INTRODUCTION
Global warming and climate change phenomena that can no longer be denied among the causes are greenhouse gas emissions so alternative energy is a bigger issue than ever and increasingly a focus of business and scientific attention this trend is also generating new interest in an invention that dates back 190 years the Stirling engine named after its inventor Reverend Robert Stirling. Robert Sterling was born on the 25th of October 1790 here the Perthshire village of Methven in Scotland his contact with engineering started at an early age in 1756 his grandfather Michael had developed the first rotary threshing machine for agriculture helping his father maintain and service the machines young Robert showed a keen interest in anything mechanical from 1805 to 1808 he attended Edinburgh University where he studied Latin Greek logic and mathematics a year later he switched to Glasgow University to study divinity and law graduating on the 4th of July 1815 he was ordained a minister of the Church of Scotland a vocation which he followed till his death
In 1878 in the first half of the 19th century Europe underwent a technological economic and social transformation in the early days of the Industrial Revolution working conditions were appalling and sterling witnessed them at first hand in his parish in the coal mines new high-pressure steam engines were introduced to drive pumps for extracting the mine water the human cost was high the boilers frequently exploded and claimed many lives among the victims children as young as six forced to work in the mines driven by the idea of making the miners work safer and easier Robert Stirling sought to develop a machine that would operate at much lower pressure and without dangerous steam although the principles of thermodynamics were not yet known Stirling was aware that gases expand with heat and contract with cold this can be demonstrated by a simple experiment if a paper lid is placed over a jar of hot jam the air inside the jar is heated this causes it to expand in the jar and force the lid to belly outwards but if the jar is placed in cold water the air inside cools down and contracts and the lid curves inwards Stirling was inspired to design a machine that exploits this property of gases
The principle on which it works has been known since antiquity in the second century BC the scholar hero of Alexandria developed a machine for opening temple doors automatically when a visitor lit a votive fire the heat spread to a sealed underground tank half filled with water and cause the air inside it to expand the resulting pressure forced the water through a pipe into a bucket because of the weight of the water the bucket descended pulling a rope that opened the doors when the fire went out the air quickly cooled and contracted a partial vacuum was created the water in the bucket was sucked back into the tank and a counterweight closed the doors like heroes hot-air machine Robert Stirling's invention exploited the property of air to expand with heat and contract with cold on the 27th of September 1816 he obtained a patent in Scotland for a hot-air machine that was later to become known as the Stirling engine
This is how it works a sealed cylinder is exposed to a continuous source of heat from below the air in the lower part of the engine gets hot expands and pushes a piston upwards now the hot air needs to be cooled again this is done by adding air fins to the farthest point from the flame and getting the air to flow towards them so the air at the bottom needs to be displaced a task that's performed by another piston which displaces the air as it descends and forces it up to the air fins there the air cools and contracts and the power piston returns to its starting point now when the displacer piston makes space for the air at the bottom the air is reheated and the cycle starts again to ensure coordinated movement the power piston and the displacer piston need to be precisely synchronized this is achieved by coupling the two Pistons with a flywheel the piston rods are anchored in such a way that the Pistons move in perfect coordination through each stage of the operation while the power piston travels upwards due to the expanding hot air the displacer piston remains virtually motionless then the displacement of air to the cold end of the cylinder begins finally the two Pistons return to their starting points the cooled air is displaced downwards to be heated and the cycle starts all over again
Robert Stirling's hot air engine was a safe alternative to the steam engine because the internal pressures involved were much lower coal was needed only to provide the required heat but apart from being a great deal safer than the steam engine Sterling's invention was more fuel efficient reason enough to develop the new technology further the first Stirling engine was used in air show in Scotland to pump water from a quarry by 1840 Robert and his brother James an engineer and foundry manager had developed and patented a number of improvements to the Stirling engine in March 1843 a 34 kilowatt unit was built to drive all the machinery at the Dundee foundry company for years in operation it consumed only 1/3 as much fuel as the steam engine it replaced its efficiency ratio 18% was never matched in the 19th century the steam engine in those days managed less than 10% Robert Sterling was not an industrialist so he spent no more time developing and marketing his idea he attended instead to his duties as a clergyman Robert Stirling remained a Presbyterian minister for the rest of his life
He died on the 6th of June 1878 at Galston in heirship he was nearly 88 but his idea the Stirling engine lived on at the dawn of the 20th century around a quarter of a million Stirling engines were in use worldwide as fans pumps or drive units for small pieces of equipment they supplied private households and small businesses with mechanical power their advantage was that they needed only heat to work so any combustible material could be used but as internal combustion engines and electric motors became more efficient and widespread Stirling engines gradually disappeared from the market then in 1937 a number of companies started looking at the Stirling engine as an option for generating electricity in remote areas
Today there are various types of Stirling engine one of the most widespread is the Alfa type here the engine is comprised of two separate cylinders and to power Pistons one hot and one cold connected by a pipe the left cylinder head is exposed to a heat source the one on the right is cooled by air fins in the first phase the left cylinder is heated and the expanding air pushes the left piston down then the right piston moves down the hot air is sucked into the cold cylinder and cooled this creates a partial vacuum in the power cylinder which pulls the power piston back to its original position at which point the cycle starts all over again owing to the alternating transfer of air between hot and cold cylinders continuous motion is achieved in the engine today there are many applications for the Stirling engine
It's simple principle and design makes it an attractive low-maintenance option and with air pollution and emissions such a major issue the closed cycle engine is experiencing a Renaissance because it can run on any kind of fuel renewables in particular even biogas or sewage gas from treatment plants could be used as an alternative to petrol diesel or the fuel originally used coal but the fuel doesn't even need to be combustible geothermal or solar energy can also be harnessed to power a Stirling engine if the heat source is non-combustible as in the case of solar the system is totally emission free in terms of versatility the Stirling engine is in a class of its own in arid areas for example lots of mechanical power is needed to pump water and lots of sunlight is available here
Dish Stirling systems are a particularly suitable technology fitted with a parabolic dish to focus the heat of the sun's rays they use a Stirling engine to power a generator for the production of electricity thanks to this capacity to generate electricity from renewable resources the Stirling engine faces a bright future as a machine for a clean world even very small differences in temperature like that between the heat of a human hand and the temperature of the air around it are enough to set a Stirling engine in motion and as long as that temperature difference is present it keeps on running
In contrast to internal combustion engines with all the noise is generated by explosions and ignitions Stirling engines run very quietly virtually vibration-free operation and considerably extends the life of the engine but the big argument in favour of the Stirling engine is resource conservation which makes up for drawbacks such as slowness and low power to weight and mass ratios so that time seems ripe for a renaissance of the Stirling engine even today as in Robert Stirling's time it could help make a considerable difference to the world we live in you.
ENGINEERING ASPECT
The physical processes by which heat is converted to work in the Stirling engine, the forced convective heat transfer combined with associated viscous dissipation or flow friction of the working fluid, the fluid dynamics involve a repetitive cyclic process of fluid momentum heat and mass transfer. Heat transferred into and out of the working fluid through metallic heat exchangers with known thermal conductivity minimizing the internal volume of the heat exchangers is necessary to keep compression ratio as high as possible for good efficiency without detracting from the heat transfer requirements per cycle the heat exchangers must exhibit low flow losses due to viscous effects overall the problem is extremely complex fortunately there are some design guidelines formulated by experts one of these is Allen Oregon plus some computational modeling software available and this is known as SNAP it stands for Stirling numerical analysis program and that was written and contributed by Allen Aultman and both of these provide very good guidelines and verification and the design of a new Stirling engine estimating realistic operational temperatures of the heater and the cooler and a target cyclic operational frequency. The working fluid the thermal components of the engine may be designed utilizing the guidelines specified in Allen Morgan's texts.
