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Selective sensing of trace hydrogen gas is highly desirable in leak detection applications, and it has recently come to the fore to an even larger degree due to the great potential of hydrogen in the current energy transition. One difficulty of room-temperature hydrogen sensing in an open environment is the potential interference from multiple other analytes, including water vapor, especially in high-humidity environments. To develop a high-responsivity, selective hydrogen gas sensor, ultrathin palladium nanosheets (PdNS) were synthesized using a hydrothermal method. Then, the PdNS were utilized as the chemical recognition layers in gravimetric resonant gas sensors for the real-time detection of hydrogen at room temperature. The nanoscale structure of the PdNS provides the material with a high amount of functional surface area; in turn, this allowed for significant surface-analyte interactions and high-performance sensing. Importantly, the PdNS active layer can selectively sense hydrogen gas in the presence of multiple interfering gases (e.g., acetone, alcohols, and water vapor) in the concentration range of 1–5% in both nitrogen and air environments. Therefore, this sensor system may potentially be of interest for future industrial applications regarding the safe handling of hydrogen gas. [ABSTRACT FROM AUTHOR]

The current testing and rating procedure for residential air conditioners and heat pumps is based on a steady-state performance measurement approach with a degradation coefficient to account for cycling losses at part-load conditions. Test equipment performance is measured under various ambient conditions with varying compressor and fan speeds, and the results are propagated through a temperature-bin method to estimate seasonal performance. Although the current rating approach offers a standardized performance metric for comparing the relative performances of different equipment, it involves disabling the native controls and, as a result, does not consider the impact of integrated controls for test units and their dynamic interactions with representative building loads. As an alternative, a load-based testing methodology (CSA EXP07) has been developed in which the dynamic performance of equipment is measured in a test facility by allowing it to respond to a simulated virtual building model. This study compares the steady-state and dynamic load-based performance measurement methodologies for use on a 5-ton residential heat pump system in order to understand the differences and their significance for the next-generation rating procedure. The differences in the two test methodologies' performance evaluation results are discussed with a causal analysis of the observed differences. [ABSTRACT FROM AUTHOR]

This paper presents a holistic building life-cycle assessment methodology that estimates the embodied and operational global warming potentials (GWPs) of a building covering the envelope, mechanical and lighting systems. The methodology relies on EnergyPlus to generate the use-phase energy consumption for any given building and incorporates a streamlined procedure to extract construction materials, which are used for building envelope GWP analysis. Embodied GWP accounting was performed for a representative packaged electric cooling and gas heating system and three types of lighting technologies, i.e., incandescent, compact fluorescent (CFL) and light-emitting diode (LED). The methodology was applied for carbon footprint analysis of five U.S. Department of Energy commercial building prototypes across seven climate locations. The results show that the operation phase has a dominant contribution (more than 74%) on the overall building environmental impact. LED and CFL lighting result in 45% whole-building energy consumption and 35% GWP reductions compared to incandescent lights. [ABSTRACT FROM AUTHOR]

Improving the energy efficiency of air conditioners and heat pumps is crucial for reducing the effects of continuously rising energy demands for space conditioning. One of the effective and tested approaches for achieving this goal has been to set energy efficiency benchmarks based on minimum energy performance standards which drive technological innovation and implementation in the market. For heat pumps, a testing and rating procedure that estimates equipment seasonal performance from laboratory measurements forms the technical basis for these standards. However, with current residential heat pumps rating standards, significant differences have been observed between efficiency measured in the lab and actual operational performance in field applications. One of the reasons for this is that with current rating approaches, the test unit native controls are overridden when measuring performance in the laboratory. As an alternative, a load-based testing approach has been developed that captures dynamic interactions of the integrated controls, equipment and building with the goal of better-representing field application conditions. In this paper, a load-based testing approach previously developed for air-conditioners is extended for heat–pump heating-mode and demonstrated for a variable-speed system. Climate-specific heating seasonal performance ratings were determined by propagating the test results through a temperature-bin method. [ABSTRACT FROM AUTHOR]

Integrating renewable energy and energy storage systems provides a way of operating the electrical grid system more energy efficiently and stably. Thermal storage and batteries are the most common devices for integration. However, it is not clear which integrated storage system performs better in terms of overall economics. Ice storage has low initial and maintenance costs, but there is an efficiency penalty for charging of storage and it can only shift electrical loads associated with building cooling requirements. A battery's round-trip efficiency, on the contrary, is quite consistent and batteries can be used to shift both HVAC and non-HVAC loads. However, batteries have greater initial costs and a shorter life. This research presents a tool, using model predictive control and optimal sizing, and provides a case study for comparing life-cycle economics of battery and ice storage systems for commercial buildings that have chillers for cooling and an on-site photovoltaic system. [ABSTRACT FROM AUTHOR]

Vapor compression cycles are not an established technology for microgravity applications yet. One reason for the low technology readiness level in microgravity is the concern of liquid flooding from the evaporator, which could reduce the lifetime of the compressor. Liquid refrigerant can accumulate in the evaporator while the cycle is turned off and slug into the suction line upon the next compressor start-up. This paper describes experiments on parabolic flights, which investigated this issue. A test stand was designed to observe liquid flooding from the evaporator upon compressor start-up. The time from compressor start-up until flooding first occurred and the duration of flooding were recorded and used as quantifiers. They showed that the occurrence of liquid flooding is mainly a function of the refrigerant mass in the evaporator before start-up and that there is a mass threshold below which flooding does not occur. Test were conducted with an off the shelf and one similarly sized transparent evaporator. The hypothesis that the threshold would be significantly smaller in microgravity was not confirmed with the data collected on the parabolic flights. Instead, flooding typically occurred at the same charge level for both evaporators and both testing environments. Flow visualizations showed that mostly stratified flow during start-up in ground testing became mostly annular flow in microgravity. Both flow regimes keep liquid close to the wall so that vapor generated during the start-up process can escape through a void tunnel. [ABSTRACT FROM AUTHOR]

Formaldehyde, a compound commonly employed in many construction materials, paints, and plastics, has been linked to deleterious health effects. Thus, monitoring the presence of formaldehyde in interior locations is increasingly important when it comes to public health. Currently, there is a crucial need for a low-cost, small-scale, selective, and sensitive indoor sensor capable of real-time formaldehyde detection. To meet these performance metrics, materials need to be incorporated onto existing gas sensor platforms to act as chemically selective recognition layers. A main challenge when addressing this issue is creating a material that can remain easily processable, can be easily synthesized, and can operate in practical environments (i.e., at common temperatures, humidity values, and in the presence of distractant analytes). Here, we show the unique properties of poly(5-carboxyindole) (P5C), an easily synthesized polymer, for practical indoor air monitoring of formaldehyde gas at concentrations as low as 25 ppm with rapid response and recovery times characterized by time constants of 27 s and 16 s, respectively. Importantly, we demonstrate that β-cyclodextrin (BCD), when blended into P5C to create a poly(5-carboxyindole) with β-cyclodextrin composite (P5C–BCD), offers distinct properties that enhance the response to formaldehyde gas in common operational conditions. Specifically, BCD adds features into the P5C such as its ability to form strong host–guest interactions with formaldehyde, its ability to buffer P5C protonation states to allow for more protonated carboxylic acid moieties on P5C which can hydrogen bond more effectively with formaldehyde, as well as creating a cylindrical morphology with the polymer film to assist the diffusion of formaldehyde into the polymer matrix. Additionally, these materials provide for chemically selective adsorption to formaldehyde gas in environments where interfering analytes exist. Due to the practical advantages these materials offer, they have the potential to unlock new avenues for future formaldehyde sensor materials. [ABSTRACT FROM AUTHOR]

Recently, linear compressor technology has gained increased attention in vapor compression systems due to its compactness and lower frictional losses compared with the conventional reciprocating compressors. Since the absence of the crank-mechanism eliminates the rotation and reduces the side loads on the piston centered inside the cylinder, it is possible to operate the linear compressors without oil lubrication. A number of advantages can be obtained in terms of the refrigerant compatibility, operation conditions and the system cost. In order to enable oil-free operation, while minimizing frictional losses and leakage flow between the piston and the cylinder wall, a gas bearing approach is applied. Little work was found in the open literature related to the numerical analysis of gas bearings coupled with a comprehensive dynamic linear compressor model. This paper presents a 1-D gas bearing model based on a Finite Volume Method (FVM) applied to a linear compressor. The dynamic characteristics of the gas bearings on the gas film pressure field are analyzed using the model. When integrated together with a comprehensive dynamic linear compressor model, it is possible to predict leakage and frictional losses within the compressor as well as the overall performance. [ABSTRACT FROM AUTHOR]

Carbon dioxide (CO2) has been linked to many deleterious health effects, and it has also been used as a proxy for building occupancy measurements. These applications have created a need for low-cost and low-power CO2 sensors that can be seamlessly incorporated into existing buildings. We report a resonant mass sensor coated with a solution-processable polymer blend of poly(ethylene oxide) (PEO) and poly(ethyleneimine) (PEI) for the detection of CO2 across multiple use conditions. Controlling the polymer blend composition and nanostructure enabled better transport of the analyte gas into the sensing layer, which allowed for significantly enhanced CO2 sensing relative to the state of the art. Moreover, the hydrophilic nature of PEO resulted in water uptake, which provided for higher sensing sensitivity at elevated humidity conditions. Therefore, this key integration of materials and resonant sensor platform could be a potential solution in the future for CO2 monitoring in smart infrastructure. [ABSTRACT FROM AUTHOR]

This paper presents a near-optimal control algorithm for minimizing energy costs in hybrid cooling plants in response to cooling loads. The approach is based upon an approximate solution to the partial differential equations involved in minimizing the cost of energy consumed by the plant with respect to two control variables: cooling tower airflow and condenser water flow. The parameters of the algorithm can be determined with design information of the chillers and cooling towers, along with some measurements of total condenser water flow and pump power consumption. In addition to reducing plant operating costs, the algorithm simplifies tower control and is more stable compared with conventional tower control strategies such as constant condenser water supply temperature or constant approach to wet-bulb. A large-scale cooling plant model was utilized to evaluate the performance of the approach compared with optimal control and two heuristic control strategies over six months. The difference between the energy costs associated with the near-optimal control approach and optimal control was 1.7%. Comparison with the heuristic control strategies shows that significant energy savings can be achieved with the proposed algorithm. [ABSTRACT FROM AUTHOR]

This paper presents the application of a multi-agent control methodology to large chiller plants. The approach, originally conceived to automate controller design and reduce engineering costs in building energy systems, consisted of a multi-agent simulation framework with distributed consensus-based optimization algorithms. To adapt the approach to this scenario, agents representing physical components of a cooling plant were developed and incorporated in the framework along with two alternative optimization methods: centralized, parallel optimization with a genetic algorithm (GA), and a combination of the GA with a quasi-newton method to handle non-linear equality constraints associated to physical component behavior. An existing cooling plant was utilized as case study to simulate the performance of the methods under different operating conditions. The results demonstrated the difficulty of the consensus-based algorithms to find optimal solutions. The GAs, on the other hand, showed that significant energy savings can be achieved through the implementation of multi-agent control with algorithms capable of handling non-convex objective functions and a combination of discrete and continuous variables, which are characteristic of central cooling systems. [ABSTRACT FROM AUTHOR]

Building audits are conducted in many commercial buildings to identify opportunities to reduce energy costs and improve building operation. Because audits require significant effort by building engineers, they are usually only affordable for larger commercial buildings. "No-touch" building audit tools have thus been developed to identify potential savings based on a simplified analysis of building energy consumption patterns via high-level energy data such as monthly utility bills. This paper presents a comprehensive and standardized methodology to evaluate the accuracy of no-touch audit tools in detecting and diagnosing building energy problems and quantifying potential energy savings. The test suite is based on output data from a well-characterized set of building energy models, and the methodology is illustrated by applying it to a representative no-touch building audit tool. Results show that the tool estimates building energy end uses with reasonable accuracy but is less accurate in identifying probably causes of high energy. [ABSTRACT FROM AUTHOR]

Developing a computationally efficient but accurate building energy simulation (BES) model is important for many purposes. Model order reduction (MOR) methods are attractive and much more reliable than identification approaches, since it directly extract a lower-dimensional model from a detailed physics-based model without any pre-simulations. However, because of computational and data storage requirements, there are challenges of applying these methods to a large-scale building. To overcome the problem, this paper introduces the Krylov subspace method to the building science field. Technical issues of applying the method to building applications are addressed and a suitable algorithm that overcomes those challenges is presented. To demonstrate the reliability of the algorithm, comparisons between the resulted reduced-order model (ROM) and a high-fidelity model from a commercial BES software for a 60-zone case study building are provided. The ROM was a factor of 100 faster than the high fidelity model but with high accuracy. [ABSTRACT FROM AUTHOR]

Buildings are responsible for more than 73% of the total electricity usage in the United States and have a significant contribution to peak electrical demand of the power grid. Thus, proper building demand management could improve the robustness and efficiency of grid operation and could also result in economic benefits for building owners. Almost half of the building electricity usage occurs in the commercial building sector where 60% of the installed cooling capacity is attributed to small commercial buildings employing packaged air-conditioning systems (e.g., rooftop units). Thus, understanding the demand response potential and economic benefits for the small commercial sector is of great importance. In particular, the effect of climate, building type, and utility structure on demand response potential is valuable information for building managers in identifying the most beneficial demand response strategies and for utilities in developing better utility structure rate incentives to achieve demand reduction. This article presents a simulation-based assessment approach and highlights the key assessment results for three demand response strategies—thermostat, shade, and lighting controls—across a range of building types, vintages, locations, and utility rate structures. The control performance is evaluated in terms of utility cost savings, peak demand reductions, and indoor comfort impacts in comparison to a baseline night setup strategy. The tradeoff between cost savings and comfort is also explicitly investigated. The assessment results show that the demand response controls lead to slightly worse indoor comfort compared to the baseline control, with an increase in the median percentage of people dissatisfied from 5.7% to 8.1%. However, significant demand reductions and cost savings could be achieved across all tested cases with the magnitudes dependent on the weather condition and building type. Annual electricity cost savings of 5 to 25 cents/ft2 (54 to 269 cents/m2) were demonstrated for time-of-use energy rates with demand charges, whereas the savings were relatively small, below 5 cents/ft2 (54 cents/m2) for most cases, when buildings subscribed to flat rates without demand charges. [ABSTRACT FROM AUTHOR]

A load-based methodology for testing and rating the performance of unitary air conditioning equipment has been developed and demonstrated using a laboratory psychrometric chamber test facility. The methodology simulates representative sensible and latent loads of a commercial building as well as the thermal and latent capacitance of buildings using psychrometric chamber test facilities. The advantage of the proposed test methodology is that there is no requirement to disable the native control systems designed by equipment manufacturers. This results in more realistic representations of the installed performance of equipment that include interaction of their embedded controls with the building. The load-based control approach is described and applied to the control system of an existing psychrometric chamber test facility. Results using the developed test methodology collected from fixed-speed and variable-speed RTUs show significant differences between actual part-load performance and steady-state performance at equivalent ambient operating conditions. A test procedure that includes interaction of the equipment controls with a virtual building could enable future equipment ratings that are both climate and building-type specific. This would not be possible for the current testing approaches that override native controls with no building interaction. [ABSTRACT FROM AUTHOR]

This article introduces a simple empirical model that can be readily implemented in building simulation programs to model the effects of refrigerant charge on air conditioner performance. Under- or over-charged refrigerant faults are relatively common and impact performance of equipment. It is important to understand their overall impacts in the context of whole building energy consumption and comfort in order to prioritize their importance relative to other building issues. This article develops an empirical model using some existing data. The model applies corrections to the rated values of cooling capacity, energy input ratio, and sensible heat ratio of existing air-conditioning equipment modeling approaches. Model predictions are compared with experimental results obtained from three air conditioners that have fixed orifices as expansion devices. The accuracy of the model is compared with a more complex empirical model and is shown to provide better accuracy for predicting cooling capacity and sensible heat ratio. [ABSTRACT FROM PUBLISHER]

This article presents a general approach for determining maximum monthly energy cost savings associated with optimal supervisory control for cooling in commercial buildings in the presence of utility rates that include both demand and time-of-use energy charges. The resulting tool has a month-long time horizon because of the nature of demand changes and is only useful for benchmarking the performance of simpler and shorter-term demand response and optimal control approaches. Attempts to solve this optimization problem using a centralized formulation failed and, therefore, the benchmarking problem was formulated as a dynamic optimization problem within a multi-agent control framework so that the monthly optimization problem is segmented into several sub-problems where each sub-problem involves system optimization for a shorter period of time, for example, a 1-day period. The daily-scale optimization involves determination of trajectories of zone set-point temperatures that minimize an integral cost function with a demand cost constraint determined by a demand agent. In order to further simplify the daily optimization, dynamics associated with the cooling system are neglected and optimal control of the cooling system is assumed to be based on heuristics determined through upfront analysis. This approach leads to significantly reduced computational requirements and more importantly, it provides guaranteed convergence in the multi-agent optimization. Results for a single-zone building case study are presented to illustrate the potential cost savings. In addition, a simpler and more practical short-term optimization approach with a demand-limiting heuristic is proposed and evaluated in comparison to the benchmarking optimization results for this case study and achieves most of the potential savings. [ABSTRACT FROM AUTHOR]

Metal hydrides have been investigated for use in environmentally friendly heat pumps. These systems operate using reversible adsorption and desorption of hydrogen from metallic compounds and can be incorporated in a cycle having a work input (compressor) or thermal energy input (generator). Some challenges faced by compressor driven metal hydride heat pumps are poor heat transfer in the metal hydride beds and high compressor discharge temperatures. To overcome these challenges, this article investigates the use of a metal-hydride slurry in conjunction with various isothermal compression techniques. Liquid-flooded, electrochemical, and liquid piston compressors were modeled and integrated into a system model in order to assess their impact on the performance of the slurry-based metal hydride heat pump system. [ABSTRACT FROM PUBLISHER]

Despite the increasing interest in organic Rankine cycle (ORC) systems and the large number of cycle models proposed in the literature, charge-based ORC models are still almost absent. In this paper, a detailed overall ORC simulation model is presented based on two solution strategies: condenser subcooling and total working fluid charge of the system. The latter allows the subcooling level to be predicted rather than specified as an input. The overall cycle model is composed of independent models for pump, expander, line sets, liquid receiver and heat exchangers. Empirical and semi-empirical models are adopted for the pump and expander, respectively. A generalized steady-state moving boundary method is used to model the heat exchangers. The line sets and liquid receiver are used to better estimate the total charge of the system and pressure drops. Finally, the individual components are connected to form a cycle model in an object-oriented fashion. The solution algorithm includes a preconditioner to guess reasonable values for the evaporating and condensing temperatures and a main cycle solver loop which drives to zero a set of residuals to ensure the convergence of the solution. The model has been developed in the Python programming language. A thorough validation is then carried out against experimental data obtained from two test setups having different nominal size, working fluids and individual components: (i) a regenerative ORC with a 5 kW scroll expander and an oil flooding loop; (ii) a regenerative ORC with a 11 kW single-screw expander. The computer code is made available through open-source dissemination. [ABSTRACT FROM AUTHOR]

In this article, virtual sensors for air-side diagnostics are developed and validated for rooftop air conditioners. The virtual air-side sensors require indoor fan differential pressure and speed measurements. Experimental data from laboratory measurements are used to evaluate the accuracy of the virtual sensors. Methodologies for improving the accuracy of air-side temperature sensors often used for outdoor-air economizer diagnostics have also been developed and evaluated. Outdoor-air temperature correction correlations improve single-point temperature inaccuracy caused by exhaust-air recirculation. A model correcting a single-point mixed-air temperature measurement for inaccuracy caused by thermal stratification and non-uniform velocity distributions at the evaporator inlet has also been described and validated. Finally, methods for estimating outdoor ventilation air fraction are presented and compared to measurements. [ABSTRACT FROM PUBLISHER]

Many approaches for reducing model complexities have been suggested for characterizing the dynamic behaviour of buildings. However, previous work has focused on buildings that have a relatively small number of zones and there have been very limited comparisons with experimental data or with the predictions and computational requirements of validated simulation models that are widely used. In particular, the applicability to large multi-zone buildings has not been addressed. This is an important application, where the cost to generate a reduced-order model (ROM) can be significant, but the benefits of ROM for analysis and design for building system can be tremendous. This paper presents a general and computationally efficient approach for generating a ROM from a detailed representation of the dynamics of commercial sized multi-zone buildings and provides comparisons of predictions and computational requirements with a commercial building simulation tool. [ABSTRACT FROM PUBLISHER]

While buildings served by multiple rooftop units are widely prevalent in retail and food industries, optimal coordination among rooftop units has received little attention. Currently, most buildings utilize plain feedback or heuristic strategies to determine rooftop unit operations. This article presents a switched dynamic programming based approach for optimal scheduling of rooftop unit operations. Using a mid-sized restaurant served by multiple rooftop units as an example, a switched affine dynamic system model is first presented. The rooftop unit coordination control problem is then formulated as a switched optimal control problem with nontrivial switching cost. A dynamic programming based method is proposed for the solution of the switched optimal control problem, which has reasonable computational complexity and can be implemented either as a one-shot solution or as part of a model predictive control algorithm. To mitigate the growing computational complexity of the proposed algorithm, the approach is extended by proposing a distributed extension of the model predictive control formulation. Finally, a simulation based evaluation of the various formulations is conducted, and the salient features of the formulation are highlighted. [ABSTRACT FROM PUBLISHER]

This article presents a generalized control heuristic to reset supply air temperature set-points for direct-expansion units with capacity modulation and variable airflow and integrates the heuristic into a simplified model predictive control framework that precools building thermal mass through zone temperature set-point adjustments. The resulting control methodology is appropriate for control of medium-sized commercial buildings that employ variable air volume air distribution with direct-expansion units for primary cooling. The heuristic is shown to work well for different direct-expansion systems with different compressor types and fan–duct combinations. To assess the integrated energy savings potential of the heuristic strategy and simplified model predictive control, a simulation platform was developed for a medium-sized commercial building. Simulations were carried out for a 100-day cooling season in different U.S. locations and under different control strategies. The heuristic control provided energy savings of between 5% and 10% relative to conventional control, depending on the system type and location. Approximately 4% additional energy savings was achieved when the heuristic was integrated into a simplified model predictive control having only moderate computational requirements. The cost savings could be significantly greater if time-of-use and demand charges were considered. [ABSTRACT FROM PUBLISHER]

Passive ceiling cooling systems can lead to reduced cooling requirements, less fan energy and downsized ductwork, compared to conventional all-air systems. Additionally, radiant cooling of occupants allows for improved comfort while allowing for higher operating temperature, improving chiller efficiency. This paper presents a comprehensive review of current modelling approaches for passive ceiling cooling systems in order to document the state of the art and identify current research gaps and modelling development needs. Modelling methods are separated in three main categories, based on the domain of interest:componentor “passive ceiling cooler” models, “indoor environment” models and “integrated” models. Simplified, detailed and empirical models are presented for each category. Different modelling approaches may be appropriate for different purposes (design vs. control analysis, and system simulation vs. whole building performance). The study summarizes useful findings, modelling limitations and applications, and presents needs for further modelling and simulation research, including passive chilled beams. [ABSTRACT FROM AUTHOR]

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]

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]

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]

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]

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]

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]

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]

This paper develops and evaluates a set of operating strategies that when implemented together provide near-optimal performance for hybrid cooling plants in terms of operating costs. A hybrid chiller plant employs a combination of chillers that are ‘powered’ by electricity and natural gas. Operating cost minimization for hybrid plants must account for effects of electrical and gas energy costs, electrical demand costs, and differences in maintenance costs associated with different chillers. Control strategy development was facilitated by separating hourly energy cost minimization from the problem of determining trade-offs between monthly energy and demand costs. A demand constraint is set for each month based upon a heuristic strategy and energy cost optimal strategies that attempt to satisfy the demand constraint are applied for cooling tower and chiller control at each decision interval. Simulated costs associated with the individual control strategies compare well with costs for optimal control. [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]

A hybrid chiller plant employs a combination of chillers that are ‘powered’ by electricity and natural gas. This paper presents an algorithm for determining cooling tower fan settings in hybrid plants in response to loadings on individual chillers. Parameters of the algorithm are evaluated using design information for the chillers and cooling tower fans. In addition to reducing operating costs, use of the open-loop control strategy simplifies the control and improves the stability of the tower control compared with the use of a constant condenser water supply or approach to wet-bulb. Simulated plant cooling costs associated with the algorithm were compared with costs for optimized settings and were within 1% of the minimum costs. The control method is general, in the sense that it also applies to cooling plants that have all electric or all natural gas chillers. [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]

The thermal performance of heat exchangers within a thermoacoustic cooler was investigated. Experimental procedures and calculation methods to evaluate the oscillating flow heat transfer coefficients were developed. Dimensionless heat transfer coefficients, or Colburn-j factors, were estimated based on the oscillating-flow variables and compared with results from steady-flow measurements. The results were also compared with heat transfer coefficients predicted from a boundary-layer conduction model, and methods that utilize steady-flow correlations with Reynolds numbers that characterize oscillating flow conditions. Although the boundary layer model is commonly employed for thermoacoustic calculations, it did not accurately predict heat transfer coefficients and the influence of Reynolds number on heat transfer performance. However, accurate predictions were obtained using a steady-flow correlation and a modified Reynolds number that accounts for the oscillating flow field, assuming a half-cycle Reynolds number correction value of 0.109. [ABSTRACT FROM AUTHOR]

The electroacoustic efficiency of high-power actuators used in thermoacoustic coolers may be estimated using a linear model involving a combination of six parameters. A method to identify these equivalent driver parameters from measured total electrical impedance and velocity-voltage transfer function data was developed. A commercially available, moving-magnet driver coupled to a functional thermoacoustic cooler was used to demonstrate the procedure experimentally. The method, based on linear electrical circuit theory, allowed for the possible frequency and amplitude dependence of the driver parameters to be estimated. The results demonstrated that driver parameters measured in vacuo using this method can be used to predict the driver efficiency and performance for operating conditions which may be encountered under load. © 2005 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

The goal of the work described in this paper was to identify an appropriate CFD model for refrigerant distributors and to apply the model to develop improved designs. There appears to have been no previous work applying CFD to two-phase flow in refrigerant distributors. As a first step, results of predictions using different two-phase modeling approaches available within commercial CFD codes were compared with experimental results from the literature for two-phase flow distribution with air and water as working fluids. The volume of fluid (VOF) and algebraic slip mixture (ASM) models failed to predict the experimental data available, whereas the inter-phase slip algorithm (IPSA) model predictions were in close agreement with the measurements in all cases. However, for a typical refrigerant distributor geometry and set of operating conditions, predictions of two-phase distribution and separation were quite similar for the different modeling approaches. As a result, the ASM is applicable for studying refrigerant distributor designs and was used to evaluate the performance of both existing and improved designs. In general, it is better to utilize a spherical distributor base as compared with other shapes and to locate the orifice close to the distributor base. These changes tend to improve the robustness of the distributor in terms of providing even flow and phase distribution in different outlet branches when the orifice and/or distributor are not oriented optimally. In addition, experiments were performed that tend to validate the general trends associated with the CFD results for refrigerant distributions. [ABSTRACT FROM AUTHOR]