5736 N. Michigan Road, Indianapolis, Indiana 46228
Phone (317) 257-6811 Fax (317) 257-1590 Email MAI
Theory of Operation
Thermoelectric cooling, or as it is sometimes called, "the Peltier Effect", is a phenomenon discovered by a French watchmaker during the 19th century. It is described as a solid-state method of heat transfer generated primarily through the use of dissimilar semiconductor materials. To understand the cooling method, it is first necessary to know how thermoelectric cooling systems differ from their conventional refrigeration counterparts.Like conventional refrigeration, thermoelectrics obey the basic laws of thermodynamics. Both in result and principle, then, thermoelectric cooling has much in common with conventional refrigeration methods - only the actual system for cooling is different.
Perhaps the best way to show the differences in the two refrigeration methods 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 above ambient level. The condenser helps discharge the absorbed heat into the ambient air.
In thermoelectric refrigeration, essentially nothing has changed. 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 condensor fins, discharging the accumulated heat energy from the system.
The difference between the two refrigeration methods, then, is that a thermoelectric cooling system refrigerates without the use of mechanical devices, except in the auxiliary sense, and without refrigerant.
Thermoelectric (Def): Semiconductor materials with dissimilar characteristics are connected electrically in series and thermally in parallel, so that two junctions are created.
The semiconductor materials are N and P type, and are so named because either they have more electrons than necessary to complete a perfect molecular lattice structure (N-type) or not enough electrons to complete a lattice structure (P-type). 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 a capacity for carrying more heat.
Heat Sinks
The design of the heat exchanger is a very important aspect of a good thermoelectric system.
The upper part of the diagram illustrates the steady-state temperature profile across a typical thermoelectric device from the load side to the ambient. In the temperature graph, the total steady-state heat which must be rejected by the heat sink to the ambient may be expressed as follows:
If the heat sink is not capable of rejecting the required Qs from the the given system, the temperature of the entire system will rise and the cold junction temperature will increase. If the thermoelectric current is increased to maintain the the load temperature, the COP (Coefficient of Performance) tends to decrease. Thus, a good heat sink contributes to improved COP.
Energy may be transferred to or from the thermoelectric system by three basic modules: 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.