Question

What is the effect of a low-e coating on the inner surface of a window glass on the \((a)\) heat loss in winter and (b) heat gain in summer through the window?

Short answer

Answer: Low-e coating on the inner surface of a window glass helps in reducing heat loss in winter by reflecting radiant heat back into the room, making the room warmer and reducing the energy required to maintain the desired temperature. In summer, the low-e coating helps in reducing heat gain by reflecting incident solar radiation back outside, making the room cooler and reducing air conditioning requirements. This leads to energy-efficient windows and significant energy savings.

Step-by-step solution

Understanding Low-E Coating

Low-emissivity (low-e) coating is a thin metallic layer applied to the surface of a window glass to reduce the amount of radiant heat transfer through the glass. It is designed to minimize the amount of infrared rays that pass through the glass while allowing visible light to transmit through.

Effect on Heat Loss in Winter (a)

In winter, low-e coating on the inner surface of a window glass helps to minimize heat loss from inside the room to the outside environment. It does so by reflecting the radiant heat back into the room. Let's break down its effect on heat loss in winter: Step 1: Radiant heat (infrared rays) from inside the room strikes the low-e coated surface of the window glass. Step 2: The low-e coating reflects a majority of the radiant heat back into the room, thus reducing the amount of heat that would otherwise escape through the glass to the outside environment. Step 3: As a result, the room remains warmer, lesser energy is required to maintain the desired indoor temperature, and heat loss through the window is reduced.

Effect on Heat Gain in Summer (b)

In summer, the low-e coating on the inner surface of a window glass helps to reduce the unwanted heat gain from the outside environment. It does so by reflecting the incident solar radiation back outside. Let's break down its effect on heat gain in summer: Step 1: Solar radiation, composed of visible light, ultraviolet, and infrared rays, strikes the outer surface of the window glass. Step 2: The window glass absorbs a portion of the solar radiation, while some of it passes directly through the glass. The low-insulated glass covered with low-e coating inner surface performs better in reduction of heat gain during summers. Step 3: The low-e coating on the inner surface of the glass reflects a significant amount of the incident solar radiation (especially infrared rays), thus reducing the amount of heat that enters the room. Step 4: As a result, the room remains cooler, air conditioning requirements are reduced, and unwanted heat gain through the window is minimized. In summary, a low-e coating on the inner surface of a window glass effectively reduces (a) heat loss in winter and (b) heat gain in summer, resulting in energy-efficient windows and significant energy savings.

Key concepts

Heat Transfer

Heat transfer is the movement of thermal energy from one object or material to another. This can occur in several ways:

• Conduction: Transfer of heat through direct contact between materials.
• Convection: Transfer of heat through fluids (liquids and gases) due to the movement of the fluid itself.
• Radiation: Transfer of heat in the form of electromagnetic waves, such as infrared radiation.

When it comes to windows, heat transfer plays a crucial role in determining the temperature inside a building. Windows can lose or gain heat and thereby affect the overall energy efficiency of a building.
Understanding how heat moves can help in designing more efficient windows, especially with the use of low-emissivity coatings that specifically target radiation.

Infrared Radiation

Infrared radiation is a type of electromagnetic radiation that is not visible to the human eye but is felt as heat. It plays a significant role in heat transfer processes.
Low-emissivity (low-e) coatings on windows are specifically designed to manage infrared radiation. The coatings are thin layers of metal used to reflect infrared rays.

• In winter, they reflect the infrared radiation (heat) back into the room, helping to keep interiors warm.
• In summer, they prevent infrared radiation from entering, thus keeping interiors cool.

This management of infrared radiation is key to making windows more energy efficient, as it minimizes unwanted heat loss or gain.

Energy Efficiency

Energy efficiency refers to using less energy to provide the same service. For windows, being energy efficient means reducing the heat loss in the winter and minimizing the heat gain in the summer.
Low-e coatings contribute significantly to a window's energy efficiency. By controlling the flow of infrared radiation:

• In the winter, they keep heat inside the building, reducing the need for heating appliances.
• In the summer, they prevent excess heat from entering, reducing the need for air conditioning.

Using windows with low-e coatings can therefore lead to lower energy bills and a smaller environmental footprint.

Window Insulation

Window insulation is essential for a comfortable indoor environment and to reduce energy consumption. Properly insulated windows minimize the transfer of heat, keeping spaces warm in winter and cool in summer.
Low-e coatings enhance window insulation by:

• Increasing the reflective properties of the glass to handle infrared radiation effectively.
• Maintaining interior temperatures by reducing heat flow.

These coatings act as an added insulation layer, allowing windows to perform better in extreme weather conditions. Improved insulation not only ensures comfort but also contributes to significant energy savings each year.