Phase change building materials - natural heat storage in buildings

When a substance melts or solidifies, it absorbs or releases energy. Materials that do that at room temperature can help regulate temperatures in homes.

© Wall Paper Craze

This article was inspired by an excellent podcast that can be found at Positive Energy, where I have yet to hear a podcast that I didn't want to listen to a second time, so check them out if you're into building science. This particular one was about phase change building materials (PCMs) and how they can be incorporated into buildings to improve energy-efficiency by harnessing a naturally-occurring physical reality, 'the melting point'.  

One product that looked promising several years ago but doesn't seem to have made it to market is Thermal Core, a drywall product said to contain microscopic acrylic capsules of a substance called Micronal, a high-purity paraffin wax that melts at 23°C (73°F). In order to make the change from solid to liquid, these beads need to absorb energy from the air. 

With very little testing information available I'm not comfortable making claims as to how well this or any other such products work, how long they will continue to work, and whether or not it is cost-effective. But I love the concept.

Wax phase change drywall
Phase change drywall © Thermal Core

That said, here's how Thermal Core works - as the air temperature warms the drywall to the melting point of the Micronal beads, they change from solid to liquid. This process absorbs energy and cools the air of your house. Energy is then stored in your walls, in the form of tiny sealed beads of liquid that used to be solid when your home was 22°C. A lot of unwanted heat is removed from the air to make that happen.

The walls at that point wouldn't be any hotter to the touch; the energy is used to cause the phase change from solid to liquid. This would help balance temperatures in a building and could reduce, if not eliminate, the need for air conditioning. And it would all happen without requiring an external input of energy.

To really wrap your head around this, think back to the days before we had refrigerants. Ice was harvested from frozen rivers in winter, stored, and delivered all summer by horse and carriage to keep the original refrigerators cold, known then as an ice box. The block of ice sucked up the heat in the icebox as it melted, keeping food cold.

Using that example on a larger scale, you could in theory cool your house by having a large chunk of ice in it that absorbed heat from the air; but that wouldn't reduce energy consumption because you would need supplemental energy to cool it again in order to repeat the process. Were ice to melt and freeze at room temperature, that process would cost you nothing. That's the beauty of how PCMs can work in buildings, by having a room temperature melting point.

More on that in a moment, but to backtrack a bit, this builder is about to take some liberties in describing the laws of physics. This could leave some engineers and physicists cringing over terminology and my somewhat freewheeling explanation of the laws of thermodynamics, but I'm going to stick my neck out here anyway.   

One thing to realize, is that there is really no such thing as cold, there is only heat and less heat. Above absolute zero (–273°C or –460°F), all molecules are moving, and what we call 'temperature' is just a measure of that. So a thermometer in your home is really a speedometer telling you on average how fast the air molecules are moving.

Understanding phase change:

A 'change of phase' occurs when water (or any other substance) freezes, melts, turns from vapour to liquid or vice versa. This process either absorbs or releases a lot of energy as the bonds between molecules are either broken or created. For example, ice will begin to melt at 0°C as it absorbs energy from its surroundings, and it will stay at a constant 0°C temperature for the entire time it takes for that melting to occur.

Ice molecules are stuck in lattice, but oscillating in a fixed position. Left in a warm environment, those molecules will absorb energy from the air, reducing the velocity of air molecules (i.e. cooling the air). As the molecular vibrations of the ice increases, it breaks the bonds in the lattice, causing the ice to melt.  

Only when the ice has completely melted will your newly-formed puddle of water begin to rise in temperature again (assuming it is in a warm environment of course) until the next phase change occurs, when it evaporates. Each phase change absorbs energy from the surroundings, meaning, it makes the air cooler in the process. The principle behind phase change building materials is to take advantage of that process.

So, if you have something in your home that changes phase at room temperature, you can to a degree, regulate the temperature of your home with no supplemental energy input. That, in layman terms, could translate into a certain amount of free cooling and heating, due to the ability to absorb heat, store and later release it.

PCMs balance temperatures in homes
PCMs can keep homes more comfortable by reducing the swing between high and low temperatures of a natural daily cycle © Mindset Online

This is not some magic bullet that will eliminate entirely the use of air conditioning in summer; that energy is still in the building in latent form, and your PCMs would need to solidify before the sun comes up the next day or you will not reap the benefits of another phase change.

This can happen in your home by opening windows at night to expel unwanted heat, but this isn't always a functional solution if you think about office towers with windows that don't open. In such commercial applications though, what it can do is postpone the need for cooling until off-peak hours.

Even just that has very tangible benefits - by shifting the cooling load to off-peak hours, you take much of the burden off the grid during times of peak demand, and in provinces with wildly-fluctuating power rates, you could drastically reduce your cooling costs by having a lot of it happen overnight.

While there is very little of this type of technology currently on the market and it's impossible to vouch for its effectiveness, it's a promising concept that can and should be experimented with. Wax for example, can very effectively be used as a phase change thermal battery

The very idea of keeping a building at room temperature no matter the season, is something of an uphill battle against the laws of physics. We can't change or break those laws, but this is a case where we could use them to our advantage. 

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