1. Effect of Using 2 TE units on Same Heat Sink
Development of a Thermoelectric Cooling Prototype
Thomas Coull and Dr. Steven Girard University of Wisconsin – Whitewater, Department of Chemistry
Temperature Controlled Fermentation
Results
Blower position
Shroud
Water Trial
Maintenance of a specific temperature range is crucial for many fermentation processes (i.e. beer making, yogurt, kombucha, etc.). Aside from commercial refrigerators, there exists no product designed specifically to handle temperatures and have a small footprint required for home fermentation. Here we are looking to develop a thermoelectric prototype capable of maintaining a broad range of temperatures and has a small footprint, making it more versatile than a conventional refrigerator.
Probe Height
Ch1
Ch2
Ch3
Ch4
Metal
Thermal Conductivity
(W/m K) (at 68 °F) Al
204 Cu
386
Thermoelectric Cooling
TE units generate a temperature difference (ΔT) when subjected to a ΔV established by a flow of current. A typical thermoelectric cooler will consist of an array of p- and n- type semiconductor elements. The array of elements is soldered between two ceramic plates, electrically in series and thermally in parallel.
Direct Current
N-Type
P-Type
Copper Conductor
Electrons
Holes
Ceramic Substrate
Heat Absorbed
Heat Rejected
Cold
Side
Hot
Heat Sink
Schematic of TE Device
Image of TE Device
Schematic of a TE Module
Data above shows the temperature vs. time on the cold heat sink as a function of spacer composition (either Al or Cu). Because of the greater thermal conductivity of Cu, we predicted the Cu spacer would cause a larger ΔT. The minimum temperature observed for the Al spacer was ~10 oC (50 oF), while for Cu it was ~6 oC (43 oF).
Left: Picture of the bucket (containing 19 L or 5 gal of water) in the insulating enclosure between the 2 cooling units.
Above: schematic of the bucket in the enclosure from above. The black dots indicating the position of the temperature probes in the water.
Increasing the power of the TE unit from 96 W to 154 W dramatically increased the ΔT seen on the surface of the cold heat sink
Using a 154 W TE unit is the more optimal design as it brings about a greater decrease in temperature.
Spacer in Contact with Cold Side
Methods
Metal spacer
TE unit
Heat Sink
Picture of TE Cooling Unit
Effect of Using 2 TE units on Same Heat Sink
We designed and built a thermoelectric cooling prototype and initially wanted to determine the variables to have it cool most efficiently (maximize ΔT). Our prior work showed:
0.4 g of thermal grease is needed to establish good thermal contact without inducing a thermal flow between the TE plates (thermal shorts).
2. A Cu spacer is used to prevent thermal shorts and insulate the hot heat sink.
Moving forward, we needed to test:
Composition of metal spacer
Use of multiple TEs or several cooling units
Power of TE unit used (96 W & 154 W)
Number of fans used and orientation
We then interfaced the cooling unit within the insulating chamber and monitored the temperature change of the air within the enclosure. Methods used to increase the ΔT include:
Increasing the # of external fans used to dissipate heat
Increase the # of cooling units interfaced with the enclosure.
Al Cold Heat Sink
Al Hot Heat Sink
Cu Spacer TE
During the course of this trial we were able to cool a 5 gallon volume of water to 4.5 °C (41°F).
While the final temperature of the water is encouraging, the rate of temperature change in the water is slow. Took approximately 50 h for the water to plateau.
Conclusion
We found that:
A Cu spacer is needed to reduce thermal shorts between the TE plates and increase insulation between the hot and cold sides.
0.4 g of thermal grease is needed at the various contact points for good thermal contact, but not induce thermal shorts.
A more powerful TE unit brings about a greater temperature difference. But only one TE unit can be used on a cooling unit.
Using more fans to dissipate the heat increases the ΔT observed within the enclosure.
Using more cooling units significantly reduces the air temperature within the enclosure compared to using just one cooling unit.
When the cooling unit is assembled in this way an interfaced within the insulating enclosure, air temperatures of ~2 °C (35 °F) and water temperatures of ~4 °C (40°F ) can be achieved
We now also have a much deeper understanding of how factors such as thermal contact, thermal shorts and relative resistance within the TE unit affect the ultimate cooling capacity of the cooling unit.
We hoped that using more TE units would further increase the observed ΔT.
Interestingly, while running two TE units on the same heat sinks, the temperature was not as low as just using one.
It could be that by adding an additional TE unit, there is additional thermal shorts, bad thermal contact, or excessive heat build up on the hot heat sink.
Above, a schematic of how the cooling unit was assembled with 2 TE units.
Fan
Heat Sink
Orientation 1
Blower fan- removes heat from the hot heat sink by circulating cooler air. Helps to preserve TE unit.
Hot Heat Sink- prevents the TE unit from over-heating and burning out. provides efficient rejection of heat through fins
Metal Spacer- Insulates cold side from the hot side.
Cold heat sink­- used to increase surface area that is cooled by the TE unit.
Cold Heat Sink
Hot Heat Sink
Metal Spacer
Blower
Fan
TE Unit
TE Unit- uses the thermoelectric effect to produce the change in temperature. In this study, we vary the power of the TE units, from 96 W to 154 W.
Thermal Grease- applied at contact points between the various components to improve thermal contact.
DC Power Source
Fan
Orientation 2
Heat Sink
Fan
Fan
Using two fans dissipated more head from the hot heat sink, allowing more thermal energy to be rejected onto the hot heat sink.
Above: Two differing fan orientations to test the efficiency of heat removal on the hot heat sink. Orientation 2 was found to cool more effectively.
Prior Work
We conducted experiments to determine if having a conductive spacer was necessary to achieve a greater ΔT.
We ran trials with and without a Al spacer to separate the heat sinks.
We found that using the spacer between the cold side and heat sink that a greater temperature change occurred on the surface of the heat sink.
Also found that the optimum mass of thermal grease between the cooling components is 0.4 g
Future Work
In the up coming weeks we intend to investigate:
How contact cooling affects the cooling efficiency of the prototype.
Does increasing the TE power improve the cooling ability
By using two cooling units we were able to achieve an average air temperature of ~ 2 oC (36 oF).
This is a significantly greater ΔT than we were able to achieve with just one cooling unit, and at a faster rate.
This temperature would be suitable for the fermentation of largers which require a temperature of 40 oF.
Current cooling units are constructed of:
One 154 W TE unit.
A Cu spacer
Two Al heat sinks
2 5 W CPU blower fans
References
J.G. Jeffrey and E.S. Toberer, Nature Materials 7, (2008)
Above: Picture of the insulating enclosure with two cooling units interfaced on opposite sides of the enclosure.
Acknowledgements
We would like to acknowledge the University of Wisconsin-Whitewater Undergraduate Research Program which supported this project.
No Spacer