
Two abiding challenges of laptop computer engineering are the antagonistically complimentary objectives of packing more and more computing and graphics power, memory, speed, and storage capacity into thinner and smaller form factors, all while keeping power consumption, heat generation and heat dispersal to tolerable levels.
In the context of cooling, desktop computer designers have the luxury of fewer space and power consumption constraints making techniques such as large heatsinks, water-cooling and multiple, high-volume fans practical solutions. Laptops provide far less latitude in terms of cooling options, the most prolific solution being the crudeness and noise pollution of small, high-RPM fans blowing or sucking air rapidly over the surface of the processor core and other internal components.
More Elegant Solution
However, Tessera, a San Jose, Calif.-based firm that develops cutting-edge technologies aimed at next-generation wireless, consumer and computing products, has developed a potentially more elegant cooling solution that induces cooling airflow by way of ionizing air particles. It calls this technology an Electrohydrodynamic (EHD) ionic wind pump.
The EHD technique involves applying a voltage to a sharp electrode which ionizes nearby air molecules that are propelled by the electric field, transferring momentum to neutral air molecules, thus creating airflow across hot internal components and hence cooling those components.
Such ionic-cooling systems have been demonstrated in research labs before, but Tessera is the first to test the technology inside a working laptop by replacing the stock rotary fans with EHD blowers.
A technological speed bump for engineers to overcome was to design a sufficiently compact voltage converter that could change the laptop battery’s 12 volts DC into the roughly 3,000 volts required to operate the ionic wind pump. They succeeded by using a power supply from a cold cathode fluorescent lamp to construct a supply that is only three centimeters square.
Advantages: Better Heat Reduction, No Noise
The payoff is that Tessera’s tests indicate that the ionic-cooling system can extract roughly 30 percent more heat from a laptop than a conventional fan and could potentially consume only half as much power to operate the cooling system. Another superiority of EHD cooling is that it’s entirely solid-state, involving no moving parts and consequently creating no noise. It’s also more compact, making it suitable for thinner, lighter portable devices.
In a paper (PDF) presented at the 25th IEEE SEMI-THERM Symposium, Tessera scientists noted that demand for ever-smaller portable devices has resulted in heat fluxes that push the limits of conventional fan-based air cooling technology, and that Electrohydrodynamic ionic wind pumps offer an attractive alternative to fans.
The researchers chose a laptop with a TDP (Total Dissipated Power) of ~60W as a test platform to demonstrate the EHD proof-of-concept thermal management system. The laptop’s two stock 65mm rotary fans were removed and replaced with two EHD blower systems and associated control electronics configured to fit in the space vacated by the fans, which was done for simplicity but does not necessarily demonstrate the full performance potential for the EHD thermal solution.
EHD vs. Fan Performance
In tests, the EHD retrofitted laptop was compared against a standard version of the same model cooled using the two stock mechanical fans. Major heat sources of the laptop were an Intel Core 2 Duo CPU, GPU, and the chipset. In operation, the retrofitted laptop performed similarly to the stock unit with no apparent impact on laptop functionality. There was no discernible impact on electrically sensitive systems such as wireless communication and trackpad human interface. Thermal performance of the prototype system was measured by running several benchmark programs including Geekbench and a looped 1080P movie trailer. Hardware Monitor, an off-the-shelf system utility application, was used to monitor the real-time temperature of major components, such as the CPU, GPU and their respective heatsinks. The CPU and GPU temperature of the retrofitted EHD cooled laptop was found to be approximately 10 degrees Celsius higher than the stock fan cooled laptop, with an overall temperature rise of approximately 60 degrees Celsius.
Benchmark results demonstrated comparable overall performance scores, with a variation of less than one percent between stock and EHD retrofitted laptops while running at an 1800MHz clock speed, with skin temperature for both the keyboard and bottom surface of the laptop, showing a temperature difference less than 5 degrees Celsius.
Optimized Prototype
Design and testing of a second-generation prototype is now in progress, with a more optimized configuration in which the laptop’s shelf spreader, heatpipe and heatsink have been removed. The researchers report that this second-generation solution dramatically increases heat transfer while reducing overall size of the cooling apparatus by using the collector as the heat-removal surface and by increasing its total area with no additional weight or volume associated with a separate heatsink, as is the case with a fan.
Comparing heat removed using EHD vs. fan-driven flow, for a given flow rate and the same temperature drop, EHD generated air flow removed up to 38 percent more heat than a fan.
Remaining Challenge — Durability
A remaining challenge of EHD technology is longevity and reliability of the electrodes, specifically some unique failure modes associated with the emitter electrode. Electrode degradation can be caused by the corona discharge surrounding the corona electrode, leading to surface degradation from effects such as metal sputtering at high electric fields and surface oxidation. Proper selection of emitter material, which can overcomes these challenges, is necessary to meet the required longevity, which in laptops is targeted for at least 30,000 hours. Dust accumulation can also degrade EHD cooling performance, and Tessara is working on making its ionic cooler no more sensitive to dust than a fan.
The researchers summarize that even with an unoptimized design, the EHD system shows promising cooling performance with reduced thermal solution volume and acoustics. And by incorporating further design optimization and modification of the cooling solution, it is expected that performance can be further improved to exceed that of laptop rotary fans. Other valuable advantages would include effectively silent operation, a more flexible cooling system form factor able to fit around electronics, and reduced thermal solution height and volume requirements.
EHD Technology in MacBooks?
I expect it may be a while before we see EHD technology in MacBooks, but given Steve Jobs’ famous dislike of fan racket, it’s a pretty safe bet that Apple would be one of the first laptop makers to adopt EHD cooling when it becomes available.
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