Analysis: here's a look at the engineering that underpins this technology and its potential for heating our homes

Heat pumps have jumped into the headlines a few times this year already. They were in the news when the Government announced new grant subsidies for home retrofits, and when Russia's brutal attack on Ukraine threw European gas supplies into uncertainty.

So what is a heat pump? Here's a look at the engineering that underpins this technology and its potential for heating our homes.

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We’ve been burning things to keep warm ever since our ancestors rubbed sticks together. Wood gave way to coal until coal gave way to oil and gas, all energy-dense fuels that conveniently lie underground in fluid form, so they can simply be pumped from A to B. We now know that the carbon dioxide produced by fossil fuel combustion is driving climate change that could be disastrous for human life as we know it.

But our main way to heat a building is to burn something. When carbon in the fuel combines with oxygen in a flame, one unit of locked-in chemical energy is released as one unit of heat energy. Energy is neither created nor destroyed; it just changes form.

Heat pumps break free of this constraint. Every unit of energy that goes into a heat pump can yield up to five units of heat energy. The other four units aren’t created out of nothing, but drawn from the cold surroundings such as outdoor air, water or the ground.

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This is exactly what a fridge does: it consumes some electrical energy, pulls some heat from its cold interior, and dumps both streams of energy in the form of heat. If you explore down the back of your fridge (I don’t recommend it), you’ll find warm condenser tubes providing a little bit of heat to the room. The only difference between a heat pump and refrigerator is the point of view: the fridge cools its contents and emits some heat as a side-effect, but a heat pump is designed to heat, and refrigerates the outdoors as a side-effect.

Like pumping water up a hill, it pumps heat "up" the temperature gradient, from low temperature outdoors to warm indoors, opposite to the spontaneous direction of flow. Whether pumping water or heat, some energy input is required.

The language evokes a time when heat was thought to be an invisible fluid, namely caloric. Caloric flowed from hot to cold like water down falls, and a steam engine was like a millwheel, somehow driven by this flow to generate motion.

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Sadi Carnot was a 24-year-old military engineer trying to really understand steam engines in the early 19th century and he established our modern ideas of energy with some brilliant thought experiments. Carnot recognised that an engine taps into a heat energy flow to deliver a different kind of energy that underpins motion and forces, energy that we now call "work". He deduced that this process could be reversed: if we put work into a sort of anti-engine, heat can flow from a low-temperature source, though the machine, to a high-temperature destination.

Fast forward to the 21st century. We feed work (as electrical power) into the motor of a heat pump. The motor drives a compressor (very similar to a reversed internal combustion engine) which raises the pressure and temperature of a special-purpose vapour that circulates in the system. The vapour is now the heat source for radiators or underfloor heating. As it does so, it condenses into liquid, and is then driven though a restriction, forcing its pressure to drop.

Because the fluid is at or near its boiling point due to a quirk of thermodynamics, the pressure drop induces a rapid drop in temperature and the fluid begins to boil. It then flows through the evaporator coils, which are positioned outdoors with a fan in typical air-source domestic heat pump. The working fluid is now well below 0°C and the outdoor air is hot enough to boil it before it cycles into the compressor again.

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This transfer of heat from cold outdoor air to much colder refrigerant is the heat that gets "pumped" and then dumped at high temperature into the building heat system. Every unit of electricity that we feed the heat pump with isn’t just converted to heat: it is collecting three, four or five units of additional heat from a cold free source, and gearing it up to high temperature. We are engineering our way beyond the rigid 1:1 economy of just burning whatever we can dig up or cut down.

Of course, there is a snag. Carnot realised that the efficiency of this process is linked to temperature. It’s easier to pump heat from 19°C outdoor air to 20°C indoor air than from 5°C to 20°C. The efficiency of the heat pump declines rapidly as the target temperature rises. Oil-fired central heating gives us radiators almost too hot to touch and heat pumps can do the same, but not efficiently.

As more and more renewable electricity sources come online, heat pumps enable us to decarbonise our heat supply

Heat pumps do their magic best when delivering heat at relatively low temperature. To get the same amount of heat, we need more radiator surface area (or using the floor as a radiator), and continuous operation, instead of short bursts of an hour or two. As a heat pump is a more complex and expensive machine than a gas or oil boiler, it makes economic sense to minimise waste by insulating the building to a high standard. Consequently, much of the cost associated with heat pumps is not in the machine itself, but in preparing the building.

Heating accounts for 44% of Ireland’s energy use and 41% of our CO2 emissions. Heat pumps can revolutionise that sector by delivering a multiple of their input energy. As more and more renewable electricity sources come online, this enables us to decarbonise our heat supply. Fortunately for us in Ireland, it’s usually windy outside when we need heat.

The views expressed here are those of the author and do not represent or reflect the views of RTÉ

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