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Phase Change Thermal Buffers for Environmental Control Unit Efficiency Improvement


OBJECTIVE: Develop and demonstrate a phase change material based thermal buffer to enable"rightsizing"of environmental control units (ECUs) and improve overall efficiency through reduced peak loads, more stabilized ECU operation, and off-peak thermal energy storage. DESCRIPTION: Military Environmental Control Units (ECUs) represent one of the dominant energy users in forward operating environments, and significant effort is being made to improving overall ECU efficiency [1]. Reducing overall Size, Weight and Power (SWaP) of military ECU"s is complicated by the transient, yet predictable, nature of the thermal demand profile over the course of a typical daily usage cycle, where cooling capacity must currently be set to match the daily peak load. This need for cooling unit exess-capacity creates trickle-down effects of increasing installed power generation requirements and logistic transport burdens. ECU capacity"right-sizing"can help meet Operational Energy Strategy requirements for improved net energy efficiency, but will demand significant changes to ECU design. Thermal Energy Storage (TES) has been shown to be effective in load-leveling the daily cooling profile for fixed facilities, providing reductions in net energy consumption in some cases due to"free cooling"[2], as well as reduced compressor cycling, more steady cooling, and reduced peak power requirements [3,4]. However, because of the compositional variability in cooled facilities in forward operating environments, implementing TES would require directly integrating the technology within mobile ECUs, a task which has received only cursory attention in the literature and would require optimization of overall size and weight in addition to reducing energy usage. Solid-liquid Phase Change Materials (PCMs) present one high-density TES option for cooling systems and environmental control [5,6], yet challenges remain in material selection, heat exchanger topology and integration strategy to maximize operational benefit. This SBIR program seeks to develop a PCM-based solution to enhance the performance of an existing ECU in the 9-18k BTUH range, addressing concerns of system energy density, material compatibility, and failure modes due to repeated thermal cycling. Offeror is expected to propose a phase change TES component that can be integrated into an existing ECU for the purpose of off-peak load leveling or demand reduction, with a target energy usage reduction of 5-10% over an average daily cycle, assuming no more than an 25F diurnal temperature variation. Unit should still be able to provide rated cooling capacity under worse case conditions in a deployed environment (design condition 125F ambient, 90F indoor dry bulb, 75F indoor wet bulb) for a period of at least 2 hours during daily peak demand period. Specification of PCM type, integration point (on refrigerant or air-side flow paths), storage temperature, and storage heat exchanger design are left up to the offeror, however those decisions and the associated impact on overall system size, weight and performance should be justified through thermodynamic analysis and system/component modeling. PHASE I: Offeror will demonstrate through simulation or experimentation the feasibility of combining thermal energy storage with an existing compact environmental control unit to achieve a 5-10% net energy usage reduction. If not demonstrated experimentally, performance simulation should convincingly account for non-ideal heat transfer, material and and thermodynamic conditions. Overall increase to ECU size and weight tradeoff with cooling capacity gain should be captured. During this phase commercialization aspects must be considered and potential plans for commercialization elucidated. PHASE II: Demonstrate a fully functioning prototype system using the concept developed in Phase I. Performance should also be measured under varying ambient conditions to ascertain performance sensitivities, and offeror should evaluate design scalability to larger capacity ECUs. Validate analytical and numerical models developed in Phase I. PHASE III: Design and develop phase change TES units for ECUs using the knowledge gained during Phases I and II, targeting integration into PM MEP Improved ECU (IECU) systems. This series of ECUs must meet military unique requirements, e.g. shock, vibration, and environmental variability.
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