  The figure of merit (FOM) is a widely used metric to characterize the
performance of concentrated solar power (CSP) receivers by comparing the amount
of solar thermal energy retained by the receiver to the incident concentrated
solar radiation. However, the FOM is a strong function of the concentration
factor and receiver temperature, thus direct comparison of FOM values measured
under disparate operating conditions is inappropriate. To remedy this problem,
the present study proposes a new metric called the receiver effectiveness
calculated by normalizing the actual FOM with its theoretical maximum. The
receiver effectiveness can be employed for comparing receiver performances
regardless of their operating conditions, and can be treated as more-like the
second law efficiency of thermodynamics. In addition, a theoretical limit of
the CSP plant efficiency is also examined by combining the maximum FOM and the
Carnot efficiency for different concentration factors and receiver
temperatures. The calculated maximum CSP plant efficiency clearly indicate that
optimizing FOM does not always lead to a better CSP plant performance. Along
with the FOM, the proposed receiver effectiveness and maximum CSP system
efficiency should be considered as complementary metrics to evaluate the
performance of the CSP system.
