22-10-2016, 03:39 PM
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Alpha-type Stirling engine. There are two cylinders. The expansion cylinder (red) is maintained at a high temperature while the compression cylinder (blue) is
cooled. The passage between the two cylinders contains the regenerator.
Beta-type Stirling engine. There is only one cylinder, hot at one end and cold at the other. A loose-fitting displacer shunts the air between the hot and cold ends of
the cylinder. A power piston at the end of the cylinder drives the flywheel.
A Stirling engine is a heat engine that operates by cyclic compression and expansion of air or other gas (the working fluid) at different
temperatures, such that there is a net conversion of heat energy to mechanical work. More specifically, the Stirling engine is a closedcycle
regenerative heat engine with a permanently gaseous working fluid. Closed-cycle, in this context, means a thermodynamic system in
which the working fluid is permanently contained within the system, and regenerative describes the use of a specific type of internal heat
exchanger and thermal store, known as the regenerator. The inclusion of a regenerator differentiates the Stirling engine from other closed
cycle hot air engines.
Originally conceived in 1816 as an industrial prime mover to rival the steam engine, its practical use was largely confined to low-power
domestic applications for over a century.
Stirling engines have a high efficiency compared to steam engines, being able to reach 50% efficiency. They are also capable of quiet
operation and can use almost any heat source. The heat energy source is generated external to the Stirling engine rather than by internal
combustion as with the Otto cycle or Diesel cycle engines. Because the Stirling engine is compatible with alternative and renewable energy
sources it could become increasingly significant as the price of conventional fuels rises, and also in light of concerns such as depletion of oil
supplies and climate change. This type of engine is currently generating interest as the core component of micro combined heat and power
(CHP) units, in which it is more efficient and safer than a comparable steam engine. However, it has a low power-to-weight ratio
rendering it more suitable for use in static installations where space and weight are not at a premium.
Explanation of cycle ;
In Stirling cycle, Carnot cycle’s compression and expansion isentropic processes are
replaced by two constant-volume regeneration processes.
During the regeneration process heat is transferred to a thermal storage device
(regenerator) during one part and is transferred back to the working fluid in another part
of the cycle.
The regenerator can be a wire or a ceramic mesh or any kind of porous plug with a high
thermal mass (mass times specific heat). The regenerator is assumed to be reversible heat transfer device.
Process:
1-2 isothermal expansion heat addition from external source
2-3 const. vol. heat transfer internal heat transfer from the gas to the regenerator
3-4 isothermal compression heat rejection to the external sink
4-1 const. vol. heat transfer internal heat transfer from the regenerator to the gas
Working Principle:
The system includes two pistons in a cylinder with a regenerator in the middle. Initially
the left chamber houses the entire working fluid (a gas) at high pressure and high
temperature TH.
Notes:
• Unlike internal combustion engines, a Stirling cycle does not exchange the working
gas in each cycle, the gas is permanent.
• The heat is supplied outside the engine, so any heat source can be used, e.g.: coal,
gas, solar energy, nuclear power, etc.
• Stirling engine can reach higher thermal efficiencies than Otto and Diesel engines,
since heat transfer occurs at constant temperatures, i.e., its thermal efficiency is the
same as the Carnot cycle:
• The pressure changes are very smooth and its torque is uniform, it has no valves,
exhaust pipes, etc. Thus, Stirling cycle is quiet and has less maintenance points.
• To achieve competitive efficiency, it needs to work on high pressures which cause
tremendous problems of sealing.
The temperature difference TL - TH should maintain high for acceptable thermal
efficiencies, this results in large thermal stresses in the cylinder (hot and cold ends).
Consequently, high strength expensive materials should be used.
• The working fluid has to be an ideal gas. Helium or hydrogen are typically used
because of their high heat conductivity and low molecular masses which lead to faster
heat transfer.
• Fast changes in power output are not easy to achieve which makes the Stirling cycle
not so attractive for automotive applications.