This illustrates that when a moderate amount of direct irradiance is available, the PTC collector has a higher potential to generate power.
In Fig. 5b the maximum power output is calculated for a
collector array of total area 15 m2 (considered a representative
average value for UK homes [46]) and for a London ‘‘annual-average’’
irradiance condition; with the PTC and ETC ability to use diffuse
irradiance taken into account. It is assumed that the ETC array
can use the entire (global) incident radiation (considered here for a
fixed plane tilted at 36 to the horizontal) whereas the PTC array
can use only the direct-normal component (considered here for a
perfect 2-axis tracking system). The maximum power output for
the PTC and ETC collector arrays are 127 W and 104 W respectively.
When compared to Fig. 5a it can be observed that, when
the differences in useable irradiance are taken into account, the
collectors are more closely matched with respect to their ideal
maximum power outputs. The optimum outlet temperatures corresponding
to maximum power for the PTC and ETC collectors
are 159 C and 104 C respectively. For comparison, the maximum
power output is also shown for a PTC collector at a fixed orientation
and tilt angle (due South and 36 respectively), revealing a
considerably lower power than both the PTC (tracking) and ETC
(fixed) arrays. A further case may be considered in which the available
solar irradiance consists entirely of the diffuse component, in
which case the PTC output will be close to zero, while the ETC collector
will retain a potential to generate power.