Heat Transmission In Different Windows

Published Date: 02 Nov 2017

Extended Essay

Subject: Physics

Topic: Heat Transmission in Different Windows

Research Question: How does the shade/ thickness of the glass/windows affect the inside temperature of a room/car that is under the influence of solar radiation from the sun?

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Name: Harshvardhan Sanghavi

Candidate Number: dsh532 (004669 â€“ 009)

Word Count:

The Cathedral Vidya School Lonavala

Research Question â€“ How does the shade/ thickness of the glass/windows affect the inside temperature of a room/car that is under the influence of solar radiation from the sun?

Abstract

Introduction

Background

One notices that when any vehicle such as a car is left in the sunlight (e.g. when it is parked), it heats up to a temperature higher than the outside temperature. This makes the conditions inside very uncomfortable when one enters the vehicle. But, when the windows are rolled down, it takes only a few minutes for the temperature to drop till it is the same as the outside temperature. This is because of "greenhouse effect" â€“ where light energy goes inside the enclosure through the glass and the re-radiated heat from inside is not able to escape through the glass. The windows allow the visible solar (shorter wavelength) radiation from the sun to pass through as they are transparent to it. But, they are opaque to the re-radiated heat (longer wavelength) and so some of it is trapped inside. This situation is somewhat similar to the case of global warming. But this is only a minor reason. The main reason of heat energy being trapped inside is that the air inside gets heated which does not let the heat escape through convection. This results in higher temperatures inside. This phenomenon of increase in temperature of a closed space, object or structure that results from solar radiation is called solar gain or passive solar gain.

Objective

The aim of this research is to find the relation between the inside temperature of an enclosure (car/room) under the radiation of the sun and the thickness/shade of the glass (window).

In order to find this relation, the thickness of the glass sheet was varied and the rise of the inside temperature of the enclosure was recorded. Even the outside temperature must be taken into account, so it was also recorded. Also, the colour of the glass sheet was varied and the rise of the inside temperature was recorded.

Different closed spaces like the inside of a car or a room get heated up to a temperature higher than the outside temperature depending on the properties of the glass windows that allow the light energy to pass through.

This experiment could be related to cases where thicknesses of these windows are to be decided or in cases where it is to be decided whether to tint/shade/colour a window in a certain way or in a certain colour.

There can be many applications to this relationship because so many vehicles and other modes of transport have windows and the inside temperature plays a major role in the comfort of the person (traveller) inside the enclosure.

Airplanes have windows and they have a specific thickness. They tend to be very thick for safety reasons, but they are also affecting how much heat energy is being able to enter or leave the body of the plane.

Finally, the windows at home play the most important role in keeping people inside warm or cool depending on outside weather. The results of this experiment can be related to this sector too.

Theory

The experiment is based on the theory of transmission, absorption and reflection of light and heat radiation. Also, it deals with the properties of glass. Since it deals with the transmission of heat, it involves the phenomena of conduction, convection and radiation.

Transmission, Absorption and Reflection of light through glass

When light comes in contact with a glass surface, some of it is reflected, some transmitted through and some absorbed by the glass. These depend on the angle of incidence of the rays, the refractive indices of the glass and the medium the light is coming from.

In transparent windows/glass sheets, most of the light hitting the surface gets transmitted and only a very small amount of it gets absorbed and reflected. Whereas in coloured glasses such as black, blue and brown, only certain wavelengths of light get transmitted and the rest are absorbed.

The windows of a car/room are transparent and allow most of the light to pass through.

Heat transfer through glass windows

Conduction is the type of heat transfer where two or more bodies have to be in contact to exchange heat due to difference in temperatures. Convection is the type where there is transfer of heat from one place to another due to movement of fluids (air). Radiation is the third kind where electromagnetic radiation is generated by the thermal motion of particles in matter. Anything with a temperature greater than absolute zero emits thermal radiation. The total heat transferred by conduction varies inversely with the thickness of the material through which it passes.

Conduction and convection come together here as through convection the warm air transfers heat to the colder surfaces of the glass window. They are the primary sources of heat loss. Heat has no sense of direction, but warm air being lighter rises. In homes, the primary source of heat loss is by exfiltration and infiltration. Exfiltration is the loss of heated air through cracks and openings. Infiltration is the introduction of outside cold air into the building. This air movement (drafts) also causes discomfort to occupants in addition to the heat loss itself. The physics behind this is that there is a difference in the pressure inside and outside the building and this causes air flow/drafts. Heat loss due to convection can be calculated:

Heat Loss = Heat capacity of air x Air volume exchanged/hour x Temperature difference

The infra-red radiation emitted by the inner surface of the box is absorbed by the glass sheet. This radiation is of a longer wavelength then the wavelength of visible light and so is blocked by the glass sheet, thus causing the inside temperature of the box to rise to more than the outside temperature. This is because the heat energy coming in from the visible light continues to heat the air (inside) and the inner surface but the air and the glass trap it in. There is always a difference in inside and outside temperature and so that is a source of heat loss through convection or conduction or both.

Wind is also a major factor in any window heat transfer process. In cars, the heating up is attenuated when they are moving because the air flow around the glass will take the heat out of it, thus reducing the heat load inside the car (because by reducing the temperature of the glasses we are also reducing the component that is transmitted inside the car). In this box, there were cracks and potential spots for infiltration and exfiltration drafts of air which could lead to loss of heat to the outside.

Transfer of heat radiation through a glass can also be affected by the inclination of the glass surface to the sun. The amount of heat entering will differ for different positions of the glass surface. In the case of the glass surface placed perpendicular to the ground (90Â°), it will get the radiation from the sun and some of the reflected radiation from the ground. When it is placed parallel to the ground and only facing the sky, it does not receive radiation from the ground.

In the case of layers of insulating material used, the overall conductance will be the sum of the individual conductance of each of the layers used. The combination will not involve materials of more than one kind. When more than one glass sheet is used there is a film of air in between the sheets and is also an insulating layer. These layers may reduce the amount of heat entering but may also reradiate this heat and it would be taken in the room/box through convection.

The two boxes used had different dimensions and so they had different surface areas. This is also a considerable factor as the wood also absorbs heat and re-radiates heat to the surrounding and has its own "U" factor (Coefficient of heat transfer). This difference would lead to different rises in temperature inside the box. Even the volumes of the two boxes are different and there is more air inside the larger box which could be a factor.

Physics behind the heat loss

The overall coefficient of heat transfer (U) measures how well a building component such as a wall, roof or a window can keep heat inside a building. In this experiment the wall of the enclosure would have a U-value and this would affect the heat transfer as it wouldnâ€™t only occur through the glass slab/window. This value has its application in cold climates as well as hot. In a hot weather the U-value can also tell one how easy it is for the component to keep the inside of the building cold. The higher the U-value, more heat flows through, so a good component would have a low U-value as one wants to keep more or less heat inside the building/enclosure depending on the weather outside. The using up of energy is also less in a building where the U-value is low. Even in a carâ€™s window the U-value plays a role as a low one would mean that the windows are efficient in keeping warmth in in cold weathers and keeping it out in warm weathers. This is considered as an important factor in the process of making a commercial vehicle as comfort is an important aspect. The U-value is actually the inverse of the "R" factor (thermal resistance) and the larger the R-value (or lower the U-value) the lower the heat loss.

A total hourly rate of heat loss through walls, roofs or glass (windows) can be calculated by:

Q = U*A*Î”T, where "Q" is heat loss, "A" is surface area of the component and "Î”T" is the change in temperature. In an enclosure like the one in this experiment, there is more than one component (wall, window, door etc.). In this experiment, there are two components, the wooden wall of the box and the glass slab. Both have a surface area and a U-value. The temperature change would be constant for both and the heat loss (rate) can be calculated by recording the change in temperature over one hour. Once both the heat loss values are calculated separately, add them and you will get the total heat loss amount. The unit for heat loss in North America is usually Btu/hour (British thermal unit/hour)

Heat loss due to infiltration and ventilation is the second type of heat loss. It is the loss of heat due to air currents and leakages. In this experiment there was scope for heat loss through air leakage. In buildings, there are three ways to measure heat loss due to air infiltration:

Air change method

Infiltration through the gaps

Number of people in the space

The letter "k" represents thermal conductivity. It is the rate of heat transfer through one inch of a homogenous material. A material is considered homogenous when it has the same "k" value throughout and its dimension does not affect its thermal conductivity.

The letter "C" represents thermal conductance which is a measure of the rate of heat transfer through a material just like conductivity. But, it differs from conductivity in one significant way â€“ thermal conductance is specific (for a given thickness of material) whereas conductivity is a heat transfer factor per inch of thickness. The lower the C value, the better the insulator.

The letter "R" represents thermal resistance; it is the ability to retard heat flow in a given thickness of material. By definition, the resistance of a material to heat flow is the reciprocal of its heat transfer coefficient. Even in a building, when the total resistance has to be calculated, all the individual resistances have to be summed.

Windows: they are useful in providing light, ventilation and in most cases passive solar heating, but are otherwise a great source of heat loss. All windows are created equally (i.e. â€“ each window is homogenous and loses the same amount of heat from every square-inch). A window can lose seven times more heat than a decently insulated wall.

The heat transmission coefficient "C" will tell how much heat a window will lose. The VT (visible transmission) represents the percentage of the available light that is allowed to pass through the windowâ€™s glass. Even clear glass is never perfectly transparent and is opaque to some light. Glazing and low-E coatings reduce this value.

There is the solar heat gain coefficient (SHGC) that is the percentage of the available solar gain that is allowed through the glass of the window. With the VT value falling with an increase in any kind of glazing and coating. Studies show that a low-E coating can stop 30-40% of solar gain from passing through.

Experimental setup

I used the help of the lab assistant and the campus carpenter to get two different boxes made. Each one is a closed wooden rectangular box with only one lateral (one of the two large ones) face open. There is an opening on one of the smaller sides where glass sheets are slid in. Once the glass sheet is put in the gap the box becomes completely closed with that one open side enclosed by the glass sheet/sheets.

There is a hole in the centre of the side with the slit where a thermometer can be inserted to measure inside temperature. A general lab thermometer was used during experimentation. A clamp stand was used to hold the thermometer in place halfway through the hole to ensure that it did not fall into the box.

The lab thermometer was also held by the second clamp stand right next to the box where the temperature was being calculated and this would give the outside temperature.

The experiments were carried out on a terrace connected to the physics lab on campus between 11:00 am to 4:00 pm on different days. All the days had relatively similar weather conditions

(no extreme cloud cover or any highs or lows in the temperature. Conditions were windy though and the wind flow was not constant.

The apparatus used was:-

A rectangular box with one face open (dimensions - )

Rectangular transparent glass sheet with dimensions â€“ thickness â€“ 3mm, length - , breadth â€“

Rectangular transparent glass sheet with dimensions â€“ thickness â€“ 4mm, length - , breadth â€“

Rectangular transparent glass sheet with dimensions â€“ thickness â€“ 5mm, length - , breadth â€“

Rectangular transparent glass sheet with dimensions â€“ thickness â€“ 7mm, length - , breadth â€“

Rectangular black shaded glass sheet with dimensions â€“ thickness â€“ 3mm, length - , breadth â€“

Rectangular brown shaded glass sheet with dimensions â€“ thickness â€“ 3mm, length - , breadth â€“

Rectangular blue shaded glass sheet with dimensions â€“ thickness â€“ 3mm, length - , breadth â€“

3 coloured rectangular glass sheets (dimensions â€“ and thickness of -) of black, blue and brown.

2 clamp stands

2 lab thermometers

Pieces of paper for insulating the system (minimizing air infiltration)

A piece of clay

Method

Procedure

These were the steps followed to carry out the experiments:

Experiment 1 - Large box (varying thickness of the transparent glass):

Set out the box on the platform being used and make it face the sun. It should be standing with the glass side perpendicular to the ground and the side with the hole facing the sky.

Place the clamp stand next to the box and suspend the thermometer from the stand. The lower half with the tip should be halfway through the box.

Place the second clamp stand and thermometer in the same way as the first only in front of the side facing the sun.

Wait for the second thermometer to get a constant temperature and then record it as the initial outside temperature into the data table.

Put the first thickness of transparent glass in the slit of the box and wait for a certain amount of time and record the temperature when it becomes constant.

Remove the glass sheet/s and place them elsewhere (preferably under some shade)

Wait for the temperature of the thermometer inside the box to stabilise to outside temperature (i.e. - temperature of the second thermometer).

Put the next combination of the thickness of glass sheets inside and wait till a constant temperature is obtained. Record this as well as the outside temperature (record the outside temperature before every experiment) into the data table.

Repeat this experiment till all the data for all the combinations of thicknesses have been tested. Repeat this again by placing the side with the sheet facing the sky.

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Experiment 2 â€“ Large box (varying colour of the glass sheet used):

Set out the box on the platform being used and make it face the sun. It should be standing with the glass side perpendicular to the ground and the side with the hole facing the sky.

Place the clamp stand next to the box and suspend the thermometer from the stand. The lower half with the tip should be halfway through the box.

Place the second clamp stand and thermometer in the same way as the first only in front of the side facing the sun.

Wait for the second thermometer to get a constant temperature and then record it as the initial outside temperature into the data table.

Put the first sheet of coloured glass (black) in the slit of the box and wait for a certain amount of time and record the temperature when it becomes constant.

Remove the glass sheet/s and place it elsewhere (preferably under some shade)

Wait for the temperature of the thermometer inside the box to drop to outside temperature (i.e. - temperature of the second thermometer).

Put the next coloured sheet of glass (brown) inside and wait till a constant temperature is obtained. Record this as well as the outside temperature (record the outside temperature before every experiment) into the data table.

Repeat this experiment till all the data for all the combinations of colours (black, brown, blue and transparent) have been tested. Repeat this again by placing the side with the sheet facing the sky.

Experiment 3 â€“ Small box (varying thickness of transparent glass sheets used)