Thermoelectric Lab for instrumentation lab PDF

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Summary

This is one of my labs for instrumentation lab - specifically the thermoelectric week - write up and all forms for this...

Description

Group 5: Robbie Crawford, Emily Lee, Hamilton Miller, Leonard Silver ME 3248-02 Thermoelectric Lab January 23, 2018

Project Abstract

This project explores the relationships between voltage, current, temperature, thermal resistance, and heat transfer. This was accomplished via applying various voltages to the thermoelectric under two different heat sink conditions--high and low resistance. As expected, the wood acted as a poor conductor, meaning that the heat could not transfer to the block. Conversely, the aluminum (low resistance/high conductivity) allowed for much greater heat transfer over a large surface area--allowing for much better functionality of the thermoelectric.

Methods

Using thermal paste on the backside of the thermoelectric, students adhesed it to one of the two heat sinks (a piece of wood and a large plate of aluminum). The probes were connected to the power source and the temperature was recorded using an infrared thermometer (due to issues with thermocouples available). Initially, the voltage and current on the power supply were set to 0. Then, the current was increased by around 0.1A until sufficient data points were collected. After one trial, the polarity was reversed by switching the positive and negative wires on the power supply such that for each heat sink both heating and cooling was measured. Once four trials in total (wood heating, wood cooling, aluminum heating, aluminum cooling) were completed, the thermal grease was removed and cleaned with Clorox wipes and the materials were placed back where they came from.

Results & Discussion

Table 1: Thermoelectric cooling with metal heat sink

Table 2: Thermoelectric heating with metal heat sink

Table 3: Thermoelectric cooling with wood heat sink

Table 4: Thermoelectric heating with wood heat sink

Figure 1: Applied Voltage vs. Temperature

Figure 2: Applied Current vs. Voltage

Figure 3: Heat Transfer of Heat Sink The recorded data is provided in Tables 1-4 and graphed in Figures 1-3. The data can be reduced to show that there is a fairly linear relationship between voltage and amperage for all data, and between voltage and temperature. For calculating heat transfer, the equation q = 2*N*[s*I*Tavg - I2*R - (k*A/L)*ΔT] is used, where N is the number of pairs of pellets, s is the Seebeck Coefficient, R is the given electrical resistance of the TED, k is the thermal conductivity of the alumina ceramic, A is the surface area and L is the length of one side of the exposed plate of the TED. Tavg is the average temperature of the air in the room and the measured temperature of the TED, in Kelvin, and ΔT was difference of the measured temperature and room temperature.

Discussion The expected trend was that the heat transfer would increase as the applied current was increased. Since the increased current also increased the voltage, the Seebeck Coefficient was also increased, which will raise the average temperature in the heat transfer equation. The data that was gathered backed up this equation and prediction because as each current increased, the heat transfer off of the TED also increased. The Seebeck Coefficient can be calculated by using the magnitude of the measured

thermoelectric voltage and dividing by the temperature change across the material. This measurement is only an approximation, but by also knowing the temperature of the material the TED is connected to can help calculate a more precise number. The maximum temperatures the TED is able to reach according to its specifications is 50oC ± 73oC. The maximum temperature we recorded was 98.9oC while the lowest temperature we recorded was -26oC, which is almost within the range of values for the device, as the minimum temperature was exceeded. The maximum voltage of 9.2 V and the maximum current of 3.7A were also not exceeded as our maximum voltage was 7.2V and our maximum current was 1.6A. The lab had us shut off the device if the temperature got too hot or the device was damaged, so all the values should be within range. Water did not boil during the experiment as the maximum temperature recorded was 98.9 oC, but had the current been increased past the point where instruments may have been damaged, water probably would have boiled or steam would be produced. Water from the surrounding air froze on the TED after the temperature dropped below 0oC during the experiment and stayed frozen after the temperature dropped further.

Conclusions Based on our experiment with the thermoelectric, we were able to observe the effects of thermal conductivity on the efficiency of the device. Using a heat sink with a high thermal conductivity enabled the thermoelectric to perform optimally, as it was able to dissipate heat over a much larger surface area than its own. When using wood as a heat sink, there was little to no thermal transfer, therefore the device was limited in its range of heating and cooling. As the device reached a certain temperature (around 38℃ in our example), it began to heat itself and the temperature would continue to rise despite no added current. This was due to the fact that the thermoelectric could not dissipate the heat due to the poor conduction to the wooden block. This was also observed in cooling, where the temperature reached a limit then began to rise again - the rise in temperature,due to the hot side, pressed against the wood, was not transferring heat to the wood and instead pumped heat back into the thermoelectric....