Published by Taylor & Francis
Print ISSN: 1078-9669
This paper describes the development and simulation of a predictive optimal controller for thermal energy storage systems. The `optimal' strategy minimizes the cost of operating the cooling plant over the simulation horizon. The particular case of a popular ice storage system (ice-on-coil with internal melt) has been investigated in a simulation environment. Various predictor models have been analyzed with respect to their performance in forecasting cooling load data and information on ambient conditions (dry-bulb and wet-bulb temperatures). The predictor model provides load and weather information to the optimal controller in discrete time steps. An optimal storage charging and discharging strategy is planned at every time step over a fixed look-ahead time window utilizing newly available information. The first action of the optimal sequence of actions is executed over the next time step and the planning process is repeated at every following time step. The effect of the length of the...
This paper describes the development of a model-based optimization procedure for the synthesis of novel heating, ventilating, and air-conditioning system configurations. The optimization problem can be considered as having three suboptimization problems: the choice of a component set; the design of the topological connections between the components; and the design of a system operating strategy. In an attempt to limit the computational effort required to obtain a design solution, the approach adopted in this research is to solve all three subproblems simultaneously. The computational effort has been further limited by implementing simplified component models and including the system performance evaluation as part of the optimization problem (there being no need, in this respect, to simulate the system performance). The optimization problem has been solved using a Genetic Algorithm (GA) that has data structures and search operators specifically developed for the solution of HVAC system optimization problems. The performance of the algorithm and various search operators has been examined for a two-zone optimization problem, the objective of the optimization being to find a system design that minimizes system energy use. In particular, the performance of the algorithm in finding feasible system designs has been examined. It was concluded that the search was unreliable when the component set was optimized, but if the component set was fixed as a boundary condition on the search, then the algorithm had an 81% probability of finding a feasible system design. The optimality of the solutions is not examined in this paper but is described in an associated publication (Wright and Zhang 2008). It was concluded that, given a candidate set of system components, the algorithm described here provides an effective tool for exploring the design of novel HVAC systems.
Example Building Heat Flux Parameters 
Reference Capacity, Rate of Interzonal Energy Transfer, and Minimum System Capacity at the Optimized Zone Temperature and Humidity Ratios 
System Control Optimization Variable Bounds 
System Control Optimization Constraint Bound 
AHU Component Capacities 
This is a journal article [© American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org)]. Reprinted by permission from HVAC&R Research, Vol. 14, Part 3. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAE’s prior written permission. It is also available at: www.ashrae.org/hvacr-research To date, the performance of alternative HVAC secondary systems has been compared using either the systems’ energy use directly or by a life-cycle cost analysis. This paper introduces the concept of rating a system’s performance by comparing its capacity at a particular operating point to the thermodynamic minimum capacity. A simple ratio, termed the “system effectiveness,” is also introduced to indicate the extent to which the system operates with the minimum possible capacity. This paper describes the calculation of the minimum system capacity as a nonlinear, single- criterion, constrained optimization problem. In particular, it describes the case for the minimization of the system capacity by use of interzonal airflow (the interzonal airflow rates and zone thermal conditions being variables of the optimization). This optimization problem is multi-modal in that the same system capacity can result from more than one interzonal airflow configuration. The optimization problem has been solved here using a genetic algorithm (GA) search method. This paper illustrates the use of the minimum system capacity as a benchmark for the performance of a typical multizone heating, ventilating, and air-conditioning (HVAC) system. The example also illustrates the potential benefit of using interzonal airflow to reduce the required capacity of a system. It can be concluded from the example that the arrangement of the system components is a significant factor in determining the thermal effectiveness of HVAC systems.
A very high accuracy, straight vibrating tube type of density flow meter has been used online to measure oil concentraiton of flowing R-134a/oil mixtures. The calibrations covered oil concentrations from 0-6 wt.% oil over the temperature range from -9.4 to 5.9°C. The oil concentrations were correlated statistically as a function of density, temperature and liquid compressibility to an average error of 0.09 wt.% oil with a 95% confidence limit of 0.21 wt.% oil. In addition, a simplified method not requiring calibration tests was developed for general industrial application of the density flowmeter to any refrigerant-oil mixture combination, with an average error of 0.22 wt.% oil and 95% confidence limit of 0.67 wt.% oil for the present data set.
Intube flow boiling experiments for refrigerant R-134a mixed with a lubricating oil are reported for a plain, horizontal tube. The tests were run at a nominal inlet pressure of 3.4 bar over a wide range of vapor qualities at mass velocities of 100, 200, and 300 kg/m2s (73.5, 147 and 220.5 thousands of lb/h ft2) for inlet oil concentrations from 0-5 wt.% oil. At low to intermediate vapor qualities (0.2 <x< 0.60), the oil tended to increase the local boiling coefficient while significant deterioration in boiling performance occurred at high vapor qualities for the higher concentrations. As opposed to similar tests with a microfin tube, no evidence of oil holdup inside the plain tube test sections was noted. However, the effect of flow pattern appears to be important while also the effect of local physical properties on the heat transfer coefficient at high vapor qualities was confirmed.
Curved channels in the form of bends are encountered in many industrial two-phase flow applications, such as U-bends in air-conditioning and refrigeration evaporators and condensers. For engineering design purposes, the evaluation of the pressure loss in two-phase flows is necessary. Basically, for the same flow conditions the pressure loss is higher in a U-bend than in a straight pipe with an equal cross-flow section and mean length. Moreover, the curved tube causes a perturbation that propagates up- and downstream of the U-bend itself. Many studies have been made on two-phase flows in bent tubes, but none have yet focused on the axial evolution of the pressure gradients before and after the U-bend or on the peripheral pressure differences that may be created. Therefore, this study reports new data obtained for R-134a at saturation temperatures ranging from 3°C to 5°C (37°F to 41°F), flowing at mass velocities ranging from 155 to 533 kg·s−1·m−2 (114·103 to 393·103 lb·h−1·ft−2) through a horizontal U-bend tube with internal tube and bend centerline diameters of 13.4 mm (0.52 in.) and 66.1 mm (2.6 in.), respectively, over a wide range of vapor qualities. The experimental results show that the differences between the peripheral inner and outer pressures up- and downstream of the bend are negligible, even very close to the bend (at 4 internal diameters). The frictional pressure gradients in the U-bend were found experimentally to be about two times that in a straight pipe. Close to the bend, up- and downstream, the pressure gradients are highly affected. For lower and medium mass velocities the perturbation caused by the U-bend does not propagate further than 6 internal diameters upstream from the bend. For the higher mass velocity, that perturbation seems to propagate further. Downstream of the U-bend the perturbed length extends over 100 internal diameters.
Significant process intensification (PI) of heat and mass transfer is indispensable in building compact and energy efficient absorption refrigeration systems. High potentials exist to achieve the required PI through (1) development of active heat and mass transfer enhancement techniques and (2) combining the active enhancement mechanism with proven and widely used passive enhancement techniques in transport processes. There is limited research on the effect of active mechanisms, such as vibration, on heat and mass transfer coefficients in absorption systems with falling film horizontal-tube absorbers. In this general survey, with the aim to enlighten the path for active mechanisms development, recorded heat and mass transfer enhancements via active mechanisms were extracted from pertinent research works, and were summarized in tables suitable for evaluation and comparison. The potential for future research on enhancing heat and mass transfer in absorption chillers was identified.
Indoor particulate contaminants can be generated in many ways, commonly from human activities, infiltration of HVAC systems, or resuspension from indoor surfaces. Most of these sources are transient and generate nonuniform particle distribution in the space. This study used experimental and numerical methods to investigate the dispersion of three different particle sizes (0.7, 2.5, and 7 μm) emitted from typical source positions. A test room and simplified thermal manikins were employed to mimic a realistic indoor environment, and experimental data were compared with particle modeling using the Lagrangian method coupled with Reynolds averaged Navier-Stokes (RANS) and large eddy simulation (LES) computational fluid dynamics (CFD) turbulence models. Particle dispersion was studied for two ventilation patterns: buoyancy-driven ventilation and well-mixed ventilation. The results provided a comparison of Lagrangian-RANS particle modeling, Lagrangian-LES particle modeling, and experimental data considering nonuniform temporal and spatial particle concentrations. Experimental and modeling results were evaluated with three different metrics: peak normalized concentration at various locations, peak-concentration occurrence time, and mean exposure defined as the averaged concentration in the occupant's breathing zone. The results show that Lagrangian-LES more accurately predicts concentration fluctuation during particle emission. Considering long-term exposure, however, both methods show similar results.
Building energy system retrofit and retro-commissioning projects present tremendous opportunities to save energy. Energy consumption in buildings, especially HVAC systems, is significantly impacted by weather conditions. However, short- or long-term climatic data are frequently missing because of data transmission problems, data quality assurance methods, sensor malfunction, or a host of other reasons. These gaps in climatic data continue to provide challenges for HVAC engineers in monitoring and verifying building energy performance. This article examines eight classical approaches that use Linear interpolation, Lagrange interpolation, and Cubic Spline interpolation techniques, and eleven approaches that use two newly developed methods, i.e., Angle-based interpolation and Corr-based interpolation, to restore up to 24 h of missing dry-bulb temperature data in a time series for use in building performance monitoring and analysis. Eleven one-year hourly data sets are used to evaluate the performance of these 19 different methods. Each method is applied to deal with artificial gaps that are generated randomly. In terms of the difference between estimated values and measured values, two types of comparisons are carried out. The first comparison is conducted with three evaluation indices: MAE, RMSE, and STDBIAS. The second comparison is based on the percentage of the total data that can be estimated by an approach within specific error thresholds, including 1°F (0.56°C), 2°F (1.11°C), 3°F (1.67°C), and 5°F (2.78°C), from measured values. The comparison results show that Linear interpolation performs best when filling 1–2 h gaps, Lagrange interpolation (Lag2L2R) outperforms other methods when gaps are 3–8 h long, and the Corr-based interpolation method (Corr1L1R24Avg) is a better technique for filling 9–24 h gaps. This article presents the first part of the research results through the ASHRAE 1413 research project. The second part of the results focuses on methods to filling long-term dry-bulb temperature gaps.
A quantitative comparison of oil retention and pressure drop characteristics of refrigerants, R1234yf and R134a with POE32 oil in 10.2 mm inside diameter horizontal and vertical suction lines at a saturation temperature of 13°C with 15°C of superheat is presented. High speed videos of the flow were taken to identify the flow regimes as the mass flux was varied. Test results show that for the same system cooling capacity R1234yf and R134a have very similar oil retention; however, the use of R1234yf results in 20% to 30% higher pressure drop. It was also found that inclined suction lines retain more oil than vertical suction lines.
Intube evaporation tests for R-407C and R-407C/oil are reported for a microfin tube. The tests were run at a nominal inlet pressure of 6.45 bar (93.5psia) at mass velocities of 100, 200 and 300 kg/m2s (73581, 147162 and 220743 lb/h ft2) over nearly the entire vapor quality range. Pure R-407C performed similar to previous pure R-134a tests at the highest mass velocity, but lower for the other mass velocities, at similar operating conditions. Any amount of oil tended to decrease local R-407C micorfin heat transer coefficients, especially at high vapor qualities where degradations of as much as 50% or more occurred. Two-phase pressure drops were increased by the presence of oil, especially at high vapor qualities. Buildup of the local oil mass fraction in the microfin test sections was observed at high vapor qualities together with the formation of slowly flowing viscous liquid films, a phenomenon becoming more acute at lower mass velocities.
In-tube evaporation tests for R-407C and R-407C/oil are reported for a plain copper tube. The tests were run at a nominal inlet pressure of 645 kPa (93.5 psia) at mass velocities of 100, 200, and 300 kg/(m²·s) (20.5, 41, and 61 lb/s·ft²) over nearly the entire vapor quality range. Pure R-407C performed very similarly to pure R-134a run previously in similar tests, at all three mass velocities. The only difference was at high vapor qualities where the peaks in the refrigerant heat transfer coefficient versus vapor quality were shifted slightly. For local vapor qualities from 10–70%, the oil tended to have little effect on local R-407C/oil heat-transfer coefficients at the lowest mass velocity, while at the higher mass velocities the effect was to increase or decrease the coefficients within ±20% of the pure R-407C values. At vapor qualities higher than 70%, the effect of the oil was very dramatic, decreasing performance by as much as 80–90%, even with small amounts of oil. Two-phase pressure drops were increased by the presence of oil, especially at high vapor qualities. A new method for predicting local boiling coefficients of refrigerant-oil mixtures is presented. Using the refrigerant-oil mixture viscosity in place of the pure refrigerant viscosity in the recent Kattan-Thome-Favrat flow boiling model and flow pattern map without further modification predicted the R-134a/oil and R-407C/oil data quite accurately. In addition, the Friedel two-phase friction multiplier was found to work adequately for pure R-134a and pure R-407C. Finally, a new local refrigerant-oil viscosity ratio was developed that accurately predicted two-phase pressure drops of R-134a/oil and R-407C/oil mixtures at high vapor qualities.
The lack of standard procedures for filling climatic data has the potential to undermine design, monitoring, and control efforts aimed at climate-responsive building design, performance monitoring, and energy efficiency. This article addresses the challenge of long-term missing gaps in dry-bulb temperature data by examining three spatial methods, namely the inverse distance weighting (IDW) method, the spatial regression test (SRT) method, and the substitution with best match data (SSBM) method, as well as two temporal methods, namely the temporal regression test (TRT) method and the temporal substitution with best match data (TSBM) method. Using these methods, missing dry-bulb temperature data with long-term gaps, ranging from 1 to 60 days, are restored for use in building performance monitoring and analysis. Three one-year, hourly datasets were used to evaluate the performance of these approaches. Each method was applied to deal with artificial gaps which were generated randomly and represented different seasons of a year. In terms of the difference between estimated values and measured values, three evaluation indices, namely mean absolute error (MAE), root mean square error (RMSE), and standard error of bias (BIASSTD), were utilized. The comparison results show that spatial methods are better than temporal methods. The confidence level of the SRT method was further investigated by applying this method to existing data and missing data, and examining its performance. The results indicate that the uncertainty of the SRT method can be predicted and at least two neighboring stations are recommended when using it. This is the second part of the research results obtained through the ASHRAE 1413 research project (in press) with a focus on introducing gap-filling methods for long-term gaps in dry-bulb temperature.
The Imanta district heating plant began operation in 1974 as one of the main heating plants for the Riga district heating system. Until now, it has undergone several reconstructions. First, the ordinary boiler plant with steam and hot water boilers was reconstructed to be a natural gas-fueled combined cycle cogeneration plant supported with hot water boilers. Then, one of the cooling towers of the cogeneration section was replaced by an absorption heat pump/chiller, thus providing the closed cooling water circulation system and recovering 2 MWth (7 MMBTU/h) low-grade heat otherwise lost in the atmosphere. The driving force for the absorption heat pump is steam produced by the steam boiler (3 MWth = 10 MMBTU/h) already located at the boiler plant. The efficiency of the absorption heat pump is closely related to the operation of the cogeneration unit, outside temperature, and district heating water temperature regimes. The case study confirms that a more efficient energy production system reduces the fuel consumption with the same amount of the produced energy, thus lowering the expenses on energy production and reducing the amount of the polluting emissions into the atmosphere.
Sensors are one of the key components in a modern building energy management system (BEMS). Accurate sensors are the prerequisite for the success of any building energy optimization strategy. As sensors are subject to environment disturbance and performance deterioration, their accuracy tends to decrease during their service lives. Sensor calibration is an efficient way to improve measurement accuracy and reliability. However, due to a large number of sensors installed in modern air conditioning (AC) systems, conventional regular calibration may be laborious while not optimal when the system energy performance are concerned. In order to improve measurement accuracy and reliability, a hybrid sensor management strategy is proposed in this article. This strategy integrates a measured variable importance ranking technique with a data fusion technique. Comparison of this strategy with a conventional regular calibration in case studies shows that this strategy improves both the energy and control performance of AC systems.
Electronic expansion valves have been used to replace conventional expansion devices in many refrigeration systems. Electronically controlled valves respond more rapidly to changes in operating conditions and improve the steady-state superheating. These valves are usually used with an automatic controller that regulates the superheating at the evaporator outlet. The controller gains (K p , T i , and T d ) must be properly tuned for efficient operation. However, these controllers can result in poor performance because they have been poorly tuned or put into operation using factory tuning. For refrigeration systems that are subject to large changes in operating conditions, the controller gains should be adjusted for each change to improve the system performance. Within this context, we developed an adaptive Proportional-Integral-Derivative controller (PID controller) in this study to regulate the degree of superheating. A dynamic model obtained from experimental tests was used in the controller design. The controller effectiveness was evaluated using computer simulations and experimental tests. In comparison to a nonadaptive PID controller, the adaptive controller provided better disturbance rejection and set-point tracking and was able to control the superheating more efficiently, demanding less servomotor effort.
This is a journal article. [© American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org)]. Reprinted by permission from HVAC&R Research, Vol. 8, Part 4. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAE’s prior written permission. It is also available at: www.ashrae.org/hvacr-research This paper presents a new adaptive weather-prediction model that can be used for on-line control of HVAC and thermal storage systems. The model can predict external dry-bulb temperature and solar radiation over the next 24 h. Because a building with a fabric thermal storage system has a slow response to thermal loads, a predictive controller is essential to operate the building and associated plant installation to respond effectively to external climatic conditions ahead of time. Three prediction methods are investigated in the paper: a pure stochastic method, a combined deterministic-stochastic method, and an expanded method for short-term temperature forecast. It has been found that the combined deterministic-stochastic method is simpler and gives the smallest prediction errors. For the prediction of solar radiation, a deterministic model is proposed. The proposed prediction algorithms for temperature and radiation are simple and efficient to conduct on a supervisory PC to predict hourly temperature and radiation profiles over the next 24 h. Updating temperature forecasts using observations available with time is also investigated in this paper.
, K, and k. Values of Five VOCs with Carpet by Three Regression Methods
The adsorption/desorption of volatile organic compounds (VOCs) on interior building material surfaces (i.e., the sink effect) can affect the VOC concentrations in a building, and thus need to be accounted for in an indoor air quality (IAQ) prediction model. In this study, the VOC adsorption/desorption characteristics (sink effect) were measured for four typical interior building materials, including carpet, vinyl floor tile, painted drywall, and ceiling tile. The VOCs tested were ethylbenzene, cyclohexanone, 1,4-dichlorobenzene, benzaldehyde, and dodecane. These five VOCs were selected because they are representative of hydrocarbons, aromatics, ketones, aldehydes, and chlorine substituted compounds. The first order reversible adsorption/desorption model was based on the Langmuir isotherm was used to analyze the data and to determine the equilibrium constant of each VOC-material combination. It was found that the adsorption/desorption equilibrium constant, which is a measure of the sink capacity, increased linearly with the inverse of the VOC vapor pressure. For each compound, the adsorption/desorption equilibrium constant, and the adsorption rate constant differ significantly among the four materials tested. A detailed characterization of the material structure in the micro-scale would improve the understanding and modeling of the sink effect in the future. The results of this study can be used to estimate the impact of sink effect on the VOC concentrations in buildings.
Air-handling system leakage reduces the amount of air delivered to conditioned spaces and in most cases wastes energy and money. Standards exist for where and how to measure system airtightness, but they tend to focus on new construction, and only on the high-pressure (1500–2500 Pa [6–10 in. w.c.])/ medium-pressure [500–1500 Pa (2–6 in. w.c.]) portions of the system. This article investigates air leakage in the low-pressure (≤500 Pa [≤2 in. w.c.]) portions of large commercial-building air-handling systems (i.e., downstream of variable-air-volume box inlet dampers). A simplified diagnostic protocol for measuring low-pressure leakage that can be used during normal system operation in an existing building is presented and utilized for this investigation. A validation of the protocol using a calibrated leak in a field installation is also presented, as are the results of applying this protocol in nine other buildings around the United States. The validation results indicate that normalized leakage can be measured to within 10 L/s at 25 Pa (20 cfm at 0.1 in w.c.), with and without the existence of significant flow through the minimum opening of the box inlet damper. The field test results indicate that low-pressure leakage varied considerably from system to system (standard deviation of 50% of the mean value), and that the average value was approximately 10% of the flow entering the low-pressure system sections. The variability of the measured results, combined with a simplified analysis of the impacts of this leakage, suggest that testing of low-pressure system leakage in commercial buildings should be economically justifiable.
Indoor air quality and thermal comfort affects working performance and efficiency, particularly for those who work in the office for prolonged periods. Poor air quality even compromises human health of the staff in the office. For the air-conditioning system using a package-type or split-type air conditioner, introducing fresh air will improve the indoor air quality of the office and affect the perception of thermal comfort. However, it will increase the energy consumption of the air-conditioning system, especially in a hot and humid climate. Attempting to improve indoor air quality and thermal comfort with less power consumption, this study conducts the case study of a total heat exchanger retrofitted in an office building in the hot and humid climate of Taiwan. Field measurement of indoor temperature, humidity, CO2 concentration, and energy consumption, along with thermal sensation votes of staff in the office, have been compared comprehensively after installing the total heat exchanger. Results indicated that the CO2 concentration decreases and thermal comfort evaluation improves satisfactorily after installation of total heat exchanger, even though power consumption increased slightly. It reveals that it is feasible to improve indoor air quality and thermal comfort effectively with less power consumption in the office specifically for the air-conditioning system using a package-type or split-type air conditioner under hot and humid weather conditions.
One of the most influential factors of the performance of a finned-tube heat exchanger is the distribution of the air passing through it; therefore, it must be known in order to produce a highly efficient design. We examined two different common style air-to-refrigerant, finned-tube heat exchangers: a single-slab coil oriented at an angle of 65° to the duct wall and an A-shaped coil with an apex angle of 34°. We used particle image velocimetry (PIV) to measure their in-situ airflow distributions. The results show that the airflow distributions for both heat exchangers are highly nonuniform with different sections being subject to vastly different air velocities. We also used a momentum resistance-based computational fluid dynamics (CFD) approach to model the airflow distributions through these heat exchangers. The modeled results agreed with the measured values, with most of the simulated velocities falling within +/-10% of the measured velocities. The results of this study show that the velocity profile for any configuration is strongly influenced by the geometry of the heat exchanger and other features in its proximity and, therefore, each installation configuration will have its own unique velocity distribution. The information presented in this paper documents the maldistribution of airflowing through finned-tube heat exchangers and highlights the sources and magnitude of the nonuniformities.
Engine oil migrating into the bleed air stream of aircraft environmental control systems occurs with enough frequency and deleterious effects to generate significant public interest. While previous work has explored the chemical makeup of the contaminants in the aircraft cabin during these events, little is known about the characteristics of the aerosol resulting from oil contamination of bleed air. This article presents particle counter data (giving both size distributions and concentration information) of the oil droplets from simulated jet engine bleed air. Four particle counters—a scanning mobility analyzer, an aerodynamic particle-sizer, an optical particle counter, and a water-based condensation particle counter—were used in the study encompassing a size range from 13 nm to 20 μm. The aerosol characterization is given for different bleed air temperatures and pressures. The data show a substantial increase of ultrafine particles as the temperature is increased to the maximum temperatures expected during normal aircraft operation. This increase in ultrafine particles is consistent with smoke generated from the oil. The pressure of the bleed air had little discernible effect on the particle size and concentration.
The passive airside cooling capability of heat pipes operating under high-temperature natural ventilation airstreams was investigated in this study. Pure water was used as the internal working fluid to ensure the system remained sustainable in its operation. The physical domain included 19 cylindrical copper heat pipes assembled in a systematic vertical arrangement. Using the monthly temperature data of Doha, Qatar, as a case-study reference, the efficiency of the heat pipe model was analyzed at fixed inlet air velocities of 1 and 2.3 m/s. At a source temperature of 314 K, the results showed a maximum temperature reduction of 3.8 K for an external air velocity of 1 m/s. A cooling load of 976 W was achieved, indicating a heat pipe effectiveness of 6.4% when the velocity was increased to 2.3 m/s. Wind tunnel experimental testing was conducted to validate the findings. A good correlation was observed between the two techniques with error variations of 10% for velocity and 28% for temperature. The present work identified the potential of sustainable pre-cooling using heat pipes in natural ventilation airstreams for regions with hot and dry climatic conditions. The concept is currently under intellectual property protection (GB1321709.6).
In this article, an all fresh air-handling unit with high sub-cooling degree is presented. In this unit, refrigerant flows through the high-pressure liquid receiver before it goes through the sub-cooler so as to ensure sufficient sub-cooling degree. Based on the experimental comparison between this unit and conventional unit, coupling relationships between condensing temperatures and sub-cooling degrees of these two units are worked out and analyzed. Experimental results and exergy analysis show that, sub-cooling degree drops with the decrease of condensing temperature, and sub-cooling degree of the designed unit is kept over 7°C when the sub-cooling degree of the conventional unit is only close to 0°C. Furthermore, a method of year-round exergy calculation is presented and applied in calculating and analyzing the year-round exergy of the all fresh air-handling unit. Calculation and analysis show that the all fresh air-handling unit designed and investigated in this article has a year-round exergy efficiency of 28.38%, which is 3.17% higher than that of the conventional unit without high sub-cooling degree.
An inverse design method is proposed to achieve the pre-set control objectives of the aircraft cabin environment. The method combines the artificial neural network and the genetic algorithm, and the training and testing data of the artificial neural network is obtained by computational fluid dynamics analysis. Both of the thermal comfort and energy consumption are considered in the inverse design. The artificial neural network is used to identify the relationship between the thermal comfort and the air supply parameters (inlet velocity magnitude and angle, inlet air temperature). The genetic algorithm coupled with the well-trained artificial neural network is used to design the aircraft cabin environment. Numerical results show that the Bayesian regularization algorithm is proved to have better generalization capability than the other training algorithms for the artificial neural network. The increase of training data quantity improves the generalization capability of the artificial neural network, while it spends more simulation time. A computational fluid dynamics database with 60 datasets is shown to be suitable to the present inverse design, and the testing error of the artificial neural network is below 8.2%. Several groups of optimal air supply parameters are found with different trade-offs between the thermal comfort and energy consumption. The best solution of thermal comfort, i.e., the percentage of zone with |PMV| >0.5 in all cabin control domains, is less than 7.8%.
Location of the anemometers in the experiment of human movement.
This article investigates the aerodynamic effects of human movement by experiment and numerical simulations. In the experiment, a life-size thermal manikin, a double-track orbit, and a trolley were used to realize human movement, and the velocity distribution of the induced airflow was measured. In the numerical simulations, dynamic meshing was used to simulate the human movement. The aerodynamic effects and flow fields under moving speeds of 0.5, 0.75, 1.0, 1.25, and 1.5 m/s were studied. The same timing relationship and tendency of the instantaneous velocity can be found between the measured and computed results, although the computed peak values are smaller than the measured ones. Apparent recirculation zones and vortices can be seen in the wake behind the human body in numerical simulations. The streamwise velocity profile and the structure of the wake depend on the profile of the human body and the moving speed. At each location, the nondimensional relative velocities of different moving speeds are substantially the same. The aerodynamic effects of human movements depend on the moving speed, moving distance, and spatial location. These results can be a good help for the studies on pollutant dispersion, control of air quality, and infectious diseases in indoor environment.
This article reports on the design and construction of a small-scale laboratory tank to study transient heat transfer in innovative ground heat exchangers designs. The tank is 1.35 m high and has a diameter of 1.4 m. It is filled with well-characterized laboratory-grade sand and instrumented with precisely positioned thermocouples. Experimental results for a 73-h heat injection period followed by a 5-day recovery period for a single U-tube borehole are reported. A comparison is performed against results obtained with an axisymmetric numerical model of the ground surrounding the borehole. It is shown that the agreement between both set of results is very good indicating that accurate experimental data can be obtained with the apparatus.
This article assesses the most common architectural and environmental strategies in Ghadames housing in Libya. Preliminary data were collected through field surveys undertaken in July 2013, the hottest and driest season in Ghadames. The surveys investigated the indoor thermal environment and efficiency of energy use in Ghadames housing. The actual mean vote scale was used to investigate occupants’ thermal feeling coupled with recording physical environment and also actual measurements of a number of existing houses. Additionally, objective surveys were conducted to (a) verify the subjective data, (b) provide an overall view of the residents’ life style in the old town, and (c) understand the most significant techniques employed in old dwellings. The subjective survey “questionnaire” distributed among nine new and eight old houses shows that the majority of respondents is satisfied with the number of architectural issues in modern housing design. This general satisfaction excludes the inherited identity of the traditional architecture embedded within the society. On the other hand, occupants are more satisfied with old buildings in regard to indoor environmental conditions, energy consumption, and construction materials. The occupants of old houses expressed their thermal satisfaction with the indoor comfort conditions, but the predicted mean vote, based on measurements and ISO 7730, implied discomfort (hot).The survey also carried out interviews with a number of locals, underlining their personal impressions and preference toward the change of the existing built environment. Findings indicate that, occupants’ satisfaction and perception toward the built environment have not been achieved in new housing developments of Ghadames owing to the lack of understanding of the sociocultural needs of the local community. In addition, a 3D digital model was created for the old town and imparted a full understanding of the building dynamics and physics, explicating the complexity of the compactness of its urban morphologies. The results also showed subjects were feeling neutral to slightly warm in old buildings even when indoor air temperatures reached 32°C.
As requirements regarding energy efficiency are getting tougher, buildings in the arctic, as well as the rest of the world, need to be more energy efficient without compromising a good indoor climate. This article presents measured moisture supply and occupancy level in a Swedish arctic multi-family apartment block. Measurements were done over 1 year every 30 minutes in a building consisting of 51 apartments located in Kiruna, at latitude 67.9°. Averages and typical variations on different timescales, year and day, are presented for the different parameters, as well as correlations between the parameters; for example, moisture supply as a function of occupancy level. The results can be used when input data for simulations of energy use, moisture conditions and indoor climate are chosen, as well as a reference to compare measurements to during verifications. In energy efficient buildings, occupant behavior generally has an extensive impact on building performance, which means that the characteristics of behavior related parameters are important to be able to describe.
This is a journal article [© American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org)]. Reprinted by permission from HVAC&R Research, Vol. 12, Part 3c. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAE’s prior written permission. It is also available at: www.ashrae.org/hvacr-research This paper reports on the energy implications of HVAC system configuration by analyzing the energy balance and psychrometrics of typical and innovative systems. Three criteria were shown to be significant: (1) the ability to minimize outside air load, (2) the ability to eliminate simultaneous cooling and heating and use mixing effectively, and (3) the availability of interzonal airflow. Configurations that meet these criteria would be able to deliver the desired indoor air quality with reduced energy consumption. The performance of ten two-zone system configurations, including single-duct, dual-duct, fan-coil-based variations, and other specialized systems in the literature, were analyzed for a number of operational conditions. The results confirmed that fan-coil-based configurations with interzonal airflow paths perform better than other configurations. The conclusion of this study may be used as a guideline for multi-zone system designs.
Experiments have been performed for evaporation of refrigerants R-134a, R-410A, and R-507A on the outside of a horizontal bundle composed of enhanced boiling Turbo-BII HP tubes. Hot water flowing through the tubes was used as the heat source. Water temperature measurements were made at numerous axial locations within the tubes to obtain the water's temperature profile and, thus, measure twelve local heat transfer coefficients in the bundle. Pool boiling tests to measure the corresponding nucleate boiling curves were also obtained. Tests were conducted with a 20-tube bundle over ranges of heat flux (8-64 kW/m 2), mass flux (4-38 kg/[m2 s]), and vapor quality (8-78%). The bundle test results showed a significant contribution of convection from rising bubbles to the heat transfer process with local heat transfer coefficients as large as 1.6 times those of nucleate pool boiling in the case of R-134a. Refrigerants R-507A and R-410A also showed bundle enhancement effects but less so than R-134a.
Experiments have been performed for evaporation of refrigerants R-134a and R-507A on the outside of a horizontal bundle of integral finned tubes. Hot water flowing through the tubes is used as the heat source. Water temperature measurements were made at numerous axial locations within the tubes to obtain the water's temperature profile and thus measure twelve local heat transfer coefficients simultaneously in the bundle. Pool boiling tests to measure the corresponding nucleate boiling curves were also performed. Tests were conducted with a 20-tube bundle over the following range of heat flux (2 to 50 kW/m2), mass flux (3 to 29 kg/(m2 s)), and vapor quality (8 to 82%). The bundle test results showed a significant contribution of convection from rising bubbles to the heat transfer process, with local heat transfer coefficients as large as two times those of nucleate pool boiling in the case of R-134a, but on average a 34% increase. R-507A showed significantly less benefit due to convection, averaging only a 10% increase.
Blade passing frequency (BPF) noise is the dominating component of the flow induced noise of centrifugal fans. The numerical methods for BPF noise prediction, based on the computational aeroacoustics (CAA), have been published for decades. However, there are a couple of challenges for accurately predicting noise for industrial centrifugal fans. The first arises from the fact that the free field hypothesis, adopted in the numerical model, has not yet been carefully studied. The second challenge stems from the current criteria for which the prediction results are compared to the measurement data. Because the test conditions do not always satisfy the requirements of the numerical model, inaccurate predictions occasionally resulted. Therefore, since the prediction results may deviate largely from the test data, the applicability of these methods is severely limited.
The current industry standard method for estimating tall building entryway infiltration rates in cold climates is found in ASHRAE Handbook—Fundamentals (2013). This method was developed in ASHRAE Research Project (RP) 763 (Yuill 1996). In the comparison of an example calculation provided in RP-763 (Yuill 1996) and reproduced in 2013 ASHRAE Handbook Fundamentals (2013) with simulation results for the same example using various building configurations, the multi-node airflow analysis tool simulations consistently calculated significantly lower flow rates than predicted by the RP-763 (Yuill 1996) method (Whitehead and Frisque 2012). Addressing the large over-prediction of infiltration rates resulting from the estimation method currently published in the ASHRAE Handbook—Fundamentals (2013), this article proposes a new method to estimate this infiltration rate. The main difference of this new method (which we refer to as limiting area method [LAM]) is that it takes into account the smallest openings in the flow path limiting airflow. Comparison of results using the LAM to those from the previous simulations show that it is much closer to the more detailed simulation calculations, with estimates ranging from 92% to 134% of simulation results for the building examples considered.
Smart grid has been drawing attention particularly when renewable generations are integrated. In order to ensure high power reliability and energy efficiency in an electrical grid, research and application has been conducted at power supply side to solve the grid critical issues: peak load and power imbalance. However, as the major end-users at power demand side, buildings can also play a significant and cost-effective role by making use of their power demand responses. Different demand response programs (e.g., time- and incentive-based) have been developed and applied for encouraging the end-users to change their energy usage behaviors expected by the grid. Generally, buildings are able to limit and/or shift the power demands according to their own considerations under the specific incentives. A comprehensive review on the building power demand response methods is still missing, although research and application has been investigated and conducted on power demand aspects concerning the building system configuration and the control strategies of power demand optimization. This article, therefore, presents a comprehensive review on the strategies, impacts, and benefits of building power demand response in a grid to systematically evaluate and make better use of their demand response potentials. The possibility of developing proper building power demand response strategies for offline and online applications of the smart grid is also discussed.
A new model for predicting the thermal and electrical performance of solid-oxide fuel cell(SOFC) cogeneration devices for residential buildings has been developed and demonstrated. This is a system-level model that considers the thermodynamic performance of all components that consume energy and produce the thermal and electrical output of the SOFC-cogeneration device. The model relies heavily upon empirical information that can be acquired from the testing of coherent systems or components and is designed for operation at a time resolution that is in the order of minutes. Hence, it is appropriate for use in whole-building simulation programs, where it can be applied to assess the energy and greenhouse gas emission benefits of this nascent technology.
Simplified hybrid-lumped capacitance data center model. 
Transient thermal events in air-cooled data centers may lead to undesirable operating conditions such as the formation of hot spots and associated degradation of equipment reliability. These transients may be caused by cooling equipment failures, server load changes, or other time-dependent scenarios in data center operations. This paper introduces a fast-executing hybrid computational fluid dynamics (CFD)/Lumped-Capacitance model for predicting server inlet temperatures resulting from common transient events such as server shutdown, partial or total chilled water interruption, or partial or total failure of the computer room air handlers (CRAH). The model uses initial steady-state CFD or experimental data in combination with several lumped-capacitance models of the various thermal masses in the data center, including the servers, the room enclosure, the CRAHs and the underfloor plenum. The inclusion of these thermal capacitances and their associated thermal conductance was found to be an important contributor to the overall transient response of the data center air-space. The model predictions have been compared with experimental data obtained in a three-rack data-center test cell and found to agree well with the experimental measurements. Examples of the application of the model to more realistic data center configurations are also given.
Faults in HVAC systems can have a significant negative impact on energy consumption, indoor thermal comfort, and air quality. Automatic fault detection and diagnosis tools can help commissioning providers, operators, and facility managers efficiently detect and diagnose faults. They also can help satisfy the increasing demand for commissioning. A model-based fault detection and diagnosis (FDD) method was developed to detect faults by comparing model prediction and measurement, and to diagnose faults using a rule-based fuzzy inferencing system. The method includes Monte Carlo analysis to improve the robustness of the fault detection and diagnosis and reduce false alarms. The Monte Carlo analysis is employed not only to predict uncertainties in reference model outputs, based on estimates of uncertainty in each of the measured inputs, but also to determine the confidence levels of fault diagnosis by combining the effects of input uncertainties at different operating points. A simulated variable-air-volume (VAV) system, including detailed component models that can simulate different faults as well as correct operation, was used to test the diagnostic rules and the Monte Carlo analysis included in the method. The effect of uncertainties on fault diagnosis is illustrated for various types of faulty operation.
This paper presents a robust fault detection and diagnosis (FDD) strategy for centrifugal chillers. The strategy consists of a model-based chiller FDD scheme and a sensor fault detection, diagnosis, and estimation (FDD&E) scheme, which handle chiller faults and sensor faults, respectively. The sensor FDD&E scheme uses a PCA-based method (principal component analysis) to capture the correlations among the major measured variables in centrifugal chillers, as it performs well even in the presence of typical chiller faults. The chiller FDD scheme has been developed based on six physical performance indices, which are capable of describing the health condition of centrifugal chillers and, thus, indicating chiller faults. Only after all the sensors whose measurements are crucial to the chiller FDD are validated by the sensor FDD&E scheme is the chiller FDD scheme used to conduct the chiller system FDD. The strategy was validated using laboratory data from ASHRAE RP-1043 and field data from a centrifugal chiller in a real building.
A plate heat pipe heat exchanger for use in a room ventilation system to cool or heat outdoor fresh air was designed in this study. One fresh air duct and one exhaust air duct were joined with the heat pipe heat exchanger to study the thermal performance of the thermal recovery system. A ratio of 1 for air volume flow rate between fresh and exhaust air was used to investigate the heat recovery effectiveness and the change in fresh air temperature. The fresh air temperature in the inlet duct was controlled in the range of 27 to 40°C under summer simulation conditions and in the range of 2 to 15°C under winter simulation conditions, whereas the temperature of exhaust air was controlled at approximately 24 and 18 C to simulate summer and winter conditions, respectively. The experimental results show that the temperature variations of fresh and exhaust air increase with increasing inlet fresh air temperature under summer conditions but decrease with increasing inlet temperature of fresh air under winter conditions. The maximum heat recovery effectiveness of the plate heat pipe heat exchanger was 58% in summer and 62% in winter. The change in effectiveness is moderate with different vacuum levels. At a vacuum of 1 × 10−3 Pa, the maximum effectiveness occurred when the filling ratio was 1:3. The experimental results showed that the heat pipe heat exchanger gained a high efficiency of heat recovery with low flow resistance. The energy conservation effect of this heat-pipe heat exchanger was evident.
Retrofitting walls with foam insulation is a common practice in residential construction to reduce heating demand; however, the implications of this practice for moisture control are less straightforward. Typically structures in cold climates have a polyethylene vapor retarder on the interior framing surface, therefore adding relatively water vapor impermeable exterior insulation greatly reduces the drying potential for the wall system. Furthermore, while condensation potential is reduced by the addition of exterior insulation, wood framing can be subject to a temperature and humidity regime more conducive to fungal growth relative to pre-retrofit conditions. To investigate the potential for exterior insulation retrofit strategies in subarctic climates to cause moisture accumulation in wood-framed structures, nine test wall sections were constructed using varying ratios of stud-fill and exterior insulation. The wall sections were tested in Fairbanks, Alaska, over two winters and were monitored for temperature, humidity, and wood moisture content. Test walls with less than two-thirds of the nominal wall R-value exterior to the framing performed poorly in terms of wood moisture content and relative humidity at the sheathing interior surface whether or not the test walls were equipped with vapor retarders. The findings are used to examine conventional moisture control frameworks.
Passive cooling strategies, such as nighttime cooling or wind-driven ventilation, provide opportunities for building energy savings, if the mechanical system operation is appropriately integrated with the building architecture. The articles in this section provide successful examples of such integration in several different locations with hot climatic conditions. Additionally, the building-occupant-preferred thermal conditions allow for increased indoor temperature settings. When all of these strategies of mechanical and architectural design are combined with the understanding of occupant preferences, the buildings in hot climates can be much more energy efficient. This section also offers insights into a couple of technological advancements for the chillers, including fault detection/diagnostics and fin design. Finally, a group of articles addresses particle deposition, its sources, and its influence on aircraft cabin design. These studies address the risk reduction of a potential airborne microbe transmission that could promote the spread of communicable diseases. Overall, the integration of different indoor space layouts and mechanical system performance can be addressed as an optimization problem with specific objective functions for the desired system performance. This optimization can be done numerically or experimentally for specific case studies as presented in the articles of this section.
Food and pharmaceutical refrigeration areas place significant demands on air temperature and air humidity control. This leads to high energy requirements on the HVAC system. In the majority of cases, the entire production hall is “over conditioned” with fresh air. However, very often the products are located in a small part of the overall production area (hall). From an energy efficiency and sustainability point of view, it makes sense to only air condition that area in which the products require refrigerated temperature control. One approach to reduce the refrigeration energy demand is to house the product in localized product cooling systems.In this study, localized product cooling systems are analyzed in order to identify the saving potentials associated with a localized HVAC refrigeration system. Experimental systems were built and evaluated. The simulation analysis highlighted that smaller localized refrigeration housing can reduce total energy demand by up to 65%.
By the stability analysis of the basic transcritical CO2 ejector expansion refrigeration cycle (EERC), the paper proposed a new system which introduces another evaporator downstream the ejector to increase the gas quality into the separator and a vapor feedback valve to decrease the exceed gas into the compressor. The two new components stand for two different cycles: two-stage evaporation cycle and vapor feedback cycle. The theoretical analysis of the new system is carried out based on the first and second laws of thermodynamics to show the effect of the parameters on the system performance, such as entrainment ratio, high-side pressure, outlet temperature of gas cooler, etc. The results by the first law show that, compared with basic EERC the new system can be used in wider range of working conditions, and the COP of the two-stage evaporation cycle is 28.6% higher and the vapor feedback cycle is lower slightly. By exergy analysis at optimum high-side pressure, it is found that the exergy destruction of ejector is the greatest part. The simulation results also give the working ranges of the two cycles, which can help to analyze the system control. Hence, the improvement in the system is a promising method to reduce the restrain in basic EERC system but more study is still needed.
A performance comparison of experimental results for CO2 trans-critical cycle is presented for an overview of the current level of technology. The published performance data were collected as research objects through comprehensive literature review on experimental research. The methods for data processing, error analysis, and performance evaluation are introduced in the research methodology section. Through the proposed research method, 28 groups of performance results from developed prototypes or test rigs are compared and analyzed using the coefficient of performance and the second law efficiency of thermodynamics. A discussion of the performance comparison between developed CO2 devices and commercial products of synthetic working fluid is also presented based on China's national standards (General Administration of Quality Supervision, Insection and Quarantine of the People's Republic of China 2001, 2003). Based on the comparison results, the state-of-art and possible research directions for CO2 trans-critical cycle technology are summarized and presented.
Surgery rooms are a space type with particularly stringent indoor environmental quality (IEQ) requirements (large airflow rates and narrow comfort windows), which translate into high energy use. Due to the unclear IEQ and infection control requirements for surgery rooms in Spain, these spaces are often designed and operated 24 hours per day and 7 days per week, to meet the most stringent recommendations (not only the requirements) in the available standards and guidelines. This paper critically reviews the Spanish mandatory requirements for surgery rooms by comparing them against their performance motivation and other international standards. Regulatory ambiguities and code-compliant energy efficiency opportunities are identified.The requirements and recommendations in the standards included in this review differ in their magnitude (particularly the airflow requirements), but are similar in their prescriptive nature. This paper identifies the performance goals associated to the prescriptive requirements, and proposes a method to adjust system operation (outdoor airflow rate, total supply air, indoor air temperature, and indoor air relative humidity) to meet IEQ performance goals while reducing energy use. Further work is required to define operation infection control requirements for the different surgery types and enable a performance based control strategy based on real time particle concentration monitoring.
A new Thermodynamic Approach for modelling mixtures of refrigerants and lubricating oils is presented. The model includes generalized methods for predicting the following thermodynamic properties of refrigerant-oil mixtures: bubble point temperatures, local oil concentrations, liquid specific heats release (enthalpy) curves are easily generated and also the effect of oil on the LMTD of evaporators can be modelled. A new definitionof the boiling heat transfer coefficient based on the bubble point temperature is also presented. The new Thermodynamic Approach will allow major advances to be made in two-phase refrigeration heat transfer research and design since now the type of oil and the effects of its physical properties can be included in the analysis.
Condensate that appears on mechanical pipe insulation systems might deteriorate the insulation thermal performance and lead to failure of the pipelines. An optimized solution that accounts for cost and system energy efficiency must consider the rate of moisture absorption at various operating conditions, and how the pipe insulation thermal conductivity varies with moisture content. This article reviews the most up-to-date work available in the public domain and observes that a controversy may exist about the similarities and differences of thermal conductivity of pipe insulation systems and flat slab configurations. Since the dissimilar behavior can be associated with the testing methodology from which the thermal conductivity values are originally derived, this article first discusses the methodologies for measuring thermal conductivity of pipe insulation systems with the intention of providing some clarification about such controversy. Steady-state and transient methods are discussed, and the measurements from these two methods are critically compared. The thermal conductivities of several pipe insulation systems are also summarized under dry operating conditions. For wet insulation, four main methods for preparing the wet samples during laboratory measurements have been identified, and it was observed that they yielded very different results. The advantages and shortcomings of each moisturizing strategy discussed at length, and the thermal conductivities of a few available pipe insulation systems in wet conditions are compared. To date, challenges still exist with the measurement of actual thermal conductivity of pipe insulation systems with moisture ingress, and future research needs in this area are discussed.
Top-cited authors
Qingyan Chen
  • The Hong Kong Polytechnic University
Arsen Melikov
  • Technical University of Denmark
Srinivas Katipamula
  • Pacific Northwest National Laboratory
Michael Brambley
  • Pacific Northwest National Laboratory
Shengwei Wang
  • The Hong Kong Polytechnic University