To mitigate the effects of climate change, as driven by emissions from the burning of fossil fuels, households are increasingly encouraged to switch to alternative, sustainable heating systems, rather than using traditional gas and oil boilers. Examples of renewable energy sources include biomass burners, solar panels and wind turbines. All of the above are fairly self-explanatory, but what of the humble heat pump: that most enigmatic and misunderstood of sustainable energy sources? 

Heat pumps are generally designed to transfer thermal energy from a cooler outdoor environment to a warmer indoor space, which, on the face of it, appears to contravene the second law of thermodynamics (the law which states that heat cannot spontaneously move from a cooler region to a warmer region). Of course, this is impossible and, to put it bluntly, since there's no such thing as a free lunch, work needs to be done for a heat pump to function.

Essentially, a heat pump is like a refrigerator only, instead of keeping food fresh, we want to further cool down the “outside” and transfer the resultant thermal energy to the warmer “inside”. The price we pay for this flagrant breach of the laws of physics is the work done by a compressor, an integral part of the process. In a typical air-to-air heat pump, a refrigerant fluid is passed through an external heat exchanger - usually a series of coils to maximise surface area. With a suitably low boiling point, this fluid will evaporate, absorbing thermal energy from the outside air. The refrigerant vapour now passes into a condenser via the aforementioned compressor which adiabatically forces the gas, under pressure, into another series of coils, where thermal energy is now transferred to the indoor air reservoir, as the refrigerant condenses back to liquid form. 

In the same way as the air in, for example, a bicycle pump heats up as it increases the air pressure in a tyre, the work done by the compressor further adds to the thermal energy that is transferred to the interior of the building: a win-win situation! After condensing, the liquid refrigerant passes through an expansion valve back to the evaporator coils in the colder air reservoir and the whole process continues. In an efficient system, the heat delivered to the interior, from the exterior, should exceed the work done by the compressor, thus giving a high coefficient of performance.

In some senses, a heat pump is the opposite of a heat engine (using work to transfer heat, as opposed to using heat to do work). However, whereas the efficiency of a heat engine is compromised by energy losses to the environment, a heat pump actually exploits that very consequence of thermodynamics, turning it to an advantage by warming the interior space. Of course, assuming that the compressor is running on electricity, it would be prudent to ensure that the domestic electricity supply itself is from a renewable source. 

Heat pumps can transfer thermal energy from air to air as above, but they can also move heat from air to water, water to air, ground to water, and so on. They are not only effective in heating single dwellings but can be installed to heat entire blocks of homes or even factories, and they continue to work even in extremely cold weather!

Before leaving the subject, it might be interesting to examine a philosophical ramification of these miraculous devices. For instance, were you able to observe a heat pump in operation but, for some reason, be unaware of the compressor, it would be akin to watching a film in reverse. This is because the second law of thermodynamics, which is being subverted by virtue of the unseen compressor, is indicative of the direction of time itself, in a way that other physical processes aren't. Jigsaw puzzles don't spontaneously put themselves together in the box, and broken plates don't suddenly come together whilst levitating toward waiters’ trays in restaurants. Another iteration of the second law of thermodynamics states that no process can lead to an overall reduction in the entropy of the Universe. Entropy is a measure of disorder, an increase of which is an inevitable result of doing work, as the resultant heat is produced at the expense of reusable energy.

​​​​​​​It's sobering to think that the very act of warming your home with a heat pump, or in fact simply being alive, is contributing ever so slightly to what cosmologists call the “heat death” of the Universe. But there's no need for guilt; the Universe won't achieve maximum entropy for many trillions of trillions of years... and, meanwhile, your heat pump will in the short term be helping to save the planet!

Images

Heat pump image: IntelligentVisualDesing from Pixabay.

Summer cooling and winter heating: HeatAndColdStorageWithHeatPump.jpg: KVDPderivative work: Fred the Oyster, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=9893432

Lego photo: Rick Mason on Unsplash