Stratum Ventilation—Advanced Air Distribution for Low-Carbon and Healthy Buildings

This chapter provides an overview of stratum ventilation as a low-carbon way to thermal comfort and indoor air quality. The background that leads to the development of the concept of stratum ventilation is first introduced. It is uncovered that stratum ventilation effectively provides thermal comfort in warm environments, and potential contributing factors to this phenomenon are investigated. By directly delivering air to the breathing zone, stratum ventilation significantly improves indoor air quality. Moreover, the implementation of stratum ventilation allows for substantial energy savings. Nine factors that play a role in achieving these energy savings are identified. Transient processes associated with stratum ventilation are explored, yielding valuable insights. The design guidelines pertaining to this specific topic are finally presented.

This chapter investigates the airflow characteristics of stratum ventilation by compared with two existing methods, i.e., mixing ventilation and displacement ventilation. This chapter measures the air velocity and temperature within the occupied zone of a classroom with multiple occupants under cooling mode. Based on the measured data, the turbulence intensity and the power spectrum of velocity fluctuations are evaluated. Additionally, thermal comfort and cooling efficiency are also evaluated. The results highlight distinct airflow behaviors among stratum ventilation, mixing ventilation, and displacement ventilation. The presence of multiple thermal manikins in the classroom exacerbates turbulent airflow fluctuations due to the mixing effects of the thermal buoyancy and supply airflow. Compared with mixing ventilation and displacement ventilation, stratum ventilation requires a higher supply air temperature for overall comfort while minimizing draft risks. Analysis of mean air temperatures in the occupied zone demonstrates that stratum ventilation exhibits the highest cooling efficiency, followed by displacement ventilation and mixing ventilation, respectively.

Ensuring an appropriate thermal comfort level is crucial in elevated room temperature environments. This chapter presents a subjective analysis of thermal comfort in a controlled environmental chamber utilizing stratum ventilation. Overall thermal sensation (OTS), local thermal sensation (LTS), overall thermal comfort (OTC), and draft thermal comfort indexes are quantified. Results of Test Series 1 indicate that room temperature has a significant effect on the four indexes mentioned above. Specifically, a ventilation rate of 10 air changes per hour (ACH) and room temperature of 27 °C provide a neutral overall thermal sensation, satisfactory overall thermal comfort, and low draft risk. However, at room temperatures of 24 °C and 29 °C, the percentage of subjects reporting comfort is below 80%. The effects of supply airflow rate and supply air temperature on thermal comfort (Test Series 2) show that increasing the supply airflow rate from 7 to 17 ACH at room temperature of 27 °C has a negligible impact on thermal sensation and draft, indicating a preference for enhanced air movement. However, to minimize complaints related to the draft, it is essential to avoid supply air temperatures below 20 °C. These findings emphasize that stratum ventilation creates a thermally comfortable environment.

Numerical methods are utilized to study the diffusion of gaseous contaminants under stratum ventilation, and experimental validation is conducted. The findings indicate that the concentration of gaseous contaminants along the supply air jet is lower compared to other areas of the room. When compared with displacement ventilation, lower formaldehyde concentrations are observed in the breathing zone when the contaminant source is situated near the occupant, while similar concentrations are found when the source is upstream of the occupant. Furthermore, concentrations in the occupied zone (within 1.9 m from the floor) are lower when the contaminant source is located on the floor. However, when the supply air temperatures are optimized for displacement ventilation, the toluene concentration in the breathing zone is higher under stratum ventilation than under displacement ventilation when the area source is positioned on the four surrounding walls of the room.

Sustainable ventilation is a potential solution for addressing climate change and reducing carbon emissions. However, there is a need for a comprehensive analysis of their environmental impact, energy performance, and economic cost-effectiveness. Such assessments should consider the combined effects of life cycle cost (LCC) and carbon emissions during both the supply-and-installation phase and the operation phase. This chapter presents a systematic approach for estimating the environmental impact of ventilation systems, which includes energy demand and CO₂ emissions throughout both phases. The approach is applied to a typical classroom setting, considering three ventilation systems: mixing ventilation, displacement ventilation, and stratum ventilation. The findings indicate that stratum ventilation exhibits the lowest environmental impact and life cycle cost among the three ventilation systems. Adopting displacement ventilation and stratum ventilation can lead to significant reductions in CO₂ emissions, up to 23.25% and 31.71%, respectively, compared to mixing ventilation over a 20 year service period. Moreover, adopting displacement ventilation and stratum ventilation can result in cost savings, with reductions in life cycle costs of up to 15.52% and 23.89%, respectively, compared to mixing ventilation over the same service period.

This chapter investigates the influence of air terminal layout on the performance of stratum ventilation using a combination of experimental and numerical methods. The experimental findings demonstrate that both the exhaust location and the supply air flow rate significantly impact the air diffusion performance. Simulation results demonstrate that the exhausts at the bottom of the wall in the occupied zone lead to improved air mixing. This configuration contributes to enhanced thermal comfort and indoor air quality. Furthermore, optimal levels of thermal comfort and air quality are achieved when the exhausts are placed at a lower level along the same wall as the supply air outlets, resulting in space savings for installation. Consequently, this particular air terminal layout is preferred for the design of stratum ventilation systems, provided that the performance requirements are met.

This chapter aims to investigate the impact of various air terminal types on the design and performance of stratum ventilation. Experimental and numerical methods are employed to examine four specific air terminal types, i.e., circular diffusers, square diffusers, perforated diffusers, and double deflection grilles. The experiments are conducted under different experimental conditions to assess thermal sensation and comfort. Six performance indicators are utilized, i.e., airflow pattern, temperature distribution, air diffusion performance index, thermal sensation, comfort feedback, local mean age of air, and CO₂ concentration. The findings demonstrate that the airflow pattern is significantly influenced by the type of air terminal. All four diffuser types exhibit effective air diffusion performance, and circular and perforated diffusers are particularly successful in providing thermal comfort. In terms of air quality, the circular diffuser displays the highest local mean age of air, while the CO₂ concentration in the breathing zone ranks second lowest and exhibits the greatest uniformity. Consequently, the circular diffuser is considered a favorable choice for stratum ventilation.

The conventional effective draft temperature (EDT) is commonly used to assess air distribution performance regarding thermal comfort. However, its applicability is limited to air velocities below 0.35 m/s. To accommodate higher air velocities of stratum ventilation for energy saving, this chapter proposes an extension EDT to cover the full range of air velocity. The transfer coefficient of air velocity to air temperature in the proposed extended EDT is determined based on the cooling effect of air movement, calculated using the standard effective temperature. The reference state, upper and lower boundaries of the proposed extended EDT are quantified based on thermal neutrality, the upper and lower boundaries of thermal comfort. Experimental investigations are conducted in a stratum-ventilated office to validate the proposed extended EDT, respectively. Results demonstrate that the conventional EDT achieves an average accuracy rate of 69.6%. The existing extended EDT outperforms the conventional EDT with an average accuracy rate of 71.7%. The proposed extended EDT exhibits significant improvement, surpassing the conventional EDT by 40.6% with an average accuracy rate of 97.8%. To facilitate practical applications, the proposed extended EDT is tabulated according to Categories I to III of thermal comfort under cooling and heating modes.

This chapter outlines guidelines for designing stratum ventilation systems. The effectiveness of stratum ventilation in various settings, including small to medium individual offices, open offices, classrooms, and retail shops, is evaluated. When designed properly, stratum ventilation can maintain a thermally comfortable indoor environment characterized by horizontal airflow at head level, minimal temperature gradient, and a high air distribution performance index. One of the advantages of stratum ventilation systems is the direct entry of supply air into the breathing zone. This shortened supply air path leads to a younger mean age of air, higher ventilation effectiveness, and improved indoor air quality within the breathing zone. By leveraging both the cooling effects of temperature and velocity of the supply air, stratum ventilation systems can achieve these benefits. As a result, specific applications of stratum ventilation require smaller capacity, leading to reduced system size, space requirements, initial costs, and energy consumption.

The energy-saving effectiveness of stratum ventilation, as compared to mixing ventilation, is attributed to the non-uniform distribution of vertical air temperature. However, accurately predicting this distribution using a multi-node model presents challenges for engineers due to its dependency on complex airflow patterns and specific ventilation designs. To address this issue, this chapter proposes a practical approach known as the Heat Removal Efficiency (HRE) based multi-node model. By utilizing HRE to represent airflow patterns, this model offers simplicity and versatility, requiring minimal understanding of airflow dynamics. Experimental results demonstrate the superiority of the proposed model, achieving higher accuracy and robustness compared to conventional models. It reduces the mean absolute error in temperature predictions for air nodes and enclosure surfaces of stratum ventilation by 0.1 °C and 0.08 °C, respectively. The improved convenience, generality, flexibility, accuracy, and robustness of the proposed model make it a practical solution for implementing energy-efficient stratum ventilation.

Accurately estimating the cooling load is vital for designing and controlling air conditioning systems. However, challenges emerge when dealing with indoor air temperature stratification of stratum ventilation. Many current building simulation tools solely rely on the fully mixed air model, neglecting this stratification. In this chapter, a method is proposed to estimate the cooling load using the concept of equivalent room air temperature, aiming to overcome this limitation and enable accurate estimation using the fully mixed air model for stratum ventilation. The equivalent room air temperature refers to the air temperature that would yield the same cooling load as stratum ventilation when employing the fully mixed air model. To accomplish this, response surface methodology is utilized to establish a mathematical model that correlates the equivalent room air temperature with the supply air temperature, supply airflow rate, and room air temperature. Case studies are conducted using experimentally validated multi-node models to demonstrate the effectiveness of the proposed method. For the constant-air-volume system and the variable-air-volume system, the mean absolute errors under stratum ventilation are 0.02 and 5.78%. Compared to the conventional method, our approach significantly enhances the accuracy of cooling load estimation by 99.94% for stratum ventilation.

Combining elevated room air temperature with increased room air velocity is commonly used to achieve thermal comfort and energy efficiency. However, excessively high room air temperatures negatively impact the energy performance of air conditioning systems by increasing the energy consumption of ventilation fans. Existing models for evaluating thermal comfort in the field of building energy performance often do not accurately account for elevated room air velocity, as most building simulation tools and management systems lack precise information on this parameter. This chapter introduces a method to optimize room air temperature to obtain desired thermal conditions and minimize energy consumption in air conditioning systems for stratum ventilation. Firstly, the Predicted Mean Vote (PMV) model is modified to include room air temperature and supply airflow rate. Secondly, a specific supply airflow rate is determined for each room air temperature to achieve the desired thermal condition by utilizing the modified PMV, and the room air temperature is optimized to minimize energy consumption. The case study results indicate that the energy consumption of the air conditioning system can be reduced by 7.8% while maintaining the intended thermal comfort conditions.

Stratum ventilation is widely recognized for its efficiency in maintaining good indoor air quality. However, the non-uniform CO₂ distribution within a stratum-ventilated room is challenging in achieving intended indoor air quality levels. This chapter introduces an optimization method to identify the optimal ratio of fresh outdoor air that minimizes energy consumption while maintaining indoor air quality of stratum ventilation. A model is developed to estimate the CO₂ concentration in the breathing zone by incorporating the CO₂ removal efficiency within the breathing zone and mass conservation principles. The model facilitates the quantification of ventilation parameters based on varying ratios of fresh outdoor air, intending to achieve targeted indoor air quality. Energy performance assessments of the air-conditioning system are conducted using building energy simulations. Experiments show that the developed model for CO₂ concentration in the breathing zone achieves a mean absolute error of 1.9%. The effectiveness of the proposed optimization approach is demonstrated using TRNSYS, leading to a 6.4% reduction in energy consumption for the air-conditioning system with stratum ventilation. This optimization method has significant potential for enhancing indoor air quality and energy efficiency for other ventilation modes.

A comparative experimental investigation is conducted to evaluate the effectiveness of stratum ventilation in comparison to mixing ventilation for space heating applications. Both objective and subjective experiments are carried out to assess the thermal performance of the two ventilation methods. The objective experiment analyzes the distribution of air temperature, air velocity, and ventilation efficiency in both stratum and mixing ventilation setups. The results demonstrate that stratum ventilation achieves a substantial 25% decrease in energy consumption compared to mixing ventilation. Moreover, the chapter examines considerations related to indoor thermal comfort. Overall thermal perception and comfort, effective draft temperature, and vertical air temperature gradient are employed as evaluation criteria. Implementing stratum ventilation for heating results in a 10.0% enhancement in indoor comfort levels while maintaining the same supply air parameters as mixing ventilation. In conclusion, stratum ventilation exhibits superior performance in terms of energy efficiency and indoor comfort for space heating applications.

This chapter aims to explore the impact of operational parameters on the heating performance of stratum ventilation in a sleeping environment. Validated Computational Fluid Dynamics simulations are employed to comprehensively assess the effects of supply vane angle, supply airflow rate, supply air temperature, and outdoor weather conditions. The chapter considers various performance parameters, including operative temperature, local partial thermal sensation, local mean age of air, air change efficiency, and energy utilization coefficient. The results demonstrate that stratum ventilation effectively achieves a temperature distribution that meets the local thermal requirements of occupants in the sleeping environment, with a warmer head zone compared to the body zone. Additionally, stratum ventilation performs favorably in terms of indoor air quality and energy utilization efficiency. The maximum bed zone air change efficiency reaches 0.80, and the maximum room zone air change efficiency reaches 0.57, surpassing the values found in conventional uniform environments (i.e., 0.5). Furthermore, the maximum energy utilization coefficient of stratum ventilation reaches 1.45, surpassing the energy utilization coefficient of the conventional uniform environment (i.e., 1.0). To optimize the implementation of stratum ventilated heating, this chapter identifies the optimal supply airflow rate, supply vane angle, and supply air temperature, considering factors such as thermal comfort, indoor air quality, and energy utilization efficiency.

This chapter tests the effectiveness of stratum ventilation to minimize the risk of exposure for enhancing the design of ventilation systems in hospital wards and protecting healthcare workers from respiratory infections. The distribution of contaminants within a two-bed hospital ward housing two patients and one healthcare worker under various ventilation strategies, including stratum ventilation, mixing ventilation, downward ventilation, and displacement ventilation, is compared. To simulate the release of exhaled and coughed contaminants by patients, a tracer gas like CO₂ is employed. The concentration of contaminants and the effectiveness of their removal under different air distribution scenarios are evaluated. The results indicate that stratum ventilation, with two patients exhaling, leads to lower contaminant concentrations in the breathing zone, and demonstrates higher effectiveness in removing contaminants. Furthermore, the study analyzes the concentration of coughed contaminants at various time intervals. The findings suggest that under stratum ventilation, coughed contaminants are rapidly diluted, resulting in a significant reduction in areas with high concentrations. Overall, this research underscores the effectiveness of stratum ventilation in mitigating the exposure risks faced by healthcare workers in hospital wards.

To effectively mitigate the risk of respiratory diseases, a significant supply of fresh outdoor air is necessary. However, this requirement results in a considerable increase in energy consumption. This chapter introduces an innovative integrated approach that combines an exhaust air heat pump (EAHP) and advanced air distribution to address this challenge. The findings demonstrate that the integration of the EAHP with advanced air distribution achieves energy savings through three key mechanisms. Firstly, by utilizing the waste heat from the exhaust air, the EAHP decreases the condensation temperature, thus enhancing the coefficient of performance. Secondly, advanced air distribution reduces the ventilation load. Lastly, advanced air distribution lowers the condensation temperature and raises the evaporation temperature, further improving the coefficient of performance. The EAHP alone achieves energy savings of 18%, while advanced air distribution contributes to energy savings of 36%. When combined, the integrated system achieves energy savings of 45%. When compared to a conventional system utilizing an outdoor air heat pump (OAHP) with mixing ventilation, the proposed integrated system of OAHP with stratum ventilation achieves energy savings ranging from 21 to 35% across various outdoor air ratios and temperatures.