The Efficiency of Thermoelectric Coolers

Although the Peltier effect was discovered nearly two centuries ago, efficiently using it for practical cooling has remained a challenge. Despite this, the continued interest in the existing and potential advantages of thermoelectric cooling has spawned technological advances in the latest generation of thermoelectric coolers that drastically improve both effectiveness and efficiency.

When we speak about efficiency, we typically refer to the amount of "work" produced by a machine in relation to the amount of power that is put into it. Efficiency in cooling is described using the term "coefficient of performance." The Coefficient of Performance (COP) is the ratio of heat energy that is removed to the amount of electrical power supplied. For the past several years, most of the thermoelectric coolers available on the market operated with a COP as low as 0.3. This means that for every 1 Watt of electrical energy invested, only 0.3 Watts of heat energy were removed, which is not very efficient at all. Thanks to design improvements, the COP of today's thermoelectric coolers can be as high as 1.2.

To further illustrate the efficiency improvements of current generation thermoelectric coolers, it's possible to use the COP figures from above to compare cooling capacities. Previous thermoelectric coolers operating with a COP of 0.3 and 100 Watts of supplied electric power had a cooling capacity of 30 Watts. With the COP of 1.2 found in some newer units and that same 100 Watts of power, the cooling capacity rises to 120 Watts. This is an increase of 400 % and a dramatic example of how far thermoelectric cooling has come in a relatively short time.

A key design improvement that has helped to increase the efficiency of thermoelectric coolers is active control, and one of the best control methodologies for efficient energy usage (and improved lifespan) is Pulse Width Modulation (PWM). With PWM, power to the thermoelectric cooler is quickly switched on and off at a constant frequency. This creates a square wave pulse of power over a constant time period with a width that can be varied to create an average voltage. Since this occurs very rapidly, the Peltier ( thermoelectric) elements do not have enough time to change temperature in response to the changing pulse.

As a result, the elements will take on the cooling capacity of the V average. Switching the voltage on and off in this manner will allow the thermoelectric cooler to deliver greater cooling capacity and efficiency -- doing more work while consuming less power. Another benefit of this design enhancement is that since the Peltier element inside the thermoelectric cooler sees a constant V average, it doesn't repeatedly stop and start -- leading to longer life and durability. Additionally, a 60 % increase in energy efficiency can be realized by using, variable speed, "soft-starting" fans, like the ones found in Rittal's thermoelectric coolers.