154 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



climate. Among the projects at M. I. T. made possible by Dr. God- 

 frey Cabot's endowment for research on utilization of solar energy 

 is one having as its first objective the determination of the perform- 

 ance characteristics of solar energy collectors of different types, the 

 performance, of course, being correlated with records of incident 

 solar energy so as to permit calculations of expected performance 

 in any locality where sunlight records are available. The first and 

 so far the only type of collector studied has been the flat plate, which 

 will now be considered briefly. 



Since each additional layer of glass and air cuts down the losses 

 from such a plate, it is apparent that with glass having perfect 

 transmission one could build a collector which, without any focusing 

 or concentrating device, would still collect efficiently at a very high 

 temperature level. But the best glass is not perfect. It doesn't ab- 

 sorb much solar energy when one picks the right glass — and there is 

 ample evidence that early experimenters were too casual in their choice 

 of glass in that respect — ^but there is a reflection loss of about 4 per- 

 cent at each surface. Consequently, as glass plates are added the 

 point is ultimately reached where the reduction in heat loss from the 

 metal plate is more than offset by the reduction in intensity of in- 

 cident radiation due to reflection losses. The optimum number of 

 plates to use will be less the more intense the sunlight, more the 

 colder the weather and the higher the temperature of collection of 

 heat. 



The controlling part played by reflection losses in the design of 

 flat-plate collectors having been brought out, the desirability of a low 

 reflecting glass was discussed with Professor Hardy, of our Physics 

 Department. The result was the invention by Drs. Turner and Cart- 

 wright of a method of processing glass to give it a permanent sur- 

 face of reflectivity approaching zero at one point in the spectrum. 

 The process has already demonstrated its importance in a great many 

 uses ranging from spectacle lenses through bomb sights to high-speed 

 cameras and the solar-corona camera which was described in the first 

 article of this series. Here is an excellent example of a need in one 

 field stimulating research, the results of which have many applica- 

 tions in other fields. The special glass has not yet been used for an 

 experimental solar-energy collector, but calculations indicate that 

 its use should make possible the attainment of temperatures up to 

 800° F. without any mirrors or lenses or so-called concentrating 

 devices. 



Another problem of flat-plate collectors is that of optimum tilt. 

 Obviously they are too cheap a type of collector system to warrant 

 being mounted to follow the sun, but they may profitably be tilted 

 permanently toward the Equator. A little consideration will in- 

 dicate that the optimum tilt depends very definitely on the use to 



