A transient response model is developed for collectors whose dynamic behavior is controlled predominantly by the absorber surface and heat-transfer fluid. Such collectors can be described adequately by two differential equations for which exact solutions are possible. The equations are solved for the case of evacuated-tube collectors, and the solutions are found to exhibit qualitatively new transient phenomena. In particular, it is found that fluid flow rates, thermal masses, and heat-transfer coefficients are important factors in determining the overall response of a collector, and that the functional form of the response depends on how these parameters are related. The model is compared with test results for four evacuated-tube collectors whose heat-transfer configurations and fluid flow rates encompass a broad range of transient response regimes. Excellent agreement is obtained in all cases. In addition to providing a detailed insight into the problem of collector dynamics, the model is a potentially useful tool for obtaining information about heat-transfer coefficients and fluid flow distribution effects.

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