21-04-2011, 04:55 PM
Presented by:
Mark Campbell
Peter Giles
Andrew Smith
Thomas Strapps
Nickolay Suther
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Executive Summary
In this report we will outline the design requirements, the options considered, and the final design selected. Detailed information will be presented about the design process, including detailed and dimensioned illustrations created in Solid Edge. Calculations will be presented which will prove our design and component selections. The current status of the build process will be stated.
Team 12 will design and construct a working thermoelectric cooler that utilizes the Peltier effect to refrigerate and maintain a selected temperature from 5 to 25 degrees Celsius. Temperature will be maintained within +/- 0.2 degrees Celsius. The Peltier effect is defined as the “production or absorption of heat at the junction of two metals on the passage of a current. Heat is generated by the passage of the current in one direction and will be absorbed if the current is reversed.”
Thermoelectric coolers are advantageous because of their low noise and vibration levels. These are important characteristics when performing experiments which require a controlled environment with no outside disturbances. Heat removed due to the Peltier effect varies with the current applied to the circuit. Within the design limits of the cooling material, the cooling power is proportional to amperage applied. This is a desirable property because it facilitates fine tuning of the temperature inside the cabinet. The property will also lend itself to the use of a Proportional Integral Derivative (PID) device to control the temperature of the cooling unit.
A tentative budget has been prepared for the cost of the unit. Our client, Greenfield Research has generously granted the sum of $2000 for the cost of the unit. If the cost of the device exceeds this amount, the team has agreed to share equally any shortfalls. All receipts for reimbursement must be submitted to Dr. Basu by March 15, 2006.
Abstract
Team 12 will design and construct a thermoelectric cooler with an interior cooling volume of 0.016 cubic meters (25cm x 25cm x 25cm). One liter of water will be placed inside the cooled volume. This water will be used to test the performance of the device. Our design requirements are to cool this volume of water to a temperature ranging from room temperature to 5 degrees Celsius and to do so within a time period of 2 hours. Temperature fluctuations for the sample are not to exceed ± 0.2 degrees Celsius from the set temperature.
To achieve this goal we have designed a rectangular frame/panel box with all components fastened inside a main case (Figure 1, Appendix A). Two inch polystyrene insulation will be used between a fiberglass inner wall and a steel case. The door will be located at the top to ensure a good thermally secure seal (Figure 7, Appendix A). Proportional Integral Derivative (PID) control will be used to reach and hold the target temperature. Proportional control amplifies the signal control to put the response in the range of the target temperature. Integral control tracks past performance of the system and adjusts the signal based on what it ‘remembers’. Derivative control monitors the rate of change of the system response and preemptively adjusts response to minimize overshoot.
Forced air convection will be used inside the box to circulate the interior air and as well on the outside to remove the heated air (Figure 17, Appendix A). This convection will occur over two aluminum (Al) heat sinks with the thermoelectric cooling module (TEC) sandwiched between them. Both heat sinks will be shrouded (Figure 20, Appendix A) and have radial fans moving the air. Radial fans were chosen because they have a compact design and are better for moving air volume through vertically mounted fins.
Introduction
The aim our project is to develop and build a constant temperature air bath. An air bath has the advantage over liquid constant temperature baths in that it does not require immersion of the test sample into a liquid. The basic design will involve a cubic volume housed inside a well insulated structure. Cooling will be provided by two thermoelectric modules utilizing the Peltier Effect. Heat sinks will be attached on both sides of the thermo electric modules. This will ensure an even temperature gradient inside the box, and aid in the removal of heat on the hot side of the thermo electric module.
The device is intended for temperature control of research experiments or temperature sensitive specimens. Examples of applications might include biological research, transplant organ storage, or controlled cooling of materials for strain measurements etc.
Design Requirements
The following section outlines the required specifications of the project as described in the Design Requirements Memo submitted on October 3, 2005.
Requirements
Design requirements for the thermoelectric cooler are:
• Utilize Peltier effect to refrigerate and maintain a specified temperature
• Perform temperature control in the range 5 to 25 degrees Celsius.
• Maintain temperature accuracy within ± 0.2 C°
• Interior cooled volume of 0.016m3 (25cm x 25cm x 25cm)
• Greenfield Research will sponsor project to a maximum of $2000 in exchange for the device upon completion of the project.
• Low Noise and Vibration Levels
• Weight Less than 50 kg
The finished product is to meet or exceed these requirements housed in an aesthetically pleasing structure.
Deliverables
Throughout the Fall 2005 semester, numerous papers and memos have been submitted to document the design process. These deliverables are:
• Design Requirements Memo Oct 3, 2005
• Design Selection Memo Nov 7, 2005]
• Build Report I Nov 21, 2005
• Dec Presentation Dec 3, 2005
• Dec. Project Report Dec 5, 2005
• Lab Books submission Dec 5, 2005
• WWW page Dec 5, 2005
Design Process
The design of the device progressed over the Fall semester, 2005. Our team proceeded with the design process through a series of steps. These steps were: identification of the problem, analyze problem, brainstorm ideas, decide upon a design selection, and implement design. Redesign if necessary.
The main design considerations were:
• Heat Transfer Methods
• Geometry
• Controller
• Materials
• The following section will discuss these considerations.
Heat Transfer Methods
There are several methods which can be employed to facilitate the transfer of heat from the surface of the thermoelectric to the surrounding. These methods are described in the following three sections.
Natural Convection
Natural convection consists of an arrangement similar to forced convection except there is no fan to drive airflow through the heat sink. This results in significantly reduced convection coefficients. The advantage of this arrangement is the fact that there are no moving parts.
Liquid Cooled
The heat is removed from the surface of the thermoelectric module through the use of a heat exchanger. Through the heat exchanger a fluid is passed to remove the heat coming from the thermoelectric. The fluid is then passed through another heat exchanger where the heat is dissipated to the surrounding environment and the cycle is repeated. This method provides the highest rate of cooling due to the superior thermal conductivity of liquid over gas.
Forced Convection
In this arrangement a finned heat sink would be directly attached to the surface of the thermoelectric module. An electrically driven fan would provide turbulent airflow over and through the heat sink to remove the heat by forced convection.
Geometry
Two main geometries were considered for the device. The first was a cube. The advantage of this choice is its simplicity to build and insulate. A door can easily be attached to one of the sides. Finally any insulation, thermoelectric modules or heat sinks are easily fastened to the sides. The second choice for cooler geometry was a cylinder. The advantage found with this shape is that it has the largest volume to surface area ratio of the two designs considered. This is a good property when the objective is to minimize heat loss.
Controller
The job of the controller is to regulate the amount of power which is being sent to the thermoelectric. It bases this amount on the results of testing the interior temperature and comparing it with a desired set point temperature. There are several different types of controllers which can be employed to regulate the power.
On/Off
On/off controllers turn on or off depending on the temperature of the system relative to a value set by the user. If the system is at a higher temperature than the desired value the thermoelectric is turned on. If the system is cooler than desired the thermoelectric is turned off. In this case the thermoelectric receives either the maximum power it can handle or no power at all. This is undesirable as this type of cycling is very hard on the physical system.
Set Point/Manual
Manual control involves setting a desired current through the thermoelectric and allowing that current to continue flowing as long as the device is operating. This method does not give very accurate control of the temperature in the system.
PID Control
Proportional integral derivative (PID) control uses a feedback signal to compare the temperature inside the temperature controlled environment with the temperature set by the user. The PID controller adjusts the current supplied to the thermoelectric modules until the set temperature is reached. It is the most accurate way of controlling the device and it should, theoretically, leave a zero steady state error.
Materials
We explored three different materials for the construction of the outer casing and frame of the device (Figure 2 and 12, Appendix A). These were: aluminum, stainless steel, and fiberglass.
Aluminum
Aluminum is a soft and ductile material which is reasonably priced and readily available. However, aluminum is an excellent heat conductor so care would have to be taken to insulate the container. Building the outer casing and frame could pose a problem as welding aluminum is difficult because the material is prone to burn through.
Stainless Steel
Stainless steel is readily available and reasonably priced. It has a high mass density which would make the device stable and less susceptible to vibrations. Welding steel is easy and could be accomplished in any standard machine shop. However, building the device out of stainless steel would make it hard to move and difficult to place on a lab bench.
Fiberglass
Fiberglass is desirable as it has a low thermal conductivity. Building the device out of fiberglass would make it very light and portable while maintaining rigidity. Fiberglass is easy to cut and drill. The outer casing and container would be made by first making a positive mold and applying a cloth coated with fiberglass resin.
