Of all of the heat engines developed today the Stirling engine has the greatest promise to be the most efficient. Theoretically, the Stirling engine has the ability to function at the full Carnot efficiency. However friction, thermal conductivity and other variables keep it from ever reaching that theoretical potential. Stirling engines, being “external combustion” engines, are very
smooth running and very quiet. Also, because the Stirling engine uses an external heat source, it can be run on whatever is available that makes heat.(1)
In this paper we are going to look at the history of the Stirling engine, how it works, and the different configurations in which it can be built. Further, we are going to look at the pros and cons of the Stirling engine, different applications, and how new technology is being developed using the Stirling model.
History:
With the dangers of early steam engines, Dr. Robert Stirling and his brother James sought to create a safer alternative. In 1816 the Stirling brothers patented the Stirling Air Engine.(2) Originally the patent was for a heat exchanger that Stirling called an “economizer”. Rolf Meijer coined the name “Stirling engine” approximately one hundred years later in order to describe all types of closed cycle regenerative gas engines.
The original patent went on to describe one possible application for the economizer, it could be used as an air engine. Robert and his brother James continued to apply for patents that covered various improved configurations of the original design. One very important improvement was pressurization, which increased the power output of the engine.
Although the Stirling engine was unable to compete with the steam engine in large-scale operations, it did find success in the latter nineteenth and early twentieth century wherever a reliable source of low to medium power was required.(3)
“These generally operated at lower temperatures so as not to tax available materials, and thus tended to be rather inefficient. Their major selling point was that they could be operated safely by anyone capable of managing a fire.”(4)
As technology improved the appeal of Stirling engine was reduced by the electric motor and was mostly relegated to toys.
How it works:
The Stirling engine moves through four basic processes, cooling, compression, heating, and expansion. The fixed mass working fluid (commonly air, hydrogen, or helium) moves between two heat exchangers, one heat exchanger to facilitate the absorption of heat and the other to discharge the heat absorbed. In the engine there are two pistons (mechanically linked) exposed to the working fluid at each heat exchanger. The change in working fluid temperature causes a change in the working fluid pressure, which in turn drives the pistons.(5)
When most of the working fluid is in contact with the hot heat exchanger, it pushes the “Hot Piston” to the bottom of its cylinder extracting work from the fluid (power stroke). The “Cold Piston” being mechanically linked (at 90° on a flywheel) to the “Hot Piston”, is pushed to the top of its cylinder (compression stroke). With the bulk of the working fluid being heated, heat from the hot side bleeds into the cold side causing some expansion and starts to push the “Cold Piston” toward the bottom of its cylinder (power stroke) extracting more work from of the hot working fluid. As the “Cold Piston” continues to travel to the bottom of the cylinder the working fluid is at its maximum volume and the “Hot Piston” begins its travel to the top of its cylinder (compression stroke) moving the bulk of the hot working fluid to the cold side. The working fluid starts to cool and the “Cold Piston” powered by the momentum of the flywheel, begins its travel to the top of its cylinder (compression stroke). As the working fluid reaches its minimum volume it absorbs heat from the hot side, the working fluid on the hot side increases in temperature, the pressure increases and starts to drive the piston to the bottom of its cylinder (power stroke). Once more placing the bulk of the working fluid in contact with the hot heat exchanger beginning the process again.(6)
Configurations:
Stirling engines are differentiated into three types; Alpha (operation explained in the preceding section), Beta, and Gamma. The differences are found in the movement of the working fluid between the heat exchangers.
The Alpha Stirling utilizes two separate cylinders and power pistons.
“The hot piston cylinder is situated inside the higher temperature heat exchanger and the cool piston cylinder is situated inside the low temperature heat exchanger. This type of engine has a very high power-to-volume ratio…”(7)
The Beta Stirling has a single power piston arranged within the same cylinder on the same shaft as a displacer piston. The displacer piston however, is fit loosely in the cylinder and extracts no power from the expanding working fluid. Typically, the Beta is attached to a flywheel to enhance its performance.(8)
The Gamma Stirling is a Beta with the power piston mounted in a separate cylinder adjacent to the displacer piston cylinder but is still connected by the same flywheel. The working fluid flows freely between the cylinders and remains in a single unit. This configuration is usually used in tandem with other Gamma Stirling engines.
Pros and Cons:
Pros:
Due to their ability to run on any heat source, the Stirling isn’t restricted to combustible fuel sources. They can operate on heat from solar, geothermal, biological, nuclear, and waste heat from industrial processes. Bearings and seals are typically found on the cool side (Beta and Gamma), they need less lubrication and have more longevity than other reciprocating engines. Stirling engines use a single-phase working fluid that maintains an internal pressure close to the design pressure. They run quietly and don’t require an air supply.(9)
Cons:
There are high costs due to the need for durable materials to handle the heat and pressure load. These materials also must be resistant to creep from the cyclical nature of the engine. Because of the external heat source, a warm-up time is required before useful power can be produced and the Stirling engine can’t change its power output quickly.(10) The dissipation of waste heat can be challenging and difficult to manage.
“The coolant temperature is kept as low as possible to maximize thermal efficiency. This increases the size of the radiators, which can make packaging difficult.”(11)
Applications:
Today, Stirling engines are used in some very specialized applications, like in submarines or auxiliary power generators, where quiet operation is important. The SES (Solar Dish Stirling) technology is well beyond the research and development stage, with more than 20 years of recorded operating history. The equipment is well characterized with over 25,000 hours of on-sun time. Since 1984, the Company’s solar dish Stirling equipment has held the world’s efficiency record for converting solar energy into grid-quality electricity.(12)
In space, solar powered Free Piston Stirling Engines (FPSEs) are producing electricity to meet the needs of astronauts without having to rely as much on a finite amount of fuel.
“…high efficiency, low mass 35 W free-piston Stirling engine design. Overall (engine plus linear alternator) thermodynamic performance greater than 50% of Carnot, with a specific power close to 100 W/kg appears to be a reasonable goal at this small power level”…”The use of high efficiency FPSEs would allow a reduction in the radioisotope fuel by a factor of roughly four compared to existing RTGs. Free-piston Stirling engines and cryocoolers are demonstrating long life capability, and significant strides in performance of Stirling cycle machines have been made in the commercial sector.”(13)
The Stirling engine has also seen success as a Heat-pump and Reverse Heat-pump. As a Heat-pump the Stirling engine pumps heat from the outside of a building to heat the interior rather cheaply.(14) Inversely, as a Reverse Heat-pump the Stirling engine removes heat from a cryo-coolers dropping temperatures to -200°C (73°Kelvin). Depending upon their design, the Stirling Reverse Heat-pump can drop temperatures as low as 60°Kelvin – 40°Kelvin.(15)
New Technology:
In an effort to cool motherboards for better performance and increased longevity, the “Air Power Cooler” is in development. It transfers the chipset heat into air momentum. When the heat from the chipset radiates into the air of the cylinder, the air expands and pushes the piston to rotate the fan and in doing so cooling the heat sink immediately. After the air moves from the bottom to top of the piston, the air will become heavy to push the up piston down. This air piston design which is called the “Electricity-less Fan”, can transfer over 70% heat power to air power thus very efficiently and effectively cooling the chipset.(16)
Currently Dean Kamen (the inventor of the Segway) is working to develop a Stirling engine that will re-charge a battery in an electric car while the car is in operation. Its range would be extended significantly.(17)
Conclusion:
In a global climate of shrinking resources, the Stirling engine has proven itself to be worth our time and effort to examine and experiment with. Incorporating a design that is elegantly simplistic, the Stirling engine has successfully stood the test of time. As we examined how the Stirling engine worked, we found it to be incredibly efficient. We also looked at the different configurations and saw a variety of very creative ways the concept has been applied.
When looking over the pros and cons we saw that there was some issues to workout for mass production, though when examining the applications there were specialized instances where the pros clearly out weighed the cons. Lastly, we looked at the “up and coming” new technology enhancing the Stirling engine and making it worthy of vernacular application. With the increasing pressure for efficiency it’s a very exciting time for the Stirling engine.
Bibliography:
1. Gurstelle, William. “Two-can Stirling engine”, Make (2007): 91-101
2. Unknown, “Stirling engine, 1.1 History”. Wikipedia, the free encyclopedia, 9 April 2008. Available from Wikipedia, the free encyclopedia, http://en.wikipedia.org/wiki/Stirling_engine. Accessed 9 April 2008.
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13. J. Gary Wood and Neill Lane, “Advanced 35 W Free-Piston Stirling for Space Power Applications”, Available from Sunpower Inc. http://www.sunpower.com/lib/sitefiles/pdf/publications/Doc0083.pdf. Accessed 9 April 2008
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17. Hank Green. “Segway Inventor Focusing on Green Cars”. Ecogeek, August 11, 2007. Available from Ecogeek. http://www.ecogeek.org/content/view/872/. Accessed 9 April 2008