  A theoretical and experimental analysis is carried out of radiative heat
transfer in the coaxial geometry of evacuated tube solar collectors. The gray
body approximation implicit in the use of an effective emissivity does not
strictly apply to evacuated tube solar collectors due to selective absorber
coating and partially transmitting outer glass in the thermal infrared,
especially when constructed from borosilicate. We develop analytic expressions
for the heat transfer through the outer envelope and show the equations no
longer follow a simple form where an effective emissivity for the system can be
defined. To test all approximations in practice, an experiment is performed
using an evacuated solar collector manufactured in the 1980s by the Nitto Kohki
company in Japan using the effective emissivity approximation to determine the
typical heat transfer characteristics using net radiative heat flows in both
directions. This method enabled a good fit to temperature-time data for cooling
and heating of the inner tube for temperatures between 10 and 85{\deg}C. The
results confirm that the effective emittance method can be used in situations
with typical glass wall thickness and temperatures. At temperatures greater
100{\deg}C, the spectral distribution of the emitted radiation falls
significantly within the transmitting region of the outer glass and can no
longer be neglected. The work has verified the stability of the vacuum in this
type of collector as the tube still functions well, maintaining a low
emissivity and good vacuum after approximately 40 years in storage conditions.
