Thermal Energy and Calorimetry
5. Thermal Energy Applications
Learning outcomes
- I can explain how thermal energy principles are used in cooking, heating, and cooling systems.
- I can describe how engineers use materials with different thermal properties.
- I can evaluate methods of improving thermal efficiency.
- I can explain the importance of thermal energy in natural and industrial processes.
- I can apply thermal energy concepts to solve real-world problems.
Thermal Energy Applications
Thermal Energy in Everyday Life
Thermal energy principles are involved whenever energy is transferred because of a temperature difference.
They are important in:
- cooking
- home heating
- refrigeration
- air conditioning
- transportation
- electronics
- manufacturing
- power generation
- weather and climate
- biological systems
Engineers often face one of two challenges:
increase heat transfer when it is useful
or
reduce heat transfer when it is unwanted
Understanding conduction, convection, radiation, specific heat capacity, latent heat and insulation allows systems to be designed for these different purposes.
Controlling Thermal Energy Transfer
There are three major methods of heat transfer.
Conduction
Energy transfers through direct interactions between particles.
It is particularly important in solids.
Convection
Energy is transferred through the movement of fluids such as liquids and gases.
Radiation
Energy is transferred by electromagnetic waves and does not require matter.
Most practical thermal systems involve more than one method at the same time.
For example, when cooking soup:
stove → conduction through pan → convection through soup
while radiation and convection also transfer energy from the stove and pan to the surroundings.
Choosing Materials
Engineers select materials according to their thermal properties.
Important properties include:
- thermal conductivity
- specific heat capacity
- melting point
- boiling point
- ability to reflect or absorb radiation
- thermal expansion
- resistance to high temperatures
The best material depends on what the system is designed to accomplish.
Thermal Conductors
Materials with high thermal conductivity transfer energy rapidly.
Metals such as:
- copper
- aluminium
- steel
are widely used when efficient conduction is required.
Applications include:
- saucepans
- heat exchangers
- radiators
- heat sinks
- boilers
- engine components
Copper has excellent thermal conductivity, but it can be more expensive and heavier than aluminium.
Engineers therefore consider more than just thermal performance.
Thermal Insulators
Materials with low thermal conductivity are used when energy transfer should be reduced.
Examples include:
- foam
- fibreglass
- mineral wool
- wood
- plastics
- cork
- trapped air
Applications include:
- building insulation
- refrigerator walls
- cooler boxes
- oven gloves
- saucepan handles
- thermal clothing
An insulating material does not completely stop heat transfer. It reduces the rate at which energy is transferred.
Cooking
Cooking involves transferring thermal energy into food.
Different cooking methods use different combinations of conduction, convection and radiation.
Frying
Energy transfers:
stove → pan → food
Conduction is particularly important.
Boiling
Energy conducts through the saucepan and then convection currents distribute energy through the water.
Baking
Hot air transfers energy by convection while hot oven surfaces transfer energy by radiation.
Conduction transfers energy through the food itself.
Grilling
Infrared radiation from the heating element or flame is particularly important.
Why Cooking Pans Use Metals
The base of a saucepan should transfer thermal energy efficiently.
Metals are useful because they generally have:
- high thermal conductivity
- high temperature resistance
- good mechanical strength
Aluminium and copper are particularly good thermal conductors.
However, the handle has a different purpose.
A metal handle could conduct energy toward the user's hand.
Handles are therefore often made from materials such as:
- plastic
- wood
- silicone
One device can therefore deliberately contain both conductors and insulators.
Convection Ovens
A convection oven uses a fan to circulate hot air.
Moving air increases the rate of energy transfer between the air and food.
This can:
- produce more even heating
- reduce cooking time
- reduce temperature differences inside the oven
This is an example of forced convection.
The same principle is used in many engineering cooling systems.
Induction Cooking
Induction cooktops use changing electromagnetic fields to produce electrical currents in suitable cookware.
Electrical resistance within the cookware produces heating.
This can be efficient because energy is produced directly in the cookware rather than first heating a conventional heating element and then transferring energy to the pan.
However, induction cooking requires compatible cookware containing suitable magnetic materials.
This illustrates an important engineering idea:
thermal efficiency often depends on both the energy-transfer method and the materials selected.
Heating Buildings
Heating systems transfer energy into buildings to maintain comfortable indoor temperatures.
Common systems include:
- radiators
- underfloor heating
- heat pumps
- electric heaters
- furnaces
A radiator heats nearby air.
The warmer air becomes less dense and rises.
Cooler air moves toward the radiator and is heated.
This establishes convection currents.
Radiators also transfer energy by radiation.
Underfloor Heating
Underfloor heating distributes energy across a large surface area.
The floor warms the air immediately above it.
The warmer air rises and produces convection.
The large heated area can provide relatively uniform room temperatures.
It can sometimes operate effectively using lower water temperatures than traditional radiators, depending on the system and building.
Heat Pumps
A heat pump transfers thermal energy from one location to another.
It can move energy:
outside → inside
for heating
or:
inside → outside
for cooling.
Heat pumps use a refrigerant that undergoes evaporation and condensation.
During evaporation, the refrigerant absorbs latent heat.
During condensation, it releases latent heat.
Electrical energy is required to operate the compressor and other components.
A heat pump can deliver more thermal energy to a building than the electrical energy it consumes because it moves existing thermal energy rather than converting all of its input electricity directly into heat.
Refrigerators
A refrigerator also uses a heat-pump cycle.
Its purpose is to transfer energy:
inside refrigerator → surrounding room
A refrigerant circulates through the system.
Inside the refrigerator, the refrigerant evaporates and absorbs energy.
Outside, it condenses and releases energy.
This explains why the coils at the back or underneath a refrigerator can become warm.
The refrigerator does not "create cold."
Instead, it removes thermal energy from its interior.
Air Conditioning
An air conditioner works using similar principles.
Energy is absorbed from indoor air and transferred outside.
The system involves:
- evaporation
- condensation
- compression
- convection
- conduction
Fans increase convection across the heat exchangers.
The indoor evaporator becomes cool and absorbs energy.
The outdoor condenser becomes warm and releases energy.
Insulation and Thermal Efficiency
Heating and cooling systems become less efficient when large amounts of thermal energy are transferred unintentionally.
Building insulation reduces these unwanted transfers.
Useful methods include:
- roof insulation
- wall insulation
- double or triple glazing
- draught proofing
- reflective surfaces
- insulated doors
- appropriate shading
The best combination depends on the climate and building design.
Double Glazing
Double-glazed windows contain two panes separated by a sealed gap.
The gap may contain air or a gas such as argon.
The trapped gas:
- conducts energy poorly
- prevents large convection currents from developing
Some windows also use special coatings to reduce radiative energy transfer.
Triple glazing uses another pane and another insulating layer.
Thermal Efficiency
Thermal efficiency describes how effectively a system uses or controls thermal energy for its intended purpose.
For many energy-conversion systems:
efficiency = useful energy output / total energy input × 100%
For example, suppose a heating system receives:
10,000 J
and transfers:
8,500 J
usefully into the intended space.
Efficiency = 8500 / 10,000 × 100%
Efficiency = 85%
The remaining energy has been transferred elsewhere.
Improving Thermal Efficiency
Thermal efficiency can be improved by reducing unwanted energy transfers.
Possible methods include:
- increasing insulation
- reducing air leakage
- using reflective surfaces
- recovering waste heat
- improving heat exchangers
- controlling temperatures more accurately
- reducing unnecessary heating or cooling
- maintaining equipment properly
However, improvements should also be evaluated in terms of:
- cost
- safety
- durability
- environmental impact
- maintenance
- expected energy savings
The most expensive solution is not automatically the best solution.
Heat Exchangers
A heat exchanger transfers thermal energy between fluids while usually keeping the fluids physically separate.
Examples are found in:
- car radiators
- refrigerators
- air conditioners
- power stations
- industrial chemical plants
- building heating systems
Effective heat exchangers often use:
- highly conductive materials
- thin walls
- large surface areas
- moving fluids
- large temperature differences
These features increase the rate of heat transfer.
Surface Area and Cooling
Increasing surface area can increase heat transfer.
This is why radiators and heat sinks often contain many thin fins.
The fins provide a large surface area in contact with the surrounding air.
This increases energy transfer by convection.
Radiation may also increase because more surface is exposed.
Cooling Electronics
Computers and electronic equipment generate thermal energy during operation.
Excessive temperature can damage components or reduce their performance.
A processor may use:
processor → thermal paste → heat sink → air
Thermal paste improves thermal contact between surfaces.
The heat sink conducts energy away from the processor.
A fan moves air across the fins and increases convection.
High-performance systems may also use liquid cooling.
Car Engine Cooling
Combustion engines generate large amounts of thermal energy.
If this energy is not removed, engine components can overheat.
A typical cooling system uses:
- coolant
- a pump
- a radiator
- a fan
- a thermostat
Coolant absorbs energy from the engine and carries it to the radiator.
The radiator transfers the energy to the surrounding air.
Its large surface area and thin metal fins increase heat transfer.
Specific Heat Capacity in Engineering
The specific heat capacity of a material affects how quickly its temperature changes.
Materials with high specific heat capacity can absorb substantial amounts of energy without experiencing extremely large temperature changes.
Water is particularly useful because of its high specific heat capacity.
Applications include:
- engine cooling
- industrial cooling
- heating systems
- thermal energy storage
For a temperature change:
Q = mcΔT
This equation allows engineers to determine how much energy a material can absorb or release.
Thermal Energy Storage
Thermal energy does not always need to be used immediately.
It can be stored for later use.
For example, hot-water tanks can store energy.
Because water has a high specific heat capacity, a large tank can store considerable thermal energy.
Suppose 200 kg of water is heated by 30°C.
Using:
c = 4200 J/(kg·°C)
Q = mcΔT
Q = 200 × 4200 × 30
Q = 25,200,000 J
Therefore:
Q = 25.2 MJ
This demonstrates why water can be useful for thermal energy storage.
Latent Heat in Technology
Phase changes can transfer large quantities of energy without large temperature changes.
This is used in:
- refrigerators
- air conditioners
- heat pumps
- thermal storage systems
- industrial cooling systems
During evaporation:
energy is absorbed
During condensation:
energy is released
The energy involved is:
Q = mL
Phase-change materials can therefore be useful when a system needs to absorb or release substantial energy within a narrow temperature range.
Thermal Energy in Power Stations
Many power stations use thermal energy to produce electricity.
A simplified sequence is:
energy source → thermal energy → steam → turbine → generator → electrical energy
Water is heated to produce high-temperature steam.
The steam drives a turbine.
The turbine drives a generator.
After leaving the turbine, the steam is cooled and condensed so that the water can be reused.
Heat exchangers and cooling systems are therefore essential parts of many power stations.
Waste Heat
Industrial processes often produce thermal energy that would otherwise be transferred to the environment.
This waste heat may sometimes be recovered.
Possible uses include:
- heating buildings
- heating water
- preheating industrial materials
- generating additional electricity
- supplying energy to another industrial process
Recovering waste heat can improve the overall efficiency of a system.
Thermal Energy in Nature
Thermal energy is also fundamental to natural processes.
Solar radiation heats Earth's surface unevenly.
This produces temperature and density differences in the atmosphere and oceans.
These differences contribute to:
- convection currents
- winds
- ocean circulation
- weather systems
- evaporation
- cloud formation
Thermal energy therefore plays an important role in Earth's weather and climate.
Evaporation and the Water Cycle
Solar energy causes water to evaporate from:
- oceans
- lakes
- rivers
- soil
- plants
Evaporation absorbs latent heat.
Water vapor can later rise and cool.
When it condenses:
gas → liquid
latent heat is released into the atmosphere.
This energy can contribute to convection and weather-system development.
Thermal Energy in Living Organisms
Living organisms must also manage thermal energy.
Humans regulate body temperature using processes including:
- sweating
- changes in blood flow
- convection
- radiation
- evaporation
Sweating is particularly effective because evaporation requires latent heat.
Energy transferred from the skin allows sweat to evaporate, cooling the body.
Animals use many other thermal adaptations, including:
- fur
- feathers
- body fat
- panting
- behavioral changes
Real-World Problem: Keeping a Drink Cold
Suppose you want to design a container that keeps a cold drink cold for several hours.
You should reduce all major methods of heat transfer.
Reduce Conduction
Use low-conductivity materials.
Reduce Convection
Use sealed spaces or a vacuum.
Reduce Radiation
Use reflective surfaces.
Reduce Energy Transfer Through the Opening
Use an insulated lid and narrow opening.
A vacuum flask combines several of these methods.
This illustrates an important engineering principle:
good thermal design often controls several heat-transfer mechanisms simultaneously.
Real-World Problem: Cooling a Computer
Suppose a processor is overheating.
Possible improvements include:
- using a more conductive heat sink
- increasing fin surface area
- improving thermal contact
- adding a fan
- increasing airflow
- using liquid cooling
- reducing unnecessary power consumption
Simply adding insulation would make the problem worse because the goal is to remove thermal energy.
Thermal engineering always begins by identifying the desired direction of energy transfer.
Real-World Problem: Making a Building More Efficient
Suppose a building requires excessive energy for air conditioning.
Possible improvements include:
- improving roof insulation
- installing better windows
- reducing air leakage
- using reflective roofing
- shading windows
- improving air-conditioning efficiency
The best solution should be evaluated using both thermal performance and practical factors.
For example, an improvement costing $5,000 that saves $1,000 per year may be more attractive than one costing $30,000 that saves only slightly more energy.
Evaluating Thermal Solutions
When evaluating a thermal technology, ask:
Does it improve the required heat transfer?
How much energy does it save?
How much does it cost?
How long will it last?
Is it safe?
What maintenance is required?
What environmental effects does it have?
A technically effective solution may not always be the most practical solution.
Engineering involves balancing these competing factors.
Common Mistakes
Mistake: "Thermal efficiency always means reducing heat transfer."
Some systems, such as radiators and heat sinks, are designed to increase heat transfer.
Mistake: "A refrigerator creates cold."
A refrigerator transfers thermal energy from its interior to the surrounding environment.
Mistake: "Insulation completely stops heat transfer."
Insulation reduces the rate of heat transfer.
Mistake: "Only one method of heat transfer occurs in a system."
Most practical systems involve conduction, convection and radiation simultaneously.
Mistake: "Metals are always the best materials."
Metals are useful when high thermal conductivity is needed, but insulators are better when energy transfer should be reduced.
Mistake: "Efficiency can exceed 100%."
For an energy-conversion efficiency defined as useful output divided by total input, it cannot exceed 100%. Heat-pump performance is commonly described using a different measure, the coefficient of performance, which can be greater than 1.
Key Terms
Thermal energy — part of a system's internal energy associated with the microscopic motion and interactions of its particles.
Thermal conductivity — measure of how effectively a material conducts thermal energy.
Thermal insulator — material that reduces the rate of energy transfer by conduction.
Thermal efficiency — measure of how effectively a system uses or controls energy for its intended purpose.
Heat exchanger — device that transfers thermal energy between fluids, usually without allowing them to mix.
Forced convection — convection produced or increased by a fan, pump or similar device.
Heat sink — device designed to transfer thermal energy away from a component.
Refrigerant — substance circulated through a refrigeration system to absorb and release energy, often through phase changes.
Heat pump — device that uses external energy to transfer thermal energy from a cooler region to a warmer region.
Specific heat capacity — energy required to raise the temperature of 1 kg of a substance by 1°C or 1 K.
Latent heat — energy transferred during a phase change without a temperature change.
Thermal energy storage — storing energy in a material for later use.
Waste heat — thermal energy produced by a process that is not part of its intended useful output.
Thermal equilibrium — condition in which systems are at the same temperature and there is no net heat transfer between them.
Key Takeaways
- Thermal energy principles are essential in cooking, heating, cooling, engineering, industry and natural systems.
- Engineers choose materials according to properties such as thermal conductivity, specific heat capacity and temperature resistance.
- Conductors are useful when rapid heat transfer is required.
- Insulators are useful when unwanted heat transfer must be reduced.
- Cooking usually involves combinations of conduction, convection and radiation.
- Refrigerators and air conditioners transfer thermal energy rather than "creating cold."
- Heat pumps move thermal energy and can provide efficient building heating and cooling.
- Building insulation reduces unwanted energy transfers and can reduce energy consumption.
- Heat exchangers are designed to transfer thermal energy efficiently between fluids.
- Heat sinks and radiators use conductive materials and large surface areas to increase cooling.
- Water's high specific heat capacity makes it useful for cooling and thermal energy storage.
- Evaporation and condensation allow large amounts of energy to be transferred through latent heat.
- Industrial systems can improve efficiency by recovering waste heat.
- Thermal energy drives important natural processes including atmospheric convection, evaporation, condensation and weather.
- Good thermal engineering requires balancing energy performance, cost, safety, durability and environmental impact.
Check Your Understanding
- Why are metals commonly used in saucepan bases?
- Why are saucepan handles often made from plastic or wood?
- Explain how a convection oven increases the rate of energy transfer to food.
- Explain how a refrigerator transfers thermal energy from its interior to the room.
- Why does insulation reduce the energy required to heat or cool a building?
- Give two design features that can increase the effectiveness of a heat exchanger.
- Explain why water is useful in many cooling systems.
- Calculate the energy required to heat 50 kg of water by 20°C. Use c = 4200 J/(kg·°C).
- Explain how evaporation and condensation are used in an air-conditioning system.
- Why do computer heat sinks contain many thin fins?
- Give two ways waste heat from an industrial process could be put to useful purposes.
- A building is overheating because of strong sunlight and poor insulation. Suggest three changes that could reduce the cooling energy required, and explain the thermal principle behind each.