Some use a displacer to shuttle the working gas back and forth from the hot region to the cold region of the engine. In order to reach constant-temperature and constant-volume processes, special pistons and cylinders are used. Stirling engines usually incorporate an efficiency-enhancing regenerator that captures heat during constant-volume cooling and replaces it when the gas is heated at constant volume. In the Stirling cycle, the working gas is alternately heated and cooled by constant-temperature and constant-volume processes. In modern high-performance Stirling engines, typical working gas temperature is above 700 ☌ and pressure is as high as 20 MPa. Stirling cycle engines used in solar dish/Stirling systems are high-temperature, high-pressure externally heated engines that use a working gas. Stirling or Brayton cycle engines may be used in solar dish systems. Kaufhold, in Comprehensive Renewable Energy, 2012 3.06.6.3.3 Characteristics of Stirling or Brayton engine Indeed, the appearance of the WhisperTech engine with an inherently low electrical efficiency, but with reliability and production cost parameters in line with market requirements, can be seen as a major landmark in the commercialisation of micro-CHP.ī. Thus, the quest for high efficiency has economic and performance implications which may be undesirable.
However, fluctuations in rotation of the working piston give rise to other complications, particularly variations in electrical output and high electrical losses as well as obvious increases in noise, vibration and mechanical stress. Practical implementation of this feature is possible using electromagnetic actuation of the displacer, and is to some extent simulated in the conventional crank arrangement of some engines. For example, it is possible to improve the Carnot efficiency of a Stirling engine by using discontinuous motion of the piston. Clearly there is little point in achieving a high efficiency if the production costs are so high that it could never be recovered from energy savings. Indeed, measures which improve efficiency may have undesirable consequences both in technical and economic terms.
It has been shown that the beta type is inherently more efficient than the others, 3 but as will be explained later, high efficiency alone is not necessarily a desirable goal. The third version is the gamma type in which the working piston is placed in a separate cylinder. In the beta type, both compression and expansion are carried out by the working piston, the working gas being shuttled between hot and cold spaces in the same cylinder by means of a (non-working) displacer. A sub-division of the alpha type is the double-acting type, where useful work is done by symmetrical pistons. One piston carries out compression in the cold space and the other, expansion in the hot space. In the alpha type, the working gas shuttles between two pistons. Stirling engines can be further characterised by the three typical configurations of the displacer and working pistons, known as alpha, beta and gamma.
In practice, the challenges of differential expansion and linear generator design have so far proved a major obstacle to commercialisation.
In theory the LFPSE (linear free piston Stirling engine) is much simpler as it contains fewer moving parts. In the majority of cases, output power is taken from a linear (usually permanent magnet) alternator attached to the piston, while the displacer is actuated by the pressure variation in the space beneath the piston.
The displacer is actuated through some form of mechanical linkage. Kinematic Stirling engines have a crank arrangement to convert the reciprocal piston motion to a rotational output, say to drive a generator. All Stirling engines fall into one of the following two basic categories: