a new chemical composite developed by researchers at MIT could provide an alternative. It could be used to store heat from the sun or any other source during the day in a kind of thermal battery, and it could release the heat when needed, for example for cooking or heating after dark.
A common approach to thermal storage is to use what is known as a phase change material (PCM), where input heat melts the material and its phase change -- from solid to liquid -- stores energy. When the PCM is cooled back down below its melting point, it turns back into a solid, at which point the stored energy is released as heat. There are many examples of these materials, including waxes or fatty acids used for low-temperature applications, and molten salts used at high temperatures. But all current PCMs require a great deal of insulation, and they pass through that phase change temperature uncontrollably, losing their stored heat relatively rapidly.
Instead, the new system uses molecular switches that change shape in response to light; when integrated into the PCM, the phase-change temperature of the hybrid material can be adjusted with light, allowing the thermal energy of the phase change to be maintained even well below the melting point of the original material.
The rate of cooling can be controlled.
Grace G. D. Han, Huashan Li, Jeffrey C. Grossman. Optically-controlled long-term storage and release of thermal energy in phase-change materials. Nature Communications, 2017; 8 (1) DOI: 10.1038/s41467-017-01608-y
(Score: 5, Informative) by Anonymous Coward on Wednesday November 22 2017, @03:45PM (1 child)
At one point I studied a number of the phase change heat storage materials. There are various salts with convenient melting points around "room temperature" which can store solar energy during the day and keep a house warm at night. One of the major problems with these is that the compounds degrade over freeze thaw cycles, separating into layers that no longer have the original thermal properties.
Clever researchers at MIT worked out that storing the phase change material in thin horizontal layers would greatly reduce or eliminate this separation/striation. They made ceiling tiles (looked like a normal hanging acoustic ceiling) with a few layers of material in thin horizontal plastic bags. Have forgotten the thickness, perhaps a few mm thick for each layer.
When combined with horizontal window blinds that reflected sunlight up to the ceiling, they built a super-insulated, passive-solar test building in Cambridge, MA. This was c.1980 and it worked well for many years. It was a little more expensive than normal construction, but perfectly practical otherwise. Not only do you save the cost of heating fuel, you also reclaim the floor space taken by furnace or boiler. I believe they installed some electric baseboard heaters for backup that might be required if there were several cold & cloudy days in a row--but because of the super-insulation, the power required was relatively low.
(Score: 0) by Anonymous Coward on Wednesday November 22 2017, @04:47PM
I think it's all really cool.
(Score: 0) by Anonymous Coward on Wednesday November 22 2017, @03:58PM (4 children)
So a phase change material is simply something that melts when heated and freezes again when cooled down? Such as … well, pretty much every material?
I suspect there must be more to it, or they would not have given an extra name to those. So what makes those phase-change materials different from, say, water?
(Score: 3, Insightful) by JoeMerchant on Wednesday November 22 2017, @04:41PM (2 children)
Water is a solid-liquid PCM at 0C, it's at a good temperature point to preserve food but has the inconvenient characteristic of expanding when frozen which makes it challenging to contain in other materials during liquid phase.
🌻🌻 [google.com]
(Score: 1) by anubi on Thursday November 23 2017, @10:53AM (1 child)
That is my number one problem with ice banks.
I keep damaging the heat exchanger that freezes the water. I end up "work hardening" the copper coil and it just disintegrates.
And I often destroy the container, same way ice formation will ruin an engine block and radiator in a car if you didn't add antifreeze.
"Prove all things; hold fast that which is good." [KJV: I Thessalonians 5:21]
(Score: 2) by JoeMerchant on Thursday November 23 2017, @01:45PM
I wonder if a flexible pipe heat exchanger might be longer lasting - not as efficient, sure, but if it's efficient enough to get the ice bank frozen using whatever process (solar is common) - I'd rather have a 30% bigger collector on a device that runs 20 years maintenance free, instead of a smaller unit that needs its coils replaced every 2-3 years.
🌻🌻 [google.com]
(Score: 2) by EvilSS on Wednesday November 22 2017, @06:34PM
Now that said, a lot of research goes into creating PCMs that are highly efficient, with specific properties and which will only really ever be used as a PCM.
(Score: 5, Interesting) by RedBear on Wednesday November 22 2017, @04:08PM (1 child)
Well that is just darned interesting. Reminds me of those reusable hand warmers filled with sodium acetate and a little metal flex disc. You boil it to liquify the contents, and it stays liquid when you cool it back down. But it's in a supercooled state basically, so when you flex the little metal disc inside it "snaps" and creates a shock wave that triggers crystallization, a phase change, and then it releases heat for a couple of hours. They get pretty hot.
So the molecules that change shape when hit with light act like the little metal disc in the hand warmer. Sort of. But also they keep the material from phase changing to a solid and releasing heat until you want it to. So the composite material can enter a sort of supercooled state that doesn't require insulation to retain the ability to release massive amounts of heat energy later when triggered. Just like the hand warmer, but maybe with tunable temperature ranges for different purposes, based on what you attach to the shape-changing molecule. Fascinating.
But... I guess that means I have to acknowledge that supercooled water also releases heat when it's triggered by a shock or a crystallization nucleus to suddenly phase change into ice. That's a very unintuitive concept but seems to be correct according to some quick googling. Physics is weird.
¯\_ʕ◔.◔ʔ_/¯ LOL. I dunno. I'm just a bear.
... Peace out. Got bear stuff to do. 彡ʕ⌐■.■ʔ
(Score: 3, Interesting) by JoeMerchant on Wednesday November 22 2017, @04:46PM
If they can keep the liquid state with some low-energy LED lighting, control is getting pretty good - and convenient: turn the heat-hold lights on at dusk, then switch them off later in the evening as it starts getting cooler inside.
What I want is something that "holds cold" around 50-60F that you can switch to heat sucking mode at will, perhaps around 3pm when the afternoon heat starts baking through the insulation. Then, the next night, reject that heat back to the atmosphere, or possibly groundwater say, from a drinking well. And it needs to last forever, cost nothing, and look good while performing its function. Or, at least outperform conventional freon based A/C systems on those parameters.
🌻🌻 [google.com]
(Score: 0) by Anonymous Coward on Thursday November 23 2017, @01:58AM
A goddamn way to make use of all the hot air around here XP