How to keep it cool

In the efforts to identify the impact of human activities on the global climate, space technology and satellites have proved immensely valuable for monitoring the planet and collecting data. But what if the same technologies that keep satellites up and running could help to reduce CO₂ emissions at more down-to-earth level too? A cooling technology that was developed at NLR could help reduce the energy needs of the very data centres that are processing the data from the satellites themselves (among other applications).

Satellites have become an essential part of our daily lives, providing access to information, ensuring communication and providing positioning across the globe. The key element for achieving all this is electronics. Back in 1969, NASA put a man on the moon with electronics that had the total power demand equivalent to a modern-day hand-held calculator. The power demand for space applications has been multiplied by a huge factor between the 1960s and 2022, and a single satellite can in some cases require the energy equivalent of several household water boilers.

With high power comes high heat generation. As the power demand increases, so does the heat generated by the electronic components. Even a mobile phone can reach high temperatures. The same counts for applications in space. Undesired temperature rises risk damaging the electronics. While on Earth you can replace the electronics if they fail, in space this would be quite a challenge. To extend the lifespan of the electronics and hence of the satellite, the electronics need to be reliably cooled – or to put it another way, you have to get rid of the heat generated.

Back in 1969, simple copper wiring was enough to remove the heat and disperse it away from the electronics it could damage. However, with the current level of heat generated, copper wiring is no longer a feasible solution. But why is cooling even needed for a satellite? In sci-fi movies, we are told that space is extremely cold, with temperatures close to absolute zero. Couldn’t just we “open a window up there?”

The issue is that in space there is no air as a carrier to dissipate the heat generated by the electronics, so satellites need cooling systems to keep the electronics at operating temperature. The principles available are the same as those used in a domestic heating system: conduction, radiation and forced convection.

In daily life, these three are easy to identify. Radiation is heat transported as electromagnetic waves, and is familiar from heaters, for example. Radiation can be noticed when there are large temperature differences: a fire, light bulbs and so on.

Conduction is heat transport by direct contact: literally one molecule passing energy to the next: as heat is energy, heating a metal rod (for instance) means the molecules start vibrating more rapidly, making neighbouring molecules move as well, bumping into each other like billiard balls. How ‘much’ a molecule can move is therefore a rule–of-thumb measure for how well the material can conduct. Metals are good conductors, whereas plastics or wood are not.

Another form of heat transport, probably the most well-known one, is convection. This is closely related to conduction, as it involves motion . While in conduction heat can be transported by single molecules, for convection it is transported by motion of a medium: hot air moving or boiling water rising to the surface. This movement is caused by an external force; for example, variation in density due to gravity, known as ‘natural convection’ (e.g. hot water is lighter than cold), but it can also be accomplished by ‘forced convection’ (e.g. using a fan or pump). So there is the catch: in space, there is no air to move or gravity to help the density difference, so natural convection cannot happen. Opening a window will therefore not help to transport the heat.

Currently available systems
At home, your house is heated up by a boiler and then the heated water is pumped through pipes to the radiators, where heat is dissipated and it warms our living rooms and bedrooms. The cooled water then is pumped back to the heater. Such a system is called “one-phase system”, as the water remains liquid. The system is pretty simple from a technological point of view, and is also a well-established system. However, it is far from precise; this motivated NLR to research how this system can be taken to a next level. To be able to dissipate heat more efficiently, NLR focuses on a more complicated process. To go back to the example of water: heating a litre of water from 20°C to 100°C takes a small amount of energy, while evaporating that same amount of water will take a larger amount of energy. Evaporation is a “two-phase” process, as water changes state from liquid to gas. Evaporating and condensing a specific amount of water involves a lot of energy at constant temperature (called an isothermal process): as long as the water is at its boiling point, the temperature will not rise above 100°C (under atmospheric conditions), as long as the pressure remains constant.

Increasing temperature is not the only way to achieve a phase change. Temperature and pressure are related. So a change of pressure via a pumped loop, instead of heating liquid, can be used to evaporate and condense the liquid to transport heat. NLR has perfected this methodology and uses it on the International Space Station (ISS) to maintain test equipment at a constant temperature while at the same time removing the heat generated by the equipment.

“Having a more stable and uniform temperature will increase the performance of the cooling system; this can be achieved by the NLR design of a two-phase system,” says Ramon van den Berg, NLR R&D engineer. In a two-phase pumped cooling system, thermal energy is transported by circulating a fluid that evaporates and condenses at constant temperature. It is a smart but sophisticated solution for using the immense amount of energy a fluid requires to transition from liquid phase to gas to cool electronics. It is a more efficient solution than simply cooling electronics with cold water. It is a more complicated system, consisting of more parts than the traditionally used heat pipes. On the other hand, it requires a smaller volume of liquid, and thus build volume of the system. This allows for easier integration of tubes and much higher heat transference. An example of a two-phase cooling system for aerospace is shown in the figure above.

This technology is currently deployed on the ISS, but beyond that, it is significantly more expensive than conventional heat pipes. And with the trend of nanosatellites, cost is a major factor. To make this technology attractive, NLR is working towards further miniaturizing components of the cooling system , such as for example the pump . NLR developed a pump that can pump a few millilitres per second. This is part of the ‘mini mechanically-pumped loop’ (MPL). It is integrated within a CubeSat, which is a name used for smaller types of satellites. Their dimensions are expressed in units (or simply “U”) of 10 x 10 x 10 cm. To get a feeling for this size, the mini MPL is displayed in the figure below in a 2U CubeSat model (corresponding to 10x10x20cm).

Cooling back on earth
Cooling satellites is just the tip of the iceberg of what can be done with the technology NLR is developing. Space research also helps address challenges that are more literally down to earth, like those related to climate change. Such cases are usually about using satellites to monitor the climate and atmospheric phenomena, sea levels and the concentrations of greenhouse gases in the Earth’s atmosphere.
But technologies developed for space applications and satellites are known to find their way into applications back on Earth. One example is applying cooling technologies from satellites to one of the major energy demand sources: data centres.

According to the International Energy Agency (IEA), data centres worldwide consumed about 200-250 TWh in 2020 . The IEA states that data centres and data transmission networks each were accountable for about 1% of global electricity consumption in 2020.
That is a lot of energy and sustainable solutions to the current trend of building data centres in cold environments (such as the Netherlands) or deep in the oceans are not enough. Several sustainable solutions – using less energy – are already available in the Netherlands.

An energy plant
So why is NLR’s idea better? “As long as the liquid is in two phases (both gas and liquid), the temperature is stable and dependent on the pressure, so it can be tuned to whatever temperature is needed. The temperature also remains stable over a long distance, so the heat can be moved away from the data centre and used for other applications, such as for house heating, for example. What is referred to as the quality of the waste heat is much higher, meaning the reuse of energy is higher,” explains Van den Berg. So, while elaborate air conditioning systems are currently used to cool all the servers in a data centre, a two-phase system means that using air conditioning can be avoided, and it is possible to reuse a large proportion of the energy generated by running the servers as heat for other applications. In short: a data centre becomes an energy plant. This sounds easier than it is because cooling on the chips requires a lot of tooling and engineering, but the technology is there.

Of course, data centre cooling is beyond the usual realm of NLR’s commercial projects and European research projects within which NLR developed the technology. However, this does not mean that NLR’s solutions cannot be tested on other sources of heat than satellites.

For this reason, NLR is currently experimenting with this concept on a small scale, using gaming computers. Temperature stability close to the limits of the electronics is interesting for gamers, who are eager to push the performance of their computers. As well as this, NLR is looking to collaborate outside its traditional aerospace domain, to investigate how this technology could play a role in a sustainable energy transition.