3.4. Cooling-down period and temperature stability
When a refrigerator is switched on, the temperature of the refrigerated cabinet decreases
exponentially from an ambient to a ‘‘set’’ temperature. It is usually convenient
to define the cooling-down period (CDP) of a refrigerator as the time
required for the refrigerator temperature to decrease from ambient to a temperature
which is 20% higher than the ‘‘set’’ temperature. Clearly, CDP is a measure of the
cooling the speed of a refrigerator. It can readily be determined by measuring the
temperature change as a function of time using a thermojunction placed in the centre
of the refrigerated cabinet. The results are shown in Fig. 9. It can be seen that the
CDP of a thermoelectric refrigerator can be shorter than that of a conventional compressor-
type refrigerator if a large input power is employed (as shown by TER-1 with
P = 215 W). The CDPs for the CCR and TER-3 are very similar. However, the
refrigerated cabinet volume of TER-3 is much smaller than that of the CCR. For
refrigerators with the same volume and thermal insulation, the CDP is mainly determined
by the heat-pumping capacity. Fig. 10 shows the temperature of TER-1 as a
function of cooling-down time for different input-powers (hence different heatpumping
capacities). It can be seen that the CDP decreases with an increase in the
input power supplied to the TER-1. This result shows clearly that the CDP of a
thermoelectric refrigerator can be controlled conveniently.
The effect of a small passive heat-load on the CDP was also investigated by placing
a can filled with 900 ml of water inside each refrigerator. Fig. 11 shows the CDP
of TER-3 in comparison with the CCR. Two thermocouples were placed inside the