Saturday, April 17, 2010

Comments of the PV Blogs

I received quite a few emails about the last two blogs. The blog on Sudhir’s home PV system drew several emails that asked why would he invest in something that has so little return? I asked Suhir to give me the dollars per year he has saved rather than the kilowatt hours and he told me the savings are averaging $2,000 a year. This is due to the fact that the utility pays him a pretty hefty price of about 33 cents per kilowatt hour he dispatches to the grid as the peak performance of his PV system corresponds to the peak demand for electricity from the utility. Electricity prices will increase at 5 or 6% a year so Sudhir will be saving $3,000 each year in six or seven years. Sudhir also made his investment in 2007 before the stock market plunged so he is pretty happy to have a PV system rather than phantom stocks. Sudhir you are a real green machine.

A few of the readers were dismayed that I scoffed at the solar plane. My disdain is not with the idea that an explorer is breaking new ground. I applaud anyone who has millions to spend on a dream and make that dream happen. My point was the idea is not practical for a mass solution for air transportation. The Cessna 150 that outperforms the behemoth PV Plane sold over 25,000 units and it is not green but is a practical small plane for many pilots. Planes simply cannot be made very green as they are heavier than air systems that need thrust and wingspan to stay aloft. The 12,000 PV cells that produce 30 kilowatts could have been sent to 10,000 villagers in Africa who each could enjoy 3 watts of power to light their mud huts with LED lights so kids can read at night.

I did this calculation about the carbon footprint of burning a wax candle versus using a LED light. A candle has the output of about 13 lumens and a LED will emit 130 lumens per watt so a candle is about equal to using a 0.1 watt LED. A candle that has a mass of 50 grams will burn for about 8 hours and will emit 150 approximately grams of CO2. The 0.1 watt LED lit for 8 hours by charge and discharge of an inexpensive nickel metal hydride battery will emit about 0.5 grams of CO2 if charged from the average electricity on the US grid. The LED is 300 times as green as the candle. Of course there are emissions associated with the manufacture of the battery, the PV system and the LED but there are also emissions associated with the manufacture of the candle. The battery can be recharged 300 times, the LED will last for 100,000 hours of operation, the PV system wil have a useful life of 25 years while the candle is single use. The best use of LEDs, rechargeable batteries, and PV cells will be in simple applications such as lights for a mud hut in Africa. 3 watts of power would be enough to power an array of LEDs that would light up a kids life. I need to come up with the best use for the candle wax but a hot waxing of Alfalfa may be one such optimum use.


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