Life Span

I have attempted to formulate an apples to apples comparison between three different light sources showing the relative cost, efficiency and durability. One is what I consider to be a best case conventional light source, the high pressure sodium vapor lamp (HPS). One I consider a worst case conventional light source, automobile headlamps. And a new hybrid light source which is composed of light emitting diodes (LEDs) and a blue luminous tube. This last one, I think, represents our best hope as a light source for use in growing plants without sunlight. For this example a 23 year time span was chosen. The normal factor is biological activity of light output of 36 lightwatts for 23 years (continuous). The HPS chosen for this example is a GE LU250/SBY. It has a secondary arc tube which becomes active when the primary tube burns out, this gives it an outstanding 40,000 hour life.

Light Power Output: 36 lightwatts (assumed 100% biological activity)
Power Requirement: 250 Watts
Efficiency: 14%
Cost (at discount): $137 (cost does not include fixture)
23 Year Supply: 5 lamps
Total Cost: $685

Typical automobile headlamps. 12 Volts, 2 amps. These lamps have a very short 300 hour lifespan and are not very efficient. However, since there will be many abandoned vehicles around you might even be able to get these for free.

Light Power Output: 72 lightwatts (assumed 50% biological activity)
Power Requirement: 1440 Watts (60 lamps used simultaneously)
Efficiency: 2.5%
Cost: $5
23 Year Supply: 40,296 lamps
Total Cost: $201,480

Next is a composite neon/LED light source that is not now produced for the consumer market, but is well within existing manufacturing technology. NASA is currently using a similar light source for research into advance life support systems for space travel. The LED component has a lifespan of several decades, it is also very efficient. The luminous tube portion provides 10% of total light output. Since this device does not exist an attempt was made to derive realistic figures.

Light Power Output: 36 lightwatts (assumed 100% biological activity)
Power Requirement: 192 Watts
Efficiency: 19%
Cost: $493
23 Year Supply: 1 (with the ability to rebuild the tube)
Total Cost: $493

These figures are believed to be realistic based upon catalog references and engineering data. The conclusion I hope you draw from this is that the LED/tube light source is superior. If you have the ability to rebuild the tube you can use this light source for many decades. One may consider just replacing the tube part from stockpiles. Even without the tube it will produce plant growth but at a reduced rate. The LEDs will not all fail at once if they are not abused. They will fail one at a time after many years giving gradually reduced output, and are very tolerant to mechanical shock. The values given are somewhat pessimistic, and can be improved upon.

The HPS or similar lamp would be more expensive to stockpile and would not give you quite the quality of biological light output as the LED/tube and is not quite as energy efficient. The HPS is fragile, and is not rebuildable, so when they're gone they're gone. Even assuming automobile headlamps are easy to come by I hope you can see by the example the disadvantages of the need for huge stockpiles and the low energy efficiency. This should discourage you from considering it a viable alternative.

Offered by Steve