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NREL - National Renewable Energy Laboratory
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Thermal Management

Hybrid electric and fuel cell vehicles need proper thermal management of the energy storage and power conditioning units for optimum performance and durability. NREL's power electronics team investigates cooling and heating of advanced vehicles by focusing on the thermal management of motor controllers, inverters, and traction motors with single- and two-phase cooling technologies. The team is also looking into advanced component modeling, fabrication, and manufacturing techniques at the laboratory. Our engineers and scientists are helping integrate emerging power electronic technologies to manage and control high-power components, which will provide rapid, bi-directional energy flow, improve performance, and lower costs.

A barrier to developing next-generation power electronics is cooling at high heat fluxes (up to 100 Watts/cm2) at high temperatures (>125°C) in compact (low-volume), lightweight power electronics packages. Advanced heat transfer techniques must be used to overcome such barriers and challenges in next-generation power electronics cooling.

Goals

  • Develop a means to improve heat rejection from power electronics (for >250 W/cm2).
  • Reduce system cost and increase reliability, specific power, power density, and efficiency.

Objective

  • Develop advanced thermal management methods and systems that will allow next-generation power electronics to operate at high heat fluxes and high temperatures in a compact (low-volume), lightweight power electronics package.

Approach

  • Develop and demonstrate the viability and advantages of single- and two-phase cooling techniques such as spray cooling, jet impingement, and microchannel/minichannel cooling.
  • Analyze the cooling and thermal control technology currently used in state-of-the-art insulated gate bipolar transistors (IGBTs) for high-power applications such as automotive traction drives.
  • Explore a variety of improved options for cooling within manufacturing constraints.
  • Collaborate with manufacturers to accommodate improved flow and thermal behavior of cooling systems.
  • Test and validate improvements on prototypes.

Project Descriptions

Thermal modeling image of spray cooling of inverter chip surface shows the liquid breaking up into fine droplets that impinge on the liquid wall, which enhances the spacial uniformity of heat removal.

Modeling Cooling Technologies—Spray Cooling

1. Researching Cooling Techniques

To reach the goals and objective, the NREL power electronics team is investigating emerging cooling techniques such as spray cooling, jet impingement, and microchannel/minichannel cooling.

Spray cooling. NREL engineers and scientists are conducting research on spray cooling-a two-phase cooling technique used to quench metals. Currently, most spray quenching studies do not apply to electronic cooling, as they involve high temperatures that correspond to the film boiling regime. Sprays break up the liquid into fine droplets that impinge on the liquid wall to enhance the spacial uniformity of heat removal and help delay the liquid separation (or dry out) from the wall during vigorous boiling. Governing parameters for this study include droplet size, distribution and pattern, orientation, surface treatment, spray behavior at critical system pressures, heat load, and vibration ranges. There are two types of sprays:

  • Pressure sprays, which are formed with liquids under high pressure through a small orifice.

  • Atomized sprays, which may use high-pressure gas streams to break up the liquid. These have superior cooling performance, but are difficult to incorporate because of the complexity of separating air or inert gas from coolants.

Jet impingement. NREL is studying the viability and advantages of jet impingement under two environments:

  • Free jet (with a gas liquid interface)
  • Submerged jet (liquid jet in liquid environment)
Thermal modeling image of jet impingement cooling of a motor inverter.

Modeling Cooling Technologies—Jet Impingement

Jet impingement is an aggressive form of cooling. It is used to quench metal alloy parts from high temperatures to achieve a desired alloy microstructure and superior mechanical properties, and to maintain relatively low temperatures in lasers, x-rays, etc., which dissipate enormous heat fluxes. Governing parameters include jet nozzle design, orientation, surface treatment, jet behavior at critical system pressures, heat load, and vibration ranges.

Microchannel/minichannel cooling. NREL researchers are looking at this technique, which may require minimal coolant flow rates and minimal dimensions for the flow channels. The small flow rate in a microchannel produces laminar flow with the heat transfer coefficient, which is inversely proportional to the channel's hydraulic diameter. It may permit liquid to partially or totally evaporate, which lowers coolant flow rates. However, microchannels are more prone to plugging. Further governing parameters include microfabrication methods, pressure drop, choice of hydraulic diameter, and fluid properties.

2. Advancing Inverter Cooling Methods

NREL is looking at advanced methods to cool and control the IGBTs and capacitors that are used inside inverters for automotive traction drives. IGBTs are voltage-controlled power transistors that have higher current densities than equivalent metal-oxide semiconductor field effect transistors (MOSFETs). They are faster and have better drive and output characteristics than power bipolar transistors. IGBTs are more cost effective in most high-voltage high-current, moderate frequency applications.

High temperatures and high heat fluxes in IGBTs and capacitors decrease their lifetimes by accelerating failure mechanisms in materials, and reduce the reliability of the inverter by accelerating failure mechanisms in connections and interfaces. Developing next-generation power electronics cooling methods for IGBTs and capacitors is critical step to overcoming high temperatures and high heat flux barriers.