Abstract:
Entropy generation in the receiver tube of a
parabolic trough solar collector can mainly be attributed
to the fluid friction and finite temperature differences. The
contribution of each of these components is investigated
under different circumstances. Mass flow rates, tube diameters
and operating pressures are investigated to obtain
good guidelines for receiver tube and plant design. Operating
pressures between 3 MPa (saturation temperature of
233.9 °C) and 9 MPa (saturation temperature of 303.3 °C)
were investigated. Results show that small diameters can
result in excessive fluid friction, especially when the mass
flow rates are high. For most cases, tube diameters beyond
20 mm will exclusively be subject to entropy generation due
to finite temperature differences, and entropy generation due
to fluid friction will be small to negligible. Increasing the
concentration ratio will decrease entropy generation, due to
a higher heat flux per unit meter. This will ultimately result
in shorter receiver tube lengths. From a simulated annealing
optimization it was seen that if the diameter is increased, the
entropy generation can be lowered, provided that the concentration
ratio is kept constant. However, beyond a certain
point gains in minimizing the entropy generation become
negligible. The optimal operating pressure will generally
increase if the mass flow rate is increased. Finally it was seen that higher operating pressures are more advantageous
when the entropy generation minimization is considered in
conjunction with the work output.
