Abstract:
This study focused on a numerical second law analysis and optimisation of a receiver tube op-
erating in a parabolic trough solar collector for small-scale application. The receiver functioned
in a Rankine cycle. The focus was on entropy generation minimisation in the receiver due to
the high quality exergy losses in this component. Water functioned as the working
uid and
was heated from ambient conditions (liquid) to a superheated state (vapour), consequently, the
receiver tube was subject to both single phase as well as two-phase
ow.
Entropy generation in the receiver tube was mainly due to nite temperature di erences as well
as
uid friction. The contribution of each of these components was investigated. Geometrical
as well as operating conditions were investigated to obtain good guidelines for receiver tube
and plant design. An operating pressure in the range of 1 MPa (Tsat = 180 C) to 10 MPa
(Tsat = 311 C) was considered. Furthermore a mass
ow range of 0:15 kg=s to 0:4 kg=s was
investigated. Results showed that beyond a diameter of 20 mm, the main contributor to the entropy generation
was the nite temperature di erences for most conditions. Generally, operating pressures below
3 MPa showed bad performance since the
uid friction component was too large for small
operating pressures. This phenomenon was due to long two-phase lengths and high pressure
drops in this region. The nite temperature di erence component increased linearly when the
tube diameter was increased (due to the increase in exposed area) if the focused heat
ux was
kept constant. However, the
uid friction component increased quadratically when the diameter
was reduced.
In general when the concentration ratio was increased, the entropy generation was decreased.
This was due to more focused heat on each section of the receiver pipe and, in general, resulted
in shorter receiver lengths. Unfortunately, there is a limit to the highest concentration ratio
that can be achieved and in this study, it was assumed to be 45 for two-dimensional trough
technology.
A Simulated Annealing (SA) optimisation algorithm was implemented to obtain certain optimum
parameters. The optimisation showed that increasing the diameter could result in a decrease in
entropy generation, provided that the concentration ratio is kept constant. However, beyond a
certain point gains in minimising the entropy generation became negligible. Optimal operating
pressure would generally increase if the mass
ow rate was increased. Finally, it was seen that
the highest operating pressure under consideration (10 MPa) showed the best performance
when considering the minimisation of entropy in conjunction with the maximisation of the
thermodynamic work output.
