An energy impact assessment of ventilation for indoor airborne bacteria exposure risk in air-conditioned offices

Treatment of fresh air in ventilation systems for air-conditioned offices consumes a significant amount of energy and affects the indoor air quality (IAQ). In this study, energy impact on the ventilation systems was examined against certain IAQ objectives for indoor airborne bacteria exposure risk in air-conditioned offices of Hong Kong. The relationship between thermal energy consumptions and indoor airborne bacteria exposure levels based on regional surveys was investigated. The thermal energy consumptions of ventilation systems operating for carbon dioxide (CO₂) exposure concentrations between 800 and 1200 ppmv for typical office buildings and the corresponding failure probability against some target bacteria exposure levels were evaluated. The results showed that, for a reference indoor environment at a CO₂ exposure concentration of 1000 ppmv, the predicted average thermal energy saving of ventilation system for a unit increment of the expected risk of unsatisfactory IAQ of 1% was 55 MJ m⁻² yr⁻¹ and for a unit decrement of 1%, the predicted additional thermal energy consumption was 58 MJ m⁻² yr⁻¹ respectively. This study would be a useful source of reference in evaluation of the energy performance of ventilation strategies in air-conditioned offices at a quantified exposure risk of airborne bacteria.     

Assessment of airborne bacteria and fungi in food courts

A study was undertaken to determine the effect of variations in temperature, relative humidity, occupancy density and location (indoor/outdoor) on the concentrations of viable airborne bacterial and fungal spores at an air-conditioned and a non air-conditioned food stall in Singapore. Typically, bioaerosols consisted of 50.5% bacteria and 49.5% fungi in the indoor environment. In contrast, for the outdoor environment, bacteria on an average only accounted for 20.6% of culturable airborne microorganisms whereas fungal concentrations were 79.4%. Results on bioaerosol size distributions revealed that 67% of indoor bacteria and 68% of outdoor bacteria, 85% of indoor fungi and 68% of outdoor fungi were associated with fine mode particulates (<3.3 μm). Occupant density was the key factor that affected indoor airborne bacteria concentrations while concentrations of outdoor airborne bacteria depended strongly on ambient temperature. Indoor fungal concentration was positively correlated to relative humidity whereas outdoor fungal concentration was positively correlated to relative humidity and negatively correlated to temperature. The study also compared the biological air quality between a non air-conditioned food stall (Stall A) and an air-conditioned food stall (Stall B). The dining area of the former had lower bacterial concentrations as compared to the latter, while fungal spore’s concentrations showed a reverse trend. The dominant airborne bacteria genera were Staphylococcus, Pseudomonas, Alcaligens, and Corynebacterium whereas Penicillium, Aspergillus and Cladosporium were the most common fungal genera and groups in both food stalls.     

Energy impact assessment for the reduction of carbon dioxide and formaldehyde exposure risk in air-conditioned offices

Treatment of fresh air in ventilation systems for air-conditioning consumes a considerable amount of energy and affects the indoor air quality (IAQ). In this study, energy impact on ventilation systems was examined against certain IAQ objectives for indoor formaldehyde exposure risk in air-conditioned offices of Hong Kong. Thermal energy consumptions for ventilation systems and indoor formaldehyde exposure concentrations based on some regional surveys of typical offices in Hong Kong were reviewed. The thermal energy consumptions of ventilation systems operating for CO₂ exposure concentrations between 800 ppmv and 1200 ppmv for typical office buildings and the corresponding formaldehyde exposure risks were evaluated. The results showed that, for a reference indoor environment at a CO₂ exposure concentration of 1000 ppmv, the average thermal energy saving of ventilation system for a unit increment of the acceptable formaldehyde exposure limit of 1 h (loss of life expectancy of 0.0417 day) was 280 MJ m⁻² yr⁻¹; and for a unit decrement of the exposure limit of 1 h, an additional average thermal energy consumption of 480 MJ m⁻² yr⁻¹ was expected. This study would be a useful source of reference in evaluation of the energy performance of ventilation strategies in air-conditioned offices at a quantified exposure risk of formaldehyde.     

Neutral temperature in subtropical climates—A field survey in air-conditioned offices

The thermal environment for air-conditioned offices in subtropical climates is examined from the prospect of maintaining an optimum operative temperature for the occupants. In this study, the optimum neutral temperature is evaluated from 422 occupants’ responses towards the perceiving thermal environment in 61 air-conditioned offices and 186 complaints of thermal discomfort in an air-conditioned office building on an electronic questionnaire, using a semantic differential evaluation scale and a dichotomous assessment scale. In particular, physical parameters for the thermal comfort study were measured by an indoor environmental quality (IEQ) logger, and the operative temperature was correlated with the occupants’ thermal responses. The probability of accepting an operative temperature for the thermal comfort of the occupants was correlated with logistic regression curves; the optimum operative temperature was derived in order to maximize the probability of thermal comfort expressed by the occupants. The results showed that the thermal neutral temperatures for air-conditioned offices in subtropical climates were 23.6 and 21.4 °C in summer and winter, respectively. The preferred thermal environment in Hong Kong should be slightly cool, corresponding to about 1 °C below the neutral temperature, in order to satisfy most of the occupants in the office space.     

Changing microbial concentrations are associated with ventilation performance in Taiwan's air-conditioned office buildings

Our study conducted serial environmental measurements in 12 large office buildings with two different ventilation designs to obtain airborne microbial concentrations in typical office buildings, and to examine the effects of occupant density, ventilation type and air exchange efficiency on indoor microbial concentrations. Duplicate samples of airborne fungi and bacteria, a total of 2477 measurements, were collected based on a scheme of conducting sampling three times a day for at least seven consecutive days at every study building. Air change rates (ACHs) were also estimated by tracer gas concentration decay method, and measured by continuous Multi-Gas monitor for each building. Most sampling sites were with total fungal and bacteria concentrations higher than 1000 CFU/m(3), an often-quoted guideline in earlier research. Significantly higher concentrations of fungi and bacteria, as well as higher indoor/outdoor (I/O) ratios across most groups of airborne microbes, were identified in buildings with fan coil unit (FCU) system than those with air-handling unit (AHU) system (Student's t test, P < 0.0001). Older buildings and higher air exchange rates were statistically associated with greater indoor bacteria levels in FCU ventilated buildings (R(2) = 0.452); a pattern not found in AHU buildings. Increasing ACH seemed to be the determinant factor for rising indoor fungal and Cladosporium concentrations in those FCU buildings (R(2) = 0.346; 0.518). Our data indicated that FCU ventilated buildings might have provided more outdoor matters into indoor environments through direct penetration of outdoor air. Results also demonstrated a quantitative association between rising numbers of occupants and increasing indoor levels of yeast in both FCU and AHU ventilated buildings. The regression model identified in this study may be considered a reference value for proposing an optimal ACH, while with adequate filtration of fresh air, as an effective strategy in lowering indoor microbial concentrations in air-conditioned buildings. PRACTICAL IMPLICATIONS: As control of indoor microbial contamination has become an increasing concern around the world, feasibility and effectiveness of adopting ventilation approach has attracted a significant interest. This field investigation demonstrated, quantitatively, critical variables to be taken into consideration while applying such a measure, including the kinds of microbes to be removed and the types of ventilation system already in place.     

A multivariate-logistic model for acceptance of indoor environmental quality (IEQ) in offices

The indoor environmental quality (IEQ) in offices is examined from the prospect of an occupant's acceptance in four aspects: thermal comfort, indoor air quality, noise level and illumination level. Based on the evaluations made by 293 occupants of the IEQ of offices in Hong Kong, empirical expressions have been proposed to approximate an overall IEQ acceptance of an office environment at certain operative temperature (T_o), carbon dioxide concentration (CO₂), equivalent noise level (L_eq) and illumination level (lux). The overall IEQ acceptance is calculated from a multivariate logistic regression model. A range of acceptance in typical office environmental conditions and its dependence on the four parameters stated above are determined for design conditions. The proposed overall IEQ acceptance can be used as a quantitative assessment criterion for an office environment and similar environment where an occupant's evaluation is expected.     

The effect of heat-pipe air-handling coil on energy consumption in central air-conditioning system

A study was carried out to investigate the effect of heat-pipe air-handling coil on energy consumption in a central air-conditioning system with return air. Taking an office building as an example, the study shows that compared with conventional central air-conditioning system with return air, the heat-pipe air-conditioning system can save cooling and reheating energy. In the usual range of 22–26 °C indoor design temperature and 50% relative humidity, the RES (rate of energy saving) in this office building investigated is 23.5–25.7% for cooling load and 38.1–40.9% for total energy consumption. The RES of the heat-pipe air-conditioning system increases with the increase of indoor design temperature and the decrease of indoor relative humidity. The influence of indoor relative humidity on RES is much greater than the influence of the indoor design temperature. The study indicates that a central air-conditioning system can significantly reduce its energy consumption and improve both the indoor thermal comfort and air quality when a heat-pipe air-handling coil is employed in the air-conditioning process.     

Indoor air quality investigation at air-conditioned and non-air-conditioned markets in Hong Kong

To characterize indoor air quality at the markets in Hong Kong, three non-air-conditioned and two air-conditioned markets were selected for this study. The indoor air pollutants measured included PM(10) (particulate matters with aerodynamic diameter less than 10 microm), total bacteria count (TBC), carbon monoxide (CO), nitric oxide (NO), nitrogen dioxide (NO(2)) and sulfur dioxide (SO(2)). The indoor and outdoor concentrations of these target air pollutants at these markets were measured and compared. The effects of air conditioning, temperature/relative humidity variation and different stalls on the indoor air quality were also investigated. The results indicated that all of the average indoor concentrations of PM(10), TBC, CO and NO(2) at the markets were below the Hong Kong Indoor Air Quality Objectives (HKIAQO) standards with a few exceptions for PM(10) and TBC. The elevated PM(10) concentrations at Hung Hom, Ngau Tau Kok and Wan Chai markets were probably due to the air filtration of outdoor airborne particulates emitted from vehicular exhaust, whereas high concentrations of airborne bacteria at Sai Ying Pun and Tin Shing markets were linked to the use of air conditioning. Correlation analysis demonstrated that indoor bacteria concentrations were correlated with temperature and relative humidity. The operation of air conditioning did not significantly reduce the levels of air pollutants at the markets. However, the higher indoor/outdoor ratios demonstrated that the operation of air conditioning had influence on the levels of bacteria at the markets. It was found that average PM(10) concentration at poultry stalls was higher than the HKIAQO standard of 180 microg/m(3), and was over two times that measured at vegetable, fish and meat stalls. Furthermore, the concentration of airborne bacteria at the poultry stalls was as high as 1031 CFU/m(3), which was above the HKIAQO standard of 1000 CFU/m(3). The bacteria levels at other three stalls were all below the HKIAQO standard. Statistical analysis indicated that there were no significant differences among the four stalls for CO, NO(x) and SO(2).     

冬季空调不加湿对室内参数的影响

基于质量守恒方程导出了冬季空调系统不加湿时房间内的含湿量计算式,综合考虑影响室内含湿量的主要因素,得出冬季空调可以不加湿时应满足的条件,考察了中国典型气候区代表性空调房间冬季空调不加湿时室内相对湿度能否满足设计要求。研究表明:中国各地区的商场,夏热冬冷地区的餐厅及三星级及以下酒店多功能厅,夏热冬暖地区的影剧院、办公室、酒店客房及多功能厅、餐厅,温和地区的办公室、三星级及以下酒店客房、酒店多功能厅及餐厅空调系统不加湿即可达到设计湿度要求;严寒地区及寒冷地区的影剧院、办公室、酒店客房、酒店多功能厅及餐厅,夏热冬冷地区的影剧院、办公室、酒店客房、四星级及以上酒店多功能厅以及温和地区的影剧院、四星级及以上酒店客房等房间的空调系统必须加湿才能达到设计要求。     

Quantifying occupant comfort: are combined indices of the indoor environment practicable?

What are the various ways in which evaluation of the several aspects of the indoor environment might combine to form an occupant's overall assessment of that environment? Data from an environmental survey of 26 offices in Europe (the Smart Controls and Thermal Comfort, or SCATs, project) are used. These show that dissatisfaction with one or more aspects of the indoor environment does not necessarily produce dissatisfaction with the environment overall. Conversely, satisfaction with one or more environmental aspect does not necessarily produce satisfaction with the total environment. Building occupants balance the good features against the bad to reach their overall assessment. Not all aspects are equally important in this subjective averaging process. Satisfaction with warmth and air quality is more important than satisfaction with the level of lighting or humidity. The relative importance of the various aspects differed from country to country, making it impossible to develop an internationally valid index to rate office environments by means of a single number. The best linear index constructed from the data failed to rank the indoor environments of the buildings in the correct order, as defined by the occupants' overall assessments. It is therefore wise to assess each of the several aspects separately rather than rely only on a combined index.     

Field study of human thermal comfort and thermal adaptability during the summer and winter in Beijing

This study was conducted during the summer and winter in Beijing. Classrooms and offices in a university were used to conduct the survey. The respondents’ thermal sensation and thermal adaptability in both seasons were analyzed. During the study, indoor environmental parameters including air temperature, mean radiant temperature, relative humidity, and air velocity were measured. The respondents’ thermal sensation was determined by questionnaire.A relationship between indoor temperature and thermal sensation was found. In the summer study, the “scissors difference” between TSV and PMV was observed in the air-conditioned environments if the temperature was out of the neutral zone. People had higher tolerance in the hot environment than PMV predicted. During winter, the outdoor temperature had a prominent influence on thermal adaptability. The low outdoor temperature made people adapt to the cold environment. When the indoor temperature was heated to a high temperature by space heating facilities, respondents felt uncomfortable since their adaptability to the cold environment was nullified.Furthermore, the differences in thermal responses between respondents from North and South China showed that the different climates of people's native regions also affected their thermal comfort and adaptability.     

Energy performance criteria in the Hong Kong building environmental assessment method

The Hong Kong Building Environmental Assessment Method (HK-BEAM), an environmental assessment method for air-conditioned office buildings, has been developed by the Real Estate Developers Association (REDA) for use in Hong Kong. Two versions of the scheme have been developed; one for new office premises and the other for existing office premises. HK-BEAM embraces a variety of issues related to the global, local and indoor environments. The efficient use of electricity forms a major part of the assessment. The basis for the energy performance assessment method, and the criteria for obtaining the relevant credits are described and discussed.     

Modeling the effect of hygroscopic curtains on relative humidity for spaces air conditioned by DX split air conditioning system

The use of hygroscopic materials for moisture buffering is a passive way to moderate the variation of indoor humidity. Through absorption and desorption, surface materials in the indoor environment, such as curtains, carpets and wall paper, are able to dampen the moisture variations. The moisture buffering capacity of these materials may be used to improve the relative humidity of the indoor environment at reduced energy costs.The objectives of this paper are threefold. The first objective is to derive a theoretical model for the transient moisture transfer between a curtain system and the indoor air for the case where the curtain is placed in front of a wall. The second objective is to conduct experiments inside environmental chambers to validate the theoretical model and to test the ability of curtains to moderate indoor humidity. It is shown that the experimental results for the curtain moisture uptake and the relative humidity inside the chamber compared well with the model simulation results. The third and final objective is to test and evaluate the model under “real environment conditions” for a case study of a hygroscopic cotton curtain, placed in a “typical” office space in the city of Beirut with an area of 25 m² that uses direct expansion (DX) air conditioning system. It is found that hygroscopic curtains maintain humidity of less than 65% during part load operation compared to the upper limit of 70% relative humidity when no curtain is used. On the other hand, it is found that the energy use, as determined by the daily electrical power consumption of the DX system, is almost the same for the two cases, (with and without a curtain), where approximately 20 kWh of energy input is required 13 kWh of sensible energy and 7 kWh of latent energy.     

Effects of a Furniture-integrated Breathing-zone Filtration System on Indoor Air Quality,Sick Building Syndrome,and Productivity

A field experiment evaluated the effect of a furniture-inte-grated breathing-zone filtration (BZF) system on indoor air quality, worker comfort, health, and productivity. The BZF system tested filters office air to remove volatile organic compounds and airborne particulates. The BZF system was installed on one floor of a 29 story air-conditioned office building. Another floor of the building served as a control. Comparisons of pre-installation and three month post-instal-lation surveys showed improvements in indoor air quality, sick building syndrome symptoms, and self-reported pro-ductivity with the BZF system References     

对热舒适、空气感觉质量及能耗的模拟研究

室内空调设计温度和新风量对热舒适，室内空气质量及能耗量有重要影响，然而对它们之间相互关系进行研究的文献却较少。通过计算机模拟空调系统在7种室内设计温度和7种新风量条件下的运行情况，得到不同的设计条件组合对热舒适、人体感觉空气质量及建筑能耗量的影响。基于这项分析，提出了此办公建筑合理的室内设计温度和新风量取值。     

Indoor air quality and energy performance of air-conditioned office buildings in Singapore

An integrated indoor air quality (IAQ)-energy audit methodology has been developed in this study in Singapore, which provides a rigorous and systematic method of obtaining the status-quo assessment of an 'IAQ signature' in a building. The methodology entails a multi-disciplinary model in obtaining measured data pertaining to different dimensions within the built environment such as the physical, chemical, biological, ventilation, and occupant response characteristics. This paper describes the audit methodology and presents the findings from five air-conditioned office buildings in Singapore. The research has also led to the development of an indoor pollutant standard index (IPSI), which is discussed in this paper. Other performance indicators such as, the ventilation index and the energy index as well as the building symptom index (BSI) are also presented and discussed in the context of an integrated approach to IAQ and energy. Several correlation attempts were made on the various symptoms, indoor air acceptability, thermal comfort, BSI and IPSI, and while BSI values are found to correlate among them as well as with IAQ and THERMAL COMFORT acceptability, no such correlation was observed between BSI and IPSI. This would suggest that the occupants' perception of symptoms experienced as well as environmental acceptability is quite distinct from IAQ acceptability determined from empirical measurements of indoor pollutants, which reinforces the complex nature of IAQ issues.     

Evaluation of lighting performance in office buildings with daylighting controls

Lighting control integrated with daylighting is recognised as an important and useful strategy in energy-efficient building designs and operations. It is believed that proper daylighting schemes can help reduce the electrical demand and contribute to achieving environmentally sustainable building development. This paper presents field measurements on daylighting for a fully air-conditioned office building in Hong Kong. Electricity consumption by the fluorescent luminaires, indoor illuminance levels and the room parameters affecting daylighting designs were recorded and analysed. The measurements covered several cellular offices facing opposite orientations with and without daylighting controls. The findings suggest that daylighting schemes can result in substantial energy savings in air-conditioned office buildings in Hong Kong. Results are presented and the design implications are discussed.     

Ventilation of air-conditioned residential buildings: A case study in Hong Kong

More and more studies reported that there were insufficient ventilation and excessive CO_2 concentration in air-conditioned residential buildings, but few solutions were provided. This study investigates the overnight evolution of CO_2 concentration in air-conditioned residential buildings and then focuses mainly on the evaluation of three ventilation strategies, including overnight natural ventilation, short-term mechanical ventilation and short-term natural ventilation. On-site measurements were conducted in a typical residential bedroom in Hong Kong in September. The indoor and outdoor CO_2 concentration, air temperature and relative humidity as well as the outdoor wind speed during the measurements were analysed. Ventilation rates were calculated based on the time series of CO_2 concentration. This study confirms that additional ventilation is usually needed in air-conditioned residential buildings. Overnight natural ventilation with even a small opening is associated with excessive energy consumption and deteriorated indoor thermal environment. Short-term natural ventilation strategies are inefficient and uncontrollable. Compared to the best short-term natural ventilation strategy, a reasonably designed short-term mechanical ventilation strategy requires only a 41% of ventilation period to complete one full replacement of indoor air and to reach a lower indoor CO_2 concentration. Nighttime case studies and a theoretical analysis suggest that a few several-minute mechanical ventilation periods could potentially maintain an acceptable indoor air quality for a normal sleeping period of 8 h.     

A comparative study of different control strategies for indoor air humidity

There has been a rising concern in controlling the high indoor humidity of hot and humid countries. When an air-conditioned space experiences only part of its design heat load, its humidity tends to rise as a result of the air-conditioning system trying to control the indoor temperature by reducing its cooling capacity. In this study, the part-load dehumidification performances of three temperature control strategies are compared, namely, chilled water flow control, bypass air control and the variable air volume control. Coil simulations are employed to study the part-load performance of these control strategies. The coil model has been validated with experimental data to within ±6.5%. The relative humidity of a space depends of factors such as design sensible heat factor of the space, temperature control strategy employed and load condition experienced by the space. Simulation results have indicated that chilled water control strategy results in the highest indoor humidity throughout the range of conditions studied while variable air volume system provides highly effective dehumidification performance of the cooling coil. Bypass air control appears to be a good option for adoption as it is able to provide an acceptable humidity over a wide range of load conditions without having to affect the air movement within the space.     

Optimization of indoor environmental quality and ventilation load in office space by multilevel coupling of building energy simulation and computational fluid dynamics

The fundamentals, implementation, and application of an integrated simulation as an approach for predicting the indoor environmental quality for an open-type office and for quantifying energy saving potential under optimized ventilation are presented in this paper. An integrated simulation procedure based on a building energy simulation and computational fluid dynamics, incorporated with a conceptual model of a CO₂ demand controlled ventilation (DCV) system and proportional integral control of an air conditioning system as the optimization assessment of conceptual model in the occupied zone, was developed. This numerical model quantitatively exhibits energy conservation and represents the non-uniform distribution patterns of airflow properties and CO₂ concentration levels in terms of energy recovery and indoor thermal comfort. By means of an integrated simulation, the long-term energy consumption of heating, ventilation, and air conditioning systems are predicted precisely and dynamically. Relative to a ventilation system with a basic constant air volume supply rate characterized by a fixed outdoor air intake rate from the ceiling supply opening, the optimized CO₂-DCV system coupled with energy recovery ventilators reduced total energy consumption by 29.1% (in summer conditions) and 40.9% (winter).