Peltier effect: heat pump

ENT 7.2 Peltier effect: heat pump Keywords Heat pump, coefficient of performance, efficiency, Peltier element, electrical energy, and thermal energy...
Author: Hannah Dennis
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ENT 7.2

Peltier effect: heat pump

Keywords Heat pump, coefficient of performance, efficiency, Peltier element, electrical energy, and thermal energy Cold water

Principle When direct current flows through a Peltier element, one side heats up and the other side cools down. Based on this characteristic, the mode of operation of a heat pump can be explained. The hot side is not only heated by the electric current, but also by the fact that the other side cools down. Energy is "pumped" from the cold side to the hot side. In this experiment, the temperatures and electric work are measured as a function of time. Based on these values, the electrical energy and thermal energy on both sides can be determined and the coefficient of performance as well as the efficiency can be calculated. Equipment *1 *4 *2 *1 *1 *1 *1 *2 *1 *1

Connector, straight, module DB Connector, angled, module DB Connector, interrupted, module DB ON/OFF switch, DB Thermogenerator, Peltier element Heat insulating sheet, felt Apparatus carrier with a fastening magnet Glass beaker, short, 250 ml Agitator rod Glass rod, l = 200 mm, d = 5 mm

Additional equipment 1 Demo physics board with a stand 1 Power supply, universal **1 Cobra4 Wireless Manager

Fig. 1:

09401-01 09401-02 09401-04 09402-01 04374-00 04375-00 45525-00 36013-00 04404-10 40485-03

**2 **1 **1 **2 **2 **1 2 2 1 1

Cobra4 Wireless-Link Cobra4 Sensor-Unit 2x Temperature Cobra4 Sensor-Unit Energy Holder for hand-held meters Immersion probe, NiCr-Ni, stainless steel measure software for Cobra4 Connecting cable, 250 mm, red Connecting cable, 250 mm, blue Connecting cable, 750 mm, red Connecting cable, 750 mm, blue

12601-00 12641-00 12656-00 02161-00 13615-03 14550-61 07360-01 07360-04 07362-01 07362-04

1 PC, USB port, XP, Vista, Win7 02150-00 13500-93 12600-00

* Included in the ENT 1 set ** Included in the Cobra4 extension set

09492-88 12608-88

Experiment set-up www.phywe.com

P9507260

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ENT 7.2

Peltier effect: heat pump

Set-up Set the circuit up as shown in Fig. 1. The switch is open. Connect the DC output of the power supply unit to the circuit via the Cobra4 Sensor-Unit Energy. Ensure that the positive terminal of the power supply unit is connected to the A socket of the Sensor-Unit. Attach the apparatus carrier to the board with the aid of the magnets and place the heat insulating sheet on top of it (do not add the beakers with the thermogenerator yet). Connect the immersion probe to the Cobra4 Sensor Unit 2x Temperature. Procedure Start the PC and Windows. Connect the Cobra4 Wireless Manager to the USB port of the PC. Start the "measure" software package on the PC. Connect the Cobra4 Wireless-Links to the Cobra4 Sensor-Units one by one. After the WirelessLinks have been switched on, the Sensor-Units will be automatically detected and assigned an ID number that will be displayed on the displays of the two Cobra4 Wireless-Links. The communication between the Cobra4 Wireless-Manager and the Cobra4 Wireless-Links is indicated by way of the respective data LED. Switch the Cobra4 Wireless-Link with the connected Cobra4 Sensor-Unit on. The Sensor-Unit and the electrical quantities U, I, P, and W are displayed as the measuring channels. Switch the Cobra4 Wireless-Link with the connected Cobra4 Sensor-Unit 2x Temperature on. The Sensor-Unit and the quantities T1 and T2 are displayed as the measuring channels. Load the experiment (Experiment > Open experiment > …). The program will now open all of the required presettings for the measurement data recording process (Fig. 2).

Fig. 2:

2

Measurement data recording

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Peltier effect: heat pump

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ENT 7.2

Fill both beakers precisely with 150 ml of water and place them on the apparatus carrier. Connect the thermogenerator to the electric circuit and place it in the beakers. Insert the two temperature sensors into the holes of the thermogenerator. T1 shall be the temperature of the negative pole (blue). Place a glass rod or agitator rod into the beakers. Adjust the current to 1 A and the voltage to approximately 3 V on the power supply unit. Switch the power supply unit on. Close the circuit by way of the switch. Start the measurement data recording process in "measure" . The system measures a value every 30 seconds. Stir the water in the beakers evenly, especially prior to a measurement. The measurement will be stopped automatically after 10 minutes. Note down the values for the electric work. Transfer the measurement values to the "measure" main program. Switch the power supply unit off.

Observations and results The negative pole of the thermogenerator heats up while the positive pole cools down. The temperature increase is slightly higher than the temperature decrease. In accordance with the measurement, the electric work is 1404.6 Ws. Evaluation When current flows through the Peltier element, its temperature and, thereby, its internal resistance changes. If the voltage is constant, this means an important change in the current and power of the thermogenerator. This is why it is recommended to use the current control of the power supply unit in order to limit the current automatically to 1 A. The thermal energy on the hot or cold side of the thermogenerator can be calculated based on the total heat capacity C and on the respective temperature differences ∆ϑ. Thermal energy

Q = C ⋅ ∆ϑ

with the total heat capacity C comprising the heat capacity of the water CW in the beaker and the heat capacity CTG of the copper leg of the thermogenerator. Q = (C W + C TG ) ⋅ ∆ϑ

The heat capacities CW and CTG are the product of the respective mass and the specific heat capacities cW and cTG . C =c⋅m

The Peltier element of the thermogenerator is located between two bent copper sheets. The sockets for the thermocouples are made of brass. The specific heat capacities of copper and brass are virtually identical within the measurement accuracy so that the same value can be used for the calculation.

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ENT 7.2

Peltier effect: heat pump

The heat capacities are stated in the table. Table: Heat capacities of one side of the thermogenerator

Fig. 3:

m g

c J/gK

C J/K

Thermogenerator (Cu, Ms)

147

0.383

56

Water

150

4.18

627

Total

---

---

638

Measurement

As shown in Fig. 3, use the "Display options" function and select "2" as the number of decimal places for the temperatures T1 and T2. Lay a straight line over the curve for T1 (left-hand axis) with the aid of the "Regression" function and determine the gradient, i.e. the average temperature rise of the water. Do the same for T2. Gradient of the straight line for T1: 0.28 K/min Gradient of the straight line for T2: 0.10 K/min Energy balance after 10 minutes: Temperature change of the water baths Hot side ∆Τ1 = 2.8 K after 10 minutes Cold side ∆Τ2 = 1.0 K after 10 minutes

4

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ENT 7.2

Peltier effect: heat pump

Required electrical energy (read off at the end of the measurement) Qel = 1404.6 Ws, with 1 Ws = 1 J

Electrical energy Thermal energy Hot side Cold side

Qel = 1.40 kJ Qw = 1.91 J Qk = 0.68 J

The coefficient of performance ε and the efficiency η of a heat pump are defined as follows: Qw Qel Qw η= Qel + Qk

ε=

Based on the energy values that were determined during the experiment, the following results: Coefficient of performance ε = 1.36 Efficiency

η

= 92%

It is a typical characteristic of a heat pump that the coefficient of performance is greater than 1. The electrical energy cools one side where thermal energy is withdrawn. As a result, the other side is heated more strongly than what would be possible with a purely electric heater.

Notes 1. For this experiment, the current should not be higher than 1 A. If the Peltier element heats up too strongly, the heat losses will become too high, the coefficient of performance will decrease, and the Peltier element can no longer be used as a model of a heat pump. 2. This experiment does not use any insulated vessel so that the student can clearly see the thermogenerator. This, however, results in heat losses. Application For heating systems, mainly compression heat pumps are used. On the hot side, energy is transferred to the hot-water system of the building, while the cold side absorbs heat from the environment. There are two different pressure levels in the circuit of a compression heat pump. As a result, the refrigerant can absorb energy on the cold side, which causes the refrigerant to evaporate. During the passage through the compressor, the pressure is increased and the refrigerant can release energy on the hot side causing it to condensate. Then, it flows back to the low-pressure side via a throttle valve.

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ENT 7.2

Peltier effect: heat pump

Notes Corresponding student experiments TESS EN 7.4 Peltier effect: heat pump (P9517400) For the execution of the experiment without a PC, the items on the list (page 1) that are marked with (**) must be replaced with the following items: Experiment P9507263 2 2 2 2 1 1 2

Cobra4 Mobile-Link Cobra4 Display-Connect, transmitter and receiver set Holder for hand-held meters Large-scale display Cobra4 Sensor-Unit Energy Cobra4 Sensor-Unit 2x Temperature Immersion probe, NiCr-Ni, stainless steel

12620-00 12623-88 02161-00 07157-93 12656-00 12641-00 13615-03

Experiment P9507200 1 1 1 1 1 2 1

6

Work and power meter Cobra4 Mobile-Link Cobra4 Sensor-Unit 2x Temperature Cobra4 Display-Connect, transmitter and receiver set Holder for hand-held meters Immersion probe, NiCr-Ni, stainless steel Large-scale display

13715-93 12620-00 12641-00 12623-88 02161-00 13615-03 07157-93

PHYWE Systeme GmbH & Co. KG © All rights reserved

P9507260