Flow stability in down-flow molten salt solar receiver panels

A theoretical formulation for the stability of heated single-phase down-flow through multiple parallel tubes connected at the top and bottom by common plenums is presented. The formulation is applied to develop a criteria for a lower bound on operation of molten salt solar receivers employing heat absorption panels with down-flow. The stability of heated down-flow is examined on the basis of the static relationship for force equilibrium for the flow. The derivative of the pressure difference required to drive the fluid through the panel with respect to flow rate is an indication of the unbalanced force that acts in the event of a flow perturbation. If this derivative is positive, a flow perturbation will be opposed by a force acting to restore equilibrium. If, however, the derivative is negative, the perturbation can grow, and flow reversal can result. The formulation of this derivative is detailed and a criteria is developed that is useful in the design of solar receivers. If this criteria can be satisfied, receivers employing serpentine flow paths can be successfully designed and operated, avoiding the alternative of costly piping to provide for up-flow in every panel.