A laptop computer can double as an effective portable knee-warmer — pleasant in a cold office. But a bigger desktop machine needs a fan. A data center as large as those used by Google needs a high-volume flow of cooling water. And with cutting-edge supercomputers, the trick is to keep them from melting. A world-class machine at the Leibniz Supercomputing center in Munich, for example, operates at 3 petaflops (3 × 10¹⁵ operations per second), and the heat it produces warms some of the center's buildings. Current trends suggest that the next milestone in computing — an exaflop machine performing at 10¹⁸ flops — would consume hundreds of megawatts of power (equivalent to the output of a small nuclear plant) and turn virtually all of that energy into heat.

Increasingly, heat looms as the single largest obstacle to computing's continued advancement. The problem is fundamental: the smaller and more densely packed circuits become, the hotter they get. “The heat flux generated by today's microprocessors is loosely comparable to that on the Sun's surface,” says Suresh Garimella, a specialist in computer-energy management at Purdue University in West Lafayette, Indiana. “But unlike the Sun, the devices must be cooled to temperatures lower than 100 °C” to function properly, he says.

To achieve that ever more difficult goal, engineers are exploring new ways of cooling — by pumping liquid coolants directly on to chips, for example, rather than circulating air around them. In a more radical vein, researchers are also seeking to reduce heat flux by exploring ways to package the circuitry. Instead of being confined to two-dimensional (2D) slabs, for example, circuits might be arrayed in 3D grids and networks inspired by the architecture of the brain, which manages to carry out massive computations without any special cooling gear. Perhaps future supercomputers will not even be powered by electrical currents borne along metal wires, but driven electrochemically by ions in the coolant flow.

This is not the most glamorous work in computing — certainly not compared to much-publicized efforts to make electronic devices ever smaller and faster. But those high-profile innovations will count for little unless engineers crack the problem of heat.

Go with the flow
The problem is as old as computers. The first modern electronic computer — a 30-tonne machine called ENIAC that was built at the University of Pennsylvania in Philadelphia at the end of the Second World War — used 18,000 vacuum tubes, which had to be cooled by an array of fans. The transition to solid-state silicon devices in the 1960s offered some respite, but the need for cooling returned as device densities climbed. In the early 1990s, a shift from earlier 'bipolar' transistor technology to complementary metal oxide semiconductor (CMOS) devices offered another respite by greatly reducing the power dissipation per device. But chip-level computing power doubles roughly every 18 months, as famously described by Moore's Law, and this exponential growth has brought the problem to the fore yet again (see 'Rising temperatures'). Some of today's microprocessors pump out heat from more than one billion transistors. If a typical desktop machine let its chips simply radiate their heat into a vacuum, its interior would reach several thousand degrees Celsius.

That is why desktop computers (and some laptops) have fans. Air that has been warmed by the chips carries some heat away by convection, but not enough: the fan circulates enough air to keep temperatures at a workable 75 °C or so.

5 Comments

1. 1. suitti 01:07 PM 12/13/12

   If a chip is 2-D, but with devices on both sides, then devices on both sides can be cooled, for example, by emmersion in a flowing liquid. If most of the heat is in the wires, then put the wires on the outside where they can be cooled. While a 3-D architecture lets devices be closer to each other, you have the issue of how to cool stuff that isn't on the surface. One possible way of fixing that is to introduce holes for coolant to pass through the body of the device. How small can they be made? Can all devices be close enough to coolant? Does the liquid have to be forced through small tubes under high pressure? Perhaps a study of capilaries can help.

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2. 2. Alchemist101 02:59 AM 12/14/12

   I dont believe in engineers !

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3. 3. David E. Hockin 03:58 PM 12/14/12

   Eniac was NOT the first "modern electronic computer" and that was "built at the end of WW2".
   The first was built at Bletchley Park in Britain DURING WW2, and was instrumental in the code-breaking work that was carried out there on coded German radio transmissions.

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4. 4. NussbeckRon 08:28 PM 12/17/12

   A solution for cooling massive array's of computers circuits looms as the single largest obstacle to computing's future may have been solved using multiple low orbit processing centers cooled by space and powered by solar radiation? Computing using Low Orbit remote data processing centers that utilize encrypted microwave transmission to user centers for distributions to subscribers is the future?
   
   Google could save $100's of Billions over Ten years of operations in energy costs it will spend to run current Data center and cooling costs. Space data centers would allow for the immediate development of future computing, exaflop machine.
   
   
   Ron Nussbeck

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5. 5. jonconnell 07:58 AM 12/19/12

   Great article - thanks. I was wondering if to the best of your knowledge anyone is actively working on a "reversible computing" means as suggested by Ray Kurzweil? That method is supposed to generate zero net thermal loss. I can sorta-kinda envisage the logical operations involved - and wondered if perhaps the new memristor semiconductor flavor or the much in-the-news electron spin storage and other pending quantum computing widgets would get us any closer to that dream?
   To clarify the reference - quoting from his "The Singularity is Near"... Quote: "Rolf Landauer showed in 1961 that reversible logical operations such as NOT (turning a bit into its opposite) could be performed without putting energy in or taking heat out, but that irreversible logical operations such as AND (generating bit C, which is a 1 if and only if both inputs A and Bare 1) do require energy. In 1973 Charles Bennett showed that any computation could be performed using only reversible logical operations. A decade later, Ed Fredkin and Tommaso Toffoli presented a comprehensive review of the idea of reversible computing. The fundamental concept is that if you keep all the intermediate results and then run the algorithm backward when you've finished your calculation, you end up where you started, have used no energy, and generated no heat. Along the way, however, you've calculated the result of the algorithm." End quote.
   He then goes on to inspiringly ask "How Smart Is a Rock?". Wondering if there any fundamental efforts towards changing the calculation process, not just the wiring and location? (This middle aged engineer needs a zetaflop before he turns 100!).
   

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