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One of the big challenges in many power-management situations is keeping “things” cool, whether that thing is a component, board or system. Many approaches are used, ranging from passive cooling aided by convection and conduction to forced-air designs to fluid-based cooling and even air conditioning, depending on the specifics of the situation. A cynic might even say that the objective of such cooling is to transfer excess unwanted heat to that magical place called “away” where it’s no longer your problem but now someone else’s.
But there’s another engineering thermal challenge in some cases: keeping a component or functional block close to a desired setpoint temperature. For example, the James Webb Space Telescope Mid-Infrared Instrument (MIRI) carries detectors that need to be at a temperature of less than 7 K to operate properly, while its 18 primary mirror segments range in temperature from 34.4 K to 54.5 K.
Of course, not all thermally sensitive applications are so esoteric. There are high-volume products that need thermal “bounding,” such as the fiber-optic electro-optical (EO) transceiver modules in many high-speed data links. The reason is simple: Any thermal deviations from the design’s nominal value result in shifts of key performance characteristics, such as optical wavelength, with many performance consequences.
The solution in many cases is to use Peltier devices as coolers. These are active, powered thermal-control modules that offer both “warming” and “cooling” depending on the applied DC polarity. With a suitable control loop, a Peltier device can stabilize the temperature to a fraction of a degree. Note that the thermal issue here is very localized at the EO transceiver rather than the larger circuit board or chassis, so a Peltier device can be fixed to each transceiver to manage its “personal” temperature. As long as the Peltier unit is supplied with DC power, it can work to maintain the desired setpoint temperature to less than a degree, within certain broader minimum/maximum limits.
However, not every application needs temperature control as tight as required by EO transceivers. Proof of this is a fascinating, very unusual application I recently saw with a much wider range of 4°C to 8°C. This is needed to maintain the viability of human organs for transplants despite transport time.
You may have seen videos of this scenario in dramatic TV shows, with a lunch cooler packed with ice used for the mission. In reality, it’s a little more complicated, but not by much. The standard arrangement consists of three plastic bags and an ice box. The first plastic bag includes the organ itself immersed in a preservation solution. This bag is put in a second bag filled with saline, and then these two are put in a third bag filled with saline, which is then put in the ice box. (For heart transplants, there’s also a tube with a special fluid that helps “nourish” the heart tissue.)
The disadvantage of this method is that the organ usually gets too cold, going down to between 0°C and 4°C, and 0°C and 2°C is especially troublesome. As a result, preservation of hearts is limited to four to six hours, while the liver, kidney and pancreas can last up to 24 to 36 hours (although their graft function may be compromised).
A new approach to carrying organs was devised a few years ago by Paragonix Technologies, with its SherpaPak family of three types of carrier models (Figure 1). The company was founded in 2010 and its device was FDA-approved in 2018, with a commercial launch in 2019. It has been used on about 3,000 donor hearts to date.
Paragonix was in the news recently when its SherpaPak Cardiac Transport System was used to successfully transport a donor heart from Juneau, Alaska, to Boston. While this is a record distance (about 2,900 miles/4,700 km) what is really significant is that the time of transport, at about eight hours, was much longer than the lunch-carrier-plus-ice approach.
You might think that the company devised a battery-powered, closed-loop cooling scheme to maintain the temperature, but it didn’t. Instead, the SherpaPak’s cooling mechanism is based on a phase-change material (PCM), which is a substance with a high heat of fusion that can store and release large amounts of energy. The SherpaPak’s PCM panels are designed to hold 5°C longer than conventional PCM cold packs (which undergo phase change at 0°C and have little heat capacity at 5°C).
What’s a phase-change material? It’s a substance that transforms from a solid state (phase) to a liquid phase as it absorbs heat. When it releases that heat, it returns to the solid phase (Figure 2). This passive phase cycle is not new, as the principles of phase change have been known and understood for well over 150 years as part of classical (non-quantum) thermodynamics. It’s really a basic principle: A liquid substance has higher energy than its solid form, giving up that excess energy (heat) when it transforms into a solid. The reverse is true as well: A solid absorbs heat energy when it transitions to liquid.
Although a non-technical person would likely be somewhat mystified by the PCM concept—after all, it’s thermal engineering—many people have seen it used in another guise, even if they didn’t recognize it for what it was. How so? In orange groves, for example, when the temperature starts to drop below about 28°F (–2°C) but is expected to get warmer soon, the grove managers spray the trees with water. If the temperature drops just below freezing for a short period, the heat given off by the water as it transitions from liquid phase to solid phase may be enough to keep the fruit from freezing.
The Paragonix system does not require electrical power to provide its function operation. Management of preservation in the SherpaPak system is the same for both kidneys and hearts, other than that the heart is attached to a connector in the inner hard-shell assembly to feed it those special preservative fluids.
Although the system for maintaining the critical temperature is non-electronic, there is a small battery-powered electronics module in the unit (Figure 3). This module measures, records and displays temperature, provides data logging via Bluetooth to the person managing the transport, tracks location via GPS and even updates the receiving hospital on status. Still, the power demands of this module are quite modest compared with what is required to keep a chamber in the critical range using active cooling and refrigeration.
The use of phase-change material for thermal management is not new, of course. Engineers have used phase-change material for many years in very specific circumstances. PCM-based sinks are a good fit with pulse loads because they can absorb the short-term excess heat and then release it more slowly (think of it as a thermal charge/discharge capacitor); this allows the overall cooling system to be sized for average rather than peak load.
They can also be used for short-term transitional storage until the overall heatsink system is available (the fan turns on or proper convection is restored), and a PCM-based sink can act as a thermal “fuse” by absorbing heat and preventing thermal overload (at least for a while) when the rest of the cooling system is offline.
So why isn’t phase-change material used for general cooling? The answer is that when it comes to energy—in this case, thermal energy—there is no “free lunch,” and the laws of physics get in the way. PCMs allow heat energy to be stored and released, but they have a finite capacity. If you keep dumping excess heat into the phase-change material, at some point, it reaches maximum capacity and can absorb no more. Then you have to figure out how to get the stored heat “away”—the usual heatsinking dilemma.
Have you ever used phase-change material to manage minimum and maximum temperature or heat energy? To what extent did they help the problem?
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