NREL is working with the solar industry to decrease the cost of solar water-heating systems and to develop systems that work in mild and cold climates helping bring these technologies to vast regions of the country where they previously were not viable. Researchers assist with the prototype development of new polymer systems through modeling and optimization, characterizing system performance, and testing the durability of the materials... In collaboration with FAFCO Inc., Davis Energy Group/SunEarth Inc., the University of Minnesota, and others, NREL is helping to develop the next generation of low-cost polymer-based solar water-heating systems for mild climates. The work to date has focused on material and thermal-performance issues related to passive systems. This includes unpressurized polymer integral collector storage (ICS) systems that use a load-side immersed heat exchanger and direct thermosyphon systems... NREL is building and testing different simulation models for the..