Using a cold junction, heat sink and DC power source, thermoelectrics provide an effective means of cooling in a range of applications.

Thermoelectric cooling, also called the "Peltier Effect," is a solid-state method of heat transfer through dissimilar semiconductor materials. To understand it, one should know how thermoelectric cooling systems differ from their conventional counterparts.

Both thermoelectric and conventional refrigeration systems obey the laws of thermodynamics. So, it follows that thermoelectric cooling has much in common with refrigeration methods -- only the actual system for cooling is different. Perhaps the best way to show the differences between in the two refrigeration systems is to describe the systems themselves.

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 of the refrigerant above ambient level. The condenser helps discharge the absorbed heat into surrounding room air.

Thermoelectric refrigeration replaces the three main working parts with a cold junction, heat sink and DC power source. 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 two refrigeration methods, then, is that a thermoelectric cooling system refrigerates without use of mechanical devices, except perhaps in the auxiliary sense, and without refrigerant. Stated as simply as possible, in a thermoelectric cooler, semiconductor materials with dissimilar characteristics as connected electrically in series and thermally in parallel, so that two junctions are created.

Thermoelectric liquid chillers incorporate an integral temperature controller, pump and reservoir.

Semiconductors

The semiconductor materials are N and P type. They are so named because of their structure: The N-type has more electrons than necessary to complete a perfect molecular lattice structure while the P-type does not have enough electrons to complete a lattice structure. The extra electrons in the N-type material and the holes left in the P-type material are called "carriers," and they are the agents that move the heat energy from the cold to the hot junction. Heat absorbed at the cold junction is pumped to the hot junction at a rate proportional to carrier current passing through the circuit and the number of couples.

Good thermoelectric semiconductor materials such as bismuth telluride greatly impede conventional heat conduction from hot to cold areas yet provide an easy flow for the carriers. In addition, these materials have carriers with capacity for transferring more heat.

Figure 1. The upper part of Figure 1 shows the steady-state temperature profile across a typical thermoelectric device.

Heat Sinks

The design of the heat exchanger is an important aspect of a good thermoelectric system. The upper part of Figure 1 illustrates the steady-state temperature profile across a typical thermoelectric device, from the load-side to the ambient.

The total steady-state heat that must be rejected by the heat sink to the ambient may be expressed as follows:

    QS = QC + (V - I) + Q1
where
QS is the amount of heat that must be rejected
QC is the heat absorbed from the load
V - I is the power input
Q1 is the heat leakage

If the heat sink cannot reject enough QS from the system, the system's temperature will rise and the cold junction temperature will increase. If the thermoelectric current is increased to maintain the load temperature, the coefficient of performance (COP) tends to decrease. Thus, a good heat sink contributes to improved coefficient of performance.

Energy may be transferred to or from the thermoelectric system by three basic modes: conduction, convection and radiation. The values of QC and Q1 may be easily estimated: Their total, along with the power input, gives QS, the energy the hot junction heat sink must dissipate.

Are Thermoelectrics Right For You?

Answering these four questions about your application can help you determine whether thermoelectrics are suitable for your application.

  • Are you cooling process equipment?

    Yes
    Thermoelectric technology can be utilized quite effectively if you need to cool electronic components, lasers, equipment or other similar applications.

    No
    Thermoelectric technology is probably not the best solution for cooling a large space such as an entire building. Buildings require a larger number of BTUs per hour than most thermoelectric units will provide.

  • What is the ambient temperature surrounding the material to be cooled?

    Moderate
    If your ambient temperature is greater than -40°F (-40°C) and less than 158°F (70°C), solid-state air conditioners and cold plates can perform well in this temperature range.

    Extreme
    If your ambient temperature is less than -40°F (-40°C) or greater than 158°F (70°C), solid-state air conditioners and cold plates are not suited for use at those ranges.

  • What is the active heat load, if any?

    Less than 500 W
    A single solid-state air conditioner or liquid chiller usually can meet these cooling requirements.

    Greater than 500 W
    Solid-state air conditioners or liquid chillers can cool these materials, but these applications would require multiple units.

  • What method of cooling will you use?

    Direct Contact
    This method is possible with thermoelectric technology.

    Air
    This method is possible with thermoelectric technology.

    Liquid
    This method is possible with thermoelectric technology.

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