ENERGY FROM THE SUN. 
Energy from the Sun.” 
Physicists tell us that if we disregard: atmospheric absorption, the 
sun delivers energy to the earth at the rate of 7,000 horse-power per 
acre; but though the fact cannot be disputed in the face of the concordant 
data as to the solar constant now available, very little has yet been 
accomplished in solving the problem of utilizing this energy in the form 
of mechanical work. Since the temperature of the sun is about 6,000 
deg. Cent. (absolute), Carnot’s theorem shows us that of the energy 
us; } 6,000 deg.—300 deg. 
received here, a proportion equal to about ~~ gogg—~=C«S«CVEE 
2 
95 per cent., should be convertible into work, at least from the theoretical 
stand-point. : 
The only methods by which it has yet been attempted to convert the 
solar energy into mechanical work have, however, involved a degrada- 
tion of the energy received from its high-grade condition as radiation 
into low-temperature heat. The immediate consequence is that the 
energy, even theoretically, convertible into mechanical work falls from 
95 per cent. of 7,000 horse-power per acre down to, say, 25 per cent. of 
7,000 horse-power, or 1,750 horse-power per acre. ‘This is, of course, 
merely a theoretical result, and is very far from realizable in practice, 
Indeed, in certain regions, the atmosphere exacts a toll of nearly 50. per 
cent. on the energy it transmits, but as this defect of transparency is 
closely associated with the amount of aqueous vapour in the air, the 
loss is very substantially less in arid areas. As indicating what fraction 
of this power is actually attainable with present appliances, the figures 
given in a paper read on Monday last before the Society of Engineers 
by Mr. A. S. E. Ackermann, B.Sc., have much interest, although they 
lead to the negative conclusion that there is little prospect of commercial 
‘snecess being achieved along these lines. “ 
The site of one of the installations tested by Mr. Ackermann was 
at Meadi, Egypt, and the trials extended over about three weeks. 
The area oceupied by the sunshine-absorbing plant was seven-eighths of 
an acre, and the highest output recorded was 19.1 pump horse-power, 
as against the 5,000 to 6,000 horse-power which would have been 
obtained from a perfect motor capable of utilizing the radiation direct. 
Somewhat better results were obtained in some previous trials near 
Philadelphia, where tests made by Mr. Ackermann, extending over a 
period of five hours, were, he claimed, equivalent to the development of. 
an average of 18.54 horse-power from’ a sunshine-absorber occupying an 
area of 0.41 acre. The maximum rate of power development during this 
- test was 26.8 brake horse-power, and the lowest 8.6 brake horse-power, 
These figures bring out well the characteristic difficulties attaching to 
‘any attempt to retrieve mechanical energy from the sunshine by con- 
verting the latter into heat. These drawbacks are the enormous area 
£ : * Extract from Engineering. 
“395 
