It is unfortunate
that cooling is required in summer as these results in reduced system efficiency!
Refrigeration
plant is equipment which converts electricity into coolth. Coolth is the same
stuff as heat i.e. thermal energy except that it has a negative value. In other
words it is heat being removed from something causing a reduction in
temperature. Most refrigeration systems are based on the vapour compression
cycle and this information panel will concentrate on this system.
The
energy consumption of a cooling system is by two principle components. Firstly
by the compressor, and secondly by any fans or pumps used to remove waste heat
from the condenser and remove coolth from the evaporator. The compressor consumes
the majority, approximately 95%, of the energy input to the refrigeration
equipment.
Coefficient
of Performance (COP)
The
efficiency of a cooling system is normally called the coefficient of
performance or COP. The COP of a real system is given by:
COP =
Cooling Capacity (kW)/Total Power Input (kW)
Typical
COP's for cooling are 1.5 to 2.0 which indicates that you can achieve 2kW of
cooling for the consumption of 1kW of electricity. This compares with a COP of
2.5 to 3.0 if the refrigeration equipment is being used as a heat pump. To
study factors affecting performance more clearly it is useful to look at the
formula for the theoretical COP. This is given by:
COP = T1/T1-T2
where T1 = Evaporator ambient air temperature (K)
T2 = Condenser ambient air
temperature (K)
From this
it can be seen that for the COP to be high the difference between T1 and T2 should
be small. Since T1 is set by system requirements the variable is T2, the
ambient temperature surrounding the condenser. Taking a split cooling system as
an example, what this means in practice is that when the outside air
temperature increases the efficiency of the system will fall. This is
unfortunate since most cooling is required in summer when high ambient
condenser temperatures prevail. However, there are a number of things that can
be done to improve this. The first is to increase the size of the air cooled
condenser and ensure that there is good airflow through the device to keep the
ambient temperature low. The next is to consider evaporative condensers these
use latent heat removal to reduce ambient temperatures. Finally, stable low
temperature heat sinks should be considered such as surface and groundwater.
Ice
Storage
The
efficiency of a chiller varies with the amount of work it is required to do. At
low loads the efficiency will be reduced. One way of overcoming this problem is
to use an ice storage system. Ice storage involves using an undersized chiller
to produce an ice slurry during the night. The ice is stored in an insulated
tank. During the day the chiller would not have enough capacity to satisfy the
cooling demands of the building. However by operating the chiller at full load
and drawing additional coolth from the ice store the building cooling demand
can be satisfied.
The
advantages of the system is that the chiller operates most of the time at full
load and hence peak efficiency, it also provides a good proportion of the
cooling requirement of the building using cheaper night time electricity
tariffs. Both of these contribute to reducing operating costs.
