An energy impact assessment of indoor air quality acceptance for air-conditioned offices

An energy impact assessment of indoor air quality acceptance for air-conditioned offices

Energy Conversion and Management 49 (2008) 2815–2819 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: ww...

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Energy Conversion and Management 49 (2008) 2815–2819

Contents lists available at ScienceDirect

Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

An energy impact assessment of indoor air quality acceptance for air-conditioned offices L.T. Wong 1, K.W. Mui *, K.L. Shi, P.S. Hui Department of Building Services Engineering, The Hong Kong Polytechnic University, Hong Kong, China

a r t i c l e

i n f o

Article history: Received 18 December 2006 Received in revised form 20 August 2007 Accepted 24 March 2008 Available online 7 May 2008 Keywords: Ventilation Energy saving System optimization Occupant load Occupant satisfactory

a b s t r a c t Treatment of fresh air in ventilation systems for the air-conditioning consumes a considerable amount of energy and affects the indoor air quality (IAQ). The ventilation demand is primarily related to the occupant load. In this study, the ventilation demands due to occupant load variations and occupant acceptability were examined against certain IAQ objectives using the mass balance of carbon dioxide (CO2) concentrations in an air-conditioned office. In particular, this study proposed a ventilation model for the consideration of the occupant load variations and occupant acceptability based on the regional survey of typical offices (422 samples) in Hong Kong. The model was applied to evaluate the relative energy performance of different IAQ objectives in ventilation systems for typical office buildings in Hong Kong. The results showed that the energy consumption of a ventilation system would be correlated with the occupant load and acceptability in the air-conditioned office. Indicative CO2 levels of 800 ppmv, 1000 ppmv and 1200 ppmv corresponding to 83%, 97% and 99.7% survey samples were shown, corresponding to the thermal energy consumptions of 1500 MJ m2 yr1, 960 MJ m2 yr1and 670 MJ m2 yr1, respectively. In regards to the monetary issue, an annual value of HK$ 762 million per year in electrical consumption could be saved in all office buildings in Hong Kong when the indoor target CO2 concentration is increased from 1000 ppmv to 1200 ppmv. To achieve an excellent IAQ following the existing design standard, i.e. to decrease the CO2 level from 1000 ppmv to 800 ppmv, 56% additional energy would be consumed, corresponding to an annual value of HK$ 1,419 million, even though the occupant acceptability is only improved from 81% to 86%. The development of the models in this study would be useful for the energy performance evaluation of ventilation systems in air-conditioned offices. Crown Copyright Ó 2008 Published by Elsevier Ltd. All rights reserved.

1. Introduction Hong Kong is one of the most densely populated cities in the world. According to the census in 2001, the population was 6.71 million with an annual growth rate of 0.9% and the usable land was 1098.5 km2 [1]. Due to the high population and limited supply of land, most of the buildings in Hong Kong are high-rises in order to cope with the rapid development of the society. As a metropolitan city located in the subtropical region, almost all of Hong Kong’s office buildings are air-conditioned. Based on the record of Hong Kong energy statistics, the energy consumption of electricity has a multiple increase of nearly nine times during the past 33year period from 1970 to 2004 [2]. Specifically, air-conditioning consumes over 50% of the amount of electricity used in the commercial sector [3]. Therefore, different types of energy saving pro* Corresponding author. Tel.: +852 27665835; fax: +852 27667198. E-mail addresses: [email protected] (L.T. Wong), [email protected] (K.W. Mui). 1 Tel.: +852 27667783; fax: +852 27667198.

tocols should be evaluated and applied to enhance the operating efficiency of air-conditioning components. The effort, however, has often resulted in energy saving which ignores the fundamental delivery of indoor satisfaction. Many engineering systems in buildings have their performances closely associated with outdoor conditions and occupant loads. Apart from the indoor design parameters, occupant load is one of the essential parameters for building designs. It relates to considerations such as structural loads [4], indoor air quality (IAQ) and the associated energy consumption [5], cooling load [6], sewage demand [7], water consumptions [8] and evacuation [9]. Occupant load surveys in various types of buildings are conducted from time to time in different countries in order to update the design practice. Design values or profiles of the occupant load in indoor spaces have been recommended in some design guides and codes of practice [10– 14]. Courtney and Houghton did a study on the occupant load in typical office buildings in USA in 1935 [10]. An average maximum occupant load factor Of (m2 person1 or denoted as m2 ps1) ranging from 6 to 15 m2 ps1 (the average was 8 m2 ps1) was recommended for evacuation designs.

0196-8904/$ - see front matter Crown Copyright Ó 2008 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2008.03.015

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Nomenclature Af Av Cpa E Ec GM GSD hfg N( ) Np Oa f p q R SD t V_ e V_ g Vf

floor area (m2) annual electricity cost of thermal energy consumption (HK$ yr1) specific heat capacity of air (kJ kg1 K1) annual energy consumption (kJ yr1) normalized annual energy consumption (MJ m2 yr1) geometric mean geometric standard deviation latent heat of water evaporation (kJ kg1) normal distribution total number of occupants (ps) occupancy factor (ps1 m2) unsatisfactory rate p-value cooling load (kW) correlation coefficient standard deviation operating time (s) ventilation rate (m3 s1) CO2 generation rate (m3 s1) space volume (m3)

Cooling of the outdoor air consumes a large portion of energy in an air-conditioning system and affects the IAQ, and the required fresh air quantity is related to the occupant load [5,6]. It is found that many existing building services engineering system designs were biased in determining the system maximum capacity [11,13,14]. Indeed, performance of those systems was strongly related to the time-variant parameter which was not considered properly at the design stage. At the design stage of a typical office building in Hong Kong, the actual occupant load profile is not disclosed to the building designers. Some engineers would assume a probable maximum occupant load profile for calculating the ventilation loads and corresponding energy consumption. For ventilation system that supplies large quantity of outdoor air to an air-conditioned space, significant amount of energy could be wasted if the fluctuation of the building occupant load is disregarded [15]. In a typical air-conditioned indoor environment, carbon dioxide (CO2) concentration significantly contributes to indoor air pollutants and thus could be selected as a surrogate indicator for assessing the IAQ [16–18]. In typical Hong Kong offices, the occupancy levels are almost ‘steady’ within working hours except for a significant drop during lunch-time. As CO2 concentration varies closely with the building occupant load, the assumption of ‘building up only’ for CO2 concentration in a space is deficient. In fact, the effectiveness of ventilation rate, and the initial state, build-up, fluctuation and decay of indoor CO2 concentration contribute to the uncertainties of the measurement quantities and should be quantified in all measurements. Monitoring of CO2 concentrations allows us to estimate the occupants’ comfort in a space and how well the installed ventilation system is operating. A number of relationships were suggested between indoor CO2 concentration and IAQ, such as the health effects of elevated CO2 concentrations, the impact of CO2 concentration on the occupants’ perceptions of an indoor environment, the relationship between CO2 concentrations and other contaminants, and the association between CO2 and outdoor air ventilation rate, respectively [19,20]. Oversupply of the fresh air supply flow rate is not the best solution to dilute the pollutant level [21]. There is a relationship between the indoor and outdoor pollutant level. Different indoor and outdoor ratios have special relevance to the control of indoor pollutant. If the indoor/outdoor ratio is below one, the ventilation

/ U U(t) U0T UT e q DTol Dgol

IAQ acceptance CO2 concentrations (ppmv) transient CO2 concentration (ppmv) target CO2 concentration (ppmv) existing CO2 concentration (ppmv) error term air density (kg m3) air temperature difference (°C) moisture content difference (kg kg1)

Superscript  of distribution function of sample value ˆ Subscript 0 e i oa max

of of of of of

initial exposure limit a pollutant outdoor air maximum

system will increase the indoor concentration of those particular pollutants. It was reported that a low CO2 concentration (i.e. a ‘high’ ventilation rate) would result in a high concentration of outdoor pollutants in some air-conditioned offices, e.g. ozone, nitrogen dioxide, etc. [22]. For some engineering designs, the design indoor CO2 concentration of 1000 ppm is one of the references which based on 350 ppm outdoor concentration. The difference between indoors and outdoors is 650 ppm with 20% of occupants’ unsatisfactory rate [16]. Some design guides suggest a lower design CO2 concentration of 800 ppm for an ‘excellent’ IAQ target [23]. In Hong Kong, the outdoor CO2 concentration is about 400 ppm, so the dilution margin for CO2 concentration is 600 ppm. Therefore, the maximum indoor CO2 concentration requirement may change to 1050 ppm. Adopting this approach, the fresh airflow rate can be further adjusted and the capacity of energy saving is increased without any changes of IAQ acceptance. Therefore, the dilution margin approach should be considered to optimize the design of the fresh air system. In this study, the impact of IAQ specified by some design CO2 levels and the probable IAQ acceptance was studied in terms of energy consumption for the ventilation system of air-conditioned offices in Hong Kong. Mathematical expressions among the indoor CO2 concentration, the IAQ acceptance and the energy consumption for IAQ acceptance in air-conditioning system were evaluated. In particular, indicative design CO2 levels corresponding to regional survey samples and current design guides were shown with the corresponding thermal energy consumptions in certain confidence levels. It helps to sustain the levels of occupant satisfaction with optimum energy consumption, and provides a rational basis of policy making with a balance of IAQ and energy consumption. These tools can let the building manager, control engineer and the user to quantify the ventilation system, to evaluate the energy reduction potential and to investigate the effect of ventilation control strategies to the indoor environment.

2. Energy consumption for IAQ acceptance For a typical office environment being perceived by an occupant, measurable parameters such as temperature [24], relative humidity, light level [25], noise level [26] and indoor CO2

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In a mechanical ventilated office, the steady state CO2 concentration was adopted in designing the ventilation system [16]. The governing parameters in determining the energy consumption are the ventilation rate V_ e (m3 s1) for maintaining certain CO2 concentration, the temperature difference DTol (°C), and the moisture content difference Dgol (kg kg1) between air entering and leaving the cooling coil. The annual energy consumption E (kJ yr1) is determined by integrating the cooling load q (kW) at the cooling coil of an air-handling unit with an annual total operating time t (s), the air density q (kg m3), the specific heat capacity of air Cpa (kJ kg1 K1) and the latent heat of water evaporation hfg (kJ kg1), where Z Z ð2Þ E¼ qðtÞdt  qV_ e ðC pa DT ol þ hfg Dg ol Þdt To calculate the energy consumption of fresh air at the cooling coil of a ventilation system as a function of the corresponding indoor air temperature (for thermal comfort) and indoor CO2 concentration (for IAQ), the above psychrometric equation was used for annual thermal energy evaluation with the following assumptions for simplicity [3,16,28,29] (1) leaving coil condition was at 95% saturation at the constant design indoor air temperature (°C); (2) ‘‘occupied hours of bin temperature of Hong Kong” was used in determining the outdoor air condition throughout a year, where the bin hours were the length of the time that the air temperature was between a given temperature range. The bin temperature used in this calculation was the midpoint of the given temperature range in Hong Kong; (3) constant specific heat capacity of air, latent heat of water evaporation, and air density were taken (Cpa = 1.023 kJ kg1 K1, hfg = 2454 kJ kg1 and q = 1.2 kg m3); (4) fan heat gain for the air-conditioning system was negligible. The transient CO2 concentration U(t) at anytime t (s) can be evaluated by Eq. (3) below, taking a mass balance in the space, at an initial CO2 concentration U(t = 0) = U0 (ppmv) and an outdoor CO2 concentration Uoa (ppmv) [16,30], where Vf (m3) is the space volume with a ventilation rate V_ e (m3 s1) and V_ g (m3 s1) is the CO2 generation rate due to occupants, ! V_ g V_ g _ _ UðtÞ ¼ Uoa þ ð1  eV e t=V f Þ þ U0 eV e t=V f ; Uðt ! 1Þ ! Uoa þ _V e V_ e ð3Þ The CO2 generation rate for a typical office occupant was taken at 0.00005 m3 s1 such that the total generation rate V_ g (m3 s1) related to the occupant load, i.e. total number of occupants Np (ps) in the space [16,30], is given by,

V_ g ¼ 0:00005N p

ð4Þ

Assuming a ‘steady’ and maximum occupant load during working hours in a typical office, the maximum probable number of occupants in the space Np,max can be determined with the floor area Af (m2) and the occupancy factor Oa (ps m2) [9,10] given, N p ¼ N p; max ¼ Oa Af

ð5Þ

It was reported for a ventilation system of a typical air-conditioned office, CO2 concentration had significant influence on energy consumption while temperature had minor effect on it [15]. At an occupancy factor, i.e. Oa = 0.0744 ps m2 (standard deviation SD = 0.0287 ps m2), the normalized annual energy consumption Ec (MJ m2 yr1) was approximated by a regression equation, with a significant correlation (correlation coefficient R = 0.9956 and pvalue, p = 0.0000), and e as the error term. e E c  9:54  108 U2 ð1 þ eÞ;

e  Nð0; 0:0555Þ

ð6Þ

3. IAQ for air-conditioned office A regional cross-sectional measurement of common air pollutant levels was conducted at 422 offices in Hong Kong [31,32]. The measurement aimed to study the overall picture of pollutant levels in local offices. The sampling sites were randomly picked from all regions for office development such that they covered a range of open-plan offices and conference rooms to individual small offices. Their sizes were from 10 m2 to 300 m2. Human activities and dress codes among them were reported similar. Furthermore, the schedules of their air-conditioning operation were well defined and sufficient for the study to focus on the indoor air contaminant parameters. The sampling methods were based on two assessment approaches, namely real-time and integrated approach to some guidelines [23]. The maximum lower detection limit was 10% of the recommended IAQ objective. Fig. 1 shows the average CO2 levels of office hours in 422 surveyed air-conditioned offices of Hong Kong. It was reported that the average and standard deviation of the CO2 levels in the surveyed were 660 ppmv and 159 ppmv, respectively. The distribution of the assessed CO2 levels was tested and described by a geometric distribution (p > 0.1, v2-test) with the geometric mean (GM) of 642 ppmv and geometric standard deviation (GSD) of 1.26 ppmv, respectively. According to the Hong Kong Environmental Protection Department (HKEPD) certification scheme [23], an indoor environment with any one of the specified air pollutant parameters i exceeding the exposure limits, i.e. Ui > Ui,e, is deemed to have ‘unsatisfactory IAQ’; otherwise, i.e. Ui 6 Ui,e, its IAQ is considered as ‘satisfactory’ [32]. It was reported that 3% of the surveyed air-conditioned offices were ‘unsatisfactory’, i.e. U > 1000 ppmv. With distributions of a concerning pollutant i in a cross-sectional measurement, the sample unsatisfactory rate ^f i

150

Number of offices

concentration [27] could be used to produce quantitative value of the environmental acceptance. Some studies showed that the IAQ acceptance / would be correlated with the indoor CO2 concentration U (ppm) [27]. In general, indoor CO2 is generated by occupants and diluted by outdoor air. It can be identified as a good indicator for the ventilation rate and occupant load in the space [12,16,17,19,20]. Occupant generated CO2 is used as the tracer gas to determine ventilation rate. Although CO2 concentration may not provide a comprehensive indication of IAQ, it can be a good indicator of the concentration of other human bioeffluents being perceived as a nuisance and be used as one to identify the acceptability of IAQ in a space by its occupants. From a recent study of 61 typical offices in Hong Kong, the percentage acceptance of indoor air quality /, given the CO2 concentration U (ppm) at working plane, is shown in Eq. (1) [27],  1 1 /¼1 2 1 þ expð3:118  0:00215UÞ  1  ; 500 6 U 6 1800 ð1Þ 1 þ expð3:230  0:00117UÞ

Expected counts GM=642 ppmv; GSD=1.42 ppmv

120 90 60 30 0 400

500

600

700

800

900

1000 1100 1200

CO2 level Φ (ppmv) Fig. 1. CO2 levels in 422 air-conditioned offices of Hong Kong.

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for the surveyed environment in the region can be approximated by, Z Ui;e ^f ¼ 1  e i dUi U ð7Þ i 1

4. Energy impact of IAQ improvement With Eqs. (1)–(6), the normalized estimation of annual energy consumption for IAQ acceptability with 99% confidence intervals was evaluated and showed in Fig. 2. A non-linear relationship between acceptability and energy consumption was found. Increasing slope of energy consumption for IAQ improvement indicates extra energy consumptions to maintain a higher IAQ acceptability. Indeed, even at the minimum CO2 level close to the outdoor ambient condition which requires ‘infinite’ thermal energy for ventilation, a small proportion of occupants would vote IAQ ‘unacceptable’. Indicative CO2 levels of 800 ppmv, 1000 ppmv and 1200 ppmv corresponding to 83%, 97% and 99.7% survey samples and the acceptability of 86%, 81% and 75% were shown in the figure; the corresponding thermal energy consumptions Ec were 1500 MJ m2 yr1, 960 MJ m2 yr1 and 670 MJ m2 yr1, respectively. It was reported that for the design CO2 concentration of 1000 ppmv for some office environment, it would require about 140 MJ m2 yr1 extra energy to award 1% increment of acceptability, and save about 70 MJ m2 yr1 to lower 1% acceptability. The highly non-linear relationships between the annual energy consumption and the acceptability would be approximated by for certain CO2 concentrations, Ec ¼ 0:66e9/ ; Ec ¼ 6:89e6:1/ ;

800 6 U < 1000

ð8Þ

1000 6 U 6 1200

ð9Þ

In order to investigate the potential saving of the air-conditioned buildings in Hong Kong, a survey was conducted to collect information in commercial buildings. According to the records at the Hong Kong Rating and Valuation Department, in 2002, there was a total stock of Af = 9,769,700 m2 of office buildings in Hong Kong [33]. Taking the total areas and the assumptions stated in this study, when the indoor target CO2 concentration increases from 1000 ppmv to 1200 ppmv, the percentage of energy saving is about 30%. This saving, in monetary terms, corresponds to the annual value Av of HK$ 762 million (about HK$ 3.8 million per ppmv per year) in electrical consumption (assuming at a rate of HK$0.97 per kWh), and with the occupant acceptability changing from 81% to 75%. The annual expenses Av (HK$ yr1) due to the thermal energy consumption of air-conditioning system in this city can be approx-

imated by, where U0T and UT are the target and existing CO2 concentrations, respectively, Av ¼ 0:269Af ½Ec ðU0T Þ  Ec ðUT Þ

ð10Þ

Besides, some design guides suggested a lower design CO2 concentration of 800 ppmv for an ‘excellent’ IAQ target [23]. Building owners and professionals follow these guides to renovate their properties, such as increasing the fresh air quantity to dilute the indoor CO2 concentration (from ‘‘good” IAQ objectives of 1000 ppmv CO2 concentration to ‘‘excellent” of 800 ppmv CO2 concentration). However, an additional amount of energy will be consumed when the indoor target CO2 concentration reduces 200 ppmv (increase of energy is about 56%, and increase of annual value is HK$ 1419 million), even though the occupant acceptability is only improved from 81% to 86%. 5. Conclusion In this study, the energy impact of IAQ of air-conditioned offices in Hong Kong specified by some design carbon dioxide (CO2) levels was studied and correlated to the probable IAQ acceptance. The ventilation demands of an air-conditioned office due to occupant load variations were examined against the IAQ objectives using the mass balance of CO2 concentrations in an office. In particular, this study proposed a ventilation model for occupant load and acceptability for typical air-conditioned offices in Hong Kong. The results showed that, for certain design conditions, the indoor CO2 concentrations and the occupant load in the space would have significant influence on the energy consumption of a ventilation system. Specifically, a significant amount of energy consumption variations of the ventilation system was resulted, if the target indoor CO2 concentration in the office was adjusted for certain IAQ objectives. In regards to the monetary issue, an annual value of about HK$ 3.6 million per ppmv per year in electrical consumption can be saved in office building in Hong Kong when the indoor target CO2 concentration is increased from 1000 ppmv to 1200 ppmv. For achieving an excellent IAQ following the existing design standard, i.e. to decrease the CO2 level from 1000 ppmv to 800 ppmv, 56% additional energy would be consumed, corresponding to an annual value of HK$ 1419 million, even though the occupant acceptability is only improved from 81% to 86%. The model output with carefully selected input parameters would be a useful source of information for the policymaker to evaluate operating strategies of ventilation system in some offices. This paper also provides a template in evaluating relative energy consumption of various ventilation strategies for air-conditioned spaces against certain IAQ objective with a CO2 concentration limit specified.

Normalized annual thermal energy consumption Ec (MJ m−2 yr−1)

Acknowledgements 10000

The work described in this paper was partially supported by a grant form the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No: PolyU5248/06E, Account code: BQ01G) and partially supported by a grant from The Hong Kong Polytechnic University (Project account code: GYE80).

CO2 level 800 ppmv 1000 ppmv 1200 ppmv 1000

References

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99% confidence intervals 0.5

0.6

0.7

0.8

0.9

1

Acceptability φ Fig. 2. Thermal energy consumption for IAQ acceptability.

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