3 methods for implementing two-phase cooling for embedded vehicle CPUs.

As vehicles become increasingly connected and autonomous, the thermal demands on embedded processors (think ECUs, ADAS, compute modules, etc) are rising exponentially. These systems must handle ever-higher workloads while maintaining reliability due to their ever more critical role in the vehicles operation. They echo the trend being seen in the data centre industry, but are arguably more difficult to cool given the environment.

Traditional cooling methods are often unable to meet the cooling needs of the next generation processors and with each year it becomes an increasingly difficult challenge to resolve. It is proving difficult for all the automotive OEMs let alone those providing autonomous vehicles like Waymo, Cruise and Zoox where the compute demand is on a different level to the latest vehicle your or I might be buying from a dealership.

The last thing any engineer wants is to encounter thermal throttling of processors as the limited functionality could directly affect the vehicles safety systems. Two-phase cooling offers a different approach to resolving this challenge and one that we believe brings direct benefits that alternative high-performance cooling methods cannot match. Remember a vehicle is not the kind of safe, pristine environment that exists within a data centre, users rely on it everyday, rain or shine.

Traditionally these systems have been enclosed in a box of some sorts and might even be upgrade-able later on. They can be placed anywhere in the vehicle, but typically they are found inside the cabin as it is well protected from the outdoors and in hot environments the additional heat can be dissipated into the ambient air inside where the air conditioning system maintains the balance. Many systems rely on a heat sink with a small air exchanger and a fan to dissipate the heat.

Two-phase technologies.

There are two types of technologies Calyos uses for these applications:

• Two-Phase Loops (including LHPs)
  System with dedicated evaporator capable of cooling extremely high heat flux components and is able to transport heat over long distances.
• Micro-Channel Heat Pipes (MCHPs)
  Thin and lightweight flat extruded aluminum cross sections that can transport heat over short distances and  easily integrate with components unlike traditional heat pipes.

Both of these can be applicable depending on the approach taken. The next section goes on to explain this.

Exploring Three Cooling Approaches

When designing two-phase cooling solutions for embedded vehicle processors, we focus on three different approaches. Each can be beneficial depending on:

• Heat load (power and heat flux)
• Location inside the vehicle
• Vehicle architecture
• Available cold source (Air, Water, Refrigerant)

1. Local cooling

Both technologies are applicable here as the transport distances are shorter - hence the use of the word local. However compared to traditional methods, they can spread the heat further across a finned air exchanger, enabling much better dissipation into the air.  This can boost the performance significantly and is similar to the switch in desktop computers from heat sinks to heat pipe heat sinks. The difference here is that the systems Calyos deploys are far more reliable.

Copper-water heat pipes are prone to degradation in performance over time, this is fine for an aging computer but not appropriate for vehicles due to their safety needs. Calyos' systems use a fluid and wetted material combinations that have been proven over time. Our experience here comes from the space business, where all devices must be ultra-reliable as getting somebody up there to replace a part is not so easy.

We are seeing more clients use MCHPs for these short distance applications. The lightweight flat design is easy to integrate inside and offers a solution much easier to design with than traditional heat pipes plus the reliability is far superior. This is a great first step as OEMs look to add more features to vehicles that are reliant on higher computing power.

2. Remote cooling with air

However some have ambitions to go a lot further than that. Here we can copy and approach we have used to cool power electronics. We can take the heat  from a high power, high heat flux device and transport it over long distances - passively.

This allows the engineer to reach a far better cold source - think a larger air exchanger with access to better airflow.  An example could be a compute module placed at the rear of the vehicle, maybe in the boot (trunk for the North Americans) and the heat can be transported to the front of the vehicle where is could use a low temperature radiator to dissipate the heat into the ambient air. This unlocks the ability to cool kilowatts rather than just a few hundred watts.

3. Remote cooling with water

However, while some engineers appreciate the separation from all other systems, another method is to utilize the global water-ethylene-glycol loop inside most EVs today. The same principles apply, we can still take the heat  from a high power, high heat flux device and transport it over long distances but the benefit here is the drastically reduced pressure drop on the water-EG system.

Of course this can be an effective alternative if airflow is limited or if simplicity is desired and it fits with our vision of a very simple (therefore low pressure drop) global pumped cooling circuit inside the vehicle with an array of additional passive loops that bring the heat from all the dissipating components to this loop.

A reminder of the philosophy.

All of our two-phase systems win by targeting two key aspects:

1. Utilizing vaporization's high HTC to manage even the most extreme heat loads
2. Transporting the heat to a large surface of exchange where it can be easily dissipated into the cooling medium

This allows a two-phase system to be effective even when used in combination with single-phase cooling systems.

But what about heat pumps?!

An often-overlooked benefit of two-phase cooling is its ability to provide passive heat recovery. If the vehicle uses a heat pump loop, rather than Water-EG then heat can be transported to specific parts of the loop to preheat the refrigerant, boosting the performance of the heat pump.

By harnessing this waste energy it enhances the sustainability of the cooling system and reduces overall energy consumption, helping manufacturers longer range vehicles.

The case for two-phase cooling.

It's been a while since I reiterated the fundamental benefits of adopting such a system so here they are:

• Passive & Pump-Free = Lower CAPEX
  When mass produced these systems have fewer parts, simpler designs and utilize less fluid than single-phase systems.
• Passive & Zero Maintenance = Lower OPEX
  Self-contained systems with no moving parts removes the risks of mechanical failure during operation.
• High Heat Transfer Coefficient = High Performance
  Transfers heat very effectively without consuming additional energy.

An example of local cooling:

Calyos has plenty of examples where we have demonstrated the capabilities of this approach. Our solution effectively handled the thermal management of two 150W components, in this case only using natural convection.

The system also boasted:

• High MTBF (Mean Time Between Failures) to maximize uptime.
• IP66 enclosures for complete protection against dust and water.
• Unmatched durability and performance in rugged environments.

‍Why thermal management matters.

As vehicles integrate more advanced compute modules to power features like ADAS, infotainment, and autonomy, thermal management becomes increasingly critical.

The latest high-power processors generate significant heat loads, and we all know where things are going in the future. When not managed effectively thermal throttling can compromise performance and reliability. Several challenges make traditional cooling methods tricky:

• Space constraints limit the implementation of bulky, traditional solutions.
• Active cooling systems like fans introduce noise, maintenance requirements, and potential failure points—unacceptable risks in safety-critical systems.
• Poor thermal design can reduce the lifetime of key components.

Empowering the future of mobility.

By integrating two-phase cooling solutions, automotive manufacturers can ensure their compute modules are ready to handle the future of mobility—reliably, efficiently, and sustainably.