The solar updraft tower is an energy converter that converts solar-generated thermal energy into wind energy.
This low-tech solar energy collector concept is over a hundred years old, but the first 50 kW working model was built only in 1982.
The chimney of this model had a 10-meter (33') diameter and was 195 meters (640') tall, while the diameter of the canopy was 244 meters (800'), about 11 acres or 46,000 m2. This prototype operated for nine years and reached a maximum production of 50 kW.
Storing and Transporting Solar Energy
The storage of solar energy is an important consideration, because storage is required to compensate for the diurnal, seasonal and weather-related variations in insolation (the amount of solar energy received on a unit area). Therefore, in order to supply the continuous energy users without interruptions, the generated electricity must be stored. On small installations, hot water tanks or high-density batteries can provide storage. On mid-sized installations, pumped hydro storage can be considered. For larger installations, the compressing of air into underground caverns has been suggested. (see Chapters 8.15 & 8.41, Vol. 2 IEH, 4th ed.).
A better option is to eliminate the need for storage. This can be achieved if an electric grid is available in the area, and the utility serving the area is required to take the excess solar electricity and supplement it when more is needed. In this case, if the solar-power plant is located close to a hydroelectric or fossil power plant, it is possible to increase or decrease the fossil or hydraulic power plants rate of generation as the availability of solar energy changes.
Storing Solar Energy as Chemical Energy
A favored method of storage is to convert solar energy into chemical energy (convert it into a fuel) and store/distribute it in that form. The carriers of this chemical energy can be gases, liquids or solids.
In one process, high-temperature solar chemistry is used. Mirrors concentrate the suns rays on zinc oxide and vaporize it at a temperature of 1200 °C. The vaporized zinc is later condensed into a powder. This zinc can than be transported, and when combined with water vapor, will produce hydrogen fuel. Recombining with oxygen creates zinc oxide. A 300 kW pilot plant at the Weizmann Institute of Science (WIS) in Rehovot, Israel, has successfully produced 45 kilograms of zinc per hour, but this storage method is not yet available commercially.
Hydrogen can be generated from ammonia, from the reforming of fossil fuels or from water by electrolysis. Naturally, when made from fossil fuels, carbon dioxide is generated, which contributes to global warming, while if hydrogen is oxidized, the product is clean water, which is much needed in arid regions.
Hydrogen as a Transportation Fuel
Hydrogen is one of the means of storing solar energy in chemical form, which allows it to be used as a fuel, but before that happens, a transition period is expected, during which ethanol and other biofuels will be used. The three main American car manufacturers plan to have half their fleets run on E85 or on biodiesel fuels by 2012.
E85 is a blend of 85% ethanol and 15% gasoline. Today, out of the 170,000 gas stations in the U.S., only 2,000 have pumps for E85, and ten times that number is needed to provide most American motorists with a pump within 5 miles of their homes.
Hydrogen is stored as a liquid or as a gas compressed to some 350 to 800 atmospheres pressure (5,000 to 12,000 pounds per square inch). At atmospheric pressures, hydrogen condenses at -423 °F (-217 °C). On a weight basis, the energy content of hydrogen is 3.4 times that of gasoline. Liquid hydrogen weighs 0.59 pounds/gallon, and the energy content of one gallon of gasoline equals the heating value of 3.58 gallons of liquid hydrogen. The heating value of a kilogram of gasoline is 39,600 BTU (41.8 mJ), while the heating value of a kilogram of hydrogen is 134,616 BTU (142 mJ).
On a volume basis, hydrogen requires three times the volume of gasoline to store the same amount of energy. Hydrogen can also be stored in solids, and these reversible solid storage processes are probably the safest, but their development is still in the experimental stage. Today they are capable only of storing small amounts of energy.
Gas Stations of the Future