Stirling Motors and Generators
By Gordon Shaw
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Stirling Motors and GeneratorsBy Gordon Shaw
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The Stirling Engine is a Marvel |
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The Stirling engine has numerous guises and applications but its full potential has not yet been realised. If you are not familiar with Stirling devices we can help you appreciate these fascinating and practical machines; they are particularly relevant at a time when carbon emissions must be reduced. The Stirling machine is a thermo-mechanical system invented c1816. In simple form it comprises a sealed gas chamber containing two interconnected piston/cylinder constructs and two heat exchangers. Working as a motor, heat energy is sourced through one exchanger, heating the gas (typically air, maybe helium) which passes through the chamber where temperature gradient and resultant pressure changes inside the chamber are used to drive the pistons enabling the extraction of mechanical energy (up to 30-40% of the total). The gas then moves to the other exchanger (the sink) where the remaining heat is removed by cooling; the surplus heat is dissipated to the ambient surroundings. The movies below show a demonstration model operating but provide only a minimal insight into its versatility. This particular assembly consists of two cylinders each with pistons connected to the flywheel (with a 90° offset). The lower cylinder is short and wide with aluminium plates at each end (the heat exchangers); the upper plate is at ambient temperature and the bottom plate is made to be either warmer or cooler. Within is a loose piston which allows air to flow around its edges. When the air is displaced upwards it merges with that in the small, upper cylinder. The piston here is a tight fit, but low friction, with little leakage. If the displaced air is warm it tries to expand pushing upwards on the piston. Conversely if the air is cool there is a contraction pulling the piston downwards. The driven piston causes the flywheel to rotate clockwise if the lower plate is warm (the heat source) but anticlockwise if that plate is cool (then the upper plate at ambient is the source). This model is designed to operate with small temperature differences. Indeed it can operate simply by standing it on your hand. However, Note that this model is a toy to demonstrate the principle of operation and beyond that it has little practical use. For an engine to produce useful work in the real world high temperature differences are essential and that means a very different physical construction. |
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In the short movies (10 secs) above, the engine is deliberately started in the wrong direction to prove it only rotates one way. |
 Small Stirling Engine rotating from heat on the hand Click to enlarge |
A major feature of the Stirling engine is that it is fully reversible. What we have is an engine that will happily rotate in either direction by creating a temperature difference one way or another between its heat exchangers, but the reversibility is greater than that. In the mode described the machine converts heat into mechanical energy; a motor with all the applications that can bring. But it can do much more; if instead, we input mechanical energy by driving the flywheel it becomes a kind of generator, moving heat energy and producing a temperature difference between the heat exchangers one of which becomes the output (the other is normally ambient). In short it can be used as a heat pump and used to cool or heat things simply by reversing the direction of drive.
Stirling motors have advantages compared to internal combustion (ic) engines. The most obvious is that the Stirling is heated from outside the cylinders: it is an external combustion (ec) device. This means that many different fuels can be used; by comparison ic engines are limited to highly volatile fluids like petrol or diesel. External heat sources include most things that can be burnt such as biomass and waste so there is the opportunity for green energy. Also ic engines use explosive techniques which are inevitably very noisy while ec engines can be powered very quietly. Then there is the complexity of ic engines which require accurate valve timing and often spark ignition systems. Additionally the ec device is a closed system in which the working gases are constant unlike ic engines which require intricate inlet and exhaust systems. External combustion technology is not without inlet/exhaust problems but they are simpler. The ic cycle is therefore more complex than the ec cycle, furthermore it is not reversible in its mode. The Stirling engine, acting as a motor, has some serious disadvantages especially where high power densities are required such as in automotive applications. The principal drawbacks are the heat exchangers at the two ends of the machine and the need for high pressure containment. Efficiency is dependent on large temperature differences between these extremes which demands special materials and design techniques resulting in very expensive hardware. The ic engine produces high temperatures but in short bursts, unlike the unremitting heat at the input of the ec device, making it more practical to keep the average temperature stresses more manageable. Furthermore the ec engine takes a relatively long time to get up and running and with current technology it is not so responsive. For example it is not practical to build a a competitive vroom-vroom vehicle which will perform a cold start and accelerate from 0 to 62 in 10 seconds or so. Consequently commonly available applications for Stirling engines are, with current technology, practically limited to lower power densities such as drives for pumps and generators, refrigerators and ship propulsion. The use of a Stirling 'generator' is worth special mention. In this mode it works as a heat pump transferring heat from a to b. The input is mechanical energy usually via a rotary drive. One important application is a ground-source heat pump. Here heat is taken from the earth by buried pipes and a circulating fluid then transferred into a building for space or other heating. The surrounding earth has the potential to deliver large quantities of solar heat stored at low temperatures. It is also possible to source heat from the outside air but the air is low density and is not at its best when most needed (ie when the weather is cold). The system is able to use the flexibility of the Stirling generator because it can easily be reversed to provide air-conditioning in the summer. This reverse mode of operation suggests that it could have use as a refrigerator but it has strong competition (Rankine) at the sort of temperatures used in common domestic applications. However for extreme cooling (cryo) the Stirling is very well suited. What is the current state of affairs for Stirling engines? Considering it was invented almost two hundred years ago we had hoped to see more actual applications. In the early years it had to compete with steam engines which were quite successful. About 100 years ago the ic engine appeared using the Otto cycle and that was an absolute winner in terms of size and weight for its power output. The extensive need for transport. especially road and air, together with cheap petroleum fuels made the ic engine the firm favourite and that trend was reinforced by enormous resources put into engine developments. Now things are changing due to the need to go green and alternative technologies are constantly explored. There are numerous research projects looking at this technology for new applications. These include use for automotive and, it seems, air transport, even space applications. Currently Stirling engines appear to be at their best for ancillary use but this does not make them worthless because they contribute to a better overall efficiency. It is worth noting that Stirling engines can work well when the ambient is cold (eg in the arctic, at high altitude or in space) unlike standard ic engines where the opposite is true. Perhaps the most topical application is in micro-Combined Heat and Power (CHP) boilers. The use of the Stirling engine to drive an electric generator accompanied with the ability to use the spare heat for domestic heating delivers highly efficient systems, typically about 90%. The Sterling engine has a competitor, namely the Organic Rankine Cycle engine: both are in trial production. The practical economic problem of getting these en masse into the public domain now seems to have been overcome and we expect to see micro-CHP boilers proliferate in the next few years. The high efficiency will be welcome both for the sake of the environment and the purse but the main drawback will be how to fire them with a green fuel. At least by its nature, external combustion makes that a plausible option worth pursuing. Just an idea! Hybrid vehicles combining ic engines with battery driven electric motors such as the Toyota Prius are at their best in mixed driving conditions. On long cruising journeys their special features do not come into their own but rely mainly on ic operation. Common ic engines deliberately throw away 60 to 70% of their energy, mainly in heat. What if, under these cruising conditions, a Stirling engine drawing heat from the exhaust was arranged to drive an electric generator charging the main batteries? We have only touched on this subject which is fraught with technical and complex problems. However we have aimed to stir your interest without being too long-winded nor pedantic. There is an enormous amount of literature on the subject but we suggest a good starting point is this Wikipedia page on the Stirling Engine. This has some imperfections but it is wide ranging and provides a wealth of references and bibliography. |
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