Thermoelectric Refrigeration System

What is it all about?

Although thermoelectric (TE) phenomena were discovered more than 150 years ago, thermoelectric devices (TE coolers) have only been applied commercially during recent decades.
The thermoelectric effect is the direct conversion of temperature differences to electric voltage and the opposite. In 1834 Jean Peltier noted that when an electrical current is applied across the junction of two dissimilar metals, heat is removed from one of the metals and transferred to the other. This is the basis of thermoelectric refrigeration.

Refrigeration is the process of moving the heat out of an insulated chamber in order to reduce the temperature inside the chamber below that of the surrounding air.
Thermoelectric coolers have two sides, and when DC electricity flows through the device, it brings heat from one side to the other, so that one side gets cooler while the other gets hotter. The "hot" side remains outside of the chamber, at the ambient temperature, while the cool side goes inside, and is below the chamber temperature. In some applications, multiple coolers can be cascaded together for lower temperature.

A single-stage TEC will typically produce a maximum temperature difference of 70 °C between its hot and cold sides. The more heat moved using a TEC, the less efficient it becomes. The amount of heat that can be absorbed is proportional to the current and time.

Thermoelectric junctions are about 4 times less efficient in refrigeration applications than conventional means

Why is this important?

Thermoelectric technology is important for doing very small cooling jobs, which would be impractical with a compressor-based system. For example cooling an individual integrated circuit, thermally cycling a test tube or cooling a very small enclosure.

TE's are also strong in products which demand both heating and cooling due to a changing operating environment, were a simple switching of TE current polarity allows the system to shift to the mode required. In addition, unlike compressor technology, TE system components can be mounted in any physical orientation and still function properly. Climate-controlled jackets are beginning to use TE elements.

Of course, one important characteristic of TE systems is that they do not require evaporative chemicals which may be harmful to the environment.

A camping/car type thermoelectric refrigerators can typically reduce the temperature by up to 20 °C (36 °F) below the ambient temperature.

How was it before? 

In a conventional refrigeration system, the main working parts are the evaporator, condenser, and compressor. The evaporator surface is where the liquid refrigerant boils, changes to vapor and absorbs heat energy. The compressor circulates the refrigerant and applies enough pressure to increase the temperature above ambient level. The condenser helps discharge the absorbed heat into the ambient air.

In thermoelectric refrigeration, the refrigerant in both liquid and vapor form is replaced by two dissimilar conductors. The cold junction (evaporator surface) becomes cold through absorption of energy by the electrons as they pass from one semiconductor to another, instead of energy absorption by the refrigerant as it changes from liquid to vapor. The compressor is replaced by a DC power source which pumps the electrons from one semiconductor to another. A heat sink replaces the conventional condenser fins, discharging the accumulated heat energy from the system. The difference between the two refrigeration methods is that a thermoelectric cooling system refrigerates without the use of mechanical devices.

Thermoelectric modules are too expensive for normal domestic and commercial applications which run only on regular household current. They are ideally suited to recreational applications because they are lightweight, compact, and insensitive to motion or tilting, have no moving parts, and can operate directly from 12-volt batteries.
Solar cell can be incorporated in this system as well.

Advantages of using this technology?

  • No moving parts so maintenance is required less frequently
  • No chlorofluorocarbons (CFC)
  • Temperature control to within fractions of a degree can be maintained
  • Flexible shape (form factor); in particular, they can have a very small size
  • Can be used in environments that are smaller than conventional refrigeration
  • Long life, with mean time between failures (MTBF) exceeding 100,000 hours
  • Controllable via changing the input voltage/current

How you can build it?

A TE module is a device composed of thermoelectric couples (N and P-type semiconductor legs) that are connected electrically in series, in parallel thermally. They are fixed by soldering, positioned between two ceramic plates which form the hot and cold thermoelectric cooler (TEC) sides. The configuration of thermoelectric coolers is shown in figures below.

Commonly, a TE module consists of the following parts:
- Regular matrix of TE elements – Pellets. Usually, such semiconductors as bismuth telluride (BiTe), antimony telluride or their solid solutions are used.
- Ceramic plates – cold and warm (and intermediate for multi-stage coolers) ceramic layers of a module. The plates provide mechanical integrity of a TE module. They must satisfy strict requirements of electrical insulation from an object to be cooled and the heat sink. The plates must have good thermal conductance to provide heat transfer with minimal resistance. The aluminum oxide (Al2O3) ceramics is used most widely due to the optimal cost/performance ratio and developed processing technique. Other ceramics types, such as aluminum nitride (AlN) and beryllium oxide (BeO), are also used. They have much better thermal conductance – five to seven times more than Al2O3 – but both are more expensive. In addition, BeO technology is poisonous.
- Electric conductors provide serial electric contacting of pellets with each other and contacts to leading wires. For most of the miniature TE coolers, the conductors are made as thin films (multilayer structure containing copper (Cu) as a conductor) deposited onto ceramic plates. For large size, high-power coolers, they are made from Cu tabs to reduce the resistance.

Thermoelectric Cooler Mounting

The mounting method and applied materials must provide good thermal contacts and minimum heat resistance.

- Mechanical mounting – the TE module is placed between two heat exchangers. This sandwich is fixed by screws or in another mechanical way. The advantage of fixing by screws lies in the possibility to make a fast and easy disassembling if required
- Soldering – this is a universal method for most of the miniature TE coolers. Usually, it is not recommended to apply soldering for TECs with linear dimensions of sides measuring more than 18mm because of thermal stress. In this case a very careful materials choice is required.
- Gluing is used widely due to simplicity. Usually, epoxy compounds filled with thermoconductive material, such as graphite powder, silver, silicon nitride (SiN) and others, are used.

Usually, to build a TE refrigerator, one should use two heat sink/fan combinations in conjunction with one or more Peltier devices. In the simplest case, the Peltier device is mounted between this 'cold side' sink and a larger sink on the 'hot side' of the system. As direct current passes through the thermoelectric device, it actively pumps heat from the cold side sink to the one on the hot side. The fan on the hot side then circulates ambient air between the sink's fins to absorb some of the collected heat.

Insulation between the hot and cold sides of the system
Insulation is highly recommended to minimize heat leakage between the hot and cold sides. For best results, use two-part, closed-cell foam.

Safety concerns
First, never power up a Peltier device unless at least one side is mounted to a suitable heat sink that can at least handle the wattage dissipated by the device. A typical Peltier device may dissipate 60 W or more internally (like 60-watt incandescent bulb). The hot side of a thermoelectric device can get even hotter. This is not just a safety issue, either a device powered without proper sinking, can destroy itself very quickly.

The other safety concern is electrical. Although the electrical hazard potential associated with most thermoelectric systems is very small, there are some issues which deserve attention. Typically, Peltier devices are mounted to either aluminum or copper hardware (sinks, liquid heat exchangers, etc.). It is possible, therefore, for debris or moisture to create a short-circuit condition between the hardware and an electrically-live part of the Peltier device. The use of autotransformers or direct wiring to an AC service line is generally not recommended, but if employed, a ground fault interrupter should be included in the design.

Detailed procedure of how to build a small home made TE refrigerator can be found here.
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1 comment:

  1. Thanks for such a very nice technology and Get solar powered refrigerator to cool things for us by solar energy and save electricity.