Compressor Less Portable Refrigerator[1]

COMPRESSOR LESS PORTABLE REFRIGERATOR BLOCK DIAGRAM

PELTIER EFFECT The thermoelectric refrigerator works on the PELTIER effect that The Peltier–Seebeck effect, or thermoelectric effect, is the direct conversion of thermal differentials to electric voltage and vice versa. Related effects are the Thomson effect and Joule heating. The Peltier–Seebeck and Thomson effects are reversible (in fact, the Peltier and Seebeck effects are reversals of one another); Joule heating cannot be reversible under the laws of thermodynamics.

Seebeck effect

 The Seebeck effect is the conversion of temperature differences directly into electricity. This effect was first discovered, accidentally, by the German physicist Thomas Johann Seebeck in 1821, who found that a voltage existed between two ends of a metal bar when a temperature difference ΔT existed in the bar.  The effect is that a voltage, the thermoelectric EMF, is created in the presence of a temperature difference between two different metals or semiconductors. This causes a continuous current to flow in the conductors if they form a complete loop. The voltage created is of the order of several microvolts per degree difference.

 Thermo-electric cooling

 Thermoelectric coolers are solid state heat pumps used in applications where temperature stabilization, temperature cycling, or cooling below ambient are required.  There are many products using thermoelectric coolers, including CCD

cameras

(charge

coupled

device),

laser

diodes,

microprocessors, blood analyzers and portable picnic coolers.  The typical thermoelectric module is manufactured using two thin ceramic wafers with a series of P and N doped bismuth-telluride semiconductor material sandwiched between them

 The N type material has an excess of electrons, while the P type material has a deficit of electrons. One P and one N make up a couple, as shown in Figure 1. The thermoelectric couples are electrically in series and thermally in parallel. A thermoelectric module can contain one to several hundred couples.

Designing of thermal emf refrigeration Th = Tamb + (O) *(Qh)

where

TH=The temperature of hot side Tamb=The ambient temperature O=thermal resistance of heat exch Qh=heat realeased heat released to the hot side of the thermoelectric (watts). Qh = Qc + Pin Where Qh = the heat released to the hot side of the thermoelectric (watts). Qc = the heat absorbed from the cold side (watts). Pin = the electrical input power to the thermoelectric (watts).

 The temperature difference across the thermoelectric (T) relates to Th and Tc according to Equation 

T = Th – Tc

 The thermoelectric performance curves in Figures 2 and 3 show the relationship between T and the other parameters.  Estimating Qc, the heat load in watts absorbed from the cold side is difficult, because all thermal loads in the design must be considered. Among these thermal loads are:  Active: I2R heat load from the electronic devices Any load generated by a chemical reaction  Passive:

1.

Radiation (heat loss between two close objects with different temperatures).

2.

Convection (heat loss through the air, where the air has a different temperature than the object)

3.

Insulation Losses

4.

Conduction Losses (heat loss through leads, screws,

etc.) 5. Transient Load (time required to change the temperature of an object)  As the thermoelectric operates, the current flowing through it has two effects:  (1) the Peltier Effect (cooling) and  (2) the Joulian Effect (heating).

 We know that joulian effect is proportional to the squire of the current so heating effect will dominates the cooling effect that why we can not increase the current to a maximum value called Imax for themo-electric. 

The thermal resistance of the heat sink causes a temperature rise above ambient. If the thermal resistance of the heat sink is unknown, then estimates of acceptable temperature rise above ambient are:

 Natural Convection20°C to 40°C  Forced Convection10°C to 15°C  Liquid Cooling2°C to 5°C (rise above the liquid coolant temperature)

 As we have done our design on

a Melcor

thermoelectric . The

specifications for the are(these specifications are at Th = 25°C): 1. Qmax = 51.4 watts 2. Vmax = 15.4 volts 3. Imax = 6.0 amps 4. Tmax = 67°C To determine if this thermoelectric is appropriate for this application, it must be shown that the parameters T and Qc are within the boundaries of the performance curves. 5. Our main aim to maintain the temperature of container 5°C which contain 16 litres of air in 0.5minute. we know 1000litres =1m3

16 litres=0.016 m3 density of air=1.293 kg/m3. mass of air =0.016*1.293=0.020688kg specific heat of air=1kJ/Kg°k As

Q=m*s*(th-tc)=0.020688*1000*(35-5)=620.64 J

this is maintain in 0.5minutes so Required power=620.64/(0.5*60)=22 watts (As we assume that the ambient temperature Tamb=25c the rise in the temperature due to sink resistance is 10°C So final temperature will be =25+10=35°C)

Performance Curve (T vs. Qc)

Performance Curve (T vs. Voltage )

 So by the graph maximum current = 3.6amp . corresponding voltage by graph between temperature and voltage voltage=10v Now we will determine the corresponding value of temperature by these values of current and voltage. We know that the temperature at hot side Th = Tamb + (O) *(Qh) Value of heat released at hot side Qh = Qc + Pin Now Pin that is the input power to produce this effect is Pin=V*I V=10volt I=3.6amp Pin=10*3.6=36watt

 And Qh=Qc+Pin =22+36 =58 watts And

Th= Tc +Rcon*Qh

Now the temperature is also rise due to its convective resistance , we assume that the convective resistance of the sink is 0.15°C So

Th=25+0.15*58=33.7°C

The calculated Th is close enough to the original estimate of T h, to conclude that the CP1.4-127-06L thermoelectric will work in the given application Material used for insulation  The material used for the assembly components deserves careful thought. The heat sink and cold side mounting surface should be made out of materials that have a high thermal conductivity (i.e., copper or aluminum) to promote heat transfer. 

However, insulation and assembly hardware should be made of materials that have low thermal conductivity (i.e., polyurethane foam and stainless steel) to reduce heat loss.

 Environmental concerns such as humidity and condensation on the cold side can be alleviated by using proper sealing methods. A perimeter seal (Figure 4) protects the couples from contact with water or gases, eliminating corrosion and thermal and electrical shorts that can damage the thermoelectric module.  Typical thermoelectric from Melcor with a perimeter seal

Single Stage vs. Multistage  Given the hot side temperature, the cold side temperature and the heat load, a suitable thermoelectric can be chosen. If T across the thermoelectric is less than 55°C, then a single stage thermoelectric is sufficient. The theoretical maximum temperature difference for a single stage thermoelectric is between 65°C and 70°C.  If T is greater than 55°C, then a multistage thermoelectric should be considered. A multistage thermoelectric achieves a high T by stacking as many as six or seven single stage thermoelectrics on top of each other. Materials used to built thermocouples.  Silicon, Bismuth, Nickel ,Cobalt ,Palladium, Platinum, Uranium, Copper, Manganese, Titanium, Mercury, Lead , Tin, Chromium, Molybdenum ,Rhodinium ,Iridium ,Gold ,Silver , Aluminium, Zinc, Tungsten, CadmiumIron, Arsenic, Tellurium, Germanium