An Investigation of the Waste Heat/Cold Recovery in Use of Thermoelectric Power Generators
Date Issued
2015
Date
2015
Author(s)
Weng, Chien-Chou
Abstract
In this thesis, the proper analysis methods were proposed for three thermoelectric power generator (TEG) applications; relevant thermal-resistance model was carefully built and the accuracy of the approximate solution for this model was discussed. The first TEG application is associated with the automotive waste heat recovery. The influences of the number and the coverage rate on the surface of the heat-exchanger of the thermoelectric couples (TE couples) were explored via FLUENT simulation. It was found that implementing more TE couples does not necessarily generate more power because the heat sinks attached to the downstream TE couples loot heat from the upstream hotter wall, resulting in a performance degradation of the upstream TE couples. Furthermore, for a given number of TE couples, the performance is better when a portion of the heat exchanger is uncovered with TE couples at the downstream side, because the uncovered part becomes hotter and consequently transfers heat from the downstream wall to the upstream TE couples. The second application is to recover waste heat from a table lamp. The table lamp integrated with TEG modules and cooled by a natural convection heat sink was investigated experimentally as well as numerically. In the simulation, the heat sink was not truly simulated but modeled by the so-called compact heat sink model. To reduce the computational amount and still accurately estimate the power generation, the open-circuit system was first simulated and a TEG thermal-resistance model was built for predicting the power generation rate based on the open-circuit temperature difference. For verifying, closed-circuit simulation which properly takes Peltier and Joule’s heats into consideration was also performed. The investigation shows the prediction from the TEG thermal-resistance model is in a good agreement with the closed-circuit simulation result; the maximum power generation rate is slightly larger and the optimal electric load is slightly smaller than the experimental measurements however. We attribute these differences to the effect of the parasitic electric resistances in the system. A same reason is employed to explain a lower maximum power generation rate observed in the parallel network than in the series network. Finally, it was concluded that the low hot-side thermal conductance is the main reason for the low power generation efficiency. The final application is about the waste cold recovery. A TEG waste-cold recovery system was studied for the cryogenic-nitrogen exhaust system. A TEG thermal-resistance model was constructed for predicting the power generation rate and experiments were performed for verification. Both the model analysis and the experiment show that using cascade TEG modules can access more temperature difference and thus generate more power; a power generation rate as high as 0.93W was obtained by the present system when four two-layer cascade TEG modules were employed at a mass flow rate of cryogenic nitrogen of 3.6 g/s. However, the measured power generation rates are less than the predictions. The ice frozen over the thermal spreader, which is not taken into consideration in the model, must take the responsibility. On the other hand, the influence of the TEG-module number was also explored and the existence of the optimal number of TEG modules was observed in the experiment. Finally, the analytical solution of a common TEG thermal-resistance model is usually unavailable. For convenience, the approximate (empirical) solutions of the maximum power generation rate and the optimal electric load for the TEG thermal-resistance model were derived and investigated in various situations. The investigation shows that the approximate solution has a better accuracy when in the TEG recovery system the hot-side and cold-side thermal conductances are low or nearly equal or when the system is operated at high operating temperatures or with a low temperature difference.
Subjects
Thermoelectric Power Generator (TEG)
Waste Heat Recovery
Waste Cold Recovery
Thermal-Resistance Model
Automobile
Table Lamp
Type
thesis
File(s)![Thumbnail Image]()
Loading...
Name
ntu-104-D99522007-1.pdf
Size
23.54 KB
Format
Adobe PDF
Checksum
(MD5):edbc77b2051f4e69adee22c01983958e