PCMs in the house and in the pocket

So the beauty of phase change materials is that they store heat at constant temperature. One of the challenges in our house, with its passive solar design and extensive south-facing windows, is the daily temperature difference in the winter, when it gets warmer from the sunshine in the day, then when the sun sets cools down from the loss of heat over the massive, thirty-degree temperature difference. Because of the insulation, it doesn't cool down so much, but insulation does not stop heat from moving—it just slows it down.

Building thermal inertia into the house is important—our slab of concrete and stuff under the floor has been doing a great job—but if the thermal mass were in phase change materials, that would be even better. When we were looking at solar thermal collectors, we looked at underground phase-change storage to transfer the heat of summer to warm the winter.

The principle is a bit like the pocket warmers we send the kids to school with in the winter. You put them in a pan of hot water until they melt. They stay molten in your pocket until you need the heat, and when you do, you click the metal strip inside. This causes the liquid to freeze, and in the process release a lot of heat. You can see the liquid freezing in the photos, taken in rapid succession.  Somewhat counterintuitively, they are giving out heat when they are freezing, and taking in heat when they melt. Because of hysteresis, the freezing point is a little bit lower than the melting point. Water will freeze when it is below freezing, and ice will melt when it is above freezing. Some hysteresis is very helpful because we want heat when it gets colder, and we want heat to be taken away when it is hotter.  As long as the liquid is not interrupted, its temperature can cool well below the freezing point, which I guess is a little over 40 degrees C. It's ready to freeze, though, just like the moisture is ready to freeze in the cold winter air. There needs to be a catalyst for the molecules to solidify around, and a clicked metal strip will act as that. Then the molecules will all freeze onto each other, like a rapidly growing crystal. A bit like snow crystals growing from the moisture in the air. As the molecules freeze, they release heat, warming up your pocket. I guess snow is releasing heat too, if the flakes are growing and taking moisture from the air. Perhaps that's why it feels warmer when it snows. I'm not sure whether snow counts as a phase change building material. You'd have to ask an Inuit.  One possibility for using PCMs in the house was inside the hot water tank. Effectively the hot water tank has a load of wax floating around in it. When heat is coming into the tank from the solar panels, the wax gradually melts, absorbing the heat but staying around its melting point. When hot water is drawn out of the tank, the water cools, forcing the wax to freeze and release heat in the process, just like the pocket warmer. The combination of hot water and PCMs floating around in it, is very effective for increasing the thermal mass of water, and avoiding some of the issues of PCMs such as super-heating and hysteresis.  I suggested that we use a hot water tank with PCMs when we were negotiating with the Solar thermal people, but they didn't seem to understand what I was talking about. I may as well have been suggesting that we build the foundation out of blancmange. Perhaps it was my language inability, or it may have been their reluctance to grasp the science. It was probably their stance as experts preventing them from listening to a technical opinion from a potential customer.

Here is a paper on a system in India using cans filled with paraffin, storing and releasing solar heat.

This is one of the tanks that I would have liked to get for our solar thermal system, which works on the same system.  I suggested this to the people who were trying to sell us the solar thermal system, but they didn't seem to understand. 

Another interesting idea with PCMs is this one from China.  Solar vacuum tubes are filled with phase change material. When the sun shines they change phase and charge up. When you run water through them, they draw off the heat and charge down. 

So the pieces are all there. They are just scattered around on the floor rather than coming together into the plans of buildings. 

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. 

Beating the heat

Just because we live in a low energy house, it doesn't mean we can completely ignore the summer. In fact there are lots of things we can do to keep the house cool, now that it's going to get over 30 degrees centigrade oustide pretty much every day for the next two months. Some of these you may be able to try at home. Some of them rely on infrastructure such as high insulation, airtightness and good windows. Having a low energy house is one thing, but being a low energy person is another. Some try to tread lightly on the planet and make their carbon footprint small, but attempts are thwarted because they are surrounded by energy inefficiency. Others have all the low energy tools at their disposal, but they have so many of them, and use them so much that their energy consumption is even higher. The goal of low energy design is to make it both impossible and undesirable to consume energy copiously.

1. Shut the blinds in the morning. Tempting though it is to shut them at night and open them in the day time, as the sun starts getting higher and further south, we really don't want it's rays to reach into the house. After the summer solstice in June, the sun starts getting lower, so as summer builds up pace, the solar gain is also increasing. External blinds are more effective at keeping heat out.

2. Shut the windows when it gets hotter outside than it is inside. Also counterintuitive, but this will keep the heat out. There is an argument that opening the windows will get some breeze going through the house, and that breeze will take the heat away from us and cool us down. This is true if the breeze is blowing past us, but in terms of temperature, a breeze of hot air is going to heat up the house. If you have an air conditioner on, it will work much better with the windows shut, just like a heater works better with the windows shut, and a fridge works better with the door shut.

3. Open the windows when it gets cool in the evening, or early in the morning.This will let some of the heat out and let cool air in.

4. Use cold water. If you have to use hot water, use as little as you can, and turn it off as quickly as you can. Hot water is going to heat up the house, and the more airtight and well insulated you are, the more it will heat your thermal envelope. If you have a smart boiler, which I think we do but it's too smart to give itself away, a secondary effect of this is that using more hot water will encourage the boiler to make more hot water, in other words keeping its water at a higher temperature. This will mean that more heat leaks into the house.

5. Switch electrics off. They're all giving out heat too. Don't use them unless you have to. Switch them off as soon as you've finished.

6. Cook at night or early in the morning, especially if you're using an oven. If you boil a kettle it should just have as much water as you need in it. Any more means more heat in the house. I get really annoyed when I see a kettle or pan of water being kept on the boil with the cooker on low. Much better to boil the water when you need it, and switch off the heat and use it as soon as it's ready. If you don't use it as soon as it's ready it could be there putting out heat for half an hour, and in fact it often is.

7. Stop using ice. It may cool you down, but the fridge puts out a lot of heat to make it. Iced tea is probably the worst as it involves boiling water then adding ice. To be honest ice makes less of a difference than the things above, and a couple of lumps is not going to make a big difference to the heating effect of your fridge, but I'm with the Chinese that you should drink hot drinks when it's hot. Cold drinks just confuse your body. I grudgingly confess that ice improves gin and tonic.

8. There's a whole load of really obvious stuff like wearing light clothes and not doing much exercise, and slightly less obvious things like not eating so much, and eating food that will cool you down rather than heat you up. Cucumbers and bananas are good. Meat and potatoes are not. 

9. Also there's probably a lot of subliminal stuff that will make you feel cooler starting with the colours, patterns and textures you can see around you. Having an electric fan in the room, even if it's not switched on, may make a psychological difference. 

10. Use LEDs. The above are all about using or not using what you have. LEDs make sense for two reasons. First, they are more efficient so they add less heat to the thermal envelope. Second, while incandescent and fluorescent lights give of heat in their radiation, LEDs just radiate light, so you won't get hot standing under an LED. Also, LED light does not attract insects, in stark contrast to incandescent and fluorescent light, so there is less threat leaving windows open at night time. It's a good idea to get other low-energy appliances too.

Tiles and heat

So far the tiles on the ground floor have been great. They feel warm on cold days, and cool on warm days. Six months after moving in, the extremities of the slab have warmed up, and it will now hopefully work as a passive heat sink.

The only thing that seems to have been a waste of time is the active system for pumping heat around the slab. I had over-ambitious hopes for the tiles to absorb direct sunlight and help bring the heat of the sun into the thermal mass of the concrete slab.

I think this was also tied up with earlier ideas using solar thermal collectors and pumping excess heat into lower levels of the slab, from where it would keep everything warm. The pumping system we have for the underfloor heating is not wasted, of course, as it still works with the heating system, which would have strained to pump hot water around the floor area we have, and would have been sending it at far too high a temperature. 

When we were choosing tiles I was interested in how much of the sun's heat they would absorb, but asking the supplier about this was like asking the fishmonger how many microgrammes of mercury are in his salmon. Perhaps something they should know, but certainly not something they do know.

The answer from the Passive House lady was more than nothing, but less than you think. This seems to have been accurate.

From a thermal point of view we should probably have got darker tiles with a matt finish. I don't know how this would have affected the aesthetics of the room though.

A hot slab of concrete

Actually it was a rather cold slab, and what we really want is a warm slab of concrete, not a hot one. Well, perhaps a luke warm slab. In fact what we really want is a room temperature slab. 

To be honest, the temperature of the slab is not a direct concern, but we want the temperature inside to be warm in the winter and cool in the summer. Because the concepts of warm and cool are relative, we may reasonably get away with cool being hotter than warm. 18 degrees may be warm in the winter when it's below freezing outside, and 25 degrees cool in the summer when it's over 35 outside.

So we want the slab to be slightly above room temperature in winter, and slightly below in Summer. If we can keep it somewhere between 20 and 25 degrees for the whole year, we should be comfortable.

The slab itself, with fifteen centimetres of concrete at the bottom, ten at the top and 70 centimetres of aggregate between, is mostly going to work as thermal mass, maintaining such a steady temperature,as I wrote before.

We have a 460 litre tank of hot water, which will be heated by an atmospheric heat pump, using cheap nighttime electricity to elevate the abundant but cool heat in the nighttime air to piping hot water. Actually, it may make more sense for us to modify this to take heat from the hot air under the solar panels in the day time, but that needs to be dealt with in a whole new blog. 

The heat pump, known as an Eco cute, is capable of controlling four heating circuits as well as providing domestic hot water and reheating the bath. But, the heating circuits send water at around 60 degrees, and keep sending it until the temperature sensor reads something like 50 degrees in the return pipes. As a 25 degree slab is going to be sufficiently warm, the heating circuits, as they are, are not much use to us. Instead they pass through a thermostatic mixing valve, which can be set to some temperature between 30 and 60 degrees, that will mix a suitable amount of hot water from the boiler to the water coming back from the slab. I suspect we will usually set this as low as possible, although as soon as we move in, we may want to get the slab up from 10 degrees as quickly as possible. 

They filled the underslab water pipes with antifreeze. It will be a very cold day when anywhere near these pipes gets anywhere near freezing, but I suppose there are advantages with protection from rusting, and there may be an increase in heat capacity. The water may need changing every couple of years, but we will see.

Temperature and heat of the slab

So we're getting all this temperature data from the thermometers in the slab. Not sure exactly what to do with it, or exactly what it all means yet!

There are ten thermometers in the slab: two in each corner, and two in the middle. One at the bottom in the foundation slab, and one in the screed floor.  They are numbered from 1 and 2 in the middle, 3 and 4 in the north-east corner, then clockwise until 9 and 10 in the north west corner. Odd numbers are at the bottom and even numbers at the top. 

You can see eight of these on the graph. The software from T&D will only show eight bits of data at a time. If you look at the graph you can see the lines at the top moving up and down rapidly, fluctuating with the temperature in the house, in turn affected by the outside temperature. The bottom lines, at the bottom of the slab, are much more sedate. 

One highlight is 6th July when the windows were installed and the fluctuations at floor level were quelled. 

The weather changed after the middle of July and it got a significantly cooler. Luckily this was just after we got back from a camping trip. You can see the peak of the temperature at the top around 20:00 on 16th July, which didn't reach the bottom of the slab until 05:00 on 19th July, two and a half days later. 

As a thermal system, I think there are nine ways in which heat can move:

Going in:

1. From the sun to the screed

2. From the boiler to the screed through the under floor heating pipes

3. From the air in the room to the screed (when the room temperature is above floor temperature)

Going out:

4. From the screed into the room (when the floor is warmer than the room)

5. From the screed through the walls around the foundation to the external air.

6. From the bottom of the foundation to the ground under the house.

Within the slab:

7. Up and down (depending on temperature difference between top and bottom)

8. North-south (especially when the sun is heating the floor in the winter.)

9. Through the underfloor heating pipes (when there is a big difference between north and south).

This heat will all pass according to the second law of thermodynamics, that you can hear more about here on you tube from Flanders and Swann.

According to the calculations in the Passive House software, for the heating season between October and April, 5,195 kWh of heat are going to come in through the windows on the south. January will get the most heat. This is a combination of fine weather and a low angle of the sun. On average there will be 31 kWh per day. January also has the coldest temperatures. 

I'm not sure how much of this heat is going to go straight into the slab. Some will hit walls or furniture, some will be reflected from the slab and the heat that does reach the slab may leave it quickly.

Also according to the Passive house software, most heat will be lost through the slab in February, and the figure it gives is 4.6 kWh per day. 

The slab is like a box, representing the structural foundation, filled with some gravel and topped with a screed floor.  According to the builder's invoice, there is around 40 cubic metres of concrete; 16.5 at the bottom, 4 standing up around the edges and 20 on the floor. At a density of 1600 kg per cubic metre, that's 65,000 kg. 

There's around 22 cubic metres of gravel in it, which amounts to 26 tonnes, if the density is 1200 kg per cubic metre. 

It's difficult to be sure of the specific heat capacity, but 0.8 kJ/kg seems a reasonable estimate. 1kWh is 3,600 kJ, so the whole slab holds around 73 kilowatt hours per Kelvin. In other words, if it drops one degree it will release 73 kilowatt hours.

In the very worst winter weather, the whole house will lose 55 kilowatt hours in one day, so even with no sunlight or heating, the temperature of the slab should drop by less than one degree. 

The temperature is being logged

Some data loggers have just arrived for the thermometers we put into the slab at a couple of levels. The recording all started back in the middle of winter at the bottom of the foundation, when we added sensors into the rebar.

Then we had all these sensor plugs growing like flowers from the wet concrete.

Most of them survived the layer of aggregate, then another sensor was added in the metal grid for the screed floor. I tried to get them to move the sensors as far as possible from the underfloor heating pipes, outlined in red.

Hopefully the sensor in the middle is around this position, although it may have moved when the concrete was poured.

After the screed floor, there's a few centimetres of wire to the plug. This is at the back of the house where the store room floor is a few centimetres lower and the slab a few centimetres thinner.

To avoid another decapitation, the carpenters very quickly made little boxes to cover the protruding sockets.

Tsukanaka-san arrived from T&D, and fixed the one we broke when the aggregate was poured in. I had tried to fix it, and stripped back the cover of the wire only to find three identical wires inside. I tried to fix them together, hoping for the best, but when Tsukanaka-san and the company president Morizumi-san plugged in the sensor, there was no reading. The president pulled my work apart, and reconnected two of the wires. One is apparently a dummy. He got the right two wires first time!

When the data loggers arrived they had to make bigger boxes.

You can see from the readings that there's already over 2 degrees difference between the temperature at the top of the slab to the bottom. There was also a difference from the west side of the slab, which gets some sunlight in the evening, while the other parts are in the shade. We can track the temperature over the next few months and get some idea of its reaction time, which should help when we start operating the underfloor heating. I'm hoping to some extent we can build up heat at the end of summer, and then release it over the winter so the slab is as cool as possible when summer hits. Probably not enough thermal inertia though. 

Each logger has an index number from 1 to 10, so we have started in the middle of the house with number 1 at the bottom of the foundation, inside the insulation, and number 2 in the concrete floor. Each logger can store 16,000 readings, or around 110 days' worth of data. The batteries last six months, so we need to fix the loggers where we can get to them. They have extension cables, but three pairs are positioned in or under cupboards, one is under the stairs and the other is in the storeroom. Maybe the cupboards should have removable floors or something. The data can be collected by a radio collector, which can then put the information into a computer.

There are fifteen channels, so we can record the room temperature and humidity in a few places, possibly outside as well. Also I was wondering about recording the temperature in the air channel under the solar panels, which I think is going to get quite hot and impair the power generation.