Electronics Protection - Spring 2015 - (Page 12)

Feature Pumped Two Phase Cooling Solutions for Challenging Thermal Management Applications Advanced Cooling Technologies, Inc. For the past few decades, excellent thermal management solutions have been achieved using well established technologies such as air cooling and pumped liquid systems. Recently, however several new thermal management challenges have emerged for which these technologies may not be adequate. These applications include high performance lasers which are temperature/wave length sensitive, mobile lasers systems that size and weight limits and medium (kW level) voltage IGBT drives for energy and other power electronics applications. Pumped two phase (P2P) cooling, is finding increasing acceptance for solving these challenging thermal management applications in both military and commercial market segments. As the technology becomes more prevalent, there is a growing need in the thermal design community for a better understanding of the fundamentals of the P2P, including its performance features and which applications can take benefit from them. Technology Overview In general, P2P systems differ from traditional pumped liquid cooling solutions because they are designed to promote boiling of the working fluid as it comes in contact with the hot evaporator device being cooled. P2P takes benefit from the fact that a fluid's latent heat of vaporization, the heat required to change the state of a substance without a change in temperature, can be two orders of magnitude larger than the sensible heat of single phase liquid cooling. This means that compared to pumped single phase cooling, pumped two phase systems can provide higher heat dissipation, more uniform evaporator temperatures with lower flow rate and power requirements. Figure 2. As can be seen in the chart, pumped two phase has significantly higher heat flux capability with pumped liquid systems typically in the 10 to 20 W/cm2 range and pumped two phase from three hundred up to even a thousand W/cm2. Single phase pumped liquid systems have a variety of potential working fluid selections including, water, ethylene glycol, combinations of water and ethylene glycol and refrigerants. Pumped two phase systems which are closed and pressurized, are usually refrigerant based, but can be used with water, methanol or combined water/methanol systems. A real advantage for weight and space confined applications is the significantly reduced pumping power and flow rate requirements for pumped two phase vs pumped liquid systems. For example in order to dissipate 80kW of heat, a pumped liquid system using PAO as the working fluid, would require a flow rate of 35 gallons per minute and approximately 5.3 kw of power. A pumped two phase system using R245fa refrigerant, would require only 6 gallons/minute and 250W of power, which is an 80% reduction in flow rate requirements and a 95% reduction in power requirements, compared to a pumped liquid system. Of course pumped liquid solutions have been around for a long time and these solutions are reliable and well understood. Pumped two phase is an emerging technology that while, a bit more complex, also has many important performance benefits. Applications Figure 1. Schematic comparing pumped two phase and pumped single phase systems. Both have the same basic components, but P2P systems have the working fluid boil as it passes across the evaporator system. As can be seen in Figure 1, P2P uses the same core components as a traditional single phase pumped liquid system. It has a pump, an evaporator, a heat exchanger and a working fluid reservoir. A key difference between pumped single phase and pumped two phase cooling is that the working fluid is close to its saturation temperature as it comes in contact with the hot device. The heat causes the working fluid to evaporate and heat is removed through vapor/liquid mixture, which has some performance benefits. To help establish and clarify a better baseline, Figure 2 compares and contrasts key attributes of pumped single vs. pumped two phase cooling, which are the two most common thermal management technologies used to dissipate kilowatt or higher heat loads. 12 Spring 2015 * www.ElectronicsProtectionMagazine.com Below are some applications that utilize P2P technologies. Laser Diode Cooling Laser Diodes typically require lots of power, most of which results in waste heat. Vertical Cavity Surface Emitting Lasers for example, can have heat fluxes in excess of 500 W/ cm2. Further, the heat loads must be dissipated while maintaining a uniform temperature Figure 3. P2P Evaporator for a Laser across the evaporator surface Diode Application. Bubbles can be In many cases pumped liqseen as the working fluid exits to uid systems are not suitable for the right of the evaporator, an indithese applications. It is diffication the boiling has occurred. cult or impossible to meet the isothermality requirements of a few °C, particularly when hundreds of laser diodes are involved. In these applications a pumped two phase cooling solution is often appropriate. One can see in Figure 3, an example of a pumped two phase evaporator. Flow is from left to right and one can see the bubbles exiting the evaporator. The formation of the bubbles as the working fluid passes through the cold plate is an http://www.ElectronicsProtectionMagazine.com

Table of Contents for the Digital Edition of Electronics Protection - Spring 2015

Editor's Choice
Predictive Modeling: The Next Frontier in Data Center Condition Maintenance
Don't Forget the Batteries
Military Aircraft Power
Pumped Two Phase Cooling Solutions for Challenging Thermal Management Applications
Identifying the Ideal Mechanical PCB Hardware for Electronic Systems
Bridging the Technology Gap: The Importance of Cyber and Physical Security within the Data Center
Waterproof Mobile Device Protection without Compromising Acoustic Quality
Industry News

Electronics Protection - Spring 2015