I'm going to put my raspberry pi zero in a water/air tight box outdoors, but I'm worried that it might overheat with it preforming tasks all day and sitting in the sun. Considering that, I want a way to cool it down without breaking the water seal. That makes a fan, not an option. Can anyone tell me how to cool it down without a fan? I don't think a heat-sink is enough to solve the problem because there is no airflow to transfer heat to.
You've not told us which RPi you are using. For example, a Pi Zero consumes less power, and generates less heat than the RPi4B. That will matter quite a lot when you get to the detailed design stage; when you need to determine what your objective Junction Temperature is, and how much cooling you need to meet that objective.
Are Heat Pipes a Solution?
If we assume here that your RPi needs to operate with a minimum of throttling in a high ambient temperature environment, and in a sealed container, perhaps the best way to cool it may be a heat pipe. This is a guess of course - but this answer will hopefully give you some information to consider as you work toward a design. All of that is just to say this answer may appear to be out of left field to you. If it is, so be it. Employing a heat pipe in your situation is absolutely a sound concept, but the realization may be more of a challenge than you had in mind.
Simply put, a heat pipe is nothing more than an efficient heat transfer device. Instead of depending upon air currents to remove heat at the source - as a heat sink does - the heat pipe uses phase transitions of a working fluid contained within a closed tube to efficiently transport the heat away from the source. This video provides a better explanation.
Heat pipes - originally developed for space applications in the early 1960s - have since found wide commercial applications. For example: portable/laptop computers where space is at a premium, and cooling efficiency is paramount. It may surprise you to know that heat pipes are readily available in a variety of shapes and sizes, and some are quite reasonably priced ($2.89 for example ). And yes - heat pipes are even available through Amazon
Using a new technology is always has its challenges; but there are resources available online (1, 2, 3, 4), including at least one practical guide on the design and usage of heat pipes.
This Is a Heat Transfer Problem - It Requires Design of a Proper Enclosure
Whether you use heat pipes or not, the problem you must solve here is the mechanical design of the housing. As you've correctly noted, a heat sink inside a small closed container isn't a real solution. Heat pipes offer one solution for moving the heat outside the enclosure. You will need to weigh that up against other, alternative designs (e.g. mounting the RPi directly to a heat sink that is effectively one side of the enclosure).
Whichever design you settle upon, you'll need to know where the heat is produced. The primary source of heat on the RPi is the SoC module and the power management chip as shown in the following picture (credits to Tom's Hardware).
And so this all boils down to understanding your requirements, and managing the Junction Temperatures of the chips on the RPi. You'll need to consider your computing load as that will define how much heat your device generates, and how much throttling you can take. You'll need to know the ambient temperature ranges in your operating environment, including any solar loading on the case.
Once you've got a handle on that, you will be ready to calculate the specification for your cooling system (heat sinks/heat pipes/etc). The specification for your cooling system is the term RθJA in the equation below. Once you have this, you're ready to design an enclosure that will evacuate the heat necessary to keep your RPi running reliably.
TJ = TA + ( RθJA x PD ).
Mull this over, and let us know if you have any detailed questions.
You can move heat in two ways and by using the plastic sealed box it gets very difficult to use either conduction or convection.
Convection (movement of heat) will literally hit a wall of the plastic box and as you rightly surmise, the temperature will just keep increasing inside the box till the Pi gives up though heat. I would say that no box is fully waterproof and it is often better to put holes in a box to allow water to run out and some airflow (with a mesh to keep bugs out). Plastic is a poor radiator - metal is better.
Given your aim for a sealed system, you are only left with conduction and you need to cope with the CPU/WiFi/USB chips as the main heat production chips and either get the heat out via something like aluminium fins through the case or connected to the case side to act as the radiator.
Metal cases with seals are available (up to IP68 rating) so I would use one of these and allow radiation to help keep it cool.
This issues you need to think about in the design are:
- Can you use an Arduino or similar as they are more rugged and run cooler.
- Does the box need to be in the sun or can you put the unit in a sheltered spot?
- Can you use vented case such as these for weather stations?
- Can you bury the case in the ground?
- Can you use a bigger box?
A long while ago, Project Curacao documented the system they had running on the tropical island and it's worth reviewing that.
I do have one hot Pi zero - it's not doing a lot at the moment but the case it's in is only two inches deep and less than quarter of an inch wider than the Pi and it's been running for over a year with no cooling fine. It gets sun for about a third of a day (British weather allowing).
tl;dr Use a white case if possible with at least one metal side shaded to conduct heat away and then radiate it off to the shade.