Heat pumps are
vapour compression systems, but they are used for space heating rather than
cooling.
It can be seen that
what the vapour compression chiller is doing is extracting heat from a low
temperature space and transferring it into an environment at a higher
temperature. This is the basis of the heat pump (below figure) which uses the
vapour compression cycle to absorb heat from outside air and convert it to
higher grade heat for indoor space heating.
The
theoretical efficiency with which the heat pump carries out this function is
very high at approximately 300%. This means for every 1kWh of electricity put
into the compressor 3kWh of heat is obtained by the building. In practice
however the operating efficiency tends to be lower. This is for two main
reasons. The first is that the highest efficiencies are obtained when the
inside and outside temperatures are similar. This is not the case in winter
when heat pumps are required for space heating. The second cause of the fall
off in efficiency occurs on cold winter days when the evaporator may become iced
up due to low temperatures. This restricts heat transfer across the evaporator.
This can be avoided by using an electrical heater on the outside coil to
defrost it or to reverse the refrigerant flow direction. Both of which reduce the
overall efficiency of the device.
Even with
this reduction in efficiency the efficiency with which the unit uses electricity
to provide heating is higher than simple resistive heating. The operating efficiency
of the device can be increased if a body of water is used as the heat source
rather than the outside air. This is because the water will have a more stable
and higher temperature than the surrounding air. Examples are canals, lakes,
ground water or warm effluent.
Reverse cycle heat
pumps are very useful pieces of equipment which
can either heat or cool a space.
This feature is obtained
by equipping the heat pump with a valve which can reverse the direction of
refrigerant flow (below figure). The direction of the refrigerant flow
determines if the coil inside the building is a cooling evaporator or heating
condenser. Two expansion valves fitted with non-return valves are also required.
Each expansion valve works in one direction only.
Reverse
cycle heat pumps are particularly useful where spaces may have a requirement
for both heating and cooling but at different times. One application is in
shops where at the start of the day heating may be required. Later in the day
as the shop fills with customers and heat is given out by display lighting, cooling
may be needed to maintain comfort.
