A Hybrid Air Conditioner Driven by a Hybrid Solar Collector
by Ali Al-Alili
The objective of this thesis is to search for an efficient way of utilizing solar energy in air conditioning applications. The current solar Air Conditioners (A/C)s suffer from low Coefficient of Performance (COP) and performance degradation in hot and humid climates. By investigating the possible ways of utilizing solar energy in air conditioning applications, the bottlenecks in these approaches were identified. That resulted in proposing a novel system whose subsystem synergy led to a COP higher than unity. The proposed system was found to maintain indoor comfort at a higher COP compared to the most common solar A/Cs, especially under very hot and humid climate conditions.
The novelty of the proposed A/C is to use a concentrating photovoltaic/thermal collector, which outputs thermal and electrical energy simultaneously, to drive a hybrid A/C. The performance of the hybrid A/C, which consists of a desiccant wheel, an enthalpy wheel, and a vapor compression cycle (VCC), was investigated experimentally. This work also explored the use of a new type of desiccant material, which can be regenerated with a low temperature heat source. The experimental results showed that the hybrid A/C is more effective than the standalone VCC in maintaining the indoor conditions within the comfort zone. Using the experimental data, the COP of the hybrid A/C driven by a hybrid solar collector was found to be at least double that of the current solar A/Cs. The innovative integration of its subsystems allows each subsystem to do what it can do best. That leads to lower energy consumption which helps reduce the peak electrical loads on electric utilities and reduces the consumer operating cost since less energy is purchased during the on peak periods and less solar collector area is needed.
In order for the proposed A/C to become a real alternative to conventional systems, its performance and total cost were optimized using the experimentally validated model. The results showed that for an electricity price of 0.12 $/kW-hr, the hybrid solar A/C's cumulative total cost will be less than that of a standard VCC after 17.5 years of operation.
http://hdl.handle.net/1903/13508
Numerical Modeling and Optimization of Single Phase Manifold-microchannel Plate Heat Exchanger
by Martinus Adrian Arie
In recent years manifold-microchannel technology has received considerable attention from the research community as it has demonstrated clear advantage over state of the art heat exchangers. It has the potential to improve heat transfer performance by an order of magnitude while reducing pressure drop penalty equally impressive, when compared to state of the art heat exchangers for selected applications. However, design of heat exchangers based on this technology requires selection of several critical geometrical and flow parameters. This research focuses on the numerical modeling and an optimization algorithm to determine such design parameters and optimize the performance of manifold-microchannels for a plate heat exchanger geometry. A hybrid method was developed to calculate the total pumping power and heat transfer of this type of heat exchangers. The results from the hybrid method were successfully verified with the results obtained from a full CFD model and experimental work. Based on the hybrid method, a multi-objective optimization of the heat exchanger was conducted utilizing an approximation-based optimization technique. The optimized manifold-microchannel flat plate heat exchanger showed superior performance over a Chevron plate heat exchanger which is a wildly used option for diverse applications.
http://hdl.handle.net/1903/13568
Transient Performance Evaluation of Automotive Secondary Loop Systems
by Magnus Eisele
Automotive air-conditioning is a high impact technology where improvements in energy consumption and environmental performance can make a significant difference in fuel efficiency and comfort. The mandatory phase out of R134a as refrigerant in the European Union has set the stage for new systems and alternative refrigerants. While some of these refrigerants, such as R152a or R290, have a low Global Warming Potential, their flammability requires secondary loop systems to be used. The added thermal mass of such systems may increase power consumption and delay cool down while benefitting thermal comfort during start/stop operation. The recent revival of electric vehicles, as well as the associated focus on air-conditioning energy consumption, provides new challenges and opportunities.
This research focuses on the performance evaluation of refrigerants R152a and R290 during transient operation in secondary loop systems, quantification of thermal storage benefits for start/stop operation, and investigation of energy saving potentials in electric vehicles through the use of advanced air-conditioning system controls and cabin preconditioning.
A test facility was built to dynamically test secondary loop systems over a wide range of pull down conditions and drive cycles using a passenger cabin model and associated controls. It was shown that R290 is a viable alternative in secondary loop systems and system performance may be on par or better compared to R134a direct expansion systems. The preservation of cooling capacity and thermal comfort during off-cycle periods were quantified for a secondary loop system, as well as a combined ice storage system. System efficiency increases with longer off-cycle periods compared to direct expansion systems. Advanced compressor control strategies and the use of cabin preconditioning can make use of this characteristic and improve energy efficiency by more than 50%. Ice storage may be used in combination with cabin preconditioning to preserve comfort for an extended driving time with reduced use of the vapor compression cycle. A Modelica model of the secondary loop system was developed and validated with experimental data. The model enables dynamic simulation of pull-down and drive cycle scenarios and was used to study the effects of coolant volume and coolant concentration on transient performance.
http://hdl.handle.net/1903/13521
Development and analysis of micro polygeneration systems and adsorption chillers
by Kyle Gluesenkamp
About a fifth of all primary energy in the US is consumed by residential buildings, mostly for cooling, heating and to provide electricity. Furthermore, retrofits are essential to reducing this consumption, since the buildings that exist today will comprise over half of those in use in 2050. Residential combined heat and power (or micro CHP, defined by <5 kW electrical generation capacity) has been identified as a retrofit technology which can reduce energy consumption in existing homes during the heating season by 5-30%. This thesis investigates the addition of a thermally-driven chiller/heat pump to a CHP system (to form a trigeneration system) to additionally provide savings during the cooling season, and enhance heating season savings. Scenarios are identified in which adding thermally-driven equipment to a micro CHP system reduces primary energy consumption, through analytical and experimental investigations. The experimental focus is on adsorption heat pump systems, which are capable of being used with the CHP engines (prime movers) that are already widely deployed. The analytical analysis identifies energy saving potential off-grid for today's prime movers, with potential on-grid for various fuel cell technologies.
A novel dynamic test facility was developed to measure real-world residential trigeneration system performance using a prototype adsorption chiller. The chiller was designed and constructed for this thesis and was driven by waste heat from a commercially available natural gas-fueled 4 kW (electric) CHP engine. A control strategy for the chiller was developed, enabling a 5-day experiment to be run using a thermal load profile based on moderate Maryland summer air conditioning loads and typical single-family domestic hot water demand, with experimental results in agreement with models. In this summer mode, depending on electrical loads, the trigeneration system used up to 36% less fuel than off-grid separate generation and up to 29% less fuel than off-grid CHP without thermally driven cooling. However, compared to on-grid separate generation, the experimental facility used 16% more primary energy. Despite high chiller performance relative to its thermodynamic limit, this result is primarily due to the electrical efficiency of the prime mover being lower than the grid. A residential trigeneration system utilizing a high temperature fuel cell is predicted to save up to 42% primary energy relative to the grid.
http://hdl.handle.net/1903/13513
Comparison of Waste Heat Driven and Electrically Driven Cooling Systems for a High Ambient Temperature, Off-Grid Application
by Christopher Philip Horvath
Forward army bases in off-grid locations with high temperatures require power and cooling capacity. Each gallon of fuel providing electrical power passes through a complex network, introducing issues of safety and reliability if this network is interrupted. Instead of using an engine and an electrically powered cooling system, a more efficient combined heat and power (CHP) configuration with a smaller engine and LiBr/Water absorption system (AS) powered by waste heat could be used. These two configurations were simulated in both steady state and transient conditions, in ambient temperatures up to 52°C, providing up to 3 kW of non-cooling electricity, and 5.3 kW of cooling. Unlike conventional AS's which crystallize at high temperatures and use bulky cooling towers, the proposed AS's avoid crystallization and have air-cooled HXs for portability. For the hottest transient week, the results showed fuel savings of 34-37%, weight reduction of 11-19%, and a volumetric footprint 3-10% smaller.
http://hdl.handle.net/1903/13497
High-Performance Tubular Evaporator Utilizing High Aspect Ratio Manifold Microchannels
by Vibhash Chandra Jha
Heat recovery using absorption chillers has not been economical for small scale applications due to high capital requirements and heavy weight/volume as deterring factors for its expanded use in waste heat-to-cooling applications. Development of advanced, high performance heat and mass exchanger components can significantly improve the competitive edge of heat activated absorption cooling systems, particularly with respect to weight reduction and size/volume of these systems. The main contribution of this thesis is demonstration of a novel high performance micro-grooved evaporator, as well as a solution heat exchanger, for use in a small-scale ammonia-water absorption cooling system. A compact tubular evaporator was developed which uses an innovative manifold/fluid feed system combined with a micro-grooved evaporator to realize substantially higher (4 to 5 fold) overall heat transfer coefficient of the evaporator; while requiring much less refrigerant charge per ton of cooling, when compared to conventional state of the art systems. The experimentally measured heat transfer coefficients reported in this study are record high, while pressure drops for the given capacity are modest.
Additional contributions of the study included a detailed numerical study of single- stage absorption cycle with multiple cycle design enhancements to identify the controlling system parameters. A single-phase numerical study for manifold microchannel design was carried out to understand the effect of important geometrical parameters in support of design and development of the evaporator. The tubular evaporator was successfully fabricated and tested to the system pressure of 500 psi on the refrigerant-side and was experimentally evaluated with several microgroove surface made of aluminum and nickel alloys, and also with different flow header enhancements using R134a/water pair. For the experiments conducted, the microchannel width was typically in the range of 30-100 µm with a maximum aspect ratio of 10. The refrigerant flow rate was varied within 5-30 g/s and water flow rate was varied within 150-600 ml/s obtaining wide range of cooling capacity between 1- 5 kW for 2-12 °C LMTDs. The overall heat transfer coefficients greater than 20,000 W/m2-K was obtained which is roughly 4-5 times higher than state of art for given application. A maximum pressure drop of 200 mbars on water-side and 100 mbars on the refrigerant-side was observed at maximum mass flow rates.
An alternative method for the evaporator design was also explored in form of flat plate evaporators which can further provide improved overall heat transfer coefficients. Manifold microchannels were used on both sides of the plates, with the aim to achieve overall heat transfer coefficient greater than 50,000 W/m2-K.
The new micro-grooved evaporator has the potential to introduce a game-changing evaporative surface, with precise flow delivery and high heat transfer coefficients, driven by a combination of thin film evaporation, as well as convective boiling on the heat transfer surface.
http://hdl.handle.net/1903/13511
Thermal and Hydraulic Performance of Heat Exchangers for Low Temperature Lift Heat Pump Systems
by Hoseong Lee
The work presented in this dissertation focused on investigating and understanding the hydraulic and thermal design space and tradeoffs for low temperature difference high performance heat exchangers for a low temperature lift heat pump (LTLHP) system, which benefits from a small difference between the condensing and evaporating temperatures of a working fluid. The heat exchangers for the LTLHP application require a larger heat transfer area, a higher volume flow rate, and a higher temperature of heat source fluid, as compared to the typical high temperature lift heat pump system. Therefore, heat exchanger research is critical, and it needs to be balanced between the heat transfer and pressure drop performance of both fluids in the heat exchanger. A plate heat exchanger (PHX) was selected to establish a baseline of a low temperature lift heat exchanger and was investigated experimentally and numerically. The traditional PHX is designed to have the identical surface area and enhancements on both fluid sides for ease of production. However, fluid side heat transfer coefficients and heat transfer capacities can be drastically different, for example, single-phase water versus two-phase refrigerant. Moreover, the PHX needs to have a large cross sectional flow area in order to reduce the heat-source fluid-side pressure drop. In the experimental test, the PHX showed a relatively low overall heat transfer performance and a large pressure drop of the heat source fluid side under LTLHP operating conditions. The CFD simulation was carried out to further improve the potential of the PHX performance. However, there were limitations in the PHX. It was concluded that the PHX was restricted by two main factors: one was a large pressure drop on the heat source fluid-side due to corrugated shape, and the other was low overall heat transfer performance due to the low refrigerant-side mass flux and resulting low heat transfer performance. A concept of a novel low temperature lift heat exchanger (LTLHX) has been developed based on the lessons learned from the PHX performance investigation for the application to the LTLHP. Geometries were newly defined such as a channel width, channel height, channel pitch, and plate flow gap. Two design strategies were applied to the novel heat exchanger development: the flow area ratio was regulated, and plates were offset. The design parameters of the novel heat exchanger were optimized with multi scale approaches. After developing the laboratory heat exchanger test facility and the prototype of the novel LTLHX, its performance was experimentally measured. Then the thermal and hydraulic performance of the novel LTLHX was validated with experimental data. The heat transfer coefficient correlations and the pressure drop correlations of both the water-side and refrigerant-side were newly developed for the novel LTLHX. The overall heat transfer performance of the novel LTLHX was more than doubled as compared to that of the PHX. Moreover, the pressure drop of the novel heat exchanger was drastically lower than that of the PHX. Lastly, the novel heat exchangers were applied to a water source heat pump system, and its performance was investigated with parametric studies.
http://hdl.handle.net/1903/12969
Application of Plate Heat Exchanger for Low Temperature Lift Heat Pump Systems
by Song Li
In this thesis I investigated the energy saving potential of a low temperature lift heat pump (LTLHP) such as water source heat pump (WSHP), and studied the utilization of a plate heat exchanger (PHE) as the LTLHP evaporator. Due to the facility limitation, I only tested and studied the evaporator for LTLHP. Since the LTLHP requires a large water flow rate, its heat source liquid-to-refrigerant PHE is operated at lower refrigerant mass flux than typical applications. I varied the vapor quality, heat flux, evaporation pressure, and refrigerant mass flux to provide unique heat transfer characteristics, and I studied their effects on evaporation heat transfer. Based on the collected data, I concluded that at a low mass flux range, evaporation heat transfer is dominated by nucleate boiling, and convective boiling has mall influence. In addition, I carried out a simulation to compare the performance of WSHP with air source heat pump (ASHP).
http://hdl.handle.net/1903/13558
Investigation of vapor injection heat pump system with a flash tank utilizing R410A and low-GWP refrigerant R32
by Xing Xu
Vapor injection technique has proven to be effective in improving heat pump system performance, especially for cooling application at high ambient and heating application at low ambient temperature conditions. Recent research on vapor injection technique has been mostly focused on the internal heat exchanger cycle and flash tank cycle. The flash tank cycle typically shows better performance than the internal heat exchanger cycle. However, the flash tank cycle control strategy is not yet clearly defined. Improper system control strategy would result in undesirable amount of liquid refrigerant injected to the compressor or poor system performance.
In this research work, a novel cycle control strategy for a residential R-410A vapor injection flash tank heat pump system was developed and experimentally investigated. The proposed cycle control strategy utilizes an electronic expansion valve (EEV) coupled with a proportional-integral-derivative (PID) controller for the upper-stage expansion and a thermostatic expansion valve (TXV) for the lower-stage expansion, and applies a small electric heater in the vapor injection line to introduce superheat to the injected vapor thus providing a control signal to the upper-stage EEV. The proposed control strategy functions effectively for both transient and steady-state operating conditions.
As global warming has raised more critical concerns in recent years, refrigerants with high global warming potentials (GWP) are facing the challenges of being phased out. R410A, with a GWP of 2,088, has been widely used in residential air-conditioners and heat pump systems. A potential substitute for R410A is R32, which has a GWP of 675. This research work also investigates the performance difference using R410A and R32 in a vapor-injected heat pump system. A drop-in test was performed using R32 in a heat pump system that is designed to utilize R410A, for both cooling and heating conditions. Through experimentation, it was found that there was improvement for capacity and coefficient of performance (COP) using R32, as compared to an identical cycle using R410A. The compressor, heat exchangers and two-stage vapor injection cycle have been modeled and validated against experimental data to facilitate an optimization study. Heat exchangers were optimized using 5 mm copper tubes and result in significant cost reduction while maintaining the same capacity. Compressor cooling was investigated to decrease the high compressor discharge temperature for R32.
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