Environmental Engineering Reference
In-Depth Information
cooling is required) and building shell performance (insulation,
glazing, orientation etc.). It is important to note that, under normal
usage, the air conditioner will spend a significant amount of time at
less than its rated capacity; in terms of efficiency, this is really only
important for variable speed drive models.
2. Capacity output: The measure of energy service for an air condi-
tioner is the rated cooling and/or heating capacity of the air con-
ditioner, usually specified in kW. (Some product brochures use
BTUs, although this is now unusual. Some retailers use compressor
'horsepower', but this has no meaning in terms of the units' capabil-
ity.) These rated values are as declared by the manufacturer and the
test conditions are defined in the Australian standard. The heating
capacity of a reverse cycle air conditioner is the heat that can be put
into a room. Similarly, the cooling capacity is the heat that can be
removed from a room. The cooling capacity is made up of the sensi-
ble component (usually the majority of the capacity), which relates to
the actual temperature reduction (cooling) of the air, plus the latent
component, which is a measure of the dehumidification effect of the
indoor air. Latent cooling capacity is sometimes expressed as mois-
ture removal capacity in litres or kilograms of water per hour (1 kg
per hour of moisture removal is equal to 683 W latent capacity).
3. How can the capacity output be greater than the power input?
Refrigerative air conditioners (the only type covered by energy label-
ling in Australia; evaporative units are not included) use a technique
called the vapour compression cycle to 'move' energy in the form of
heat from one space to another. This is generally a very efficient pro-
cess, and the amount of heat moved is typically two to three times
(or more) the energy required to run the compressor system. This
ratio is called the energy efficiency ratio (cooling) or coefficient of
performance and is used as the basis for determining the star rat-
ing of an air conditioner (see following). The efficiency of the sys-
tem depends on the components used (their design and how well
these are matched) and the temperature difference between inside
and outside (as the temperature difference increases, the system
becomes less efficient). The system uses a refrigerant (which exists
as a gas at low pressure and as a liquid under compression) that is
compressed and liquefied, allowed to cool in a condenser, and then
allowed to expand to become a gas in an evaporator (the expansion
is accompanied by a strong cooling effect). In this operation, the con-
denser becomes warm and the evaporator becomes cold as the heat
is moved from the evaporator to the condenser.
The principle is the same as used in a normal refrigerator, which 'moves'
heat from the inside of refrigerator to the outside. In the case of an air
