Environmental Engineering Reference
In-Depth Information
(i) sealing problems on the hot cylinder; (ii) lubrication issues due to the need to
lubricate both the pistons and the mechanism; and (iii) high specific volumes derived
from the particular geometry of the engine and by the presence of two pistons. The
latter is a factor that increases the specific costs of the system and precludes its use for
applications requiring compact solutions. The Eurodish collector discussed in Section
14.3 exploits the Alpha scheme.
The
Beta configuration
is characterized by the presence of a single cylinder and two
components moving inside the same cylinder. The first is the piston, responsible for the
phases of compression and expansion, and the second is the displacer accomplishing
the processes at constant volumes. The displacer is a mechanical device similar to the
piston but, unlike the latter, it is not equipped with seals: its surfaces must support only
the pressure difference related to the fluid dynamic losses in the regenerator. Piston and
displacer may have a different rotating mechanism. In general, they have a sinusoidal
motion, associated with a phase shift of 90
◦
between piston and displacer. Recently, a
novel configuration called “Beta free piston'' has been developed by some manufactur-
ers and is characterized by the absence of kinematic connection between piston and dis-
placer (Thombare & Verma, 2008). The motion of piston and displacer is entrusted to
resonance phenomena and regulated by the natural frequency of a linear generator, con-
nected to the grid directly or through inverters. This technology is employed, for exam-
ple, by Infinia in its 3.2 kWe solar dish collector described in Section 14.3. In general,
the main advantages of the Beta configuration are: (i) its compactness due to the pres-
ence of a single cylinder for both the compression and expansion phase; (ii) high power
density; (iii) reduced dead volumes; and (iv) the possibility of developing solutions
intrinsically sealed with the adoption of a free-piston mechanism (i.e. it is possible to
pressurize both the working fluid and the chamber containing the generator, avoiding
leakages). Figure 14.5.8 shows a schematic view of a Beta free-piston Stirling generator.
It can be noted that the whole engine is encapsulated to avoid leakages of fluid.
Another option is the
Gamma configuration
. This differs from the Beta configu-
ration by having two separate cylinders which contain, respectively, the displacer and
the power piston. The Alpha configuration also has two cylinders but both feature a
power piston to implement the compression and expansion phases. Like the Beta con-
figuration, the displacer has to overcome only the pressure losses generated during the
displacement of the fluid from the hot side to the cold side of the engine, and vice versa.
The power piston generates the phases of compression and expansion. The Gamma
configuration is rarely used because of its modest efficiency, low specific power and
high vibrations generated by the kinematics required to operate the system.
In addition to these three classic configurations there is one further scheme that is
a variant of Alpha, namely the
double-effect Alpha configuration.
This configuration
derives from the Alpha with three or four interconnected pistons: the lower part of each
piston acts as volume of compression and is interconnected to the expansion chamber
(upper volume) of the next piston. Figure 14.5.9 shows the working principle of an
engine based on a double-effect Alpha configuration. This configuration retains the
simple design feature of the Alpha engine, bringing about some major improvements,
namely: (i) an increase in the specific power (W/cm
3
) owing to the double action of
the pistons, which allows the displacement of the motor to be halved: (ii) a reduction
in leakages (the only seal towards the outside is the ring which allows sliding of the
connecting rods of each piston); (iii) high continuity in the movements; and (iv) reduced
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