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The market for variable-speed ductless heat pump (DHP) systems has grown in North America in recent years. However, the tools available for modeling their performance within building simulation programs have not kept pace. In general, simple empirical models are typically used for characterizing equipment performance for use in system simulation. However, unlike more conventional single-speed systems, DHPs have complicated control algorithms for managing electronic expansion valve opening, compressor speed, fan speed, and defrost operation. Very little work has been done in the development of empirical models that explain the impact of these characteristics on the performance of DHP systems. In this article, an empirical approach is introduced to model the heating and defrost performance of DHP systems under a wide range of conditions. The model incorporates separate relationships for performance associated with maximum, minimum, and intermediate (part-load) heating capacities. The defrost operation of the systems is also modeled empirically. The approach was tested on data from two DHP systems and the agreement is very good. The performance of the systems under different building loads was simulated and it is shown that the performance is highly dependent on the system control characteristics. [ABSTRACT FROM AUTHOR]

Originally created and developed for space applications, several commercial terrestrial technologies still permeate our society today. Examples include solar cells, Global Positioning Systems, and less expensive methods of carbon nanotube manufacture. Given a long and successful history of spinoffs, there might exist opportunities for the transfer of modern spacecraft thermal control technologies to terrestrial HVAC&R applications. First, this paper presents a broad overview of spacecraft thermal control. Next, a review of several modern spacecraft thermal control subsystem technologies is provided, each including an assessment of their potential use for terrestrial applications. [ABSTRACT FROM AUTHOR]

Refrigerant pressures are critical measurements for monitoring, control, diagnostics, and optimization of vapor compression cycle equipment. Direct refrigerant pressure measuring practices are expensive and more than often problematic. This paper describes a method, termed virtual pressure sensing, wherein refrigerant pressure values are indirectly derived from low-cost temperature sensors that can be surface-mounted. In this manner, physical pressure sensors are eliminated and pressure sensing can be achieved at a much lower cost and in a non-invasive way. Five virtual pressure sensors are developed to obtain the five most important pressures in vapor compression cycle equipment: compressor discharge line pressure, condensing pressure, liquid line pressure, evaporating pressure, and suction line pressure. The performance of the proposed virtual pressure sensors, in terms of accuracy in estimating pressures and inferring liquid line subcooling, suction superheat, compressor power consumption, and refrigerant flow rate, is evaluated extensively using laboratory data collected from four systems. These systems include air conditioners and heat pumps, split and packaged systems, refrigerants R-22 and R-410α, fixed-orifices and thermal expansion valves, and reciprocating compressors and scroll compressors. Ultimately, the virtual sensors are used as a part of a decoupling-based fault detection and diagnosis (FDD) technique to diagnose multiple simultaneous faults. The impact of the virtual pressure sensors on the FDD performance is evaluated extensively using the laboratory data collected from the four various systems. [ABSTRACT FROM AUTHOR]

Proper refrigerant charge level is critical for vapor-compression cycle refrigeration systems to operate efficiently and safely. The current practice of accurately determining charge levels is very time consuming and costly. Technicians must stop and evacuate the system, weigh the removed charge, and add the correct amount of charge to the system using a scale. This paper presents a method for obtaining accurate estimates of refrigerant charge level using noninvasive temperature measurements obtained while the system is operating. The method could be used as part of a permanently installed control or monitoring system to indicate charge level and/or to automatically detect and diagnose low or high levels of refrigerant charge. It could also be used as a stand-alone tool by technicians to determine existing charge and during the process of adjusting the refrigerant charge. The accuracy of the virtual refrigerant charge sensor is evaluated using laboratory data for seven different systems and over a wide range of operating conditions with and without the presence of other faults. The evaluation and demonstration verify that the virtual refrigerant charge level gauge has very good performance in terms of accuracy and robustness and can be easily implemented and installed in terms of both hardware and software. [ABSTRACT FROM AUTHOR]

Accurate mixed-air temperature (MAT) measurements are notoriously difficult to obtain in packaged air-conditioning equipment for small commercial applications because of space constraints and the use of small mixing chambers. However, MAT is important for control of economizers and in diagnostic algorithms for both economizers and vapor compression equipment. This paper demonstrates that a single-point measurement of MAT can provide accurate results when combined with a correlation for bias error that depends on damper control signal and difference between outdoor and return-air temperature. The correlation could be determined using a self-calibration procedure with measurements of mixed, outdoor, return, and supply air temperatures. A system was tested in a laboratory over a wide range of outdoor conditions and damper positions, and the single-point measurement errors were reduced by about a factor of five when using the bias correction. Furthermore, errors in outdoor air fraction (OAF) determined from temperature measurements were reduced by about a factor of four. [ABSTRACT FROM AUTHOR]

Most of the previous diagnostics research for vapor-compression equipment has been performed for packaged air conditioners and chillers. This paper describes application of a decoupling-based diagnostic technique that was originally developed for air conditioners to equipment used in walk-in coolers and freezers. Testing was performed on a small, restaurant- style walk-in cooler and a small walk-in freezer for faulted and unfaulted conditions. The data was then used to characterize how the fault features respond to faults and to evaluate the performance of the decoupling-based diagnostic technique. [ABSTRACT FROM AUTHOR]

This paper presents the development and validation of a computer model for hermetic two-stage rotary compressors that can be used for design optimization. The model considers the effects of refrigerant leakage between the compressor chambers, heat transfer between the cylinder wall and the refrigerant gas, and heat loss by natural convection to the surroundings. The model numerically solves mass and energy balances for each of the compressor's chambers to predict the pressure and temperature variations over a crankshaft revolution. The compressor power input and refrigerant mass flow rate are calculated and used to determine the isentropic and volumetric efficiencies. The model can operate both with and without intercooling between the stages to allow for study of the effects of intercooling on compressor performance. The model results are compared to the results of external performance tests that were conducted using a prototype compressor. The intermediate temperature and pressure, discharge temperature, power consumption, and mass flow rate were recorded for 17 different operating conditions. The model predicts the compressor power consumption and mass flow rate within ±5% of the experimental results. [ABSTRACT FROM AUTHOR]

Adjustable throat-area expansion valves (ATAEVs), including thermostatic and electric expansion valves, are used commonly in air-conditioning equipment and lead to improved system performance as compared to the use of fixed-orifice expansion devices. However, ATAEV modeling literature is limited. Typically, an ATAEV is modeled by specifying the superheat entering the compressor or by empirically correlating experimental data. In order to model system behavior more accurately and effectively over a wide range of conditions, more accurate ATAEV models are necessary and a more general modeling methodology is desired. This paper presents a general model format that utilizes manufacturers' rating data. Model structures for three types of valve geometries are derived. Two model formats and corresponding parameter estimation procedures using manufacturer performance rating data are considered. The proposed method is validated using experimental data and compared with results from the literature. Both the experimental validation and the theoretical analysis demonstrate that the proposed method is more accurate and more generic than other methods presented in the literature. [ABSTRACT FROM AUTHOR]

This paper presents a detailed experimental validation of a simple transient model for cooling and dehumidifying coils that is developed in a companion paper (Zhou and Braun 2007). In all cases, the model provided very accurate predictions of exit air and water states. This appears to be the first experimental validation of a transient coil model under conditions where moisture condensation occurs. [ABSTRACT FROM AUTHOR]

Fin efficiencies are used within cooling coil models to simplify heat and mass transfer calculations associated with the use of air-side fins. These models utilize relationships developed for dry fins (i.e., no condensation) in determining convective heat transfer and modified fin efficiencies for estimating combined heat and mass transfer. However, the fin temperature distribution is different for wet and dry fins and, therefore, the use of a dry fin efficiency relationship for convective heat transfer is not strictly correct under wet conditions. This paper develops a correction factor for existing fin efficiency relationships that allows a better estimate of convective heat transfer fin efficiencies under wet conditions. The improved method is validated using results obtained from a two-dimensional numerical analysis and experiments performed on an eight-row cooling coil. [ABSTRACT FROM AUTHOR]

Dynamic coil models are useful in the development and evaluation of feedback controllers and fault detection and diagnostics algorithms. Existing models in the literature have not been validated fully using experimental data and have not been evaluated in terms of trade-offs between model accuracy and computational requirements. This paper develops a computationally fast, distributed model for the transient behavior of cooling and dehumidifying coils. The model utilizes several steady-state performance indices that simplify dynamic modeling, including fin efficiency, air-side effectiveness, and water-side effectiveness. Predictions from the simplified model were compared with a detailed reference model and with other simplified models from the literature. Comparisons with detailed experimental measurements are given in a companion paper (Zhou and Braun 2007). The model provides very accurate predictions of coil transient performance and executes more than 1000 times faster than real-time on a 1.2 GHz personal computer. As a result, it is very practical for implementation within system simulation programs that execute over long time periods, such as a year. [ABSTRACT FROM AUTHOR]

A near-optimal control method was developed for charging and discharging of cool storage systems when real-time pricing (RTP) electric rates are available The algorithm requires relatively low-cost measurements (cooling load and storage state of charge), requires very little plant-specific information, is computationally simple, and ensures that building cooling requirements are always met (e.g., storage isn't prematurely depleted). The control method was evaluated for ice storage systems using a simulation tool for different combinations of cooling plants, storage sizes, buildings, locations, and RTP rates. The simplified method worked well in all cases and gave annual costs within approximately 2% of the minimum possible costs associated with optimal control. [ABSTRACT FROM AUTHOR]

Existing methods addressing automated fault detection and diagnosis (FDD) for vapor-compression air-conditioning equipment have good performance for faults that occur individually but have difficulty handling multiple simultaneous faults. In addition, these methods either require high-cost measurements or measurements over a wide range of conditions for training reference models, the development of which can be time consuming and cost prohibitive. This paper formulates model-based FDD in a generic way and demonstrates that decoupling is the key to handling multiple simultaneous faults. To eliminate a cost-prohibitive overall system model, an alternative physical decoupling methodology to mathematical decoupling is developed. During the mathematical development, a previously developed FDD method termed the statistical rule-based method is reexamined and cast within the general mathematical framework. The paper also includes an evaluation of the FDD method in terms of both sensitivity and robustness. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive model for a hermetic scroll compressor. The model combines models of the compression process including leakage and local heat transfer within the scrolls, frictional losses, motor inefficiencies, and heat transfer within the shell and to the ambient. The model is useful for investigating the impact of design changes on compressor performance. Some sample results are presented in this paper. Detailed validation and an investigation of compressor design improvements are described in a companion paper. [ABSTRACT FROM AUTHOR]

Current air-conditioning system models seek to balance the level of code complexity with the speed of execution. Ideally, the simplest code possible that can accurately predict system performance is desired. To further understand this issue, two methods of modeling unitary air-conditioning systems are compared: a detailed, local approach and a simplified, global approach. These methods have been applied to two commercial unitary air conditioners for which extensive experimental data were obtained. The global approach predicted absolute system performance as well as or better than the local approach. In a separate analysis, the effects of two-phase modeling parameters on system model predictions were considered. The two-phase friction factors, heat transfer coefficients, and void fraction models were independently varied. The resulting system performance predictions were sensitive to variations in the air-side heat transfer coefficients and the void fraction model and not as sensitive to variations in the friction factor. Both the void fraction model and the condenser parameters mainly affect the compressor discharge pressure and the compressor power consumption. The evaporator parameters mainly affect the refrigerant mass flow rate and the heat transfer rates in both heat exchangers. [ABSTRACT FROM AUTHOR]

The author reflects on the trend and development of heating, ventilation, air-conditioning & refrigeration (HVAC&R) equipment in the U.S. The author cites the trend strated in 1980s wherein intelligent buildings are developed promising effective operational cost savings. The author explains that the said trend has encouraged manufacturing companies to design innovative and environmental friendly HVAC&R equipment systems for intelligent buildings.