Monday, 24 September 2012

Phase change materials - an introduction

Under the category of things we thought about doing, didn't, and now wish that we had, comes PCMs, or phase change materials. You probably think you don't know what they are, but I'm sure you have used them many times. The planet has also been using them to stabilise its climate, although perhaps it won't be for much longer. In fact that I'm just talking about one phase change material: ice.

We should probably also call it water, as the point of phase change materials is that they change from one phase to another, for example ice turning to water, steam condensing to water, or dry ice subliming into gaseous carbon dioxide. What is happening in all these cases is a transaction of heat at well above the regular exchange rate per degree change in temperature. 

Some languages don't have separate words for the different phases of water. For example, Malay apparently has the word air for water, confusingly enough, and air batu, roughly "solid water" for ice. Strong evidence for the Sapir Whorf theory that language depends on culture. Japanese, on the other hand, has four words for di-hydrogen oxide. Kohri is ice, mizu is cold water, (o)yu is hot water and jouki is steam. Actually you could argue that jouki is not really one word, but two Chinese characters, hence as foreign a concept as when Malay speakers say aisu. But I digress.

The beauty of phase change materials is that they save up heat, either in credit or in debt, and release it at a constant temperature. So when you put some ice cubes in a gin and tonic, you're ensuring that the temperature of the beverage, as it reaches your lips, will be the same from the moment it reaches the table, until the last drop pours out of the rattling ice. That is as long as you finish drinking it before the ice melts. My grandmother was never a big fan of ice as it diluted the alcohol. I seem to be digressing again. 

What is happening in your gin and tonic, or even lemonade, is that the ice, being ice, stays at around zero degrees centigrade. If you work in the Farenheit system, then all you need to know is that zero corresponds to the freezing point of water and 100 to the boiling point. The glass and liquid in it set up some kind of equilibrium so that the temperature of the water is between room temperature and ice temperature. Heat, dutifully obeying the second law of thermodynamics, flows into the liquid from outside, leaving drops of sweat from condensed airborne humidity in its wake. It then flows from the water into the ice. Rather than changing the temperature of the ice, part of it changes from ice into water. 

On a molecular level, this means that rather than sitting in neat rows, either in starry crystals or glass-like blocks, those di-hydrogen oxides are all getting up and boogying around. Getting them all up takes a lot of energy. When they all sit down in their neat rows again, that energy is released. A similar level of energy is needed to change them from the pedestrian liquid state, where the molecules are at least in close proximity with each other, to the jet-set gaseous state where they fly around at great distances to each other. 

The United States have been instrumental in the propagation of ice around the planet. In the nineteenth century there was a huge trade harvesting ice from New England lakes and transporting it as far afield as India and Australia. This trade was finished off by the 1920s at the hand of plant ice, based on technology that went in to the refrigerator that became a part of every kitchen from the 1930s in the US, and in later decades around the world. 

As a phase change material, ice works by first having the heat taken out of water, either in a cold winter or with the refrigerating cycle of a heat pump. Later it absorbs heat from its environment, bringing down the temperature accordingly. Steam can also work in the opposite way, as used in heating systems of large buildings, taking in heat when the water evaporates in the boiler, and releasing heat when it condenses in radiators around the building. 

As a phase change material for buildings, water is rather limited as the freezing point, zero degrees C, is much too cold for the building, and the boiling point, 100 degrees C, is much too hot. There are other materials with melting points around ambient temperature, for example chocolate. More about that later.