Free piston Stirling engine for rural development R. Krasensky, Intern, Stirling development, r.krasensky@rrenergy.nl W. Rijssenbeek, Managing director, w.rijssenbeek@rrenergy.nl Abstract: This paper presents the first results obtained on the development of a Free Piston Stirling Engine (FPSE). This engine/generator is designed so that it can be used by anyone (mainly in developing countries, where no grid is present) on a biomass cooking stove to generate electricity. It can also use a solar concentrator for the heat generation. The output power that is aimed at, is 100W. INTRODUCTION Some years ago a renewed interest in free-piston Stirling engine converters (FPSE) for use in space power applications was demonstrated by NASA, It combines a simple design and very high efficiency. On the other hand, these systems are demonstrating long life capability with low maintenance. The design presented in this paper is rough and primary. It was developed by a group students from TDO, Techniques for Sustainable Development at Eindhoven University, Netherlands, based on the design ideas and materials handed over by RR Energy. The student most engaged in the design was Gilles Hommen. However, the Stirling engine prototype made by these students did not function when tested. In the next stage, at RR Energy, we studied the design and made a number of small improvements and finally got it running. It first ran at a hot temperature of 450 C, but we were able to have it running at 280 C after further improvements. Components size and characteristics will be presented, as well as how they work. Further development will be made on this engine to improve its efficiency as well as to link it with a linear alternator. This paper is not intended to explain how a Stirling engine works (refer to http://en.wikipedia.org/wiki/stirling_engine for this, or this to understand free piston engines: http://www.adthree.com/engine/assembly.pdf). We will however give a short preview of the working principle of this engines. For questions, comments or advices, don t hesitate to mail (r.krasensky@rrenergy.nl w.rijssenbeek@rrenergy.nl) WORKING PRINCIPAL A Stirling engine is a machine which converts heat energy into mechanical power and belongs to a category of heat engines known as external combustion engines. This means that the fuel is burnt outside of the engine cylinder, rather than inside the cylinder like an internal combustion engine as, for example, the engine in a car. The principle of external combustion has some big advantages, of which the fact that one can use any fuel which is available is the most important one for this project. These fuels can range from biomass to solar heat. The Stirling engine relies on the principle that a quantity of gas (usually air, but sometimes helium or hydrogen) that is heated by an external source will expand. Because the gas is sealed in a container, the pressure will rise. When cooled, again by an external source, the gas contracts, the volume decreases and thus the pressure will drop. By this expansion and compression, a cylinder or piston can be moved, which generates mechanical power. The Stirling engine has a heated and a cooled side, and uses the potential energy difference between these ends to establish a cycle of a fixed amount of gas expanding and contracting within the engine, thus converting a temperature difference across the
machine into mechanical power. The greater the temperature difference between the hot and cold sources, the greater the potential for power production is. FPSE PARTS 1- Displacer: it is made of Calcium-silicate which has a low density (0.25g/cm ) and is heat resistant. This material is used to isolate furnaces which are used for heat treatment of metal. The displacer has to be a good insulator, as we want the heat to stay in the hot part of the engine (bottom part) and the cold part (top part) has to remain cool. Its roll is to move the gas from top (cold part) to bottom (hot part). 2- Regenerator: it is composed of two different parts; first, the outside of the displacer, then three holes are present in the middle of the displacer which allows the air, which is the working gas, to flow from the cold part to the hot part, and vice versa. The regenerator has to take maximum heat from the gas when it is flowing from the hot part to the cold part and when the gas flows backward (from cold to hot part), the regenerator has to give maximum of the heat taken back. This is why it needs to have good heat conduction. The three holes were filled with steel wool, and the outside of the displacer was covered with a thin layer of copper. On the next steps, we plan to reduce the gape on the side of the displacer and to have the regenerator located on the outside of the displacer. The gas would flow in a gap made between the glass cylinder and another cylinder (probably stainless steel). This tends to get closer to the FPSE design from the NASA (http://www.grc.nasa.gov/www/tmsb/stirling.html). This is supposed to make the displacer lighter. Whatever mass is moving inside the engine is less power output. 3- The gas spring: it is made of a rubber below which was taken from a car steering system. It does not need to resist heat since it is exposed to only 100 C. This gas spring is what activates the displacer: the pressure in the engine is changing in cycles; ant the pressure difference between the gas inside the gas spring, and inside the engine is going to change the length of this gas spring. With a larger volume of air linked to the gas spring, the pressure inside it can be considered to remain constant. Therefore, the pressure difference has a stronger effect on the change of length of the gas spring. For the experiment on this first prototype, we have installed a Schroeder valve (auto valve) which was linked to a plastic bottle. The ratio of the radius between the gas spring and the power piston should be about 3. This is not the case on the engine, but it will be changed in the next steps of improvements. We also plane to use the volume inside the displacer (which requires a hollow sealed structure) to have a large volume of air linked to the gas spring. A leakage has to be present between the gas spring, and the working volume of the engine, that way the pressure inside the gas spring will be the average pressure of the working gas, and the displacer will remain at the right position inside the engine. 4- The power piston: it is also made of a rubber below taken from a car steering system, but the diameter is bigger than the one of the gas spring. Since it is located on the top part of the engine (cold part), there is no problem of heat resistance. However, fatigue has to be considered: this is why the stroke should not be too high in order not to put too much stress on the rubber. This bellow has a spring effect; therefore it has a resonance frequency, so there is no need to add a spring to it. Changing the load is sufficient to adjust the frequency in order to have the best efficiency (slightly lower than the one of the displacer). 5- The rocking arm: it was put in place to guide the power piston. Other wise, it does not have a linear movement, and tends to oscillate on the sides, and no power can be extracted then. It was first made of steel, but was changed to aluminium to have less weight on the power piston. 6- Top and bottom plate: they are made of stainless steel. They will be changed for more adapted material (regular steel for bottom plate, and aluminium and top) with better heat transfer coefficient. They are tightened on each sides of the glass cylinder by means of eight stainless steel bolts of 8mm diameter. Insulating, heat resistant washers were used on the bottom and Teflon ones were used on the top in order to limit the heat transfer through those bolt from top to bottom, which is causing
significant drop in efficiency. A gasket ring was used on the bottom, and a rubber ring was used on top in order to seal the engine. Silicone is used as well to improve the sealing, but eventually, it will have to be replaced. 7- The glass cylinder: it is special glass which resists heat. It has been chosen to be made of glass because it enables use to see what is happening inside the engine, to make sure that frequency and stroke are good. Also, it is a good insulator. Eventfully, this will be replaced by stainless steel. 8- Flat springs: They are made from steel. Three of them were used to maintain the displacer in the right place. They form an equilateral triangle. They determine the frequency of the machine, as well as the weight of the displacer (F=1/2*(k/m)^(1/2)). They then have to be designed carefully. Also, it is important to notice that the displacer hanging from them has to remain in the middle at rest. This is why they can not be too weak, otherwise, the displacer will hang at the bottom. An alternative for this is to place those springs at the bottom, but then they receive a lot of heat, and their elasticity changes a lot within a couple of hours. This is why they should be stiff enough, but then, the frequency of the machine becomes higher, and heat transfer has to be good. Figure 1: Scaled drawing of the FPSE
Figure 2: Scaled drawing of the displacer FPSE PARTS SIZE All the values given are in metres, or m³ for the volumes. The strokes given are the total displacement: for example, the power piston has a stroke of 5cm, which means that it will go up to 2.5cm above the rest position and 2.5cm down the rest position. The 2 nd version of flat springs work much better and they allow a bigger stroke for the displacer. Since they were quite weak, the had to be bend before use, so the displacer could push them and stay in the middle of the cylinder at rest (7,5mm on each side of the displacer). However, like said above, they heat up, and much more stress is put on them, this is why they don t last very long. Cylindre Displacer Diameter 0,25 weight (kg) 0,684 thickness 0,008 Diameter 0,227 Height 0,08 Height 0,065 Volume 0,003440418 Volume 0,002630599 Actual volume 0,002529895 Gas spring Stroke 0,015 mean inside diameter 0,038 volume displaced 0,000607061 height at rest 0,0425 Volume 4,81998E-05 Power piston Gas spring hole mean inside diameter 0,0545 Diameter 0,05 height at rest 0,05 Height 0,036 spring constant (N/m) 50 Volume 7,06858E-05 Stroke 0,05 Volume at rest 0,000116641 Regenerator holes (3) Max volume 0,000174962 Diameter 0,014 Min volume 5,83207E-05 Height 0,065 Change in volume 0,000116641 Volume of the 3 3,00179E-05 Total volume (m3) 0,000978965 Hot/Cold plate Total volume (L) 0,978964604 Thickness 0,004
Flats springs Flats springs 2 nd version Length 0,14 Length 0,17 Thickness 0,0005 Thickness 0,0005 Width 0,019 Width 0,01 Table 1: Size of the FPSE components CONCLUSION This prototype is a first try to make a FPSE run. This step was successful in that way. It has enabled us to understand the mechanisms better, and further improvements are possible from that. The next prototype will soon be built and here is a list of changes that will be made: Top plate made from aluminium: 8mm thickness. Bottom plate made from regular steel: 6mm thickness. Hollow structure type of displacer with connection to gas spring to provide larger volume of air to the gas spring. It will also make the displacer lighter Centralized power piston. The diameter will also be enlarged to 12,5 cm, but we expect the stroke to remain the same. Gap between displacer and cylinder reduced Double wall cylinder: new type of regenerator, using the double wall feature. RECOMMENDATIONS At RR Energy, we want this engine to be mass produced by those who have already good manufacturing capabilities and market channels, reaching the people in developing countries. We want to create a platform with students and R&D people to exchange knowledge on this topic to further the FPSE, so it becomes a low cost renewable energy alternative for those 2 billion not connected to the grid. If you which to join, check the web site www.rrenergy.nl for further information or give us a e-mail!.