Thermal Runaway

2.     Hot, Cold and Weather

Thermal Runaway

The art of lighting a bonfire

Any fool can light a bonfire when the temperature is 28 degrees C and you have a nice pile of dry combustible material. All you have to do is drop a lighted match and stand back. Or wait for lightning to strike – a subject I shall return to later on ecology. In some parts of the world this is a common occurrence. I have recently returned from a trip to Australia, and it was very noticeable how many restrictions there are on lighting any kind of fires outside, because of the huge risk of forest fires.

But when its 8 degrees C and you have a pile of mainly damp material, it is another matter altogether. I have spent many frustrating hours peering forlornly at a pile of miserably smouldering garden waste. But over the years I have learned roughly how to solve these problems. Here, for what its worth, are some golden rules for lighting bonfires.

1.                  Wet stuff doesn’t burn.

2.                  You need heat to dry out wet stuff

3.                  Dry stuff burning makes heat

4.                  Heat can escape or be contained

Pretty obvious you might think, but believe me, it has taken years of painful study to apply this knowledge. The trick is to have the right balance of wet and dry stuff on the fire at any time. You need some dry stuff to get the fire going at all, and to keep it going, and to create enough heat to dry out the wet stuff. You need some wet stuff so it gets dried out by the dry stuff burning, but not enough so that it overwhelms the heat being produced and cools the whole mess down so that the fire goes out.

You also need to keep the heat in so it dries out the wet stuff rather than disappearing rapidly into the upper atmosphere. I have sometimes managed to create a spectacular mass of flames rising high enough to threaten the branches hanging over my garden from Mr. Brown next door, only for them to burn out rapidly leaving the familiar pile of half burnt damp waste again. You need to contain that heat by a judicious wet/dry covering of other material. Sometimes it pays to apply the Ronald Reagan principle – don’t do something, just stand there. A fire may go out, but retain enough heat that it will in time dry out enough to be relit and really get going.

OK, so what does all this have to do with feedback? Well, we have a kind of loop here.

 Heat dries out wet stuff. Dry stuff burns. Burning makes heat.

This is positive feedback, and boy do we need it here. But of course, things can get out of hand as they often can when positive feedback appears.

Firestorms

There is another kind of feedback that happens with big fires. We all know, don’t we, that hot air rises. And from a big fire, it rises pretty darn fast. All that air rising sucks in cold air from low down. And what does air contain? – Oxygen, and what does a fire need apart from dry stuff? Oxygen. So we have another kind of loop.

Hot air rises. More air is sucked in. More heat is produced. More hot air rises.

Its positive feedback again, and in extreme cases it can create the horrendous kind of firestorms that destroyed Hamburg and parts of Tokyo in WWII.

The positive feedback that we see in fires is an example of thermal runaway, but there are other places where this kind of thing can happen.

Thermal runaway in electronics

All electronic products produce heat. Anyone who owns a modern laptop can vouch for that. It surprises me that in a world where you can successfully sue McDonalds because you pour hot coffee over your lap, that no one has sued for getting their lap burnt by a laptop. Perhaps they have.

A lot of electronics (and electrics) revolves around something called resistance, and components called resistors. Anything has a resistance to an electric current. Metals, which are generally good conductors, have a low resistance. Things like plastic, glass, wood have a high resistance. Some things like carbon have a middling resistance.

Now be brave, I am going to show you an equation. It’s really not hard.

            V         =          I.R        (known as Ohm’s Law)

What does it mean?

The Voltage (V) across a resistor is equal to the current through it (I) multiplied by the value of the resistor. So high resistance leads to low current, low resistance leads to a high current. Don’t ask me why current is always called I – its just one of those traditional engineer things.

Here is one more equation:

            W        =          V.I

This means that the power (W for watts) generated in a resistor is equal to the current (yup, I again) multiplied by the Voltage.

So what happens to this power? It turns into heat. And the resistor gets warm, or hot. How hot it gets depends on how much heat, and how good it is at getting rid of it.

Now if we combine the two equations, we get one more (last one, honest):

            W        =          I².R

Which means that the power is proportional to the square of the current. So if the current doubles, the power goes up by four times. So you can see that if the current increases, the power (and heat) is going to go up fast.

Now there is a complication. Resistance isn’t fixed, it can vary as the temperature varies. For instance the resistance of most metals rises as they get hotter. This is OK, in fact it’s an advantage. Why? Because it gives us some negative feedback, which you will recall, is generally a good thing.

Otherwise light bulbs might explode. What happens here is that as the wire in a light bulb gets hot, its resistance gets higher, so the current falls, so it gets cooler (this all happens quickly so you don’t see it). So its stable, and everything is hunky dory.

But some electronic components have a resistance that can fall as the temperature rises. This is bad news, because it leads to positive feedback.

Temperature rises. Resistance falls. Current increases. Component gets hotter.

If this is not properly controlled by the circuit design and construction, the component can get hot enough to burn it out. That’s thermal runaway. It can be avoided by having sufficient cooing so that, while the component generates a lot of heat, it does not actually get too hot. Modern microprocessors have large heat sinks with fans attached - like miniature car radiators. But those laptops still get pretty hot.

Chemical Thermal runaway

Some chemical reactions actually generate heat, these are called exothermic reactions. They can lead to thermal runaway under some conditions. If the heat produced can’t be removed fast enough, the chemicals get hotter, and this makes the reaction take place faster. Most chemistry happens quicker at higher temperatures – just think about lighting the bonfire for example.


The amount of heat which is can be removed rises as things get hotter, but it doesn’t rise as fast as the chemical reaction, so things can get out of hand.
If this happens inside a closed vessel, as in a chemical plant, then very nasty things can happen, such as fire and explosions.


The horrendous accident at Bhopal in 1984 was caused by water leaking into a storage tank and then starting an exothermic reaction, which led to a runaway until a safety valve lifted and released a deadly cloud of poisonous gas.


These problems are treated very seriously, as you can imagine, and if you are interested in further examples, you can find them on the UK Health and Safety Directorate web site at:
http://www.hse.gov.uk/comah/sragtech/techmeasreaction.htm

Runaway Batteries

Thermal runaway is often referred to in battery management. Nasty accidents can and have occurred when batteries get too hot. Many of these situations do not rely on positive feedback, so are not true thermal runaway in my book, but there is at least one example that is.

Lithium-ion batteries, are commonly used in laptops, cell phones, digital cameras and PDAs where their power weight ratio is valuable. But lithium ion in most cases uses cobalt oxide, which has a tendency to true thermal runaway. If it gets over heated beyond a certain point for some reason, it can start to self-heat and then catch fire or even explode. The internet is awash with companies offering ‘safe’ technologies to avoid exploding laptops and cell phones. Such things have happened, but only rarely.

  

Containment

As positive feedback can be dangerous, we often take steps to contain the effects. For example, we light fires in fireplaces, or stoves. Then the amount of combustible material that the fire can reach is limited, (as is the air supply in a stove), and so the positive feedback cannot get out of control. If we are not careful enough, things can get out of hand. I had such an experience recently when I lit a large pile of dry garden waste in an incinerator near my wooden compost cage. The heat was such that the compost cage started burning vigorously, and I had to resort to buckets of water to stop it.

Cold Runaway ?

Water is strange stuff, as Phillip Ball explores in his book H₂0. For one thing, water expands when it freezes, which is contrary to the general rule of things contracting as they get colder. Actually, water contracts until 4 C, then it expands, which is why we get pipes bursting in freezing weather.


Ice is also peculiar; it has many different configurations as it gets colder. This is why the inhabitants of Siberia can tell how cold the ice is by the sound it makes when they strike it with an axe. Very cold ice is denser, and more brittle. Some time ago there was an idea going around that maybe there was another, undiscovered, form of ice that could form at a temperature above freezing. This could be rather nasty, as Kurt Vonnegut explored with ‘ice 9’ in Cat’s Cradle.


The point is that if an ice could form at higher temperatures, it could start a chain reaction, gradually turning all the water on earth to ice. The formation of ice, and snowflakes, is a form of crystallization, where one crystal acts as a template for forming another. More and more crystals form, and so form more crystals. This only happens when the conditions are suitable, and the new structure is at a lower energy level. So the change in structure is like something running downhill. This process is not quite like thermal runaway, where there is a more definite feedback process, but it is an example of the avalanche type, where one event triggers further ones.


Don’t worry, though. It turned out that there was no such form of ice. Or at least, not yet.

Weather Systems

As I mentioned above, snowflakes form from a crystallization process, and raindrops also form in a similar way. Although raindrops don’t form in the complex patterns of snowflakes, they have a more complex formation in another sense. It turns out that raindrops travel up and down through cloud structures, getting bigger as they go, until they are too heavy to stay aloft, and fall as rain.
                       
Other weather systems also build up as feedback processes. Tornadoes and tropical storms extract energy from their surroundings and grow in size. As they grow, they extract even more energy. So in a sense, they behave like other growing things, only faster and bigger. And, as is often the case, the bigger they are, the harder they fall.

Avalanches, Mud Slides and Sand Piles

I talk about avalanche like processes elsewhere, in electronics and in epidemics, but real avalanches are another example of positive feedback leading to trouble. What starts of as something small rapidly grows out of control, because each additional particle that starts sliding downhill can trigger the movement of more particles. The whole process then grows rapidly as the amount of movement grows exponentially until the limit is reached because the process runs out of loose material.


The same kind of thing happens in mud slides, just the material is different. Both events however happen when a large amount of loose material is in a critical state. Avalanches don’t start on the prairies. They need a pile of snow that is ready to be triggered. What does that mean? Well, let’s talk a bit about sand piles.


Sand piles have received quite a lot of attention in the past decade or two because they turn out to exhibit chaotic behaviour. If you drop loose dry sand into a pile, it develops a natural conical shape, where the angle of the slope is fixed. Excess sand runs down the slope in smaller or larger little movements, keeping the angle more or less constant. If however the sand is a little sticky, the angle can build up above the ‘natural’ one until a mini avalanche is created.


So all these processes rely on a pile being in a critical state, where the slope is greater than the natural loose slope. Any breakdown in the ‘glue’ that is keeping the whole thing aloft leads to the avalanche type effect.


This ‘super critical’ slope is similar to other ‘super cool’ effects in liquids that I will talk about in Chemistry.