Air Conditioning Hybrid Electric Vehicles while Stopped in Traffic

HEVs idle their engine during the stops to meet the cooling and heating needs. But, idling decreases fuel economy and increases engine wear and emissions. The report explores alternative strategies for air conditioning the HEV during the stop times. Simulation analyses are used to identify fundamental differences and new technology tradeoffs encountered in HEVs. An analysis of cooling and heating loads on a car under typical weather and driving conditions is combined with efficiency estimates for an advanced a/c system to compare different cooling strategies in terms of fuel usage and overall system COP. Options considered include belt and electrically driven compressors, with thermal and electrical storage technologies. The results of this parametric analysis narrow the range of cooling and heating strategies to be considered for detailed analysis and prototype testing.Air Conditioning and Refrigeration Project 14

Development, Validation, and Application of a Refrigerator Simulation Model

This report describes the further development and validation of the Refrigerator/Freezer Simulation (RFSIM) model. The reports also describes the first major application of the model as an analysis tool for new refrigerator designs; several aspects of multi-speed compressor operation were examined with the model. Several improvements were made to the model that facilitated the validation process and the examination of multi-speed compressors: the model was made more general so that it could operate in numerous configurations in addition to the original design and simulation modes; many improvements were made in the modeling logic and robustness of the capillary tube-suction line heat exchanger model; and the equation-of-statebased property routines that calculated the thermodynamic properties were replaced with interpolation routines that were much faster. The RFSIM model, in design and simulation mode, was validated with data from two refrigerators. In both modes, the average model errors were less than ??5% for several important variables such as evaporator capacity and coefficient of performance. The errors of the simulation mode were reduced from the previous model validation primarily by using a different void fraction correlation in the refrigerant charge equations. The results from the validated RFSIM model indicate that a two-speed compressor could yield energy savings of 4% to 14% due to the increased steady-state efficiency at the low speed and an additional 0.5 to 4% savings due to the decreased cycling frequency. The results also showed that the capillary tube-suction line heat exchanger, when designed for the low speed, did not adversely affect the pull-down capacity when the compressor operated at the high speed. Lastly, it was found that a refrigerator operating at low ambient temperatures could actually benefit from a decrease in the condenser fan speed. This change in fan speed increased the evaporator capacity by reallocating charge to the evaporator and subsequently reducing the superheat at the evaporator exit.Air Conditioning and Refrigeration Project 6

Multiple Heat Exchangers Simulation Within the Newton-Raphson Framework

A general framework is proposed for simulating complex heat exchanger geometries in a manner suitable for sequential solution of the refrigerant- and air-side equations for mass, momentum and energy. The sequential solution enables the algorithm to be applied to a single module of a complex heat exchanger, and then integrated with other modules within a simultaneous equation solver employing a Newton-Raphson approach. This report also describes the integration of component subroutines into system simulation models for air conditioners and refrigerators. The modular approach is illustrated by describing its application to a dual-evaporator refrigerator simulation.Air Conditioning and Refrigeration Project 6

Optimization of Heat Exchanger Design Parameters for Hydrocarbon Refrigerant Systems

Hydrocarbon refrigerants (HC's) are one alternative to hydrofluorocarbons (HFC???s) since they have zero ozone depletion potential and negligible global warming potential. However, due to their flammable nature, the amount of refrigerant used in systems is regulated for safety reasons. This report presents simulation results for a 3-ton R290 (propane) air-conditioning system, and identifies the optimum heat-exchanger geometries that would minimize system charge while trying to retain the same system efficiency. An existing R410A microchannel system simulation served as the base case, and then the geometries were optimized for the R290 system, and the results were compared to the base case. The model was then analyzed for the off-design conditions, and the conclusions presented. The optimal condenser geometry tended to have smaller port diameter and core depth with thicker webs between the ports. Also, the fins tended to be taller, thinner and more densely packed. Similar results were noted for the evaporator geometry. The optimal design reduced the combined heat exchanger charge by more than a factor of 5. The system efficiency was reduced by 3% in the process, but the loss could be recovered because the pressure drop was low enough to permit increasing the air-flow rates. The off-design behavior of the R290 microchannel system is very different from a traditional R410A round-tube plate-fin system. Typically with the increase in ambient temperature, charge from the evaporator and the liquid line moves to the condenser. In the R290 system, because of the oil/refrigerant solubility characteristics, charge from the compressor sump also moves to the condenser. In the microchannel systems, the heat exchangers account for only 20% of the system charge as opposed to 70% in the tube fin systems. At higher ambient temperatures, the additional charge flowing from the other components, provides the condenser with the additional ~7% charge it needs at hot ambient conditions. However, due to the small internal volume of the heat exchangers in microchannel systems, an additional 60% charge flows into the condenser, resulting in high values of subcooling, thus reducing system efficiency. One solution to this problem would be to install a receiver at the outlet from the condenser, to retain high levels of efficiency across a wide range of operating conditions.Air Conditioning and Refrigeration Project 14

Assessment of Factors Contributing to Refrigerator Cycling Losses

Thermal mass effects, refrigerant dynamics, and interchanger transients are three factors affecting the transient and cycling performance of all refrigeration and air conditioning equipment. The effects of refrigerant dynamics, including refrigerant/oil solubility, off-cycle migration, and charge redistribution, were found to be the most important. These effects are quantified for a refrigerator instrumented with immersion thermocouples, pressure transducers, and microphones. The analytical methods, however, are applicable to other types of refrigeration and air conditioning systems, including those with capillary tube/suction line heat exchangers.Air Conditioning and Refrigeration Center Project 3

An Evaluation of Heat Exchangers Using System Information and PEC

This report describes analyses aimed at integrating component optimization and system design by developing heat-exchanger performance evaluation criteria (PEC) that account for the system-level performance impacts of heat exchanger design. It builds on earlier studies that used relatively simple PEC to capture some of the component-level tradeoffs, but which usually ignore the system impact of component design. This report evaluates four PEC-j/f, heat transfer/pumping power (8), heat transfer/(pumping + compressor power) (n), and system COP. It is shown that j/f and 8 are better used as comparison criteria for existing heat exchangers of equal heat duty rather than as design criteria. The other two PEC, n and COP, include the system effect of compressor efficiency and therefore can be used more effectively in heat exchanger and system design. Through a combination of PEC and system optimization techniques, a method is developed to evaluate and design heat exchangers for maximum system performance.Air Conditioning and Refrigeration Project 9

Simulating the Performance of a Heat Exchanger During Frosting

Factors affecting frost distribution are explored using a finite element model, developed and validated using a full-scale 8-row heat exchanger in a wind tunnel. The heat exchanger is typical of the type used in supermarket display cases; so face velocities and air inlet temperatures were varied from 0.5-2.3 m/s and 0 to -20 ??C, respectively, and inlet humidities from 70-90%. In order to focus on frost distribution, the prototype was designed to have a simple geometry and single-phase refrigerant to provide maximum certainty on parameters not directly related to frost. Measured and predicted total and sensible heat transfer agreed within RMS 6% and 8%, respectively, over the range of operating conditions. For latent heat, there was more scatter due to frost nonuniformities induced by the experimental apparatus. The simulation model was used to illustrate how the point of maximum frost thickness moved from the front to the rear of the heat exchanger, depending on face velocity, inlet humidity and fin surface temperature. Heat transfer and pressure drop were calculated from standard correlations, with fin thickness and tube diameter increasing as a function of frost thickness. The model was further extended to simulate the performance of the heat exchanger under the effect of a fan curve. A comparison is made between DX and indirect refrigeration system performance with respect to capacity, pressure drop and air flow variations under frosting conditions.Air Conditioning and Refrigeration Project 10

Fault Detection and Diagnosis in Air Conditioners and Refrigerators

A fault detection and diagnosis (FDD) method was used to detect and diagnose faults on both a refrigerator and an air conditioner during normal cycling operation. The objective of the method is to identify a set of sensors that can detect faults reliably before they severely hinder system performance. Unlike other methods, this one depends on the accuracy of a number of small, on-line linear models, each of which is valid over a limited range of operating conditions. To detect N faults, N sensors are needed. Using M>N sensors can further reduce the risk of false positives. For both the refrigerator and air conditioner systems, about 1000 combinations of candidate sensor locations were examined. Through inspection of matrix condition numbers and each sensor's contribution to fault detection calculation, the highest quality sets of sensors were identified. The issue of detecting simultaneous multiple faults was also addressed, with varying success. Fault detection was verified using both model simulations and experimental data. The results were similar, although in practice only one of the two would probably be used. Both load-type faults (such as door gasket leaks) and system faults were simulated on the refrigerator. It was found that system faults were generally more easily detectable than load faults. Refrigerator experiments were performed on a typical household refrigerator because it was readily available in a laboratory, but the results of this project may be more immediately useful on larger commercial, industrial or transport refrigeration systems. Air conditioner experiments were performed on a 3-ton split system. Again, the economic benefits of this type of fault detection scheme may also be more feasible for larger field-assembled systems.Air Conditioning and Refrigeration Project 8

Environmental controls, morphodynamic processes, and ecogeomorphic interactions of barchan to parabolic dune transformations

The transformation of barchans into parabolic dunes has been observed in various dune systems around the world. Precise details of how environmental controls influence the dune transformation and stabilisation mechanism, however, remain poorly understood. A ‘horns-anchoring’ mechanism and a ‘nebkhas-initiation’ mechanism have previously been proposed and selected environmental controls on the transformation have been explored by some modelling efforts, but the morphodynamic processes and eco-geomorphic interactions involved are unclear and comparison between different dune systems is challenging. This study extends a cellular automaton model, informed by empirical data from fieldwork and remote sensing, to fully explore how vegetation characteristics, boundary conditions, and wind regime influence the transformation process and the resulting dune morphologies. A ‘dynamic growth function’ is introduced for clump-like perennials to differentiate between growing and non-growing seasons and to simulate the development of young plants into mature plants over multiple years. Modelling results show that environmental parameters interact with each other in a complex manner to impact the transformation process. The study finds a fundamental power-law relation between a non-dimensional parameter group, so-called the ‘dune stabilising index’ (S⁎), and the normalised migration distance of the transforming dune, which can be used to reconstruct paleo-environmental conditions and monitor the impacts of changes in climate or land-use on a dune system. Four basic eco-geomorphic interaction zones are identified which bear different functionality in the barchan to parabolic dune transformation. The roles of different environmental controls in changing the eco-geomorphic interaction zones, transforming processes, and resulting dune morphologies are also clarified