For the engineers and techs in the energy & sustainability sector, I am here for you.
This is your bread and butter!
And I hope to draw it for you with uncommon beauty.
What is a Chiller Plant?
So.. air conditioning takes up about 50% of energy consumption in our commercial buildings.
The cold is costly!
The energy which powers Air Conditioning simply moves machines that throw human heat outside of a building.
This sounds simple, but actually involves a long chain of machines, passing the heat to each other.
This magical process works mainly because of one principle:
Hot things transfer heat to cold things.
In a building, this happens a few times:
- Your muscles and organs (37°C) heat your skin (36°C) in your body.
- Your skin (36°C) heats the supply air (18°C) from your air-conditioning, making return air (28°C).
- The return air (28°C) heats a chilled water supply (7°C) in the aircon unit to a chilled water return (12°C) .
- The chilled water (12°C) heats a very cold refrigerant (5°C) in the chiller’s evaporator, to 6°C.
- The refrigerant is compressed (6°C -> 50°C) into a high pressure vapor in the chiller’s compressor.
- The hot refrigerant (50°C) heats condenser water (30°C) in the chiller’s condenser to 35°C.
- The condenser water (35°C) heats the ambient air (29°C) in the cooling tower…
- … which is where all our heat goes! The ambient. The sky. The outside.
Is this bad for the environment?
Technically, without air conditioning, we would walk around heating the air anyway.
What these machines do is essentially increasing the gradient between hot and cold.
That’s no problem, right?
The thing is, not all machines are efficient – they produce their own heat while working.
And the energy they consume from power plants.. requires more heat to generate too! Ideally, 100% of the electricity we supply to machines is converted into the useful work we use the machines for. However, some of this heat is inevitably converted to heat and sound produced by friction between mechanical parts and electrical resistance.
The more machinery we run to operate an air-conditioning system, the more heat is generated in the world. Sounds kinda funny, but we don’t really feel the effects of this when we’re chilling indoors!
On to the machines themselves!
1. The Air Cycle (Air Handling Units)
- Humans do stuff. Their muscles and organs get warm, and this heat is transferred to their skin, which is transferred to the air around their skin.
- This hot membrane of air transfers heat all over the air! Hotter air particles have more kinetic energy, and jiggle faster than their cold neighbouring particles. These hotter particles rise above their inactive neighbours who remain relatively still (AKA Meritocracy). At the ceiling of a room, a fan sucks in these hotties, blowing them past a very cold chilled water coil inside an Air Handling Unit (AHU). This coil’s water is at about 7°C. That’s cold!
- The hot jiggly particles collide against this coil, losing their jiggliness to the coil surface. The cold water inside the coil takes gets heated from 7°C to 12°C. Meanwhile, the air is cooled from ~28°C to about 18°C. The resulting average room temperature is somewhere around 24°C, which is the temperature your thermostat tries to maintain.
- The cycle repeats. Cold air hits human, human heats air, hot air heats water, hot air becomes cold. Ugga bugga.
2. The Water Loop
- Chilled water just took some heat, and needs to drop it quickly. It flows through the Chilled Water Return pipes to the chiller plant, where it will be cooled back to 7°C again, by the chiller.
- Chillers have a simple function – Make chilled water cold again.
- They do this by passing the 12°C water through super-cold refrigerant fluid, which is about 5°C in the evaporator.
- After the 12°C water transfers its heat to the refrigerant, it leaves the chiller at 7°C, on its merry way back to the AHUs.
3. The Refrigerant Loop
- Air passed heat to water. Water passed heat to refrigerant. Refrigerants are fluids with ideal properties for containing and releasing energy rapidly, but I’ll be skipping those details for now (low boiling temperature! Rapid phase-change material!),but in summary is a fluid that can be very cold at room temperature. This enables it to be the ‘colder thing’ which the already-very-cold 12°C water can give its heat to.
- Once the refrigerant takes this heat, it is compressed into a hot, high-pressure vapor, which enables it to now be the ‘hot thing’ to transfer heat to another water loop, the condensor water loop.
The whole operation of a chiller looks like this:
Which I hope makes sense of this!
4. The Condenser Water Loop – Throwing it all out
- In the chiller’s condenser, hot refrigerant now passes its heat to water (Yes, a separate circuit). This water goes to a cooling tower, those huge grilled-boxes you see on top of many buildings in Singapore.
- Cooling towers do exactly what you do to cool soup – blow air over it. The hot water is trickled down the serrated walls of metal, and air is sucked through these walls by a huge fan in the centre of the box’s ceiling.
- The water that collects at the bottom is at about outdoor temperature, where it is pumped back to the chiller to be heated again.
- This stage is called Heat Rejection, the final stage of any air conditioning cycle. It is where the heat of hundreds of warm humans is amalgamated and tossed back out into the air, where it is more easily forgotten.
Is this all necessary?
We’ve all suffered the heat here – air-con definitely makes it feel better. While I don’t have air-conditioning at home, I seem to survive quite well with fans. Even a cool breeze outdoors on a sunny day makes me feel comfortable enough to forget the humidity.
I’m not so sure what the best way is – what do you think?