Energy Conversion in the Peltier Device

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Laboratory exercise 4 Energy Conversion in the Peltier Device Preface The purpose of this exercise is to become familiar with the Peltier effect. Students will observe Peltier device working as a heat pump that cools down temperature sensor and cools down electric device. Peltier device could also be used as a thermoelectric generator and this phenomenon would be also measured. Physical principles of Peltier device are: Peltier effect Joule effect Heat conduction in solid material Thomson effect Seeback effect In this lab manuals all of these phenomenon will be discussed. Figure 1: The drawing of Peltier device. (from http://www.kelk.co.jp/english/useful/thermo.html) 1

1 Peltier effect In 1834 Peltier has joined together two wires made from different material - copper and bismuth. Then he connected them to a battery. When he closed the circuit, the current pass through the junction and one of the two wires became hot, while the second was cooled down. The amount of heat that was transferred by the device depends of current value and materials type that were used in the device. A typical module consists of p and n semiconductor elements and ceramic substrates. Bismuth telluride is an example of semiconductor commonly used in module. In appearance, it is a device in which pairs of semiconductor elements are sandwiched between ceramic surfaces and the elements are electrically series-connected, P-N, P- N. Its size is generally 10-40 mm square and 3-6 mm thick. The Peltier effect is created in p-n junctions. The structure is pictured in Fig. 1. The most important fact should be, that p and n materials are parallel to each other and the junctions create both surfaces of a Peltier device. Figure 2: The structure of typical thermoelectric device [from http://ixbtlabs.com/articles/peltiercoolers/] The electric current that occurs in semiconductors is created by electrons movement. In p-type semiconductor electric current is created by hole movement. The hole is created by a process of doping the semiconductor material. One of electron is missing from one of the four covalent bonds normal for the silicon lattice. In semiconductors n-type there is an excess of electrons. Previously mentioned energy band is completely filled and the electrons have high energy. That means that the electron has the possibility to separate easier than in not doped material. When electron jumps to higher orbit from the lower one, it needs some energy. In the Peltier device the electrons require higher energy, to pass from the semiconductor p-type into the n-type. This energy has to be taken from anywhere. It is taken from environment in form of heat energy. That phenomenon generates lower temperature on Peltier device surface. Then electrons flow from n-type to p-type. In that junctions, electrons reject energy because they flow from higher energy to lower. That energy heats the second surface - it is the "hot" surface. The junctions of 2

p-n type absorbs heat and of n-p type rejects heat. When direction of current is changed, the hot and cold surfaces exchange places. 2 Joule heat This effect occurs when electric current passes through a material of resistance value greater than zero. The current generates heat which is proportional to value of material and a product of the square of the current: P=R I 2 The problem with Joule heat in case of the Peltier device is that it has influence on the Peltier phenomenon. Joule effect creates the limitation for Peltier device, by generating a heat inside the device. This heat creates a additional energy which has to be transported by electrons. This is the reason that on hot surface there is more energy than was absorbed by electrons on colder surface. Peltier effect transports heat which is proportional to value of current. However the opposite phenomenon generates heat of value proportional to square current. When value of electric current increases, it causes faster growth of Joule heat, than the heat transported by the Peltier effect. The state when both kind of energy are equal means, that the energy that is transported by electrons, is the heat that is generated by Joule effect. More heat could not be transported from cold surface. Figure 3: Joule heat (red) and heat transported with Peltier effect (green) for electric current values This state was pictured on Fig. 3. The most important parameter for any Peltier device is the value of maximum current and the highest possible heat power to be transported with current. The highest possible transport power value for given Peltier device was marked on Fig. 3 with a square. 3

3 Heat conduction phenomenon in the Peltier device Another important phenomenon, besides the previously described phenomena, that occurs in a thermoelectric device is a heat conduction phenomenon. According to the second law of thermodynamics, temperatures difference causes a flow of a heat energy from a hotter object to a colder. In Peltier device, heat is transported from the cold surface to the hot one. Heat conduction phenomenon causes heat flow in the opposite direction. That means that it is the second phenomenon which counteracts the Peltier principle. The value of heat power that is transported from the hot to cold surface is given by the equation: P= t hot t cold W peltier which means that it depends of temperature difference on both surface and a value of heat resistance of Peltier device. The heat resistance is almost constant during a temperature changes. The value of heat power is almost entirely dependent only on temperature difference. If this difference is high enough which is also a result of high value of electric current passing through semiconductors junctions, the heat conducted and Joule heat become both significant. That counteracts Peltier phenomenon. To prevent of returning heat to cold surface, sufficient cooling of hot side should be use, which takes heat energy from the device surface. The heat sink are typically used, but it do not has enough cooling power. Water cooling of hot side of Peltier device is much better. The advantage of use water are its significant specific heat which means that water absorbs a lot of energy. The water cooling system that is used during laboratory exercise allows water to pass through water block, that is placed under the Peltier device. The water absorbs heat and returns to hydraulic system of the building. This is circuit of open type, which delivers a cold water all time and does not need an additional pump. During the laboratory exercise, students will observe how much impact has sufficient cooling of hot surface for efficiency of Peltier device. 4 Thomson effect This effect was predicted by Lord Kelvin. He stated, that additional electromotive force will be generated in a conductor, in which a electric current passes, at the presence of a temperature gradient. It means, that a different temperatures were formed on the surfaces of conductor. If electric conductor is connected with a galvanometer, which measures very small electric current, and something hot is being moved along conductor (for example a cigarette lighter), a electric current passing by this conductor would be observed by galvanometer. This effect will also occurs if we hold in a place the heat source during electrons movement. It means that additional voltage (electromotive force) will be created. Some of electrons will flow in the normal direction of source voltage that was used. But some of them will flow in the opposite direction. That phenomenon is Thomson effect and it happens 4

with the occurrence of temperature gradient. These electrons will cool down a part of conductor, in which they are moving. That means that heat power is taken from environment the heat is transferred from hot to cold in the similar way as in the Peltier phenomenon. So additional electromotive force could absorbs or rejects heat. That depends of material type and current direction and which electric potential has higher temperature. In case of the lead, this effect is neglected. This effects is so small, that it could be neglected in a Peltier device. 5 Seeback effect The last of mentioned in the preface effect is the Seeback effect. The three phenomena: Peltier effect, Thomson and Seeback are treated as thermoelectric effects. The latter was discovered by Seeback who thought, that he actually did discover thermomagnetic phenomenon. On the junction of two different metals under the influence of temperature difference, a voltage is generated. That phenomenon is used in temperatures measurement. Special temperature sensor that is a junction of two metals is called thermocouple. If junction is placed in different temperature than both ends of thermocouple, between both metals a potential difference will be generated. This is also used to identify a components of alloy in industry. The phenomenon is sometimes also use in special circumstances to generate electric energy. It is used in some space probes or satellites in form of a radioisotope thermoelectric generator. In the device, decay of radioactive material generates heat, which generates electric energy by many thermocouples which are commonly connected in series. 6 The plan of exercises During the lab exercise students measure the temperatures: on top surface of Peltier device and on inlet and outlet of water. The voltage value which is fed to Peltier and a value of electric current also have to be measured. The latter is measured by a read of value of voltage drop on a special measurement resistance. The bottom surface of Peltier device is cooled by a water, which passes by a water block which is placed under the Peltier device. The water flow is measured with a rotameter. EXPERIMENTAL PROCEDURES: 1. Get several sheets of paper for writing down measured data and create a lab results report. Write down all names of students from your group. 2. Check laboratory circuit. Do you know what is the purpose of all of used devices? Draw a picture of the circuit in your group results report. 3. You are going to get an introduction from the teacher. During it, you 5

could be asked simple questions about this lab exercise. 4. The first part of your measurement should be made for different electrical power feeds to the Peltier device. Water flow should be selected and maintained at a constant level. This will make different temperature on both surfaces of the device. Get results for PWM of values: 5%, 10%, 20% 100%. 5. After reaching 100% of PWM value, students should set different water flow, and check it's influence on temperatures on Peltier device. Check this for lower and higher water flow. 6. In the next part of the exercise we will check how much Peltier influence the temperature of a object in which Joule effect generates some heat. 7. In the last part of this experiment, we observe how a temperature difference generates electrical power. 7 Report preparing The most important issues, that should be in report: describe the measurement system and elements of the test and measurement procedure, prepare some charts and tables showing the function of temperature of cold surface and a current value, discuss a influence of water cooling on the device efficiency, how much Peltier device change temperature of hot object? write down some conclusions. 8 Bibliography 1. http://www.kelk.co.jp/english/useful/thermo.html 2. http://www.magnet.fsu.edu/education/tutorials/java/thomsoneffect/index.htm l 6