Thermal modelling of parabolic trough collectors - Solar Energy Sciences and Engineering Applications - page 171

Thermal modelling of parabolic
trough collectors
Soteris Kalogirou
Department of Mechanical Engineering and Materials Science and Engineering,
Cyprus University of Technology, Limassol, Cyprus
6.1 INTRODUCTION
As shown in Figure 6.1.1, in a parabolic trough collector (PTC) a sheet of reflective
material is formed in parabolic shape. At the focal point of the parabola a metal black
pipe, covered with a glass tube to reduce thermal losses, is placed. When the axis
formed by the centre of the parabola and the receiver faces the sun, the parallel rays
incident on the reflector are reflected and focused onto the receiver tube. In this way
the concentrated radiation reaching the receiver tube heats the receiver pipe and thus
the fluid circulating through it, transforming the solar radiation into useful heat.
As long collector modules are usually produced, a single axis tracking of the sun
is used (Kalogirou 2004, 2009). The collector can be orientated in a north-south
direction, tracking the sun from east to west, or orientated in an east-west direction,
tracking the sun from north to south. The advantages of the latter tracking mode is
that very little collector adjustment is required during the day and the full aperture
always faces the sun at noon time but the collector performance during the early and
late hours of the day is greatly reduced due to large incidence angles (cosine loss).
North-south orientated troughs have their highest cosine loss at noon and the lowest
in the mornings and evenings when the sun is due east or due west. Over the period of
one year, a horizontal north-south trough field usually collects a little more energy than
a horizontal east-west one. However the north-south field collects a lot more energy in
summer and much less in winter. On the other hand the east-west field collects more
energy in winter than a north-south field and less in summer, providing a more constant
Figure 6.1.1 Schematic diagram of a parabolic trough collector and receiver detail. The collector com-
prise a sheet of reflective material in parabolic shape together with a metal black pipe,
covered with a glass tube to reduce thermal losses, placed along its focal line.