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Impact of high-performance glass on energy-efficiency of commercial buildings

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Why choose the right glass?

There is a growing concern about energy consumption in buildings and its implications on the environment. Potential for energy conservation exists in all building typologies but it becomes more of a concern in energy-intensive buildings such as air-conditioned offices and shopping centers. As windows contribute to a significant portion of the heat transfer in a building, the glazing should be well considered as it plays a vital role in the total heat gain to the building; because the inappropriate or inefficient use of glass may result in unwanted heat gain. When choosing a glass for any application, a number of factors should be taken into account, including solar control, light transmission, color, sound insulation, safety properties, aesthetics, and thermal insulation in buildings.

Case study objectives

  • To estimate the heat gain and cooling energy in a heat gain model of a commercial building (for two different glass products) and the impact of coated glass as a passive solution.
  • To calculate the reduction in heat flow into the building, its resultant benefits on cooling systems and energy savings.

Model Building

  • Location: Chennai
  • Total build up area: 1200 Sq.ft
  • Window-to-Wall ratio: 25%
  • Total glazing: 300 Sq.ft
  • U-value of walls: 0.5 W/m2k
  • U-value of roof: 0.4 W/m2K
  • Type of occupancy: Office space
  • Glass used: SGG Antelio plus ST 467 and SGG EvoliteET 450

Heat Gain

Various heat transfer processes take place between a building and the external environment through walls, windows, and roofs from higher to lower temperature in three ways – conduction, convection, and radiation. Conduction occurs through walls, windows, and roofs while solar radiation is transmitted through transparent windows and absorbed by the building’s internal surfaces. The presence of human occupantsand the use of light and equipment adds further heat to the space.

Indirect Heat Gain

Of the total solar energy incident on the outer surface of the wall, a part of it is reflected to the environment and the remaining part is absorbed by the wall, which is then converted into heat energy and conducted to the room’s interior surface. The inner surface transfers heat by convection to the air inside the room, thus raising its temperature. This accounts for the indirect heat gain of the building. The rate of indirect heat transfer through any element such as the roof or wall can be written as:

Q = A U Δ T

Where,

A  =  Surface area (m2)

U  =  Thermal transmittance (W/m2-°C)

Δ T = Temperature difference between inside and outside air (°C)

                                        

              Fig.1 Heat Gain through walls                                                                           Fig .2 Heat Gain through windows

Direct Heat Gain

Direct incident solar energy contributes to the direct solar heat gain of the building through the fenestration system. The transmitted radiation becomes instantaneous room heat gain. The heat gain due to direct solar radiation can be represented as:

Q = A I SF

Where,

A = Surface area (m2)

I = Intensity of solar radiation (W/m2)

SF = Solar factor of glass

Result and Discussion

The indirect and direct heat gain through the various components of the envelope is calculated and the results are compared with the base case as follows.

Component

U value (W/m2K)

RHG (W/m2)

Wall

0.5

6

Roof

0.4

4.8

Clear glass

5.7

68.4

ST 467

5.6

67.2

ET 450

5

60

                 Table.1 Relative Heat Gain due to Temperature Difference

                Fig.3 Relative Heat Gain due to Incident Solar Radiation on Windows

                        Fig.4 Monthly Variation in Chiller Electricity Consumption

                       Fig.5 Annual Energy Consumption

  Clear ST 467 ET 450
Annual Electricity Consumption (kWh) 8022 7094 6802
Savings in electricity(kWh) base 928 1220
AC running cost per year(Rs) 56153 49659 47612
Savings in AC running cost per year(Rs) base 6494 8541
Efficiency base 12% 15%
Pay back Years base 2.2 2.6

                       Table.2 Comparison with Base Case

  • High-performance glass saves up to 20% on capex on cooling system
  • ST 467 performs 12% more efficiently than the base case with a payback of 2 years and 5 months. One sq. m. of glazing can save up to Rs.206 a year
  • ET 450 is 15% more energy-efficient than the base case with a payback of 2 years and 7 months. One sq. m. of glazing (anti-reflection coating) can save up to Rs.217 a year
  • Results show that one sq. m. of high-performance glass can save up to Rs.30/per month on energy

Conclusion

Glass windows are commonly used in buildings as a physical and visual connection to outsiders and aesthetical reasons. Hence, glass selection should be suitable to specific climate with higher selectivity to harvest more light into the space without compromising on the energy ingress into the space. The latest state-of-the-art products from Saint-Gobain Glass India with double layers and silver-based coatings ensure the excellent selectivity to the glass with marginal reflection values.

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