How Heat Pumps work

Andrew on January 25, 2022

Heat pumps are common & efficient

Electric heat pumps are the essential part of standard household equipment:

  • In fridges, a heat pump moves heat from inside the fridge into your kitchen.
  • In reverse cycle air-conditioners, the heat pump cools a room by moving heat from the room to outside the house. When you reverse the cycle, the heat pump heats the room by transferring heat from outside your home to inside the room.

Heat pumps use electricity to run a fan and compressor – and can be amazingly efficient. A high-performance residential heat pump can use one unit of electrical energy to move six times as much heat energy into your house, an efficiency of 600%.

By comparison:

  • Electrical heaters have an efficiency of 100% as they can convert all the electrical energy into heat, and
  • Gas heaters have an efficiency of between 50% and 95%, as some of the heat generated heats the house exterior.

A Heat pump hot water service

One use of a heat pump is to heat your hot water. You can run the heat pump when the sun is up and use electricity from photovoltaic solar panels to power the heat pump. After installing the panels and heat pump, you heat your water for free, generating no emissions. 

The heat pump heats your hot water by moving heat from the air outside your house into your hot water tank. It uses a refrigerant, a liquid that boils at a low temperature like minus 26 C and works in a cycle as follows.

(The numbered points in the following text further explain the below diagram.)

  1. In the heat pump evaporator, the refrigerant is intially a liquid at low pressure.
  2. Boiling the refrigerant: An electric fan warms the refrigerant by blowing air from outside the house over a heat exchanger, which transfers the heat to the refrigerant. While the air temperature is above minus 26 C, it will heat and boil the liquid refrigerant. The refrigerant evaporates like water becomes steam in a pot on the stove. The cold air of a freezing day will still boil the refrigerant because boiling point of the refrigerant is so low. However, on a cold day, the system is less efficient as the fan must work longer to boil the refrigerant.
  3. In the compressor, the refrigerant is initially a cool gas at low pressure.
  4. Compressing the refrigerant gas: A valve closes, shutting the refrigerant gas in the compressor where an electric pump compresses the gas. As the pressure builds, the gas gets hotter, just as a bicycle pump gets hot as you pump up the tyres. The gas can heat to 95 C.
  5. In the condensor, the refrigerant gas is initially hot and at high pressure.
  6. Producing hot water & cooling the refrigerant: The hot, pressurised gas then passes its heat, via another heat exchanger, to where you want it, to the water in your hot water tank. In doing this, the refrigerant gas cools and condenses. The refrigerant is under pressue so the condensing / boiling point is higher than it was at room pressure in the evaporator.
  7. Pressurised Liquid: Now, this refrigerant is a warm liquid, still under high presssure.
  8. Dropping the pressure: The expansion valve opens and the pressure drops, causing the liquid refrigerant to vapourise. It mixes with the liquid refrigerant at room pressure in the evapourator.
  9. Liquid refrigerant at room pressure: The refrigerant is back where it started in the evaporator, ready to repeat the heat harvesting cycle shown below.

The Heat pump cycle

Refrigerant liquid under low pressure.
(1) & (9)		 Evaporator: Fan blown air boils the refrigerant (2)
Expansion valve: Release the pressure (8)Refrigerant gas, cool & at low-pressure (3)
Refrigerant warm liquid at high pressure (7)Compressor: Pump pressurises the refrigerant gas (4)
Condenser: Producing hot water cools the refrigerant (6) Refrigerant gas at high temperature & high-pressure (5)

Further information

Updated: 29 Jan 2022

Australia’s strong sun

Andrew on April 30, 2018

Australia could become a renewable energy superpower.  One factor behind this is that Australia gets stronger sun than most developed countries.

Two world maps superimposed

Here is a strange map that demonstrates this.
http://www.dailymail.co.uk/sciencetech/article-4707544/Antipodes-Map-lets-dig-virtual-tunnel-Earth.html

It is two maps superimposed on one-another. The first is a normal map of the world.  The second map shows, for each point on the first map, where you would be if you drilled straight down through the centre of the Earth to the other side.

The closer a place is to the equator: (1) the more the sun is directly overhead, (2) the more sunshine it gets, (3) the more electricity is generated from each solar panel, and (4) the cheaper it is for that place to generate electricity from the sun.  This is ignoring other factors like how cloudy a place is.

From the map you can see that:

  • Australia is closer to the equator than the developed countries in northern Europe, northern Asia and northern America.
  • Northern Greenland, Canada, Alaska and Russia are as far from the equator as northern Antarctica
  • Melbourne is as far from the equator as southern Spain, so most of Australia gets stronger sun than Spain and most of Europe.
  • Northern Australia is as far from the equator as the border between the Sudan and Egypt.
  • The southern border of the USA is about the same distance from the equator as Port Macquarie (half way between Sydney and Brisbane). So northern Australia gets more sun than the south of the USA.

Manchester in England is as far north as Macquarie island is south

Here is another way of understanding how strong Australian sun is, compared to Europe.  People often think of Macquarie Island, which lies far south of New Zealand, as being in the Antarctic.  Well Manchester in England is as far north (latitude 54 degrees north) as Macquarie island is south (latitude 55 degrees south).  Northern Europe gets very weak sun and if it were not for the Gulf Stream, much of Europe would be very cold.

Australia has quality solar resources

So, considering only the factor of sun strength (closeness to the equator), Australia has better solar resources than most developed countries – and we have other advantages too, which mean that Australia could become a renewable energy superpower.