solar designers used water stored in large containers as the heat transfer medium. While water is cheap, the containers and the space they take up are quite expensive. Some designers have moved to containers filled with stones, using them as reservoirs for thermal mass. It should be borne in mind that to maintain the same amount of heat, stones will be required three times more than water. However, the humid environment that forms in places where water containers are installed causes a strong unpleasant odor and is a favorable environment for the reproduction of fungi and bacteria. These issues have undermined the reputation of this passive solar building option.

Storing heat with water and stones requires complex control systems, pumps, and fans. Such a heat preservation process is almost never used today. The main reason for this is that the functioning of such systems depends on electricity, these systems require maintenance, are subject to periodic breakdowns and, accordingly, require repair.

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