The Opportunity for 100% Passive Cooling Systems for Energy and Industrial Applications

The shift to advanced components like SiC and GaN isn't just an upgrade – it's a revolution in power electronics. Plus for batteries new chemistry's are always being considered and solid-state remains a 'hot' topic. But with these advancements in power comes great thermal responsibility.

Here's what we're dealing with:

1. Smaller Packages:

Modern components are like flying Ryanair – packing more into less space. New technologies enables these compact designs, but fitting effective cooling into these tight spaces is becoming increasingly challenging.

‍2. Higher Power Dissipation:

While these new technologies are incredibly efficient, their smaller size means more power on a smaller surface area. This heat flux means traditional cooling methods are often left gasping for breath trying to keep up.

3. Thermal Sensitivity:

These components need to be just the right temperature to perform their best, and we are hearing about significant boosts in efficiency when cooled effectively. Even slight overheating will shorten their lifespan.

4. Regulatory Pressures:

Regardless of any current political environment, the direction of travel long term is clear - sustainability regulations are going to tighten their grip, pushing us toward greener cooling solutions. It's must be about both performance and sustainability.

The opportunity to remain passive

Fans and pumps are the easy options. Nobody is getting fired for adding a fan to assist a heat sink. But in reality, you are introducing a component prone to failure in even the most safe and clean environments. Let alone when you put it in a factory or worse, out at sea.

By integrating two-phase systems into a system you can drastically boost the thermal performance without impacting reliability or maintenance. As I said before - alot of this tech comes from the space industry where reliability and zero maintenance are a must, not a nice to have.

Let's break down why it's feasible:

Superior Heat Management:

We all know that by using both liquid and vapor phases to move heat away from sensitive components, two-phase systems achieve impressive heat transfer coefficients.

Self-Regulating Operation:

Here's the beautiful part – these systems run themselves. It activates only when heat is applied relying purely operate on a temperature delta. No pumps, no complex controls – just physics doing its thing.

Compact Yet Powerful:

Using technologies like vapor chambers and micro-channel heat pipes, these systems pack serious cooling power into small spaces. In the case of MCHPs they evacuate heat effectively using tiny channels of fluid to transport and dissipate it.

Design Considerations: Avoiding the Pitfalls  

However in order to realize this opportunity it is helpful to avoid a few things. Here's what to watch out for and some helpful tips when designing:

1. Material Selection:

Choosing the right working fluids isn't just about cooling performance – it's about long-term compatibility. You need fluids that play nice with the wetted materials (all materials that come into contact with the fluid) over the long haul. Water is particularly prone to the generation of non-condensible gases, especially if there are any contaminants inside - this will quickly degrade the performance. Hence why the copper-water heat pipe inside your laptop is not performing as well as the day you got it.

2. Harnessing Gravity:

Every passive two-phase loop either uses gravity or capillary (or both) action to move fluid (and therefore heat) around the system. Placing the condenser section above the evaporator will boost the fluid return, improving thermal performance. This is easily achieved in static systems to ensure maximum performance of the two-phase cooling.

If you are using two-phase plates: Providing space and clear vertical pathways for the heat to be dissipated above.

If you are using two-phase loops: Collating heat sources together to utilize few evaporators and placing components vertically to provide a clear path for the vapor is important.

3. Natural Convection:

This can be used to dissipate large volumes of heat passively (no fans is the goal remember!), here are a few tips:

• Design optimization with vertical fins and open structures to maximize natural airflow.
• Selecting materials with high thermal conductivity to enhance the dissipation into the air.
• Spreading the heat using two-phase fluids results in an isothermal condenser providing the ability to use the full surface area to dissipate heat.

Three examples that are ready to be converted to being fully passive.

SubStation Cooling

Calyos has long provided cooling systems for Rail as show in the image above. Utilizing a manifold heat pipe system to boost performance and dissipate the heat into the air above. However fans remain required due to the design of the air exchanger.

It would be easy to convert this air exchanger to one optimised for natural convection. Yes it would need to be larger and likely sit above the cabinet in order to access the ambient air, but surely this is worth it in pursuit of a 100% passive system - do you agree?  

Cylindrical Cell Cooling

For many applications it is enough to simply cool the bottom of each cylindrical cell. When placed horizontally as in the images above, this can work very effectively with an MCHP cooling system.

Now in the projects Calyos has previously worked on this used a water cold plate and an 'L' shape MCHP, however this could easily be convered to a 'I' straight MCHP with a small finned heat sink. This could dissipate the heat through natural convection directly above the cells - ideal for a BESS.

Invert Cooling

This system, while similar to the one above, relies on a two-phase loop to transport and dissipate the heat. This was introduce primarily due to the mechanical design constraints and the desire to transport the heat over a longer distance, horizontally.

This system would be even easier to convert, as the LHP provides the ability to place the air exchanger where you wish. While of course it could benefit from gravity to boost performance the air exchanger could be at the back or side of the unit instead - providing more flexibility and making it easier to achieve a fully passive unit.

The Road Ahead

The evidence is clear: passive two-phase cooling systems offer the opportunity to provide fully passive thermal management of static power electronics and BESS systems. They offer the right blend of performance, reliability, and sustainability that modern applications demand.

I hope to see more real world examples of these systems, not only from Calyos, in the coming years.