Passive heating and cooling techniques are strategies for maintaining comfortable indoor temperatures without relying on active heating or cooling systems, such as HVAC units or fans. These techniques work by using the natural properties of the building and its surroundings to regulate the indoor temperature.
- Passive solar design refers to the use of the sun’s energy for the heating and cooling of living spaces by exposure to the sun.
- When sunlight strikes a building, the building materials can reflect, transmit, or absorb the solar radiation.
- In addition, the heat produced by the sun causes air movement that can be predictable in designed spaces.
- These basic responses to solar heat lead to design elements, material choices and placements that can provide heating and cooling effects in a home.
1) Passive Heating Techniques
- Passive heating is the spontaneous warming effect resulting from the absorption of solar radiation wherein solar energy is exploited to induce heat flow from the affected surface to indoor air, as well as promote heat storage within the building structure.
- Passive heating technique generally used in cold climates.
- In the climatic zones requiring indoor space heating, it may be explored to use the following strategies:
i) Direct gain method
- Controlled sun may be permitted into the habitable spaces through an opening to directly heat the floor, walls or other internal components and objects, which, in turn, heat the air within the room.
- Some examples of building materials that enable direct solar gain by acting as thermal storage mass are concrete, bricks, stone and water.
- The high thermal mass is usually located in the internal or external walls, floors or other built- in structures that receive sun directly.
ii) Indirect gain
- A thermal storage wall may be placed between the glazing and habitable space which prevents solar radiation from directly entering the living space.
- The solar energy incident on wall is absorbed by the wall and then indirectly transmitted to the habitable space over a longer time.
iii) Trombe wall
- It is a thick solid wall with vents at its lower and upper ends. This wall may be placed directly behind the glazing with an air gap in between.
- The vents act as inlets of warm air into the room and as outlets for flushing out cool air from the room.
iv) Solar chimneys
- This system is a kind of modified trombe wall that is incorporated into the roof.
- A solar chimney is essentially a collector panel with minimum thermal inertia on the south facade of the building.
- It absorbs incident solar radiation and heats up the air inside the space.
- It is an integration of direct gain and thermal storage concepts.
- Solar radiation admitted into the sunspace heats up the air, which by convection and conduction through the mass wall reaches the habitable space.
- It essentially consists of a greenhouse constructed on the south side of the building with a thick mass wall linking the two.
- Sunspace types according to the type of partition between the sunspace and the adjacent room: T1 – sunspace with a thermal storage wall and a direct system; T2 – sunspace with a transparent partition; T3 – sunspace with a thick thermal storage wall; T4 – sunspace with a Trombe wall
2) Passive Cooling Techniques
- Passive cooling is especially useful in hot and humid or hot and dry climates.
- Passive cooling systems rely on natural heat sinks to remove excess heat energy from a building.
- They derive cooling directly from evaporation, convection and radiation without using electrical energy.
- All strategies rely on diurnal changes in temperature and relative humidity.
- The applicability of each system depends upon the prevailing climatic conditions.
- The building envelope can be designed to effectively exchange heat with the surrounding ambient air.
- In order to have an appreciable net heat flux between two bodies, the temperature difference should be significant (typically at least 7°C). Some of the techniques are as follows:
i) Nocturnal cooling
- Nocturnal cooling, also called night sky cooling, is the cooling of a building by radiation to the night sky.
- The roof of a building absorbs the greatest amount of solar radiation during summer.
- On a hot summer day, the surface temperature of the roof can rise as high 65°C.
- The roof also has the largest exposure to the sky and hence can be used effectively for nocturnal cooling.
- The roof of a building can be used both as a natural radiator and also as a cold store. It is often an economical solution to architectural problem. During the night the roof is exposed to the night sky, losing heat by long-wave radiation and also by convection. During the day, the roof is externally insulated in order to minimize the heat gains from the direct receiving solar radiation and the ambient air.
- In this technique, night sky cooling may be very effectively used to dissipate the heat stored in building envelope so that it is regenerated to store the day heat gain.
- Such buildings will require high thermal mass and application is ideally suited for day use buildings such as offices.
ii) Roof pond with movable insulation
- A water body on the roof may provide cooling where during summers it is covered with insulation with a surface finish of low absorptivity.
- During the day time, this minimises the solar radiation impact on the roof, as the water in the pond holds the heat gain and further increases the time lag.
- During the night, insulation is removed and the heat stored in the day time is exchanged with the night sky. In winter, the operation of the movable insulation is reversed to allow heat gain in daytime and reduce heat loss during the night.
- All such provisions shall however be without prejudice to the need of compliance to the requirement of structure safety as prescribed in this Code.
- Due to incident solar radiation in courtyards, the air gets warmer and rises. Cool air from ground level flows through louvered openings of rooms thereby inducing airflow.
- The three periods in the working mechanism of a courtyard.
Passive heating and cooling techniques can be used in combination with each other and with active heating and cooling systems to provide a comfortable and energy-efficient indoor environment.