Performance Evaluation of a High Solar Fraction CPC-Collector System

One of the most important goals on solar collector development is to increase the
system’s annual performance without increasing overproduction. The studied
collector is formed by a compound parabolic reflector which decreases the collector
optical efficiency during the summer period. Hence, it is possible to increase the
collector area and thus, the annual solar fraction, without increasing the
overproduction. Collector measurements were fed into a validated TRNSYS
collector model which estimates the solar fraction of the concentrating system and
also that of a traditional flat plate collector, both for domestic hot water production.
The system design approach aims to maximise the collector area until an annual
overproduction limit is reached. This is defined by a new deterioration factor that
takes into account the hours and the collector temperature during stagnation
periods. Then, the highest solar fraction achieved by both systems was determined.
The results show that, at 50° tilt in Lund, Sweden, the concentrating system
achieves 71% solar fraction using 17 m2 of collector area compared to 66% solar
fraction and 7 m2 of a flat plate collector system. Thus, it is possible to install 2.4
times more collector area and achieve a higher solar fraction using the load adapted
collector. However, the summer optical efficiency reduction was proven to be too
abrupt. If the reflector geometry is properly design, the load adapted collector can
be a competitive solution in the market if produced in an economical way.