We investigated the gas heat conduction in two types of evacuated tubular solar collectors for a wide range of Knudsen numbers. For tube-in-tube collectors, we generalized a solution of the gas kinetic Boltzmann equation, which has been obtained by the four-momentum method, to polyatomic gases. The resulting equation coincides with Sherman’s interpolation formula. For a plate-in-tube collector, we measured the stationary heat loss for gas pressures varying between 10−2 and 104 Pa. The accuracy of an earlier experiment was improved. For analysis we applied the temperature jump method: a heat conduction equation with boundary conditions of the third kind involving the temperature gradient and the pressure was numerically solved. The results with the temperature jump method agree with the experimental values nearly within the error bands. We also applied Sherman’s interpolation formula and found, as expected, that the heat conduction as function of the pressure is too steep. For both types of collectors, the influence of geometric parameters was theoretically studied.

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