THERMOELECTRIC WATER COOLER

ABSTRACT
Thermoelectric cooler (TEC), or Peltier Cooler is a solid-state heat pump that uses the Peltier effect to move heat. The modern commercial TEC consists of a number of p- and n- type semiconductor couples connected electrically in series and thermally in parallel. These couples are sandwiched between two thermally conductive and electrically insulated substrates. The heat pumping direction can be altered by altering the polarity of the charging DC current.
Hear we will summaries the steps to cooling water using the TEC , according to rate of flow of heat side temperature to the atmosphere through the hot side fins the cool side temperature varies ,if more heat is flown out side through the fins the cold side temperature can be bring down more effectively
In our device we have achieved to bring down the temperature of water from the ambient temperature to a difference of 20 ºC , and This project aims to bring down the temperature of water below the ambient temperature for drinking purposes





INTRODUCTION

A typical thermoelectric module consists of an array of Bismuth Telluride semiconductor pellets that have been “doped” so that one type of charge carrier—either positive or negative— carries the majority of current. The pairs of P/N pellets are configured so that they are connected electrically in series, but thermally in parallel. Metalized ceramic substrates provide the platform for the pellets and the small conductive tabs that connect them. The pellets, tabs and substrates thus form a layered configuration. Module size varies from less than 0.25” by 0.25” to approximately 2.0” by 2.0”. Thermoelectric modules can function singularly or in groups with either series, parallel, or series/parallel electrical connections. Some applications use stacked multi-stage modules.
In our water cooler we use 1 TEC module for the transfer of temperature to the medium, the cooling ratio will be depends up on the ambient temperature from water. The chip we used hear is TEC1-12705








THE PELTIER EFFECT
By connecting 2 wires of different electrically leading materials at the 2ends and by applying additionally a voltage, a current flows, which transports heat of one junction point to the other .In the consequence one junction point becomes cold and the other one warm.



For thermoelectric modules materials are applicable with a high electrical conductivity and a
Small thermal conductivity. Unfortunately good electrical conductors are also good heat conductors. The best efficiency is obtained with semiconductors.






A BRIEF HISTORY
Early 19th century scientists, Thomas Seebeck and Jean Peltier, first discovered the phenomena that are the basis for today’s thermoelectric industry. Seebeck found that if you placed a temperature gradient across the junctions of two dissimilar conductors, electrical current would flow. Peltier, on the other hand, learned that passing current through two dissimilar electrical conductors, caused heat to be either emitted or absorbed at the junction of the materials. It was only after mid-20th Century advancements in semiconductor technology, however, that practical applications for thermoelectric devices became feasible. With modern techniques, we can now produce thermoelectric “modules” that deliver efficient solid state heat-pumping for both cooling and heating; many of these units can also be used to generate DC power in special circumstances (e.g., conversion of waste heat). New and often elegant uses for thermoelectrics continue to be developed each day.



THE THERMOELECTRIC MODULE
TEC are solid-state devices that convert electrical energy into a temperature gradient, known as the "Peltier effect" or convert thermal energy from a temperature gradient into electrical energy, the "Seebeck effect." Thermoelectric modules used as TE generators or TEGs are rather inefficient and little power is produced. Typical applications of this type include NASA supplying power to space craft with a radio isotopic thermo electric generator and electronic equipment along fuel pipelines where fuel may be burned off. TE modules may also be used as thermocouples for temperature measurement. This discussion will focus on the use of thermoelectric modules TEMs for cooling TECs and for temperature stabilization.


With no moving parts, thermoelectric modules are rugged, reliable and quiet heat pumps, typically 1.5 inches (40 x 40mm) square or smaller and approximately ¼ inch (4 mm) thick. The industry standard mean time between failures is around 200,000 hours or over 20 years for modules left in the cooling mode. When the appropriate power is applied, from a battery or other DC source, one side of the module will be made cold while the other is made hot. Click here to see how they work Interestingly, if the polarity or current flow through the module is revered the cold side will become the hot side and vice versa. This allows TE modules to be used for heating, cooling and temperature stabilization.
Since TE modules are electrical in nature, in a closed-loop system with an appropriate temperature sensor and controller, TE modules can easily maintain temperatures that vary by less than one degree Celsius. Simpler on - off control can also be produced with a thermostat. Because the cold side of the module contracts while the hot side expands modules with a footprint larger than 1.5 - 2 inches square usually suffer from thermally induced stresses, at the electrical connection points inside the module causing a short, so they are not common. Long, thin modules want to bow for the same reason and are also rare. Larger areas than an individual module can maintain are cooled or have the temperature controlled by using multiple modules.
We know from the second law of thermodynamics that heat will move to a cooler area. Essentially, the module will absorb heat on the "cold side" and eject it out the "hot side" to a heat sink. The addition of a heat sink to a module creates a thermoelectric device or TED. In addition to the heat being removed from the object being cooled, the heat sink must be capable of dissipating the electrical power applied to the module, which also exits through the modules hot side.
As any Electrical Engineer will tell you the resistive or "Joule heat" created is proportional to the square of the current applied (I2 R). This is NOT the case with thermoelectric modules. The heat created is actually proportional to the current (amperes x volts) because of the flow of current is working in two directions (the Peltier effect). Therefore, the total heat ejected by the module is the sum of the current times the voltage plus the heat being pumped through the cold side.
To understand the capabilities of a thermoelectric module, and related assembly, it is necessary to understand what TE module specifications represent and their implications. The four standard specifications for a module are 1.) The heat pumping capacity or Qmax in watts 2.) The maximum achievable difference in temperature between the hot and cold sides of the module known as the Delta Tmax or Tmax, usually represented in degrees Celsius 3.) The maximum (optimal) input current in amps or Imax 4.) The maximum input voltage or Vmax when the current input is optimal (Imax).
As a practical matter it is only possible reach either heat pumping capacity in watts or to obtain the maximum temperature differential in degrees. In other words, the DTmax is the maximum temperature difference between the hot and cold side of the module when optimal power is applied and there is no heat load (Q=0). As a thermal load Q is added, the difference in temperature between the two surfaces will decrease until the heat pumping capacity or Qmax value is achieved and there is no net cooling (DT=0). Since your application will likely require net cooling of an object with a thermal mass, the actual heat pumped or Q will be less than Qmax and the actual difference in temperature will be less than the DTmax
LIST OF COMPONENTS USED.
 peltier chip TEC1-12705
 12v 4amps power supply
 hot side ,cold side alluminium fins
 12v exhaust fan
 temperature measuring unit
 Polarity switch
 water tank
HEAT SINK SELECTION
After learning what power is required for an appropriate module to reach the desired level of cooling it is necessary to focus on the assembly required, specifically heat sink selection, in order to allow the module to maintain the desired results. The actual temperature achieved, with a given level of cooling or DT on the module, in an assembly is derived by subtracting the temperature of the cold side Tc from the temperature of the hot side Th. Naturally, the cooler the hot side of the module, the cooler the cold side will be. Many people not familiar with thermoelectrics assume that the temperature of the hot side will be the same as the ambient temperature. This is probably not the case. As mentioned earlier, as soon as power is applied to the module the hot side of the module will begin ejecting this as heat to the heat sink causing it to rise in temperature. The ability of the heat sink to dissipate this heat as well as the heat being pumped through the cold side will determine the actual operating temperature of the hot side thus, the cold side.
This brings us to the importance of selecting an appropriate heat sink. In general, the better (the lower the thermal resistance of) the heat sink the easier it is to keep the hot side temperature from increasing. Liquid heat sinks typically have the lowest thermal resistance however they are relatively expensive and plumbing is required. The use of a liquid heat sink assumes that a "house water supply" or chiller is available to cool the water or liquid being circulated through the heat sink. The most common type of heat sink used in thermoelectric applications is made from a thermally conductive material like aluminum or copper and has fins that are perpendicular to a base.

A typical extruded heat sink profile
It is recommended that you select the largest (greatest surface area) heat sink that you can accommodate. In general, to reduce the thermal resistance of a heat sink by 50% it is necessary to increase it's volume by 400%.
In most TE applications that our modules will be appropriate for, a heat sink alone will not be able to remove a sufficient amount of heat by natural convection keep the hot side at an acceptably low temperature. In order to help the heat sink remove heat on and around the heat sink fins, a fan or blower must be attached which forces ambient temperature air over the fins and exhausts the heat to ambient. This is known as a forced convection heat sink. Even with a forced convection heat sink it is common that the hot side will stabilize at 10 - 15°C above ambient temperature
PERFORMANCE SPECIFICATION OF TEC1-12705
Hot Side Temp(ºC) 25ºC 50ºC
Qmax (Watts) 43 49
Delta Tmax (ºC) 66 75
Imax (Amps) 5.3 5.3
Vmax (Volts) 14.2 16.2
Module R (Ohms) 2.40 2.75

Where: A=40,B=40,C=4.2
Ceramic Material: Alumina (Al2O3)
Solder temperature: 138ºC
Conductor: Copper
• n-type: bismuth-telluride-selenium (BiTeSe) compound
• p-type: bismuth-telluride-antimony (BiTeSb) compound
• Max. Operating Temperature: 138Oc
• Life expectancy: 200,000 hours
• Failure rate based on long time testing: 0.2%.

The cooling will be more efficient if we use a water block to remove the hot side temperature the figure below shows a aluminium fabricated water block





CONSTRUCTION
The thermoelectric module is powered by a power supply of 12v and 4amps.
The 2 faces of the module is fixed with temperature radiating fins made of aluminium. In between the fins thermal grease is applied to transfer temperature ore efficiently
The cold side fins is kept in such a way that the fins will get in contact with the water in the can A thermometer is employed in the water can to measure the cold side temperature
variation in the system



WORKING
When DC voltage is applied to the module, the positive and negative charge carriers in the TEC absorb heat energy from one substrate surface and release it to the substrate at the opposite side. The surface where heat energy is absorbed becomes cold; the opposite surface where heat energy is released becomes hot. Using this simple approach to heat pumping with an heat sink attached with an exhaust fan the heat generated at one end is pumped out
The voltage and current ratio used in this system is 12v,4amps The cold side of the TEC is attached with an aluminium fins which helps to radiates the cold side temperature to the water to be cooled The fins are attached to the TEC by using a medium in between them for a better temperature transfer using the thermal grease The thermal grease absorb more temperature to be transferred The cold and hot side of the TEC can be changed by interchanging the polarity of the power supply to the TEC module If the temperature of the hot side if radiated more efficiently using a power full exhaust fan or by a stream of flowing water , and hence as a result of effective heat removal in the hot side the cold side temperature can be bring much lower to -8ºC ,in our device we can lower the temperature of water to a temperature difference of 20ºC from the ambient temperature of water from the initial stage Thus by the process of removing the cold side temperature through the fins in contact with the water ,the water get cooled corresponding to the temperature transferred through the fins in contact with it


SCHEMATIC REPRESENTATION OF TEC WATER COOLER


1. Our design of a thermo electric water cooler to cool the water below the ambient temperature




























A COMPARISON OF COLINGTECHNOLOGIES
The flow of heat with the charge carriers in a thermoelectric device, is very similar to the way that compressed refrigerant transfers heat in a mechanical system. The circulating fluids in the compressor system carry heat from the thermal load to the evaporator where the heat can be dissipated. With TE technology, on the other hand, the circulating direct current carries heat from the thermal load to some type of heat sink which can effectively discharge the heat into the outside environment.
Each individual thermoelectric system design will have a unique capacity for pumping heat (in Watts or BTU/hour) and this will be influenced by many factors. The most important variables are ambient temperature, physical & electrical characteristics of the thermoelectric module(s) employed, and efficiency of the heat dissipation system (i.e., sink). Typical thermoelectric applications will pump heat loads ranging from several milli watts to hundreds of watts.




RELIABILITY OF TEC
Thermoelectric systems are highly reliable provided they are installed and used in an appropriate manner. The specific reliability of thermoelectric coolers tends to be difficult to define though because failure rates are highly dependent upon the particular application. Thermoelectric modules that are at steady state (constant power, heat load, temperature, etc.) can have mean time between failures (MTBFs) in excess of 200,000 hours. However, applications involving thermal cycling show significantly worse MTBFs, especially when TE coolers are cycled up to a high temperature. With thermal cycling, a more appropriate measure of reliability is not time but rather number of cycles.

All materials expand or contract as they are heated or cooled. Different materials will expand at different rates. The rate of expansion is given by the material property called the coefficient of thermal expansion (CTE). Generally, as the cold side of a module gets colder, it will shrink, and as the hot side gets hotter, it will expand. This flexes the thermoelectric elements and their solder junctions. Furthermore, because the module is constructed of several different materials, there is added stress simply because the materials themselves are expanding/contracting at different rates. After repeated thermal cycling, the solder junctions within the module fatigue, and the electrical resistance increases. Cooling performance is reduced, and eventually the module becomes inoperable. The "failure point" is thus a function of operating temperature, the amount of temperature cycling, and how much degradation the particular system can tolerate before performance becomes unacceptable. All thermoelectric modules (regardless of manufacturer) experience the same stresses of operation, but how they tolerate these stresses is a question of build quality—selecting a manufacturer with good, strong solder junctions is a must!
A similar phenomenon occurs when a module is soldered or adhered with epoxy to a heat sink. The "zero-tension" point (that is, the point where there is no internal stress resulting from mismatches in CTE) will freeze between the ceramic substrate and the heat sink when the solder or epoxy becomes rigid at some temperature which is typically different from the operating temperature. In other words, the module is pre-stressed when the module and solder cool back down to room temperature (assuming the module is soldered to a heat sink)





CHARACTERISTIC VALUES OF A COOLER

1 POWER SUPPLY
1.1 VOLTAGE
Basically the cooling capacity depends on the current. The cooling units are usually built for using at constant dc voltage e.g. 12V, 24V. We advice you to reduce the maximum ripple to 10%, preferably to 5% for an optimal operation. If the voltage rises over the nominal value, the increase of the cooling performance is small or even declines and the efficiency drops intense.
If the voltage is reduced, the maximum temperature difference cannot be achieved any more. The
cooling power reduces in equal measure, but the COP rises
The use of adjustable DC supplies makes a rough adjustment for the temperature possible. If an
exact temperature is required, a controller must be used
Please note that the fans have always to be operated with rated voltage.
By reversal of the polarity one heats instead of cools. So the cooling unit can be used as air conditioner.
Please note that the polarity of the fans may not be inverted (=> separate supply)
1.2 CURRENT
The initial current is larger than the current in continuous operation. Consider this for the dimension of the power supply. With increasing temperature difference at the cooling unit the current decreases.

1.3 EFFICIENCY
The efficiency of a thermoelectric cooling unit is indicated as the COP (Coefficient of Performance).
It is defined as follows:
COP =Qc/Pel
Qc: cooling power
Pel: electrical power
The COP depends on the temperature difference. The higher the temperature difference the smaller is the COP.

ALSO USED AS A HEATER
Thermoelectric coolers can indeed be used for very effective and efficient heating. Since thermoelectric coolers are solid-state heat pumps, they can actively pump heat from the ambient in addition to the heating effect that comes from the electrical resistance of the cooler itself. So, the thermoelectric cooler can be more efficient than a resistive heater (within limits). The heating can be so effective that you could very easily cause the module to reach the melting point of the solder! Care must be taken to ensure that the module does not overheat.
ADVANTAGES OF TEC
 thermoelectric module have no moving parts
 highly reliable
 It is ideal in cooling devices that are sensitive to mechanical vibrations
 Very precise temperature control is possible
 Small size and weight
 electrically quite operation
 operating in any orientation
 convenient power supply
 environmentally clean
 free from dangerous gases like CFC
 available in various size and working range
 can be also used in construction of portable refrigerators
 low cost
 Low power consumption
 high life expectancy 200,000 hours
 Failure rate based on long time testing: 0.2%.
 Low maintenance ,very fast response

MORE APPLICATIONS OF TEC
 Used in portable refrigerators
 Military based cooling systems
 Used in air conditioners
 In portable beverage cooler
 In CPU cooling systems for small and large applications
 Used n super computer and main frame computer cooling systems
 Used to as a generator to produce DC power with waste thermal energy
 In Ecofan Stovetop Fans
 High temperature peltier effect water distiller
it uses a TEC to evaporate water and hence with the help of a condensation system it condense the evaporated water and distilled

.CONCLUSION
Through our project we have succeeded in developing a water cooler cum heater to cool or heat the water below or above the ambient temperature with a low power dc supply , which is free from all mechanical , noice and environmental disturbances





ACKNOWLEDGEMENT
We sincerely acknowledge Mr. Rajeesh Kumar R.C of Hindustan Unilever laboratories for his extensive financial and personnel support in the development of our project .Without which it wouldn’t have been possible for us to finish it on time.


REFERANCE
• www.electracool.com/modulespecs
• http://www.thermoelectrics.com/introduction.htm
• www.wikipedia.com
• www.youtube.com
• http://www.educypedia.be/electronics/thermoelectric.htm
• http://www.peltier-info.com/info.html
• And other online libraries