What is a Heat Pump?
‘Heat pump’ is very much an eco-buzz phrase, and has been for some years now.
Everybody wants one but who really knows why or what a heat pump even does?
We have ground source heat pumps, air source heat pumps and even water source heat pumps.
For a while so was I. It all sounds a bit too good to be true, magic low energy heating, all made possible by this elusive thing called a heat pump.
I regularly mentor students studying architecture at universities and I’m truly inspired by their passion and vigour, yet equally dismayed when it comes to realising the gap between their aspirations and fundamental knowledge about the issues they promote.
And it’s not just students.
I interview graduates and experienced architects regularly, and when the ‘eco’ conversation comes up I ask the question.
Not once has one of them demonstrated that they know what a heat pump actually does.
To be fair, it’s not an easy thing to explain, and it was only through me constantly nagging mechanical engineers that I managed to extract an intuitive explanation that sunk in. Uncannily, it was the same engineer that made me ‘get’ the concept of heat recovery ventilation that I shared in the April issue of
First of all, heat pumps are not new. All refrigeration and air conditioning relies on heat pumps, and the first ever refrigeration system was built way back in 1834. The first commercial ice making machine appeared in 1854, and refrigerators for domestic use in 1913. But when refrigeration and air conditioning is all about cooling, where does the ‘heat’ bit come in? It’s simply about moving heat from one place to another. In the case of a fridge it’s from the air inside the fridge to the air in the room outside.
Basic secondary school chemistry and physics tells you this: When you compress a gas it gets hot, and when you decompress it it gets cold. Take a bicycle tyre as an example. Pump it up and the valve heats up. Let it down and the valve gets cold. Release the contents of an aerosol can and the nozzle gets cold. All the same stuff.
It works like this. Imagine a closed vessel containing some form of gas. Crank up the compressor and the gas bounces around in its confined environment and gets hot. Potentially very hot. Then reverse the cycle by decompressing the gas and it gets cold. The trick is to extract the heat at the compression stage and do something with it. When the gas returns to its natural state it will be cooler than it was when the cycle began. Without meaning to complicate things, heat pumps actually use fluids that evaporate and then condense back to liquid form easily (refrigerants). This makes moving them around more efficient.
Take a ground source heat pump for example. The temperature at a metre below the surface of the Earth is fairly constant at about 10 degrees Celcius. Not exactly hot, but when you consider that absolute zero is minus 273 degrees, in cosmic terms, 10 degrees is a furnace. Then create a loop of pipes that go underground to tap into this 10 degree ‘furnace’, compress the refrigerant to fire it up to 50 degrees or more, suck out as much heat as you can while you can, then let the refrigerant revert to its natural state. Instead of it reverting back to 10 degrees, energy was pillaged during the process so it will revert back to something like 8 degrees.
Then, send it back underground to warm up to 10 degrees, and do it all over again.
Energy efficiency? In order to extract energy from the ground, air or water using a heat pump requires electricity to run the compressor. Typically a ground source heat pump operates at something like 400% efficiency. This means that for every 1kW of electricity used to run the heat pump up to 4kW of heat is recovered. Not bad, but the cost of installing such equipment doesn’t come cheap. Plus, at today’s prices, gas is a quarter of the cost of electricity on a per kW basis which evens things out. This is why government incentives are needed to encourage such investment or it simply wouldn’t happen.
Predictions are that the gap between the price of non-renewable natural gas and potentially renewable electricity will close up, thereby eventually making use of technologies such as this the rule rather than the exception.
So while you’re at it, install some photovoltaic panels to provide electricity to power your heat pump as a form of future proofing.