CHEN 205: Project Stirling Engine 11 August 2014 Samuel Adams, Ashley Bender, Matthew Carlin, Monica Cuerno
BACKGROUND Robert Stirling was a Scotsman and clergyman born in Cloag, Perthshire, on October 25, 1790. Because of his faith and the fact that steam engine boilers tended to explode and kill people, Stirling set out to build a safer and more efficient engine. On September 27, 1816, the same year as his ordination, he and his mechanical engineering brother, James, applied for a patent for their economizer, which they would later develop into what we now know as the Stirling Engine. One of the original patent sketches is shown in Figure 1 for this hot-air engine 1. In 1818, the engine was modified for use in a quarry to pump water; it generated 2-horsepower and was 2 feet in diameter. After being used for two years, the cylinders eventually wore out 2. Starting in 1824, the Stirling brothers continued to perfect the engine, applying for additional patents in 1827 and 1840. James Stirling came up with the idea of increasing the internal pressure to increase the power output, but Robert Stirling developed the first practical piece that made this engine so efficient: the regenerator. Many other hot-air engines were similar in structure, but lacked this key component. This allowed the Stirling engine to be essentially equal to theoretical Carnot efficiency and be much more practical to build and use than a Carnot engine. Other advantages included its quiet running cycle and straightforward design. Yet, its large size and weight were negative features. In 1843, the brothers replaced the steam engine in the Dundee Iron Foundry with their Stirling engine to run all of its machinery. After four years and three replacements of the hot 2
cylinders, the factory returned to steam engine power 2. This illustrated the major problem with the Stirling Engine -- because of the need for extreme temperature differences for maximum power output, failure of the cylinders were inevitable and frequent, preventing it from surpassing the steam engine in production and use. The Stirling engine transitioned to smaller uses, such as pumping water and domestic applications. Its reliability for low to medium power outputs generally outweighed its inefficiencies by lower, less strenuous temperatures. In 1983, Professor Ivo Kolin of the University of Zagreb, Croatia, developed a differential Stirling engine, which ran on a temperature difference of 100 C, slowly decreasing to a 20 C differential. In the late 1980 s, Professor Senft of the University of Wisconsin, working closely with Professor Kolin, produced the Ringbom engine, which depends on pressure differences to move the displacer instead of a flywheel. Professor Senft also designed for NASA the N-92, a hand-held differential Stirling engine run off a 6 C temperature difference 3. Today, the Stirling engine is not widely used, but its applications are far reaching. It has been used in some hybrid cars and can be powered by solar energy to produce electricity. It is also commonly a novelty item. DESCRIPTION Unlike the engines that power the average car or other heavy machinery, the Stirling engine is a heat engine that is powered by an external heat source. Much like the Carnot cycle, the Stirling cycle has four basic steps: isothermal compression, isochoric heating, isothermal expansion, and isochoric cooling. In the first step, the working fluid originally designed to be air, although now often helium is compressed in the hot side of the cylinder at constant temperature. During the second step, the gas is heated and kept at constant volume by the piston in the cylinder. For the third step, the working fluid is allowed to expand isothermally by pushing 3
on the piston. Finally, the gas is cooled at constant volume, setting the stage for the cycle to begin again. This process is the most basic theoretical explanation for the Stirling cycle. Based on these principles, Robert Stirling built a working engine. The Stirling cycle, while simple, was inefficient if it performed exactly as previously stated. Due to the inefficiency found in the theoretical cycle, Robert Stirling made three main changes when dealing with a real working engine. The three main differences were: a displacer piston between the power piston and the cylinder head, a cylinder that was simultaneously heated at one end and cooled at the other, and a regenerator that the working fluid passes through to move from one side of the cylinder to the other. The displacer piston was invented by Stirling to push the gas from one side of the cylinder to the other through a tube connecting the hot side of the cylinder to the cold side. With the power piston set at a specific point inside the cylinder, the displacer piston is able to move the working fluid freely without much resistance. The displacer piston allows the cylinder to be kept hot at one side and cold at the other. This mechanism allows the hot gas to expand and perform work on the power piston. The gas is then forced to the cold side by the displacer piston and compressed by the power piston. The compressed cold air is then forced through the tube to the hot side of the cylinder. This cycle repeats indefinitely as long as heat is continually supplied to the hot side of the cylinder and heat is removed from the cold side. While the displacer piston and the simultaneous hot and cold temperatures greatly improved the efficiency of the engine, Stirling realized that heat was still being lost. He noticed that once the gas was heated and moved to the cold side, the heat was simply removed and lost. In order to capture this loss of heat, Stirling added a regenerator to the tube that transferred the gas from the hot side to the cold side. A regenerator is simply a porous metal that saves heat in 4
order for it to be used later. The best way to imagine a regenerator is to think of it as many ball bearings inside the middle of the tube 4. As the hot gas passes over the bearings from one end, the heat from the gas is stored and the gas slowly cools to the lower temperature. When the gas is forced back to hot side, it gets back most of the heat stored in the bearings. The temperature gradient within the ball bearing packing allows most of the internal heat to be saved and makes the engine significantly more efficient. Refer to Figure 2 below 5 : APPLICATION: A heat engine can be applied anywhere there is a temperature difference. A common source of such a temperature difference is a household hot water heater. A Stirling engine could be placed so that the typical heat source of the hot water heater is applied directly to the engine, and the waste heat is rejected to heat the water. Calculations of this application were made with certain assumptions and specifications. It is assumed that the water is an incompressible fluid, and the hot water heater is at a steady state with a consistent flow of 2.5 gallons per minute (the 5
maximum shower head flow rate by law) that comes in at 79.3 F and leaves at 60 C. The flame is considered to provide a temperature of 1027 C. Although the Stirling engine is typically 70% efficient overall, these assumptions have led the authors to carry out these calculations with the actual performance of the engine as half the predicted maximum Carnot thermal efficiency. Even so, this conservative approach leads to surprising results that will be discussed following the calculations performed in the separate pdf. These calculations indicate that a modern Stirling engine could be a practical solution for the energy demands of 24 homes. The beauty of the Stirling engine is the variety of fuel sources that it can accommodate without a compromise in performance. If in an urban community, the likely fuel of choice is natural gas. Using this fuel source at $3.97 per MMBtu leads to a single home s monthly electric bill being less than the average American s phone bill. Wooden pellets cost roughly five times this, meaning it is not a practical urban fuel source at all. However, if one were to live in a rural area without access to natural gas, wood could then be considered a viable option for fueling the Stirling. Fifteen tons of wood is needed each month. It may be practical to spread the workload to provide this wood over the 24 families that the engine supports. An alternative is to use the engine to power a wood mill, reducing the amount of manual labor necessary and the number of homes the engine could support. Additionally, this eliminates the task of finding 24 families dedicated to chopping wood every day. No matter the fuel source or what community it is implemented in, the Stirling engine shows promise to meet energy demands around the globe. 6
CONCLUSION Even though three major changes were successfully made in order to make the Stirling engine more efficient, the Stirling engine has not reached commercial success like less efficient engines. This is because the problem of extreme temperature differentials needed to produce useful amounts of power beyond that of boutique uses has not yet been solved. However, there are companies that are interested in making the Stirling engine a mass produced product, such as Infinia Corporation. Infinia Corporation s approach is to reduce the cost of production and emphasize the advantages of the Stirling engine 10. By doing so, the Stirling engine could have the opportunity to be used for applications like the hot water heater. The Stirling engine has potential, but may never be widely used. Only time will tell what becomes of the Stirling engine. 7
Works Cited [1]Meijer, Roelf J., Benjamin Ziph. Sterling Engines. Wiley Online Library. Web. 3 August 2014. [2] Robert Stirling. AccessScience. Web. 3 August 2014. [3] A brief Stirling engine history. Stirlingengine.co.uk. Web. 3 August 2014. [4]Holtzapple, M. Class Lecture, Topic: Chapter 8: Production of Power from Heat. JEB 111, Texas A&M University, College Station, Texas. 1 August 2014. [5]Helsing, V. File: Beta stirling animation.gif. http://commons.wikimedia.org/wiki/file:beta_stirling_animation.gif. Web. 3 March 2007, [8 Aug. 2014] [6] How much electricity does an American home use? http://www.eia.gov/tools/faqs/faq.cfm?id=97&t=3. Web. 9 August 2014. [7] Natural Gas prices. http://www.bloomberg.com/energy/ Web. 9 August 2014. [8]Rapier, Robert. The Price of Energy. http://www.forbes.com/sites/energysource/2010/01/26/theprice-of-energy/. Web. 26 January 2010, [9 Aug. 2014] [9]Yang, Xuejiao (Snow), Albert R. George. Biomass Heating. http://cardi.cornell.edu/cals/devsoc/outreach/cardi/programs/loader.cfm?csmodule=security/getfile& PageID=1118149. Web. 26 January 2014, [9 Aug. 2014] [10] Qnergy: Overview http://www.qnergy.com/-overview. Web. 16 July 2014, [8 Aug. 2014] 8