Passive solar energy is based on the use of solar energy directly without the need to use external support mechanisms. By these mechanisms we refer to the use of electric motors to guide solar panels or similar systems.
The objective of this type of renewable energy is to obtain personal thermal comfort without external energy inputs. Passive systems are sometimes combined with other active systems such as the installation of photovoltaic panels.
To use passive solar energy requires a series of energy exchanges, transferring energy from one system to another. For example, transferring the energy of solar radiation to a water tank to heat it and obtain sanitary hot water.
This transfer of thermal energy is basically carried out by three thermodynamic processes: radiation, conduction and convection. Let's see what each of these concepts means:
Heat transfer by conduction
Heat transfer by conduction is based on the transport of heat from one body to another through a third element. This third conductive element can be air or water. In the field of passive solar energy, the transport of energy through the air is very important.
In terms of passive solar energy, heat transfer by thermal conduction can be beneficial or detrimental. That is, in winter we need to obtain heat but in summer an excess of heat can generate too high temperatures in the house.
Uncontrolled air infiltration can contribute up to 40% of heat loss during winter. However, strategic placement of windows or vents can improve convection, cross ventilation, and cooling in summer.
Filtered energy recovery ventilation systems can be helpful in removing unwanted moisture, dust, pollen, and microorganisms from unfiltered ventilation air.
Natural convection that causes hot air to rise and cooler air fall can cause uneven stratification of heat. This stratification can cause:
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Uncomfortable variations in temperature in space.
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It can serve as a method of venting hot air.
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A natural convection airflow circuit can be designed for passive distribution of solar heat.
Convection heat transfer
Thermal transmission by convection is based on the exchange of heat between two bodies at different temperatures that are in contact. This process is governed by the first principle of thermodynamics.
An example of this thermodynamic process is when we touch a radiator with our hand, at this moment, the radiator (hotter) transfers thermal energy to our hand (colder) by convection.
In buildings constructed following the parameters of passive architecture, it is very important to use constructive elements with a high thermal insulation capacity. Thermal bridges should be avoided in exterior door and window coatings.
Radiation heat transfer
Radiation heat transfer occurs through electromagnetic waves without any element involved. In this way, solar energy can travel from the Sun to the Earth through empty space.
Solar radiation is absorbed mainly through the roof and windows. Thermal radiation moves from a warmer surface to a cooler one.
Radiation energy can move into a window during the day and out the same window at night.
Solar radiation is also important on cold, clear days. If you want to reduce the heat entering through the windows, it can be reduced by insulated glazing, shading and the orientation of the windows.
The sun can provide heat and lighting, but it is vulnerable to overheating in summer if it is not in the shade. On the other hand, the low noon sun easily admits light and heat during winter. In contrast, it can be easily shaded with appropriate overhangs or angled blinds during the summer and summer shade trees that shed their leaves in the fall.
Another passive solar design principle is that thermal energy can be stored in certain building materials. When the heat gain facilitates the stabilization of the diurnal temperature variations it can be released.