Compact Absorber for Advanced Absorption Heat Pumps
by Stefan Bangerth
Almost half of all energy contained in primary energy carriers is discarded as low temperature waste heat. One of few application areas for low temperature waste heat recovery is to drive absorption cooling systems for conversion of waste heat to cooling energy. However, absorption chillers are often not economical due to their bulky, and hence expensive, heat and mass exchangers; the absorber heat/mass exchanger being the largest among them. This dissertation introduces original contributions to advance next generation, more economical absorption chillers by utilizing a novel, highly compact absorber. The novel absorber designed in this work enhances absorption performance by combining rotation of the heat transfer surface for solution-side heat and mass transfer enhancement with innovative high-performance heat transfer technology on the water-side. A numerical model was developed to describe the absorption process and promote design optimization. The replacement of gravitation force by the stronger centrifugal acceleration thins and mixes the solution film and thereby decreases solution-side thermal and mass transfer resistance. The development of an original adaptation of manifold-microchannel technology leads to significant water-side heat transfer enhancement. This dissertation includes the first publication of an experimental characterization of exothermic absorption on a spinning disk. The overall and film-side heat transfer coefficients were 4.7 and 5.5 times higher, respectively, than conventional horizontal tube banks. The absorption rate increased by a factor of 4 to 10 folds over those of the conventional tube absorbers. The power required for spinning the disk was modest and ranged between 1.1% and 2.3% of the cooling capacity. The results suggest that a spinning disk absorber could substantially reduce the size of absorber in the absorption machines. The technology developed in this dissertation can lead to more compact and hence more economical absorption chillers, thereby easing higher market penetration of absorption chillers which in turn can reduce the amount of primary energy spent on cooling applications. Spinning disk absorbers may be especially useful if combined with a new generation of absorbents that promise improved system efficiency and/or expanded application range but exhibit challenging thermophysical properties.
Doctoral Dissertation
http://hdl.handle.net/1903/21359
CO2 Transcritical Refrigeration With Mechanical Subcooling: Energy Efficiency, Demand Response and Thermal Storage
by John Bush
This dissertation examines two important concepts: improvements to transcritical carbon dioxide (CO2) refrigeration systems being deployed in supermarkets, and their potential use for demand response and load shifting in a utility-connected application. As regulatory pressure increases to reduce the use of ozone depleting and greenhouse gases as refrigerants, the heating, ventilation, air conditioning and refrigeration (HVAC&R) industry is moving towards alternative refrigerants including natural substances such as carbon dioxide. CO2 has already gained traction as the refrigerant of choice for supermarket applications in some countries, but deployment in warmer climates has been slower due to concerns over efficiency when the cycle operates in transcritical mode. Among the cycle enhancements considered to overcome these concerns is the use of dedicated mechanical subcooling. Laboratory testing was performed on a transcritical booster system with mechanical subcooling to quantify the system performance with and without the subcooler. Data was used to develop and validate transient models, which in turn were used to study the system-wide effects of demand response, particularly short-term shedding of medium or low temperature load. Systems can provide value to the electric grid if they can be responsive to changes in electric utility generation, as indicated by direct calls to shed load or price signals. To further expand the potential usefulness of the refrigeration cycle in grid-interactive operation, the integration of thermal storage is considered. In particular, the integration of thermal storage into the subcooling system is investigated. The mechanical subcooler is used to “charge” a storage media (such as water or another phase change material) overnight, and the storage media allows the subcooler to turn off during peak hours. This allows the system to shift load and allow temporary reduction in electric power usage without a reduction in delivered refrigerating capacity. These two paths are potentially complementary: the load shifting of the integrated thermal storage provides long-term load reduction, while direct load shedding in evaporators allows more agile, short-term reductions. The models developed and validated with laboratory data and expanded upon with thermal energy storage and demand response approaches provide new learnings into enhanced load shifting and demand response capability. The findings of this work show that particularly in time-of-use rate structures with a high ratio of on-peak to off-peak pricing, the thermal storage and load shedding strategies here can provide a reduction in total refrigerating energy cost, even though the changes proposed introduce a slight increase in daily energy under the simulated conditions. In a simulated hot day for Baltimore, Maryland, the energy consumption was 2.6% higher using the thermal storage system than without. In the most extreme case, comparing an aggressive real-world Time-of-Use rate with thermal storage and load shedding against a flat-rate case from the same utility and no controls or storage, a cost savings reduction of 21% was calculated. Comparing baseline operation against a controlled load-shifting strategy under the same time-of-use rate plan, the cost reduction was in the range of 2.8-8.7% depending upon the specific plan.
Doctoral Dissertation
http://hdl.handle.net/1903/20846
Dynamics of Vapor Compression Cycle with Thermal Inertia
by Rohit Dhumane
The use of heating, ventilation, air conditioning, and refrigeration (HVACR) systems is always increasing. Reducing energy consumption has become necessary in modern times for environmental, economic and legislative reasons. Thus, there is ongoing research to improve the performance and reduce the negative environmental impact of these systems. HVACR systems are normally sized for peak load conditions. As a result, these systems operate under off-peak conditions most of the time by on-off cycling. The average efficiency of the system during cycling is lower due to transient losses caused by refrigerant migration and redistribution. This motivates a detailed understanding of the dynamics of vapor compression systems (VCS) for their improved design and performance. The dissertation contributes towards reducing energy consumption from HVACR by exploring both sides: improving the performance of current systems and developing highly energy efficient personal conditioning systems (PCS) which reduce the load of HVACR systems altogether. PCS reduce the energy consumption of building HVACR by up to 30%. Multi-physics modeling for thermo-fluid, electricity and mechanical domains is conducted to compare performance of four PCS employing different thermal storage options. The dissertation then focuses on vapor compression system based version of PCS called Roving Comforter, operating cyclically between its cooling and recharge mode. Exhaustive study of design space including optimization of thermal storage, operation with a natural refrigerant and alternate recharge modes is conducted to improve its overall coefficient of performance. The dissertation then presents comprehensive dynamic validated modeling of air-conditioning systems operating in cyclic operations to characterize cyclic losses. Parametric study with different operating conditions is carried out to provide guidelines for reduction of these cyclic losses. Secondly, a physically based model of the test setup for quantifying the cyclic losses of air-conditioning systems is developed and used to understand its influence on the cyclic losses. A new term called “Thermal Inertia Factor” is defined to enable more uniform rating of equipment from various test centers and help selection of actual energy efficient air-conditioners.
Doctoral Dissertation
http://hdl.handle.net/1903/21342
Plate Heat Exchanger Improvements with Shape Optimization
by Radia Eldeeb
Plate Heat Exchangers (PHXs) are used in a wide variety of applications including, but not limited to Heating, Ventilation, Air-Conditioning, and Refrigeration (HVAC&R). PHXs are favored by the HVAC&R industry due to their compactness, flexible sizing, close approach temperature, and good heat transfer performance. PHXs are increasingly utilized and becoming very competitive in two-phase flow applications due to their desirable thermal-hydraulic characteristics. Pillow plate heat exchanger (PPHX) is a type of PHXs that consists of wavy plates that are welded together with a certain pattern using spot welding, sealed at the edges, and then inflated in a hydroforming process. PPHXs have an economic advantage over other types of PHXs due to their simple manufacturing process. Additionally, the complex wavy structure of the pillow plates creates an excellent heat transfer medium and thus, if their performance is optimized, they can potentially replace other types of PHXs in a wide range of applications. However, very limited research is done regarding the use of PPHXs in the HVAC&R and no research on their optimization is found in literature. The first objective of this thesis is the optimization of PPHXs using four design parameters including their basic geometry parameters using Parallel Parameterized Computational Fluid Dynamics (PPCFD) and Approximation Assisted Optimization (AAO). The potential improvement in thermal-hydraulic performance is expected to be at least 50% as compared to existing designs. The second objective is to perform a comprehensive multi-scale analysis with topology and shape optimization integrating Non-Uniform Rational B-Splines (NURBS) to obtain a novel PPHX design with at least 20% improvement in thermal-hydraulic performance as compared to optimal chevron PHXs designs. This will improve energy efficiency significantly on the component level and potentially on the system level. Finally, a comprehensive literature survey shows a significant gap regarding PHXs modeling with respect to combining robustness, accuracy, flexibility, and convenient speed into a single model. A current PHX computer model in literature is significantly improved in the aforementioned aspects using a novel algorithm. The model is also used as a component on a system level modeling to evaluate the performance of PHXs on the system level.
Doctoral Dissertation
http://hdl.handle.net/1903/21336
Design and Performance Characterization of an Additively-Manufactured Heat Exchanger for High Temperature Application
by Xiang Zhang
In its early stages of development, additive manufacturing was used chiefly for prototyping, but over the last decade, its use has evolved to include mass production of certain products for numerous industries in general, and speciality industries such as biomedical and aerospace industries in particular. Additive manufacturing can be used to fabricate unconventional/complex designs that are difficult and time-consuming through conventional fabrication methods, but offer significant performance advantage over state of the art. One such example is high temperature heat exchangers with complex novel geometries that can help improve the heat transfer density and provide better flow distribution, resulting in more compact and efficient designs and thereby also reducing materials costs considering fabrication of these heat exchangers from the suitable super alloys with the conventional manufacturing techniques is very difficult and laborious. This dissertation presents the results of the first high-temperature gas-to-gas manifold-microchannel heat exchanger successfully fabricated using additive manufacturing. Although the application selected for this dissertation focuses on an aerospace pre-cooling heat exchanger application, the results of this study can still directly and indirectly benefit other industrial sectors as heat exchangers are key components of most power conversion systems. In this work, optimization and numerical modelling were performed to obtain the optimal design, which show 30% weight reduction compared to the design baseline. Thereafter, the heat exchanger was scaled down to 66 × 74 × 27 mm3 and fabricated as a single piece using direct metal laser sintering (DMLS). A minimum microchannel fin thickness of 165 μm was achieved. Next, the additively manufactured headers were welded to the heat exchanger core and the conventionally manufactured flanges. A high-temperature experimental loop was next built, and the additively manufactured heat exchanger was successfully tested at 600°C with ~ 450 kPa inlet pressure. A maximum heat duty of 2.78 kW and a heat transfer density close to 10 kW/kg were achieved with cold-side inlet temperature of 38°C during the experiments. A good agreement between the experimental and numerical results demonstrates the validity of the numerical models used for heat transfer and pressure drop predictions of the additively manufactured heat exchanger. Compared to conventional plate-fin heat exchangers, up to 25% improvement in heat transfer density was achieved. This work shows that additive manufacturing can be used to fabricate compact and lightweight high temperature heat exchangers, which benefit applications where space and weight are constrained.
Doctoral Dissertation
http://hdl.handle.net/1903/21697
Refrigerant Charge Distribution in Unitary Air-conditioning Units
by Tianyue Qiu
To address the global warming, reduction of refrigerants charge in HVAC systems is becoming an essential topic. An experimental setup was designed and built for investigating the cyclic degradation performance and the refrigerant charge distribution of a unitary air-conditioning unit. A code tester featuring two parallel ducts was developed to properly control air flows according to the ASHRAE standard test conditions. First, baseline tests were carried out under ASHRAE standard test conditions. System performance was compared to manufacturer’s specifications to validate the accuracy of the experimental setup. Second, the transient performance of the unit was analyzed and presented, and the degradation coefficient of the unit was evaluated. Third, an approach of refrigerant charge distribution measurement was proposed and implemented. Refrigerant R410A charge distribution tests were conducted under steady-state conditions and the results were compared with published results of the refrigerant charge distribution in various types of air-conditioning units. It was concluded that the system charge distribution was significantly influenced by various component combinations and the length of refrigerant copper tubing.
Master's Thesis
http://hdl.handle.net/1903/21413
Two-Phase Flow Regimes and Heat Transfer in a Manifolded-Microgap
by David Deisenroth
Embedded cooling—an emerging thermal management paradigm for electronic devices—has motivated further research in compact, high heat flux, cooling solutions. Reliance on phase-change cooling and the associated two-phase flow of dielectric refrigerants allows small fluid flow rates to absorb large heat loads. Previous research has shown that dividing chip-scale microchannels into parallel arrays of channels with novel manifold designs can produce very high chip-scale heat transfer coefficients with low pressure drops. In such manifolded microchannel coolers, the coolant typically flows at relatively high velocities through U-shaped microgap channels, producing centripetal acceleration forces on the fluid that can be several orders of magnitude larger than gravity. Furthermore, the manifolded microchannels consist of high aspect ratio rectangular channels, short length to hydraulic diameter ratios (L/Dh < 100), and step-like inlet restrictions. The existing literature provides only limited information on each of these effects, and nearly no information on the combined effects, on fluid flow and heat transfer performance.
This study provides fundamental insights into the impact of such channel features and coupled fluid forces on two-phase flow regimes and their associated transport rates. Moreover, because the flows in manifolded microchannel chip coolers are very small and optically inaccessible, a custom visualization test section was developed. The visualization test section was supported by a custom two-phase flow loop, which incorporated three power supplies, two chillers, and two dozen thermofluid sensors with live data monitoring. The fluid flow and wall heat transfer in the visualization test section was simultaneously imaged with a high-speed optical camera and a high-speed thermal camera. The results were reported with maps of the flow regime, wall temperature, wall temperature fluctuation, superheat, heat flux, and heat transfer coefficients under varying heat fluxes and mass fluxes for three manifold designs. Variations in flow phenomena and thermal performance among manifold designs and between R245fa and FC-72 were established. The post-annular flow regime of annular-rivulet was associated with a precipitous decline in wall heat transfer coefficients. The current experimental campaign is the first in the open literature to study the thermal and hydrodynamic characteristics of manifolded-microgap channels in such detail.
Doctoral Dissertation
http://hdl.handle.net/1903/21681
System Performance Enhancement of Mobile Cooling System With Thermal Battery and Thermosiphon Recharge
by Darren Thomas Key
A personalized mobile cooling device was modified and tested with different components to deliver better system performance. The device uses a miniaturized vapor compression cycle (VCC) to deliver approximately 165 W of cooling to an individual. The device stores the waste heat from the VCC condenser in a phase change material (PCM) carried on-board the device. The PCM is then recharged by rejecting heat stored in the PCM with a thermosiphon recharge cycle. The PCM was enhanced with copper and graphite matrices. The system was tested with the goal of increasing the coefficient of performance (COP) of the VCC and decreasing the PCM recharge time. This study found that a copper enhancement provided the highest COP at 4.43, an improvement over the baseline COP of 2.41.
Master's Thesis
http://hdl.handle.net/1903/20794
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