''Two-tank'' seasonal storage concept for solar space heating of buildings

An analysis of a novel two-tank water storage system is presented, consisting of a large primary water tank for seasonal storage of solar energy plus a much smaller secondary water tank for storage of solar energy collected during the heating season. The system offers the advantage of high collection efficiency during the early stages of the heating season, a period when the temperature of the primary tank is generally high. By preferentially drawing energy from the small secondary tank to meet load, its temperature can be kept well below that of the larger primary tank, thereby providing a lower-temperature source for collector inlet fluid. The resulting improvement in annual system efficiency through the addition of a small secondary tank is found to be substantial - for the site considered (Madison, Wisconsin), the relative percentage gain in annual performance is in the range of 10 to 20%. A simple computer model permits accurate hour-by-hour transient simulation of thermal performance over a yearly cycle. Results are presented of detailed simulations of collector and storage sizing and design trade-offs for solar energy systems supplying 90 to 100% of annual heating load requirements.