Study of energy demand in openings and windows and influence on the thermal envelope.
One of the key points in the envelope of a building or property and that decisively influences the energy demand are the holes or windows. These are made up of woodwork and glass.
In the first place, we must analyze and understand how the composition of the gap can influence energy demand. I am going to try to make a tour between all the concepts that define them (referring to energy demand).
This will make our knowledge reach an acceptable level of decision, according to the proposed object; In other words, we will help reduce CO2 emissions, since we will need less non-renewable energy to achieve the theoretical comfort within the spaces of our buildings.
With this premise, we will seek that the composition of our holes is such that in summer not much heat enters through them into our spaces and that in winter, the heat from the heating systems does not escape to the outside. We will bear in mind that making a rational decision in this matter is not an easy task, since factors that directly or indirectly affect heat transmission intervene in said analysis:
- Size and surface
- Climate of the place
- Solar orientation of the facades
- Shading devices
- Destination and mode of use of the building
- Etc.
How can heat be transmitted or conducted inside the spaces of a building?
Starting from the fact that all bodies are interacting with the environment, needing a balance; We affirm that the process of heat transmission it always occurs from a warmer space or body to a less warm one.
The exterior will always be at a different temperature than the interior of our buildings; the heat will be transmitted from the hottest space to the less hot one through the elements that make up our windows. This form of heat transmission is calleddriving.
When the sun's rays hit our windows directly, part of the heat will be transmitted to the interior of the building. This form of heat transmission is calledradiation. The air can also transmit heat to the interior or exterior of our buildings, calling this form forconvection.
When we are clear about the concepts outlined, we can define thethermal transmission or transmittance (U), as the amount of heat that is exchanged between the interior-exterior in the unit of time, either by conduction, radiation or convention, when there is a difference in temperature between the exterior and interior surfaces.
Therefore, the lower the thermal transmittance, the lower the energy transfer between both faces, and therefore the better insulating capacity the hole or window will have. It is measured inW / m2K (quantity of heat per hour, expressed in watts, transmitted through a surface of 1 m2 for each degree kelvin of difference between the interior-exterior).
Heat is not transmitted in the same way through glass as through plastic. Glass conducts heat faster than plastic. We could also say that glass offers less resistance to heat transmission than plastic.
This fact tells us that there is an intrinsic characteristic of the materials. This is known ascoefficient of thermal conductivity (λ). Each material, depending on its composition, has a coefficient that characterizes it, transmitting or resisting more or less heat.
It is measured inW / mK(Amount of heat, expressed in watts, that passes through the unit area of a material sample, of infinite extension, planar parallel faces and unit thickness, when a temperature difference equal to one is established between their faces).
Energy demand in the solar factor and absorptivity.
The Sun transmits energy to the outside through a set of electromagnetic radiation or waves called solar radiation. These electromagnetic waves or radiation can manifest themselves in various ways, such as radiated heat, visible light, X-rays or gamma rays.
In the set of these radiations or energies emitted by the Sun, there is a group that the human eye can perceive and another group that is unable to capture. It is known as the visible and invisible spectrum respectively. Within the visible spectrum we have visible light.
In the invisible spectrum we have the non-visible light, which differs into two groups; infrared rays (infrared rays, television signals, radio signals, microwaves, thermal radiation) and ultraviolet rays (ultraviolet rays, X rays, gamma rays). The color of objects depends on what happens when light (part of the solar radiation that can be perceived by the human eye and interpreted by the brain in different colors) falls on it.
Materials absorb some colors and reflect others. The colors we see are the reflected colors.
We add as an example a green leaf, it absorbs all colors except green, which is reflected, captured by the human eye and interpreted by the brain in that color. Black materials absorb all colors and reflect none (no color). In contrast, white materials reflect all colors.
Consequently, we can say that materials absorb and emit energy. (We can see more of the color from this article)
- Absorptivity
It is the property of a material that determines the amount of incident radiation it can absorb. Its value is in the range 0<α<1><α<100% un="" cuerpo="" negro="" absorbe="" toda="" la="" radiación="" incidente="" sobre="" él,="" es="" un="" absorbente="" perfecto="" (α="1" ó="">
- Solar factor.
The relationship between the total energy that enters a room through a glazing and the solar energy that affects said glazing. This total energy is the sum of the solar energy that enters by direct transmission and that given by the glazing to the interior of the premises as a consequence of its energy absorption.
Thus, a glass that has a solar factor of 40% »means that only 40% of the solar energy is allowed to pass through. Therefore, the lower the percentage of solar factor of a glass, the greater the protection it provides against solar energy.
A heat transfer medium could be air as we saw earlier, therefore, an important concept to consider would be the permeability of the carpentry to this transfer medium. We define theair permeability, like the amount of air that passes through a closed window. It is measured in m3 / h.
If we look at the table, for a window to be classified as Class 4, it must not have an infiltration greater than 3m3/ h (per square meter surface) and 0.75 m3/ h (per linear meter of joint).
Now, we have enough knowledge to be able to interpret the data that characterizes the composition of our holes, and to be able to decide, which of the existing systems we need to improve the energy demand of our buildings.
To conclude and in summary, say that thewindow frame It represents between 25% and 35% of the window surface and its main property is Thermal Transmittance.
The most common materials are metallic, metallic with thermal break, wood, PVC and mixed (wood-aluminum, polyurethane with metallic core, metallic with thermal break filled with insulating foam, etc.).
- Monolithic or simple.Formed by a single glass or by 2 or more glasses joined together over its entire surface (called laminar). We can find it colorless, colored, printed and security.
- Low emissivity. They are monolithic glasses, on which a very thin layer of metallic oxide has been deposited, thus reducing the transfer of heat by radiation (it reduces the entry of solar radiation, improving insulation in the summer season).
- Double glazing. Set of two or more monolithic glasses separated from each other by one or more air chambers, hermetically closed. This type of glass limits heat exchange by convection and conduction. If we also incorporate low-emissivity glass, the insulating capacity is enhanced.
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Article prepared by Gustavo A. Fdez. Bermejo (Technical Architect and Energy Advisor) Access to its website… http://gustavoafernandezbermejo.blogspot.com.es/. OVACEN Collaborator
