Friday, March 21, 2008

Nitrogen-filled tires: myths and reality

Recently, I went tire shopping for my Mazda 626 and came across the idea of nitrogen-filled tires. A brochure (from a company that shall remain unnamed) I picked up at the tire shop claimed that:

1. N2 is a dry gas and free of moisture
2. The moisture in "wet oxygen" deteriorates rubber, which dry N2 does not.
3. N2 is a bigger molecule than O2 and therefore N2 leaks 3-4 times slower.
4. This loss of tire pressure leads to reduced fuel economy, reduced tire life and increased tire maintenance.

Out of these claims, #3 is often misunderstood. Oxygen molecule (O2) has a molecular weight of 32 while nitrogen molecule (N2) has a molecular weight of 28 (amu). It is clear that O2 is a bigger molecule compared to N2. However, the effective size of the oxygen molecule (called the kinetic diameter) during diffusion through the rubber tire to the surrounding air is smaller than N2. Some of the concepts involved are explained here.

It might be true that oxygen leaks out 3-4 times faster than pure N2, but the air we breathe is mostly ~79% N2 and 21%O2. Therefore, the actual beneficial effects of filling nitrogen in the tire are actually lesser (since oxygen accounts for only 21% of the tires volume, the rest being N2). If we take these two values and assume that O2 leaks 4 times faster than N2 out of a tire, then the tire filled with N2 will hold its pressure for ~40% longer time, compared to the air-filled tire. Ultimately, the choice of filling N2 in tires will come down to personal preference. If you are a normal car user, check tire pressures regularly and ensure that the tires remain properly inflated, then N2 filling may not be the best choice (considering that one has to pay for this). However, if you want the comfort of knowing that your tires will remain inflated for longer time periods, N2 filling may be an option.

Some more links related to the effects of nitrogen filled tires:


  1. Gas mileage
  2. Nitrogen Filled Tires: a Scam? « Hot Cup of Joe
  3. Cops using N2-filled tires
  4. More on gas mileage
  5. A more critical article on nitrogen-filled tires
  6. Tires - Nitrogen air loss study: Consumer Reports Cars Blog

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Tuesday, March 11, 2008

The Unclean Side of Biodiesel: How Some manufacturers Pollute

I just read this article, about pollution from biodiesel plants, in the Herald Tribune, something that I do NOT see carried in several mainstream newspapers. Sad to see that so called green initiatives are screwing up the environment because of some (or many?) greedy manufacturers.

Pollution is called a byproduct of a 'clean' fuel - International Herald Tribune

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Oil prices: how high will they go?

Why are oil prices increasing even when US economy is slowing down? The answer partly lies in market speculation. Following the Fed's interest rate cuts, investors in the market are betting on hard commodities such as crude and gold to assure returns greater than the market average. This is partly contributing to high prices. From the NPR :
"Analysts believe oil's underlying supply and demand fundamentals do not support such high prices, and argue that crude's rise in recent months is mostly due to the falling dollar. Crude futures offer a hedge against a falling dollar, and oil futures bought and sold in dollars are more attractive to foreign investors when the dollar is weak."
Although widely criticised for not increasing oil production, the OPEC contends that the US economy is to blame for the high oil prices. MSNBC has an article on the OPEC's viewpoint on this issue. NPR also has a somewhat outdated, but good primer on oil prices. It mentions that oil companies are price takers and do not set oil prices per se.



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Exxon's Efforts Into the Hybrid Car Market

Exxon-Mobil Corporation, US's largest refiner and one of the world's largest oil companies has been carrying out research in technologies that can improve Lithium-ion batteries - the mainstay of many hybrid car propulsion systems. Exxon scientists are basically developing a plastic film separator that serves as a major component in Li-ion batteries. Li-ion batteries are lighter than the commonly used Li-metal hydride batteries, but Li-ion batteries have overheating problems that have been causing explosions in laptop batteries. Anyhow, once the heating issue is resolved, the market for Li-ion should pick up and that is where Exxon's focus happens to be.
If the R&D into alternative technologies by companies like Exxon grow to be a sizeable part of their R&D then that augurs well for "green technology".
Read more about the article below.
Guess Who Hopes to Help Power New Hybrid Cars - WSJ.com

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Sunday, March 2, 2008

Redwoods vs solar panels

A CNN article on the conflicts between growing more trees and getting lesser sunlight for solar panels was interesting. Some bare facts:
  1. Mark Vargas, who owns solar panels (an investment of 70,000 (2001 USD)) contends that his neighbour, Treanor's redwood trees are blocking sunlight for his solar panels
  2. After 6 years, a judge cites CA's obscure sunlight availability law to rule in favor of Mark Vargas.
  3. As alternate energy sources become more mainstream, many experts predict that conflicts similar to this case, will happen.
In response to Treanor's claims that the redwoods help capture CO2, Vargas "counters it would take two or three acres of trees to reduce carbon dioxide emissions as much as the solar panels that cover his roof and backyard trellis."
Legal issues aside, this case represents an interesting conflict. For starters, if we assume that the solar PV panels indeed help prevent more CO2 emissions than the redwoods (which turns out to be valid, see the appendix for back-of-the-envelope calculations), the question for environmental economists would be which option would generate the least negative externalites ? In other words, if we could put a price/ton on every pollutant (including CO2) evolved, which option would result in a lower price? Can the amount of services provided by redwoods be quantified in monetary terms?

Appendix
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Analysis of net CO2 emissions prevented by using solar PV and amount of CO2 fixed by redwood trees over the course of a year:
The data on photosynthetic CO2 fixation in coastal redwoods was taken from this paper (Osborne and Beerling, 2003). The authors of the above study used 3 year old trees to measure photosynthetic rates in a controlled atmosphere. Only results for coastal redwoods at pCO2 (partial pressure of CO2)=40 Pa (pascal) (corresponding to 395 ppm CO2 in the atmosphere) will be used in this analysis.
The PVWatts website was used to estimate costs and energy production from a single solar PV panel. San Francisco was chosen as the location (due to its proximity to Sunnyvale). The DOE web page puts the amount of CO2 emitted/unit of energy produced for coal power plants at 2.12 lb/kWh.
In Fig.7 (Osborne and Beerling, 2003) show the average rates of photosynthetic fixation (milli moles of CO2/plant/day) for different species. The area under the curve (in Fig.7 of the paper) for coastal redwoods gives the total CO2 photosynthetic fixation over the entire year. It comes to ~1650 mmol CO2/plant/year. This amount is however, for 3 year old plants. Let us assume that the amount of CO2 fixed increases linearly with the leaf growth and the plant age. If we consider a 12 year old tree, ~3350 mmol CO2/year are fixed by a single redwood tree. This amounts to 0.15 kg CO2/tree/year.

In comparison, Vargus installed 70,000 $ worth of solar PV panels. I assumed the average 2001 costs of solar PV panels to be 0.2$/kWh (current costs from the PVWatts website are 0.125 $/kWh). Per a 4 kW panel, the energy produced/year (from the PVWatts wevsite) would be ~5800 kWh. This represents an investment of ~1150 $/4kW panel. Since Vargus invested ~70,000$, he probably has 60 solar PV panels installed.
In a year, the AC electricity produced from all the panels (from the PVWatts website) would be 464000 kWh.If we assume that only 70% of the panels are operational at any given time, the figure becomes a little lower, ~330,000 kWh/year. I took electricity from thermal power plants as the basis for comparing CO2 emissions. Using the value of 2.12 lb CO2/kWh generated (and not accounting for transmission and distribution costs), I calculate that emissions of ~312 T of CO2/year will be avoided with the 60 solar PV panels.

Comparing the above two values (CO2 fixed/plant/year and CO2 avoided/year), we can clearly see that there is orders of magnitude difference between these two quantities. 60 solar PV panels prevents more CO2 emissions than ~1000 redwood trees. However, this is not the end of the story.

Solar PV panels are produced in a energy-intensive process which leads to CO2 emissions. For a complete CO2 emissions analysis, the emissions for solar PV manufacturing also need to be taken into account. However, this will not change the results here by very much as there is clearly differences in orders of magnitude between solar PV panel and a redwood tree.


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