I. semiconductor material. The temperature of the interconnecting

I.
Introduction

Although
thermoelectric (TE) phenomena was discovered more than 150 years ago,
thermoelectric devices (TE coolers) have only been applied commercially during
recent decades. For some time, commercial TECs have been developing in parallel
with two mainstream directions of technical progress – electronics and
photonics, particularly optoelectronics and laser techniques. Lately, a
dramatic increase in the application of TE solutions in optoelectronic devices
has been observed, such as diode lasers, super-luminescent diodes (SLD),
various photo-detectors, diode pumped solid state lasers (DPSS), charge-coupled
devices (CCDs), focal plane arrays (FPA) and others. The effect of heating or
cooling at the junctions of two different conductors exposed to the current was
named in honor of the French watchmaker Jean Peltier (1785–1845) who discovered
it in 1834. It was found that if a current passes through the contacts of two
dissimilar conductors in a circuit, a temperature differential appears between
them. This briefly described phenomenon is the basis of thermoelectricity and
is applied actively in the so-called thermoelectric cooling modules.
Thermoelectric devices (thermoelectric modules) can convert electrical energy
into a temperature gradient––this phenomena was discovered by Peltier in 1834.
The application of this cooling or heating effect remained minimal until the
development of semiconductor materials. With the advent of semiconductor
materials came the capability for a wide variety of practical thermoelectric
refrigeration applications. Thermoelectric refrigeration is achieved when a
direct current is passed through one or more pairs of n and p-type
semiconductor materials. In the cooling mode, direct current passes from the n
to p-type semiconductor material. The temperature of the interconnecting
conductor decreases and heat is absorbed from the environment. This heat
absorption from the environment (cooling) occurs when electrons pass from a low
energy level in the p-type material through the interconnecting conductor to a
higher energy level in the n-type material. The absorbed heat is transferred
through the semiconductor materials by electron transport to the other end of the
junction TH and liberated as the electrons return to a lower energy level in
the p-type material. This phenomenon is called the Peltier effect. We studied
working system of HVAC system and observe temperature and pressure in this
system. Our aim is to introduce the new HVAC system using thermoelectric couple
which shall overcome all the disadvantages of existing HVAC system. If this
system comes in present HVAC system, then revolution will occur in the
automotive sector also. With population and pollution increasing at an alarming
rate TEC (thermoelectric couple) system have come to rescue as these are
environment friendly, compact and affordable. Conventional compressor run
cooling devices have many drawbacks pertaining to energy efficiency and the use
of CFC refrigerants. Both these factors indirectly point to the impending
scenario of global warming. As most of the electricity generation relies on the
coal power plants, which add greenhouse gases to the atmosphere is the major
cause of global warming. Although researches are going on, better alternatives
for the CFC refrigerants is still on the hunt. So instead of using conventional
air conditioning systems, other products which can efficiently cool a person
are to be devised. By using other efficient cooling mechanisms we can save the
electricity bills and also control the greenhouse gases that are currently
released into the atmosphere. Although Thermoelectric (TE) property was
discovered about two centuries ago thermoelectric devices have only been commercialized
during recent years. The applications of TE vary from small refrigerators and
electronics package cooling to Avionic instrumentation illumination control and
thermal imaging cameras. Lately a dramatic increase in the applications of TE
coolers in the industry has been observed. It includes water chillers, cold
plates, portable insulin coolers, portable beverage containers and etc.

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Fig.
1. Schematic of thermoelectric module operation

1. (a)
cooling mode

1. (b)
heating mode

 

II.
Methodology and Experimentation

 2.1 Thermoelectric Module: A standard module consists of a number of thermocouples
connected in series and sandwiched between two ceramic plates (See Figure 3). By
applying a current to the module one ceramic plate is heated while the other is
cooled. The direction of the current determines which plate is cooled. The
number and size of the thermocouples as well as the materials used in the
manufacturing determine the cooling capacity. Cooling capacity varies from mill watts to several thousand watts. Different types of TEC modules are single stage, two
stage, three stage, four stage, centre hole modules etc. A typical single stage
is shown in Figure 2.

Fig. 2 A typical single stage
thermoelectric module.

 

 

 

 

 

 

 

 

Fig. 3 A Classic TE Module Assembly

While
designing a TEC cooling system, the designer must consider the following
parameters:

1. Temperature to be maintained for the object that is
to be cooled.

2.
Heat to be removed from the cooled object.

3.
Time required to attain the cooling after a DC power is applied.

4.
Expected ambient temperature.

5.
Space available for the module and hot side heat sink.

6.
Expected temperature of hot side heat sink.

7.
Power available for the TEC.

 8. Controlling the temperature of the cooled
object if necessary

 

 2.2
Parameters of a Thermoelectric Module: Once it is decided that thermoelectric cooler is to be
considered for cooling system, the next step is to select the thermoelectric
module or cooler that can satisfy a particular set of requirements. Modules are
available in great variety of sizes, shapes, operating currents, operating
voltages and ranges of heat pumping capacity. The cross section of a TEC is
shown in Fig4. The minimum specifications for finding an appropriate TEC by the
designer must be based on the following parameters.

Fig. 4 Cross section of Thermoelectric Module

 

§  Cold
side temperature (Tc )

§  Hot
side temperature (Th )

§  Operating
temperature difference, which is the temperature difference between Th and
Tc .

§  Amount
of heat to be absorbed at the TEC’s cold surface. This can also be termed as
heat load. It is represented as (Qc ) and the unit is Watts.

§  Operating
current (I) and operating voltage (V) of the TEC.

2.2.1 Cold
side temperature-If
the object to be cooled is in direct contact with the cold surface of the TEC,
the required temperature can be considered the temperature of the cold side of
TEC (Tc). The aim is to cool the air flowing through the heat sinks.
When this type of system is employed the cold side temperature of the TEC is
needed to be several time colder than the ultimate desired temperature of the
air.

 

2.2.2 Hot
side temperature-The hot side
temperature (Th) is mainly based on the two factors. First parameter is
the temperature of the ambient air in environment to which the heat is been
rejected. Second factor is the efficiency of the heat sink that is in contact
with the hot side of TEC.

 

2.2.3 Temperature
difference-The
two temperatures Tc and Th and the difference between them ?T is
a very important factor. ?T has to be accurately determined if the cooling
system is expected to be operating as desired. The following equation shows the
actual ?T.

Actual ?T is not
same as the system ?T. Actual ?T is the difference between the hot and cold
side of the TEC. On the other hand system ?T is the temperature difference
between the ambient temperature and temperature of the load to be cooled.

 

2.2.4 Cooling
Load- The most difficult and important factor
to be accurately calculated for a TEC is the amount of heat to be removed or
absorbed (Qc) by the cold side of the TEC. In this project Qc was
calculated by finding the product of mass flow rate of air, specific heat of
air and temperature difference. Here the temperature difference system ?T in
the difference between the inlet temperature and outlet temperature of the
cooling system. The mathematical equation for Qc is as shown below.

2.2.5-Thermoelectric
Assembly – Heat Sinks Thermoelectric
Assemblies (TEAs) are cooling or heating systems attached to the hot side of
the TEC to transfer heat by air, liquid or conduction. TEAs which dissipate
heat from the hot side use heat exchangers. TEC requires heat exchangers or
heat sinks and will be damaged if operated without one. The two ?T s, actual ?T
and system ?T depend on the heat sinks fitted at the hot sides or cold sides of
TEC. The thermal resistances of the heat sinks could vary the ?T across the TEC
for a set ambient temperature and cooling load temperature. Therefore the
thermal resistance of the heat sinks could increase the current flowing through
the TEC. The three basic types of heat sinks are: forced convective, natural
convective and liquid cooled, where liquid cooled is the most effective. The
typical allowances for ?T at the hot side heat sink of a TEC are

1. 10 to 15 °C
for a forced air cooling system with fins – Forced convection

 2. 20 to 40 °C for cooling using free
convection – Natural convection.

3. 2 to 5 °C for
cooling using liquid heat exchangers – Liquid cooled.

There are several
different types of heat exchangers available in the market. As far this project
is concerned a forced convection type of heat sink was be used based on the ?T.

The main heat
sink parameter for the selection process is its thermal resistance. Heat sink
resistance can be termed as the measure of the capability of the sink to
dissipate the applied heat. The equation is as follows.

 

R is
the thermal resistance (in oC /W or K/W) and Th, the hot side
temperature and T? ambient temperature respectively. Qh is the heat load
into the heat sink which is the sum of TEC power Pe and heat absorbed.

 

The
goal of a heat sink design is to lessen the thermal resistance. It can be
attained through exposed surface area of the heat sink. It may also require
forced air or liquid cooling.

2.2.6-Power
Supply and Temperature Control: Power
supply and temperature control are two added items that must be considered
wisely for a successful TE system. TEC is a direct current device. The quality
of the DC current is important. Current and voltage of a TEC can be determined
by the charts provided by the manufacturer. TEC’s power is the product of
required voltage and current. (P = IV).

Temperature
control is generally categorized into two groups. One is open loop or manual
and the other is closed loop or automatic. For cooling systems normally cold
side is used as basis of control. The controlled temperature is compared to the
ambient temperature. An on-off or a control using thermostat is the simplest
and easiest techniques to control the temperature of a TEC.

 

 

2.3-TEC Selection:

The
TEC was selected considering few factors such as dimensions, Qc, power supply
and etc. The model of TECs used in this project was manufactured in China by
Hebei I.T (Shangai) Co. Ltd. The model no. of the module is TEC1-12706. The
idea was to select a TEC which has a cooling power greater than the calculated
TEC. TEC1-12706 operates with an optimum voltage of 12V. It has maximum voltage
of 15.4V. At 12V it draws and maximum DC current of 4 A. The minimum power
rating or the cooling power is 37.7 W. The maximum power is 48W. It has a
maximum operating temperature of 200°C of the TEC are 68 when hot side
temperature is 25 °C. The charts from the TEC manufacture were also
analyzed while choosing the TEC. It had been decided to use 1 module.

 

2.4-TEC Arrangement

The TEC is sandwiched between two heat sinks which
are fitted with 12V, 0.185A fans.

 

Hot
Side heat sink: The hot side heat sink used
in the project was a single long one installed on the top side of the TECs. As discussed,
thermal resistance of a heat sink is an important factor while designing a
system.

 

Therefore a forced convection heat sink had to be used.
When selecting hot side heat sink for the project other factors such as
dimension to fit into the whole assembly, budget and availability were also
taken to consideration. The heat sink was bought from a local shop and there
was no thermal resistance or datasheets available for the product. The
alternative was to calculate Rt from the resistance of the unfinned area
(Rb) and the resistance offered by the fins (Rf). Since both of these
resistances are acting in parallel, total resistance was found using the
equation

 

The
calculated value was 0.0145K /W. The calculated thermal
resistance of the heat sink was lesser than the required. But when considered
the dimensions of the cooling system the selected heat sink was very apt.

 

IV.
CONCLUSIONS

A
Thermoelectric Air cooling & heating system was designed and built which
can be used for personal cooling & heating. By using 1 TEC with a DC power
supply through external power supply (dimmer stat) cooling was achieved. The
cooling system is capable of cooling & heating the air when air was
circulating between the heat sinks and it was blown to the object by fans.

Comparing
the cost, size, flexibility, and environmental friendliness we conclude that it
is better to reliable on TECs than present HVAC with certain modifications and
using better materials for heat sinks.