Effects of Control Sensor Drift on Annual SDHW System Performance

The determination of proper control set points for active solar energy systems has been discussed by numerous authors. Varying the differential-off set point has been thought to contribute only additional parasitic energy costs. This work shows that the effect of varying the differential-off set point is much more significant than that. Laboratory testing at the Solar Energy Research Institutehas shown that sensor response can drift by as much as 12 degrees C. The sensor testing and results are briefly reviewed. A negative differential-off set point results in an interesting phenomenon. The continued pump circulation keeps the collector sensor warmer than the ambient air and, in fact, will keep it very close to the storage tank temperature. As a result, the pump will continueoperating with no irradiation leading to high thermal losses. Analytical expressions have been developed and are presented that determine the ambient air temperature required to turn the pump off as a function of the (negative) differential-off set point and storage tank temperature. The annual effect of this for five differential-off set points was determined for Albuquerque, Fort Worth,Madison, and Washington, D.C., using TRNSYS. The results show that a negative differential can effectively reduce the net collected energy by 50% and increase the auxiliary energy requirements by 300%.