The story of solar gains, when buildings are concerned, is primarily the story of windows. The topics of interest to us architects include: What makes one window better than another in terms of energy savings; How much money is lost annually because of windows; How to choose windows when designing a building; What the key parameters are when comparing windows in terms of their energy efficiency; If installing a sun-shading device is advisable, and if so, which one; How much money used for heating is lost annually if too large window area is put on the north façade, etc. The questions are numerous because the choice of windows and determining their size and position on different façades influence to a great degree the final appearance and quality of a building.
We’ll begin with considering the sources from which a building receives thermal energy. Naturally, the sun is the most significant and most crucial energy source for people and the living world and for buildings. Solar thermal energy comes in through transparent parts of the building, windows, and other similar surfaces. In this part of the calculation, we are primarily interested in windows, because depending on their characteristics, we calculate the amount of heat really obtained from the sun. In addition, the sun is the source of daily light, which we already mentioned in the lesson on lighting.
How to calculate the amount of heat energy, usually denoted as Qsol, obtained from the sun?
We know that the sun emits a certain amount of heat, called solar irradiance, usually denoted as Is and expressed in W/m2. How much energy will be received from the sun primarily depends on the site where the building is located, i.e., on climatic data? These data also contain information on the amount of solar energy that reaches us every day, i.e., month, year. Information on solar irradiance is part of the official data of each country and, in part, can also be found on many online portals dealing with this topic.
Next, we need information about windows regarding their orientation because it will influence the quantity of heat energy the building receives. Logically, the north-oriented window won’t receive the same amount of heat as the one on the south façade. That’s why the information on solar irradiance is always provided by months and for all points of the compass, i.e., depending on the window’s position for which solar gains are calculated.
Just to clarify, when referring to a window, we mean all the transparent surfaces, such as roof windows, skylights, etc.
As for windows, in addition to orientation, we need information on their surface area, type of glazing, and the number of glasses. Information on how much heat energy passes through a particular glazed area (depending on the type of glazing and number of glasses) is usually contained in tables, and we’ll provide you with one as a bonus to this lesson.
Now, we’ll see how these data are expressed in the calculation formula. Therefore, Qsol equals the multiplied figures of glass surface area, solar irradiance, and the time the window is exposed to radiation, usually a month. However, this would not give an exact result because some coefficients reduce that “ideal” value due to real factors. For example, usually, we have available information on the total dimensions of the window with a frame; that figure should be reduced by the frame factor, which is approximate to the ratio of window surface area and the frame surface area. If, for example, the total frame surface area of a given window is 30%, then the frame factor is 0.7, and the above result should be multiplied by it. All of this can be calculated manually, and we have prepared, as a bonus for you, a file for a manual, precise calculation based on the dimensions of the window and the frame width. However, you won’t make a mistake if you reduce the total window surface area by a frame factor of about 0.7.
The total amount of heat energy a building receives through transparent surfaces can be reduced, for example, by another building standing in front of the existing one or by an obstacle that casts a shadow and reduces sun gains. In that case, the so-called shading correction factor, usually 0.9, is added, which means that the initial calculation is reduced by 10%.
The following reduction in solar heat gains maybe since the sun rays strike the surface at a certain angle, and it is necessary to consider this slight reduction by using a factor due to oblique incidence of radiation, which is 0.9 and is denoted as Fw.
Let’s go on. Imagine that the windows are not perfectly clean, causing less solar energy to be received, in case of which the initial “ideal” value should be multiplied by the coefficient of dirt depreciation factor or Fv, reducing that result by 10% at most.
Furthermore, if windows have some kind of sun protection, such as a canopy or curtains, they will naturally get less sunlight, and the initial value should be multiplied by a coefficient depending on the type of external sun protection. This coefficient is usually provided in tables, and you will get one such table with this lesson. This coefficient is referred to as Fc. In some cases, the coefficient equals up to 0.5, which means that the solar heat gain will be reduced by 50% if, for example, the window is blocked by a terrace or console.
Finally, another quite important parameter by which solar thermal gains are multiplied is the total energy transmittance of glazing, denoted as g^. This data is used without considering solar protection because that correction has already been included in the correction factor for shading – Fc. The total energy transmittance of glazing shows how much solar radiation can pass through each glazing system, depending on its type and the number of glasses. For example, for single glazing glass, this coefficient amounts to 0.87, while for triple glazing insulating glass, it is 0.5, which means that the former will let a more significant amount of solar heat pass in because of multiplication by a higher coefficient. However, at the same time, this glass is far more unfavorable when it comes to heat losses, so it won’t be acceptable, despite a slight advantage in this case.
This shows that a building must be considered and understood as a whole and from all aspects, both from energy gains and energy losses.
At the end, when everything is multiplied, we obtain a value in kWh, which represents the amount of thermal energy received through the transparent surfaces. Later on, in the lesson on heat losses, we’ll see how much heat energy is lost through the same openings.
In modern architecture, a lot of attention is devoted to utilizing the sun heat, but we mustn’t forget there is a thin line between the desired additional heating of buildings in winter and the excessive sun exposure in summer. This is one of the biggest challenges for us architects. Sun protection is an essential element of design and mustn’t be forgotten when designing a building, especially in parts of the world where summers are long and warm. This especially applies to the southern and partly eastern and western parts of the building’s façade. Sun protection systems well balanced with external conditions ensure good working and living conditions in a building.
The sun protection elements can be installed on the facade or in the interior.
These elements can be fixed or movable, sliding, rolling, or automated. They can be placed as individual vertical or horizontal elements or as plates, in both cases outside or inside. These elements, which should be light, are installed in a substructure separated from the load-bearing structure of the building.
The materials that the sun protection elements are made from are usually aluminum (extruded, anodized, sandblasted), wood (resistant to external conditions), various fabrics (fiberglass, impregnated, or natural material).
Useful sun protection elements are canopies or porches of different depths placed on the south façade that prevent the sun from entering in summer and let it in during winter. Usually, horizontal elements are installed on the south façade because the sun falls at a higher angle in summer, and the horizontal surface can repel it. The winter sunsets at a slight angle and passes inside through the horizontal elements.
Vertical elements are installed on the west and east sides that can scatter the sun rays because the western sun always falls slightly. However, the correct orientation of the building, i.e., a grouping of rooms by purpose, contributes most to the sun protection.
This topic is exciting, and we’ll probably cover it in more detail in the next course dedicated to solar architecture.
