Of the existing solar energy technologies, non-concentrating solar thermal systems to heat water, to warm buildings and to provide heat for industry and agriculture have arguably the greatest near-term potential to displace fossil fuels.  However, solar thermal systems have not achieved market potential.  Technology has seen modest change since the 1980’s and initial cost has remained high relative to gas and electric water heaters.  The seminar will highlight work at the University of Minnesota on polymer based systems, which are hoped to be half the cost of conventional systems.  The seminar will touch on some of the material issues, including mechanical behavior and scaling of polymer tubes, but the emphasis will be on our work to characterize buoyancy driven flow and convective heat transfer in indirect storage systems that use an immersed heat exchanger.  Natural convection flow in the collector is interpreted from a transient scale analysis and three-dimensional computational model as well as measured water temperature distributions in a laboratory collector of aspect ratio of 9.3:1 and inclined at 30 degrees to the horizontal.  Average natural convection heat transfer rates are measured for isothermal and stratified collectors with both adiabatic and uniform heat flux boundary conditions.  The presence of the collector enclosure is shown to enhance natural convection heat transfer rates compared to those for tubes in an unbounded fluid.  The heat transfer enhancement is attributed to the large scale circulation rates in the enclosure.  For tube bundles, generally Nusselt numbers increase with increasing pitch-to-diameter ratio and applied heat flux.  The results are interpreted to provide guidance on heat exchanger geometry.