318 Electrical Power Systems Technology
heat pumps. Most of these systems use forced air to circulate the heat.
Some electric heaters have individual thermostats, while others are con-
nected to one central thermostat that controls the temperature in an entire
building. The possibility of having temperature control in each room is an
advantage of electric heating systems.
Heat Pumps
In recent years, the heat pump has become very popular as a com-
bination heating and cooling unit for buildings. The heat pump is a heat-
transfer unit. When the outside temperature is warm, the heat pump acts
as an air conditioning unit and transfers the indoor heat to the outside of
the building. This operational cycle is reversed during cool outside tem-
peratures. In the winter, the outdoor heat is transferred to the inside of
the building. This process can take place during cold temperatures, since
there is always a certain amount of heat in the outside air, even at subzero
temperatures. However, at the colder temperatures, there is less heat in
the outside air.
Thus, heat pumps transfer heat rather than produce it. Since heat
pumps do not produce heat, as resistive-heating units do, they are more
economical in terms of energy conservation. Heating and cooling are re-
versible processes in the heat-pump unit; thus, the unit is self-contained.
The reversible feature of heat pumps reduces the space requirement for
separate heating and cooling units. Another advantage is that the change-
over from heating to cooling can be made automatically. This feature
might be desirable during the spring and autumn seasons, in the many
areas where temperatures are very variable. In extremely cold areas, the
heat pump can be supplemented by an auxiliary resistance-heating unit.
This auxiliary unit will operate when the outside temperatures are very
cold, and will be useful in maintaining the inside temperatures at a com-
fortable level. Air is circulated past these heating elements into the heat
vents of the building.
Heat pumps are used for residential as well as commercial and in-
dustrial applications, and they are being used more extensively each year.
Figure 12-7 shows a simplified circuit arrangement of a heat pump, in
which a compressor takes a refrigerant from a low-temperature, low-pres-
sure evaporator and converts it to a high temperature and a high pressure.
The refrigerant is then delivered to a condenser, in much the same way as
in a refrigerator.