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How Does a Passive Solar Heating System Work?

How do Passive Solar Heating Systems Work

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Passive solar heating is the process of using a certain building system to regulate internal temperature carefully and benefit from the sun’s heat energy.

So, the purpose of a passive solar heating system is to store the sun’s heat energy during days within the building’s elements or materials and use it during the night. 

A comfortable temperature inside the living space should be maintained during the absorption of the sun’s heat energy. This means preventing possible overheating of the living space during the daytime.

Passive solar heating offers a comfortable living area, reduces energy consumption, and minimizes maintenance. It is applicable and suitable for small buildings, especially if the envelope design of the building controls the energy demand.

What is Passive Solar Heating System?

The essential elements of a passive solar heating system include facing a number of windows placed in the house toward the direction where the sun’s heat strikes the most. For instance, facing windows toward the south for North America is best. The availability of thermal mass to absorb, store, and distribute heat is another vital element of a passive solar heating system. 

The thermal mass is made of a construction material that is capable of holding heat energy, such as concrete, bricks, ceramic tiles, and stone. The stored heat can be dispersed by radiation, convection, or conduction into the living space.

Radiation is the movement of heat in the form of waves. Convection is the transfer of heat through the air, which can be seen as ragged lines in the air during hot days, and conduction is the transfer of heat from storage to another mass. For example, touching a hot surface and burning is the transfer of heat through conduction.

A passive solar heating system is suitable for low-rise buildings in a temperate and cold climate, barracks, lobbies, hallways, break rooms, and large maintenance facilities. This system can reduce heating energy consumption by 25-75% compared with conventional structures.

Passive Solar Heating Techniques

Three approaches are available to implement passive solar heating:

Direct Gain

In the direct gain system, the living space collects, absorbs, and distributes the sun’s heat energy. 

The south-facing glass allows solar energy into the living space, where it directly and indirectly strikes thermal mass materials like masonry walls and floors. 

The thermal mass takes solar radiation during the daytime and emits heat energy at night into the living space. The direct gain uses around 60-75% of the sun’s heat that strikes the window.

Figure-1: Direct Solar Gain System

Indirect Gain

In this system, the thermal mass is between the sun and living space. The thermal mass absorbs heat energy from the sun and conducts it to the living space. 

The direct gain system utilizes 30-45% of the sun’s heat energy that strikes the window. Three indirect gain solar passive heating techniques are available namely:  thermal storage wall systems (or Trombe wall), water wall, and roof pond.

Trombe wall

The thermal storage wall system absorbs and stores heat during the daytime. Excess heat is transferred by air between wall and glass through the thermosyphon principle into the living space. During the night, vents of the trombe wall are closed, then heat energy is dispersed into the interior space.

Figure-2: Working Principle of a Trombe wall
Figure-3: Trombe Wall at Zion Visitor Center at Zion National Park in Utah. The Trombe Wall is the Lower two Panes of the Lowest Level of Glass

Water Wall

The water wall consists of transparent containers filled with water. During the day, the water absorbs and stores the sun’s heat and disperses it into living space at night.

Roof Pond

Solar passive heating roof pond system uses water which is encased in plastic beds and placed on roofs, see Figure-4 and Figure-5. The system is the only indirect gain approach that provides heating and cooling.

Figure-4: Roof Pond Panels
Figure-5:Roof Pond Passive Cooling and Heating System

During the daytime, in hot seasons where the cooling effect is desirable, movable cover or insulation covers the roof pond to minimize solar heat absorption from outside. As a result, water absorbs heat from the living space which subsequently provides a cooling effect. 

During the nighttime, the movable insulation is removed, and the water disperses heat outside the living room.

Figure-6: Roof Pond Cooling System in Summer

In the winters, when solar heating is desirable, the movable insulation cover is removed during the day to allow water to absorb and store heat. During the nighttime, moveable insulation is used to cover the roof pond to prevent heat loss. As a result, the water releases heat in the living room and raises the temperature.

Figure-7: Roof Pond Heating Passive System

Isolated Gain

The integral parts of the isolated passive solar heat gain system are isolated from the main living area. The isolated gain uses solar energy to passively move heat to or from the living space through water or air by natural or driven convection. 

An example of an isolated passive solar heat gain system is a sunroom, see Figure-8. The sunroom uses both direct gain and indirect gain features. It consists of a solar collector, storage area, and transfer medium. 

The flat plate solar collector uses air or liquid to collect the heat energy from the sun. The collected heat is moved through pipes or ducts by natural convection to the storage area, where cooled fluid or air is displaced and driven back to the collector.  

The storage consists of a tank when liquid is used as medium transfer or rock known as bin when air is used as a medium transfer.

If air is used as a transfer medium, the heated air is directed into a bin where a rock or other masonry material absorbs heat energy. As the air loses heat to the rock, it cools and falls to the bin bottom and is returned to the collector, hence completing the cycle. During the nighttime, radiant energy stored in the rock is emitted to heat the living space.

If water is employed as medium transfer, the heated water is driven into a tank where it forces cool water out of the tank and directs it to the collector. If the process is done without mechanical help, the living space should be above the storage tank.

Figure-8: Isolate Heat Gain System through Integrated Solar Collector for Isolated Heat Gain


What is a passive solar heating system?

Passive solar heating is the process of using a specific building system to regulate internal temperature carefully and benefit from the sun’s energy.

What are the approaches or different technologies for a passive solar heating system?

1. Direct gain
2. Indirect gain
2. Isolated gain

Is it necessary to insulate a building for passive solar heating design?

A building must be well-insulated for a passive solar heating system to work properly.

What are the types of buildings suitable for a passive solar heating system?

1. Small buildings such as low-rise buildings in a temperate and cold climate, Barracks, 
2. Lobbies, 
3. Hallways, 
4. Break rooms, 
5. Large maintenance facilities.

Is passive solar heating expensive?

Incorporating a passive solar heating system into a building will increase its total cost by around 0-3%.

Read More

South-facing glass in a passive solar building: how to determine its optimum size? 

What is thermal mass in a passive solar building?

How to plan a building site for solar access?

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