PASCO ET-8782 Energy Transfer– Thermoelectric - Manual (Page 34)

Energy Transfer –Thermoelectric Coefficient of Performance
34 ®
Procedure
Refrigerator
Before you start, the knife switch should be in the neutral position (straight up) and the fan should
be switched off. Make sure that the foam insulator is on the cold block, and that the heat sink is on
the hot block.
Set the DC Voltage to about 5 volts.
Click the Start button.
Observe the temperatures of the hot and cold blocks; they should be within 0.1 °C of each other.
(If they are not, turn on the fan and wait until the temperatures have equalized. Then turn the fan
off and proceed.)
Set the knife switch to Heat Pump mode. Allow the heat pump to run for 10 to 15 seconds, then
open the switch. Watch the temperature graphs; once the temperatures have peaked out, stop data
recording. You need to give the blocks a few seconds to reach a maximum or minimum before
you stop recording.
Heat pumps are rated by the Coefficient of Performance, k. In the case of a heat pump used for
cooling (such as a refrigerator) the Coefficient of Performance is
(for cooling)
The Coefficient of Performance expresses how much heat the heat pump removes from the cold
side compared to how much energy it uses to move the heat.
1) Use your values for the heat pumped out of the cold block (Q
cold
) and the area under the
Power versus time curve (W) to calculate the Coefficient of Performance, k, for your model
refrigerator.
2) For real heat pumps k is usually expected to be greater than 1. Is this the case for your model?
3) Your heat pump can also be thought of as a model air conditioner, a device used to keep the
inside of a building cooler than the outside air. In terms of moving heat, what does an air
conditioner do to keep a building cool? (Keep in mind that an air conditioner does not
necessarily move air into or out of the building.)
4) Compare your model to a building being cooled by an air conditioner. What does the peltier
represent? What does the cold block represent? What does the hot block represent?
5) If you were selecting an air conditioner to keep your home cool, would you choose one with a
high or low coefficient of performance? Explain why.
k
Q
cold
W
------------ =