Thursday, February 19, 2009

energy release calculation for the Victorian bushfires

David Packham estimated that the energy released in the recent Victorian bushfires was equivalent to 660 Hiroshima bombs. This surprised me and the figure also supports the argument for the need for fuel reduction.

I did my own calculations based on the Victorian fires and found that Packham seems to be roughly correct. I obtained the figure of 1290 Hiroshima bomb equivalents compared to Packham’s estimate of 660. I'm not saying that my figures are more accurate, there is some guess work involved, but my figure is in the same ballpark as that estimated by David Packham

Here are my calculations.

David Packham:
More than 330,000 hectares were destroyed in Victoria’s “hell on earth” bushfires and according to Mr Packham each hectare contained 30 tons of bushfire fuel — adding up to 9.9m tons. ”That equates to the energy release of 660 Hiroshima bombs” ...
- Victoria's bushfires compared to Hiroshima
Yield of Hiroshima Bomb: 1 Hiroshima Bomb is roughly 20 Kilotons TNT Equivalents of 1 Kiloton of TNT: 1 Kiloton TNT equals 1.15 x 10^6 Kilowatt-hours. KT means Kiloton.
- The fission product equivalent between nuclear reactors and nuclear weapons
The amount of energy in wood is a tricky one to calculate accurately. It can vary between fresh wood which has only 2000 Btu/pound of energy and the "high heat value" of 8660 Btu/lb, which is obtained only with perfectly dry wood (0% moisture content) and only in an atmosphere of pure oxygen. Given that the Victorian fires were fuelled mainly by partially or largely dried out dead fuel then it is fair to take an intermediate figure, let us say 5000 Btu/pound
- The Amount of Energy in Wood Fuel

1 Btu (thermochemical) = 0.00029287508333 kilowatt-hours
http://www.onlineconversion.com/energy.htm

1 pound = 0.00045359237 ton (metric)
http://www.onlineconversion.com/weight_all.htm

By our estimate, wood burns to create 5000 Btu/pound. To convert to Kilowatt-hours / ton then multiply by 0.00029287508333 and divide by 0.00045359237

Hence wood burns to create roughly 3300 kilowatt-hours / ton (rounded). For 9 million tons burnt –> 9 * 10^6 * 3300 kilowatt-hours energy = VB (victorian bushfires)

1 Hiroshima bomb = 20 * 1.15 x 10^6 Kilowatt-hours = HB (Hiroshima bomb)

To calculate Hiroshima bomb equivalents divide VB by HB = 1290 Hiroshima bomb equivalents

David Packham's qualifications:
OAM, MAppSci, worked for 40 years in bushfire research with CSIRO, Monash University and the Australian Emergency Management Institute. He was responsible for fire weather services in the Bureau of Meteorology. His extensive research concentrated on the physics of bushfires, and he applied this research to practical issues including the development of aerial prescribed burning, non-evacuation of properties, modelling of fire behaviour, and forensics

4 comments:

Tony Forster said...

In the emotional times following the Victorian fires, it would be good to examine the issues dispassionately.

The comparison of the fires to the Hiroshima bomb, though technically correct, is an attempt to cloud clear thinking with emotional reaction.

It reminds me of Godwin's law http://en.wikipedia.org/wiki/Godwin's_Law where clear thinking is inevitably clouded by references to Nazis in an arguement.

We can do other calculations like 10 minutes of "clear fresh sunlight falling on Australia" or the "output of 1 billion fireflies on a warm night" with much different emotional overtones.

The blame game is starting and there will be a lot of ill informed comment as the community looks for scapegoats for the tragedy.

Bill Kerr said...

hi tony,

Emotional invocation is an important topic. My aim was not that but to demonstrate that there is an enormous amount of fuel in the Victorian forests and so fuel reduction ought to be taken more seriously. I was surprised that the amount of energy release was so high.

I did that particular calculation partly out of curiosity - I wanted to check whether the energy output was indeed that high. Also in another discussion someone argued that David Packham couldn't do his maths and I wanted to check that.

It would be more dispassionate to say that the energy output of the Victorian bushfires was 2*10^10 kilowatt-hours. However, that figure means very little to me (and many others) whereas the 660 Hiroshima bomb equivalents, although misleading in some respects, does enable some sort of visualisation.

How is it misleading? That the energy release is dissipated over a large area, mainly not inhabited by humans, rather than being concentrated on one city, Hiroshima.

Yes, the blame game has started. In every Royal Commission investigation bushfires since 1939 the recommendation has been to increase preventative burning. The fact that that has not happened yet indicates to me that someone is to blame.

Tony Forster said...

I have some sympathy for those responsible for implementing controlled burns based on my own experience burning off. It rapidly goes from being too hot and dry to too cold and wet with a very narrow window for burning off.

They will find it hard to avoid the twin criticisms of not burning enough and creating fires with controlled burns that get out of hand, (as happened with the 2005 fires at Wilson's Prom)

Bill Kerr said...

As I see it tony the choice is between quite intensive and as you say, difficult, preventative burning and irregular disasters. Hence, the need to stress the amount of energy stored in our forests. It seems that as a society we choose irregular disasters over regular inconvenience.

Nevertheless, we have an organised group (the Stretton group ) which is expert, has been warning us and has outlined the necessary preventative measures.

After the 1939 fire, Royal Commissioner Stretton recommended fuel reduction but I think this has since been consistently fudged. Apart from misguided but influential green thinking (but not all greens) there are other reasons:

(1) Regular fuel reduction is expensive

(2) Regular fuel reduction is often unpopular because of smoke, ugliness and messiness

(3) Because these disasters are irregular and many people have short memories it is politically possible for governments to avoid reality and get away with it. Government and media tend to present them as unavoidable tragedies.

(4) Many people have a poor understanding of the issue and the laws of probability - you may live your whole life in a house which cannot be defended from bushfires and yet be lucky enough to not suffer. Hence consistent pressure from below is not applied to solve the problem and implement Stretton’s 1939 advice.

(5) Control of these matters needs to be in the hands of a centralised, expert land management authority with real power and not in the hands of local emergency or fire management authorities.

Even if we focus on saving human life by stricter regulations and building underground bomb shelters in threatened country towns then it would still be true that far more environmental damage to plants, animals and with CO2 release would occur through the irregular disasters.

Australian plants are fire loving and thrive off a regular fire regime, as acknowledged by environmentalist, Tim Flannery.

Intense fires release far more CO2 into the atmosphere than preventative burning in the non fire season and are exempt from the Kyoto Protocol