An additional difficulty in measuring the efficiency follows from the presence of highly stratified temperature conditions in the collector (by design) and a definition for the standard measure of solar collector efficiency which assumes unstratified temperatures. Still, there is very good reason to believe the performance would be unusually efficient because the largest of the normal heat loss pathways is completely removed (convection from the absorber to the glazing). The approximations used were also done with care and are believed to have been fairly accurate.
In the design of the collector, heat rising from the absorber is drawn into a slit along the top. When the rate of flow through the exhaust slit is correctly adjusted the heat currents farther away follow directly behind the ones nearest the exhaust slit, and none travel anywhere else. This is directly observable with smoke tracers. As a result it is reasonable to expect energy transfer efficiencies missing the normal heat loss provided by convection from the absorber surface to the glazing . More references to the design and investigation of controlled current devices are available. The following graphs are scanned from old hand drawn plots. The original data and lab notes are missing.
The unusual behavior of the collector can be seen in the temperature distribution found during operation, figure 3. On this occasion there is no air temperature rise until less than 1" from the absorber surface, consistent with observing that there was no movement of hot air from the absorber surface back out into the collector space. The standard measures of collector efficiency, however, assumes an absence of extreme horizontal or vertical gradients of this kind, i.e. that temperatures A and B are both measures of an assumed 'ambient' collector temperature. Direct observation of this unusual condition is made with smoke tracers. Air movement in the collector space is remarkably quiet and a filament of smoke will drift undisturbed toward the absorber until little pieces suddenly dart in toward the surface to be drawn up out of the collector with the surface hugging convection sheet. This behavior was observed when the collector was operated both as an active (fan driven) and passive (convection loop driven) solar collector with equal success.
The standard efficiency calculation (Kreith & Kreider 1977 "Solar Heating and Cooling") assumes a direct correlation between collector temperature and solar intensity. The ratio of in-out temperature differential to average insolation/sf is the horizontal axis. For the structured current collector this linkage is broken and both efficiencies need to be calculated separately. Full day collection efficiencies were based on the temperature rise of storage over the day in conditions of constant temperature difference with variable solar intensity (1) and of constant solar intensity with variable temperature difference (2) .
The theoretical maximum efficiency is about
90% of solar flux, with 5% reflected at the glazing and 5% lost by reflection
or re-radiation by a good selective surface absorber, making no allowance
for conduction loss. The peak theoretical maximum for full
day efficiency (top solid line) is about 85%. For the controlled
current collector, when solar intensity is high and temperature differential
is low the peak efficiency is about 80%. Not even the best
of the engineered hot air collectors, the Owens Corning evacuated tube
collector (2nd solid line) starts the efficiency curve that high.
It's peak is efficiency is about 70%. The efficient passive
convection loop cells made by Steve Morris had a peak efficiency of only
60%. As mentioned the accuracy of these measures can
not be checked and they did include reasonable estimates. Still
they are consistent with the logical analysis that the observed performance
characteristics must necessarily result in unusually high efficiencies.
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