Solar Hot Water Controller Design - Part 2
by Dennis L. Feucht
Innovatia Laboratories

In Part 1 of this series, the schematic diagram and heat flow rate equation were introduced. In Part 2, the rest of the basic design equations are derived and applied.

Solar Collector ΔT

The solar collectors raise the temperature, T_L, of inlet fluid, assumed here to be water, by transferring solar heat, Q_λ, to the fluid, resulting in an exit temperature of T_H. The temperature difference is:

ΔT = T_H-T_L

The relationship between change in heat, ΔQ, and ΔT is a thermodynamic quantity called the heat capacity. The per-mass (or specific) heat capacity is called the specific heat. In this case, the inlet and exit pressures are equal, and the quantity of relevance is the constant-pressure specific heat, c_p. The total heat capacity (not per-mass) is C_p. (In thermodynamics, lower-case quantities are per-mass.) Then:

DΔQ = C_p·ΔT

Download the entire Part II here in Adobe Acrobat PDF format (261 kb)
Download Part I here in Adobe Acrobat PDF format (102 kb)

Contact the author

• Dennis Feucht