Hereby, Cth is the heat capacity of the ZnO blocks. The system can be characterized as thermally stable if ˙Q eff ≥ 0 for all time instants, i.e., if the cooling process dominates or is in
equilibrium to the electric heat generation. The above relation can be rewritten with the total heat transfer coefficient αeff as ˙Qeff = αeff(T −T0). By combining the latter equation and (7),
one obtains as stability criterion, where η defines the system cooling rate.