Review: ActiveCool AC4G Thermoelectric cooler

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April 24, 2004 by Russ Kinder

AC4G Thermoelectric cooler
Market Price

The ever increasing heat output of CPU's has been a topic of much debate of late. Each new chip introduction drives the wattage up another notch, only to be followed soon after by yet another increase. The runaway heat race has more and more people asking the questions, "How hot is too hot?" and "Is this as hot as it can get?"

With each generation of chips someone theorizes that we've reached the end of what can effectively be cooled with conventional air cooled heatsinks. While we probably haven't reached the maximum yet, clearly the cost and complexity needed to keep the hottest of CPUs cool with conventional air cooling are greater and greater. As heat output continues to rise, the need for a more effective — and cost effective — cooling technique becomes more pressing. With the Intel Prescott P4s at ~110W and climbing , the need is here and now.

The AC4G system from ActiveCool employs one of the likely alternatives to the conventional heatsink and fan: Thermoelectric Cooling. Thermoelectric Coolers (TEC) are nothing new, but Active-Cool aims to put the technology to work in a new way, to keep both the temps and the noise down.


The concept of thermo-electric cooling dates back more than a century. Thermo-electric coolers operate on the Peltier effect, first documented by Jean Peltier in 1834. Hence their common name of "Peltier Coolers". A TEC is essentially a solid-state heat pump. When a current is applied to the unit heat is transferred from one side to the other, resulting in a temperature gradient of as much 50°C from one side to the other. For more technical information, Active-Cool has an excellent explanation of thermoelectric cooling on their site: Understanding Thermoelectric Cooling

In theory, TECs have a couple of distinct advantages over conventional HSFs:

  • They can cool the CPU to below ambient temperature.
  • They allow the heatsink to be at a higher temperature than the CPU.

For our purposes, the second characteristic is particularly important. With a conventional air cooler the heatsink is always cooler than the CPU. For a heatsink, the heat transferred is proportional to the difference between the heatsink temperature and the air temperature. All other things held constant, doubling the differential doubles the heat transfer. In other words, getting the heatsink hotter lets you move more heat (watts) with the same CFM (and noise), thus making the heatsink perform more efficiently.

Sounds great, more heat transfer, same noise.... but there's a catch. (Isn't there always?) TECs are not perfect conductors of heat. For every watt that they move from the cold side to the hot side, they must also consume energy, which is also released on the hot side. The measurement for this is referred to as a TEC's coefficient of performance (CoP). The CoP is defined as: "the amount of heat energy being moved divided by the amount of supplied electrical power". For a typical TEC, the coefficient of performance is between 0.4 and 0.7. That means that to move 60 watts from the cold side you will be releasing between 85 and 150 watts from the hot side. That extra 25 to 90 watts works against the efficiency gains made by increasing the temperature of the heatsink, and there is also the issue of producing that wattage. It has to be produced by the PSU (with its accompanying efficiency heat loses), and then be removed from inside the case.

ACTIVE-COOL's unique proposition is that by instantly adjusting the power to the TEC in direct response to demand, even very hot processors can be cooled effectively without creating constant excess heat. Unlike a conventional TEC, the AC4G does not stay at full power all the time, but only when needed with high CPU load. It combines this thermally-controlled variable power TEC with a sophisticated fan controller as well. If everything is executed well, the AC4G promises effective cooling of the hottest processors without the price of constant high fan noise.

Let's find out how close the Active-Cool AC4G gets to this ideal.

The AC4G-B comes in a nice full-color retail cardboard box, with just enough techno-marketing-babble on the back to get you interested.

Removed from the box we see the components of the Active-Cool system.

The kit is composed of 2 components, the heatsink/fan assembly and the power supply/control unit. The kit comes complete with a packet of generic silicon goop and an AC cord for the power unit.

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