According to Duff (1996), the flow in the collector loop should be in the
range of 0.2–0.4L/min-m2 of collector aperture area. The effect of low flow rate
is examined in Chapter 4, Section 4.1.1. In effect, the penalty for low flow rate
is a reduction in collector efficiency due to higher collector temperature rise for
a given inlet temperature. For example, for a reduction from 0.9L/min-m2 to
0.3L/min-m2, the efficiency is reduced by about 6%; however, the reduction of
the inlet temperature to the collectors because of the improved stratification in
the tank more than compensates for the loss of collector efficiency. The pumps
required for most of the active systems are low static head centrifugal (also
called circulators), which for small domestic applications use 30–50W of electrical power to work.