Air conditioner

Some air conditioner related metrics.

Theory of operation

Air conditioners, refrigerators and heat pumps are based on the following basic principles:

1. Evaporation (turning a liquid into gas) requires energy and condensation (turning a gas into a liquid) releases energy.
2. The boiling point of a liquid depends on the pressure: The higher the pressure, the higher the boiling point;
   In a vacuum water boils at room temperature. Just look for 'boiling water at room temperature' on YouTube. This short shows it really well: Cooling down water by boiling it!
   And an erupting underwater volcano, deep underneath the ocean, can't make the water boil: If the are any steam bubbles, the high water pressure squeezes them into liquid water again. (The difference between a gas an a liquid is the distance between the molecules.)

In the diagram below a compressor pumps a refrigerant through a metal pipe. At the inlet of the compressor the pressure is low. At the outlet the pressure is high. The refrigerant flows through a reduction valve which does exactly the opposite: It reduces the pressure.

              Evaporator
              Low pressure thus
              low boiling point
              Liquid evaporates
                →            →
 Liquid      ┏━━━━━━━━━━━━━━━━━━┓   Vapor
 + vapor     ┃                  ┃
           ↑ ┃                  ┃ ↓
            ┏┻┓                ┏┻┓
 Reduction  ┃↑┃                ┃ ┃  Compressor
 valve      ┃ ┃                ┃↓┃
            ┗┳┛                ┗┳┛
           ↑ ┃                  ┃ ↓
             ┃                  ┃
 Liquid      ┗━━━━━━━━━━━━━━━━━━┛   Vapor
                ←            ←
              Condenser
              High pressure thus
              high boiling point
              Vapor condenses

At the low pressure side the the boiling point is low which evaporates the liquid. And since evaporation requires energy the pipe in which the evaporation takes place cools off.
At the high pressure side the boiling point is high which condenses the gas. And since condensation releases energy the pipe in which the condensation takes place warms up.
At the cool side the pipe runs in a zigzag through thin metal plates. A fan blows air between the plates which cools off the air. This bit is called the evaporator.
At the warm side there is also a zigzag through thin metal plates. Here the air blown between the plates gets warmed up. This part is called the condenser.
An air conditioner has the evaporator indoor and the condenser outside. A heat pump has the condenser indoor and the evaporator outside.
Most modern air conditioners have a system of valves which make it possible to do both.

Links

• Air conditioning
• Vapor-compression refrigeration

Required cooling power

The above values are required cooling power, not the electrical power consumption. And since 1 Watt equals 1 Joule per Second, the above numbers are the the amount of heat (thermal energy) expressed in joules that is removed from your room every second.
The actual power consumption is much lower. The ratio between the two determines the air conditioner efficiency:

                      Watts cooling power
 Efficiency = ────────────────────────────────────
               Watts electrical power consumption

As air conditioner technology improves, this number gets higher, which reduces your electricity bill. So you want this number to be as high as possible.

Theoretical efficiency

Below the theoretical maximum performance:

      Indoor temperature in Kelvin
 ────────────────────────────────────────
  Outside - Indoor temperature in Kelvin

And the same for degrees Celsius:

   Indoor °C + 273.15
 ────────────────────────
  Outside °C - Indoor °C

Below the theoretical maximum performance as a graph for a 24°C and a 27°C indoor temperature:

The actual efficiency is always less than this number and may be as small as 1/10^th of the theoretical maximum.
However, once your room is cool a modern air conditioner will reduce the compressor speed and very little power is needed to keep it cool. This is usually done by powering the compressor from a (three phase) inverter (circuit which turns DC into AC): Reducing output frequency reduces the compressor speed.
And the better the thermal insulation for your room, the less power your air conditioner needs to keep it cool.

Units of measure

Non standard units.

Area vs Volume

Sometimes room sizes are expressed as m² instead of m³. This usually assumes a ceiling height of 2.6 m.

BTU/hour vs Watt

And yet an other SI vs imperial source of confusion!
A BTU or British Thermal Unit is 1055.1 Joule. And since one hour equals 3600 seconds, a BTU per hour is 1055.1 / 3600 ≈ 0.2931 Joules per Second or Watt:

A conversion table:

When the cooling power is expressed in BTU/h, this somehow is always a whole multiple of 1000. My air conditioner for instance, is specified as both 9000 BTU/h and 2.7 kW. And 3.412 x 2700 does not precisely equal 9000 (it's 9212.4). So these are rounded numbers!
Doing things this way may be more visually appealing, but doesn't provide you with accurate information. So always look at the cooling power expressed in kW.

Sometimes the air conditioner efficiency is expressed as BTU/h cooling power / Watt power consumption (which equals BTU / Watt x hours). This gets you a number that's a factor 3.412 higher than the above method, in which case you have to divide the air conditioner efficiency value by 3.412 to get a more realistic number.
Always make sure you're not dealing with inflated numbers!

Efficiency

Different ways to express how efficient an air conditioner is.

EER

Energy Efficiency Ratio
Efficiency measured with an outside temperature of 35°C and and an indoor temperature of 27°C at 50% humidity.

COP

Coefficient Of Performance
This the the efficiency of a heat pump. And since most air conditioners can also be used as a heat pump, COP is often specified as well.

SEER

Seasonal Energy Efficiency Ratio
This is more or less the average efficiency over a whole year; It's weighted average efficiency that compensates for seasonal differences. You can think of it as the total cooling provided in a whole year divided by the total electricity consumption in a whole year.
In a modern air conditioner, this number tends to be a lot higher than EER.

The total SEER is the weighted sum of the EERs at the above values. So 1/4 of the EER at 35°C plus 1/4 of the EER at 30°C plus 1/4 of the EER at 25°C plus 1/4 of the EER at 20°C.

Some websites specify SEER in BTU/Wh instead of W/W (those inflated numbers again). Below a conversion table:

So, if somebody advertises a SEER of 20, it's probably BTU/Wh and not W/W!
And from W/W to BTU/Wh:

SCOP

Seasonal Coefficient Of Performance
This is more or less the average efficiency of a heat pump over a whole year; It's weighted average efficiency that compensates for seasonal differences.

ESEER

European SEER:

EU Energy Label

Energy labels may get as low as 'G', but air conditioners with a label below 'B' are not allowed.
I have never seen any air conditioners with a label below 'A' though. Most are 'A+' or higher.
Note: Efficiency classes may change as regulation gets more strict!

Links

• Driving Efficiency Through Air Conditioning Assessments (PDF)
• European seasonal energy efficiency ratio

Noise

For comparison below some sound levels.

Indoor units tend to be quieter than outdoor units. And keep in mind that a fan sounds less annoying than a compressor.