The Stirling Cycle part 1 (Stirling Cryogenics)

this is an early print of a patent for a hot air engine submitted in 1816 by Robert Stirling a Scottish minister today with the help of modern materials and with new knowledge of flow and heat transfer phenomena a highly efficient engine has been developed but the fundamental principles of the Stirling cycle are unchanged what then are these principles at the heart of the engine are the cylinders in each cylinder a certain quantity of gas is retained by a piston assume that the gas pressure on either side of the piston equals one atmosphere no force on the piston results with the piston held in position but the head of the cylinder be heated pressure of the gas rises this force results now let the piston be released it is driven by the expanding gas until the pressure to the left of the piston reverts to one atmosphere if the cylinder head is cooled the pressure of the gas decreases further a force is now exerted on the piston in this direction a force which drives the piston back to its original position let us relate this cycle to a PV diagram we start with this given volume and a pressure of one atmosphere when heat is supplied to the cylinder the pressure rises this distance is proportional to the force acting on the piston when the gas expands its pressure drops the force becomes smaller this area is proportional to the energy delivered by the gas during cooling the pressure of the gas in the head becomes lower than one atmosphere the piston driven now to the Left delivers yet more energy the work done in one cycle is proportional to the area enclosed by the related curves this cycle turns heat into mechanical energy but the power of the simple device considered so far is small and its efficiency low the power can be increased by raising the pressure on both sides of the piston to a larger value in the modern Stirling engine this may be 100 atmospheres the force is induced by heating and cooling will increase correspondingly so will the energy delivered per cycle to show this on the PV diagram the pressure scale has to be condensed now the cycle starts with a pressure of 100 atmospheres the area indicating the energy per cycle has increased so much but at the same scale the original area can hardly be seen so far it has been assumed that the pressure to the right of the piston remains constant throughout the cycle this doesn't apply in practice the volume to the right of the piston obviously changes so does the pressure however the pressure is kept reasonably constant by including a cavity known as the buffer space the power of the cycle has been raised now let's turn to the efficiency as well as the gas the cylinder wall is alternately heated and cooled the heat so expanded is wasted but it can't be neglected as the heat capacity of the cylinder wall is much higher than that of the gas the genius of Robert Stirling solved the problem with the so called displacer piston this piston can drive the gas from one end of the cylinder to the other hardly demanding energy this is because the pressure is on either side of the piston are almost equal the cylinder head can now be kept constantly at a high temperature and the other end at a low temperature the whole cycle can be performed without wasting heat energy on the cylinder wall first the gas is displaced to the cylinder head where it is heated then the gas is expanded and work is done on the power piston the gas is driven to the right and therefore cooled then compressed and again work is done but even here heat is being wasted it's being wasted on the gas itself a certain amount of heat is being supplied to the gas and yet a half cycle later it's being removed again and lost the engines efficiency could be increased if only this heat could be conserved somewhere to be used later in the cycle sterling again provided the basic solution we call it a regenerator a chamber filled with a porous metal which acts like a sponge for this chamber the gas gives up heat during one half of its cycle only to take heat back on its return with this modern version more than 99% of the heat otherwise lost is recovered there is now only one essential reason left to supply heat to the cylinder head which is to compensate for the tendency of the gas to cool during expansion conversely is also one reason to remove heat from the other end which is to compensate for the tendency of the gas to heat up when compressed but now to the amounts of heat to be supplied and removed are much smaller than before the efficiency as well as the power of the original Stirling cycle are increased still further by raising the expansion temperature and by using hydrogen or helium as a working gas instead of air the movement of the displacer piston is regulated by the power piston through the so called rhombic drive these are the four phases of the cycle first the gas is driven to the hot space where it is expanded driven back to the cold space and ultimately compressed with the use of the rhombic drive the four phases of the PV diagram will partly overlap resulting in this shape the work done is still proportional to the area enclosed this is how the rhombic drive is built up the mechanism comprises two pairs of connecting rods of equal length one pair is connected with the yoke of the power piston and the other pair with the yoke of the displacer piston the yokes are of equal length as well due to the symmetrical build up of the drive there are no resulting inertia forces in the horizontal direction furthermore the vertical inertia forces acting in the mechanism will not be affected by the assumption that the masses of the Pistons are concentrated in their respective yokes if these masses are equal then the center of gravity of this rhombus will coincide with the geometrical center the center of gravity is displaced vertically the inertia forces which result can be compensated by two counter weights whose combined center of gravity is in counter movement to the center of gravity of the rhombus the masses of these counter weights can be evaluated so that the center of gravity of the whole system remains static this means that the single cylinder hot gas engine is inherently balanced with this balance it is possible to run the engine at high speed this leads to high-power with both high speed and high power the modern Stirling engine needs an improved heat transfer system this can be achieved by placing more heat exchange units in parallel efficient cooling is ensured by subdividing every cooler tube into many smaller ones all these are embedded in the cooler housing through which water is pumped the heater is improved by extending the heater tubes increasing their number and adding fins the exact dimensions of the heat transfer system result from a thorough investigation of the Stirling cycle and have been evaluated electronically to minimize the sum of all the losses in section it can be seen how a burner is placed inside the heater the actual combustor is suitable for a wide range of gaseous and liquid fuels which are fed in at the top the fuel and air associate to become the hot combustion gases which circulate around the heater tubes the efficiency is increased by using a pre heater in this the air is led to the burner against the flow of the combustion gases absorbing heat from them and thus reducing chimney losses the heating system being external is very flexible indeed and can be adapted to use nuclear or solar energy this is the end result a powerful quietly running engine with a high efficiency nearly 40 percent it has turbine properties in as much as it can be completely balanced and for a model with only four cylinders and extremely study torque is obtained the Stirling cycle is

Channel: DH Industries

Views: 592,653

Rating: 4.8551793 out of 5

Keywords: dh industries, stirling cryogenics, stirling engine, cryocooler, cryogenerators, cryogenics, liquid nitrogen

Id: GqIapDKtvzc

Length: 12min 14sec (734 seconds)

Published: Tue May 08 2012

Reddit Comments

[u/i_drah_zua]

Here is part 2 ot the video.

[u/WiseCynic]

I built a "coffee cop" model. The damned things are awesome!

I want to marry a Beta to a parabolic mirror and point that thing at the sun. Dunno how much work I'll get out of it (yes, size is important here), but even if it turns only a small generator - I can store the power for use at night in an LED array, etc.

[u/MrMasterplan]

I love these old educational videos! An this one is on my favorite engine! thanks for posting.

[u/stereomind]

Hell Yes! here's a coffee cup toy me and my buddy made some years ago.

[u/DingDongSeven]

Love 'em. Got a car for a friend, great gifts.

[u/J4k0b42]

I did my senior project on Stirling engines, I actually made a working one out of a few cans (though it had to have dry ice in the top).

Edit: I also talked to engineers from the Idaho National Lab who are building a Stirling engine version of the Radioisotope Thermoelectric Generator (RTG) like the one on Curiosity rover and Voyager. The engine would use two opposed beta Stirling engines with floating pistons and electrical resistance feedback control mechanisms, so there are only two moving parts and the cylinder is entirely sealed and filled with hydrogen. The finished design is estimated to be three times more efficient than the current chemical-electric model at the same weight, while also using les plutonium, which is in short supply.