Hi Russell, Do you have any drawings or photographs that can illustrate this kind of regenerator? Best regards Luis -----Original Message----- From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of Russell McMahon Sent: 31 August 2006 16:51 To: Microcontroller discussion list - Public. Subject: Re: [OT]:Sandia, Stirling to build solar dish engine power plant > When you talk of a regenerator on the Stirling Engine, what kind of > thing are you talking about? What materials do you use? The "Regenerator", which is arguably the heart of any serious Stirling Engine, is a thermal store interposed between the hot and cold spaces. Gas flows through it from cold to hot and expands thereby raising overall system pressure in all spaces, or from hot to cold and contracts and thereby lowers the overall system pressure in all spaces. The regenerator is ideally a matrix with high thermal capacity per volume, high to infinite resistance to thermal "heat" flow in the longitudinal direction (ie in the direction of gas flow) and high thermal conductivity in the transverse direction. It has ideally zero dead space and no pneumatic resistance. The best material is nonexisteum. Practical alternatives are wire screens packed together under pressure. These can be sintered together to make a rigid block with appropriate properties. . At low temperatures (more for Stirling coolers) material such as lead shot has been used. With wire screens heat can flow across each screen through the wires but less easily from screen to screen as they are not in total contact at all points and points of contact are not necessarily optimum for heat transfer. Lead shot is used in Stirling cryo coolers as lead retains more thermal capacity at cryogenic temperatures than almost any other substance. Other unlikely substances such as cotton wool have been used successfully in some regenerators. Imagine a cylinder filled with hundreds of circular screens each a good fit to the circular cross section. These are pressed in tightly and end caps fitted. Place a "hot space" at one end and a "cold space" at the other. Blow gas from hot to cold via the holes in the screens. The first screen will be heated to the temperature of the hot gas and there gas will be slightly cooled. The next screen will be slightly cooler and this effect will progress with the temperature of each dropping and the gas being cooled. If properly designed the gas will enter the cool end totally cooled to the cool end temperature. Now reverse the flow. Cold gas entering the screen pack will encounter increasingly warm screens and will get hotter as it flows towards the hot end. If an equal quantity of gas flows from cold to hot as flowed from hot to cold then it will enter the hot space at about the same temperature as the other gas left. The screen temperatures will be decreased. If the thermal capacity of the screens is several ties or more of the thermal content of one charge of gas then the screen temperatures will not vary too widely during each cycle. If work is done at the cold end by expanding he gas to drive a piston then a net flow of "coolth" will flow up the regenerator and the hot end must supply a net downwards flow of heat energy to balance this. But overall the energy stored in the regenerator is several times at least the energy flow per piston cycle. If the regenerator has "dead space" in it then system pressure increases which are intended to drive the piston will instead go to increasing the pressure in these dead spaces. So a R will ideally have zero dead space. However, as R "dead space" falls the air passages get smaller and pneumatic resistances rise and losses increase. As with most aspects of Stirling engines, actual dead space is a compromise. Dead space also occurs in the hot and cold ends when the "displacer" which sweeps the gas into or out of the space, is not a perfect fit. The displacer also needs some wall clearance and this is also dead space. Also, large gaps around the displacer encourage gas to flow to and from past it and bypass the regenerator. Some systems actually use the walls of he displacer tube as the regenerator, while others have the regenerator inside the displacer with the gas flowing through the inside of the displacer as it goes from hot to cold and back. Heat which flows from hot to cold or coolth which flows the other way (same thing) via the regenerator structure, and not carried by gas, is waste energy which will increase overall system energy consumption without benefits. Russell -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist