Theoretical analysis of the thermal performance of all-glass evacuated tube solar collectors with absorber coating on the outside or inside of the inner tube

Theoretical efficiencies (η) and thermal behaviour of all-glass Evacuated Tube solar Collectors with an Internal Absorber Film (ETCIAF), i.e. the absorber film deposited in the inner surface of the inner tube, are compared and contrasted with the traditional design of all-glass Evacuated Tube solar Collectors with an External Absorber Film (ETCEAF), using the absorber film on the external surface of the inner tube. The values of η of the ETCIAF are unacceptably lower than that of ETCEAF for any particular value of the heat transfer coefficient (h_b) for the annular space, except in the case of a highly leaky ETCEAF, with h_b > 2.6 W/m² K. However, it is shown that the use of a transparent conductive coating with moderately low emittance ∼0.1-0.25 on the outside of the absorber tube of ETCIAF can offer efficiences ∼0.75-0.63, respectively, for f = 0.1 °C m²/K, competing well (η = 0.76) with the ETCEAF design operating under best conditions (α = 0.91, ε{lunate} = 0.05, and h_b = 0.026 W/m² K).