When solar electricity generation expands beyond peak daytime usage, energy storage comes into play. The most attractive storage method of solar electricity is hydrogen production via the electrolysis of water. Highly efficient hydrogen fuel cells can produce electricity for nighttime. Hydrogen will be the primary fuel for motor vehicles in the post-petroleum age that will soon be upon us. Because this fuel is carbon-free, a hydrogen economy will not contribute to greenhouse emissions.
The highly efficient multi-junction solar cells at the heart of the PYRON Solar Generator produce 2.8V direct current. By a happy alignment this is a little more than twice the 1.3V of electrolysis, so that voltage-mismatch losses are minimized. Each PYRON platform can have a co-located Electrolyser using the same cooling water as the cells. The 77% electrical efficiency of electrolysis means that every 3.9 kWh of solar electricity produces 1 m3 of hydrogen, with an energy content of 3 kWh. When hydrogen is liquified its energy content is 33.3 kWh/kg four times that of gasoline. Hydrogen fuel cells are three times as efficient as the internal combustion engine, so that overall energy-storage efficiency is above 60%. Electricity not used during the day will be saved as hydrogen for nighttime loads.
Hydrogen instead of gasoline?
U.S. gasoline consumption is 400x109 kg/a (kilograms per year).
That eqals an energy of 3680x109 kWh/a (kilowatthours per year).
Hydrogen has a calorific value of 38.4 kWh/kg (Oil has only 9.2 kWh/kg), thus, using hydrogen would reduce the mass needed to provide the same amount of energy to 95.8x109 kg/a.
Electrolysis works with an efficiency of 76%, so the electric energy needed to produce that amount of Hydrogen would be 4.8x1012 kWh/a.
As fuel cells work 315% more efficient than conventional combustion engines, the energy needed to power those is reduced to 1.54x1012 kWh/a.
At an average electricity output of 500x106 kWh/km2, a PYRON Solar Powerplant could provide the energy needed to power a fuel-cell based U.S. traffic on an area of just 3080 km2. That's 1203 mi2 or 34.7 by 34.7 miles.
As there are 134 million cars in the U.S. this equals a (desert-)area of only 23 m2 or 15.7 by 15.7 feet per vehicle!