ENERGY FROM THE SUN. 
developing only 29.6 horse-power, with steam supplied at 15.8 absolute, 
and with a vacuum in the condenser of 28.07 in. For a small output 
such as this, the reciprocating engine is no doubt superior to a steam 
turbine; but were it commercially practicable to construct a large 
sun-power plant, it would obviously be desirable to substitute a steam 
turbine, with which a materially higher efficiency ratio would then 
be readily realized. 
At the Meadi tests, it appears that the engine did not work satis- 
factorily, so that the steam consumption was exceedingly high; but, 
even assuming that as good results are obtainable as with a large 
exhaust steam, the output in brake horse-power would still, on the 
Meadi figures, not amount to an average of 70 brake horse-power per 
acre, so that even a 1,000-kw. plant would require a gathering ground 
of nearly 14 acres. It would probably not be possible to generate at 
the above rate for more than 3,000 hours per year, even under very 
favorable climatic conditions, and the unwieldy character of the plant 
is thus sufficiently obvious. 4 
The output would, no doubt, be raised were the steam pressure in- 
creased to 200 lbs. per square inch; but in the exposed position in which 
a sun boiler must needs be placed, it is quite possible that the increased 
convection losses would go far to offset any particular gain in this 
direction. If, however, the apparatus could be sheltered from wind, 
very high temperatures could, no doubt, be attained, but there would 
be obvious difficulties in utilizing them. Since the temperature of 
the solar radiation is that of the source, there is no physical bar to 
the attainment of extremely high temperatures, with a corresponding 
gain in the Carnot efficiency. In short, theory shows that a body 
exposed to the solar radiation, prevented from losing heat to the sur- 
rounding space, would attain the temperature of the sun, even without 
any concentration of the rays of the latter, and it would be of consider- 
able interest to determine the temperature thus attainable inside a 
well-silvered vacuum flask. In practice with high temperatures, how- 
ever, the gap between the Carnot efficiency and that actually attainable 
rapidly widens. Gas-engine makers discovered this long ago, the loss 
of heat to the cylinder walls increasing more rapidly at high tempera- 
tures than does the gain from the greater availability of the heat energy. 
Inside a gas engine cylinder temperatures of 2,000 deg. Cent. can be 
dealt with fairly successfully; but, whilst there should be no serious 
difficulty in attaining a temperature of this order from concentrated 
sunlight, there are no obvious means to hand by which energy supplied 
at this temperature could be successfully converted into work. If the 
solar engine is tobe a heat engine, it would seem that it must be a 
low-temperature engine, and this, of course, involves an enormous waste 
of availability, and this waste will, in practice, be notably more than 
indicated by the theory. Bad as the solar-heat engine is, it is, however, 
probably at least equal to Nature’s own method of converting sunlight 
into a form permitting of its utilization for the production of mechanical 
work. A few tons (largely water) per acre is the net result of many 
months’ conversion of radiation into chemical energy, and the 
1,000,000,000 tons of coal now produced annually is the product of 
millenniums of sunshine over vast areas. The energy thus stored 
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