Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 468

Figure 14.5.5 Ideal Stirling cycle representations in p-V (a) and T-s (b) diagrams.

processes occurring in the engines are not ideal, leading to lower efficiency than Carnot.
The main penalties of a real engine that cause performance decay (Walker, 1980) are:

• Heat exchange losses : (i) non-isothermal processes (engines are adiabatic in nature;

isothermal processes are unfeasible in practice); (ii) T in heat exchangers due to
finite surfaces;

• Fluid dynamic losses : (i) pressure losses (especially in the regenerator); (ii) losses

due to the dead volumes (volumetric efficiency); (iii) leakages of fluid;

• Losses due to kinematics : isochores are not perfectly executed in normally adopted

mechanisms;

•

Electrical and mechanical losses .

As regards the mechanical device, a reciprocating piston-cylinder arrangement is
normally used. The kinematic transmission is based on a rod-crank mechanism in
which the pistons move according to a sinusoidal law. Another kinematic transmis-
sion involves a Wobble-Joke or a Swash-plate mechanism, with similar effects. As a
result the four transformations that compose the cycle are partially overlapped in a
real cycle, with consequent reduction of overall efficiency (Figure 14.5.6).

14.5.4 Stirling configurations
Depending on the mechanical scheme adopted, existing Stirling engines can be classi-
fied as one of three basic arrangements (Martini, 1983; Stine, 2007), as represented in
Figure 14.5.7.
The Alpha configuration is characterized by the presence of two distinct cylinders
with two corresponding pistons. Both pistons have the same pressure at any given
time. The crank mechanism is generally kinematic, usually of a rod-crank type. In this
way, a sinusoidal motion is induced on the pistons, with a phase shift close to 90 ◦ .