stirling engine is an unpopular but not-yet-obsolete engine. it was patented by robert stirling back in 1816! one major drawback of this engine is that it needs an external heat source - its not an internal combustion engine.

the msi cpu cooler works on this principle. the heat from the processor drives the stirling engine. the stirling engine drives a fan which is cools the cpu. so instead of having a dc motor driven fan, the msi cooler uses the stirling engine.

thermodynamics of the stirling cycle

the stirling cycle is the only practical cycle that can achieve near carnot efficiencies. carnot efficiency is the maximum theoretical efficiency a thermodynamic cycle can achieve.

carnot efficiency is given by, η_carnot = 1 - (T_sink / T_source)

left: schematic representation | right: pv diagram

the processes are as follows

• 1-2: constant volume heat addition
• 2-3: isothermal expansion (T₂ = T₃). heat is supplied from an external source.
• 3-4: constant volume heat rejection
• 4-1: isothermal compression (T₄ = T₁).

heat from the constant volume process 1-2 is rejected during the constant volume process 3-4. the gases mix and exchange heat ideally in the regenerator.
therefore: Q_1-2 = Q_3-4 and thus: c_v(T₂ - T₁) = c_v(T₄ - T₃).

Q_2-3 = W_2-3 = mRT₂ ln (p₂ / p₃)
Q_4-1 = W_4-1 = mRT₁ ln (p₁ / p₄)

note

work done in an isothermal process is given by: W= mRT ln (p₂ / p₁)

W = - ∫ mp dv
pv = RT
W = - ∫ mRT/v dv = mRT ln (v₁ / v₂)
RT = p₁v₁ = p₂v₂
W = mRT ln (p₂ / p₁)

net work = W_2-3 - W_4-1 = Q_2-3 - Q_4-1
heat supplied = Q_2-3

cycle efficiency, η = net work / heat supplied = (Q_2-3 - Q_4-1) / Q_2-3 = 1 - (Q_4-1) / Q_2-3)

for the constant volume processes 1-2 and 3-4:
p₁ / T₁ = p₂ / T₂ and p₃ / T₃ = p₄ / T₄

p₁ / p₂ = T₁ / T₂

from the isothermal processes 2-3 and 4-1: T₂ = T₃ and T₄ = T₁

p₃ / T₂ = p₄ / T₁
p₃ / p₄ = T₂ / T₁

p₁ / p₂ = p₄ / p₃
ln (p₁ / p₄) = ln (p₂ / p₃)

thus, η = 1 - [mRT₁ ln (p₁ / p₄) / mRT₂ ln (p₂ / p₃)]= 1 - [T₁ / T₂] = η_carnot

the compression ratios of the two cylinders are quite small. and thus the mean effective pressure is quite low. therefore weight to power ratio of stirling engines are much lower than internal combustion engines. this is another factor which has made these types of engines unpopular.

stirling cycle engines can operate on very low temperature gradients. the cpu surface is around 75°C while the environment is 30° C (say). the temperature gradient is only 45K and this is sufficient to drive the cooling fan. other heat engines require very high temperature gradients. an automobile engine operates between 30°C and about 1200°C - a temperature gradient of over 1100K!

the process explained

the stirling engine consists of two gas chambers. one is always heated and the other is always cooled. the regenerator is a chamber where gases from the other two chambers mix. the gas in the regenerator is used to transfer heat from one chamber to the other.

press release: World's First Powerless Air Cooler on a Mainboard!

MSI presents the "Air Power Cooler" chipset cooler with ECO concept at CeBIT 2008!

Taipei, Taiwan – Micro-Star International (MSI), one of the leaders in mainboard technology, will bring new life to old technology with new concept on mainboard products. While everybody is looking forward to have a more ECO friendly design, more mainboard makers are trying to save energy on the mainboard side with PWM design, MSI already has very efficient power design, but we research more than only PWM design enhancement, trying to offer the total ECO solution we now give you the heat transfer power concept on mainboard cooler design!

MSI unique ECO cooler concept, over 70% heat transfer!

The amazing of nature power has taught humans a lot, with the earth's resources running out and fuels such as petroleum to run-out in this century, now is the time to work harder and find the replacement resources and use what we have learnt from earths nature. After MSI concept solar power notebook system, MSI continued to survey in the pursuit of innovation & ECO power to find other new technology for mainboards, today we introduce to you a solution that's heat and air power!

Maybe you have already heard about "Stirling Engine Theory", but in Taiwan, we have great companies that not only work hard on selling products, but also try to investigate future energy solutions. Thanks to MSI and our great partners, we would like to introduce the "Air Power Cooler" based on Stirling Engine theory with great support from Polo-Tech Taiwan, the exclusive heat transfer technical design company with their own patent rights.

The "Air Power Cooler" transfers the chipset heat into air momentum, when the air becomes hot, the air will expand then push the fan to rotate and In doing so cooling the heatsink immediately. After the air moves from the bottom to top of the piston, the air will become heavy to push the up piston down. The better air piston design can transfer over 70% heat power and transfer to air power, that's great efficiency transfer from Stirling engine theory. In a comparison with solar power the transfer rate is only around 20~30% requiring more surface and as a result cost.

MSI "Air Power Cooler", more then just saving watts!

MSI will present a live demo of our "Air Power Cooler" concept product in MSI's ECO area block on the MSI booth in CeBIT Hall 21 B34. You will see the electricity-less fan which was powered only by the movement of heat and air, the fan speed will change relative to the chipset temperature. The prototype "Air Power Cooler" is our first attempt to investigate how the Eco concept can work in tandem with the evolution of the mainboard "ECOlution" and joins the theme for MSI at Cebit08. We will continue to co-work with our great partners and try to turn this early concept into mass production in the near future.

MSI believe that "less is more" and currently use less power but with more efficiency. MSI philosophy for good mainboard design is not about simply adding more components to PWM phases, to then turn off some phases to make a power saving. Today MSI introduces a new ECO concept using recycled energy giving you real-world environmental and power savings! For features. MSI once again shows our market leading credentials and why our slogan is "Innovation with Style".

More detail about the Stirling Engine!

• The history of the Stirling engine.
– The Stirling engine theory and prototype was invented during1600's.
– In 1816, Robert Stirling of Scotland revised and then make it as "economizer".
– With that addition the early age air engines started to use Stirling's idea.
– Many other applications adopted the principle of the Stirling engine to replace existing applications.

• The theory of the Stirling engine.
– The Stirling engine, cycles through four main processes: cooling, compression, heating and expansion.
– This is accomplished by moving the gas back and forth between hot and cold heat exchangers.
– The hot heat exchanger is in thermal contact with an external heat source, e.g. Chipset, and the cold heat exchanger being in thermal contact with an external heat sink.
– The heat transferred the outside becomes air heat and pushes up the inside pump.
– The warm air cools, and the inside pump falls down again.
– The cycle restarts again.

• Usages of the Stirling engine.
– The striling engine is still used in some submarines and to power toy airplanes.
– The evolution of the Stirling engine has become a replacement energy solution.
– Heat transfer power, small electrical generator and internal power engine.

• Advantages of the Stirling engine.
– Max transfer rate over 70% heat transfer capability, solar power is only 20~30%.
– Smaller size than solar power, only heat, no extra fuel needed when working.
– Lower total cost and continued power output with continued heat resource.

some more videos from youtube