The heat gain components through glass consists of solar radiation and conduction. Solar radiation is considered in two parts - direct and diffuse (or scatter). Diffuse radiation is the solar radiation that is absorbed, stored and scattered in the atmosphere. The glass can be in the sun (direct and diffuse radiation) or in the shade (diffuse or scatter radiation). Conduction heat gain occurs due to the difference in temperature on either side of the glass. Conduction heat gain is positive if the outdoor air temperature is greater than indoor air temperature and it is negative (heat loss from the space) if the indoor air temperature is greater. Solar radiation is always positive.
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Direct solar radiation is the vector component of the absolute (total) solar radiation that is perpendicular to the glass surface. The Solar Cooling Load (SCL) Factor for a window is based on this value. So for any given hour, the SCL values for windows with different azimuth and tilt angles will have different SCLs although the absolute solar radiation is the same for all windows.
Q-solar = A * SC * SCL
Q-cond = A * U * CLTD
A = Glass Area , SC = Glass Shading Coefficient, U = Glass heat transfer coefficient.
CLTD = Cooling Load Temperature Difference. CLTD for glass depends mainly on the difference between indoor and outdoor temperatures but as with walls and roofs it is affected by the mass and properties of the glass material.
When solar radiation strikes a glass surface, some of it is transmitted, some of it is absorbed and some of it is reflected. The absorbed component increases the temperature of the glass and the heat is slowly conducted (released) to the outside and inside depending on the difference in temperature. Unlike walls that are thick and have high densities, the absorbed portion of the solar radiation is relatively small compared to transmitted and reflected components.
For example, a particular tinted glass has a transmissivity of 0.6 (60% of radiation is transmitted), reflectivity of 0.3 (30% of radiation is reflected) and absorbtivity of 0.1 (10% of radiation is absorbed). The direct solar radiation value is the component of the absolute solar radiation that is perpendicular to the glass surface.
Shading Coefficient (SC) is the ratio of the solar heat through a given glass type under specific conditions to the solar heat gain through a standard reference unshaded glass that was used to determine Solar Cooling Load (SCL) factors. The reference glass is one-eighth thick inch clear double strength single glass and it has an SC = 1 under the specific conditions.
SC = Solar heat gain through given glass type Solar heat gain through reference glass type
Solar Heat Gain Coefficient (SHGC) is the ratio of the measured solar heat through a given glass type to the incident solar heat on the glass. The measured values are affected by the air films on either side of the glass, absorbtivity and by other factors. SHGC is therefore less than SC (about 10% to 15%). SHGC values are used in manual calculations. Input to energy computer programs is usually SC, and the program calculates SHGC based on conditions on either side of the glass. ASHRAE Standard 90.1 specifies SHGC values for different climates because it is based on tested measured values of different glass types and not on theoretical values.
Radiant heat entering through the glass does not directly affect the room space air through which it passes. The radiant heat is first absorbed by the interior surfaces (walls, floor, ceiling) of the space and the contents (furniture and other objects) in the space. The absorbed heat is released to the air in the space through conduction and convection due to the difference in temperature.
Solar Cooling Load (SCL) factors are based on the solar radiation heat gain entering through the glass and the effect of the room surfaces and furnishings in absorbing and transmitting the radiant heat. There is therefore a time lag between the solar radiation entering the space through the glass and when it affects the temperature of the air in the space.
Visible Light Transmission (VLT) factor is the ratio of amount of light (lumens) transmitted through the given glass type to the amount of light transmitted by the standard reference glass type. Visible light and radiation heat are part of the electromagnetic spectrum and vary from each other in wave length. The is therefore a correlation between SC and VLT for different glass types but they are not the same. For building energy efficiency in summer you want to reduce the SC and increase the VLT. This reduces the cooling load due to radiation heat gain and reduces it even further by reducing the heat gains from lighting. Glass manufacturing research and technology tries to develop glass that optimizes the properties of glass for building energy efficiency.
In summary, energy efficient glass depends on it's U-value, SC, SHGC and VLT. Glass manufacturer's data provides this information.
ASHRAE tables for Solar Cooling Load (SCL) factors are therefore based on approximate groups and combinations of different types of room surfaces and furnishings. It is also based on floor levels (ground, middle, top) since this affects the inside surfaces of the space.
ZONE TYPES
The ASHRAE Cooling Load Temperature Difference (CLTD), Solar Cooling Load (SCL) and Cooling Load Factor (CLF) method was developed so that heating and cooling loads can be calculated manually. It consists of building performance tables for different latitudes and different building configurations. The tables were generated using the DOE2 computer program that used more intensive and accurate calculation procedures (example Transfer Function Method for Walls). These tables are published in the ASHRAE 1981 Handbook.
- Room or space zone types were developed for:
- Solar Heat Gain through Glass
- Internal Heat Gains from People, Lights, and Equipment
ASHRAE Zones for Solar Cooling Load (SCL) Factors for Glass are based on:
- Floor Level and Room Location
- Single Story Building (Table 8.8-A)
- Top Floor (Table 8.8-B)
- First / Ground Floor (Table 8.8-C)
- Middle Floor (Table 8.8-D)
- Interior Rooms (Table 8.8-E)
- Number of Walls in the Space ( 1 & 2 or 3 & 4 or greater)
- Floor Type ( Concrete or Wood )
- Ceiling Type ( With or Without Suspended Ceiling )
- Floor Covering ( Carpet or Vinyl )
- Partition Type (Gypsum or Concrete Block )
- Inside Shades ( Full, Half or None )
The figure below shows space heat gain time delay.
The tables show :
Direct solar radiation intensities for different vertical surface azimuths and for a horizontal surface for 24o North latitude on July 21st and January 21st. Note that solar radiation on the south surface peaks in winter. Heating energy can be reduced by increasing the percent glass on the south.
how the SCL (time delayed) values vary for different vertical surface azimuths and for a horizontal surface for 24o North latitude on July 21st.
EXAMPLES OF ZONE TYPES
for Glass SCL and People, Equipment & Lights CLF
Conduction Heat Gain through Glass (Windows and Skylights)
Q = A * U * CLTD
Q = conducted heat gain through glass
A = glass surface area
U = U-value of glass
CLTD = Cooling Load Temperature Difference for glass
The glass CLTD values in the table above are for based on the following conditions:
Indoor Room Temperature = 78oF Outdoor Design High Temperature = 95oF Average Outdoor 24-Hour Day Temperature = 85oF Daily Temperature Range = 21oF
ASHRAE design weather conditions give T-maximum and T-daily_range.
T-average can be calculated from:
T-average = T-maximum - (T-daily_range) / 2
CLTD values have to be corrected for location conditions other than the values shown above.
CLTD (corrected) = CLTD (table) + ( 78 - T-room ) + (T-average - 85 )
T-room = Actual design temperature of the room
T-average = Actual average summer design temperature of the location.
Example:
Location = Dubai.
Design outdoor temperature = 115oF, Daily range = 20oF , T-average = 115 – 20/2 = 105
Design indoor temperature = 72oF,
CLTD (corrected) for hour 3:00 PM =
CLTD(table) + ( 78 - 72 ) + ( 105 - 85 ) = 14 + 6 + 20 = 40oF
Glass is one of the most popular and versatile building materials used today, due in part to its constantly improving solar and thermal performance. One way this performance is achieved is through the use of passive and solar control low-e coatings. So, what is low-e glass? In this section, we provide you with an in-depth overview of coatings.
In order to understand coatings, it's important to understand the solar energy spectrum or energy from the sun. Ultraviolet (UV) light, visible light and infrared (IR) light all occupy different parts of the solar spectrum – the differences between the three are determined by their wavelengths.
- Ultraviolet light, which is what causes interior materials such as fabrics and wall coverings to fade, has wavelengths of 310-380 nanometers when reporting glass performance.
- Visible light occupies the part of the spectrum between wavelengths from about 380-780 nanometers.
- Infrared light (or heat energy) is transmitted as heat into a building, and begins at wavelengths of 780 nanometers. Solar infrared is commonly referred to as short-wave infrared energy, while heat radiating off of warm objects has higher wavelengths than the sun and referred to as long-wave infrared.
Low-E coatings have been developed to minimize the amount of ultraviolet and infrared light that can pass through glass without compromising the amount of visible light that is transmitted.
When heat or light energy is absorbed by glass, it is either shifted away by moving air or re-radiated by the glass surface. The ability of a material to radiate energy is known as emissivity. In general, highly reflective materials have a low emissivity and dull darker colored materials have a high emissivity. All materials, including windows, radiate heat in the form of long-wave, infrared energy depending on the emissivity and temperature of their surfaces. Radiant energy is one of the important ways heat transfer occurs with windows. Reducing the emissivity of one or more of the window glass surfaces improves a window's insulating properties. For example, uncoated glass has an emissivity of .84, while Vitro Architectural Glass' (formerly PPG glass) solar control Solarban® 70 glass has an emissivity of .02.
This is where low emissivity (or low-e glass) coatings come into play. Low-E glass has a microscopically thin, transparent coating—it is much thinner than a human hair—that reflects long-wave infrared energy (or heat). Some low-e's also reflect significant amounts of short-wave solar infrared energy. When the interior heat energy tries to escape to the colder outside during the winter, the low-e coating reflects the heat back to the inside, reducing the radiant heat loss through the glass. The reverse happens during the summer. To use a simple analogy, low-e glass works the same way as a thermos. A thermos has a silver lining, which reflects the temperature of the drink it contains. The temperature is maintained because of the constant reflection that occurs, as well as the insulating benefits that the air space provides between the inner and outer shells of the thermos, similar to an insulating glass unit. Since low-e glass is comprised of extremely thin layers of silver or other low emissivity materials, the same theory applies. The silver low-e coating reflects the interior temperatures back inside, keeping the room warm or cold.
Low-e Coating Types & Manufacturing Processes
There are actually two different types of low-e coatings: passive low-e coatings and solar control low-e coatings. Passive low-e coatings are designed to maximize solar heat gain into a home or building to create the effect of “passive” heating and reducing reliance on artificial heating. Solar control low-e coatings are designed to limit the amount of solar heat that passes into a home or building for the purpose of keeping buildings cooler and reducing energy consumption related to air conditioning.
Both types of low-e glass, passive and solar control, are produced by two primary production methods – pyrolytic, or “hard coat”, and Magnetron Sputter Vacuum Deposition (MSVD), or “soft coat”. In the pyrolytic process, which became common in the early 1970’s, the coating is applied to the glass ribbon while it is being produced on the float line. The coating then “fuses” to the hot glass surface, creating a strong bond that is very durable for glass processing during fabrication. Finally, the glass is cut into stock sheets of various sizes for shipment to fabricators. In the MSVD process, introduced in the 1980’s and continually refined in recent decades, the coating is applied off-line to pre-cut glass in a vacuum chambers at room temperature.
Because of the historic evolution of these coating technologies, passive low-e coatings are sometimes associated with the pyrolytic process and solar control low-e coatings with MSVD, however, this is no longer entirely accurate. In addition, performance varies widely from product to product and manufacturer to manufacturer (see table below), but performance data tables are readily available and several online tools can be used to compare all low-e coatings on the market.
Coating Location
In a standard double panel IG there are four potential surfaces to which coatings can be applied: the first (#1) surface faces outdoors, the second (#2) and third (#3) surfaces face each other inside the insulating glass unit and are separated by a peripheral spacer which creates an insulating air space, while the fourth (#4) surface faces directly indoors. Passive low-e coatings function best when on the third or fourth surface (furthest away from the sun), while solar control low-e coatings function best when on the lite closest to the sun, typically the second surface.
Low-e Coating Performance Measures
Low-e coatings are applied to the various surfaces of insulating glass units. Whether a low-e coating is considered passive or solar control, they offer improvements in performance values. The following are used to measure the effectiveness of glass with low-e coatings:
- U-Value
is the rating given to a window based on how much heat loss it allows.
- Visible Light Transmittance
is a measure of how much light passes through a window.
- Solar Heat Gain Coefficient
is the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed & re-radiated inward. The lower a window's solar heat gain coefficient, the less solar heat it transmits.
- Light to Solar Gain
is the ratio between the window's Solar Heat Gain Coefficient (SHGC) and its visible light transmittance (VLT) rating.
Here’s how the coatings measure up by minimizing the amount of ultra-violet and infrared light (energy) that can pass through glass without compromising the amount of visible light that is transmitted.
When thinking of window designs: size, tint and other aesthetic qualities come to mind. However, low-e coatings play an equally important role and significantly affect the overall performance of a window and the total heating, lighting, and cooling costs of a building.
For complete technical information about designing with low-e glass, read Vitro Architectural Glass Technical Document TD-131. For any other glass questions, please contact Vitro Glass or call 1-855-VTRO-GLS (1-855-887-6457).
Learn more about our full line of Low-E Glass products.