Adrienne B. Little

A REVIEW OF EJECTOR TECHNOLOGY FOR REFRIGERATION APPLICATIONS

This paper provides a comprehensive review of ejector technology for refrigeration applications, combining an understanding of basic fluid flow fundamentals within the ejector with application in cycle-level development. An ejector is a passive device that requires no external mechanical input or moving parts. A high-velocity motive stream produces a low-pressure region into which a suction flow is entrained, resulting in a pressure rise of the suction flow and mixing between the two streams to provide a pumping effect. The first part of this review addresses the progression from experiment-based analytical models to computational modeling of the ejector itself from the early 1950s to 2009. Included is an assessment of the most recent work in CFD modeling, and an exploration into what is needed to develop these models further. Suggestions for future research include better modeling of shock phenomena and the effects of two-phase flow in ejectors. The second part of this review focuses on ejector applications in refrigeration cycles with special emphasis on the vapor-jet refrigeration cycle. Important connections are made between ejector component and system level studies, an understanding of which would enable improvement of system level performance to the extent where they could be used in some niche applications instead of conventional refrigeration systems.

Waste Heat Recovery in Data Centers Using Sorption Systems

Of the total electricity consumption by the United States in 2006, more than 1% was used on data centers alone; a value that continues to rise rapidly. Of the total amount of electricity a data center consumes, about 30% is used to cool server equipment. The present study conceptualizes and analyzes a novel paradigm consisting of integrated power, cooling, and waste heat recovery and upgrade systems that considerably lower the energy footprint of data centers. Thus, on-site power generation equipment is used to supply primary electricity needs of the data center. The microturbine-derived waste heat is recovered to run an absorption chiller that supplies the entire cooling load of the data center, essentially providing the requisite cooling without any additional expenditure of primary energy. Furthermore, the remaining waste heat rejected by the data center is boosted to a higher temperature with a heat transformer, with the upgraded thermal stream serving as an additional output of the data center with negligible additional electrical power input. Such upgraded heat can be used for district heating applications in neighboring residential or commercial buildings, or as process heat for commercial end uses such as laundries, hospitals, and restaurants, depending on the location of the data center. With such a system, the primary energy usage of the data center as a whole can be reduced by up to 23% while still addressing the high-flux cooling loads, in addition to providing a new income stream through the sales of upgraded thermal energy. Given the large and fast-escalating energy consumption patterns of data centers, this novel, integrated approach to electricity and cooling supply, and waste heat recovery and upgrade will substantially reduce primary energy consumption for this important end use worldwide.

Visualization and Validation of Ejector Flow Field with Computational and First- Principles Analysis

Passive, heat actuated ejector pumps offer simple and energy-efficient options for a variety of end uses with no electrical input or moving parts. In an effort to obtain insights into ejector flow phenomena and to evaluate the effectiveness of commonly used computational and analytical tools in predicting these conditions, this study presents a set of shadowgraph images of flow inside a large-scale air ejector and compares them to both computational and first-principles-based analytical models of the same flow. The computational simulations ...

Optical validation of ejector flow characteristics predicted by computational analysis

Passive, heat actuated devices can offer simple and energy-efficient options for a variety of end uses. An ejector pump is one such device that provides reasonable pressure head with no electrical input or moving parts. Useful for a wide range of applications from nuclear reactor cooling to vapor compression in waste-heat-driven heat pumping and work recovery systems, the flow phenomena inside an ejector must be understood to achieve improvements in component design and efficiency. In an effort to obtain insights into the flow phenomena inside an ejector, and to evaluate the effectiveness of commonly used computational tools in predicting these conditions, this study presents a set of shadowgraph images of flow inside a large-scale air ejector, and compares them to computational simulations of the same flow. On-design and off-design conditions are considered where the suction flow is choked and not choked, respectively. The computational simulations used for comparison apply k-e RNG and k-ω SST turbulence models available in ANSYS FLUENT to 2D, locally-refined rectangular meshes for ideal gas air flow. Experimental and computational results show that on-design ejector operation is predicted with reasonable accuracy, but accuracy with the same models is not adequate at off-design conditions. This is attributed to an inability of turbulence models to predict shock/expansion interaction with the motive jet boundary, as well as the strength and position of flow features. Exploration of local flow features shows that the k-ω SST model predicts the location of flow features, as well as global inlet mass flow rates, with greater accuracy. It is concluded that to provide a rigorous validation of turbulence models for the application of modeling ejector flow, it is necessary to rely on off-design data where more complex phenomena occur, such as flow separation, strong boundary layer/shock interaction, and unsteady flow. Such validation will help refine turbulence models for future ejector design purposes, and allow for more efficient ejector operation.© 2012 ASME

Comparative Assessment of Alternative Cycles for Waste Heat Recovery and Upgrade

Thermally activated systems based on sorption cycles, as well as mechanical systems based on vapor compression/expansion are assessed in this study for waste heat recovery applications. In particular, ammonia-water sorption cycles for cooling and mechanical work recovery, a heat transformer using lithium bromide-water as the working fluid pair to yield high temperature heat, and organic Rankine cycles using refrigerant R245fa for work recovery as well as versions coupled to a vapor compression cycle to yield cooling are analyzed with overall heat transfer conductances for heat exchangers that use similar approach temperatures for each cycle. Thermal-to-mechanical conversion efficiencies of ∼9%, upgraded heat delivered at 150°C, or cooling coefficients of performance of 0.5–0.7 are realized for source temperatures of 120°C, with a nominal 1 kW of heat extracted from the waste heat stream. Ambient sink temperatures of 35°C are used, as well as indoor return air temperatures of 27°C for cycles that produce cooling at 8°C. Comparative assessments of these cycles on the basis of efficiencies and system footprints will guide the selection of waste heat recovery and upgrade systems for different applications and waste heat availabilities. The increased implementation of such waste heat recovery systems in a variety of applications will lead to decreased primary source inputs and sustainable energy utilization.Copyright

A New Energy Frugal Paradigm for Data Centers

Of the total electricity consumption by the United States in 2006, more than 1% was used on data centers alone; a value that continues to rise rapidly. Of the total amount of electricity a data center consumes, at least 30% is used to cool server equipment. The present study conceptualizes and analyzes a novel paradigm consisting of integrated power, cooling, and waste heat recovery and upgrade systems that considerably lowers the energy footprint of data centers. Thus, on-site power generation equipment is used to supply primary electricity needs of the data center. The microturbine-derived waste heat is recovered to run an absorption chiller that supplies the entire cooling load of the data center, essentially providing the requisite cooling without any additional expenditure of primary energy. Furthermore, the waste heat rejected by the data center itself is boosted to a higher temperature with a heat transformer, with the upgraded thermal stream serving as an additional output of the data center with no additional electrical power input. Such upgraded heat can be used for district heating applications in neighboring residential buildings, or as process heat for commercial end uses such as laundries, hospitals and restaurants. With such a system, the primary energy usage of the data center as a whole can be reduced by about 23 percent while still addressing the high-flux cooling loads, in addition to providing a new income stream through the sales of upgraded thermal energy. Given the large and fast-escalating energy consumption patterns of data centers, this novel, integrated approach to electricity and cooling supply, and waste heat recovery and upgrade will substantially reduce primary energy consumption for this important end use worldwide.Copyright