The nighttime losses from an integral collector storage (ICS) system were investigated. The significance of the sky temperature, wind speed, and ambient temperature on the losses were examined. Outdoor data was taken on several nights to characterize the thermal performance of an ICS system under various environmental conditions. Indoor tests were then performed under an artificial "nighttime sky" environment, with a simulated wind, in an attempt to duplicate the heat losses which occurred outdoors.

The standard rating procedure which specifies the conditions for the heat loss tests for ICS systems was analyzed to see how well it characterizes the collector performance at night. Experimental results indicate a synergistic effect between the sky temperature and wind speed. The effects of wind on the losses from the ICS system overshadow the effects of small changes in sky temperature, but larger changes of sky temperature, with a constant wind speed, have a pronounced effect.

It is recommended that both of these parameters be taken into account in heat loss tests in standard rating procedures. Indoor tests can duplicate outdoor heat loss results within 8 per cent. The minimum requirement for SRCC rating tests should be to monitor, record, and report the sky temperature.