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By Noel Leon-Rovira and Arturo Moreno-Lehmann
The Stirling engine has existed for more than 180 years.11 The fundamental principle of the Stirling engine is to constantly heat and cool a gas in order to expand and compress it. This change in volume produces work over a set of pistons. Since the working gas never leaves the engine and any heat can drive the Stirling, this machine is also known as the external combustion engine. Other technologies, however, like the internal combustion engine, have offered better power per mass unit. The internal combustion engines have also offered faster response to speed control.
The Stirling never reached a level of worldwide popularity to be applied in the energy generation field; however, this technology offers an alternative to scarce oil resources as well as to environmental situations that make it a great opportunity for market. Since any source of heat can make the Stirling rotate, there is a chance of using clean sources of energy such as the Sun. This work presents the analysis of a solution based on the Theory of Inventive Problem Solving (TRIZ) methodology where the Stirling engine is combined with the Brayton cycle in order to produce a more efficient machine. The result is a system that operates in a closed cycle and obtains energy from external sources.
Stirling motors have been considered a more secure alternative to the steam engine since their inception even though they never became popular due to the arrival of the internal combustion engine.1 The last Stirling motor was capable of producing more power per mass unit and more operation control. Due to environmental situations and scarce oil resources a better and more ecological alternative than the Otto or diesel engine is desirable. For that reason, the Stirling engine is resurging because it has the characteristic of being able to use any source of heat in order to operate including solar energy.
The resurgence of the Stirling engine is relatable to the TRIZ methodology. The advance in science and the availability of information has increased significantly since the initial development of the Stirling engine. In research reports the application of TRIZ reveals the results of the new technology achieved when using TRIZ tools. The following gives a preliminary introduction of the operation of Stirling engines.
The fundamental principle of the Stirling engine is creating a cycle where a gas is continually heated at a constant pressure. In order for it to expand and move to another section of lower temperature it cools and is compressed again for continuing the cyclical process. When heat is applied or extracted the gas pressure is increased producing a volumetric change in the gas that causes a piston generating mechanical power. In this way the Stirling operates with the same gas because the gas never leaves the interior of the engine. These engines are also known as external combustion engines.2,3
One of the most important elements of the Stirling engine is the regenerator. The regenerator flows hot and cool gas in a counter flow. Its function is to extract part of the thermal energy of the hot gas that is flowing to the cold cylinder in order to pre-cool it. Later, the cold gas returns to the hot side and is preheated with the stored energy in the regenerator. The idea is a part of the thermal energy used is recycled.4 The table below shows the Stirling engine process.
| The Complete Alpha Type Stirling Cycle | |
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Phase 1 | Phase 2 The gas is now at its maximum volume. The hot cylinder piston begins to move most the gas into the cold cylinder where it cools and the pressure drops. |
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| Phase 3 Almost all the gas is now in the cold cylinder and cooling continues. The cold piston powered by a flywheel momentum (or other piston pairs on the same shaft) compresses the remaining part of the gas. | Phase 4 The gas reaches its minimum volume and it will now expand in the hot cylinder where it will be heated once more driving the hot piston in its power stroke. |
Several types and different configurations have been developed for the Stirling engine. In general, there are three principal categories: alpha, beta and gamma.
One of the limits in efficiency of the Stirling engine is the section of gas that is never heated or cooled because it is not displaced completely. This is known as dead space. An increased proportion of dead space reduces the efficiency of the engine.
The Stirling engine also has a physical problem. There are only two instances where the gas is completely heated or cooled. During the rest of the cycle, there is a fraction of the gas cooling or heating. Consequently, there is an opposition of forces that reduces the overall magnitude of the engine power. Some designs exist where the transmission and one piston moves at a constant speed while the other one rotates at a variable speed.
Another limit to the Stirling efficiency also exists. When rotating at speeds of 1,500 rpm to 5,000 rpm, the gas only has 0.012 to 0.04 seconds to absorb or transmit energy. When the gas does not absorb enough external energy, the engine does not produce work. This situation reduces the overall efficiency.
The friction produced by the gas through the cylinders does not represent a problem at low speeds. The friction increases exponentially as the speed levels increase. In the case of the beta configuration, the displacer is designed to have a minimal distance to the cylinder walls. In this case the dead space is reduced but the friction increases.5,6
Due to the efficiency limits mentioned for the Stirling engine, the use of tools to aid the process of innovation were required. The first tool applied was the morphological matrix. It organized different conceptual variants to the Stirling engine.
The morphological matrix helps with the organization of different conceptual solutions for each of the product functions. The matrix has a structure where the first column corresponds to a specific product function where a solution has been worked. The following cells propose a solution concept in order to keep generating ideas. At the end, the different solutions may be combined and applied. Figure 1 shows the morphological matrix used to arrive at the solution.10
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One of the functions shown in Figure 1 illustrates that the Stirling engine must perform in order to operate by heating the working gas. Since the Stirling engine may use any source of thermal energy, there are a variety of options such as waste steam, natural gas and solar power.
The Stirling engine, however, needs to cool the gas and keep a temperature difference. Figure 1 shows several methods that can be applied to cooling.
The last row of Figure 1 highlights the spinning of the rotor in order to produce work. Actual Stirling engines have a crank design, but the use of a turbine such as in a Brayton cycle could also produce work and requires other functions of cooling and heating.
Based on the last morphological matrix, it is possible to enhance the concept solutions by applying the TRIZ methodology. The solar collector, for instance, has a parabolic design. It is used in several solar Stirling engines. This device, however, has to track the Sun with the heavy weight of the Stirling engine and results in an application with too high a cost for the technology.
By identifying technical contradictions from the contradiction matrix a principle may be found which says: 8,9
a. Remove or separate a part or property unordered, from an object or:
b. Extract only a necessary part or property.
Example:
In order to keep birds away from an airport, a sound that scares birds may be reproduced.
Another principle of the contradiction matrix says:
a. Instead of implementing an action dictated by the specifications of the problem, implement an opposite action.
b. Make a movable part of the object or exterior ambient immovable and the make unmovable into movable.
c. Turn the object upside down.
Example:
Clean parts that are cleaned abrasively using vibrations.
By examining the essence of a parabolic mirror, it can be concluded that the main objective is to concentrate light. The principle of inversion leads one to think that instead of reflecting the light to a specific focal point, the Sun's rays may be deviated using a Fresnel lens to a specific point on the floor. A tracking mechanism is no longer necessary as the application cost reduces itself to an extraordinary amount.
Instead of moving a piston back and forth, the use of a turbine may lead to a continuous movement instead of a reciprocating movement. A more detailed description of the solution using a turbine instead of a crank and pistons is explained in the next section.
To counteract some of the deficiencies of the Stirling engines, the innovation presented in this article combines the advantages of the Brayton technology with the fundamental properties of the Stirling engine.
In contrast to the Stirling cycle, the Brayton cycle is a process that operates in an open system. That means that the gas used, typically air, is renewed at the beginning of every cycle. A Brayton engine is basically composed of a compressor, a combustor and a turbine.
Initially the gas enters the compressor where its volume is reduced adiabatically. Later the gas is mixed with some kind of combustible inside the combustor where it ignites producing a change of temperature at a constant volume increasing its pressure. The gas then passes through the turbine where the pressure is converted into mechanical energy. The gas is expulsed to the atmosphere where its temperature reduces until it reaches normal conditions. After that, the cycle begins again.
The Brayton technology is typically used in jet planes. The Stirling engine also has several environmental advantages. For instance, the conversion of biogas in electric energy is accomplished using a turbine. The heat wasted by industrial processes can be used as the energy source to increase the temperature of the fluid in a Brayton cycle. The Brayton engines, therefore, are one of the most efficient and powerful processes that exist.7
By combining the advantages of the Stirling and Brayton engines, a Stirling/Brayton cycle device is developed. This innovation consists of modifying the Brayton cycle to operate in a closed cycle. The possibility of using any source of thermal energy is opened. In addition, by using alternative sources of energy, there is the possibility of using the technology in places where the presence of air is scarce (for instance, the mining and aerospace industries).
The combination of both cycles, however, resolves some of the problems related to the efficiency of the Stirling engine. First, the cycle stops reciprocating and the process turns into a steady state. This means that the temperature, pressure and volume in each section will remain constant. Note: the gas flows in only one direction.
The actual Stirling/Brayton cycle follows the same phase diagram pattern that the Brayton cycle does. The only difference is that heat is applied externally just the same way as it is applied to a Stirling engine. Figure 2 shows the phase diagrams of a typical Brayton cycle.
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The present research looks at the development of technology to be affordable and offered in the residential market. For this to happen, the engine parts are selected from commercial providers. Some of these components consist of compressors and turbines obtained from car turbochargers, which are not expensive.
The information presented explains the analysis of a solution based on the TRIZ methodology where the Stirling engine is combined with the Brayton cycle in order to produce a more efficient machine. The result is a system that operates in a closed cycle and obtains energy from external sources.
The experience obtained from this project can conclude that TRIZ has proved to be an effective tool for innovation in the field of technology and science. For this reason, it is encouraged that engineers and academics offer extended use and training of TRIZ.
Note: This paper was originally presented at The Altshuller Institute's TRIZCON2009.
Leon-Rovira is a research Professor of the Center for Product Design & Innovation. Contact Noel Leon-Rovira at noel.leon (at) itesm.mx.
Arturo Moreno-Lehmann is doing Masters studies in Automation Engineering from Tecnologico de Monterrey, Mexico. Contact Arturo Moreno-Lehmann at arturomoreno (at) itesm.mx or visit www.moleh.com.
