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ABSTRACT
The paper presents the novel design of double glazing helical coil solar cavity receiver for solar thermal applications. Performance model has been developed for the experimental setup based on energy balance equations. The results obtained were compared with horizontal tube receiver for the same experimental setup. The result shows that the 87.96% improvement in the convective heat transfer coefficient for the double glazing helical coil solar cavity receiver. Maximum conversion efficiency achieved is 21% more than that would be obtained for horizontal tube receiver. This paper also investigates how the quality of vacuum degraded with the temperature of the glass cover.
Acknowledgments
The authors would like to show their gratitude to the MHRD, New Delhi for their financial support. We are also immensely grateful to Professors, IIT (BHU), Varanasi for sharing their pearls of wisdom with us during the course of this research and for their comments on an earlier version of the manuscript, although any errors are our own and should not tarnish the reputations of these esteemed.
Nomenclature
| Q5= | = | convection heat transfer between inner surface of absorber pipe to heat transfer fluid |
| hi= | = | HTF convection heat transfer coefficient at T5 (W/m2-K) |
| di= | = | inside diameter of the absorber pipe (m) |
| T5= | = | mean (bulk) temperature of the HTF (°C) |
| T4= | = | inside surface temperature of absorber pipe (°C) |
| Nudi= | = | Nusselt number based on |
| K5= | = | thermal conductance of the HTF at T5 (W/m-K) |
| f2= | = | friction factor for the inner surface of absorber pipe |
| Pr1= | = | Prandtl number evaluated at the HTF temperature, T5 |
| Pr2= | = | Prandtl number evaluated at the absorber inner surface temperature, T4 |
| Q4= | = | conduction heat transfer through the absorber wall |
| K4= | = | absorber thermal conductance at the average absorber temperature (T3+T4)/2 (W/m-K) |
| T4= | = | absorber inside surface temperature (K) |
| T3= | = | absorber outside surface temperature (K) |
| do= | = | absorber outside diameter (m) |
| Q6= | = | convection heat transfer between receiver and 1st glass cover per unit length of receiver |
| T3= | = | absorber outside surface temperature (K) |
| Tg1i= | = | inner surface temperature of 1st glass cover (K) |
| Q7= | = | radiation heat transfer between receiver and 1st glass cover per unit length of receiver |
| D1i= | = | inner diameter of 1st glass cover (m) |
| ε3= | = | emissivity of receiver |
| εg1i= | = | emissivity of 1st glass cover |
| D1o= | = | outer surface diameter of 1st glass cover (m) |
| D2i= | = | inner surface diameter of 2nd glass cover (m) |
| h12= | = | convection heat transfer coefficient for the annulus air at Tg12 (W/m2-K) |
| Tg1o= | = | outer surface temperature of 1st glass cover (°C) |
| Tg2i= | = | inner surface temperature of 2nd glass cover (°C) |
| kstd= | = | thermal conductance of the annulus gas at standard temperature and pressure |
| (W/m-K)= | = | 0.02551 |
| b= | = | interaction coefficient =1.571 |
| λ= | = | mean-free-path between collisions of a molecule (cm)= 88.67 |
| a= | = | accommodation coefficient |
| γ= | = | ratio of specific heats for the annulus gas = 1.39 |
| Tg12= | = | average temperature (Tg1o + Tg2i)/2 (°C) |
| Pa= | = | annulus gas pressure (mm of Hg) |
| δ= | = | molecular diameter of annulus gas (cm)= 3.53×e−8 |
| k12= | = | thermal conductance for the annulus air at Tg12 W/m-K) |
| Tg12= | = | average temperature (Tg1o + Tg2i)/2 (°C) |
| β= | = | volumetric thermal expansion coefficient (1/K) |
| Pr12= | = | Prandtl number |
| RaD1O= | = | Rayleigh number evaluated at D1O |
| β= | = | 1/Tg12 |
| Q12= | = | convection heat transfer from 2nd glass cover to ambient per unit length |
| D2o= | = | outer diameter (m) of 2nd glass cover |
| Tg2o= | = | outer surface temperature of 2nd glass cover (°C) |
| Ta= | = | ambient temperature (°C) |
| h2ga= | = | convection heat transfer coefficient for air at (Tg2o – Ta)/2 (W/m2-K) |
| NuD2o= | = | average Nusselt number based on the glass envelope outer diameter |
| σ= | = | Stefan-Boltzmann constant (5.670×10−8) (W/m2-K4) |
| εg2o= | = | emissivity of the 2nd glass cover outer surface |
| Ts= | = | effective sky temperature (K) = Ta− 8 |
| θ= | = | solar incidence angle normal to the collector aperture |
| η2gc= | = | effective optical efficiency at the 2nd glass cover |
| α2gc= | = | absorptance of the 2nd glass cover (Borosilicate glass) |
| Κ= | = | incident angle modifier |
| η1gc= | = | effective optical efficiency at the 1st glass cover |
| α1gc= | = | absorptance of the 1st glass cover (Borosilicate glass) |
| τ2gc= | = | transmittance of the 2nd glass cover |