a shows a comparison of the maximum power
available per unit area from the PTC and ETC collectors under
steady-state direct normal irradiance (DNI) equal to 500 W/m2
.
In essence this figure, together with Fig. 5b, describes the compromise
between the two terms on the RHS of Eq. (28).
At high mass flow-rates the temperature rise of the fluid flow through the
collector is low,
leading to low collector temperatures and thus
high collector efficiencies (described by the first term). Nevertheless,
the high mass flow-rate also causes high losses due to
irreversible heat transfer (described by the second term). At the
other extreme, low flow-rates lead to high temperatures and low
collector efficiencies, but also reduced losses due to heat transfer,
such that an optimum can be identified at some intermediate
flow-rate and temperature. The PTC collector has a maximum exergy
output of 96 W/m2
, available at an outlet temperature of 394 C;
whereas the ETC collector has a maximum exergy output of