Techniques for reducing thermal conduction and natural convection heat losses in annular receiver geometries

Techniques studied for reducing conduction heat losses in annular receiver geometries for solar collectors include evacuation of the annulus gas, oversizing of the annular space while maintaining slight vacuum levels and utilization of gases other than air in the annular space. For the geometry considered, total heat reductions of 10-50% may be obtained depending on the means by which conduction heat loss is limited. In addition, numerical analysis indicates that highly nonuniform temperature distributions are required to affect significantly natural convection between concentric cylinders and that rather large eccentricities cause only a slight increase in natural convection heat transfer.