Zhang Lin

City University of Hong Kong | CityU · Division of Building Science and Technology

For improving the indoor thermal comfort and air quality, floor radiant heating and cooling combined with fresh air systems have been widely used in recent years. These systems require two heat sources/sinks at different temperatures. To satisfy the requirement of two heat sources/sinks, an ejector-compression dual-heat source/sink air source heat...

Evaluation of airborne infection risk with spatial and temporal resolutions is indispensable for the design of proper interventions fighting infectious respiratory diseases (e.g., COVID-19), because the distribution of aerosol contagions is both spatially and temporally non-uniform. However, the well-recognized Wells-Riley model and modified Wells-...

To reduce the energy consumption of the heating, ventilation and air conditioning system, a novel dual heat source warm-air heating system (DWAHS) was developed in this study. A dual heat source air source heat pump was applied to satisfy the warm air segmented handling process first time. The theoretical model of the system was established, and ec...

Predicted Mean Vote (PMV) is a prevailing thermal comfort model adopted by thermal comfort standards. To extend its ability in explaining thermal adaptations, the PMV is multiplied by an extension factor. However, the original extended PMV (ePMV) cannot account for thermal adaptations around thermal neutrality, resulting in deviation around thermal...

A stratum ventilated heating system is applied to satisfy the thermal requirements in a sleeping environment. Several parameters and configurations significantly influence the performance of the heating system. Among these parameters, the supply vane angle, supply air terminal, and exhaust location are critical factors that influence the energy con...

High-efficiency air source heat pumps are of great importance for energy saving and environmental protection, especially for the cold area in north China. In this study, the thermo-economic performance of the dual-temperature ejector-compression air-source heat-pump (DAHP) cycle, is conducted wherein R1234yf, R290, and R134a are used as refrigerant...

Reducing airborne infectious risk is crucial for controlling infectious respiratory diseases (e.g., COVID-19). The airborne transmissibility of COVID-19 is high so that the common ventilation rate may be insufficient to dilute the airborne pathogens, particularly in public buildings with a relatively large occupancy density. Reducing occupancy can...

To improve the living and sleeping quality for human beings, air conditioning is widely applied to maintain a comfortable indoor environment in bedrooms. However, most current air conditioning methods in bedrooms are based on mixing ventilation, which is low efficiency due to the uniform environment created by this ventilation method. In this study...

A suitable outdoor thermal environment can encourage people to partake in outdoor activities, which, in turn, reduces building energy consumption. This can be achieved by accurately predicting the outdoor thermal environment. Most existing prediction models of the outdoor thermal environment focus on the relationship between environmental parameter...

Predicted Mean Vote (PMV) predicts thermal sensation for a group of people from the human thermal load, and is well recognized by thermal comfort standards for energy-efficient design of thermally comfortable buildings. However, the oversimplified skin evaporative heat loss in the thermal load calculation contributes to the frequently reported disc...

Evaluation of airborne infection risk with spatial and temporal resolutions is indispensable for the design of proper interventions fighting infectious respiratory diseases ( e.g ., COVID-19), because the distribution of aerosol contagions is both spatially and temporally non-uniform. However, the well-recognized Wells-Riley model and modified Well...

This study presents a novel dual-temperature air-source heat pump system with an ejector using zeotropic mixture R1270/R600a. The system provides a high-temperature heat source (around 60 °C hot water) and low-temperature heat source (around 35 °C hot water) respectively. The system improves its performance via the application of an ejector. The ma...

In order to improve the ventilation design for hospital wards and protect the healthcare workers from respiratory infections, stratum ventilation is proposed to reduce the exposure risk. A numerical (CFD) approach is applied to investigate contaminant distribution in a two-bed hospital ward with two patients and one healthcare worker under stratum...

The accurate prediction of thermal comfort is crucial for optimally designing buildings with thermal comfort and energy efficiency. Predicted Mean Vote (PMV) is widely recognized by national and international standards for the prediction of thermal comfort. However, the low accuracy of the PMV has been criticized by various studies under different...

Enhancing ventilation effectiveness can improve indoor air quality (IAQ). Both contaminant removal effectiveness (CRE) and air change effectiveness (ACE) are widely used to evaluate ventilation effectiveness. They reflect two different aspects of IAQ, and an analysis based exclusively on ACE or CRE has important limitations. Firstly, this study is...

Stratum ventilation with proper operation can provide thermal comfort and good inhaled air quality with energy efficiency. Previous studies mainly focused on stratum ventilation for cooling or heating in separate seasons, while little attention on stratum ventilation for whole year operation. Proper year-round operation of stratum ventilation is mo...

Occupants show different thermal requirements in the sleeping environment in comparison with the activity environment. A new sleeping environment provided by stratum ventilation (SV) is proposed to satisfy the requirements on indoor air quality, thermal comfort and energy efficiency. This study comprehensively investigates the effects of operation...

Air distribution is widely used for thermal comfort. Air distribution is inevitably non-uniform with some subzones in the occupied zone cooler than the others, and the thermal preferences of occupants are differentiated. Non-uniform air distribution may deteriorate thermal comfort because occupants with warmer thermal preferences might sit in coole...

Thermal adaptations mitigate thermal discomfort and thereby reducing building energy consumption. Adaptive-rational thermal comfort modelling modifies the rational model using the adaptive approach to enable the rational model to fully account for the thermal adaptations for improved thermal comfort prediction. However, the applicability of the ada...

To provide a highly efficient heating for the residential buildings, different heating systems, including the floor and ceiling heating, wall radiator heating and stratum ventilation heating, are compared based on a typical sleeping environment in this study. Performance indices, including occupied zone predicted mean vote (OPMV), energy consumptio...

Thermal adaptations, as feedbacks of occupants to physical stimuli, extend thermal comfort zone thereby reducing building energy consumption effectively. The rational approach models thermal comfort from the perspective of the body’s heat balance, but is limited in explaining the thermal adaptations. The adaptive approach of modelling thermal comfo...

Accurate prediction of thermal comfort is the core of thermal comfort and energy efficiency of buildings. The rational approach (e.g. PMV - Predicted Mean Vote) is used prevailingly for thermal comfort prediction, but could result in large errors because of its deficiency in explaining thermal adaptations. To improve the rational approach for therm...

The previous subzone control method of air distribution divides the occupied zone into subzones and controls the thermal conditions of the subzones to satisfy the respective thermal preferences. The previous method improves thermal comfort efficiently by considering the non-uniform thermal environment and differentiated thermal preferences of the o...

This paper proposed a novel dual-chilling-source refrigerating system using R407C applied in the temperature and humidity independent control system. The theoretical model was established to analyze the performance of this system regarding the coefficient of performance, exergy loss and exergy efficiency in comparisons with a single-chilling-source...

Inaccurate thermal comfort prediction would lead to thermal discomfort and energy wastage of overcooling/overheating. Predicted Mean Vote (PMV) is widely used for thermal comfort management in air-conditioned buildings. The metabolic rate is the most important input of the PMV. However, existing measurements of the metabolic rate are practically in...

To explore one reasonable ventilation system that can provide a comfortable and healthy indoor environment becomes urgent with the deterioration of outdoor environment. Objective and subjective tests were both employed to investigate the difference in indoor air quality, air pollutant control, thermal performance and thermal comfort. In the objecti...

Ventilation and air distribution methods are important for indoor thermal environments and air quality. Effective distribution of airflow for indoor built environments with the aim of simultaneously offsetting thermal and ventilation loads in an energy efficient manner has been the research focus in the past several decades. Based on airflow charac...

Accurate prediction of thermal comfort is the premise of proper management of the indoor environment to avoid overcooling/overheating discomfort with energy saving. Adaptive Predicted Mean Vote (aPMV) has been verified for the thermal comfort prediction in both free-running and air-conditioned buildings, and has been stipulated in the thermal comfo...

Outdoor microclimatic conditions strongly affect the thermal comfort of pedestrians. A transversal field survey was conducted in Guangzhou, together with micrometeorological measurements. The outdoor physiological equivalent temperature (PET) varied from 3 to 59 °C. Regression lines were obtained to establish correlations of the mean thermal sensat...

The non-uniform indoor thermal environments of stratum ventilation and displacement ventilation help to reduce the energy consumption, but challenges the cooling load estimation for the energy performance evaluation of air conditioning systems during the design and operation. Co-simulations coupling the building energy simulation and indoor thermal...

Macro-models provide a fast simulation tool for exposure assessments and design of control interventions, yet there are concerns about their accuracies. Single-zone based models are known to be inadequate for predicting exposure to near-source emissions, while the complexity of sequential box models (SBM) limits their application to steady-state co...

Air distribution (e.g., mixing ventilation) is widely used to provide indoor thermal comfort. The thermal deviation is defined as the deviation between the thermal condition and thermal preference, and a thermal deviation closer to zero indicates a more comfortable thermal environment. The thermal deviation is significantly affected by uncertaintie...

In this paper, a comparative experimental study on the performance of mixing ventilation and stratum ventilation for space heating was carried out. Objective and subjective experiments were both conducted to explore the difference of the thermal performance between the two types of ventilation. In the objective experiment, air temperature distribut...

The conventional method for air distribution (e.g., mixing ventilation and stratum ventilation) controls the averaged thermal condition in the occupied zone to satisfy the averaged thermal preference of a group of occupants. However, since the thermal environment cannot be absolutely uniform, the microclimates of occupants can be distinct from the...

When modelling the non-uniform thermal environment of the energy efficient air distributions (e.g., stratum ventilation), the wall temperature (i.e., the inner surface temperature of an exterior wall) is conventionally required as the input to reflect the weather condition. Recently, the exit air temperature has also been used to reflect the weathe...

Heat stress negatively influences human health and performance, leading to lower efficiency in daily activities. In particular, strenuous training in outdoor hot environment conditions increases the potential for developing heat illness, such as heat cramps, heat exhaustion, and heat stroke. Few investigations have been conducted on heat stress dur...

The conventional control method of a collective ventilation (e.g., stratum ventilation) controls the averaged thermal environment in the occupied zone to satisfy the averaged thermal preference of a group of occupants. However, the averaged thermal environment in the occupied zone is not the same as the microclimates of the occupants, because the t...

Comfortable and healthy outdoor microclimates are beneficial to sustainable urban development. Based on a comprehensive comparison of some currently frequently used thermal comfort indices, including PMV, WBGT, PET, SET*, and UTCI, the differences among these indices are significant in dealing with the fundamental energy balance model, descriptive...

The information (e.g., indoor air temperature and velocity distributions) of the non-uniform thermal environment is the base of the proper operation control of stratum ventilation for thermal comfort and energy efficiency. The current practice of modeling the non-uniform thermal environment requires inputs of the wall temperatures/heat fluxes, whic...

Lift-up buildings are advantageous in improving the wind circulation in a congested and compact city. However, the wind conditions in the void underneath a lift-up building, also known as the lift-up area, are vital for wind comfort of occupants of lift-up buildings. This study tested 28 lift-up buildings in a boundary layer wind tunnel to assess t...

Stratum ventilation (SV), a new indoor air distribution strategy, has been promoted for applications in different building premises in recent years. Compared to the conventional mixing ventilation (MV), the prominent advantage of SV is that indoor thermal comfort can be satisfied with a relatively high supply air temperature, hence less energy cons...

This study investigates adaptive thermal comfort in air conditioned classrooms in Hong Kong. A field survey was conducted in several typical classrooms at the City University of Hong Kong. This survey covered objective measurement of thermal environment parameters and subjective human thermal responses. A total of 982 student volunteers participate...

Due to vertically thermal stratification in indoor spaces, the cooling load calculation is always a challenge in the design of stratified air distribution systems (STRAD), which in turn is crucial to determine the supply air flow rate and the cooling load to be removed by the air conditioning system. In this paper, several cooling load calculation...

Outdoor activities are believed to provide citizens with various benefits, including stress reduction, livability improvement and energy conservation. Encouraging outdoor activity becomes an effective way to evocate positive emotions. Thus, numerous studies were established to enhance respondent outdoor thermal comfort by investigating landscape de...

Urban microclimatic conditions affect human body energy balance and individuals’ thermal perception. Therefore, it is necessary to find an appropriate index to evaluation of outdoor space. Predicted Mean Vote (PMV) always use for evaluation of outdoor thermal environments. However, PMV model was developed based on the indoor chamber experimental da...

An ejector cooling system with thermal pumping effect (ECSTPE) consumes no electricity but suffers severe thermal energy waste problems. This study innovatively reveals that different from the conventional ejector cooling where the designed coefficient of performance (COP) increases with an increasing set generating temperature , the designed COP o...

With the development of the urban city, increasing attention has been paid to outdoor thermal comfort. In this paper, an analysis of the sensitivities of different factors, including the personal factors and physical parameters of the thermal environment was conducted. The results showed that there was a strong linear relationship between the Physi...

Elevated room air temperature is normally accompanied by elevated room air velocity to provide thermal comfort and save energy. One problem is that an excessively high room air temperature would deteriorate the energy performance of the air conditioning system due to the increased energy consumption of the ventilation fans. Another problem is that...

A sustainable ventilation method is one of the possible solutions to mitigate climate change and carbon emission. This method shall involve an analysis of the environmental impact, energy performance, and economical cost-effectiveness. There are still few studies concerning the life cycle assessment (LCA) of various alternative ventilation systems...

This study investigates, using in-situ and numerical simulation experiments, airflow and hygrothermal distribution in a mechanically ventilated academic research facility with known cases of microbial proliferations. Microclimate parameters were obtained from in-situ experiments and used as boundary conditions and validation of the numerical experi...

The first decade of research into stratum ventilation is reviewed in the current manuscript. The background which midwifed the concept of stratum ventilation is introduced. Stratum ventilation is found to provide thermal comfort in warm conditions. Possible reasons behind this are explored. Because air is directly delivered to the breathing zone, t...

A growing number of people spend their leisure time and vacations in outdoor activities. Therefore, it is important to look for appropriate evaluation tools for outdoor thermal comfort. Most of the outdoor thermal comfort indices were developed by indoor thermal comfort model. So the analyses of thermal comfort model adaption in HongKong is very ne...

The laptops are popularly used by the students in classes. This study is to experimentally investigate the effect of the use of laptops on thermal comfort in a stratum-ventilated classroom with three-row seating. The distributions of air velocity and temperature in the occupied zone are measured. Twenty-four college-aged students are recruited to e...

The purpose of this study is to investigate the loads and lengths of the pull-down periods (the time used to achieve a comfortable thermal environment before a room is ready for occupation) with mixing ventilation, displacement ventilation and stratum ventilation. In a typical classroom in Hong Kong, experiments begin with the same initial hot ther...

Stratum ventilation, which could provide quality air in breathing zone and stratified thermal comfort, is a promising technology to meet the challenge of energy saving nowadays. This study is to find the influence of movements on airflow under stratum ventilation.

The paper describes an experimental study from forty-eight subjects about the acceptable thermal conditions under three ventilation strategies with six exhaust configurations, including mixing ventilation (MV), displacement ventilation (DV), and four exhaust types of stratum ventilation (ceiling-level-exhausted "SV", rear-low-level-exhausted "SV-1"...

Residents wish to have outdoor spaces to enjoy walking, cycling, and other recreational activities, which are often hindered by the unfavorable thermal comfort conditions, especially in the summer. High building densities lower the average wind speed and this intensifies the urban heat island effects at city scale. The conscientious use of building...

The amount of energy consumed by air conditioning of buildings is large and ever-increasing. Guidelines for elevated room temperature settings in summer have been issued in several East Asian Judiciaries, especially in Hong Kong (25.5°C) and the Chinese Mainland (26°C). These new temperatures are significantly higher than the previous values. The f...

As a new energy-saving air supply mode, stratum ventilation system has not been widely applied due to inadequate theoretical support. A mathematical model based on the characteristics of the indoor air flow was established to predict the vertical temperature profile in a stratum ventilated room. By combining the characteristics of indoor air flow w...

Room occupants' comfort and health are affected by the airflow. Nevertheless, they themselves also play an important role in indoor air distribution. This study investigated the interaction between the human body and room airflow under stratum ventilation. Simplified thermal manikin was employed to effectively resemble the human body as a flow obst...

Air-source heat pump (ASHP) often operates with substantial frost formation on the outdoor heat exchanger at low ambient temperature in winter, it insulates the finned surface and also reduces heat transfer rate, leading to performance degradation or even shutdown of ASHP systems. Although several defrosting methods have been reported, the frosting...

Unlabelled: The motivation of this study is stimulated by a lack of knowledge about the difference of airflow characteristics between a novel air distribution method [i.e., stratum ventilation (SV)] and conventional air distribution methods [i.e., mixing ventilation (MV) and displacement ventilation (DV)]. Detailed air velocity and temperature mea...

Distributed pumping system demonstrates advantages in energy saving and hydraulic stability. This paper analyzes the mechanism of the distributed pumping system for energy saving and studies the characteristics of energy consumption under the design condition, as well as the flow regulation operations in the conventional pumping system and in the d...

Air-source heat pumps (ASHPs) are being widely used in residential and commercial buildings and have a huge potential market and development foreground for its energy-savings, high efficiency and environmental friendliness. However, at low temperatures, frost can accumulate on the surface of the finned outdoor coil which will decrease the heating c...

This study uses a computational fluid dynamics (CFD) program to compare indoor air quality and local thermal comfort level in a full-size classroom under three ventilation methods: mixing ventilation, displacement ventilation and stratum ventilation. Since fresh air can be efficiently distributed to the breathing zone with stratum ventilation and d...

Stratum ventilation has been recently proposed to accommodate the elevated room temperatures recommended by several governments in East Asia. This study evaluates the performance of stratum ventilation with slot diffusers in a classroom using computational fluid dynamics (CFD), which is validated by the experimental data. Slot diffusers with substa...

Three human test series were conducted to evaluate the uniformity of the thermal environments in a stratum ventilated chamber with dimensions of 8.8 m (L) × 5.1 m (W) × 2.4 m (H). In all, nineteen conditions were generated by adjusting the room temperature, supply airflow rate and supply terminal type. An air diffuser performance index (ADPI) of at...

Fabric air terminal made of polymer was found to create a clean, health and comfort indoor environment. This paper describes a numerical investigation of fabric diffuser applied in a classroom with stratum ventilation. The computational fluid dynamics (CFD) simulation was validated by experimental data. Performance of the fabric diffuser was evalua...

Stratum ventilation has been recently proposed to accommodate the elevated room temperatures recommended by several governments in East Asia. This study evaluates the performance of stratum ventilation with slot diffusers in a classroom using computational fluid dynamics (CFD), which is validated by the experimental data. Slot diffusers with substa...