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Everything you need to know about the wild world of heat pumps

We’re entering the era of the heat pump.

The concept behind heat pumps is simple: powered by electricity, they move heat around to either cool or heat buildings. It’s not a new idea—they were invented in the 1850s and have been used in homes since the 1960s. But all of a sudden, they’ve become the hottest home appliance, shoved into the spotlight by the potential for cost savings and climate benefits, as well as by recent policy incentives.

Simple though the basic idea may be, the details of how heat pumps work are fascinating. In the name of controlling your home’s temperature, this device can almost seem to break the laws of physics. Heat pumps are also getting better: new models are more efficient and better able to handle cold weather.

So let’s dive in and uncover what makes a heat pump tick.

How does a heat pump work?

At a high level, a heat pump gathers heat from one place and puts it in another place. We’ll mostly talk about heat pumps in the context of heating, but they can also be used for cooling, gathering heat from inside and sending it outside like an air conditioner. Many heat pumps can actually be run in reverse, either heating or cooling depending on what’s needed.

The hero in a heat pump is the refrigerant: a fluid that moves in a circuit, soaking up and releasing heat as it goes. Electricity powers the system, pushing the refrigerant around the cycle.

As the refrigerant moves through the heat pump, it’s compressed and expanded, switching between liquid and gas forms to allow it to gather and release heat at different points in the cycle. (If this is enough detail for you, feel free to skip to the next question. Otherwise, join me on a journey inside a heat pump to understand how this all works.)


INTERNATIONAL ENERGY AGENCY

Picture this: it’s a chilly winter day, say 25 °F (-5 °C). You’re sitting on the couch in your living room with a good book, and your cat is curled up nearby. You look over at the thermostat, which is set to 68 °F. Sensible, but a little chilly. You walk over and bump it up a bit, to 70 °F.

Your heat pump has been quietly humming along in the background. Now it kicks things up a notch to raise the temperature: the fan and compressor inside speed up, and the refrigerant starts moving faster to transfer more heat from outside to inside.

It may seem counterintuitive to collect heat from outside when it’s so cold out, so let’s follow the refrigerant for one cycle to see how it works. For most heat pumps, the trip takes just a few minutes.

Heat pump refrigerants have very low boiling points, typically below -15 °F (-25 °C). So at the beginning of our journey, the refrigerant is around that temperature, and in liquid form. Even in the coldest places, a refrigerant in this state is usually significantly colder than the outside air (in our case, more than 40 degrees colder).

In the first stage of its trek, the refrigerant flows through a heat exchanger, past that outside air and warms up enough to start boiling, changing from a liquid to a gas.

The second phase of its journey is a trip through the compressor. The compressor squeezes the refrigerant into a smaller volume, increasing its pressure and boiling point (this will become important in a minute). This also warms it further, so by the time the refrigerant is past the compressor, it’s warmer than the room indoors.

The third leg of the refrigerant’s journey takes it through another heat exchanger. But by now, the refrigerant is a warm gas, above 100 °F, and it’s flowing past a relatively colder room. As it transfers some of that heat into the room with the help of a fan, it starts turning back into a liquid.

Finally, in the fourth stage, the liquid refrigerant will go through an expansion valve, releasing the pressure. Just as squeezing a material heats it up, expanding it allows it to cool down again, so now the liquid is back to a low temperature and ready to absorb more heat to bring inside.

Do heat pumps work in the cold?

The claim that heat pumps don’t work well in really cold weather is often repeated by fossil-fuel companies, which have a

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By: Casey Crownhart
Title: Everything you need to know about the wild world of heat pumps
Sourced From: www.technologyreview.com/2023/02/14/1068582/everything-you-need-to-know-about-heat-pumps/
Published Date: Tue, 14 Feb 2023 19:59:16 +0000

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