Test-and-Rate Methods for Thermosiphon Solar Water Heaters

Rating of thermosiphon systems (TS) in the U.S. has required that a complete system be assembled, instrumented and tested. This is a relatively expensive and time-consuming process, compared to the method for active systems, where only the key components are tested. To determine if the component test method can be used for TS, TS were tested by component test and system test methods. The resulting models were validated with 8 weeks of normal operation data. The flow model predicted measured collector loop flow rate within 20%. Simulated savings agreed with the validation data saving better than 6% in all cases. Simulations also show that annual savings vary less than 3% for factor-of-two error in collector loop flow. It is concluded that TS can be rated by the more efficient component-test method, similar to active systems. Methods are needed to extrapolate flow-ΔP data to other temperatures and, for collectors, other sizes of the same basic design.