Report Thermoelectric Refrigerator 113w5f

Many individual have proudly influenced us during our Studies (B.TECH)

at

PUNJAB

COLLEGE

OF

ENGINEERING

&

TECHNOLOGY, Lalru-Mandi, Mohali, Punjab. And it is pleasure to acknowledge their guidance and . At PCET, we learned many things like the project training is mainly aimed at enabling the student to apply their theoretical knowledge to practical as "The theory is to know how and practical is to do how" and to appreciate the limitation of knowledge gained in the class room to practical situation and to appreciate the importance of discipline, punctuality, team work, sense of responsibility, money, value of time, dignity of labour. We would will like to express my gratitude towards Er.GURMEET SIR who took keen interest in our project, who helped me in every possible way and is source of inspiration for all the group . I would also like to thanks Dr. Beant Singh (HOD), Mechanical Engineering who motivated us to complete our project with enthusiasm and hard work.

THANK YOU

ABSTACT

The purpose of our project is to use this electricity or DC voltage generated from a battery source or a solar , to directly convert the DC voltage applied on the Thermoelectric refrigerator into refrigeration effect on the other side based on peltier effect and Thermo Electric Cooling. This kind of thermoelectric refrigerators are one of the latest advancements in modern world and its maximum applications are in transporting small elements/ medicines from one place to another. The main principle of operation of our Thermoelectric Refrigerator is to produce a positive and negative temperature effects on its either junctions by supplying an electrical supply voltage to it and with extra heat available on the plate through solar heat to generate a cooling effect on the other side. This refrigerator has an added advantage over existing cooling devices and refrigerants that it is a very fast device; it could produce a cooling effect of upto -8 .C in 1-2 seconds. Also, this solar refrigerator is also applicable as one of the best heat sinks for microprocessors of U’s and other electrical devices. In industries, thermoelectric Refrigerators are used for the storage of medicines and solar based cooling devices. Also, complete material required for air and water cooling devices are very large in amount including the tubes and containers used for carrying the water as a coolant and its circulation inside the coolers. The use of highly effective, solar energy efficient and compact peltier plate based thermoelectric Refrigerator working on thermo-electric effect has been presented as an advantage over older techniques of cooling.

INTRODUCTION 1. AIM Aim of our project is to design a Thermoelectric Refrigerator that could generate a cooling effect on one side and heat sinking on other side with DC voltage as input. The complete Thermoelectric Refrigerator would be based on Peltier Effect. The purpose of our project is to use this electricity to directly convert voltage applied on one side of the surface of our Thermo electric element into refrigeration effect on the other side based on peltier effect and Thermo Electric Cooling. 2. INTRODUCTION Thermoelectric refrigerators are one of the latest advancements in modern world and its maximum applications are in transporting small elements/ medicines from one place to another where a only solar light is available to us and this solar heat is used to generate enhanced cooling effect on the other side of the thermo-electric element. The main principle of operation of our Solar Refrigerator is to produce a positive and negative temperature effects on its either junctions by supplying an electrical supply voltage to it and with extra heat available on the plate through solar heat to generate a cooling effect on the other side. In industries, thermoelectric Refrigerators are used for the storage of medicines and solar based cooling devices. The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice-versa. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when a voltage is applied to it, it creates a temperature difference. At the atomic scale, an applied temperature gradient causes charge carriers in the material to diffuse from the hot side to the cold side, similar to a classical gas that expands when heated; hence inducing a thermal current. This technology is far less commonly applied to refrigeration than vapor-compression refrigeration is. The main advantages of a thermo electric cooler (compared to a vaporcompression refrigerator) are its lack of moving parts or circulating liquid, and its small size and

flexible shape (form factor). Its main disadvantage is that it cannot simultaneously have low cost and high power efficiency. Many researchers and companies are trying to develop thermo electic coolers that are both cheap and efficient. A thermo electric cooler can also be used as a thermoelectric generator. When operated as a cooler, a voltage is applied across the device, and as a result, a difference in temperature will build up between the two sides. When operated as a generator, one side of the device is heated to a temperature greater than the other side, and as a result, a difference in voltage will build up between the two sides (the Seebeck effect). However, a well-designed thermo electric cooler will be a mediocre thermoelectric generator and vice-versa, due to different design and packaging requirements.

3. PERFORMANCE OF THERMOELECTRIC ELEMENTS Thermoelectric

junctions

are

generally

only

around

5–10%

as

efficient

as

the

ideal refrigerator (Carnot cycle), compared with 40–60% achieved by conventional compression cycle systems (reverse Rankine systems using compression/expansion). Due to the relatively low efficiency, thermoelectric cooling is generally only used in environments where the solid state nature (no moving parts, maintenance-free, compact size) outweighs pure efficiency. thermoelectric cooler performance is a function of ambient temperature, hot and cold side heat exchanger (heat sink) performance, thermal load, thermo electric module (thermopile) geometry, and thermo electric electrical parameters.

Figure 2: Thermo electric element schematic. Thermoelectric legs are thermally in parallel and electrically in series

Construction TERs are constructed using two dissimilar semi-conductors, one n-type and the other p-type (they must be different because they need to have different electron densities in order for the effect to work). The two semiconductors are positioned thermally in parallel and ed at one end by a conducting cooling plate (typically of copper or aluminum). A voltage is applied to the free ends of two different conducting materials, resulting in a flow of electricity through the two semiconductors in series. The flow of DC current across the junction of the two semi-conductors creates a temperature difference. As a result of the temperature difference, Peltier cooling causes heat to be absorbed from the vicinity of the cooling plate, and to move to the other (heat sink) end of the device. A typical TEC inner view and actual picture is shown below:

TEC inner view The heat is carried through the cooler by electron transport and released on the opposite ("hot") side as the electrons move from a high to low energy state. When the two materials are connected to each other by an electrical conductor, a new equilibrium of free electrons is established. Potential migration creates an electrical field across each of the connections. When current is subsequently forced through the unit, the attempt to maintain the new equilibrium causes the electrons at one connection to absorb energy, while those at the other connection release energy. In practice many TEC pairs (or couples), such as described above, are connected side-by-side, and sandwiched between two ceramic plates, in a single TEC unit.

Figure 2: Working of a TEC The heat pumping capacity of a cooler is proportional to the current and the number of pairs in the unit.

ADVANTAGES OVER EXISTING COOLING TECHNIQUES Air Cooling, Oil Cooling, Water Cooling, etc. are some of the techniques that have been implemented in commercial transformers for reducing the heating losses and increasing the transformer efficiency and life. Due to some of the disadvantages of such coolants like slow effects, bulky material requirement, large space coverage and cost, thermoelectric effect cooling can be devised and used for the same. This fundamental difference gives solid-state thermoelectric coolers the following advantages over compressors: 

No moving parts. Therefore they require little or no maintenance. Ideal for cooling parts that may be sensitive to mechanical vibration.



No refrigerants, such as potentially harmful CFCs. Therefore environmental and safety benefits.



Enables reduced, low-noise operation of cooling fans, while providing greater cooling power.



Suitable for manufacture in very small sizes. Therefore ideal for microelectronics.



Lightweight.



Long life. Exceeds 100,000 hrs MTBF (Mean Time Between Failures).



Controllable (by voltage / current).



Small size.



Fast, dynamic response.



Enhanced ration between heat sink and target element.



Can provide cooling below ambient temperature.

APPLICATIONS OF THERMO ELECTRIC BASED COOLING TECHNIQUE Numerous applications on refrigeration and cooling can be mentioned on this latest technique. Some of practical implementation of the same has been mentioned below: 1. Thermo-electric coolers

2. Increasing efficiency of various electrical devices by efficient heat sinking 3. Solar Based Refrigerators Design 4. Thermoelectric AC 5. Waste Heat Utilization System Design for Refrigeration 6. For Cooling Car Seats 7. For Cool Satellites And Space Craft 8. For Making Humidifiers 9. For Laser Cooling 10. Car Batteries

HEAT SINK & THERMAL PASTE:In electronic systems, a heat sink is a ive component that cools a device by dissipating heat into the surrounding air. Heat sinks are used to cool electronic components such as high-power semiconductor devices, and optoelectronic devices such as higher-power lasers and light emitting diodes (LEDs). Heat sinks are heat exchangers such as those used in refrigeration and air conditioning systems, or the radiator in an automobile. A heat sink is designed to increase the surface area in with the cooling fluid surrounding it, such as the air. Approach air velocity, choice of material, fin (or other protrusion) design and surface treatment are some of the factors which affect the thermal performance of a heat sink. Heat sinks are used to cool computer central processing units or graphics processors. Heat sink attachment methods and thermal interface materials also affect the eventual die temperature of the integrated circuit. Thermal adhesive or thermal grease fills the air gap between the heat sink and device to improve its thermal performance. Theoretical, experimental and numerical methods can be used to determine a heat sink's thermal performance. Basic heat sink heat transfer principle A heat sink is an object that transfers thermal energy from a higher temperature to a lower temperature fluid medium. The fluid medium is frequently air, but can also be water or in the

case of heat exchangers, refrigerants and oil. If the fluid medium is water, the 'heat sink' is frequently called a cold plate. In thermodynamics a heat sink is a heat reservoir that can absorb an arbitrary amount of heat without significantly changing temperature. Practical heat sinks for electronic devices must have a temperature higher than the surroundings to transfer heat by convection, radiation, and conduction. To understand the principle of a heat sink, consider Fourier's law of heat conduction. Fourier's law of heat conduction, simplified to a one-dimensional form in the x-direction, shows that when there is a temperature gradient in a body, heat will be transferred from the higher temperature region to the lower temperature region. The rate at which heat is transferred by conduction,

, is

proportional to the product of the temperature gradient and the cross-sectional area through which heat is transferred.

Design factors which influence the thermal performance of a heat sink:-

Thermal resistance For semiconductor devices used in a variety of consumer and industrial electronics, the idea of thermal resistance simplifies the selection of heat sinks. The heat flow between the semiconductor die and ambient air is modeled as a series of resistances to heat flow; there is a resistance from the die to the device case, from the case to the heat sink, and from the heat sink to the ambient. The sum of these resistances is the total thermal resistance from the die to the ambient. Thermal resistance is defined as temperature rise per unit of power, analogous to electrical resistance, and is expressed in units of degrees Celsius per watt (C/W). If the device dissipation in watts is known, and the total thermal resistance is calculated, the temperature rise of the die over ambient can be calculated. The idea of thermal resistance of a semiconductor heat sink is an approximation. It does not take into non-uniform distribution of heat over a device or heat sink. It only models a system in thermal equilibrium, and does not take into the change in temperatures with time. Nor

does it reflect the non-linearity of radiation and convection with respect to temperature rise. However, manufacturers tabulate typical values of thermal resistance for heat sinks and semiconductor devices, which allows selection of commercially manufactured heat sinks to be simplified. The most common heat sink materials are aluminium alloys Aluminium alloy 1050A has one of the higher thermal conductivity values at 229 W/m• K but is mechanically soft. Aluminium alloys 6061 and 6063 are commonly used, with thermal conductivity values of 166 and 201 W/m• K, respectively. The values depend on the temper of the alloy. Copper has around twice the conductivity of aluminum , but is three times as dense and, depending on the market, around four to six times more expensive than aluminium . Aluminium can be extruded, but copper cannot. Copper heat sinks are machined and skived. Another method of manufacture is to solder the fins into the heat sink base. Diamond is another heat sink material, and its thermal conductivity of 2000 W/m• K exceeds copper five-fold] In contrast to metals, where heat is conducted by delocalized electrons, lattice vibrations are responsible for diamond's very high thermal conductivity. For thermal management applications, the outstanding thermal conductivity and diffusivity of diamond is an essential. Nowadays synthetic diamond is used as sub mounts for high-power integrated circuits and laser diodes.

Fin efficiency Fin efficiency is one of the parameters which makes a higher thermal conductivity material important. A fin of a heat sink may be considered to be a flat plate with heat flowing in one end and being dissipated into the surrounding fluid as it travels to the other. As heat flows through the fin, the combination of the thermal resistance of the heat sink impeding the flow and the heat lost due to convection, the temperature of the fin and, therefore, the heat transfer to the fluid, will decrease from the base to the end of the fin. Fin efficiency is defined as the actual heat transferred by the fin, divided by the heat transfer were the fin to be isothermal (hypothetically the fin having infinite thermal conductivity). Equations 6 and 7 are applicable for straight fins.

Fin efficiency is increased by decreasing the fin aspect ratio (making them thicker or shorter), or by using more conductive material (copper instead of aluminum, for example). Another parameter that concerns the thermal conductivity of the heat sink material is spreading resistance. Spreading resistance occurs when thermal energy is transferred from a small area to a larger area in a substance with finite thermal conductivity. In a heat sink, this means that heat does not distribute uniformly through the heat sink base. The spreading resistance phenomenon is shown by how the heat travels from the heat source location and causes a large temperature gradient between the heat source and the edges of the heat sink. This means that some fins are at a lower temperature than if the heat source were uniform across the base of the heat sink. This non uniformity increases the heat sink's effective thermal resistance. To decrease the spreading resistance in the base of a heat sink: Increase the base thickness Choose a different material with better thermal conductivity Use a vapor chamber or heat pipe in the heat sink base.

A pin-, straight- and flared fin heat sink types A pin fin heat sink is a heat sink that has pins that extend from its base. The pins can be cylindrical, elliptical or square. A pin is by far one of the more common heat sink types available on the market. A second type of heat sink fin arrangement is the straight fin. These run the entire length of the heat sink. A variation on the straight fin heat sink is a cross cut heat sink. A straight fin heat sink is cut at regular intervals.

In general, the more surface area a heat sink has, the better it works. However, this is not always true. The concept of a pin fin heat sink is to try to pack as much surface area into a given volume as possible. As well, it works well in any orientation.. Although the pin fin has 194 cm2surface area while the straight fin has 58 cm2, the temperature difference between the heat sink base and the ambient air for the pin fin is 50 °C. For the straight fin it was 44 °C or 6 °C better than the pin fin. Pin fin heat sink performance is significantly better than straight fins when used in their intended application where the fluid flows axially along the pins rather than only tangentially across the pins. Another configuration is the flared fin heat sink; its fins are not parallel to each other, as shown in figure 5. Flaring the fins decreases flow resistance and makes more air go through the heat sink fin channel; otherwise, more air would by the fins. Slanting them keeps the overall dimensions the same, but offers longer fins The heat transfer from the heat sink occurs by convection of the surrounding air, conduction through the air, and radiation. Heat transfer by radiation is a function of both the heat sink temperature, and the temperature of the surroundings that the heat sink is optically coupled with. When both of these temperatures are on the order of 0 °C to 100 °C, the contribution of radiation compared to convection is generally small, and this factor is often neglected. In this case, finned heat sinks operating in either natural-convection or forced-flow will not be effected significantly by surface emissivity. In situations where convection is low, such as a flat non-finned with low airflow, radiative cooling can be a significant factor. Here the surface properties may be an important design factor. Matte-black surfaces will radiate much more efficiently than shiny bare metal in the visible spectrum. A shiny metal surface has low effective emissivity due to its low surface area. While the emissivity of a material is tremendously energy (frequency) dependent, the noble metals demonstrate very low emissivity in the NIR spectrum. The emissivity in the visible spectrum is closely related to color. For most materials, the emissivity in the visible spectrum is similar to the emissivity in the infrared spectrum; however there are exceptions, notably certain metal oxides that are used as "selective surfaces".

In a vacuum or in outer space, there is no convective heat transfer, thus in these environments, radiation is the only factor governing heat flow between the heat sink and the environment. For a satellite in space, a 100 °C (373 Kelvin) surface facing the sun will absorb a lot of radiant heat, since the sun's surface temperature is nearly 6000 Kelvin, whereas the same surface facing deepspace will radiate a lot of heat, since deep-space has an effective temperature of only a few Kelvin.

Engineering applications Processor/Microprocessor cooling Heat dissipation is an unavoidable by-product of all but micro power electronic devices and circuits. In general, the temperature of the device or component will depend on the thermal resistance from the component to the environment, and the heat dissipated by the component. To ensure that the component temperature does not overheat, a thermal engineer seeks to find an efficient heat transfer path from the device to the environment. The heat transfer path may be from the component to a printed circuit board (PCB), to a heat sink, to air flow provided by a fan, but in all instances, eventually to the environment. For each interface between two objects in with each other, there will be a temperature drop across the interface. For such composite systems, the temperature drop across the interface may be appreciable. This temperature change may be attributed to what is known as the thermal resistance. For very large heat sinks, there is no substitute for the threaded standoff and compression spring attachment method. A threaded standoff is essentially a hollow metal tube with internal threads. One end is secured with a screw through a hole in the PCB. The other end accepts a screw which compresses the spring, completing the assembly. A typical heat sink assembly uses two to four standoffs, which tends to make this the most costly heat sink attachment Thermal resistance occurs due to the voids created by surface roughness effects, defects and misalignment of the interface. The voids present in the interface are filled with air. Heat transfer is therefore due to conduction across the actual area and to conduction (or natural convection) and radiation across the gaps . If the area is small, as it is for rough

surfaces, the major contribution to the resistance is made by the gaps. To decrease the thermal resistance, the surface roughness can be decreased while the interface pressure is increased. However, these improving methods are not always practical or possible for electronic equipment. Thermal interface materials (TIM) are a common way to overcome these limitations, Properly applied thermal interface materials displace the air that is present in the gaps between the two objects with a material that has a much-higher thermal conductivity. Air has a thermal conductivity of 0.022 W/m• K, while TIMs have conductivities of 0.3 W/m• K and higher. When selecting a TIM, care must be taken with the values supplied by the manufacturer. Most manufacturers give a value for the thermal conductivity of a material. However, the thermal conductivity does not take into the interface resistances. Therefore, if a TIM has a high thermal conductivity, it does not necessarily mean that the interface resistance will be low. Selection of a TIM is based on three parameters: the interface gap which the TIM must fill, the pressure, and the electrical resistivity of the TIM. The pressure is the pressure applied to the interface between the two materials. The selection does not include the cost of the material. Electrical resistivity may, or may not, be important, depending upon electrical design details. Methods to determine heat sink thermal performance In general, a heat sink performance is a function of material thermal conductivity, dimensions, fin type, heat transfer coefficient, air flow rate, and duct size. To determine the thermal performance of a heat sink, a theoretical model can be made. Alternatively, the thermal performance can be measured experimentally. Due to the complex nature of the highly 3D flow in present in applications, numerical methods or computational fluid dynamics (CFD) can also be used. This section will discuss the aforementioned methods for the determination of the heat sink thermal performance.

THERMAL PASTE Heat sinks operate by conducting heat from the processor to the heat sink and then radiating it to the air. The better the transfer of heat between the two surfaces (the U and the heat sink metal) the better the cooling. Some processors come with heat sinks glued to them directly, ensuring a good transfer of heat between the processor and the heat sink. Heat sinks that are attached using clips normally sit rather loosely on top of the processor. It may feel like it is attached securely, but there will be a gap between the U and the heat sink, and that gap of air them makes for poor heat transfer, even if it is very small. Air is a poor conductor of heat compared to most liquids or solids. To improve the thermal connection between the processor and heat sink, a special chemical called heat sink compound should be used. A thin layer of this is spread between the two, which greatly improves heat transfer and the cooling of the processor. Heat sink compound is typically a white paste made from zinc oxide in a silicone base. Very little of the substance is needed, just enough to fill the gap between the U and heat sink. Using more will not make it work better, it will just make a big mess when you press the heat sink down onto the U, much like putting too much strawberry jam in your PB&J sandwich. :^) The use of this compound is strongly recommended for those who want to cool their processors properly.

THERMAL PASTE All modern appliances produce enough heat that they need a heat sink. Almost all of them need a heat sink with a fan. Many heat sinks come with some sort of thermal transfer thingy pre-applied a patch of grease, or a square of chewing-gum-like semi-solid material, or just a rubbery pad for the low performance units. Most appliances don't produce enough heat that the stuff to put between the chip package and the heat sink matters very much, as long as the appliances case has decent ventilation and the ambient temperature isn't sauna-like. There just has to be something between appliances and heat sink. The reason why there has to be something there is that the two mating surfaces of processor and sink aren't flat. They may look flat. They may have a mirror polish. But, on the microscopic scale, they look like a scale model of the Andes. And the mountains on one item do not match the valleys on the other. Without thermal transfer compound, everywhere heat sink metal doesn't mate with appliance package material is a teeny-tiny air gap. Air is a good thermal insulator. As long as your heat sink looks flat when you lay a ruler on it then there'll be a decent amount of actual , of course, but the amount of heat that'll actually make it around the air gaps may be surprisingly small. Hence, thermal compound. It's grease with lots of minuscule thermally conductive particles mixed into it, basically. It doesn't conduct heat as well as direct , but it's a heck of a lot better than air gaps.

A popular view among those of us who've spent more time cleaning thermal grease off our hands than we'd care to is that it doesn't really matter much what kind of thermal grease you use. Plain cheap white zinc-oxide grease, fancy silver grease, ultra-fancy super-exotic betterthan-the-stuff-NASA-uses grease; they're all much the same. As long as you apply the stuff reasonably sparingly, you'll be fine. I'd never actually tested this, though. Perhaps the marketing bumf for the current crop of exotic super-greases was right; perhaps they really are spectacularly better than plain cheap white thermal goop. Perhaps the fancy greases have advantages beyond their thermal performance, too.

PELTEIR AND THEMO-ELECTRIC EFFECT:The Peltier effect is a temperature difference created by applying a voltage between two electrodes connected to a sample of semiconductor material. This phenomenon can be useful when it is necessary to transfer heat from one medium to another on a small scale. The Peltier effect is one of three types of thermoelectric effect; the other two are the Seebeck effect and the Thomson effect. In a Peltier-effect device, the electrodes are typically made of a metal with excellent electrical conductivity. The semiconductor material between the electrodes creates two junctions between dissimilar materials, which, in turn, creates a pair of thermo couple voltage is applied to the electrodes to force electrical current through the semiconductor, thermal energy flows in the direction of the charge carriers. Peltier-effect devices are used for thermoelectric cooling in electronic equipment and computers when more conventional cooling methods are impractical. The Peltier effect is the presence of heat at an electrified junction of two different metals and is named for French physicist Jean-Charles Peltier, who discovered it in 1834. When a current is made to flow through a junction composed of materials A and B, heat is generated at the upper junction at T2, and absorbed at the lower junction at T1. The Peltier heat junction per unit time is equal to

absorbed by the lower

where ΠAB is the Peltier coefficient for the thermocouple composed of materials A and B and ΠA (ΠB) is the Peltier coefficient of material A (B). Π varies with the material's temperature and its specific composition: p-type silicon typically has a positive Peltier coefficient below ~550 K, but n-type silicon is typically negative. The Peltier coefficients represent how much heat current is carried per unit charge through a given material. Since charge current must be continuous across a junction, the associated heat flow will develop a discontinuity if ΠA and ΠB are different. Depending on the magnitude of the current, heat must accumulate or deplete at the junction due to a non-zero divergence there caused by the carriers attempting to return to the equilibrium that existed before the current was applied by transferring energy from one connector to another. Individual couples can connected in series to enhance the effect. Thermoelectric heat pumps exploit this phenomenon, as do thermoelectric cooling devices found in refrigerators The Peltier effect can be used to create a refrigerator which is compact and has no circulating fluid or moving parts; such refrigerators are useful in applications where their advantages outweigh the disadvantage of their very low efficiency.

Peltier effect describes the temperature difference generated by EMF and is the reverse of See beck effect. Finally, the Thomson effect relates the reversible thermal gradient and EMF in a homogeneous conductor. Peltier thermo-element is a device that utilizes the peltier effect to implement a heat pump. A Peltier has two plates, the cold and the hot plate. Between those plates there are several thermo couples. All those thermo couples are connected together and two wires comes out. If voltage is applied to those wires, the cold plate will be cold and the hot plate... hot.

The device is called a heat pump because it does not generate heat nor cold, it just transfers heat from one plate to another, and thus the other plate is cooled. It is also called a thermo-electric cooler or TEC for short. Because TECs have several thermocouples, a lot of heat is transfered between the plates. Sometimes it can reach a temperature difference of 80 degrees Celsius or more! What are Peltier elements made of? Peltier thermo-elements are mainly made of semi conductive material. This means that they have P-N s within. Actually, they have a lot of P-N s connected in series. They are also heavily doped, meaning that they have special additives that will increase the excess or lack of electrons. The following drawing shows how the P-N s are connected internally within a Peltier TEC:

Now, imagine tens or hundreds of those P-N material between two plates. The following drawing shows how can many P-N s exist in a rectangular area like a Peltier TEC.

The P and N material are connected in series together to implement a long strip of P-N junctions. The top plate is the hot plate and the bottom is the cold plate. When power is applied to the two wires, the heat will be transferred from the cold plate to the hot plate and thus the cold plate shall cold.

THERMOELCTRIC EFFECT The thermoelectric

effect is

the

direct

conversion

of temperature differences

to

electric voltage and vice-versa. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when a voltage is applied to it, it creates a temperature difference. At the atomic scale, an applied temperature gradient causes charge carriers in the material to diffuse from the hot side to the cold side, similar to a classical gas that expands when heated; hence inducing a thermal current.

This effect can be used to generate electricity, measure temperature or change the temperature of objects. Because the direction of heating and cooling is determined by the polarity of the applied voltage, thermoelectric devices are efficient temperature controllers. When two dissimilar metals such as iron and copper are ed at both ends to form a closed circuit, and one of the junctions is at a higher temperature than the other (Fig. 128), a current is set up. The e.m.f. driving this current is called a 'thermoelectric e.m.f.', and the phenomenon is known as the thermoelectric effect or See beck effect after the German physicist who discovered it in 1821.

THERMO ELECTRIC COOLING

Usually a thermoelectric e.m.f. is very small, only a few millionths of a volt. For a copper-iron circuit it is found to be about 7lV for every degree Centigrade of temperature difference between the junctions; for antimony and bismuth it is as high as 100 lV per deg C, while for copper and Constantan (55 per cent copper, 45 per cent nickel), the two metals most often used in practice, it is 40 lV per deg C of temperature difference. A pair of dissimilar metals welded together at their junction forms what is called a thermocouple. By arranging several thermocouples in series, as shown in Fig. 129, the e .m .f. s add together to give an appreciable output; this arrangement is known as a thermopile.

figure of Thermopile Although thermopiles have been constructed to deliver e.m.f.s of a few volts, the thermoelectric effect is rarely used at present as a source of energy. Its main application lies in the measurement of temperature. A form of hotwire ammeter (p. 135) for measuring alternating currents incorporates a thermocouple whose junction is heated indirectly by the current being measured. The thermal e. m. f. in the junction sets up a direct current which is measured by a moving-coil galvanometer. In thermo electric, conductor generates a voltage when subjected to a temperature gradient. To measure this voltage, one must use a second conductor material which generates a different voltage under the same temperature gradient. Otherwise, if the same material was used for the measurement, the voltage generated by the measuring conductor would simply cancel that of the first conductor. The voltage difference generated by the two materials can then be measured and related to the corresponding temperature gradient. It is thus clear that, based on See beck's principle, thermocouples can only measure temperature differences and need a known reference temperature to yield the absolute readings.

THERMOELETRIC ELEMENT

THERMOELECTRIC COOLING Thermoelectric cooling uses the Peltier effect to create a heat flux between the junction of two different types of materials. A Peltier cooler, heater, or thermoelectric heat pump is a solid-state active heat pump which transfers heat from one side of the device to the other, with consumption of electrical energy, depending on the direction of the current. Such an instrument is also called a Peltier device, Peltier heat pump, solid state refrigerator, or thermoelectric cooler (TEC). The Peltier device is a heat pump: when direct current runs through it, heat is moved from one side to the other. Therefore it can be used either for heating or for cooling (refrigeration), although in practice the main application is cooling. It can also be used as a temperature controller that either heats or cools. This technology is far less commonly applied to refrigeration than vapor-compression refrigeration is. The main advantages of a Peltier cooler (compared to a vapor-compression refrigerator) are its lack of moving parts or circulating liquid, and its small size and flexible shape (form factor). Its main disadvantage is that it cannot simultaneously have low cost and

high power efficiency. Many researchers and companies are trying to develop Peltier coolers that are both cheap and efficient. (See Thermoelectric materials.) A Peltier cooler can also be used as a thermoelectric generator. When operated as a cooler, a voltage is applied across the device, and as a result, a difference in temperature will build up between the two sides. When operated as a generator, one side of the device is heated to a temperature greater then the other side, and as a result, a difference in voltage will build up between the two sides (the See beck effect). However, a well-designed Peltier cooler will be a mediocre thermoelectric generator and vice-versa, due to different design and packaging requirements. Thermoelectric

junctions

are

generally

only

around

5–10%

as

efficient

as

the

ideal refrigerator (Carnot cycle), compared with 40–60% achieved by conventional compression cycle systems (reverse Rankine systems using compression/expansion). Due to the relatively low efficiency, thermoelectric cooling is generally only used in environments where the solid state nature (no moving parts, maintenance-free, compact size) outweighs pure efficiency. Peltier (thermoelectric) cooler performance is a function of ambient temperature, hot and cold side heat exchanger (heat sink) performance, thermal load, Peltier module (thermopile) geometry, and Peltier electrical parameter.

USES Peltier devices are commonly used in camping and portable coolers and for cooling electronic components and small instruments. Some electronic equipment intended for military use in the field is thermoelectrically cooled. The cooling effect of Peltier heat pumps can also be used to extract water from the air in dehumidifiers. Peltier elements are a common component in thermal cyclers, used for the synthesis of DNA by polymerase chain reaction (PCR), a common molecular biological technique which requires the

rapid heating and cooling of the reaction mixture for dnaturation , primer annealing and enzymatic synthesis cycles. The effect is used in satellites and spacecraft to counter the effect of direct sunlight on one side of a craft by dissipating the heat over the cold shaded side, whereupon the heat is dissipated by thermal radiation into space. Photon detectors such as CCDs in astronomical telescopes or very high-end digital cameras are often cooled down with Peltier elements. This reduces dark counts due to thermal noise. A dark count occurs when a pixel generates an electron because of a thermal fluctuation rather than because it has received a photon. On digital photos taken at low light these occur as speckles (or "pixel noise"). Thermoelectric coolers can be used to cool computer components to keep temperatures within design limits, or to maintain stable functioning when over clocking. However, due to low efficiency, much more heat is generated than normally, necessitating a very large and noisy fan or a liquid cooling system. In fiber optic applications, where the wavelength of a laser or a component is highly dependent on temperature, Peltier coolers are used along with a Thermistor in a loop to maintain a constant temperature and thereby stabilize the wavelength of the device. A Peltier cooler with a heat sink or water block can cool a chip to well below ambient temperature. Peltier devices are used in recent products that chill beverages. Some products can also reverse the current to heat the beverage. Products such as the one pictured draw power from the USB port found on computers. However, these products' ability to heat and cool is limited, as the USB 2.0 standard guarantees only 500 m A of current (900 m A in the USB 3.0 standard).

DC BRUSHLESS FAN

Figure: Brushless DC Fan 12 V Brushless DC electric motor (BLDC motors, BL motors) also known as electronically commutated motors (ECMs, EC motors) aresynchronous motors that are powered by a DC electric source via an integrated inverter/switching power supply, which produces an AC electric signal to drive the motor (AC, alternating current, does not imply a sinusoidal waveform but rather a bi-directional current with no restriction on waveform); additional sensors and electronics control the inverter output amplitude and waveform (and therefore percent of DC bus usage/efficiency) and frequency (i.e. rotor speed). The motor part of a brushless motor is often a permanent magnet synchronous motor, but can also be a switched reluctance motor, orinduction motor. Brushless motors may be described as stepper motors; however, the term stepper motor tends to be used for motors that are designed specifically to be operated in a mode where they are frequently stopped with the rotor in a defined angular position. This page describes more general brushless motor principles, though there is overlap. Two key performance parameters of brushless DC motors are the Motor constants Kv and Km (which are numerically equal in SI units)

BRUSHLESS VS BRUSHED Brushed DC motors have been in commercial use since 1886. Brushless motors, on the other hand, did not become commercially viable until 1962. Brushed DC motors develop a maximum torque when stationary, linearly decreasing as velocity increases. Some limitations of brushed motors can be overcome by brushless motors, they include higher efficiency and a lower susceptibility of the commutator assembly to mechanical wear. These benefits come at the cost of potentially less rugged, more complex, and more expensive control electronics. A typical brushless motor has permanent magnets which rotate and a fixed armature, eliminating problems associated with connecting current to the moving armature. An electronic controller replaces the brush/commutator assembly of the brushed DC motor, which continually switches the phase to the windings to keep the motor turning. The controller performs similar timed power distribution by using a solid-state circuit rather than the brush/commutator system. Brushless motors offer several advantages over brushed DC motors, including more torque per weight, more torque per watt (increased efficiency), increased reliability, reduced noise, longer lifetime (no brush and commutator erosion), elimination of ionizing sparks from the commutator, and overall reduction of electromagnetic interference (EMI). With no windings on the rotor, they are not subjected to centrifugal forces, and because the windings are ed by the housing, they can be cooled by conduction, requiring no airflow inside the motor for cooling. This in turn means that the motor's internals can be entirely enclosed and protected from dirt or other foreign matter. Brushless motor commutation can be implemented in software using a microcontroller or computer, or may alternatively be implemented in analogue hardware or digital firmware using an FPGA. Commutation with electronics instead of brushes allows for greater flexibility and

capabilities not available with brushed DC motors, including speed limiting, "micro stepped" operation for slow and/or fine motion control, and a holding torque when stationary. The maximum power that can be applied to a brushless motor is limited almost exclusively by heat; too much of which weakens the magnets, and may damage the winding's insulation. A brushless motor's main disadvantage is higher cost, which arises from two issues. Firstly, brushless motors require complex electronic speed controllers (ESCs) to run. In contrast, brushed DC motors can be regulated by a comparatively simple controller, such as a rheostat (variable resistor). However, this reduces efficiency because power is wasted in the rheostat. Secondly, some practical uses have not been well developed in the commercial sector. For example, in the radio control (RC) hobby arena, brushless motors are often hand-wound while brushed motors are usually machine-wound. Brushless motors are more efficient at converting electricity into mechanical power than brushed motors. This improvement is largely due to the brushless motor's velocity being determined by the frequency at which the electricity is switched, not the voltage. Additional gains are due to the absence of brushes, alleviating loss due to friction. The enhanced efficiency is greatest in the noload and low-load region of the motor's performance curve. Under high mechanical loads, brushless motors and high-quality brushed motors are comparable in efficiency. Environments and requirements in which manufacturers use brushless-type DC motors include maintenance-free operation, high speeds, and operation where sparking is hazardous (i.e. explosive environments) or could affect electronically sensitive equipment. VARIATIONS IN CONSTRUCTION Brushless motors can be constructed in several different physical configurations: In the 'conventional' (also known as inrunner) configuration, the permanent magnets are part of the rotor. Three stator windings surround the rotor. In the outrunner (or external-rotor) configuration, the radial-relationship between the coils and magnets is reversed; the stator coils form the center (core) of the motor, while the permanent magnets spin within an overhanging rotor which surrounds the core. The flat or axial flux type, used where there are space or shape limitations, uses stator and rotor plates, mounted face to face. Outrunners typically have more poles, set up in

triplets to maintain the three groups of windings, and have a higher torque at low RPMs. In all brushless motors, the coils are stationary. There are two common electrical winding configurations; the delta configuration connects three windings to each other (series circuits) in a triangle-like circuit, and power is applied at each of the connections. The Wye (Y-shaped) configuration, sometimes called a star winding, connects all of the windings to a central point (parallel circuits) and power is applied to the remaining end of each winding. A motor with windings in delta configuration gives low torque at low speed, but can give higher top speed. Wye configuration gives high torque at low speed, but not as high top speed.[6] Although efficiency is greatly affected by the motor's construction, the Wye winding is normally more efficient. In delta-connected windings, half voltage is applied across the windings adjacent to the driven lead (compared to the winding directly between the driven leads), increasing resistive losses. In addition, windings can allow high-frequency parasitic electrical currents to circulate entirely within the motor. A Wye-connected winding does not contain a closed loop in which parasitic currents can flow, preventing such losses.

From a controller standpoint, the two styles of windings are treated exactly the same, although some less expensive controllers are designed to read voltage from the common center of the Wye winding. APPLICATIONS Brushless motors fulfill many functions originally performed by brushed DC motors, but cost and control complexity prevents brushless motors from replacing brushed motors completely in the lowest-cost areas. Nevertheless, brushless motors have come to dominate many applications,

particularly devices such as computer hard drives and CD/DVD players. Small cooling fans in electronic equipment are powered exclusively by brushless motors. They can be found in cordless power tools where the increased efficiency of the motor leads to longer periods of use before the battery needs to be charged. Low speed, low power brushless motors are used in direct-drive turntables for gramophone records.

Transport High power brushless motors are found in electric vehicles and hybrid vehicles. These motors are essentially AC synchronous motors with permanent magnet rotors. The Segway Scooter and Vectrix Maxi-Scooter use brushless technology. A number of electric bicycles use brushless motors that are sometimes built into the wheel hub itself, with the stator fixed solidly to the axle and the magnets attached to and rotating with the wheel. Heating and ventilations There is a trend in the HVAC and refrigeration industries to use brushless motors instead of various types of AC motors. The most significant reason to switch to a brushless motor is the dramatic reduction in power required to operate them versus a typical AC motor. While shadedpole and permanent split capacitor motors once dominated as the fan motor of choice, many fans

are now run using a brushless motor. Some fans use brushless motors also in order to increase overall system efficiency. In addition to the brushless motor's higher efficiency, certain HVAC systems (especially those featuring variable-speed and/or load modulation) use brushless motors because the built-in microprocessor allows for programmability, better control over airflow, and serial communication. Industrial engineering The application of brushless DC motors within industrial engineering primarily focuses on manufacturing engineering or industrial automation design. In manufacturing, brushless motors are primarily used for motion control, positioning or actuation systems. Brushless motors are ideally suited for manufacturing applications because of their high power density, good speed-torque characteristics, high efficiency and wide speed ranges and low maintenance. The most common uses of brushless DC motors in industrial engineering are linear motors. servomotors, actuators for industrial robots, extruder drive motors and feed drives for CNC machine tools. Motion control systems Brushless motors are commonly used as pump, fan and spindle drives in adjustable or variable speed applications. They can develop high torque with good speed response. In addition, they can be easily automated for remote control. Due to their construction, they have good thermal characteristics and high energy efficiency. To obtain a variable speed response, brushless motors operate in an electromechanical system that includes an electronic motor controller and a rotor position sensor. Brushless dc motors are widely used as servomotors for machine tool servo drives. Servomotors are used for mechanical displacement, positioning or precision motion control. In the past DC stepper motors were used as servomotors; however, since they are operate with open loop control, they typically exhibit torque pulsations.Brushless dc motors are more suitable as

servomotors since their precise motion is based upon a closed loop control system that provides tightly controlled and stable operation. Positioning and actuation systems Brushless motors are used in industrial positioning and actuation applications.[14] For assembly robots, brushless stepper or servo motors are used to position a part for assembly or a tool for a manufacturing process, such as welding or painting. Brushless motors can also be used to drive linear actuators. Actuators that produce linear motion are called linear motors. The advantage of linear motors is that they can produce linear motion without the need of a transmission system, such as a balland-lead screw, rack-and-pinion, cam, gears or belts, that would be necessary for rotary motors. Transmission systems are known to introduce less responsiveness and reduced accuracy. Direct drive, brushless DC linear motors consist of a slotted stator with magnetic teeth and a moving actuator, which has permanent magnets and coil windings. To obtain linear motion, a motor controller excites the coil windings in the actuator causing an interaction of the magnetic fields resulting in linear motion. Model engineering

A microprocessor-controlled BLDC motor powering a micro radio-controlled airplane. This external rotor motor weighs 5 grams, consumes approximately 11 watts and produces thrust of more than twice the weight of the plane. Brushless motors are a popular motor choice for model aircraft including helicopters. Their favorable power-to-weight ratios and large range of available sizes, from under 5 gram to large motors rated at thousands of watts, have revolutionized the market for electric-powered model

flight, displacing virtually all brushed electric motors. They have also encouraged a growth of simple, lightweight electric model aircraft, rather than the previous internal combustion engines powering larger and heavier models. The large power-to-weight ratio of modern batteries and brushless motors allows models to ascend vertically, rather than climb gradually. The low noise and lack of mess compared to small glow fuel internal combustion engines that are used is another reason for their popularity. Legal restrictions for the use of combustion engine driven model aircraft in some countrieshave also ed the shift to high-power electric systems.

Radio controlled cars Their popularity has also risen in the radio controlled car sector. Brushless motors have been legal in North American RC car racing in accordance to ROAR since 2006. These motors provide a great amount of power to RC racers and if paired with appropriate gearing and highdischarge Li-Po (Lithium Polymer) batteries, these cars can achieve speeds of up to 100 miles per hour (161 km/h).

RESULT, CONCLUSION AND FUTURE SCOPE Thermoelectric Refrigerator Designed has been working efficiently and The idea behind this project was to utilize a dc voltage to produce temperature difference between the two junctions of the peltier plate thereby producing cooling effect on one side in few seconds. EFFICIENCY OF THERMOELECTRIC REFRIGERATOR Currently, Thermoelectric Refrigerator are about 5% efficient. However, advancements in thinfilm and quantum well technologies could increase efficiency up to 15% in the future. The efficiency of an Thermoelectric Refrigerator is governed by the thermoelectric conversion efficiency of the materials and the thermal efficiency of the two heat exchangers. The ATEG efficiency can be expressed as: [email protected] ζOV = ζCONV х ζHX х ρ Where: ζOV : The overall efficiency of the ATEG ζCONV : Conversion efficiency of thermoelectric materials

ζHX: Efficiency of the heat exchangers ρ : The ratio between the heat ed through thermoelectric materials to that ed from the hot side to the cold side

REFERENCES [1] Heat Loss from Electrical and Control Equipment in Industrial Plants: Part-Methods and Scope, Warren N. White, Ph.D, 2004 [2] Solar refrigeration using the Peltier Effect J C. Swart Cape Technikon, 1996 [3] Efficiency Performance of a Refrigerated Plate based on the Peltier Effect Potentially Supplied by Solar Energy, M. S. Carvalho [4] Solar Powered Refrigeration for Transport Applications, David Bergeron [5] Thermo electric effect, Wikipedia [6] Reiyu Chein, Guanming Huang – “Thermoelectric cooler application in electronic cooling”, Applied Thermal Engineering (2004), ELSEVIER; [7] H. Sofrata – “Heat rejection alternatives for thermoelectric refrigerators”, Energy Conversion & Management 37 (1996) 269-280, PERGAMON; [8] P. Corrèges, E. Bugnard, C. Millerin, A. Masiero,, J.P. Andrivet, A. Bloc, Y. Dunant – “A simple, low-cost and fast Peltier thermoregulation set-up for electrophysiology”, Journal of Neuroscience Methods 83 (1998) 177-184, ELSEVIER;

[9] Incropera, P. Frank, De Witt, P. David – “Fundamentals of Heat and Mass Transfer”, 5 th Edition, Wiley & Sons; [10] Ioffe, Af – “Semiconductor and thermoelectric cooling”, London: Infosearch, 1957; [11] John Merchant, Mikron Instrument Company, Inc – “Infrared Temperature, Measurement Theory and Application” – Omega Handbook; [12] Jun Luo, Lingen Chen, Fengrui Sun, Chih Wu – “ Optimum allocation of heat transfer surface area for cooling load and COP optimization of a thermoelectric refrigerator”, Energy Convertion and Management 44 (2003) 3197-3206, PERGAMON; [13] Ken Sato, Haruhiko Okumura, Satarou Yamaguchi – “Numerical Calculations for Peltier current lead deg”, Cryogenics 41 (2001) 497-503, ELSEVIER; [14] Lawton, B. and Klingenberg, G. – “Transient Temperature In Engineering and Science”, Oxford Science Publications, 1996.