Q2.Is it possible to use lightning as an energy source?
A2. It appears to be impractical to utilize lightning energy, as illustrated in this section. Each cloud-to-ground lightning flash involves an energy of the order of 109 J. This is approximately equal to the energy required to operate five 100-W light bulbs continuously for one month:
5 x 100 W x 3600 s x 24 x 30 = 1.3 x 109 J
or about 360 kilowatt-hours (1 kW-hr = 3.6 x 106 J), probably comparable to the monthly energy consumption of an average household. Even if it were possible to capture all flash's energy (the bulk of this energy is not delivered to the strike point since it is lost to heating the air and producing thunder, light, and radio waves), one would need to attract 12 flashes to the energy storage facility in order to operate these five light bulbs for one year. The probability of lightning strike to a given point on ground is very low. For example, a 1 m2 area in Florida is struck by lightning on average, once in 105 years. A grounded structure protruding above earth's surface is more likely to be struck by lightning. A 60-m tower located in Florida is expected to be struck by lightning once every other year. Thus, one needs 24 such towers covering a large area of 1 km2 or so to operate five 100-W light bulbs, which appears rather impractical. Most of the U.S. experiences a factor of 2 to 3 lower lightning activity than in Florida. As a result, the number of lightning capturing towers needed to operate only five 100-W bulbs in areas of moderate lightning activity would be 48 to 72. Thus the two main problems with the utilization of lightning energy can be formulated as follows:
(1) The power associated with a lightning flash is very high, but it is released in pulses of short duration (of the order of 10-4-10-5 s). As a result, lightning energy, the integral of high power over a short period of time, is rather moderate, comparable to the energy consumption by a typical household (the integral of relatively low power over a long period of time). This energy is equivalent to that released in the burning of 20 to 30 kg of oil.
(2) The capturing of lightning strikes would require the use of a large number of tall towers, which is rather impractical. Additionally, as noted above, not all the lightning energy is delivered to the strike point. Using a typical measured value of energy per unit resistance (action integral) for negative lightning of 105 A s2 and an assumed range of effective resistances at the strike point of 10 to 100 Ohm, we estimate a range of the lightning energy delivered to the strike point to be from 106 to 107 J, which is only 10-2-10-3 of the total energy.
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02-08-2007, 09:21
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