Thursday, March 20, 2008

Thank Gandhi

Green Thursday TGIT – Thank Gandhi It’s Thursday

This week’s blog is about a co-worker named Sudhir. Sudhir is originally from India and that is why I am thanking Gandhi for this week’s episode. Sudhir and I were talking at the water fountain about energy and the environment. Unlike me who only blogs about energy, Sudhir has taken action. He has put his money toward being green. He has installed a 4.2 kilowatt PV system on his roof of his home in Redwood City California. The 4.2 kilowatts is the gross capacity expressed in direct current (DC). The capacity expressed in alternating current (AC) is 3.8 kilowatts. The system became operational last August and ever since Sudhir has been a self generator of electricity. PG&E our local utility has installed a smart meter at Sudhir’s home that measure energy consumption or generation in both peak and off-peak periods. Peak period energy is much more valuable than off-peak energy so the utility credits Sudhir much more money when his system is generating power at 2 pm of a weekday when the grid is experiencing peak demand. At midnight on a Sunday, electricity has much less value.

As solar radiation is at it’s peak in the early afternoon Sudhir has actually generated more power during the peak periods than his home has consumed during the same peak period. Over the past six months Sudhir has spun his meter backward by some 480 kilowatt hours during peak electric demand periods. During off-peak periods over the past six months Sudhir has bought 4,275 kilowatt hours from the utility. Sudhir is a well off person with a rather large home for the bay area having some 3,000 square feet of living space in his house. His mother as well as his wife and kids live in this home. He instructed them all to run the dryer and dishwasher in off-peak periods so that his PV system is generating the maximum amount of energy to the grid during peak periods and he is therefore receiving the maximum credit from PG&E. The energy he buys during off-peak periods is much less expensive and so it makes economic sense to shift his power load to off-peak periods.

Sudhir is also a smart investor. He has calculated that his after tax rate of return for the system based on a 25 year life and modest inflation rate of 3% in PG&Es’s electric tariff, he should enjoy a 7.3% internal rate of return. This is better than 11% before tax and given the low risk of his investment he has invested wisely. It is quite likely the PG&E tariff will increase far faster than 3% per year and his rate of return will correspondingly increase with higher inflation rates.

Our friends from across the pond at BP have an interesting web link that helps folks determine the quantity of energy they can generate from photovoltaic systems. BP produces and sells such systems. All one has to do is set the size of the system and provide your ZIP Code and viola the BP calculator gives you the expected net generation of the system.

Yahoo had this article that a diesel BMW 5 series had better gas mileage than a Prius on a 545 mile trip from London to Geneva. Well any good thermodynamics student would have predicted this. First diesel has more energy content per gallon than gasoline. This is because diesel has a higher specific gravity than gasoline and there are more pounds of diesel in a gallon than there are of petrol (the English word in the article for gasoline). Diesel engines are also more efficient than gasoline engines and on the 545 mile highway trip from London to Geneva the Prius would hardly have used it hybrid battery system except to stop a few times for food, fuel and going to the restroom. Diesel has 130,000 BTU/gal (US) versus 115,000 BTU/gal (US) for petrol. This equals 155,880 BTU/gal (UK) for diesel an 137,890 BTU/gal (UK) for petrol. The BMW got 41.9 MPG and the Prius got 41 MPG on the 545 mile trip. Therefore the BMW consumed 13.01 gallons (UK) of diesel or some 2,027,556 BTUs while the Prius consumed 13.29 gallons (UK) of petrol or some 1,832,928 BTUs. The Prius is the winner on the basis of lower real energy consumption for the trip

The word of the day is acerbic or sharp in temper. I have become acerbic ever since I learned that the Wall Street Bonks gave their brokers over $30 billion in bonuses two months ago and now the Feds have to bail them out. acerbic \uh-SUR-bik\, adjective:Sharp, biting, or acid in temper, expression, or tone. But more than that, he is a social critic, and an efficient one, acerbic and devastating.-- Benoit Aubin, "Quebec's King of Comedy", Maclean's, August 27, 2001Since I started out as a writer many years ago, I have built a reputation as an acerbic, mean-spirited observer of the human condition.-- Joe Queenan, My Goodness: A Cynic's Short-Lived Search for SainthoodJoey gained a reputation as a smart aleck adept at delivering acerbic one-liners.-- "Joseph Heller, Author of 'Catch-22,' Dies at 76", New York Times, December 14, 1999Acerbic comes from Latin acerbus, "bitter, sour, severe, harsh." Entry and Pronunciation for acerbic


  1. Thanks for the math explanation for the BMW v. Prius. I had suspected it was not quite as it was made up to be, merely due to the fact that the strong suit of the diesel engine is highway (higher speeds) while the strong suit of the Prius is city driving (start-stop).
    Thanks also for the real life case study on PV - and ways to see if others can make similar sound investments!

  2. Catherine

    You are a good student of thermo. may the force of hydrocarbons always be behind you