The Navy is developing a vehicle mounted directed energy (DE) weapon system in their Ground Based Air Defense (GBAD) Directed Energy On-The-Move (OTM) program. As part of this initiative, Advanced Cooling Technologies, Inc. (ACT) has been awarded a $1.2M, three year contract to develop an efficient, lightweight, compact thermal storage and management subsystem. The cooling system will remove waste heat from the laser commensurate with available power and other engineering constraints. The technology being developed will be used to suppress unmanned aerial vehicles (UAV’s).

Background

High energy lasers, microwave emitters and other pulsed power systems deliver a burst of high energy over a prescribed time interval. Due to in ef fi cien cies in these systems, a key challenge is to rapidly and efficiently remove excess heat generated during operation. This is important to maintain the required device operating temperature and ensure reliable and repeatable performance. Lasers, for example, are particularly sensitive to their operating temperature as emission wavelengths are strongly temperature-dependent.

In many of these systems, the ultimate heat sink is ambient air, which as most thermal engineers know is a poor heat transfer medium and thus, drives the need for large heat transfer areas to compensate for otherwise low heat transfer coefficients. Depending on operating temperatures, a vapor compression system may also be needed to provide the necessary temperature lift. Vapor compression systems are attractive since they have high coefficients of performance (COP) and are able to transport heat over large distances (evaporator and condenser can be far apart) yet unfortunately require large compressors and condensers when sized to handle peak heat loads. This is a particular challenge for airborne and mobile applications where Size, Weight and Power (SWaP) are limited and simply not available.

Compact, Lightweight, Efficient Cooling Systems

To address this need, compact, lightweight and efficient cooling systems with integrated thermal energy storage have been developed. The heat produced during high power operation is stored in the thermal energy storage system and dissipated during the off/low heat load period to the heat sink.

Figure 1. Applications having pulsed heating with a period of high heating followed by a longer period of lower/ no heating can benefit from integrated thermal storage devices. This allows the heat rejection system to be sized for the time-averaged rather than the peak heat loads.
With thermal energy storage, the cooling system needs to be sized to handle a lower time-averaged heat load, rather than the peak heat load as illustrated in Figure 1. This is a big deal, especially for systems that have long duty cycles. In air-cooled systems, for example, the use of thermal storage enables the fin volume (owed to a reduction in required heat transfer area) to be reduced while in liquid cooled systems, the pump size and associated power can be reduced. In space systems, the radiator area, for example, can be reduced. In all systems, a smaller cooling system is traded against the addition of the thermal energy storage component.

With that said, the size, weight and power of the overall system is typically much smaller for systems having thermal storage. Complete systems have been developed at ACT with overall weight reductions on the order of 40- 60%. The actual savings depends on the heat load, the duty cycle, the heat sink conditions and other design parameters.

Thermal Energy Storage

The Navy is developing ground based air defense systems that require compact and efficient thermal management solutions.
Regarding the thermal energy storage component, it must be engineered to: (1) maximize the use of available thermal storage capacity during the peak heat load period, (2) minimize the overall size and weight of the component, and (3) ensure that the thermal storage material can be regenerated within the available down time. To address optimized thermal storage, clever engineering solutions have been developed to improve heat transfer into thermal storage materials that usually have otherwise poor thermal properties. These enhanced heat transfer strategies include the use of internal structures, additives, and other technologies.

Regarding the required space claim for these thermal management solutions, substantial savings can be realized by selecting the appropriate phase change material combined with an efficient thermal design so that the available PCM is melted during the operational period while ensuring that all of the PCM can be re-generated within the available down time. In addition, it is important to ensure minimal degradation through multiple heat storage/ regeneration cycles.

The integration of thermal storage capability within the thermal management solution is a key enabler in improving the efficiency and reducing the size of the overall heat sink. Appropriately sized and combined vapor compression, pumped liquid, and thermal storage systems are leading the way to help the Navy deploy mobile directed energy devices.

This article was written by Pete Ritt, VP, Technical Sales; Devin Pellicone, Lead Engineer, Custom Products Group; and Howard Pearlman, Senior Engineer; Advanced Cooling Technologies, Inc. (ACT) (Lancaster, PA). For more information, Click Here .


Aerospace & Defense Technology Magazine

This article first appeared in the October, 2015 issue of Aerospace & Defense Technology Magazine.

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