A computer program using a finite difference technique is developed to study the steady-state and transient performance of an evacuated solar collector suitable for applications in the 120–250°C temperature region. The collector is modeled as an equivalent flat plate collector, with water, oil, and air being the heat transfer fluids studied in detail. Nonflow and laminar flow conditions are considered for the three fluids, together with turbulent flow for air. Step and sinusoidal solar inputs are modeled. Experimental transient tests on a module of evacuated collectors were made and were used to show that the accuracy of the theoretical model developed is adequate for design purposes.

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