Carbon Emissions from Planes: I eat my words

A Boeing 747: How much fuel does it use?

At the end of March I will be attending the 9th International Temperature Symposium, a conference that takes place once every 10 years. Sadly the event takes place at DisneyLand, California, roughly 5,500 miles away . I am looking forward to spending a week discussing arcane details of temperature measurement and taking morning coffee in the Magic Kingdom East Foyer, but I am concerned at just how much carbon dioxide I will emit travelling to the conference.

I had worked this out before – and my answer was very roughly that the flight to LA from Heathrow emitted around 2 tonnes of carbon dioxide per passenger. But at the last Protons for Breakfast someone questioned this – and said it was nearer to half that! It took me ages to figure out that the reason we disagreed was simply in our estimation of how much fuel a Boeing 747 uses. I have looked hard to find the answer and… – I was wrong. The flight will ‘only’ emit around 1 tonne of carbon dioxide per passenger each way. Great!

The key information that led me to this conclusion was a post on this chat site that sounded trustworthy – it seemed to be from a 747 crew member.

Fuel burn (planning) 747-200s or 747-300s…

• Taxi/Takeoff : 1 tonne
• Assuming a heavy aircraft at 377 tonne takeoff weight, it will burn some 15 tonnes to initial cruise.
• First hours of cruise, expect some 13 tonnes per hour
• As the plane gets lighter, this will decrease to under 10 tonnes per hour.
• Descent, approach, landing, generally burn 3 tonnes.
• Taxi-in will be anywhere from 0.500 to 1 tonne
• Our company policy is to plan landing at destination with 14 tonnes reserve.
• The capacity of these airplanes is generally 155 tonnes with the 7 tanks configuration, or 165 tonnes  with 9 tanks…

So for a flight to LA from Heathrow taking roughly 11 hours and 5500 miles. Fuel use is therefore:

• Taxi/Takeoff : 1 tonne 
• 15 tonnes to initial cruise.
• 9 hours of cruising at an average of 11 tonnes per hour = 99 tonnes
• Descent, approach, landing, 3 tonnes.
• Taxi-in 1 tonne
• Total is approximately 120 tonnes 

I had previously assumed that the fuel used was nearer to 200 tonnes. We can work out the carbon dioxide emissions from this much fuel in several stages as follows:

• Fuel 120 tonnes = 120,000 kg
• Fuel is kerosene which has an approximate formula C12H26
• Molecular weight is 12 x 12 + 1 x 26 = 170
• Carbon fraction is roughly 144/170 ~ 84.71% by weight
• Mass of Carbon in fuel 101,640 kg
• Mass of CO2 from carbon in fuel 373,019 kg. This is a factor 44/12 = 3.67 larger than the mass of carbon in the fuel because of the addition of oxygen.
• Passengers on the plane is typically 350
• CO2 per passenger 1066 kg, or just over 1 tonne.

After thoughts. Firstly, I had checked that calculation several times, but my error was in the very first step: I >am< an idiot. Secondly, one tonne is still a large amount of carbon dioxide to emit in 11 hours. And thirdly, it’s important to get these things right. If the Protons’s attendee is reading this: sorry.

A ‘Safe’ Dose?

Is there such a thing as a 'safe' exposure level to a hazard?

In many areas of life we are exposed to hazards in the form of a substance or a process which may harm us. Mobile phones use microwaves – physically similar to those used for microwave cooking – but at a much lower power. We know that microwaves at high power are bad for us – think of microwaved meal . ANd we know that mobile phones are pretty safe – but is the exposure level from using a phone completely safe?

To the best of my knowledge, there is not a single case of a person having been harmed by using a phone (aside from as a result of the inattention they cause). But many people are still convinced that they are harmful. Indeed, for the people for whom this is a concern, the microwave radiation used by phones is intrinsically ‘bad’. And they would consider any level of exposure – no matter how low – to be dangerous. And that led me to wonder if the concept of a ‘safe dose’ really made sense?

One problem with microwaves, is that we can’t see them! And we never receive a large enough dose to sense them directly. This can lead us to either ignore the hazard (“Its all nonsense!”), or to become anxious that we are secretly being over-exposed (“They want to build a phone mast only 100 metres form my children’s school!”). So let’s first learn some lessons from two hazards with which we are more familiar: sound and light. Hazards? Surely exposure to sound and light is completely safe? Well, no. Exposure to loud sounds can cause deafness, and exposure to bright light – such as the Sun – can cause blindness. But it is not just excessively large exposures that cause problems.

Exposure to modestly loud noises does not cause immediate deafness, but prolonged exposure does reduce our hearing acuity, something we don’t usually notice for many decades. “I SAID WE DON’T NOTICE IT FOR DECADES!”. Similarly, exposure to normal sunlight causes a yellowing of the cornea which affects our vision in old age. So it is sensible to consider that exposures to sound and light should be limited so that in the course of our normal lives we are not unduly damaged.

But we also benefit from exposure to light and sound – indeed to withdraw exposure would be – quite literally – torture. As best we can tell, a whole lifetime of listening to sounds which are not-too-loud and exposing ourselves to light which is not-too-bright does us no harm and brings us wonderful benefits. So sound and light are not intrinsically ‘bad for us’ or ‘good for us’. What determines whether exposure to these ‘hazards’ is ‘safe’ is judged quantitatively, not qualitatively. For a hazard of this kind, the concept of a ‘safe dose’ makes sense, we get to enjoy the benefits and avoid the downsides.

What are the numbers for microwaves? The intensity of the microwaves inside a microwave oven is around 100,000 watts per square metre – definitely bad for you. By comparison the intensity of visible light from the full summer Sun is roughly 1,000 watts per square metre. And a mobile phone? A few millimetres from the phone surface the maximum intensity is around 0.1 watts per square metre.

Sound and light are familiar to us, and so it is easy to understand the balance required in terms of reducing our exposure to avoid future harm.  However microwaves are invisible and unfamiliar. So although the concept of determining a ‘safe dose’ quantitatively is reasonable, it is easy to understand why people feel suspicious that we are being secretly harmed. Trusting your health to a regulatory authority is not quite as reassuring as being able to trust your own judgement. But I can’t think of any alternative.

The Elements versus The Periodic Table

Two iPhone Apps. Theodore Gray's 'The Elements' (left) or Merck's PSE-HD (right). Both apps are shown displaying basic information for Technetium.

Did I mention I had a new iPhone? Well obviously the first kind of app I downloaded was a periodic table of the elements – it just gave me a comfortable feeling knowing that I had that information close at hand. And what could be better than a Periodic Table app? Well, obviously two Periodic Table apps! And so here I compare the free Merck(TM) PSE HD app with the £6.99 app called ’The Elements’.

'The Elements' page for Technetium. Touching the image allows you to rotate it and the text scrolls for an engaging description of the element.

The Elements is an app written with love by Theodore Gray, co-founder of Wolfram Research who make Mathematica. It is a simple app to understand and navigate. It has an introductory essay, a picture of the periodic table with animated pictures of each element, and splendid animation of the ‘The Elements’ Song by Tom Lehrer. Tapping on an element brings up a screen with a rotatable picture of the element, and an engagingly written essay by the author. I did find a tiny error on the page about Helium, but Theodore said he would fix that in the next release. Each essay links the physical properties and the history of the element and has links to the pages for other relevant elements (this makes it easy to browse and ‘get to know’ the elements).

The main attraction of this app is its accessibility – it is simple to use and a pleasure to interact with – one of those apps you will show your friends to make them think that your insanely expensive mobile phone might not have been a complete waste of money. On the down side, it is a bit limited and expensive as apps go. It does have links to Wolfram Alpha’s database on the elements, which is nice, but then you are back to browsing web pages.

Link to UK App Store

The Merck PSE (HD) has more data built-in to the app than The Elements, and it has many different ways to display and interact with the data. The main screen is a picture of the periodic table, and touching an element brings up a panel with basic information. Touching the panel causes it to flip and on its reverse are different categories of information shown in considerable detail, from the history and discovery of the element, to technical data. At this point the app is just ‘really useful’. But there is more: touching the Merck ‘M’ on the home screen brings up a new way of viewing the data where one can view how a property varies across the periodic table.

Turning the control increases the temperature, and the periodic table graphically shows which elements are solid, liquid, and gas at that temperature.

For example, selecting ‘state at room temperature’ allows one to view the periodic table colour-coded as to the state (solid, liquid or gas) of the elements. A rotary control allows one to change the temperature and see visually which parts of the table melt and then vaporise in which temperature range: it is delightful. Similarly, selecting electronegativity shows how this property varies across the periodic table. Not sure what electronegativity is? Then select the glossary tab to find out.

The periodic table graphic shows how the electronegativity of a substance varies with position in the table.

My favourite is the discovery tab which allows you to scroll back through time and see when each element was discovered – and the app displays an image of the discoverer. And there is lots more too.

The faces of the people who discovered each element.

This app is data rich and carefully thought out. Having it on your phone gives you that comfortable feeling, knowing that even when your network connection is down, you will still have access to melting point data for the elements

Link to UK Apple App Store and  Android marketplace

Which is best? For people who enjoy tech lore, or if you’re studying Physics or Chemistry, then the Merck PSE app – for free – is a must have . For people who have £6.99 to spare – and are perhaps curious about science – but not professionally involved, ‘The Elements’ is a real pleasure to own.

A Silent National Disaster

At Protons for Breakfast I try to make the world of science accessible to ‘normal’ people i.e non-scientists. I use no maths, because the course is about ideas, but I do use words. However many adults in this country can’t read. And so there is universe of ideas to which these people don’t have access.

People who can’t read generally don’t make a fuss: they are embarrassed. And after they have left school, their whole lives are in no-small-part defined by their inability to read. And one in five adults in the UK is functionally illiterate.

Two fantastic friends of mine – Nick Ainley and Libby Coleman – have written a book and started a practical campaign to help any one person teach one other person to read. It’s called Yes we can read. They aim to reach non-readers one by one. At the moment their work is still a ripple, but I think it will become a wave.

Nick and Libby: You are my heroes.

Ground Source Heat Pumps are Solar Powered

A simplified estimate of how the temperature of soil at selected depths varies through the year. Notice that at depth, the variations are minimal and lag behind the variations at the surface. However the average temperature is roughly the same as the average surface temperature, in this case 10 °C. Data are guesstimates adapted from here : please do not trust them! Click for larger version.

Sometimes I astonish myself with how stupid I can be.

At Protons for Breakfast last autumn I stated that geothermal energy supplies could only ever extract around 0.1 watts per square metre of ground. Why? Because that’s the average rate at which heat rises through the Earth. This is a ridiculously low figure, and I couldn’t understand how it made any sense. And then it clicked.

• Geo-thermal energy isn’t sustainable, at least on a strict definition. It’s a one-shot operation that cools a reservoir over a decade or so, and then waits a few decades for the reservoir to heat up again.
• Ground Source Heat Pumps don’t capture Geo-thermal energy – they capture Solar Energy and there is on average over the Earth around 240 watts per square metre of solar energy available.

I realised that I had confused these two sources for years, and browsing the web makes me think a lot of other people have too. Let me explain.

Geothermal energy is the heat flowing outward from the centre of the Earth. It arises in part from the radioactive decay of elements within the Earth, and in part from heat left over from the formation of the Earth. The Earth’s crust is an excellent insulator and the heat only flows out slowly. Across the UK, the average heat flow is just 0.038 watts per square metre, which means that in order to generate the 10 kW one needs to heat a house, one requires an area of around 500 metres x 500 metres – and drilling down doesn’t increase this figure at all. However there are two situations in which we can extract this heat and use it.

1. In areas of the Earth where the heat flow is stronger than average (e.g. Iceland) it would be perverse not to exploit the gift of heat.
2. The second situation is more common and involves drilling down to around 3 km depth where the temperature is around 100 ºC. If the rocks are porous at this depth then we can pump cold water in to the rocks, and extract hot water. The porosity of the rock allows the water to accept heat from a large volume of rock. This is not strictly sustainable in that the block of rock will cool down over a few years, and we will need to leave it to warm again. But it will keep warming up for millions of years to come.

Both these schemes extract genuine geothermal energy, which originated on Earth, roughly one half it, nuclear in origin.

Ground Source Heat pumps collect heat in the top few metres of the Earth and here the temperature is strongly affected by the temperature of the surface. And  this is determined by the  local average heating from the Sun, typically around 240 watts per square metre. Even in areas with cold winters, if one digs down more than a few metres, the soil stays warm , and a heat pump can be used to extract this heat. The pump chills a fluid such as ammonia to around 3 °C and then passes it through the buried pipes. Being colder than the surrounding soil, the ammonia absorbs heat from the soil, and evaporates. When compressed, the heat is released at a temperature of around 20 °C which can be used to heat a house.

However the source of the energy is the Sun not the Earth, and so 100% of it comes from nuclear fusion.