That feeling of walking into an air conditioned building from the stifling heat outside is a joy of the modern age. The scientific principles that allow air conditioning to remove this heat and humidity are also fascinating. It takes energy to remove heat. This may sound counter-intuitive, but it’s part of the laws of thermodynamics. Moving heat from one area (inside a building) to another (outside the building) requires consuming energy. You can’t do it for nothing. In this post, we will look at two of the fundamental concepts related to how much energy cooling systems must use: sensible heat and latent heat.
Sensible heat is concerned with temperature changes. Let’s say the air outside is 95 degrees and your office building is pumping 55 degree supply air into the occupied spaces. For the purpose of understanding these concepts, it does not matter whether your building has DX units or chillers. In either case, the total sensible heat that the cooling system needs to deliver is the difference between those two temperatures (95-55) or 40 units of cooling. Now let’s say the outdoor air temperature drops to 75 degrees and the building is still pumping 55 degree supply air. Then the sensible heat change the cooling systems needs to provide is simply half as much as was needed before, or 20 units (75-55=20).
Latent heat relates to humidity. It is a measure of the energy stored in the water vapor molecules in the air. Pint for pint, latent heat packs much more punch than sensible heat. The energy needed to heat one gram of water from 80 degrees to 81 degrees is 1 calorie (the common unit of food energy we use is actually based on units of heating up water). But while, it takes just one calorie to heat water by one degree, it takes 540 calories to evaporate one gram of 100 degree water to 100 degree water vapor (steam).
The reason for this enormous difference in energy needs is in the chemistry of molecules. Sensible heat is the energy needed to speed up molecules. Latent heat, on the other hand, is the energy needed to rip apart the bonds holding water molecules in liquid form.
The thermodynamic tool for understanding how much latent and sensible heat a cooling system needs to remove is the Psychrometric chart. Though these charts are complex (for a thorough explanation see here: http://www.powerknot.com/how-to-read-and-use-a-psychrometric-chart.html), at their heart they are simply a map of temperature and humidity.
So which type of heat, sensible or latent, requires more energy to remove? It turns out sensible heat is the winner. Latent heat account for 20-35% of the work a cooling system has to do. This is because the amount of water vapor in our air is very low: 1% of the air at 77 degrees and average humidity. So while there’s tons of energy stored in each unit of water vapor, there are far fewer of them.
In summary, both sensible and latent heat play important roles in cooling systems. When a building is cooled, the energy consumed is not only used to lower the temperature from 95 to 55, but also to remove the energy stored in the evaporated water molecules. And while the age old summertime complaint “it’s not the heat, it’s the humidity” certainly feels true, from an energy perspective, it’s actually the sensible heat, the temperature heat, that takes more work to get rid of.