D. Energy and Society · Thermal and Nuclear EnergyHeat Pumps, Refrigerators and Thermal Technologies
D1.1 D1.2 D3.8 D2.1 A1.3 A1.9About 70 minutes
Learning goals- I can explain how a refrigerator or heat pump moves heat from a cold place to a warm place, using changes of state.
- I can describe the energy transformations in a heat pump and calculate its coefficient of performance.
- I can compare air-source and ground-source heat pumps with other ways of heating a home.
- I can assess the social and environmental costs and benefits of a thermal technology.
Words to know
refrigerant
A fluid that evaporates at a low temperature and is cycled through a refrigerator or heat pump to carry heat.
evaporator / condenser
Coils where the refrigerant evaporates (absorbing heat) / condenses (releasing heat).
compressor
A pump that uses electrical energy to squeeze refrigerant gas, raising its pressure and temperature.
heat pump
A device that uses work to move heat from a colder place to a warmer place, to heat (or cool) a building.
coefficient of performance (COP)
For a heat pump in heating mode: heat delivered ÷ electrical energy used. A COP of 3 means 3 J of heat for every 1 J of electricity.
ground-source (geothermal) heat pump
A heat pump that exchanges heat with the ground through buried loops of pipe.
A machine that pumps heat uphill
Your fridge is colder inside than your kitchen. Yet heat, on its own, always flows from warm to cold, so kitchen heat is constantly leaking into the fridge. To keep the inside cold, the fridge must keep pushing that heat back out, from cold to warm. That is like pumping water uphill: it takes energy.
Put your hand near the back or bottom of a fridge and you will feel the heat it is pumping out. More and more Canadian homes now use the same trick in reverse: a heat pump pulls heat out of the cold outdoor air or ground in winter and pumps it into the house.
The refrigeration cycle
A refrigerator or heat pump uses a refrigerant, a fluid chosen because it boils at a very low temperature. The refrigerant goes around a closed loop and changes state twice. It uses the latent heat you studied in Changes of State and Latent Heat.
- Evaporator (cold side): cold, low-pressure liquid refrigerant flows through coils on the cold side. It is colder than its surroundings, so heat flows into it, and it evaporates, absorbing latent heat of vaporization. This takes heat Q_cold out of the fridge (or out of the outdoor air, for a heat pump).
- Compressor: an electric pump squeezes the gas. Doing work W on it raises its pressure and its temperature until it is hotter than the warm side.
- Condenser (warm side): the hot gas flows through coils on the warm side. It is hotter than its surroundings, so heat flows out of it and it condenses, releasing latent heat. Heat Q_hot goes into the kitchen (or into the house).
- Expansion valve: the high-pressure liquid passes through a narrow valve, its pressure drops suddenly, and it becomes cold again, ready to repeat the cycle.
Energy is conserved, so the heat released on the warm side is the heat collected on the cold side plus the work done by the compressor:
Q_hot = Q_cold + W
The laws behind it
Two big ideas from thermodynamics are at work:
- Energy is conserved (the first law of thermodynamics): Q_hot = Q_cold + W. No energy is created; it is moved and transformed.
- Heat flows naturally only from hot to cold (a form of the second law of thermodynamics). To move heat from cold to hot, something must do work. That is why your fridge has to plug in.
You will use these same two laws in The CANDU Reactor: Inside a Nuclear Power Plant, where heat flows the other way, from a hot reactor to cool lake water, and some of it is turned into work along the way.
Heat pumps for homes
A heat pump for a house is a refrigerator turned inside out: the evaporator is outdoors and the condenser is indoors. In summer, valves reverse the flow and it works as an air conditioner.
How good is a heat pump? We compare the useful heat it delivers to the electrical energy it uses:
COP = Q_hot ÷ W
An electric baseboard heater turns 1 J of electricity into 1 J of heat (COP = 1). A heat pump also delivers the heat it pulls from outside, so its COP is usually greater than 1, often around 2 to 4. The COP gets lower as the outdoor temperature drops, because the pump has to move heat across a bigger temperature difference. Older air-source heat pumps needed a backup heater on very cold days; newer cold-climate models are designed to keep working at much lower outdoor temperatures.
Ground-source (geothermal) heat pumps collect heat from the ground instead of the air, through loops of plastic pipe buried in trenches or in deep vertical boreholes. A water-and-antifreeze solution circulates through the loops. A few metres down, the ground's temperature stays fairly steady all year, warmer than winter air and cooler than summer air, so a ground-source heat pump keeps a good COP even in January.
Worked exampleExample 1: Coefficient of performance
In one hour, a heat pump uses 3.6 × 10⁶ J (1 kWh) of electrical energy and delivers 1.08 × 10⁷ J of heat to a house. (a) Find its COP. (b) How much heat did it collect from outdoors? (c) How much electrical energy would a baseboard heater need to deliver the same heat?
- (a) COP = Q_hot ÷ W = 1.08 × 10⁷ ÷ 3.6 × 10⁶ = 3.0.
- (b) Q_cold = Q_hot − W = 1.08 × 10⁷ − 3.6 × 10⁶ = 7.2 × 10⁶ J. Two-thirds of the heat came from outdoors, free.
- (c) A baseboard heater has COP = 1, so it would need the full 1.08 × 10⁷ J (3 kWh).
COP = 3.0; 7.2 × 10⁶ J came from outdoors; a baseboard heater would use three times as much electricity.
With a COP of 3, a heat pump buys one unit of electrical energy and moves two more units of heat in from outside.Worked exampleExample 2: Analysing a technology (D1.1)
Analyse a ground-source heat pump for a home in rural Ontario using energy-transformation principles, and list one benefit and one risk.
- Energy input: electrical energy to run the compressor and the loop's circulating pump.
- Transfers and transformations: thermal energy flows from the ground into the cold loop fluid (conduction), is carried to the house (forced convection), is absorbed by evaporating refrigerant, is raised to a higher temperature by the compressor's work, and is released in the condenser to the house's air or water.
- Useful output: heat delivered to the house. Losses: heat from the motor, some heat lost through ducts and pipes.
- Benefit: with a COP around 3 or more, it uses much less purchased energy than electric resistance heating, and it burns no fuel on site, so it can lower greenhouse gas emissions compared with a fuel-burning furnace, depending on how the electricity is generated.
- Risk / cost: installation is expensive (drilling or trenching), and if a buried loop leaks, its antifreeze solution could contaminate soil or groundwater. That is why installers follow provincial rules and standards and often choose less-toxic antifreeze solutions.
A ground-source heat pump moves ground heat into the house using electrical work; it saves energy and emissions but costs more up front and needs care to protect groundwater.
Other thermal technologies worth knowing
- Green roofs (plants growing on a roof) add insulation, and the evaporation of water from plants and soil absorbs latent heat, so the roof stays cooler in summer.
- Passive solar design: large south-facing windows let in sunlight in winter; thick floors of concrete or stone store the energy (high heat capacity) and release it at night.
- Fast-freezing in the cold chain: many frozen foods are frozen very quickly with very cold air or liquid nitrogen. Fast freezing forms smaller ice crystals, which do less damage to food cells, so the texture is better after thawing.
- Laser printers use heat: a heated roller melts toner powder onto the paper.
Just for funYour fridge doesn't make "cold"; it moves heat. That means if you leave the fridge door open on a hot day hoping to cool the kitchen, the kitchen actually gets a little warmer: the heat just goes around in a loop, and the compressor's work is added to the room as extra heat.
Hands-on activityHeat-pump detective and home energy audit
Grown-up note: do not move the fridge, open any panels or touch fans, wiring or the outdoor unit. Only look and hold a hand nearby.
- The fridge. Hold your hand near the back, bottom grille or sides of your fridge while it is running (you will hear the compressor hum). Then touch the inside wall of the freezer. Where is heat being absorbed, and where is it released? Draw the refrigeration cycle for your fridge and label where Q_cold, W and Q_hot are.
- Time the cycle. Listen for when the compressor turns on and off. Time three on-periods and three off-periods. Predict how they would change if the door were opened often, or if the kitchen were hotter. (If you can, test it on a hot day.)
- Home heating audit. Find out how your home is heated (furnace, baseboards, heat pump, boiler, wood stove...). With a grown-up, find the fuel or energy source and describe the energy transformations from source to warm room. Note one place heat escapes (windows, doors, attic) and one improvement.
- Research. Using two reliable sources (for example, Natural Resources Canada or a provincial energy agency), find the typical COP of an air-source heat pump and one cost or concern people raise about heat pumps. Record your sources.
What to look for: warm air near the fridge's grille or sides (Q_hot), a cold freezer (Q_cold), and a compressor that runs longer when more heat leaks in. In the audit, clear source → transformation → use → loss steps.
Your turn
1In the refrigeration cycle, where does the refrigerant absorb heat, and by what change of state?
In the evaporator (cold side), by evaporating (absorbing latent heat of vaporization).
2What is the job of the compressor?
It does work on the refrigerant gas, raising its pressure and temperature so that it is hotter than the warm side and can release heat there.
3A heat pump delivers 9.0 × 10⁶ J of heat using 3.0 × 10⁶ J of electrical energy. Find its COP.
COP = 9.0 × 10⁶ ÷ 3.0 × 10⁶ = 3.0.
4For the heat pump in the last question, how much heat came from outdoors?
Q_cold = 9.0 × 10⁶ − 3.0 × 10⁶ = 6.0 × 10⁶ J.
5Why does a refrigerator have to be plugged in, if it just moves heat?
Heat flows naturally only from hot to cold. Moving heat from the cold inside to the warm kitchen requires work (the second law of thermodynamics).
6Why does a ground-source heat pump usually have a better COP in January than an air-source heat pump?
In January the ground a few metres down is much warmer than the outdoor air, so the heat pump moves heat across a smaller temperature difference, which takes less work per joule of heat.
7Give one environmental concern about ground-source heat pump loops.
If a loop leaks, the antifreeze solution could contaminate soil or groundwater. Drilling and trenching also disturb land.
More practice: warm-up
1Name the four main parts of a refrigeration cycle.
Evaporator, compressor, condenser, expansion valve.
2What is the COP of an electric baseboard heater?
1 (1 J of heat for each joule of electricity).
3In a heat pump heating a house, is the condenser indoors or outdoors?
Indoors (it releases heat into the house).
4Write the energy equation for a heat pump.
5What change of state happens in the condenser?
Condensation (gas → liquid), releasing latent heat.
More practice: on your own
1A heat pump with a COP of 2.5 must deliver 5.0 × 10⁷ J of heat in a day. How much electrical energy does it use?
W = Q_hot ÷ COP = 5.0 × 10⁷ ÷ 2.5 = 2.0 × 10⁷ J.
2Explain why a heat pump's COP decreases as the outdoor temperature drops.
It must move heat across a larger temperature difference (from colder air into a warm house), which takes more work per joule of heat delivered.
3Why do refrigerants need to boil at very low temperatures?
They must evaporate in the evaporator at a temperature colder than the space being cooled, so that heat flows into them.
4A green roof helps keep a building cool in summer. Name two heat-transfer effects involved.
Soil and plants add insulation (slowing conduction), and evaporation of water from leaves and soil absorbs latent heat. Plants also shade the roof from direct radiation.
5Suppose electricity costs 15¢ per kWh (a value chosen for this question). How much would it cost to deliver 30 kWh of heat with a baseboard heater and with a heat pump of COP 3?
Baseboard: 30 kWh × $0.15 = $4.50. Heat pump: 30 ÷ 3 = 10 kWh × $0.15 = $1.50.
Challenge
1A freezer removes 2.0 × 10⁵ J of heat from food. Its compressor does 8.0 × 10⁴ J of work. How much heat is released into the kitchen? Why is this more than the heat removed from the food?
Q_hot = Q_cold + W = 2.0 × 10⁵ + 8.0 × 10⁴ = 2.8 × 10⁵ J. It is more because the compressor's electrical work also ends up as heat on the warm side (energy is conserved).
2Fast-freezing makes smaller ice crystals in food than slow freezing. Using what you know about freezing, explain why rapid heat removal matters for the cold chain.
When heat is removed very quickly, the water in the food freezes before large crystals have time to grow. Many tiny crystals form instead, which damage fewer cell walls, so the food keeps its texture and quality after thawing. Removing heat quickly means removing the latent heat of fusion quickly, which needs very cold air or liquid nitrogen.
3A family is choosing between replacing an old gas furnace with a new high-efficiency gas furnace or with an air-source heat pump. List two questions about energy and two about society or the environment that they should research before deciding.
Energy: What COP does the heat pump have at our coldest winter temperatures? How much electricity and gas will each use per year? Society/environment: What are the up-front and running costs, and are there rebates? What greenhouse gases are produced by each option, including how our province generates electricity? (Other reasonable questions: noise, backup heating, lifespan, refrigerant leaks.)
Quiz
1. In a refrigerator, heat is absorbed from the food compartment when the refrigerant:
2. Which statement is a form of the second law of thermodynamics?
3. A heat pump delivers 1.2 × 10⁷ J using 4.0 × 10⁶ J of electricity. Its COP is:
4. For the heat pump in the previous question, how much heat came from outdoors?
5. Why do ground-source heat pumps keep a good COP in winter?
6. Which is a genuine environmental risk of a ground-source heat pump?
Big idea: expectation D1.1 asks students to analyse a technology that uses thermal energy transfer, and D1.2 to assess the social and environmental effects of thermal and geothermal technologies (the ministry's sample issue is exactly ground-source heat pumps: lower heating costs versus the risk of buried antifreeze tubing).
How to help: Do the fridge and home-heating audit together. Talk about how your own home is heated and what changes your family has considered. Encourage your teen to back up claims with reliable sources (government energy agencies, utilities) rather than ads.
Common difficulties:
- Thinking a fridge "makes cold". It moves heat out.
- Thinking a COP above 1 breaks conservation of energy. It does not: the extra heat comes from outdoors (Q_hot = Q_cold + W).
- Mixing up which coil is indoors and outdoors when a heat pump heats versus cools.
Safety: in the activity, only look and hold a hand near the fridge. Do not move appliances or open panels.