PCM Heat Sinks, Proven Design Considerations For Space

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Hello, My name is Bryan Muzyka and welcome to PCM Design Considerations for Space. So today we’ll be talking about phase change material, heat sinks and everything, and anything you need to know to integrate them into your space systems.

So, it is my privilege to introduce two extremely well-versed aerospace engineers here at ACT. Ryan Spangler, He’s joining us, with years of experience here at ACT. In Aerospace engineering he has done many of these phase change material heat sinks (PCM) for spaceflight consideration, qualification programs, and testing programs. So, he brings a lot of experience here.

And the other speaker is Clayton Hose. We tend to give him, myself and customers included, a lot of fires to put out and he has a way of solving them very quickly.

Moderator: Again, with that, I’ll give a quick overview of ACT, just for those who are joining us for the first time. And then we will pass it over to Ryan and Clayton before we go into live Q&A at the end.

So, a quick background on Advanced Cooling Technologies (ACT). We are a thermal management solutions provider here in Lancaster, Pennsylvania. We’re a growing small business, with over 240 employees, and over 140,000 square feet. We do the full gamut of thermal solutions, so everything from very low cryogenic temperatures to very high liquid metal temperature ranges in all types of diverse applications, but we will be talking today about space-type heat sinks. We also do military-grade, highly reliable, heat sinks and heat-transfer devices, as well as industrial applications. So, we have a lot of fielded and proven technologies, all focused on kind of high reliability and high-performance thermal management solutions. You can see some of the examples we have there over to the left side of the screen. We do focus on highly innovative, cutting-edge solutions.

Our core values here are innovation, teamwork, and customer care. So, we like to work very collaboratively and very closely with our customers to bring newer technologies to the industries and go through the entire gamut in Space Technology. Working closely with the space industry, we are very well versed in understanding the design requirements conceptually and coming up with the most appropriate thermal solution, and then going through all the qualification, acceptance testing requirements to make sure that it’s spaceflight ready.

So, with that, I want to thank everyone for joining us today, and I will pass it on to Ryan and Clayton to give insight into everything you need to know about PCM (phase change material) heat sinks.

All right, so as Bryan said, I’m Ryan Spangler. I’m going to walk you through the first portion of the presentation here, I’m going to start off with some space considerations that are a little bit unique relative to typical terrestrial applications.

Ground Vs. Space Thermal Considerations

So, in space, I guess the most obvious constraint or unique design criteria is that you don’t have any sort of convection. The ultimate heat sink you have to move all the energy to the radiator panels where it’s dissipated into space via radiation.

That applies to or operates under a kind of a Stefan Boltzmann equation. The higher the temperature that you’re operating your Radiator Panel at, the more effective or efficient they’re going to be at dissipating that heat.

Phase Change Material (PCM) Heat Sink Design for Space

I guess one consideration for the PCM, then, is because it operates between the sources that you’re trying to cool and the radiator panel, typically, it’s going to actually operate at a lower temperature. So, you want to be a high temperature for the PCM to enable a lot of dissipation to space, but you have to make sure you’re below Max electronics temperature once all of the delta T stack up is considered. So, that requires you to have a lot of information regarding max component temperatures, and the overall stack up back to the sink, Obviously, reliability is key. We can’t send any sort of maintenance crews to space and we touch on that a little bit more later.

So, you have to have a good understanding of your overall mission profile, which generally, if you’re at this point, you have a pretty good idea of at least worst-case, and you want to avoid any moving parts that are higher risk or tendency for failure.

So, again, just like any other application, whether you’re flying terrestrially or out in orbit, you want to have low mass and volume so that you can sub more of the important components in there to get more capability out of your system.

So just a little bit of background on what exactly phase change material and heat sinks are. They are a fully passive two-phase heat storage device, kind of like a thermal battery.

Typical Phase Change Materials (PCM)

Generally, it operates with a paraffin wax, which is the most common type within an enclosed system. It’s designed to have a phase change either solid or liquid, or liquid-solid, to absorb heat energy without allowing a significant temperature rise at the source or degradation at the source.

The way it does that, is it actually uses that latent heat as it goes through the phase change, which is orders of magnitude higher than the specific heat of the material.

Some paraffin waxes actually have multiple phase changes, and will go from solid to solid, and then solid to liquid. So, identifying where those phase changes are, and how much energy is absorbed over that temperature is critical for the design of the system. But the whole time that it’s going through that phase change, it’s somewhat preventing a terrible temperature rise or fall if you’re trying to prevent it from getting too cool at the source.

So, as I said, the most common PCM used is paraffin wax for the temperature ranges that we see. There are also salt hydrates. They present certain challenges being that it’s a salt-based phase change material. So, it does have a tendency to be more corrosive, therefore material selection becomes much more important.

And then you also have metals. You know, solders are technically phase change materials, and depending on the temperature that you want that phase change to occur and how much energy you want to store any match requirements, metals might actually be the ideal choice for you.

Thermal Conductivity in PCM Heat Sinks

PCM heat sink solutions do come with a challenge in that at least the wax-based and the salt hydrants have a fairly low conductivity, even sub-one watt per meter Kelvin. So, getting the heat effectively into the PCM volume generally is the most critical or difficult task of the full system design.

So, there is some optimization between how much PCM you can have versus additional conductance or conductive elements into the wax, typically fins, either folded fins or machined fins.

And so, here, iterating the thickness of those fins, and the quantity or spacing of those versus how much PCM you have helps to determine what are the appropriate choices. That’s something we have a lot of experience in and are able to do fairly simply.

When do you want to use PCM heat sinks?

There are a few different criteria that would fit the envelope or the mold for these types of systems.

  • If they are to be short, and that’s relative speed and short duration applications. So, let’s say it’s a fail-safe and you need to keep something. from over-temping for 30 minutes until you have the opportunity to shut off all your electronics, that might be a good opportunity to utilize phase material system.
  • Duty Cycle operation; if you’re below 50% duty cycle, at or below 50% duty cycle, it may be a good technology to investigate because it’s able to absorb the heat over that 50% and then it has sufficient time to resolidify for the remainder of that operational cycle,
  • For redundancy. I’ve already talked about the thermal runaway, and there are also instances where, let’s say you’re trying to dock to a system, where you’re trying to prevent the temperatures from falling too far before you’re able to re-heat the system I another good use for these in both directions.
  • There are potential applications for solder fatigue dampening. So, in some systems, you’re going to have fairly large temperature swings at the payload. So your electronics components are soldered in place. You’re seeing those and the CTE mismatches are causing strain within your solder joints. This can help dampen the magnitude of those temperature swings.
  • We see instances where it can be useful for both cubesats, small satellites, geo, or large satellites, and various scientific missions.

Why would you use PCM heat sinks?

  • They’re reliable and passive
  • And as we said, before, you know, Clayton has yet to get his license to be an astronaut repair man, and I certainly will not be going up because I’m scared of heights and so, you want your thermal solution to be reliable and passive
  • Enable a lot more capability out of your spacecraft by allowing you to use higher power amplifiers or just more powerful components than you would typically be able to use because you’d be constrained by your radiator panel surface area, the amount of power that it can actually put out and reliably dissipate.
  • It’s got no moving parts–  it’s stable over thousands to tens of tens of thousands of cycles
  • Weight and volume benefits. So, in instances where you do have the opportunity that you can size radiator panels for peak load, putting a PCM volume in there to drastically reduce that load for a duty cycle application and can help reduce additional design work, to have those additional radiator panels, ending up cheaper, lighter, and occupying less volume on your satellite.

Orbit Considerations

So, again, there are certain on-orbit considerations, depends on where you’re actually going to be pulsating or, where this needs to operate. Are you seeing an eclipse? And that is something that we would take a look at in terms of the thermal design with an orbital profile, the worst-case, both hot and cold.

Radiator panel size is dictated by the thermal load

So, integrating that PCM can drastically reduce the size of radiative panels needed, because you can size it for actual time-averaged dissipation rather than the peak load.

So, one example we’ll get into in a little bit is if you have a 100-minute orbit, you’ve got a 10-minute long period, and you’re trying to dissipate 100 watts. Your radiator panel goes from needing to dissipate that hundred watt pulse load to a time-averaged approximate 10 watt load over the full orbit.

The heat source for survival. Again, if you have long-duration cold states, one example might be docking to a system where you’re disconnected from the electronics of the actual craft, PCM can help prevent it from decaying to a worst-case temperature. And there may be some applications associated with lunar night survival.

You want to take some next steps, at the top there, above the plot actually has this opportunity where you can, we print out the PDF of it and submitted even to HCT.

Combining Technologies

Discrete Heat Loads

So you either have discrete loads or you can’t put your source directly at the sink. So in some cases for the discrete heat loads, for example, let’s say you have that four-inch by four-inch box, but your heat load is a half-inch by half-inch square right in the center. So you’re going to have to have either a very thick PCM module, where most of that 4 by 4 area is not being utilized, and then you will have a tremendous Delta T through it, or you need something to help you conduct in-plane before it gets to the PCM to help maintain a low or reasonable delta T. In which case, one option that you have is utilizing heat pipes for spreading in-plane there, before it gets into the PCM. Those heat pipes, since they’re in front of the PCM, and the resistance at work, are going to have to see and move that full 100 watt of power.

Transport to Radiator

In another application, you do have your PCM at the source, but it’s a good distance away from the sink. You can use either a CCHP or a copper water heat pipe to take that heat from the PCM, to the Radiator Panel and even spread it in-plane to get a nice efficient dissipation out of the panel. In that case, you’d see time average power. So, calling back, again to the verbal math that I did earlier, and the example we just touched on hundred watts over 10 minutes. You have a 100-minute orbit, which is a 10% duty cycle assuming you’re also dissipating as it’s storing. That results in about 10 watts for the heat pipe to transport. So, a significant reduction obviously on what the heat pipe and thus the radiator panel see.

Typical Technology Combinations

First is kinda the old word workhorse for these systems: aluminum-ammonia CCHP which with ACT have over 50 million on-orbit hours without failure currently, they’re very good at transporting a lot of energy very long distances.

There are some limitations in terms of ground testing. They have what is referred to as an axial grooved extrusion. So, it’s able to move a lot of power whenever there’s not a gravity head, but it’s just not able to wick well against gravity or pull liquid back up to the evaporate. So, there’s a certain constraint that that will put on your integration and testing side of things.

You are also somewhat limited in the heat fluxes. Generally, once you get to the bus or radiator panel side of things, it’s reasonable there, and that’s where these typically see a lot of use. But if you’re directly at the source, they’re generally like a max of 5 to 10 watts per centimeter square, which can be inhibitive.

The other option, which I talked to, was the copper water heat pipe, something that we do have space flight heritage with as well, and are seeing operation currently. They don’t offer quite the distance to transport heat, however, they do afford you the opportunity to, in some cases, test on the ground in any orientation. And they’re generally good for the temperature ranges your electronics components are going to work in. And if you have significantly higher heat flux, they’re able to receive that relative to the CCHP (constant conductance heat pipes), where they were 5 to 10 watts per centimeter squared these SCWHPs can handle about 50 watts per centimeter squared.

 

PCM HEAT SINK DESIGN CONSIDERATIONS

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