the heat flowing in an upward direction along the closed louvers 10 would first flow upwardly over the trailing louver ends 10a before encountering the leading louver ends 10b in order for heat to flow between the contacting surfaces of adjacent louvers 10 in being lost from the collector 2 the heat would thus have to reverse its direction of flow heat which had flowed upwardly in contact with the trailing end 10a would then have to in effect turn a corner and flow downwardly in order to pass between the trailing end 10a and the leading end 10b of the adjacent louver 10 since rising heat would have a tendency to flow in a straight line and not to reverse its flow direction the position of the trailing and leading louver ends 10a and 10b coupled with the normally angled position of the collector 2 functions to reduce heat loss from the collector fig3 is a vertical sectional view in which the the collector 2 is viewed from its opposite side from that of fig2 thus the tubes 82 appear as tube pairs in fig3 while the same tubes 82 appear as curved widened portions in fig2 where the individual tubes of each tube pair are connected together additionally as shown in fig3 collector 2 is in its cooling mode with the louvers 10 rotated about their shafts 18 to an opened position accordingly the backing surface 84 may then radiate heat directly to a darkened sky further with opening of the louvers 10 there is no impediment to heat losses from the enclosure 8 by means of thermal convection it should be understood that the effect of the louvers 10 in transmitting or.