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The Stirling Engine
On page 9 we made reference to the Stirling cycle as one of the many thermodynamic cycles used to generate Work from Heat. The original Stirling Engine was invented and patented in 1816 by Reverend Dr Robert Stirling but has remained, largely, a curiosity from that time. The Otto and Diesel cycles were adopted as main work horses for engines and only recently has interest being renewed in the Stirling engine. The main difference of the latter engine from the Otto and Diesel engines ( internal combustion engines ) is that an external heat source is needed for operation and therefore a flexibility of fuel source is available to the user. An index of Stirling engines that have been built (present and historical) is given at
www.stirlingengines.org.uk/manufact/post.html .
A comprehensive account of the Stirling cycle is given at http://en.wikipedia.org/wiki/Stirling_cycle and graphical data is presented for an idealised engine. In any thermodynamical analysis of engines the Pressure - Volume (P-V) graph or indicator diagram is a useful starting point and such a diagram for the Stirling cycle is shown below:
A fixed amount of gas is subjected to the above cycle and takes in heat from an external source to produce mechanical power. The numbers on the curve are the angles (in degrees) of the crank with respect to the horizontal.
From an educational point of view the Stirling Engine is a marvel. A laboratory experiment can be set up with such an engine without disturbing the whole class - just an innocent chug-chug for the duration of the experiment. Internal combustion engines on the other hand have to be housed in a sound-proof booth. So many experiments can be carried out with the engine - it can be an engine or a refrigerator or a heat pump - so, it is a valuable apparatus for any science or engineering laboratory.
At the start of my teaching career, in the late 1960's most University laboratories had a Leybold Stirling engine. Now, if one does a Google search of "stirling engine and Leybold " one gets a huge supply of references: Experiment TH1 from University College, London, Experiment GP48 from Oxford University, and Leybold www.leybold-didactic.de supply the old engine and a mini engine for experimentation.
More than three decades ago Cussons Technology Ltd , Manchester, www.cussons.co.uk manufactured a Stirling engine for student experiments (current reference number is P5691) as illustrated in the following photograph:
Fairly basic experiments could be performed like energy input to heater (red glow on right hand side) versus shaft power output measured with a breaking dynamometer. For students taking a course in thermodynamics, however, it was felt that sensors should be added to the apparatus to enable a P-V diagram to be displayed. An article in Physics Education, 1980 showed how this was achieved. An aluminium strip was attached to the piston as shown in the following diagram:
This strip is on the reverse side of the engine (opposite side to flywheel ). Since the bore (cross-sectional area) of the cylindar is constant the volume of each stroke may be determined by a displacement measurement.
Access to the cylindar chamber is via a tapped hole and this facilitates changes of the working gas, say, from air to nitrogen or helium. The hole can then be sealed with a blind screw and copper washer to make the cylindar air-tight. It was a relatively simple matter to fit a pressure sensor in place of the screw and hence a P-V diagram could be displayed. A page from the article is given below and the full article is found at
https://sth-se.diino.com/f.thompson/migrated_data/EandH
The file is named Stirling-Eng1.pdf but the article in the Physics Education Journal ( Phys.Ed., 15, 244-246, 1980 ) was entitled "An inexpensive linear transducer using a photovoltaic cell".
Recent developments
Since the 1960's Stirling engine technology has always been applicable to niche markets and NASA has frequently used the technology on its space-craft. However, applications may come to main-stream markets with products from DISENCO which stands for DIstributed ENergy COmpany, www.disenco.com . This company is developing a combined heat and power (CHP) unit for the home which relies on a Stirling engine. On page 5 it was shown that we are loosing 3 Exo Joules of energy per year or 90 GWatts of power in distribution losses and power station inefficiency. The Danish government has taken steps to reduce this loss (page 4) but UK has shown little progress in this area. Now, Disenco is offering a CHP unit which produces electricity and heating at the same time - a micro energy source - and this unit could well be powering our home in a few years time. The "waste heat" of normal electrical generation goes directly into heating our homes. This is what they say:
We look forward with interest to this new venture which may start an entirely new way of using energy in our homes. For instance, if we have a peak electrical output of 3 kW, given above, and each home uses the Stirling generator for 10 hours per day then the energy supply would be 30 kWhour. Typically, one third of this would be needed for the home and two thirds could be fed into grid. We have, therefore a situation where 2 kW is available for a 10 hour period. If the generators were automatically switched on (harmonised) at "peak" times then a boost to the grid supply would be given when the demand is most critical. This would eleviate many problems in our electrical supply chain and avoid power cuts. CHP in both homes and factories could minimise the collosal waste of energy in central power sations ( thermodynamic efficiency of one third and two thirds waste heat delivered to the atmosphere) and give added flexibility where renewable energy sources are used. So, instead of building new power stations we equip each home and factory with its own CHP power station.
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