Science Buddies: "Ask an Expert"

Hi,

I have a group project to design a better fridge door (easy access to food and high energy efficiency). We are considering a transparent door made of argon gas in between two panes of glass similar to low energy windows. A clear door would let the user view the food before opening the door, saving energy. Condensation might be an issue but this is simply a concept design. Obviously it would be reasonable to keep it low cost. Here are some questions we have had trouble answering.

1. Will argon within glass prevent enough heat from passing into the fridge?
2. Will heat from sunlight heat up the fridge?
3. Is there a way of quickly tinting glass that could be used to tint it until the time someone wants to get something? Then they could press a button to clear the glass and view the contents.
4. I've heard a vacuum will not allow heat through, so could we maintian a vacuum inside of two panes of transparent material and use that as a door?

If you can help with any of these questions it would be really helpful

Thank you!

R. Rainville

That's a fairly complex topic. Either argon or vacuum could be helpful in a double-layered design.

Air is a pretty good insulator - it doesn't conduct or store heat nearly as well as metal or liquids or even most solids (this is why windows in cold areas are often made with two panes of glass) but it has a problem that it can conduct heat by convection. Convection is where a bit of the air heats up, and then moves because its density changes (hot air rises, cold air falls). This has the effect of transmitting heat, and sometimes this is undesirable, sometimes useful.

I would expect a noble gas like Argon to have similar properties to air, but it could be a better or worse insulator - the usual reason a noble gas like Argon is used is because it isn't reactive - air (or anything with oxygen in it) can be corrosive to metals or have other undesired effects with sensitive equipment. Geiger counters, for example, use a noble gas inside partly because ordinary air will react with the wire+current that is supposed to be affected only by radiation and skew the results. I'm not aware of the insulative properties, but the information is probably out there if you look for it.

Vacuum is immune to convection, there is no medium to transmit the heat. That can make it useful as an insulator. However it does nothing to stop radiated heat (the sun's heat reaches the Earth just fine, after all).

A refrigerator with a transparent door would have to be kept out of direct sunlight, as visible light would pass through the door. Any radiation not stopped by the door would eventually be stopped by the contents of the refrigerator - and it would then transform into heat. (this effect is part of why your automobile heats up so fast in the sun - ultraviolet and visible rays are let in, they are absorbed by the opaque leather/plastic/metal of the car and radiate heat. Heat then is stopped by the glass windows, because they're fairly decent insulators and are not transparent to infrared radiation).

Plexiglas is pretty reactive - that's useful when you want to shape it and attach it together - you seal plexiglas with substances that dissolve it and when the "glue" evaporates away, the two pieces are just as strong bonded as they would have been if they were created as one piece...that also makes it fairly easy to seal. It's also soft and easy to machine. But I don't think you'd need argon (as opposed to air) with plexiglas, what it reacts to are organic solvents. That could be a problem with a refrigerator door - alchohol is an organic solvent and if somebody spilled high proof spirits like vodka or whiskey on your door, it might cause it to discolor/go cloudy instead of clear. (it takes a lot to burn through enough plexiglas to make a refrigerator door, but not much to ruin the transparency). Window-quality plexiglas would be ok with respect to reacting to the sun - it isn't like some plastics that would degrade quickly.

Plexiglas is a decent structural material, but it behaves kind of like wood - it isn't easy to keep it transparent if you want it in a funny shape. You'd probably want a plastic or metal frame, with plexiglas panels, rather than the whole door made of plexiglas. No matter how you do it, you need to think about how you'd suck the air out (vacuum) or introduce gas (argon) if you want an insulator other than air between the panels.

All that said - a lot of oven doors have small windows (double paned) and it works out ok. Perhaps a design with windows in key locations instead of the whole door transparent might work better.

Anyway, that's some stuff to think about.

Ah...and doing it with glass instead of plexiglas helps in some areas (reactivity, resistance to scratching etc) and makes it harder in others (you would absolutely need to go with a window-approach, glass is not a good enough structural material to make a whole fridge door out of - too heavy, too fragile.)

If it was my design, I'd go with glass, swipe the initial design from how oven windows are done and think about how to place the windows so you can still have storage space in the refrigerator door, but see the contents of the refrigerator.

Thank you for your input it was extremely helpful, especially about the reactive properties of plexi-glass. That is a consideration that none of us thought about.

After talking with a chemistry teacher today we decided on using a vacuum as opposed to argon or another gas. He mentioned that there might be a coating to counteract the passage of the sun's heat through glass. We found one example of something similar to what we want at:

http://www.glacierbay.com/ultra-r.asp

The models on that site do not have clear exteriors but they appear to be low maintainance and probably our design will incorporate part solid door and part clear area to see the food.

Another idea that we came upon was using a vertial slider door. We were considering how opening a standard fridge door creates suction when opening and can allow warm air to rush in. You mentioned that it probably wouldn't be possible to make a full size panel vacuum anyway so we thought of making two panels that were partially glass, partially solid (each containing a vacuum). These 'combo' panels would be one over the other. The bottom one would slide up behind the top one and visa versa. This would avoid the use of hinges for moving the heavy glass and instead we could use sealed greased bearings and possibly a spring system to help in raising and lowering the doors. Unfortunately it wouldn't allow for putting food on the door but hopefully the energy savings would outway this inconvenience.

Thank you again for your help and if you have any more suggestions they would be greatly appreciated.

R. Rainville and Group

A slider mechanism is interesting - it certainly would be preferable to putting a hinged window on the door, but I'm not sure you'll be able to replace the whole door with one. Refrigerator doors are heavy and there are space considerations (a vertical slider needs enough "headroom" to slide...or "footroom" if you slide down).

Yes, you can treat or coat glass to block UV rays and thus most of the heat energy coming from sunlight. There is a lot of interest in transmitting visible light and not transmitting other kinds of radiation, so there has been a lot of research on this topic. Most modern sunglasses, for example, have UV opaque coatings to help protect your eyes from bright, direct sunlight. I've seen some early studies on doing similar things for cars, to reduce how fast they heat up, but that was 15 years ago and I don't know if anything came of it.

I don't know if cost enters into your design, but a lot of things don't get done for that reason. Another thing to think about is the vertical refrigerator/freezer plan vs horizontal one and how this would affect your design. People who have trouble reaching high places often go for this kind of design, even though they're not as energy efficient as the traditional model (the long vertical divide between the freezer and refrigerator is harder to insulate than the floor of a top-mounted freezer).

Hi...

When engineering devices, it helps to have some numbers for the possible materials handy. Here are the thermal conductivities for the gasses that might be inbetween your two plates of glass:

Ar (argon): 17.72mW/m*K (milliWatts per meter-Kelvin)

N (nitrogen) 25.83mW/m*K

O (oxygen) 26.58mW/m*K

Air is 20% Oxygen and 80% Nitrogen. Notice that the thermal conductivity of the two is very similar... but Argon is significantly lower.

I hope this helps in understanding why argon is used for thermal insulation.

I would go to the home renovation store first. Look at double-pane window glass panels that are already mounted in a frame. There are some models that also include a thin plastic sheet between the glass sheets. The plastic sheet has a vacuum deposited layer of metal that acts as a selective reflector. Visible light can pass but infrared cannot. The assembly also has argon gas in it.

Warming, quality is proportional to price.

The vacuum ideas is good, but has some limitations. The two panes will be at atmospheric pressure on one side and zero pressure on the other. A decent-sized window would require one-inch thick glass for structural strength. Not a good choice for a household refrig.

Regarding the condensation: Go study the refrigerator case at the grocery store. Usually clear while closed, they get fogged over when open since the cold inner surface is below the dew point of the room air. When you close the door, the fog evaporates inside the case, since that air has low humidity from the action of the refrigeration system.

Another idea is a micro-sized camera inside and a flat panel display on the outside.

Robert Reavis

Everyone's postings have been very helpful in putting together this project. This afternoon a group of us are going over to Home Depot to investigate different types of glass and then tonight we are meeting to put together a power point presentation of our project. We will be presenting next week and I know this posting has really helped us to be prepared,

Thanks again,

rainviller and group

Hi,

It is unlikely tht very much UV light will get to the kitchen. The more wavelengths (colors) of light that you block, the better. Optimally, you would like to pass green light, to which your eyes are most sensitive and block everything else. In particular, you will want to block infrared light which you can't see but which is emitted by warm objects. Incandescent lights, for example, mostly emit heat (infrared light), only a modest amount of visible light, and essentially no UV light.