Solar Engineering of Thermal Processes, 4th Edition

6.6 Temperature Distribution in Flow Direction

The useful gain per unit flow length as calculated from Equation 6.5.17 is ultimately transferred to the fluid. The fluid enters the collector at temperature and increases in temperature until at the exit it is . Referring to Figure 6.6.1, we can express an energy balance on the fluid flowing through a single tube of length as

6.6.1

where is the total collector flow rate and is the number of parallel tubes. Dividing through by , finding the limit as approaches zero, and substituting Equation 6.5.17 for , we obtain

6.6.2

If we assume that and ...