Prince Pondicherry and the Phase Change Palace

Those familiar with the works of Roald Dahl, or even just the Johnny Depp film, will no doubt remember Prince Pondicherry, who had a palace in India made out of chocolate. Every part was made from chocolate. Even the taps, from which would flow hot chocolate. Mister Wonka did warn the prince that he should start eating quickly as it would likely melt in the heat of summer, which sure enough it did.
A couple of pages before this palace is mentioned, Dahl tells us that Willy Wonka could make ice cream that wouldn't melt, even in hot sunshine. So the obvious question is, why not make a tropical palace out of chocolate that didn't melt?
The only sensible answer is that he was using the phase change of chocolate to maintain room temperature in the palace. While the phase changes of water, at zero degrees to ice and 100 degrees to steam, are too high and too low for a comfortable ambient climate or efficient heat production, chocolate changes its phase at a much more useful temperature.
The melting point of the best chocolate is around 34 degrees Celsius. This is an ideal temperature from a culinary perspective as it is low enough to melt in your mouth and high enough not to melt in your pocket. Unlike water, which is predominantly H2O molecules, chocolate contains a variety of substances in various forms of crystalinity, so the melting point will vary. The art of great chocolate making is to get the right kind of crystals of fat in there, but hearing about this suddenly makes its taste much less appealing.
There's also likely to be a lot of hysteresis. This means that it may melt at 34 degrees, but you have to cool it down below 28 degrees to get it to solidify again. I usually put it in the freezer.
So if Mr Wonka, who elsewhere exhibits a  great mastery of the physical sciences, was indeed trying to make a phase change building, it would work something like this.
The temperature inside would stay below the melting point of the chocolate. On a hot day the chocolate would start melting. As it melted, it would absorb heat from its surroundings, therefore having a cooling effect. Later, the melted chocolate would solidify, emitting heat into the inevitable coolness of the night.
If he was trying to make a phase change palace, though, he didn't do a very  good job, and shouldn't really have been advising Prince Pondicherry to eat this thermodynamic wonder.
In the winter, chocolate could work in the opposite way, absorbing heat from the sun in the day time, and melting in the process. As it cooled later inside the building, it would solidify and release heat as it changed its phase.  The best thing to do with this chocolate would be to line a south-facing internal wall with it, so that it would catch the sun coming through south-facing windows. Painting a wall with chocolate is probably not to be recommended as it would run down the wall when it melted, and all end up at the bottom. Fixing chocolate in sealed bars, preferably in heat-absorbant black, would be much more effective.
And if it got really cold, you could always eat the chocolate.

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. 

Outdoor goods - don't use them outdoors

It's been windy the last few days. Not a typhoon or anything, but a strong wind blowing right through our garden.

The house doesn't seem to mind this, but the tarps we've been using to provide shade have been flapping like they're literally three sheets to the wind. That expressions refers to the setting of sails--a sheet, as is not at all obvious to the non-nautical, is a piece of rope tied to the loose end of a sail. This can change the sail from being close-hauled, almost in line with the ship when you're sailing upwind, or let right out for a broad reach or a dead run.

The house doesn't look drunk, but the strong wind has been pulling pegs out and letting the tarps wave around.

The wind was blowing so strongly the other evening that I undid the guy ropes from the pegs, went up to the balcony and furled the tarp, rolling it up and tying it along the top of the railing so that it would not flap any more. I started thinking about getting a more ship-shape arrangement out there, so the tarps can more easily be drawn in and out to suit the sunshine and wind.

The sound of wind is not a bad sound, but there is a lot of energy in the wind, and hopefully the balcony which the tarps are tied to will not sail away from the house. At night we don't need the tarp, but if it's a sunny and windy day in September, we want to keep the sun out. Perhaps I can rig up some halyards for next year, so that the tarps can go up and down with the weather, and possibly be set differently for the sun. Low and in the East for breakfast; high and to the south for lunch; letting the stars in at night.

One issue is that the guy ropes have been breaking a lot over the past few days. This is probably a combination of the heavy wind and fatigue over a month of holding the tarp through all weather. The points of failure were not at places with friction, at the knots, in the eye connecting to the tarp or at the peg, but in the middle of the rope.

I wonder also whether UV radiation has been breaking down the material of the rope. I know that UV can be very damaging. This would be ironic as the tarp is called a "UV Hexagon Tarp" and claims to "protect UV". No doubt "UV" is more of an advertising slogan than an optical description. And anyway the tarp claimes to protect anyone sitting under it from UV; it does not claim that the ropes holding it up are UV resistant. Or perhaps "protect UV" is literal, and it is doing as little as it can to impede the ultraviolet raise on their long journey from the sun to our subcuticular carcinomas.

Anyway, I'd hoped all the parts of these outdoor goods would last more than a month outside, rather than being good for a couple of weekends and being stored away in a cupboard for years in between.

The first tarp we used tore in two on a very windy day at the beginning of August. We'd had it for years and it didn't owe us much. It was a good excuse to buy a newer tarp to add to our camping inventory. After some sewing we can now use the original as two different tarps, which I suspect will be less susceptible to wind than one large piece of cloth.

A certified ventilation system

A side benefit of the reinstalled ventilation system is that we now have a unit that has been certified by the Passive House institute. They replaced the Stiebel Eltron LZW 170 with an LZW 270 Plus. The "plus" means that it has a bypass function, which is the part we wanted to help get rid of that summer heat. The 270 is more powerful than the 170 so it can ventilate more volume, although we already had enough with the smaller system.  The other features are the same and it's difficult to beleive the efficiency has changed in any way.

As far as data entry is concerned, though, if you're entering manufacturer data for parts that have not been certified by PHI, you need to take off something like 12%. This is probably entirely justified, but in our case we can now increase the heat recovery efficiency from 78% to 83%. This doesn't sound a lot but brings our score down from 14.6 to 13.8 kWh/m₂a.

Further investigation online shows that Stiebel give a heat recovery up to 90%, and they say that the Passiv Haus Institute has a figure of 86%. The database on the PHI website passiv.de, on the other hand, gives a figure of 83%. There is a mistake somewhere!

I suppose a lesson to learn, if the calculations are to be believed, is that ventilation and air tightness make a big difference, and having a well insulated house is not enough.

Talking about my generation

August was definitely a good month. About 150 kWh more than May, which had been the best month before then. You can see how well we're doing here, with the actual monthly generation in green and the simulated monthly below it, for the whole year. As I write the September generation is just based on the first week, so it may get better or worse. The simulation is quite a lot worse than August, although August was not the best month in the simulation.


There is quite a variation between the simulation and the actual results, and we are doing much better in the summer, and were doing a little worse in the winter, so the overall the simulation seems pessimistic. Of course this year's weather may have been exceptional, and the differences could be accounted for by that. We'll need a few years' data before we can really know how accurate the simulation was.

To get an idea of why August was so good, you can see from this chart that there were just very few days with low generation. Also there was relatively little limitation of the power output, in fact the least since records began. Less than two hours over the whole month, compared with almost twenty hours in May.

Guaranteed power for quarter of a century

On the same day that the 54,000 yen electricity bill came, we got the warranties from the installers of our solar panels. The power conditioners are guaranteed for ten years, and the panels are guaranteed to produce electricity for 25 years. If the house falls down, and all the bits in it stop working, at least we will still have a small power plant, which should be worth something unless somebody starts making free electricity. The guarantee is for between 5.5 kW and 10 kW, which means at least 60% of the rated value.

Of course a guarantee is only valid as long as the company giving the guarantee is still there. That company is Suntech, which according to Wikipedia is the largest manufacturer of solar panels in the world, with an annual production capacity of 1.8 giga watts, and over 13 million panels sold in 80 different countries. 

For a while we were looking at solar thermal, or to put it in earthy Germanic terms rather than fancy Latin, hot water from the sun. We would have been lucky to get a five year guarantee on a system, and probably would have had engineers coming round every month or two to fix it, or at least pretend to fix it while each time making it worse.

With no moving parts, photovoltaics are much more reliable, and these will probably be generating electricity long after I stop generating carbon dioxide.