Thermoelectric Effects ⚡ How a Peltier Cell and a Thermocouple Work - en

a peltier cell is a component usually flattened shape capable of using a

voltage to generate a temperature gradient i.e decrease the temperature of

one of its faces while increasing the temperature of the opposite face

this thanks to what is known as the peltier effect

given these characteristics they are used as cooling systems

or even as a device to generate energy by performing the process in reverse

they are interesting but it is only a small demonstration of something much

bigger in this video we will talk about the

thermoelectric effects how thermocouples work and how a peltier cell works so

prepare your neurons so that you don't get confused first i

want to clarify several terms that you will surely hear when talking about this

type of devices first the seebeck effect refers to the phenomenon that occurs

when a temperature gradient between the junctions of two different materials

generates a voltage secondly the peltier effect refers to

the generation of a temperature gradient when a voltage is applied between two

different materials that are in contact thirdly the thompson effect refers to

the increase or decrease of temperature in a conductor with a temperature

gradient through which a current is passing

and finally fourthly thermoelectric effects which are a generic way of

calling all the previous cases the only difference being that in the

first ones references made to the scientists who discovered each

phenomenon thomas johann seebeck jean peltier and william thompson the latter

being also known as lord kelvin let's start analyzing the seabeck effect

since chronologically this was the first to be discovered

more specifically in the year 1821 seebeck realized that by positioning two

plates of different metals welded at their ends

that is forming a closed circuit and positioning a compass between them it

varied its direction if a heat source was applied at one of the ends

and not only that when performing the reverse action using ice or some other

element at low temperature the direction in which the compass was pointing was

also reversed initially seebeck assumed that the temperature generated a

polarization in metals which produced a magnetic field directly but as we have

already seen in several previous episodes this behavior is related to the

experiment of hans christian urstead performed just a year earlier in 1820

in which he demonstrated empirically that an electric current passing through

a wire was able to affect the direction of the magnetized needle of a compass by

generating a magnetic field to understand what is happening in this

system and why an electric current is generated by creating a temperature

gradient we must first understand how electric charges move through a

conductive material under normal conditions

conductive materials such as metals are able to transmit electricity

because they have charge carriers that can move freely through them

the most common being electrons negatively charged subatomic particles

although there are also positive charge carriers which we will talk about later

if we have only one piece of metal and nothing else the electrons could move

freely but since in general the composition is

relatively homogeneous they will tend to equilibrium and there will be no current

however if we connect two different conductive materials the electrons will

tend to move towards one of them due to their different chemical compositions

generating a potential difference which in turn generates an electric current

just like in luigi halvani's experiment that we saw in the video on how

batteries work unfortunately with only two metals this

electric current will not last long because the electrons will not be able

to keep moving and the set will quickly come to equilibrium now if we close the

circuit when we connect the metals in both ends nothing will happen either

because the potential difference generated in the second end will be of

equal value to the first one but with the opposite sign

that is the circuit will have a net voltage equal to zero and there will be

no current by increasing the temperature of one of

the junctions we will be delivering more energy to the free electrons of each

material while increasing the potential difference that existed between them

therefore now that one of the ends will have a greater potential difference

the net voltage of the circuit will be different from zero and a current will

be generated due to this behavior the seebeck coefficient appears which is an

intrinsic property of each material but is usually expressed as a value relative

to a second material such as platinum because as we saw two different

materials are needed to generate the effect

the unit of measurement of the seebeck coefficient

is the microvolts per degrees kelvin and helps us to calculate what voltage we

should get as a result depending on the temperature difference between the two

ends this coefficient does not have a linear

behavior but for certain materials and temperature ranges this simplification

can be made now that we know how to generate

electricity using just two pieces of metal we are ready to build our own

thermoelectric generators [Music]

unfortunately as i already mentioned the seebeck coefficient is usually measured

in microvolts per degrees kelvin so we would need a lot of pairs connected in

series or a large temperature difference between the two ends to generate just a

few millivolts but that does not mean that we cannot

take advantage of this phenomenon as we will be able to relate a

temperature difference with the generation of a voltage

and therefore if we position one end in a place with a known temperature and the

other end in a place with an unknown temperature

we can know its value by measuring the voltage generated dividing it by the

seebeck coefficient of the pair of materials and adding the reference

temperature these temperature measuring devices are

known as thermocouples in fact they are quite common in chances

or you can find some of these in your own homes

as control devices and other products whose temperature varies

if you ever want to use a thermocouple some of the properties you should

consider are its sensitivity to temperature

differences which is directly related to the seebeck coefficient of the pair

because the higher it is the easier it will be to measure differences in

voltage that we can later relate to a temperature

the temperature ranges at which they can be used

since each material has different melting points and therefore will melt

above certain temperatures as well as certain characteristics that

limit their operation at extremely low temperatures

and finally the specific range in which we need more precision since as i

mentioned before the seebeck coefficient does not have a linear behavior over its

entire functional range now before moving on to talk about the

peltier effect in peltier cells i want to make a small digression about another

device for measuring temperature that is also composed of a couple of different

metals but works in a completely different way

and that's why i don't want you to confuse them

i'm talking about bimetallic foils the principle of its operation is quite

simple if we put two sheets of different

materials together and heat them since they will have different coefficients of

thermal expansion one of them will expand more than the

other causing the set to bend to one side

and if we exaggerate this effect by winding the sheet in the shape of a

spiral we can create a temperature control system in which the passage of

electricity depends on the angle of one of the ends

more specifically we can position a glass container with two wires

and a small amount of mercury which is a metal capable of being in a liquid state

at room temperature therefore when the temperature is low

and the foils are contracted the mercury stays on the opposite side of the wires

and the current cannot pass between them while on the other hand when the

temperature is higher the spiral expands and the mercury moves towards the wires

closing the circuit and allowing the electric current to pass through

with that out of the way we are ready to talk about the peltier effect which acts

in the opposite way to the seabeck effect

that is if we use again a pair of metals joined at the ends whose temperature is

equal everywhere and we make a current pass through them

one of the ends will start to increase its temperature while the other will

start to decrease this is because when electrons pass from

one material to the other the energy they had before and after are

different which goes against the law of conservation of energy at least until we

include heat as a variable if an electron passes from a state of

more energy to one of less energy this loss of energy is not actually being

lost but transformed into heat which increases the temperature of the

junction in a similar way to how light emitting

diodes work on the other hand if an electron goes

from a lower energy state to a higher energy state it needs to get that energy

difference from somewhere and as you can imagine it gets it from the heat that

previously existed in the junction reducing its temperature

in theory with this alone we could already build our cooling system however

we will still have two problems in our way

the first is the joule effect which tells us that when a current passes

through a conductor this dissipates energy as heat commensurate to the

voltage and current passing through the conductor

which in practical terms generates a contradiction

because if we want to increase the thermal gradient we need more current

and the current generates more heat in the system

and i wish it were as simple as that remember the thompson effect which i

mentioned at the beginning it said that when a current passed through a

conductor with a temperature gradient there was an increase or decrease in the

temperature of the conductor so if for example we had a series of

metal pairs to amplify the effect three things would happen at the same time

first by the peltier effect one end will increase its temperature while the other

end will decrease it second by the joule effect the whole

system will dissipate energy in the form of heat

and finally by the thompson effect because the current moves through

temperature gradients that are reversed each time they pass through a junction

it will alternate periodically between sections that increase and decrease

temperature but that's not all

if you thought we were done with the problems that was only the first one

even if we managed to reduce the joule effect and the thompson effect to turn

this into a viable product we would have to optimize the use of materials and the

easiest way to do this would be to reduce the intermediate sections since

ultimately the temperature transfer is generated only at the junctions between

the two materials unfortunately by doing this we will reduce the distance between

the ends and since metals besides being good electrical conductors are usually

also good thermal conductors the temperature gradient that we are trying

so hard to create will not last long as the system will tend to equilibrium

quickly the missing piece that made it possible

to solve this problem with semiconductors because they have two

very interesting characteristics the first one is that they can be doped

a process in which by including impurities their internal structure can

be altered to create n-type semiconductors with an excess of

electrons and p-type semiconductors with a lack of

electrons or simply holes the interesting thing about this is that

both electrons and holes are charge carriers but with opposite signs

this means that the thermoelectric effects are also reversed

and the second characteristic is that a semiconductor can be both a good

electrical conductor and a bad thermal conductor

considering all of that it was possible to create a device like this commonly

known as a peltier junction or peltier cell in which we will have an n-type

semiconductor and a p-type semiconductor in the internal part

plus metal plates at the top and bottom all connected in series

by applying a potential difference the upper part will increase its temperature

while the lower part will decrease but let's analyze the path of a single

electron as it passes through this system paying special attention to the

junctions between materials for this we are going to rely on a diagram with the

energy level of the electron along its path when the electron is passing

through the metal it will have a certain amount of energy that we will use as a

reference when passing to the n-type semiconductor

since it will have an excessive electrons our electron will have to move

through higher energy orbitals so that it can pass through and as we mentioned

before to compensate for this increase of energy in the electron the material

decreases its temperature later when it reaches the second

junction the opposite will occur the electron will pass to a state of lower

energy in the metal and to compensate it will increase the temperature

then when passing from the metal to the p-type semiconductor since it has a lack

of electrons our electron will need less energy to pass through it

that is again heat will be generated as a result

and finally at the last junction the electron will pass to a more energetic

state when entering the metal decreasing its temperature to compensate for the

change in this case always uses a reference the movement of an electron

but remember that it is completely valid to represent the movement of the charges

in the p-type semiconductor as if they were positive and moving in the opposite

direction which is probably what you will find on the internet if you do some

research on the subject as you can imagine based on everything

we have seen so far a peltier cell can act in four different states generating

two different thermal gradients depending on the polarization of the

voltage source that is being used and generating two potential differences

with opposite signs depending on the thermal gradient applied to the ends of

the cell however this design also has other

benefits given the characteristics of

semiconductors they have much higher peltier and seabed

coefficients than other materials which makes them much more efficient

either to generate thermal gradients or to generate electricity in addition

since the semiconductors used are poor thermal conductors we will have an

insulating layer between the sections in which we do want to modify their

temperature allowing the development of much more

compact designs than if we used only metal

and finally it is possible to make a raise of hundreds of these cells

connected electrically in series and thermally in parallel

to further enhance their performance however if we want to use them to reduce

the temperature of another component we will need to include a heatsink in the

section that will increase its temperature because although

semiconductors work as thermal insulators

at a certain temperature they will no longer be as effective

in general if we compare peltier cells with respect to other cooling methods

the truth is that they are not as efficient however they are compact have

no moving parts which means a lower probability of failure and also can

reverse its operation without major complications

which together can make them perhaps the only viable option for some particular

cases and the same applies if we wanted to use

them as a power generation device there are much more efficient methods to

generate electricity but for extremely particular cases this may be the only

option for example generating electricity for a

flashlight using only the difference in temperature between the environment and

the user's hand or powering a satellite like voyager that must operate in space

for 50 years at distances so far away from our sun that even photovoltaic

cells would not be useful more specifically peltier cells are an

essential component of radioisotope thermoelectric generators used in space

missions but we will see that in a future video

this video required many hours of research and animation so if you think

it's worth it i remind you that you can support me through patreon with which i

can hire help in the research stages or by 3d models like this which ultimately

translates into more videos and with better quality plus other benefits

that's all for now and see you in the next video