Refrigeration or Cooling may be defined as the process of removing heat. This technique may be achieved by using one of the refrigeration systems; vapor compression, absorption or thermoelectric refrigeration systems. The first two systems need high and low pressure edges of a working substance to complete the refrigeration pattern. The thermoelectric refrigeration system, however, uses electrons somewhat than refrigerant as a heating carrier. (Davis, 2005)
Thermoelectric coolers are greatly needed, especially for the producing countries situation where extended life and zero-maintenance are needed. In such a aspect, thermoelectrics can't be challenged, in spite of the fact that their coefficient performance is not as high for a vapor compression routine. Thermoelectric refrigerators hold the features of being small, compact, rugged, reliable, and insensitive to orientation, noiseless, portable and low priced in mass production. (Davis, 2005) Thermoelectric cool has been trusted in armed forces, aerospace, device, and industrial or commercial products, as a cooling device for specific purposes. This technology has been around for about 40 years. (Riffat, 2000) Many experts are worried about the physical properties of the thermoelectric materials and the making technique of thermoelectric modules. As well as the improvement of the thermoelectric material and module, the system analysis of a thermoelectric refrigerator is evenly important in creating a high-performance thermoelectric refrigerator. (Huang, 2000)
The heating flux generated in the processor chip is rising day by day at an extremely fast rate with development because of reduction in CPU sizes and large amount of heat load produced at the chip. Consequently, it is now a challenging process for researchers to take care of such enormous amounts of heat fluxes.
Moore's had proven that quantity of transistors on the included circuit is increasing exponentially every year. So heat weight in the CPU also boosts at the same rate with the increase in the speed in addition how big is the chip today which we normally talk about is of the order of mm which is subsequently making problem more difficult. The high temperature era inside the CPU may cause slowing the computation acceleration, failure of the processor chip, gate oxide break down, effect on screen resolution and so many more electrical failures as well as mechanised failures (Davis, 2005)
Presently in CPU very complicated designs of air cooled high temperature sinks are used which dissipates high temperature to the surroundings by moving large amounts of air. These heat sinks have two major shortcomings.
Due to space constrains air should be thrown at very high velocities and maintain such velocities big size lover needs to be used.
Also, the environment streaming at high velocities creates a great deal of noise.
Moreover, in air cooled devices there is no active chilling device so we can not go below the ambient temperature. Because of this working at high speeds in the hot ambient conditions possessed become extremely difficult.
Chip cooling down is one of the bottlenecks in the high density consumer electronics. There is certainly need of some better cooling approaches for the same. So, now a day analysts will work a whole lot on liquid cooled systems, because they have practically 10 times (Davis, 2005) heat transfer coefficient than that of air cooled ones. In normal water based liquid cooling down systems, the heat is pumped to water stop by some cooling down device that water eliminates the heat to finally chuck it in the surroundings. The mostly used device to pump heat to water block in such system is TEC (Thermoelectric coolers). TEC consumes their own electric power and cool off the chip by extracting heat from it and moving it to this inflatable water block. Water operates inside the channel in the water block and eliminates heat from it. The hot water is further cooled in the condenser. With the help of Thermoelectric water coolant system the chip heat range can be easily designed to go below ambient heat range which is extremely hard by the prevailing systems, and so the CPU can be made to operate at high speeds and higher loads in even hot ambient conditions. Thus TEC have potential opportunities for chip chilling and can establish very effective if an effective system is developed for the same.
1. 2 Problem statement
Chip cooling is one of the bottlenecks in high density gadgets. An enormous amount of warmth flux is generated by the present day processor chip. Nowadays many complicated designs of air cooled temperature sinks are being used, but off past due heat fluxes have achieved such an even that to take care of them large volume move rate of air is required. So scheduled to space constraint, in order to accomplish large move rates, air should be blown at high velocities which lead to increased degrees of noise. Another major drawback of air cooling is that we can't go below ambient temp and as a consequence, propensity of chip failure in the personal computers employed in ambient condition of about 35C - 45C rises a whole lot.
For each one of these reasons it is becoming apparent that the heat fluxes reach such a level that air cooling can't deal with them efficiently. Thus the present scenario necessitates the use of active cooling devices. Thermoelectric coolers to be able to cool below ambient and having good thing about being compact, light-weight, free of moving parts and precise heat range control have high potentials for chip air conditioning.
It is known that the heat of the thermoelectric component is the main criterion for its reliability and performance. The heat range rise of the hot side above ambient is dependent on the thermal amount of resistance of the road that heat sink. Lowering the thermal resistance of heat sink plays a part in the reduction of the thermal level of resistance of the path and hence a rise in the performance. So a liquid high temperature exchanger with spiral flow passage having dimples is used. Dimples result in effective heat transfer by creating turbulence and thus boosting the performance of the machine.
1. 3 Research Objectives
Understanding the basics of Thermoelectric coolers, working of Thermoelectric Air conditioning Systems and parameters that governs the performance of such systems
Design, fabrication and development of an efficient thermoelectric cooling system for computer chips
Carrying out experimentation and research of the performance of the developed system
1. 4 Methodology
1. 5 Work Plan
These are some of the important duties that might be performed during this research
Understanding the essential ideas of thermoelectric cooling
Study of the prevailing CPU cooling techniques
Literature review about the topic and review about the result due dimples over the move of water
Deciding the many parameters for which system should be designed
Deciding about the thermoelectric component that may produce the desired cooling effect
Design of the experimental create and recognition of the various gadgets to be required
Market survey for all your required equipments
Procurement of the equipments
Design and fabrication of the dimpled water block
Design and fabrication of heat exchanger
Preparation of the experimental collection up
Carrying out experiments and acquiring the results
Analysis of results
Checking out the performance of the thermoelectric module used
Comparison of the designed normal water stop with some commercially existing normal water block
Discussions and conclusion
Report writing
1. 6 Expected Outcomes
An knowledge of the application of thermoelectric cooling systems would be developed. Important features of the thermoelectric cooling down systems in current circumstance of high density gadgets would be shown. The entire thermoelectric cooling unit for CPU chip would be designed, fabricated and analyzed for the desired loads
Chapter 2:
LITERATURE REVIEW
2. 1 THE ANNALS of Thermoelectrics
In 1821, Thomas Seebeck found out that a constantly flowing current is created when two wiring of different materials are signed up with together and heated up at one end. This notion is known as the Seebeck Impact (Number 1. 1). The Seebeck result has two main applications including temp measurement and vitality generation. (Global Techno Scan, nd)
Figure 1. 1 Seebeck Effect
S= dV / dT;
S is the Seebeck Coefficient with products of Volts/K
S is positive when the route of electric current is same as the course of thermal current
In 1834, a French watchmaker and part time physicist, Jean Peltier found that an electrical current would produce a heat gradient at the junction of two dissimilar metals. This result is recognized as the Peltier Impact. This idea forms the basis for the thermoelectric refrigerator (Global Techno Scan, nd)
Figure 1. 2 Negative Peltier effect
a) For <0; Negative Peltier coefficient
When current is permitted to go through n-type semiconductor shown in above circuit, high energy electrons move from to left leading to cooling of far off end. Thermal current and electric current flow in other directions (Global Techno Check out, nd)
Figure 1. 3 Positive Peltier effect
b) For >0; Positive Peltier coefficient
When current is allowed to go through p-type semiconductor shown in above circuit, high energy slots move from still left to right leading to heating of very far end. Thermal current and electric energy circulation in same path (Global Techno Check out, nd)
q=*j, where q is thermal current density (Warmth flux) and j is electric current density.
Also, = S*T (Volts) Peltier coefficient
Where, T is the Complete Temperature
Scottish scientist William Thomson (later Lord Kelvin) found out in 1854 that when a temperature difference is accessible between any two details of any current carrying conductor, warmth is either evolved or utilized depending upon the materials. If such a circuit absorbs temperature, then high temperature may be evolved if the course of the existing or of the temperatures gradient is reversed.
2. 2 Thermoelectric Refrigeration
A thermoelectric device is one which operates over a circuit that contains both thermal and electronic effects to convert heat energy into electrical energy or electrical energy to a temps gradient. Thermoelectric elements perform the same cooling function as Freon -structured vapor compression or absorption refrigerators. Energy is extracted from a region therefore reducing its temp. The vitality is than turned down to a temperature sink region with an increased temp. Thermoelectric elements are in a completely solid state, while vapor circuit devices have moving mechanised parts that want a working smooth (Tellurex, nd)
Thermoelectric modules are small, durable, quiet heat pumps managed with a DC vitality source. They usually last about 200, 000 hours in heating mode or about 20 years if left on cooling method. When power is supplied, the top where heat energy is absorbed becomes cold; the opposite surface where heat energy is released becomes hot. In the event the polarity of current circulation through the module is reversed, the frigid side can be the hot area and vice-versa. Thermoelectric modules can also be used as thermocouples for temps measurement or as generators to provide capacity to spacecrafts and electric powered equipment. (Tellurex, nd)
Thermoelectric devices can even be used as refrigerators on the bases of the Peltier result. To create a thermoelectric refrigerator, high temperature is absorbed from a refrigerated space and than declined to a warmer environment. The difference between both of these quantities is the web electrical work that needs to be offered. These refrigerators aren't overly popular because they may have a minimal coefficient of performance. The coefficient of performance for thermoelectric refrigerators can be computed by dividing the cooling effect by the task source. (Tellurex, nd)
2. 3 Semiconductors
The semiconductor materials are N and P type, and are so named because either they have significantly more electrons than essential to complete a perfect molecular lattice framework (N-type) or not enough electrons to complete a lattice framework (P-type). The extra electrons in the N-type materials and the holes kept in the P-type material are called "carriers" and they're the agents that move the heat energy from the cold to the hot junction. Temperature utilized at the frigid junction is pumped to the hot junction for a price proportional to carrier current passing through the circuit and the number of couples. Good thermoelectric semiconductor materials such as bismuth telluride greatly impede regular temperature conduction from hot to wintry areas, yet provide a simple stream for the carriers. Furthermore, these materials have carriers with a convenience of transferring more temperature. Since semiconductors were found to have large Seebeck coefficients, good electrical conductivities, and poor thermal conductivities, you have made a discovery in the utilization of the Peltier-effect in thermoelectric devices to create refrigeration. Presently, thermoelectric refrigerators, manufactured from semiconductor, materials, have many interesting applications for their small size, simplicity, quietness and stability.
2. 4 Basic Working Rule of Thermoelectric Coolers
Thermoelectric coolers are semiconductor devices which works on the process of Peltier result (Haung, 2005) i. e. whenever a current is exceeded between the junctions of two dissimilar materials a temperature difference is created between your two junctions. In thermoelectric coolers we have a P type and an N type semiconductor connected together whenever we pass the existing, at the wintry junction electrons travel from P aspect to N side as P type coming to lower energy level and N type at higher vitality. So when electron vacations it absorbs energy at the frigid side. In the same way at the hot area electron moves from N part to P aspect thereby releasing the power. And in this way a temperatures gradient is established between hot side and cold side (Haung, 2005)
2. 5 Advantages of Thermoelectric Coolers
Thermoelectric coolers have some unique advantages over other cooling systems. The many advantages are (Chien, 2004)
Ideal for localized cooling credited to small size
Highly controllable cooling power
Convenient electric power supply
Precise heat control
Sub-ambient cooling capacity
Spot cooling
Compact, Quite, and free from moving parts
Low maintenance
2. 6 An average thermoelectric coolant system:
Fig. 1 shows working of an thermoelectric system. The heating and cooling functions of the thermoelectric system can be interchanged by reversing the polarity of the direct current put on it. Capacity control in a thermoelectric system can be achieved by differing the voltage put on the couples either by way of a varying voltage control or by turning series and parallel circuits. As the voltage drops, the heat range difference between the hot and cold area is reduced. (Chien, 2004)
On the frosty side of the module we have the heat source from which heat is to be removed and on the hot side we have a heat sink which finally throws heat in to the ambient. Design of heat sink can be an important parameter for improving the performance of the thermoelectric component. For many applications, the advantages of TEC outweigh its main drawback of low coefficient of performance. (Chien, 2004)
Figure 2. 1 Thermoelectric chiller (Chien, 2004)
2. 7 Thermal Parameters Regulating Performance of TEC
The collection of a thermoelectric for a particular application is principally dependent on the three important variables. These are temperatures of the hot surface (Th), the temp of the cool surface (Tc) and the quantity of temperature to be extracted at the chilly part of the module (Qc) (Haung, 2005).
The heat kitchen sink is fastened at the hot part of the component where the high temperature gets released when the DC power is applied to the component. The hot aspect heat range of the module while using a air cooled heat kitchen sink whether natural or obligated convection, can be found out using below equations (Haung, 2005)
Th = Tamb + Rth(Qh) (1. 1)
Qh = Qc + Qp (1. 2)
The temperatures rise above ambient, of the hot part, takes place due to thermal resistance of the heat. If we know the thermal amount of resistance of heat sink then your general estimates of the surge in temp above ambient are as explained below: (Haung, 2005)
20C to 40C in case there is Natural Convection
10C to 15C in case there is Forced Convection
2C to 5C in case of Liquid Cooling (In cases like this is the rise above the liquid coolant temps)
The performance coefficient for a thermoelectric cooling system can be determined with the help of the following formula:
2. 8 Request of thermoelectric cooling
TEC (Thermoelectric Cool) differs from classic compression refrigeration; there are no moving parts. Since there are no moving parts, there is certainly nothing to wear out and there is nothing generating noise. There is no refrigerant to contain so the problem of managing a two-phase change over is simplified. Pressure restricted tubing is substituted by electric powered wiring. There is absolutely no ozone layer risk (Melcor, nd). Thermoelectric coolers provide potential to enhance the cooling of electronic module packages to reduce chip operating temperature or to allow higher module capabilities. Thermoelectric coolers also offer the benefits of being compact, reliable, and their degree of cooling may be handled by the existing supplied. Unfortunately, compared to vapor-compression refrigeration, they can be limited in heat flux that they can accommodate and exhibit a lesser coefficient of performance (COP). Both of these restrictions have generally limited thermoelectrics to market.
The thermoelectric coolers are used in the electronics of the luxury cruise missile, critical equipment on aircraft, critical camera components in a pod aircraft navigation system and many military applications. Thermoelectric coolers provide small heat exchangers that aren't attitude-sensitive, nor contain excessive tubing and fittings that can be vunerable to vibration
2. 9 Earlier Work done at International level
Till today air coolers are get together the needs of CPU cooling, with the increased size of heat sinks and a rise in fan swiftness. The typical resistance of air coolers with high fan rates of speed is 0. 2C/W (Bar Cohen, 2000). But with further upsurge in the heat flux, air cooling down techniques seems to be diminishing because of the limitations already mentioned. The next best answer to the situation is the use of liquid air conditioning techniques as the liquids have relatively high convective heat copy coefficients then air and so lessening the thermal amount of resistance. The liquid cooling systems will involve "water stop" for useful heat transfer to the liquid.
Experiments have been completed by mounting drinking water blocks directly over the CPU chip and they have shown to be very successful then the air cooled techniques. With the use of immediate water-cooling techniques the chip temp can be kept at 30C for an ambient condition of 25C with a CPU insert of 60W whereas with air air conditioning it goes to 45C (Bar Cohen, 2000).
But with the introduction of thermoelectric coolers it experienced made possible to use the chip temperatures even below ambient. Thermoelectric Coolers have unique advantages over other chilling devices
Chein and Huang (2004) analyzed utilization of thermoelectric cooler for electronic cooling down. The cooling capacity, junction temperature, coefficient of performance (COP) of TEC and the mandatory heat sink level of resistance at the hot aspect were computed. They discovered that the cooling down capacity could be increased as Tc is increased and ‹T is reduced. The utmost chilling capacity and chip junction temp obtained were 207 W and 88C, respectively. The mandatory heat sink level of resistance on TEC hot aspect was found to be. 054C/W. A micro channel heat kitchen sink ( with size of 55mm x 55mm with channel width of 0. 3 mm) using drinking water or air as coolant was also demonstrated to meet the low thermal warmth sink resistance requirement of TEC run at maximum cooling down capacity conditions.
Huang et al in 2005 analyzed the distribution of temp for a thermoelectric cool under the effects of Joule heating up, Fourier High temperature conduction, Thomson impact and convection and radiation heat transfer. They tried out to enalyze and explore some of the important things like Thomson effect's affect on the syndication of temps, on the amount of heat that flows back to the cold aspect, the maximum temperatures difference accomplished and the maximum amount of high temperature extracted etc. They finally figured other than enhancing the thermoelectric materials for increasing the cooling down efficiency of the component the other possible way is to adopt the advantage of Thomson result this also helps in enhancing the chilling efficiency.
Researches already are happening for incorporating thermoelectric normal water cooling down systems for the CPU. The main element factor in using such kind of systems is to possess highly efficient drinking water blocks with low thermal resistances also to have thermoelectric modules with proper Qmax. Many complicated water block design are present in today's market. The typical thermal resistances of this blocks used presently for such systems are 0. 08C/W (Bar Cohen, 2000).
