THE SUN’S ENERGY—DANIELS 245 
116,000 liters falling through a height of 10 feet. If a hydroelectric 
plant or an abandoned mine or a large diving bell in a lake or ocean 
is available the cost may not be so great. At Austin, Tex., a steam 
powerplant pumps water back from a low reservoir to a high reservoir 
when electricity is not much in demand and then the full hydroelectric 
power is available for peak loads. 
Standard electric power grids are well suited to help overcome the 
intermittent nature of solar energy. When the sun is shining the coal 
supply or the dammed-up water supply can be conserved. The de- 
mand for air conditioning is creating new problems with the peak 
loads of electrical power systems. The sun is usually shining 
brightest when the air conditioning is needed most. 
Power can be stored also in the form of high-temperature heat. 
One kilowatt-hour of heat is equivalent to 860 kilocalories which 
is equivalent to a 10-degree C. drop in the temperature of 86 liters 
of water. Heat to run a steam engine for a while after sundown 
might be stored in a large insulated hot-water tank or in fused mate- 
rial such as urea with its melting point of 125 degrees C. and its 
heat of fusion of 50 calories per gram. Huttig has proposed to 
store heat for operating an engine in an iron sphere heated electri- 
cally to a high temperature. The ratio of heat storage to radiating 
surface is comparatively low. It must be remembered that 1 kilo- 
watt-hour of heat gives only about 0.1 kilowatt-hour or less of 
work in a small heat engine. 
The electrolysis of water and storage of the hydrogen and oxy- 
gen in underground gas tanks with water seals offer possibilities. 
The gases may be combined to operate a high-temperature gas tur- 
bine. One kilowatt-hour of heat is equivalent to the heat of com- 
bustion of about 10 cubic feet of hydrogen. The stored hydrogen 
and oxygen may be used directly to operate a hydrogen-oxygen 
“fuel cell” with nickel electrodes in a fused salt bath at a high tem- 
perature. Good laboratory progress is being made in obtaining 
efliciencies up to 60 and 70 percent with fuel cells. 
HOUSE HEATING 
House heating is one of the theoretically simple uses of solar energy 
because the temperature does not have to be high. Flat plate col- 
lectors give satisfactory heat traps and focusing devices are not 
necessary. However, large surfaces are necessary and architectural 
difficulties arise. The storage of heat may be accomplished with 
pebble beds, hot-water storage tanks, or chemicals which undergo 
fusion or transition in crystal form. Competition of solar heating 
against cheap coal, oil, and gas is difficult, but inasmuch as about 
one-third of the fuel consumption in the United States goes for 
