Heat pumps


COP - Coefficient of Performance

(Heat pump efficiency)

If we use electricity to heat a wire, and then have it heat water, then one joule of electrical energy will produce one joule of warming. Obvious.

By a quirk of thermodynamics, a heat pump does much better. We can put in 1 joule of energy, and get 4 joules of heating. they can be 400% efficient. Counter intuitive.

A heat pump works by compressing a gas, thereby heating it. (We experience this when we feel a bike pump getting hot.) This hot compressed gas, then heats water.

When the gas is allowed to expand again, it become cooler. If heat has been removed from the hot side, then the expanded gas will become cooler than the surrounding air. So  one side is hot and the other is cool. If the cold pipes are in a box, it is a refrigerator. If the hot side is in a box, it is a heater. We call one a refrigerator, the other a heat pump. In fact they are both  heat pumps, but we are stuck with the name.

The graph below shows a heat pump is most efficient when temperature difference is low. (The rich get richer)

COP = Heat gained / energy used

Theoretical COP - A graph of the formula COP = Thot /(Thot - Tcold)    Temperatures are in deg Kelvin or Rankin.

Air source heat pumps

So they work best when they source their heat from warm air. Not only more efficient, but there is leass heat to be pumped. If a heat pump coils are on the warm side of the house and run during the hottest part of the day, the pump will use the least amount of power. But during the day power can cost 4 times as much as off peak. So you may not save money. And the power supply industry still has not solved the problem of peak load.

Heat pumps make a lot of sense if they are powered by a PV system during the day when the air is warm, and the power is being exported for almost nothing. (I'll try not to use the word "theft".)

The pipes on the cool side are the heat collectors. The warmer they are, the more efficient the heating will be. It makes sense to install it in a hot ceiling space, glass house, or engine room, etc.

Ground source heat pump

In a cold climate the air is far colder than the ground, so the heat collecting pipes can be buried deep in the warmer soil or rock.

This is called a ground-source heat pump. Horizontal pipes are laid up to 1.8 metres deep. The other option is to drill several holes straight down to a depth of at least 80 metres. This is the most expensive method but uses only a comparatively small area. If there is some sort of water body available, such as a dam, pond, river or the ocean, this can also be used, but the pipes have to be covered by at least three metres of water.   Ref

COP - practical vs theoretical

In practice heat pumps never reach the theoretical COP. 

Unfortunately, manufacturers often exagerate their claims and quote a COP without any mention of air temperature. It is a meaningless figure. It's like saying a water pump can pump 100 litres of water per min, without saying how far it has to lift it.

REC - STC Small-scale Technology Certificate

The best way to compare heat pumps (and solar systems) is to see how many RECs the government clean energy regulator awards them.  

1 REC (Renewable Energy Certificate) is awarded for 1 MWh of electricity saved over the life of the system.

To find the RECs awarded to a hot water system see:

Register of solar hot water and heat pump systems

(If the website has changed, search the site for "Air source heat pumps, then download the .xlsx file.)

There are no RECs available in colder parts of Australia as heat pumps are not very efficient there. Zone 5 includes Tasmania, great dividing range, Canberra, Orange, etc.

Some of their tables are copied below:

Calculating COP

COP = Heat gained / energy used

Another quirk is that heat pumps are more efficient at heating than cooling, because the work of compressing has to be added to the cooling load.

COP = Heat delivered/Energy used by machine

COP heating = Thot/(Thot -Tcold) Heating air of water

COP cooling = Tcold/(Thot -Tcold) - Refrigeration

Temperature in these calculations are in degrees Kelvin or Rankin (fareheit scale)



Why does compressing heat air?

Temperature and energy in a gas due to movement of molecules. Faster the molecules, the more kinetic energy they have. We feel heat as fast molecules hitting our skin.

If the gas is compressed, such as in a bike pump, we feel the molecules hitting more often, so it feels hotter. So temperature can be increased by simply compressing it.