23-09-2014, 09:59 AM
Thermoelectric Cooling
[attachment=68316]
Abstract
Team 12 designed and constructed a thermoelectric cooler with an interior cooling volume of 0.016 cubic meters (25cm x 25cm x 25cm). The thermo electric cooler was equipped with on/off control which was found to be adequate to meet the required precision of +/- 0.2 degrees Celsius put forth in the project requirements.
One liter of water was placed inside the cooler to test the performance of the device. We tested the maximum performance of the device by cooling a sample down to 5 degrees Celsius. Temperature control was also tested by cooling one liter of water from room temperature down to 10 degrees Celsius. On/off control was found to give adequate performance and we met or exceeded all of our project requirements set forth in the fall semester of 2005.
Introduction
Temperature controlled environments are crucial in biomedical research laboratories and medical institutes. Storing, preparing, analyzing samples, or culturing bacteria needs precisely controlled temperature environments. Currently, most commercial products are liquid baths. This requires the sample to be enclosed in a watertight test tube or petry dish in order to perform the experiment.
Team 12 took on the challenge of designing such a device using an air bath. This eliminates the need for the experimental specimen to be placed in a well sealed container before the experiment can commence. As laboratory equipment, the device must be easy to use, aesthetically pleasing, and control the temperature precisely and accurately. The final device also has to be free of vibrations to avoid disturbing any samples which may be put in the temperature controlled environment.
Thermo Electric Modules (TEMs) are effectively heat pumps that transfer heat from one side of the module to the other when a current is applied. This phenomenon is called the Peltier effect. The goal of this project is to utilize this phenomenon to build a temperature controlled environment free of vibration and meet all the project requirements
Bending and Crimping
The sheet metal equipment in the machine shop was used extensively to form the panels for the thermoelectric cooler. When using the metal shear care was exercise to keep fingers out of the clamp and blade portion of the machine. When using the brake to bend edges for the panel, care was exercised to keep fingers from being pinched and to stay out of the way of the bending lever if someone else was using it. When using the band saw, it was important to keep fingers away from the blade and not to use excessive force. Of course eyewear must be worn at all times in the machine shop.
Cooling fans
The fans which cool the device spin at speeds up to 3000 RPM. Therefore, it is important that the fan shrouds and guards be in place prior to operation.
Soldering
Many of the wire connections needed to be soldered. It was important to wear gloves and eye protection when using a soldering iron to prevent injury.
Although this device has relatively few moving parts, there are several topics of safety which should be addressed when using the thermoelectric cooler. The most dangerous component of the machine is the power supply. The power supply converts a 120V AC current, which is unusable by the components of the thermoelectric device into 12V DC which is what most components of the cooler use. The exception to this is the two 50 mm fans on the inside of the box which run on 5V this problem is solved by wiring these two fans in series so each only gets half of 12V. They then run close to their recommended voltage. 120V AC is dangerous. 12V DC is not dangerous at the current load our device runs at. Fortunately, the power supply is sealed off and electrically isolated. As long as the power supply is not disassembled and a proper 3 prong plug connector is employed, it should not pose a safety threat. Whenever electrical components are being worked on, ensure that the power cord is disconnected from the wall plug.
Another safety issue when using the box is its weight. The box weighs about 40 lbs and care must be taken to put it on a flat level surface when being used. The surface should also be stable. This will ensure that the box will not fall when being used possibly causing injury to the user and definitely damaging the box.
Some of the edges on the box are somewhat sharp. As much as possible, we have grinded and sanded down the edges of the panels. Both the top panel and the sheet metal on the lid have had the seams TIG welded by Albert so that no sharp edges or corners would be exposed when the box is assembled. However, if the box is disassembled to change the thermo-electrics or to make changes to the controller, special care must be exercised. The fit of the panels is quite snug so attention must be paid when popping panels into place.
This device contains sensitive electrical equipment. It should be operated in an area where static electricity is not present. The user should ensure that they are well grounded and not statically charged when handling any of the electrical components of the device. Although there is no danger posed to the user by static electricity, components could be damaged or ruined, requiring replacement and or reprogramming.
Finally, the temperature at the thermoelectric modules is a concern. At temperatures above 70 °C, the bismuth telluride material will melt and the modules will subsequently fail. To avoid this potential problem, a thermostatic switch has been installed in the device. This switch is to be attached to one of the external heat sinks on the cooler. It is wired inline with the power supply to the thermo electrics. If the temperature of the heat sinks exceeds 50 °C, the power to the thermoelectric is cut and the unit stops producing heat. This feature is essential to ensure long life of the thermoelectric modules.
6 Recalibration
It has been noted in testing that the device maintains and displays a temperature that is not exactly representative of the actual temperature inside the chamber. To rectify this problem the device must be recalibrated using accurate voltage and temperature data measured over the entire operating temperature range of the device. The new more accurate calibration curve would then have to be put into the micro-controller programming in place of the existing calibration curve.
8.3.7 Portability
It could be possible to make this device more useful if it were capable of being operated from a battery pack or a solar collector. This option would allow for the device to be used in areas without electricity as well as in vehicles. For this to be possible the performance and energy efficiency of the device would have to be maximized. There are two key ways that the efficiency of the device could be increased. Firstly the ON/OFF control could be replaced with a PID control in order to use the thermoelectric modules at a lower power while they are on. This will increase the efficiency since thermoelectric modules generate excess heat as the power input is increased lowering their performance. Secondly additional thermoelectric modules could be added to decrease the load on each individual module. The lower the load the more efficiently the modules will run.
[attachment=68316]
Abstract
Team 12 designed and constructed a thermoelectric cooler with an interior cooling volume of 0.016 cubic meters (25cm x 25cm x 25cm). The thermo electric cooler was equipped with on/off control which was found to be adequate to meet the required precision of +/- 0.2 degrees Celsius put forth in the project requirements.
One liter of water was placed inside the cooler to test the performance of the device. We tested the maximum performance of the device by cooling a sample down to 5 degrees Celsius. Temperature control was also tested by cooling one liter of water from room temperature down to 10 degrees Celsius. On/off control was found to give adequate performance and we met or exceeded all of our project requirements set forth in the fall semester of 2005.
Introduction
Temperature controlled environments are crucial in biomedical research laboratories and medical institutes. Storing, preparing, analyzing samples, or culturing bacteria needs precisely controlled temperature environments. Currently, most commercial products are liquid baths. This requires the sample to be enclosed in a watertight test tube or petry dish in order to perform the experiment.
Team 12 took on the challenge of designing such a device using an air bath. This eliminates the need for the experimental specimen to be placed in a well sealed container before the experiment can commence. As laboratory equipment, the device must be easy to use, aesthetically pleasing, and control the temperature precisely and accurately. The final device also has to be free of vibrations to avoid disturbing any samples which may be put in the temperature controlled environment.
Thermo Electric Modules (TEMs) are effectively heat pumps that transfer heat from one side of the module to the other when a current is applied. This phenomenon is called the Peltier effect. The goal of this project is to utilize this phenomenon to build a temperature controlled environment free of vibration and meet all the project requirements
Bending and Crimping
The sheet metal equipment in the machine shop was used extensively to form the panels for the thermoelectric cooler. When using the metal shear care was exercise to keep fingers out of the clamp and blade portion of the machine. When using the brake to bend edges for the panel, care was exercised to keep fingers from being pinched and to stay out of the way of the bending lever if someone else was using it. When using the band saw, it was important to keep fingers away from the blade and not to use excessive force. Of course eyewear must be worn at all times in the machine shop.
Cooling fans
The fans which cool the device spin at speeds up to 3000 RPM. Therefore, it is important that the fan shrouds and guards be in place prior to operation.
Soldering
Many of the wire connections needed to be soldered. It was important to wear gloves and eye protection when using a soldering iron to prevent injury.
Although this device has relatively few moving parts, there are several topics of safety which should be addressed when using the thermoelectric cooler. The most dangerous component of the machine is the power supply. The power supply converts a 120V AC current, which is unusable by the components of the thermoelectric device into 12V DC which is what most components of the cooler use. The exception to this is the two 50 mm fans on the inside of the box which run on 5V this problem is solved by wiring these two fans in series so each only gets half of 12V. They then run close to their recommended voltage. 120V AC is dangerous. 12V DC is not dangerous at the current load our device runs at. Fortunately, the power supply is sealed off and electrically isolated. As long as the power supply is not disassembled and a proper 3 prong plug connector is employed, it should not pose a safety threat. Whenever electrical components are being worked on, ensure that the power cord is disconnected from the wall plug.
Another safety issue when using the box is its weight. The box weighs about 40 lbs and care must be taken to put it on a flat level surface when being used. The surface should also be stable. This will ensure that the box will not fall when being used possibly causing injury to the user and definitely damaging the box.
Some of the edges on the box are somewhat sharp. As much as possible, we have grinded and sanded down the edges of the panels. Both the top panel and the sheet metal on the lid have had the seams TIG welded by Albert so that no sharp edges or corners would be exposed when the box is assembled. However, if the box is disassembled to change the thermo-electrics or to make changes to the controller, special care must be exercised. The fit of the panels is quite snug so attention must be paid when popping panels into place.
This device contains sensitive electrical equipment. It should be operated in an area where static electricity is not present. The user should ensure that they are well grounded and not statically charged when handling any of the electrical components of the device. Although there is no danger posed to the user by static electricity, components could be damaged or ruined, requiring replacement and or reprogramming.
Finally, the temperature at the thermoelectric modules is a concern. At temperatures above 70 °C, the bismuth telluride material will melt and the modules will subsequently fail. To avoid this potential problem, a thermostatic switch has been installed in the device. This switch is to be attached to one of the external heat sinks on the cooler. It is wired inline with the power supply to the thermo electrics. If the temperature of the heat sinks exceeds 50 °C, the power to the thermoelectric is cut and the unit stops producing heat. This feature is essential to ensure long life of the thermoelectric modules.
6 Recalibration
It has been noted in testing that the device maintains and displays a temperature that is not exactly representative of the actual temperature inside the chamber. To rectify this problem the device must be recalibrated using accurate voltage and temperature data measured over the entire operating temperature range of the device. The new more accurate calibration curve would then have to be put into the micro-controller programming in place of the existing calibration curve.
8.3.7 Portability
It could be possible to make this device more useful if it were capable of being operated from a battery pack or a solar collector. This option would allow for the device to be used in areas without electricity as well as in vehicles. For this to be possible the performance and energy efficiency of the device would have to be maximized. There are two key ways that the efficiency of the device could be increased. Firstly the ON/OFF control could be replaced with a PID control in order to use the thermoelectric modules at a lower power while they are on. This will increase the efficiency since thermoelectric modules generate excess heat as the power input is increased lowering their performance. Secondly additional thermoelectric modules could be added to decrease the load on each individual module. The lower the load the more efficiently the modules will run.