Principles of passive solar energy transfer
Passive solar energy is based on the use of solar energy without the need to use external support mechanisms. By these mechanisms we refer to the use of electric motors to orient solar panels or similar systems.
The objective is to obtain personal thermal comfort. Personal thermal comfort is a function of personal health factors (medical, psychological, sociological and situational), ambient air temperature, average radiant temperature, air movement (thermal sensation, turbulence) and relative humidity (which affects human evaporative cooling).
To use passive solar energy it is necessary to carry out a series of energy exchanges, to transfer energy from one system to another. For example, transfer the energy of the solar radiation to a water tank to heat it and obtain sanitary hot water.
This transfer of thermal energy is basically done by three thermodynamic processes. In buildings, thermodynamic exchange takes place through thermal convection, thermal conduction and thermal radiation through the roof, walls, floors and windows.
Heat transfer by conduction
The conduction heat transfer is based on the transport of heat from one body to another through a third element. This third conductor element can be air or water. In the field of passive solar energy the transport of energy through the air is very important.
Uncontrolled air infiltration due to poor air conditioning / waterproofing / waterproofing can contribute up to 40% of heat loss during winter. However, the strategic placement of operable windows or vents can improve convection, cross ventilation and cooling in summer when outdoor air has a comfortable temperature and relative humidity.
Ventilation systems with filtered energy recovery can be useful to remove moisture, dust, pollen and unwanted microorganisms in unfiltered ventilation air.
The natural convection caused by the rise of hot air and the descent of cooler air can cause an unequal stratification of the heat. This can cause uncomfortable variations in temperature in the upper and lower conditioned space, serve as a method for venting hot air or be designed as a natural air convection circuit for the passive distribution of solar heat and temperature equalization.
Natural cooling by perspiration and human evaporation can be facilitated by the movement of convective air natural or forced by fans, but ceiling fans can disturb stratified insulating air layers in the upper part of the room and accelerate the transfer of heat from a hot penthouse or by the nearby windows. In addition, high relative humidity. Inhibits cooling by evaporation of humans.
Convection heat transfer
Thermal transmission by convection is based on the exchange of heat between two bodies that are in contact and that each of them has a different temperature. This thermodynamic process we observe when we touch a radiator with the hand, at this moment, the (hotter) radiator transfers thermal energy to our hand (colder) by convection.
In buildings built following the parameters of the passive architecture it is very important to use constructive elements with a large thermal insulation capacity. It is advisable to avoid thermal bridges in door cladding and outdoor sales.
The heat transfer by radiation
The transfer of heat by radiation occurs through electromagnetic waves without there being any element involved. In this way, solar energy can travel from the Sun to Earth through an empty space.
The main source of heat transfer is radiant energy, and the main source is the Sun. Solar radiation occurs predominantly through the roof and the windows (but also through the walls). Thermal radiation moves from a warmer surface to a cooler one. The roofs receive most of the solar radiation delivered to a house. A cool roof, or a green roof, plus a radiant barrier, can help keep your attic from getting hotter than the maximum outside air temperature in summer.
Windows are a ready and predictable site for thermal radiation. The energy of the radiation can move towards a window during the day and leave through the same window at night. Radiation uses photons to transmit electromagnetic waves through a vacuum or translucent medium.
The use of solar radiation is also important on cold and clear days. If you want to reduce the heat entering through the windows, you can reduce it by means of isolated glazing, shading and orientation of the windows. The windows are particularly difficult to isolate compared to the ceiling and walls. The transfer of heat by convection through and around the window coverings also degrades its insulation properties. When shading windows, external shading is more effective in reducing heat gain than internal window coverings.
The western and eastern sun can provide heat and lighting, but it is vulnerable to overheating in summer if it is not in the shade. In contrast, the low midday sun readily admits light and heat during winter, but can be easily shaded with appropriate protrusions or angled blinds during summer and summer shade trees that shed their leaves in the fall. The amount of radiant heat received is related to the latitude of the location, the altitude, the cloud cover and the seasonal / hour incidence angle.
Another principle of passive solar design is that thermal energy can be stored in certain building materials and released again when the heat gain facilitates the stabilization of daytime temperature variations (day / night). The complex interaction of thermodynamic principles can be contradictory for first-time designers. Precise computer modeling can help avoid costly construction experiments.
Last review: February 6, 2019