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Lecture 10: Thermoelectric Heat Pump

check here for info: http://www.melcor.com/handbook.htm

Thermoelectric Heat Pump - How they work

Solid state - no moving parts
No gases, refrigerant or liquid required
High reliability
Maintenance free, Reliable solid-state operation - no sound or vibration (lifetimes of more than 200,000 hours!)
Operation in any orientation
Precision temperature control capability
Heat or cool - depending on current direction, Heat or cool by changing direction of current flow
High resistance to shock and vibration
Operation in zero gravity and high-G level
Small, lightweight package
Minimum, if any, electrical noise (DC current)
Multistage cascades to below -100°C, standard or designed to specifications

Structure and Function

Since thermoelectric cooling systems are most often compared to conventional systems, perhaps the best way to show the differences in the two refrigeration methods is to describe the systems themselves.

A conventional cooling system contains three fundamental parts - the evaporator, compressor and condenser. The evaporator or cold section is the part where the pressurized refrigerant is allowed to expand, boil and evaporate. During this change of state from liquid to gas, energy (heat) is absorbed. The compressor acts as the refrigerant pump and recompresses the gas to a liquid. The condenser expels the heat absorbed at the evaporator plus the heat produced during compression, into the environment or ambient.

A thermoelectric has analogous parts. At the cold junction, energy (heat) is absorbed by electrons as they pass from a low energy level in the p-type semiconductor element, to a higher energy level in the n-type semiconductor element. The power supply provides the energy to move the electrons through the system. At the hot junction, energy is expelled to a heat sink as electrons move from a high energy level element (n-type) to a lower energy level element (p-type).

Thermoelectric Coolers are heat pumps, solid state devices without moving parts, fluids or gasses. The basic laws of thermodynamics apply to these devices just as they do to conventional heat pumps, absorption refrigerators and other devices involving the transfer of heat energy.

An analogy often used to help comprehend a T.E. cooling system is that of a standard thermocouple used to measure temperature. Thermocouples of this type are made by connecting two wires of dissimilar metal, typically copper/constantan, in such a manner so that two junctions are formed. One junction is kept at some reference temperature, while the other is attached to the object being measured. The system is used when the circuit is opened at some point and the generated voltage is measured. Reversing this train of thought, imagine a pair of fixed junctions into which electrical energy is applied causing one junction to become cold while the other becomes hot.

Figure 1: Cross Section of a typical TE Couple

Thermoelectric cooling couples (Fig. 1) are made from two elements of semiconductor, primarily Bismuth Telluride, heavily doped to create either an excess (n-type) or deficiency (p-type) of electrons. Heat absorbed at the cold junction is pumped to the hot junction at a rate proportional to current passing through the circuit and the number of couples.

Figure 2: Typical TE Module Assembly

In practical use, couples are combined in a module (Fig. 2) where they are connected electrically in series, and thermally in parallel. Normally a module is the smallest component commercially available.

Modules are available in a great variety of sizes, shapes, operating currents, operating voltages and ranges of heat pumping capacity. The present trend, however, is toward a larger number of couples operating at lower currents. The user can select the quantity, size or capacity of the module to fit the exact requirement without paying for excess power.

There is usually a "need" to use thermoelectrics instead of other forms of cooling. The "need" may be a special consideration of size, space, weight, reliability and environmental conditions such as operating in a vacuum. If none of these are a requirement, then other forms of cooling should be considered and in fact are probably desirable.

Once it has been decided that thermoelectrics are to be considered, the next problem is to select the thermoelectric(s) that will satisfy the particular set of requirements. Three specific system parameters must be determined before device selection can begin.

These are:

T_C Cold Surface Temperature
T_H Hot Surface Temperature
Q_C The amount of heat to be absorbed at the Cold Surface of the T.E.

In most cases the cold surface temperature is usually given as part of the problem - that is to say that some object(s) is to be cooled to some temperature. Generally, if the object to be cooled is in direct intimate contact with the cold surface of the thermoelectric, the desired temperature of the object can be considered the temperature of the cold surface of the T.E. (T_C). There are situations where the object to be cooled is not in intimate contact with the cold surface of the T.E., such as volume cooling where a heat exchanger is required on the cold surface of the T.E.. When this type of system is employed the cold surface of the T.E. (T_C) may need to be several degrees colder than the ultimate desired object temperature.

The Hot Surface Temperature is defined by two major parameters:

1) The temperature of the ambient environment to which the heat is being rejected.
2) The efficiency of the heat exchanger that is between the hot surface of the T.E. and the ambient.

These two temperatures (T_C & T_H) and the difference between them (DT) are very important parameters and therefore must be accurately determined if the design is to operate as desired. Figure 3 represents a typical temperature profile across a thermoelectric system.

Figure 3: Typical Temperature Relationship in a TEC

The third and often most difficult parameter to accurately quantify is the amount of heat to be removed or absorbed by the cold surface of the T.E.. All thermal loads to the T.E. must be considered. These thermal loads include, but are not limited to, the active or I²R heat load from electronic devices and conduction through any object in contact with both the cold surface and any warmer temperature (i.e. electrical leads, insulation, air or gas surrounding objects, mechanical fasteners, etc.). In some cases radiant heat effects must also be considered.

Single stage thermoelectric devices are capable of producing a "no load" temperature differential of approximately 67°C. Temperature differentials greater than this can be achieved by stacking one thermoelectric on top of another. This practice is often referred to as Cascading. The design of a cascaded device is much more complex than that of a single stage device, and is beyond the scope of these notes. Should a cascaded device be required, design assistance can be provided by Melcor personnel.

Once the three basic parameters have been quantified, the selection process for a particular module or group of modules may begin. Some common heat transfer equations are attached for help in quantifying Q_C & T_H.

There are many different modules or sets of modules that could be used for any specific application. One additional criteria that is often used to pick the "best" module(s) is Coefficient of Performance (C.O.P.). C.O.P. is defined as the heat absorbed at the cold junction, divided by the input power (Q_C / Q_in). The maximum C.O.P. case has the advantages of minimum input power and therefore, minimum total heat to be rejected by the heat exchanger (Q_H = Q_C + Q_in). These advantages come at a cost, which in this case is the additional or larger T.E. device required to operate at C.O.P. maximum. It naturally follows that the major advantage of the minimum C.O.P. case is the lowest initial cost.

Power supply and temperature control are additional items that must be considered for a successful T.E. system. A thermoelectric device is a D.C. device. Any A.C. component on the D.C. is detrimental. Degradation due to ripple can be approximated by:

DT / DTmax = 1 / (1+N²), where N is % current ripple.

Melcor recommends no more than a 10% ripple.

Temperature control can be generally considered in two groups: Open Loop and Closed Loop, or manual and automatic. Regardless of method, the easiest device parameter to detect and measure is temperature. Therefore, the cold junction (or hot junction in heating mode) is used as a basis of control. The controlled temperature is compared to some reference temperature, usually the ambient or opposite face of the T.E..

In the Open Loop method, an operator adjusts the power supply to reduce the error to zero. The Closed Loop accomplishes this task electronically. The various control circuits are too numerous, complex and constantly being upgraded to try to discuss in this text. There are several manufacturers of control circuits and systems that are better equipped to give expert counsel in this specific area. Suffice it to say that the degree of control, and consequent cost, varies considerably with the application.

Estimating Heat Loads (from Marlow Industries):

Before the cooler or heat sink can be selected, the cooling requirements must be defined. This includes determining the amount of heat to be pumped. By minimizing the heat load, the cooler will be able to achieve colder temperatures or it will require less power to reach the same cooling level. The following describes the techniques used to estimate active and passive heat loads, and applies only to steady state heat loads. If the heat load is of a transient nature or involves more complex factors such as air or fluid flow, we suggest that you call one of our applications engineers for assistance.

Heat load

The heat load may consist of two types; active or passive, or a combination of the two. An active load is the power which is dissipated by the device being cooled. It is generally equal to the input power to the device. Passive heat loads are parasitic in nature and may consist of radiation, convection or conduction.

Active Heat Load

The general equation for active heat load power dissipation is:

Q_active = V²/R = VI = I²R

where:

Q_active = active heat load (watts)
V = voltage applied to the device being cooled (volts)
R = device resistance (ohms)
I = current through the device (amps)

For example, a typical lead selenide (PbSe) infrared detector is operated at a bias voltage of 50 volts and a resistance of 0.5 megohms. The active load therefore, is .005 watts.

Radiation

When two objects at different temperatures come within proximity of each other, heat is exchanged between them. This occurs through electromagnetic radiation which is emitted from one object and reaches the other object. The hot object will experience a net heat loss and the cold object a net heat gain as a result of the temperature difference. This is called thermal radiation.

Radiation heat loads are usually considered insignificant when the system is operated in a gaseous environment since the other passive heat loads are typically much greater in magnitude. Radiation loading is usually significant in systems with small active loads and large temperature differences, especially when operating in a vacuum environment.

The fundamental equation which describes radiation loading is:

Q_rad = F e s A (T_amb⁴ - T_c⁴)

where:

Q_rad = radiation heat load (watts)
F = shape factor (worst case value = 1)
e = emissivity (worst case value = 1)
s = Stefan-Boltzman constant (5.667 X 10^-8w/m²K⁴)
A = area of cooled surface (m²)
T_amb = Ambient temperature (Kelvin)
T_c = TEC cold ceramic temperature (Kelvin)

Example Calculation: A Charge Coupled Device is being cooled from an ambient temperature of 27C to -50C. The known parameters are:

Detector surface area - (includes 4 edges + top surface)

A = 8.54 X 10 -4 m²

T_c = -50C = 223K
T_amb = 27C = 300K
For a worst case example, or if no information is available: F = e = 1

From the equation above:

Q_rad = (1)(1)(5.66X10^-8 w/m²K⁴)(8.45 X 10^-4m²)[(300K)⁴-(223)⁴] Q_rad= 0.270 watts

Convection

When a fluid (in this case, a gas) flows over an object while the temperatures of the gas and the object are different, heat transfer takes place. The amount of heat transfer may vary depending on the rate at which the fluid is flowing across the object. Convective heat loads on TECs are generally a result of natural (or free) convection. This occurs when the flow of a gas is not artificially induced with a fan or pump, but rather from the density difference in the gas caused by the temperature difference between the object being cooled and the gas.

The convective loading on a system is a function of the exposed area, and the difference in temperature between this area and the surrounding gas. Convective loading is usually most significant in systems operating in a gaseous environment with small active loads, or large temperature differences.

The fundamental equation which describes convective loading is:

Q_conv = h A (T_air - T_c)

where:

Q_conv = convective heat load (watts)
h = convective heat transfer coefficient (w/m²C)

(typical value 21.7 for a flat, horizontal plate in air at 1 atm)

A = exposed surface area (m²)
T_air = temperature of surrounding air(C)
T_c = temperature of cold surface (C)

Example calculation: A 0.01 square meter plate is being cooled from 25C to 5C. The top and four sides are exposed surfaces. The plate is 0.006 meters thick.

From the Convection equation:

Q_conv = (21.7 w/m²C (0.0124 m²)(25C - 5C)

Q_conv= 5.4 wattsIt is very important to avoid allowing condensation to form when cooling below the dew point. This problem may be avoided by enclosing the cooling system in a dry gas or a vacuum environment.

Conduction

Conductive heat transfer occurs when energy exchange takes place by direct impact of molecules from a high temperature region to a low temperature region.

Conductive heat loading on a system may occur through lead wires, mounting screws, etc., which form a thermal path from the device being cooled to the heat sink or ambient environment.

The fundamental equation which describes conductive loading is:

Q_cond= k A/L DT

where:

Q_cond = conductive heat load (watts)
k = thermal conductivity of the material (w/m C)
A = cross-sectional area of the material (m²)
L = length of the heat path (m)
DT = temperature difference across the heat path(C)

(usually ambient or heat sink temperature minus cold side temperature).

Table I
Thermal Conductivities of Various Wire Material

Material

Thermal Conductivity (w / mC)

Aluminum

205

Copper

386

Gold

315

Platinum (90%)
Indium (10%)

31.1

Platinum

70.9

Manganin

22.2

Example Calculation : A TEC is used as a black body reference. A temperature sensor is attached to the cold surface of the TEC. It has two platinum leads which have a diameter of 25m and are 12 mm long. These leads are attached to pins on the heat sink. The cold plate is at -20C while the heat sink is at 30C.

The known parameters are:

k = 70.9 w/mC, from Table I
DT = [30 - (-20)] = 50C
A = pi d² / 4 = 3.14159 (25 m^-6)² / 4

A = 4.91 X 10^-10 m² A(2 wires)= (2)(4.91 X 10 ^-10m²) = 9.82 X 10^-10 m²

L = 12mm = .012m

From the equation above:

Q_cond = [(70.9 w/mC)(9.82 X 10^-10 m²)] (50C) / (.012m) Q_cond= 0.0003 watts

Since the conductive load is inversely proportional to the length of the wire, the conductive load can be reduced by using longer wires.

The following equation can be used for estimating heat losses due to convection and conduction of an enclosure.

Q _passive = (A x DT)/(x/k + 1/h)

where:

Q = Heat load (watts)
A = Total external surface area of enclosure (m²)
x = Thickness of insulation (m)
k = Thermal conductivity of insulation (w/m C)
h = Convective heat transfer coefficient (w/m² C)
DT = Temperature change (C)

Table II Typical Values of convection heat transfer coefficient

Process h (w / m^{2 oC})

Free Convection - Air 2 - 25

Forced Convection - Air 25 - 250

Table III Typical Values of thermal conductivity for insulation

Product Thermal Conductivity (w / m ^oC)

Styrofoam 0.031

Polystyrene 0.037

Polyurethane 0.039

Transient

Some designs require a set amount of time to reach the desired temperature. The following equation may be used to estimate the time required:

t = [(rho) (V) (C_p) (T₁ - T₂)]/Q

where:

t = Time (seconds)
rho = Density (g/cm³)
V = Volume (cm³)
C_p = Specific heat (J/g C)
T₁-T₂ = Temperature change (C)
Q = (Qt_o + Qt_t) / 2 (J/s, J/s = watts)

Qt_o is the initial heat pumping capacity when the temperature difference across the cooler is zero. Qt_t is the heat pumping capacity when the desired temperature difference is reached and heat pumping capacity is decreased. Qt_o and Qt_t are used to obtain average values.

Heat loads may consist of one or more of four modes: active, radiation, convection or conduction. By utilizing these equations you can estimate your heat loads. The numbers can be used in conjunction with the Thermoelectric Cooler Selection Guide to select a suitable TEC for your application.

Selection

Assembly and Integration

Thermoelectric coolers (TECs) are mounted using one of three methods: adhesive bonding, compression using thermal grease, or solder.

In general, for a TEC with a ceramic base of 19 mm or less, you can solder or adhesive bond without fear of failure due to thermal stresses. If the TEC base is larger than 19 mm, we recommend the compression method because thermal grease is not rigid and does not transfer thermal stresses.

A thin layer of copper metallization on the hot and/or cold ceramic allows soldering as a means of attachment. Keep in mind a TEC that has no metallization on either side cannot be mounted using solder. Adhesives and greases are prone to outgassing, therefore they are not as appropriate for use in a vacuum package.

Surfaces Preparation

Surface preparation is important when using any of the assembly methods. No matter which method is used, the mounting surface should be flat to less than 0.08mm over the TEC mounting area. In addition, the surface should be clean and free from oil, nicks and burrs. When multiple TECs are placed in parallel thermally between common plates, the TEC thicknesses should vary no more than 0.05mm.

Mounting with Adhesive Bonding

When to Use: When you want to permanently attach the TEC to your heat sink; when mounting with solder is not an option; when the TECs need to be lapped to the same height after mounting; when moderate thermal conductivity is required.

Step One: Because of the short amount of time needed for epoxy to set up, be certain to have your TECs cleaned and ready to mount before mixing epoxy. Clean and prepare mounting surfaces on both the TEC and heat sink using methanol, acetone, or general-use solvent.

NOTE: It is recommended that acetone and cotton swabs be available so any excess or spilled epoxy (uncured) may be quickly removed.

Step Two: Use Marlow Industries Thermally Conductive Epoxy. Follow the instructions on the package carefully. Be certain to mix the two pouches thoroughly or the epoxy will not cure properly.

Remove the epoxy pack from the protective pouch.
Remove the divider.
Knead well until thoroughly mixed.
Cut a corner and dispense. The epoxy working time is approximately one hour.

CAUTION: Avoid prolonged or repeated breathing of vapor, and use with adequate ventilation. Avoid contact with eyes, skin or clothing. In case of contact with eyes or skin, flush immediately with plenty of water and get medical attention.

Step Three: Coat the ceramic of the TEC with approximately a 0.05mm thick layer of epoxy.

Step Four: Place the TEC on the heat sink and gently rotate the TEC back and forth, squeezing out the excess epoxy.

Step Five: Using a clamp or weight, apply pressure (less than 689,465 N/m2), and cure for two hours at 65C for maximum thermal and physical properties. Curing time at room temperature is 24 hours.

Mounting with the Compression Method

When to Use: When a permanent bond is not desired; when multiple TECs are used; or when your TEC is larger than 19mm.

Step One: Prepare heat sink and cold sink surfaces by machining the module area to within +/-0.03mm.

Step Two: Locate bolt holes in your assembly such that they are at opposite sides of the cooler between 3.2mm to 12.7mm from the sides of the thermoelectric. The bolt holes should be in the same plane line as the heat sink fins to minimize any bowing that might occur.

Step Three: The recommended hardware that should be used are: #4-40 or #6-32 stainless steel screws. Belleville or split lock type washers as well as a fiberinsulated washer to insulate the screw head from the heat sink.

Step Four: Remove all burrs. Then, clean and prepare mounting surfaces, with either methanol, acetone or general use solvents. .

Step Five: Apply a thin 0.05mm layer of Marlow's Thermal grease to the hot side of the TEC. Place the TEC on the heat sink and rotate the TEC back and forth, squeezing out the excess thermal grease until resistance is felt.

Step Six: Repeat step 5 and rotate cold plate back and forth, squeezing out the excess thermal grease.

Step Seven: In a two module system, torque the middle screw first. Be careful to apply torque in small increments, alternating between screws. In general, apply less than 1,034,198 N/m2 (N/m2 = Pascal) per square meter of TEC area.

Mounting with Solder

When to Use: When you need minimal outgassing; when the TEC is smaller than 19mm; when you need a high-strength junction; when high thermal conductivity is required.

IMPORTANT: The device to which the TEC is being soldered should be placed on a thermal insulator. This will allow the device to become hot enough to reflow the solder. If necessary, the device may be placed on a hot plate set at 100C to help heat it to the solder melting point.

Step One: Clean the surfaces to be soldered with methanol, acetone, or a general use solvent to remove oils and residues which would inhibit soldering.

Step Two: With a soldering iron and a new tip, pre-tin the bottom of the TEC (the side with lead wires) using Marlow Industries' Solder 96°C or 117°C and General Purpose Acid Flux. Use small amounts. You can heat the soldering iron to a maximum of 150°C, but extreme care must be taken since most TEC's are constructed with 138°C (min.) solder.

CAUTION: Do not mix solders. Use a separate soldering iron (or a new tip) for each solder.

Step Three: : With soldering iron, pre-tin the header or heat sink with the same solder and flux as used in pre-tinning the TEC. Use small amounts.

Step Four: To minimize flux residue, clean both the header and TEC. Rinse them first in hot water, then scrub with Marlow Industries' Cleaning Solution and rinse again with hot water, brushing away any excess flux residue. Finally, wash with methanol and use forced air to blow dry.

Step Five: Prior to mounting the TEC to the header, add a small amount of Marlow Industries' Blue Mounting Flux to the mounting site on the header.

Step Six: Hold TEC with tweezers and align on header. While doing this, maintain a steady, downward pressure.

Step Seven: While holding the TEC in place, put the soldering iron to the header near the solder seam. When the solder junction flows, remove the soldering iron. The downward pressure on the TEC will expel excess solder.

REMEMBER: The solder which holds the TEC together flows at 138°C (min.), so if you are using the 117°C solder do not leave the soldering iron on the header surface too long, or you will melt the TEC solder as well.

Step Eight: Continue holding the TEC in place until the solder solidifies.

Step Nine: Check along all four edges of the TEC, looking for voids, cracks, or bubbles. A smooth seam insures there will be proper thermal conduction.

Connecting Lead Wire to Header

Step One: Trim the excess wire from the TEC. Wrap the lead wires 3/4 of a turn around the connector posts on the header.

Step Two: Using solder and Blue Mounting Flux, solder the lead wires to the wire posts. You should be able to see outlines of the wires, but they should be well covered. Wick off any excess solder with the soldering iron.

Final Cleaning and Inspection

Step One: Rinse both the header and TEC in hot water, then scrub with cleaning solution and rinse again with hot water, brushing away any excess flux residue around the pins. Wash with hot water and dry with forced air. To insure complete removal of moisture, dry the entire assembly in an oven for 30 minutes at 60°C. If an oven is not available, the forced-air blower is adequate.

Step Two: Check the solder joints for cracks or bubbles.

Lead Wire Attachment

Some thermoelectric coolers use standard 2.8 mm (0.110) spade lug connectors for lead wire attachment. The spade lugs are easily attached by hand. When designing your wiring harness, we recommend that you design the female spade lug connector into the harness. The AMP part number for this female 2.8 mm spade lug connector is 42398-1.

Insertion Procedure: : Insert female spade lug over the lead tabs. Use a side-to-side motion to secure the lug on the tab. DO NOT USE an up-and-down motion, for this can damage the tab or the tab solder joint. Insert the lug until it seats onto the tab detent.

Preventing Problems

Do not use excessive amounts of solder. This can short the power leads and/or inhibit a good thermal interface.
Use the proper solder and flux. Marlow Industries' General Purpose Acid Flux is recommended. Without it, outgassing or overheating during soldering may occur.
Be sure to clean the TEC thoroughly to prevent outgassing.
Do not overheat the TEC with the soldering iron. Because of the narrow temperature differential between the mounting solder (117°C) and the solder used in the TEC (138°C min.), care must be taken not to overheat the TEC and reflow the solder.
During soldering, be sure the surface on which the soldering is being done is composed of a low thermal conductivity material. This will prevent the solder iron heat from being drawn away, which can cause difficulties with reflowing the solder.
When pre-tinning a large area of the TEC, pre-tin in small sections or purchase the coolers pretinned by Marlow Industries.
If a TEC is being soldered to a large header, it may require that the header be placed on a 100°C hot plate. This will minimize heat conduction away from the solder point.

GIVEN USE TO GIVE

SI Unit Factor English Unit

Watt .2930711 BTU / hr

M 39.37 in

°C 1.8(°C) + 32 °F

N / m² (Pa) .145E-03 lbs/in²

TEC References:

Melcor: Thermoelectric Coolers, Aztec Design Software (downlaod), http://www.melcor.com/

Marlow Thermoelectric Coolers, Design Guide (downlaod), http://www.marlow.com/