Solar Film

I couldn't resist.

All kidding aside, a timely opening and closing of curtains will help.

Oh my

I would use diffusing glazing materials and monolithic warming from stone interior....so the windows scatter the suns rays to the walls and floor made of stone, the stone retains the heat and releases it slowly overnight....a passive heating system

https://azsolarcenter.org/passive-solar-heating-cooling-manual-part-2-of-4

Great User Name for this topic, You have the right Idea,

Here I will give the exact use I am needing this for.

This is for one of our Student High altitude balloon experiments. Like this oner we did for several hundred Boy scouts.

https://youtu.be/HJ0IT4ZwtSo

Most of these flights only last 3 or 4 hours so the terrible cold temps are not a terrible Issue.

But we are working on a long duration flight. and during the day it would be nice to be able to absorb as much solar energy as we can before the sun goes down, and it really gets cold.

90+% of the heat loss because of the very high vacuum is lost due to radiation and a little conduction.

so thinking of a film that will allow the full spectrum in to be absorbed, but then blocks any heat from being re-radiated back to space.

Joe

I would use a chemical reactor for heat and styrofoam for insulation...

https://www.amazon.com/UniHeat-Shipping-Warmer-hours-pack/dp/B00ANPY5TY...

Some of the students thought of that, But with the lack of O2, they woult do anything. Those need O2 to generate the heat.

Maybe an oxygen generator could be designed, now there's a challenge...

...."The oxidizer core is sodium chlorate (NaClO₃), which is mixed with less than 5 percent barium peroxide (BaO₂) and less than 1 percent potassium perchlorate (KClO₄). The explosives in the percussion cap are a lead styphnate and tetrazene explosive mixture. The chemical reaction is exothermic and the exterior temperature of the canister will reach 260 °C (500 °F). It will produce oxygen for 12 to 22 minutes.^[2][3] The two-mask generator is approximately 63 mm (2.5 in) in diameter and 223 mm (8.8 in) long."...

https://en.wikipedia.org/wiki/Chemical_oxygen_generator

https://www.collinsaerospace.com/what-we-do/Commercial-Aviation/cabin/Oxygen-and-PSU-Systems/Passenger-Oxygen-Systems/Chemical-Oxygen

For a few hours, there are some exceptional low temperature insulators which could ditch the heating requirement.

For higher temperatures, Microtherm insulation is pretty good, but I have seen a low temperature material which is multiple layers of reflective foils.

The other great material is aerogel.

Sorry, I misread the duration.

How about very good insulation combined with a suitable insulated heat pipe, as used on hot-water heaters but with an appropriate working fluid such as a hydrocarbon.

As this is a balloon project and double pane glass would be prohibitively weighty, consider using low-e window film.

There are numerous manufacturers/retailers. Here is one example: https://www.3m.com/3M/en_US/thinsulate-window-films-us/

All,

I am surprised!! Truth... was the only person to mention low-e window films. This technology has been known to the solar energy crowd for 20+ years.

The basis for its performance is choosing a coating on the film that has a very low emissivity (and thus a very high reflectivity) at the IR black-body temperatures corresponding to typical room temperature (close to 300 deg-K) and very high emissivity (and a correspondingly low reflectivity) at the IR black-body temperature of sunlight (close to 6,000 deg-K). This stuff is commercially available and works very well.

--JMM

This looks like a fascinating project. I hope you will let us know how it comes out.

Here are my thoughts:

I would have an outer container, transparent and double-walled to minimize conduction. Inside this would be a material colored flat black to absorb the radiant energy from the sun and to convert it to heat. Almost any transparent material is opaque to long-wave infrared (heat), so the heat will mostly be trapped inside. Clear plastic is light and double-walled clear plastic containers might be available commercially or could be fabricated.

Inside the container, your payload should be thermally insulated. Styrofoam is an excellent insulator. The thermal mass of the payload combined with the thermal insulation would tend to even out the extreme day/night temperature fluctuations, similar to the action of an RC electrical circuit. The insulation would keep it from getting too hot in the daytime and too cold at night.

These are just back of the envelope ideas. You might want to test it here on earth by putting a temperature sensor with your payload, leaving it out in the sun in the daytime and stashed in a freezer at night.

I am looking for a film, That will allow as much of the Sun's heat energy to pass into a space,

Yet after the Sun goes down, any of this heat inside is blocked from escaping the inside area back to the outside.

Would not a greenhouse do what you want?

Visible light, when it is absorbed, is converted to heat. Glass is transparent to visible light but opaque to far-infrared radiation (heat). Double-paned glass is even better as it blocks heat conduction as well as radiation.

Would bubble wrap suit your purpose for I have seen aluminium foil on one side and white on the other? Maybe you could experiment with different coatings to ascertain the heat transmission and retention?

However I do not know what the low air pressure outside the balloon would have on the small air cells at high altitudes, maybe just pop.

Small cell wrap is ok they do not pop, large cell every one pops.

There may be a photochromatic coating that behave in the reverse of most modern glasses, so that when UV is present, it is clear and when the sun has set, the reduced UV would then darken the coating.

An optometrist might be able to provide company details.

Just put a small heating element inside your package (right where you need it most)

The type found in a heated shoe inserts, sock, glove, jacket etc.

Use a lightweight solar panel to power the element and insulation to retain the heat.

Many of these heat pads have a USB cord and are powered by those mini phone chargers..

Which can also be changed by solar.

Good luck!

May I suggest a clear panel such as glass and a second panel some convenient distance apart where the void between can be filled during non daylight hours to insulate. Common products are soap bubbles as used in green houses, maybe Styrofoam beads or some other insulating material that can be blown in and out as desired. Maybe even an insulating gas?

The trouble with any cellular insulator such as bubbles, they will expand at altitude and possibly burst.

Just use Rayban glasses reversed.

Simply Amazing,

I contacted 3M and describes what we are looking for here, and this is their reply

==========

Dear Joe,

Thank you for contacting 3M, where we apply science to life. We appreciate your interest.
In evaluating your specific application, we currently do not manufacture or market a product which would meet your needs.

If you have additional questions and/or concerns please don't hesitate to reach out.

Regards,

========

I replied to that with like Honestly? A plain Glass window can do this, I was expecting that 3M had something similar that performs better than a plain old piece of Glass.

no answer from them yet.

Wow!

You might ask them about their product Thinsulate 75. It may not be ideal but it is a low-e film, normally designed for application to glass. For you, you may plan to apply it to a different transparent film, such as mylar. This type of film has a emissivity near 1 in the visible and near IR ranges and close to 0.1 in the far IR range (indoor temperatures). There was another product I saw when I did a search on "Low-e window film", but I couldn't find any reference for its manufacturer.

Some posts say regular glass is good for this, but its emissivity is still close to 1 even into the far IR range. For your balloon work you wouldn't want to use glass anyway because of its weight.

--JMM

Solar panel + small battery + heating element + insulation. = About $100 / success

A window? Is that how they heat components on any known satélite? (Don't Say earth)