# Why peltier elements have smaller heatsink on cold side?

I have seen in nearly all configurations that Peltier elements are mounted with huge heat sinks on hot side as compared to tiny ones on small side.

Is this just a way of explaining the need of heat sink on hot side? Or are there any real benefits of smaller heat sink on cold side? Would an element work better(produce lower temperature on cold side) with equally sized heat sinks?

The Peltier cell generates far more heat on the hot side than it consumes on the cold side; the difference being simply the power you are pumping it with. At a minimum, about 2.5x more, and at large temperature differences, more than that.

The different sizes of heatsink simply compensate for that.

This is a simplified explanation, however it should help explain why the cold sink is smaller than the heat sink:

A peltier cooler pumps heat from the cold side to the hot side, and in addition a peltier cooler generates heat in addition to the heat its pumping.

A peltier unit's Qmax (the amount of Watts it can pump) is rated at zero delta T (the difference in temperature between the cold and hot sides). Additionally, a peltier cooler's ability to pump heat diminishes as the delta T rises.

At some point (depending on the peltier unit design, etc.) the peltier will pump zero watts (btu) of heat when delta T rises too high.

So, to get the coldest temperatures possible on the cold side, you want to be able to remove as much heat as possible on the hot side. This needs to include any heat created by the peltier.

This coupled with the diminishing heat pumping ability when delta T rises, means that the cold sink is smaller than the heat sink, so the system has a high (or higher) coefficient of performance (COP) than a system with same size cold and hot heat sinks.

Hope this explains it.

A Peltier element creates a heat flux from the cold side to the hot side of it. This means that:

• it is "absorbing" heat energy from the cold side
• it is transferring heat energy to the hot side
• it is taking in electrical energy to do that

Heat and current are the input energy sources. In order to work it must dissipate (output) at least what gets in. Otherwise, the remaining extra energy will overheat and destroy the element.

That's why the heatsinks on the cold side are smaller than the ones on the hot side.

• But I never asked why heat sinks are larger on hot side! The question is why the heat sink is usually smaller on cold side? Is a small heat sink on cold side enough? Would it be better if they were the same size? Is it absolutely necessary for heat sink on hot size to be bigger or is it just cost efficient? Commented Jun 1, 2014 at 18:12
• The Peltier element must be able to dissipate more heat than it takes in. That's why hot side heatsinks are larger than cold side heatsinks (and cold side heatsinks are smaller than hot side hatesinks). Commented Jun 1, 2014 at 18:17
• If you make the cold side heat sink larger, it will absorb more heat to emit that heat. As such, the hot side heatsink needs to be larger again. I realize there'll be a limit somewhere where the cold side wont absorb more heat, but assuming someone designs a minimum cost, you'd want the smallest heatsink that still works.
– RJR
Commented Jun 2, 2014 at 0:53
• Sure, done - but I think I only clarified Cornelius' answer; he seems to be trying to say the same thing.
– RJR
Commented Jun 3, 2014 at 1:27
• FYI, Asking "Why heat sinks are larger on hot side" is the same as "Why the heat sink is usually smaller on cold side". Commented Jun 3, 2014 at 3:54

There's really no justification for it. The two heatsinks could be any size at all. It's just a question of how much thermal resistance you want to have between each surface of the Peltier device and its respective "ambient" environment.

The coldest point in a Peltier-based system is the cold face of the device itself; the thing that it is cooling must necessarily be warmer; otherwise there would be no flow of heat into the face. Similarly, the hot face is the hottest point in the system.

The size of each heatsink determines its thermal resistance. The thermal resistance multiplied by the heat flow determines the delta-T between the Peltier device and the environment. Generally, you want that delta-T on each side to be as small as possible, so you use the biggest heatsink that fits.

It often comes down to the fact that the space being cooled is fairly compact, and so only a small heatsink fits on that side. Sometimes a small fan is used on that side to improve its effectiveness. A secondary issue is that if you have large heatsinks on both sides of the device, it can be difficult to insulate the space between them effectively — if heat from the hot-side heatsink flows directly to the cold-side heatsink, the overall efficiency of the system is reduced.

• The coldest point in a peltier system does not necessarily require the cold face to be the coldest point. As long as the cold side is above absolute zero, there is thermal energy available to be transfered to the hot side. Commented Jun 3, 2014 at 3:59

If you make the cold side heat sink larger, it will be able absorb more heat. To emit that heat, the hot side heatsink needs to be larger again.

There will be a limit somewhere where the cold side won't absorb more heat, but assuming someone designs a minimum cost, you'd want the smallest heatsink that still works. This means that the hot-side heatsink will be larger than the cold side one.

Prob a simpler answer as if it gets too hot it will get damaged, too cold doesn't matter. Hot side they have carefully matched the heatsink with module heat capacity and cold just stuck on what ever was cheap and avail.