The world spends about 15% of its energy on heat. Most of that comes from burning fossil fuels in homes and businesses. Cutting emissions from space heating remains one of the biggest challenges if we are to reach zero emissions and avoid catastrophic climate change.

A humble solution to the problem is the heat pump, which was invented in 1856 and first commercialized in 1945. Heat pumps, which are powered by electricity, never became widely used because it’s usually cheaper to burn fossil fuels for indoor climate control.

That is now changing slowly. In 2017, global sales of heat pumps jumped by 30%, to nearly $12 billion.

In its latest World Energy Investment report, the International Energy Agency (IEA) notes that the electricity load of new heat pumps (about 3.75 GW) in 2017 exceeded that for new electric cars. At the same time, the average cost of heat pump installations fell by about 8%.

A heat pump is a refrigerator, but in reverse. That idea may be a little confusing at first, but it’s simple physics.

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A fridge works by using a refrigerant (usually a chemical) in a closed-loop system. The refrigerant starts its life as a gas (or vapor), and electricity is used to run a compressor and condense the gas into a liquid. Physics dictates that when gases are compressed, they release heat. In a refrigerator, that heat is directed outside (usually into your kitchen). The liquid then runs through an expansion valve, where it becomes gas again, and that expansion absorbs heat. In the process, this cools the inside of the refrigerator. The now-warm refrigerant gas is then sent back into the compressor and the cycle repeats.

The heat pump does this in reverse. The heat from the condenser is directed into the house and the evaporator absorbs heat from the outside. And because it uses ambient heat from outside, it produces three units of heat for every unit of electricity consumed. (Electric heaters can, at best, produce one unit of heat for every unit of electricity.)

The major reason for the jump in heat-pump installations in recent years is policy change. As countries look to cut emissions, they are turning to electricity over fossil fuels for heating.

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