2.7 Make Your Building Energy Efficient

Heat, thermal energy and thermal resistance

Kinetic energy of atoms

In solids, liquids and gases, atoms and molecules have “kinetic energy”, which is the energy of motion related to vibration, rotation or movement.

SOLID LIQUID GAS

Kinetic energy of atoms 2

In solids the atoms can only vibrate, but in liquids and gases the atoms also rotate and move.

SOLID LIQUID GAS

Thermal Energy

The thermal energy of a substance is the total amount of kinetic energy due to the motion of its atoms. It also includes the “potential energy” stored in chemical bonds.

Thermal Energy Units

Temperature depends on average kinetic energy

Relation between T and molecular KE

Temperature and Thermal Energy

Mixing Water of Different Temperature

Heat

Heat is the amount of thermal energy that is transferred between two objects because of a temperature difference.

Heat has the same units as thermal energy (Joules in the metric system, British Thermal Units (BTU) in the British system).

An object does not “contain” heat---it contains thermal energy. Heat is the amount of thermal energy transferred.

Thermal Energy Flow

When a hot object is placed in contact with a cool object, thermal energy flows from the hot object to the cool object until their temperatures are the same.

This flow does not depend on the total thermal energy of either object (i.e. which is related to their sizes); it depends only on the difference in their temperatures.

Energy Flow Example

When a hot brick is put contact with a cold brick, the more rapidly vibrating molecules in the hot brick collide and transfer kinetic energy (temperature) to the uppermost molecules in the cold brick, which in turn transfer the KE downward through the cold brick.

Thermal Equilibrium

Energy transfer continues between two objects in contact until they reach the same temperature. When this happens, the amount of energy transferred between the objects is the same in either direction, and the system is in thermal equilibrium. Although the KE of individual particles is still exchanged, the net transfer of KE is zero and the total thermal energy of each object remains the same.

A system is in thermal equilibrium when its temperature is constant and there is no net transfer of thermal energy to it or from it.

Thermal Energy Transfer

In order for an object’s temperature to change, thermal energy must be added or removed.

The three primary ways in which heat is exchanged is by:

• Conduction

• Convection

• Radiation

Conduction

Conduction is the transfer of thermal energy directly through a material by collisions between molecules. Higher energy molecules collide with lower energy molecules, transferring kinetic energy. This is how heat is transferred through solids.

Example: If you hold one end of a metal rod and put the other end into a fire, it will soon start to feel warm. The warmth is conducted along the length of the rod by collisions between particles.

Conduction

If you hold one end of a wooden stick and put the other end into a fire, the far end will likely catch fire before the near end starts to feel warm. Thus conduction depends on the type of material involved. Materials that are good at conducting thermal energy are called thermal conductors, while those that conduct thermal energy poorly are called insulators.

Conductors and Insulators

Metals tend to be good conductors of both thermal energy and electricity because of their freely moving valence electrons which can transfer both.

Non-metals are generally good insulators. Air is a good insulator because gas molecules are far apart.

Example

You get up in the morning and walk barefoot from the bedroom to the bathroom. In the bedroom you walk on carpet, but the bathroom floor is tile. The tile feel colder than the carpet. Why?

Both are at the same temperature. Your feet are warmer than both. Carpet is a better insulator than tile. Less heat flows from your feet to the carpet than to the tile. Thus the tile feels as if it were colder---it makes your feet colder quicker.

Heat Transfer: Conduction

Conduction

R-values

Here, l is the thickness of the material.

R-values

Since R-values already take into account the thickness of the material, they can be added directly. For instance, 4 inches of fiberglass added onto ½” plywood would have R = 12.6.

R-values Note that heat flow through a window or wall will often create a thin “boundary” layer of air on either side of it of similar temperature, which then effectively acts as a further layer of insulation, if the air is still.

However, if there is a strong wind, this extra layer of insulating air is rapidly swept away and replaced by fresh cold air, and heat loss increases. Thus heat exchange across a surface also depends on wind speed, which is related to the process of convection.

Convection

Convection transfers thermal energy by physical movement of molecules from one place to another (translation). This occurs in gases and liquids.

Example: An electric heater’s hot coils warms the air in direct contact by conduction, but then the hot air rises, moves away and warms the whole room by convection. The molecules with high kinetic energy (temperature) move to other locations in the room and mix in.

Convection

Although liquids and gases are generally not good thermal conductors (due to their low number of molecules per volume relative to solids), they can flow and transfer heat rapidly by convection.

Convection may be natural (buoyancy-driven) or forced (advection).

Both figures below are examples of natural convection. The heated fluid undergoes thermal expansion; this increase in volume lowers its density (mass/volume), and it rises due to its buoyancy relative to the surrounding fluid, creating a circulation pattern.

Forced Convection

An example of forced convection is a forced hot-air heating system. These often have a fan that blows the air out of registers (openings), rather than relying completely on natural convection.

Our body temperature is regulated by the blood, which is pumped by the heart, and thus is another example of forced convection.

Radiation

All objects give off energy in the form of radiation. That is, objects emit electromagnetic waves relative to their temperature---infrared waves, and if hot enough, visible and ultraviolet waves. These electromagnetic waves carry energy through space---even through a vacuum---and can be absorbed by and heat a distant object.

Example: The warmth we receive from a fire is mainly radiation. The side of you facing the fire may feel very hot while the side of you facing away remains cold due to the surrounding cold air.

Radiation

The most familiar example of radiation is our own Sun, which has a surface temperature of 6000 K.

As a result of its high temperature, the Sun emits infrared waves,

visible light and ultraviolet light that

travel to and heat the Earth.

Radiation

Angle of the Solar Radiation

Angle of the Solar Radiation

Radiation

On cloudy days, the solar radiation reaching the ground is typically half that of sunny days.

White or shiny (reflective) objects are not good at emitting or absorbing electromagnetic waves, while dark (non-reflective) objects are both good absorbers and good emitters.

Seeing Temperature

Thermal Imagery

Images of infrared radiation flux are

useful in many engineering applications.

Combined Heat Transfers

In most situations, two or all three types of heat transfer occur simultaneously.

Our blood carries heat by convection. Once it reaches the surface of the skin, the heat is released through evaporation, radiation and convection of the surrounding air.

The sun heats a car by radiation. Heat spreads through the solid parts of the car by conduction, and into the air inside and outside of the car by convection.

Thermal Equilibrium

Despite whether heat is transferred by conduction, convection or radiation, once the objects involved reach the same temperature, the net transfer of heat between the objects will be zero.

Heat and Phase Change

An input of thermal energy (heat) is required to change a solid to a liquid, and a liquid to a gas.

SOLID LIQUID GAS

Hexagonal Structure of Ice

In this figure, oxygen atoms are red, and hydrogen atoms are blue.

Changing from one phase to another requires heat

Evaporation cools your skin

Consider a droplet of sweat on the skin. The highest-energy molecules escape from the liquid, such that the mean KE of the remaining liquid drops. As the temperature of the sweat become lower than that of the skin, it conducts heat from the skin, cooling the body.

Evaporation keeps you cool

This heat flow from the skin to the cooled sweat increases the mean KE of the liquid, allowing more evaporation to occur, and continues the flow of thermal energy and cooling of the body.

Thus sweating is an effective means of drawing thermal energy from the body and shooting it off into the surrounding air in the form of high-speed water molecules.

Humid air reduces the rate of evaporation---and hence cooling by sweating is less effective then.

Extreme evaporation

Although the body temperature (average KE) of this athlete is below the boiling point, his warm sweat readily evaporates into the cool, dry air. Note that water vapor is invisible; the “steam” in this photo is water that has cooled and condensed back into the liquid state.

Refrigerators and Air Conditioners

Refrigerators and air conditioners use latent heat to transport heat. An alternating air compressor puts a special liquid (formerly CFCs) under low pressure so that it evaporates to a gas. In doing so it absorbs heat inside the refrigerator. The gas is then pumped through coils outside the refrigerator and put under high pressure so that it condenses back to a liquid. When it does so, it releases the heat. It is then pumped back into the refrigerator and the process repeated.