Thursday, 4 October 2012

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.