Window operation and impacts on building energy consumption

Operable windows provide occupants’ ability to control over local environment and satisfy human’s expectation to access to outdoor environment. Operation strategies for operable windows can have significant impacts on indoor thermal comfort and energy consumption of building performance. It is not uncommon that building facility managers complain that operable windows were left open in buildings with a conventional HVAC system. However, optimum control strategies of window operation can improve thermal comfort and reduce energy consumption for building using natural ventilation or mixed mode ventilation. The study focuses on the investigation of the impacts of window operations on building performance for different types of building systems including natural ventilation, mixed mode ventilation, and conventional HVAC systems in a medium-size reference office building. Building performance simulation tool―EnergyPlus is used to simulate window operations for each system in three different climates. Various control strategies of window operations for building operation systems, implemented using (EMS) in EnergyPlus, are evaluated based on the criteria of thermal comfort and energy consumption. The results highlight the impacts of window operations on thermal comfort and energy use and identify that hybrid ventilation for perimeter zones has 12–20 % saving potentials of annual HVAC site energy consumption.KeywordsNatural ventilationMixed modeWindow operationEnergyThermal comfort

From simulation to monitoring: Evaluating the potential of mixed-mode ventilation (MMV) systems for integrating natural ventilation in office buildings through a comprehensive literature review

A holistic investigation into the seasonal and temporal variations of window opening behavior in residential buildings in Chongqing, China

Towards sustainable, energy-efficient and healthy ventilation strategies in buildings: A review

Past research on natural ventilation has revealed that the application of pure natural ventilation systems may be limited in the United States by issues such as climate suitability, humidity control, and reliability. However, hybrid (or mixed-mode) ventilation systems offer the possibility of attaining energy savings in a greater number of buildings and climates through the combination of natural ventilation systems with mechanical equipment. The objective of this study is to investigate the potential energy and indoor environmental performance of natural and hybrid ventilation alternatives in low- to mid-rise US commercial buildings in a variety of US climates. In this effort, the National Institute of Standards and Technology (NIST) reviewed hybrid ventilation approaches and conducted simulations to predict and compare the indoor environmental and energy performance of natural, hybrid, and mechanical systems in an otherwise similar building. Due to the strong interaction of airflow and heat transfer in naturally ventilated buildings, a coupled multi-zone airflow and thermal simulation tool was used to model the systems in a five-story office building in five US cities. Overall, the natural ventilation system performed adequately in San Francisco and Los Angeles, although some tolerance for imperfect thermal and indoor air quality (IAQ) control is required. Natural ventilation system performance was poor in the more challenging climates of Boston, Minneapolis, and Miami due to poor thermal control, unreliable ventilation, or high heating loads. The hybrid ventilation system improved on the performance of the natural ventilation system in all climates, with dramatic improvements in some. Compared to the mechanical system, the hybrid system saved significant amounts of fan energy, reduced cooling loads, or reduced both fan and cooling loads in all climates but often resulted in higher heating loads. Although the hybrid system provided acceptable thermal control, the mechanical system provided more consistent control, as expected. The hybrid ventilation system provided better IAQ control, as indicated by CO2 concentrations, in most but not all cases.

Ventilation behavior in residential buildings with mechanical ventilation systems across different climate zones in China

Building energy use, thermal comfort, natural ventilation, and indoor air quality are influenced by the occupant behavior related to the opening and closing of windows in residential buildings. Studies about window opening and closing behaviour focused mainly on environmental variables (indoor temperature and air quality, climatic factors) and contextual parameters (season, time of the day). This paper investigates the influence of factors related to window design and environmental variables on the frequency of opening and closing the windows and the duration of windows in the open position. The impact of window opening behavior on residential energy consumption is also explored in this study. Data related to window characteristics, ease of operation, hours of windows in open/closed state, and frequency of opening and closing the windows are collected through a questionnaire survey from 365 residences. Energy consumption data is obtained from utility bills and weather data from the meteorological department. Among 365 residences, window opening and closing behaviour were monitored in three residences with loggers for a year to validate the data collected through the questionnaire survey. This study reiterated the influence of environmental variables on the windows’ open duration and the frequency of opening and closing the windows. The results reveal that the window characteristics influence the windows’ open duration and the frequency of opening and closing the windows. The study divulged that energy consumption is influenced by the hours the window is in an open state and the frequency of opening and closing the windows.

Mixed-mode ventilation combines natural ventilation and mechanical ventilation to improve building energy efficiency and indoor air quality. However, in practice, mixed-mode ventilation buildings do not always achieve better performance than mechanically ventilated buildings, largely due to inappropriate window operations. Therefore, the sequences of operation for terminal devices serving zones with operable windows should be designed in recognition of these risks, which in turn should be informed by research investigating occupants' window and thermostat use behaviour. This research analyzes window and thermostat use data collected from two mixed-mode ventilation buildings in Ottawa, Canada. Based on this analysis, the control algorithms are developed, and 3-16% of energy reductions could be achieved based on building performance simulation (BPS). It is also found that the unregulated window operations could increase the heating load up to 21% and cooling load by 33% relative to identical buildings with fixed windows in a cold climate.

Natural ventilation via open windows is a common practice widely applied to dilute aerosols in dental offices for all year around in China, which, however, would modifies air distribution and leads to extra energy consumption for cooling and heating. This study intends to evaluate aerosol removal efficiency and energy consumption in a multi-chair dental office with both mechanical ventilation (MV) and natural ventilation (NV), namely, mixed-mode ventilation (MMV). It numerically investigates the effect of ventilation mode and environmental condition on indoor aerosol distribution and fallow time (FT) duration, as well as energy consumption. The results show that introducing fresh air via open windows in such a mechanically ventilated space results in enhanced airflow mixing and particle dispersion. Compared to six air changes per hour (ACH) MV mode, MMV mode with less than 4 ACH NV does not ensure a reduction in suspended particle count indoors. When NV rate reaches 6 ACH, all compartments show an average reduction of 64.6% in particle counts. However, energy consumption for MMV mode with 6 ACH NV is 3.5 times higher during heating seasons and 2.2 times higher during cooling seasons compared to 6 ACH MV mode. Compared to MV mode with recommended FT of 18–21 min between appointments, MMV mode with 4–6 ACH NV has the required FT of 14–16 min. This study is intended to provide references for clinic managers to balance air quality improvements with energy consumption, thereby achieving a sustainable indoor environment and optimizing operational costs in multi-chair dental offices.

Individual-based insight into occupants' interaction with windows in apartments in Beijing

The mixed-mode ventilation (MMV) system is an energy-friendly ventilation technique that combines natural ventilation (NV) with mechanical air conditioning (AC). It draws in fresh air when the outdoor conditions are favorable or activates otherwise the AC system during occupancy hours. To improve performance of the MMV system, it is proposed to integrate it with an intermittent personalized ventilation (IPV) system. IPV delivers cool clean air intermittently to the occupant and enhances occupant thermal comfort. With the proper ventilation control strategy, IPV can aid MMV by increasing NV mode operational hours, and improve the energy performance of the AC system by relaxing the required macroclimate set point temperature. The aim of this work is to study the IPV+MMV system performance for an office space application in terms of thermal comfort and energy savings through the implementation of an appropriate control strategy. A validated computational fluid dynamics (CFD) model of an office space equipped with IPV is used to assess the thermal fields in the vicinity of an occupant. It is then coupled with a transient bio-heat and comfort models to find the overall thermal comfort levels. Subsequently, a building-performance simulation study is performed using Integrated Environmental Solutions-Virtual Environment (IES-VE) for an office in Beirut, Lebanon for the typical summer month of July. An energy analysis is conducted to predict the savings of the suggested design in comparison to the conventional AC system. Results showed that the use of IPV units and MMV significantly reduced the number of AC operation hours while providing thermal comfort.

Mixed-mode ventilation can effectively reduce energy consumption in buildings, as well as improve thermal comfort and productivity of occupants. This study predicts thermal and energy performance of mixed-mode ventilation by integrating computational fluid dynamics (CFD) with energy simulation. In the simulation of change-over mixed-mode ventilation, it is critical to determine whether outdoor conditions are suitable for natural ventilation at each time step. This study uses CFD simulations to search for the outdoor temperature thresholds when natural ventilation alone is adequate for thermal comfort. The temperature thresholds for wind-driven natural ventilation are identified by a heat balance model, in which air change rate (ACH) is explicitly computed by CFD considering the influence of the surrounding buildings. In buoyancy-driven natural ventilation, the outdoor temperature thresholds are obtained directly from CFD-based parametric analysis. The integrated approach takes advantage of both the CFD algorithm and energy simulation while maintaining low levels of complexity, enabling building designers to utilize this method for early-stage decisionmaking. This paper first describes the workflow of the proposed integrated approach, followed by two case studies, which are presented using a three-floor office building in an urban context. The results are compared with those using an energy simulation program with built-in multizone modules for natural ventilation. Additionally, adaptive thermal comfort models are applied in these case studies, which shows the possibility of further reducing the electricity used for cooling.

Climate change impact on energy savings in mixed-mode ventilation office buildings in Brazil

The potential for office buildings with mixed-mode ventilation and low energy cooling systems in arid climates

Energy security, climate change and economic growth are matters of critical international importance in an effort to achieve a sustainable future. Energy consumption in buildings contributes to higher greenhouse gas emissions than the industrial or transportation sectors combined. In India, the energy in the residential sector accounts for almost 50% of the total energy consumption. The need for comfortable internal environments, healthy indoor air quality and the consequences of global warming are all contributing factors to the high reliance on mechanical cooling and ventilation systems. In recent years, financial growth and increase in disposable income in India, have accelerated purchases of such mechanical systems. In metropolitan cities of India with extreme climates (hot and dry, warm and humid), the use of these systems increases by 30% every year. This upward trend is likely to continue in response to occupants’ higher comfort expectations and the continuous increase of the outside temperature during the summer months due to climate change. This could further impact the climate and the electricity grid. Innovative solutions should establish reliable strategies for cooling purposes by utilizing the use of natural ventilation. Mixed-mode buildings rely on both mechanical and natural systems to maintain comfortable conditions. Although the performance of mixed-mode buildings has already been studied and there is evidence for its positive impact on the reduction of energy demand, there is still a lack of knowledge on the best methods for controlling mixed-mode buildings. Today, the majority of the available algorithms for the control of mixed-mode systems are very simplistic and at a primitive stage of development. Typically, the control algorithms “make the decision” based on a predefined static set-point temperature, disregarding other important parameters, such as relative humidity, the position of windows and activity of occupants. Control algorithms that would account for a variety of parameters are of paramount importance to achieve energy savings whilst maintaining thermal comfort conditions. The aim of this research was to investigate the impact on thermal comfort and energy savings of novel and sophisticated control algorithms in mixed-mode residential buildings in India.Initially, it was important to identify all the control parameters that were important to be included in the control algorithms. Then the control algorithms were designed and presented in flow charts. To analyse the performance of the proposed control algorithms, computer simulations were performed, whilst a validation analysis was conducted to provide evidence of the validity of the control algorithms. Computer modelling comprised of co-simulations, using Dynamic Thermal Modelling (DTM) (EnergyPlus) and equation-based tools (Dymola using the Modelica language). The coupling of these was achieved using the Functional Mock-up Interface (FMI) for model exchange. The co-simulations enabled to examine the energy saving potential that can be achieved by the proposed control algorithms. In order to evaluate the ventilation performance of the proposed control algorithms, the ventilation rates and ventilation effectiveness of the systems were analysed using Computational Fluid Dynamics (CFD). This allowed the final analysis which included the evaluation of the ventilation performance of the control algorithms by calculating the ventilation effectiveness. To provide evidence of the proposed control algorithms and simulation approach, a validation study was done using data from an experimental chamber in India. This research has contributed to the existing body of knowledge by providing four main conclusions concerning the design and control of mixed-mode ventilation and cooling systems: i) to deliver comprehensive guidelines on the design and control of mixed-mode buildings, and the ways in which the co-simulations can be implemented to improve the existing control algorithms that can be found in the literature; ii) the use of the co-simulations showed that the developed control algorithms, when dampers/windows and ceiling fans are used, can improve the predicted hours of thermal comfort by up to 1900h compared to the scenarios when the ceiling fans were turned off, while achieving up to 55% energy reduction depending on the city; iii) the CFD simulations predicted that cross ventilation with the maximum opening areas for windows and dampers in combination with the operation of the ceiling fans can dillute the contaminants and/or heat in the building resulting in comfortable internal environments resulting in heat removal effectiveness of 1.65; and iv) the accurate and validated control algorithms that were developed in this research can be used for any study that requires control of mixed-mode buildings regardless of the geometry of the building. The use of co-simulations provides great flexibility since the same control algorithms can be used in any geometry or building location without the need for any modification of the code.