Heat Pump Magic

Thermodynamic Heat Crime

“Setting fire to chemicals like gas should be made a thermodynamic crime. If people want heat they should be forced to get it from heat pumps. That would be a sensible piece of legislation.” — Professor David MacKay (source)

Burning things is now a heatcrime. Doubleplus ungood!

Never fear. An upcoming alternative in order to heat water and rooms in households in colder climates is the Heat Pump. It’s not that heat pumps are a particularly new technology, they just haven’t ever been particularly popular. People prefer to burn things in order to heat their homes, because this makes sense. Humans have been burning things since the stone-age in order to keep warm, so it’s something that is easily understood and everyone is comfortable with.

Air source heat pumps take heat from cold air outside and use it to warm the inside of your house.

Read that again. It makes no sense. Especially considering that certain heat pumps can operate at outside temperatures as low as -20°C, and heat water inside to 90°C. We are used to hot things heating cooler things. If the room is cold, we turn on a radiator. The radiator is hotter than the air around it, and it is this difference in temperature that slowly heats the room. Heat pumps use something cold to make something warm. Insane science fiction! Surely?

Heat Pump Magic Demystified

  • You already have a heat pump in your house. You call it a fridge.

Refrigerate!

It’s in your kitchen, purring away as it keeps your food fresh and cool. When you first switch on your fridge the temperature inside the fridge is the same as the temperature inside the kitchen. As the fridge runs the temperature inside the fridge starts to drop. The heat is pumped out of the fridge into the kitchen.

Have you felt the back of a fridge? It is quite a warm place. Although the back of the fridge is warm, the temperature in the kitchen doesn’t increase noticeably. The heat flows into the kitchen, but because the kitchen is so much bigger than the inside of the fridge, it doesn’t make a significant difference.

We are comfortable with fridges. Not because humankind has had fridges since the stone-age, but because they’ve been around as long as anyone has been alive. We don’t question how they work, but actually fridges don’t make any sense either. They are taking heat from air inside the fridge to cool the inside the fridge which also warms the inside of your kitchen (but not enough to notice).

The secret to what is going on with fridges is explained with this thought experiment.

Imagine an electric kettle filled with water. Using electricity, the element in the kettle adds heat to the water, raising the temperature. After enough heat is added the water boils. At sea-level the atmospheric pressure is 101.3 kPa and the boiling temperature is 100°C.

Now imagine if you could somehow boil the water without heating it. You don’t turn on the kettle. It just starts boiling the water, seemingly spontaneously. What would happen then? Instead of heat being added to the water, it would leave the water, cooling it down and dropping the temperature. The kettle wouldn’t get hot, it would get cold.

Can we make something boil without heating it? Absolutely. It doesn’t happen spontaneously though. It requires a drop in pressure. A drop in pressure causes the liquid to boil and the temperature to drop. This is demonstrated in this video.

That’s basically how the fridge gets cold. Instead of water, the refrigerant inside the pipework of the fridge evaporated, cooling itself and its surroundings inside the fridge.

Unlike in the video above, the fridge doesn’t throw the gas away. It sends it through the pipework to the compressor. The compressor takes the gas and squashes it, increasing the pressure. Like blocking the outlet of a bicycle pump while pushing down on the plunger, the increase in pressure raises the refrigerant’s temperature.  It gets hot.

The hot refrigerant gas is then condensed into a liquid, causing the liquid to lose its heat. The heat has to go somewhere, so it passes into the kitchen via the pipes at the back of the fridge.

The refrigerant is now back where it started, as a warm liquid, ready to be evaporated again to cool the inside of the fridge.

  • Get a really big fridge. Heat your home with it and call it a heat pump.

With a fridge, the goal is to cool the inside of the fridge. It doesn’t heat the kitchen much because it isn’t designed to do that.

With a heat pump, the goal is to heat the inside of the house. By carefully selecting the refrigerant, operating pressure and temperatures, it is possible to design the heat pump to sufficiently warm a home during winter. It doesn’t cool the outside much (it does a little), because it isn’t designed to do that.

For more on principles and equipment used in refrigeration, see HowStuffWorks or my own explanation.

Isn’t CO2 bad?

You might be wondering about CO2 and greenhouse gas emissions. The Eco Cute CO2 air supply heat pump [pdf] claims to reduce CO2 emissions, but how is that possible if it uses CO2?

I’ll explore this issue in next Thursday’s edition.