An experimental tool for research and educational applications on HVAC systems

This paper describes an experimental tool elaborated at the building sciences laboratory (LASH/DGCB). The objective of this tool is to develop control strategies for heating, cooling and ventilation, and test them on heating, ventilating and air conditioning equipment. The tests are conducted in a large room of the laboratory equipped with a double-flow ventilation system and a global control/data acquisition system. The tool is used for research and some experiments are led by engineering students. It enables the identification of optimal parameters for each developed control strategy, in order to reach good thermal comfort, acceptable indoor air quality and good stability of actuators at the least energy cost. In this paper we present an example of global control strategy for heating and cooling periods based on interior temperature control (fuzzy regulator). This strategy includes an indoor air quality controller (proportional integral or fuzzy regulator) based on indoor CO₂ level control. A measurements campaign during an occupied period is shown with an overview of the actuators’ behaviour. This tool also highlights the key role of advanced control techniques to manage heating, cooling and ventilation systems.     

Optimal decision making in ventilation control

In this paper, a two-mode ventilation control of a single facility is formulated as a scheduling model over multiple time horizons. Using the CO₂ concentration as the major indoor air quality index and expected room occupancy schedule, optimal solutions leading to reduced CO₂ concentration and energy costs are obtained by solving the multi-objective optimization model formulated in the paper. A modified evolutionary strategy algorithm is used to solve the model at different time horizons. The optimized ventilation schedules result in energy savings and maintain an acceptable level of indoor CO₂ concentration.     

Variable air volume ventilation control strategies analysed in six climate zones

This paper presents eight ventilation control strategies and their annual energy and indoor air quality simulation results for an academic building as if it were situated in each of six geographic locations. The results show that without tempering at the terminal boxes, no ventilation strategy could satisfy the outdoor air requirements when the thermal loads are low. The fixed outdoor air percentage method is the worst one. From an economic perspective, strategies using optimization techniques minimize the operating energy demand and consumption. Supply air temperature (SAT) and primary airflow rate are the two proper optimization parameters on the air side of heating, ventilating, and air‐conditioning systems. In addition to control strategies, geographic locations or weather patterns influence the benefits of optimization. Generally, a mild‐dry climate intensifies the advantages of the SAT reset and encourages the primary airflow optimization. Inversely, hot‐humid weather minimizes the benefits. Copyright © 1999 John Wiley & Sons, Ltd.     

Numerical study of a hybrid ventilation system for single family houses

The main objective of this study is to develop and test hybrid ventilation systems and control strategies that are suitable for residential buildings. Two ventilation systems were modelled: a mechanical extract ventilation system (called the reference system) and a hybrid low pressure ventilation system that can support two different types of demand control strategies (occupancy detection and CO₂ concentration). The newly developed models were assembled with the existing thermal models of the SIMBAD Building and HVAC Toolbox developed by the CSTB.A single family house located in Athens (Greece), Nice (France), Trappes (France) and finally Stockholm (Sweden) was considered as the case study. Yearly simulations were performed to assess the performance of the hybrid ventilation control strategies. The assessment criteria used are related to indoor air quality, thermal comfort, energy consumption and stability of control strategies. The results show that the low pressure ventilation system can improve the indoor air quality and reduce the fan energy consumption compared to the reference system while maintaining the same building energy consumption for heating.     

被动式太阳房的降温研究

通风的目的是保证室内良好的空气品质,采用自然通风方式是生态建筑设计的重要内容,自然通风形成的室内热环境有利于人体舒适性.太阳能烟囱管能强化房间的自然通风,可以有效地改善室内的空气品质和室内热环境,但尚未达到舒适的温度.     

Fluid, heat and contaminant transport structures of laminar double-diffusive mixed convection in a two-dimensional ventilated enclosure

The objective of the present work is to investigate the characteristics of the airflow and heat/contaminant transport structures in the indoor air environment by means of a convection transport visualization technique. Laminar double-diffusive mixed convection in a two-dimensional displacement ventilated enclosure with discrete heat and contaminant sources is numerically studied. Based on the governing equations, the fluid, heat, and contaminant transport processes are respectively described by the corresponding streamfunction, heatfunction, and massfunction. Attentions are given to analyze the effects of the main factors—the strength of heat source indicated by the Grashof number (Gr), the strength of contaminant source by the buoyancy ratio (Br), the strength of ventilation by the Reynolds number (Re), and the ventilation mode—on the indoor air environment. Numerical results, presented by the contour function lines, namely, streamlines, heatlines, and masslines, illustrated that the indoor air, heat and contaminant transport structures are mainly determined by the interaction between the internal buoyancy natural convection induced by the discrete heat/contaminant sources and the external forced convection driven by the mechanical ventilation. It is found that the convection transport method could explicitly disclose the complicated philosophy of indoor air environment, and thus provides a simple but practical approach to see the indoor airflow and heat and contaminant transport structures.     

变风量空调系统中的实时优化节能控制

在对变风量空调系统局部控制的分析基础上,利用其变风量末端风门的开度作为各区域相对负荷的指示信号,提出送风静压的实时优化控制方案;同时,针对新的ＡＳＨＲＡＥ通风标准,还提出了基于室内人数检测和焓控制的新风实时优化控制方案。试验结果证明,同常规的控制方案相比,在保证室内热舒适性和空气质量的前提下,这两个方案分别有较好的节能作用。综合采用两种优化方案,系统不仅能够达到节能的目的,而且在较小负荷情况下能够提高室内空气的品质。     

Optimal control strategy for a multi-zone air conditioning system using a genetic algorithm

This paper examines optimal control strategies of variable air volume air conditioning system. The control strategies included a base control strategy of fixed temperature set point and two advanced strategies for insuring comfort and indoor air quality (IAQ). The first advanced control adjusts the fresh air supply rate and the supply air temperature to maintain the temperature set point in each zone while assuring indoor air quality. The second strategy controls the fresh air rate and the supply air temperature to maintain an acceptable thermal comfort and IAQ in each zone. The optimization problem for each control strategy is formulated based on the cost of energy consumption and constrained by system and thermal space transient models. The optimization problem is solved using genetic algorithm. The optimization scheme/model is applied to a case study for a building floor in Beirut weather. The thermal space and system component models were validated for the base strategy using Visual DOE 4.0 software [Architectural Energy Cooperation, San Francisco, USA; 2005 〈www.archenergy.com〉]. Energy savings up to 30.4% were achieved during the summer season of four months with the optimized advanced strategies when compared with the conventional base strategy while comfort and IAQ were satisfied.     

Integration of variable refrigerant flow and heat pump desiccant systems for the cooling season

Energy saving and indoor air condition enhancing potentials by integrating the variable refrigerant flow (VRF) and heat pump desiccant (HPD) systems were investigated in a field performance test during a cooling season. Three different operating modes: non-ventilated, HPD ventilation assisted and HPD ventilation–dehumidification assisted VRF systems were investigated. The HPD systems operated in the ventilation–dehumidification mode dehumidify the outdoor air and supply it to the indoor air during the ventilation. It was found that the VRF systems provided an average of 97.6% of the total cooling energy for the HPD ventilation assisted mode. The remainder was the recovered cool by the HPD systems during ventilation. The VRF systems provided an average of 78.9% of the total cooling energy for the HPD ventilation–dehumidification assisted mode. The remainder was covered by the HPD systems which provided additional sensible and latent cooling. Overall, among the three operating modes, it is concluded that the HPD ventilation–dehumidification assisted VRF outdoor units consume less energy than the HPD ventilation assisted ones, but more than the non-ventilated ones, while providing the best indoor thermal comfort and indoor air quality conditions. For the total system, the HPD ventilation–dehumidification assisted VRF systems consume less energy than the HPD ventilation assisted ones.     

Multi-objective optimization of HVAC system with an evolutionary computation algorithm

A data-mining approach for the optimization of a HVAC (heating, ventilation, and air conditioning) system is presented. A predictive model of the HVAC system is derived by data-mining algorithms, using a dataset collected from an experiment conducted at a research facility. To minimize the energy while maintaining the corresponding IAQ (indoor air quality) within a user-defined range, a multi-objective optimization model is developed. The solutions of this model are set points of the control system derived with an evolutionary computation algorithm. The controllable input variables — supply air temperature and supply air duct static pressure set points — are generated to reduce the energy use. The results produced by the evolutionary computation algorithm show that the control strategy saves energy by optimizing operations of an HVAC system.     

A model-based optimal ventilation control strategy of multi-zone VAV air-conditioning systems

This paper presents a model-based optimal ventilation control strategy for multi-zone VAV air-conditioning systems aiming at optimizing the total fresh air flow rate by compromising the thermal comfort, indoor air quality and total energy consumption. In this strategy, one scheme is used to correct the total fresh air flow rate dynamically by utilizing the unvitiated fresh air from the over-ventilation zones based on the detected occupancy of each zone and the related measurements. At the meantime, another scheme is developed to optimize the temperature set point for the temperature control of critical zones with the aim at reducing the variation of the required fresh air fractions among all the zones and further reducing the total fresh air intake from outdoors for energy saving when the first scheme is implemented. This scheme is based on a constructed cost function relating thermal comfort, indoor air quality and total energy consumption together while the cost function is calculated based on the prediction of system responses using dynamic simplified models. Genetic algorithm is used for optimizing the temperature set point of critical zones in the optimization process. This strategy was evaluated in a simulated building and air-conditioning environment under various weather conditions.     

Use of laplace transforms and block diagrams for indoor air quality problems with perfectly mixed zones

The prediction of the quality of indoor air is important because standards for ventilation rates have been raised, thus potentially increasing energy usage. If acceptable air quality can be provided by control methods, the amount of outside air introduced into a building may be reduced. This paper presents a method of modeling unique ventilation systems with perfectly mixed zones. Laplace transforms for various system components are developed, and the block diagram method for the solution of specific indoor air quality problems is presented. An example problem with a single zone and variable outside air fractions is solved to yield the response to a step change in inlet concentration.     

Pressure peaking phenomenon: Model validation against unignited release and jet fire experiments

The aim of this study is validation of pressure peaking phenomenon models for unignited and ignited releases of hydrogen in enclosures with limited ventilation, e.g. residential garages. The existence of “unexpected” peak in the pressure transient during release of a lighter than air gas in a vented enclosure was observed by Brennan et al. (2010) by carrying out theoretical and numerical research. The amplitude and duration of this pressure peak vary depending on the enclosure volume, vent size and leak flow rate. The peak can significantly exceed the steady-state overpressure, which is reached when the enclosure is fully occupied by leaking with a constant rate gas. The pressure peaking phenomenon can jeopardise a civil structure integrity in the case of accident if it is ignored at the design stage of hydrogen-powered vehicles. This could cause serious life safety and property protection issues that requires development of prevention and mitigation strategies and innovative safety engineering solutions. The experimental validation of the phenomenon was absent up to this work. The previous model for unignited release and developed in this study model for ignited release (jet fire) have been validated against experiments performed in a vented enclosure of 1 m³ volume with three different gases: air, helium, and hydrogen. The model for unignited release reproduces closely the experimental pressure peak and the pressure dynamics within the enclosure. The model for ignited release reproduces the pressure peak with acceptable engineering accuracy, and the simulation of pressure dynamics after the peak requires the increase of the discharge coefficient due to the change of vent flow from heavier air at the start to lighter hot combustion products afterwards and ultimately hydrogen. The methodology to calculate the pressure peaking phenomenon in two steps is described in detail. Examples of pressure peaking phenomenon calculation for typical hydrogen applications are presented. The phenomenon is relevant to most of indoor applications, when release of lighter than air gas is possible in an enclosure with limited ventilation. It must be considered when performing safety engineering design of inherently safer hydrogen systems and infrastructure.     

Active desiccant integration with packaged rooftop HVAC equipment

Current research indicates a direct correlation between indoor air quality and fresh air ventilation rates which supports requirements for building ventilation standards calling for continuous supply and increased amounts of ventilation to help assure safe and healthy interior air environments [O. Seppänen, W.J. Fisk, M.J. Mendell, Ventilation rates and health, ASHRAE Journal (August) (2002) 56–58; C.C. Downing, C.W. Bayer, Classroom indoor air quality vs. ventilation rate, ASHRAE Trans., 1993, Vol. 99, Part 2, Paper Number DE-93-19-1, pp. 1099–1103. [1] and [2]]. Off-the-shelf, packaged rooftop equipment used to air condition most facilities is not designed to handle the increased or continuous supply of outdoor air necessary to comply with building ventilation codes written to this new standard [American Society of Heating, Refrigerating, and Air Conditioning Engineers, Inc. (ASHRAE), Ventilation for acceptable indoor air quality, Standard 62-1989, 1791 Tullie Circle, NE, Atlanta, GA 30329. [3]].Integration of a rooftop, unitary air conditioner with an active desiccant module (ADM) allows the use of a standard rooftop air conditioner with a thermally regenerated active desiccant component to provide a compact, cost-effective, and simple-to-use packaged system for efficiently pre-treating and supplying ventilation air adequate to ensure healthy indoor environments. By designing a combined vapor-compression/active desiccant system with the desiccant component positioned after a conventional cooling coil, the dehumidification effectiveness of the desiccant is significantly enhanced because it operates on cold, saturated, or nearly saturated, air leaving the evaporator. “Post-coil” rather than the normally used, “pre-coil” desiccant arrangement also minimizes the regeneration temperature required for the active desiccant, allows for partial bypass and post-cooling of the desiccated air after recombination, and dramatic decreases in the overall size for the pre-conditioning unit.     

Application of ANN to explore the potential use of natural ventilation in buildings in Turkey

Indoor natural ventilation provides both the circulation of clear air and the decrease of indoor temperature, especially, during hot summer days. In addition to openings, building dimensions and position play a significant role to obtain a uniform indoor air velocity distribution. In this study, the potential use of natural ventilation as a passive cooling system in new building designs in Kayseri, a midsize city in Turkey, was investigated. First, indoor air velocity distributions with respect to changing wind direction, magnitude and door openings were simulated by the FLUENT package program, which employs finite element methods. Using the simulated data an artificial neural network (ANN) model was developed to predict indoor average and maximum air velocities. The simulations produced by FLUENT show that the average indoor air velocity is generally below 1.0 m/s for the local prevailing wind directions. The simulations results suggest that, in addition to the orientation of buildings in accordance with prevailing wind directions, a proper indoor design of buildings in the area can significantly increase the capability of air ventilation during warm summer days. It was found that a high correlation exists between the simulated and the ANN predicted data indicating a successful learning by the proposed ANN model. Overall, the evaluation of the network results indicated that the ANN approach can be utilized as an efficient tool for learning, training and predicting indoor air velocity distributions for natural ventilation.     

A robust control strategy for combining DCV control with economizer control

Combining demand controlled ventilation (DCV) control and economizer control achieves acceptable and even better indoor air quality with minimum coil energy consumption. The control instability during the transition processes between different control modes are among the major difficulties faced when utilizing economizer control and when combining DCV control with economizer control in applications. A robust control strategy, using “freezing”, gain scheduling, integral term reset and feedback transition control for different transition processes, is developed for addressing these problems. They are evaluated on an air handling unit (AHU) using various simulation tests. The test results demonstrate that the robust control strategy allows stable and robust AHU control.     

Night ventilation control strategies in office buildings

In moderate climates night ventilation is an effective and energy-efficient approach to improve the indoor thermal environment for office buildings during the summer months, especially for heavyweight construction. However, is night ventilation a suitable strategy for office buildings with lightweight construction located in cold climates? In order to answer this question, the whole energy-consumption analysis software EnergyPlus was used to simulate the indoor thermal environment and energy consumption in typical office buildings with night mechanical ventilation in three cities in northern China. The summer outdoor climate data was analyzed, and three typical design days were chosen. The most important factors influencing night ventilation performance such as ventilation rates, ventilation duration, building mass and climatic conditions were evaluated. When night ventilation operation time is closer to active cooling time, the efficiency of night ventilation is higher. With night ventilation rate of 10 ach, the mean radiant temperature of the indoor surface decreased by up to 3.9 °C. The longer the duration of operation, the more efficient the night ventilation strategy becomes. The control strategies for three locations are given in the paper. Based on the optimized strategies, the operation consumption and fees are calculated. The results show that more energy is saved in office buildings cooled by a night ventilation system in northern China than ones that do not employ this strategy.     

Chilled ceiling and displacement ventilation system for energy savings: A case study

This paper studies the design and performance of cooled ceiling and displacement ventilation (CC/DV) systems application for buildings in Beirut for the purpose of saving energy. The transient thermal response of spaces cooled by the combined CC/DV system is needed for performance assessment. For that reason, the plume‐multi‐layer model of CC/DV cooled spaces is extended to transient applications. A design procedure for the combined CC/DV system in Beirut humid climate and buildings is developed to insure that both indoor air quality and comfort are satisfied within the conditioned zone. The contribution of the proposed procedure is that it guarantees that the stratification height (occupied zone) is at 1.1 m taking into consideration the plumes from internal sources and non‐isothermal walls. The design procedure is applied to a case study in Beirut to design a system for a typical office space at 85 W m^?2 sensible cooling load. The CC/DV system size is compared with the size of a conventional mixed convection system. It is found that the size of the CC/DV system is 10.2 kW compared to conventional system size of 7.9 and 13.4 kW at the 30 and 100% fresh air supply, respectively. For the same indoor air quality and thermal comfort level, the CC/DV system consumed 21% less cooling energy than the conventional 100% fresh air system over the cooling season. The initial cost of the CC/DV system is higher, but the pay back period based on transient operation is less than 5 yr. Copyright © 2006 John Wiley & Sons, Ltd.     

Simulation of a localized heating system for broiler brooding to improve energy performance

The aim of this work is to evaluate the performance of an innovative localized solar‐assisted pen heating system for brooding using a 3D computational simulation model of the heated space. The warm air‐curtained pen ensures acceptable temperature, air velocity, relative humidity, and air quality that meet the ventilation and heat requirements for a typical pen of 100 chicks as recommended by the American Society of Heating Refrigeration and Air Conditioning Engineers and American Society of Agricultural and Biological Engineers. The supply flow characteristics and the simulated velocity and temperature field of the curtained region were determined such that they meet the ventilation requirements and comfort criteria. Results show that air supplied at 40°C is capable of delivering the desired microenvironment at bird level while the heat input to the unit is 685 W when outdoor temperature is ?5°C. The system's energy performance was then analyzed using a prototype of 16 pens. The energy consumption of the new heating scheme consumed one third of the energy required by conventional non‐localized system. Moreover, integrating the new design with a solar system utilizing parabolic concentrators provided 72% of the power load from solar energy during a winter flock operation and 100% during other seasons. Copyright © 2013 John Wiley & Sons, Ltd.     

An approach to optimised control of HVAC systems in indoor swimming pools

Indoor swimming pools are recognised as having a high level of energy consumption and present a great potential for energy saving. The energy is spent in several ways such as evaporation heat loss from the pool, high rates of ventilation required to guarantee the indoor air quality, and ambient temperatures with expressive values (typically 28?30°C) required to maintain conditions of comfort. This paper presents an approach to optimising control of heat ventilation and air conditioning systems that could be implemented in a building energy management system. It is easily adapted to any kind of pool and results in significant energy consumption reduction. The development and validation of the control model were carried out with a building thermal simulation software. The use of this control model in the case study building could reduce the energy efficiency index by 7.14 points (7.4% of total) which adds up to an energy cost saving of 15,609€ (7.5% of total).