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Assessing the natural ventilation cooling potential of office buildings in different climate zones in China
Renewable Energy 34 (2009) 2697–2705
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Assessing the natural ventilation cooling potential of office buildings in different climate zones in China Runming Yao a, *, Baizhan Li b, Koen Steemers c, Alan Short c a
School of Construction Management and Engineering, the University of Reading, Whiteknights, PO Box 219, Reading RG6 6AW, UK The Faculty of Urban Construction and Environmental Engineering, Chongqing University, PR China c The Martin Centre for Architectural and Urban Studies, Department of Architecture, University of Cambridge, UK b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 July 2008 Accepted 8 May 2009 Available online 4 July 2009
This paper presents an investigation of the natural ventilation cooling potential (NVCP) of office buildings in the five generally recognised climate zones in China using the Thermal Resistance Ventilation (TRV) model, which is a simplified, coupled, thermal and airflow model. The acceptable operative temperature for naturally conditioned space supplied by the ASHARE Standard 55-2004 has been used for the comfort temperature setting. Dynamic simulations for a typical office room in the five representative cities, which are Harbin, Beijing, Shanghai, Kunming and Guangzhou, have been carried out. The study demonstrates that the NVCP depends on the multiple impacts of climate, the building’s thermal characteristics, internal gains, ventilation profiles and regimes. The work shows how the simplified method can be used to generate detailed, indoor, operative temperature data based on the various building conditions and control profiles which are used to investigate the NVCP at the strategic design stage. The simulation results presented in this paper can be used as a reference guideline for natural ventilation design in China. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Natural ventilation cooling potential (NVCP) Thermal Resistance Ventilation (TRV) model Climate zone Office Strategic design
1. Introduction Natural ventilation is often considered to be the most energy efficient and healthy solution for ventilating a building, with the potential to reduce the energy cost required for air conditioning. Adequate ventilation within a building is necessary to maintain the occupants’ health and comfort. Natural ventilation is one way of providing this, allowing the provision of a comfortable working environment coupled with the potential for low energy usage and low operating costs [1,2]. Ventilation, whether mechanical or natural, may be used for the following: Air quality control: to control building air quality by diluting internally generated air contaminants with cleaner outdoor air. Direct cooling: to directly cool building interiors and occupants by replacing or diluting warm indoor air with cool outdoor air to enhance the convective transport of heat and moisture, when conditions are favourable, thus lowering the indoor air temperature.
* Corresponding author. Tel.: þ44 118 3788606; fax: þ44 118 9313856. E-mail address: [email protected] (R. Yao). 0960-1481/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2009.05.015
Indirect night cooling: to directly cool building interiors by precooling the thermal mass of the building fabric or a thermal storage system with cool, nighttime, outdoor air. When applicable, natural ventilation can offset cooling energy consumption, the associated energy costs and CO2 emissions. However, potential cooling energy saving depends on the climate (temperature and wind), ventilation profiles and types, internal heat gains, the building’s thermal characteristics and human expectations of thermal comfort. In recent years, research on the climatic suitability and potential of natural ventilation in different climate zones has been conducted, for example, the potential of using natural ventilation in the United States for residential buildings [3] (cited from Ref. [4]). Emmerich proposed a method for evaluating the climatic suitability of a given location for direct ventilation cooling and complimentary nighttime ventilation cooling of a building’s thermal mass, and the climatic suitability statistics for 10 Californian locations has been demonstrated [1]. Axley and Ghiau have studied the methods of analysing climate suitability for natural ventilation, or more specifically the building’s effective pressure difference for natural ventilation [6,7] (cited from [5]). Yang [5] proposed a model of the potential of natural driving forces for natural ventilation in residential buildings in China. Graca studied wind-driven, ventilating,
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Nomenclature Aglaze C1 C2 C3 g12 g1a g23 g3a n V T1 T2 T3 Ta
a1 a2 Vauxiliary Vpc Vsolar Vlight
area of glazing, m2 thermal capacity of the room, which includes its contents, Wh/ C thermal capacity of the shallow surface material of the room, Wh/ C thermal capacity of deep material of the room, Wh/ C conductance between N1 and N2, W/ C conductance between N1 and node A (infinite capacity), W/ C conductance of the massive material, W/ C conductance between N3 and NA, W/ C air change rate, ac/hour room volume, m3 temperature at Node 1, C temperature at Node 2, C temperature at Node 3, C external air temperature, C proportion of the gain delivered to room node, 70% proportion of the gain delivered to surface node, 30% artificial lighting gain entered to room, W casual gain entered to room, W solar gain entered to room, W lighting gain entered to room, W
cooling systems for residential buildings in Shanghai and Beijing using BLAST and CFD programs[8]. To summarise, the research on the potential of natural ventilation has two main categories. (1) Calculation of the driving force
(wind-driven and stack effect) considering indoor and outdoor climate data, and (2) calculation of the internal air temperature using comprehensive, coupled, thermal and airflow simulation. The first category considers the factors of climate such as external air temperature, wind velocity, etc and the internal air temperature, but ignores the heat storage potential of the building itself. It usually calculates the cooling potential based on a fixed, constant, indoor air temperature. However, in a naturally ventilated building, the indoor air temperature fluctuates according to the outdoor air temperature. Therefore these kinds of methods may cause inaccurate estimations. The experimental work by Straaten shows that whenever the total area of the ventilation apertures in the building facade exceeds 20% of the total area of the facade, the uncoupled thermal and airflow approach is not adequate [8]. The second category combines climate information and building information to perform a detailed assessment of natural ventilation. Usually, the detailed thermal simulation program and the CFD program are applied to calculate simultaneously indoor and outdoor airflow, but this requires a powerful computer and is timeconsuming. This approach is not suitable for the strategic design stage. There is a great need for a robust method that allows architects to perform a strategic analysis of natural ventilation in the early stages of a design. To solve this problem, a simplified, coupled, thermal and airflow method has been developed by integrating the British Standard natural ventilation calculation method within a four-node thermal resistance network model, which is called the Thermal Resistance Ventilation (TRV) model [9]. It is very useful when assessing the potential of natural ventilation at the predesign stage with adequate accuracy. It takes into account not only the local climatological conditions, but also building type, ventilation and occupancy profiles. In addition, the expectation of thermal comfort varies from region to region, thus the adaptive thermal comfort concept has been applied in this paper.
Fig. 1. The climate zones for building thermal design in China [21].
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Table 1 Climatic characteristics of the representative cities, source [13,17]. Zone
City
Main index
Complementary index
Latitude
Annual wind speed, m/s
Very cold Cold Hot summer Cold winter Mild
Harbin Beijing Shanghai
NDAT5 145 days NDAT5 ¼ 90–145 days NDAT5 ¼ 0–90 days NDAT25 ¼ 40–110 days NDAT5 ¼ 0–90 days
39.56 31.0 25.51
3.6 2.83 3.0
25.01
2.5
Hot summer Warm winter
Guangzhou
ATCM ¼ 10 C ATCM ¼ 0–10 C ATCM ¼ 0–10 C ATHM ¼ 25–30 C ATCM ¼ 0–13 C ATHM ¼ 18–25 C ATCM 10 C ATHM ¼ 25–29 C
NDAT5 ¼ 100–200 days
25.01
2.2
Kunming
Note: ACTM ¼ average temperature in the coldest month; ATHM ¼ average temperature in the hottest month; and NDAT5 ¼ number of days that average temperature is below 5 C; NDAT5 ¼ number of days that average temperature is above 25 C.
The climate of China is extremely diverse and variable with a tropical climate in the south and a sub-arctic one in the north. The design codes are also different for the five thermal zones. Based on previous research [10,11], this paper assesses the natural ventilation cooling potential using the Thermal Resistance Ventilation (TRV) model for office buildings in the five climate zones in China. The method can perform a quick assessment and therefore is very useful for strategic natural ventilation design.
Winter Zone and has a North Asian hot monsoon climate, with four distinctive seasons. The city has a lot of sunshine. Kunming is located in the Mild Zone and its climate is characterized by the distinctive feature of only a slight variation in annual temperatures. Guangzhou is located in the Hot Summer and Warm Winter Zone with a subtropical monsoon climate. The climate characteristics of the five cities are listed in Table 1 [13]. The hourly meteorological data generated from the Meteonorm software package are used for climate data inputs in the program.
2. Research method 2.1. Climate zones in China China is a large country with an area of about 9.6 million km2. About 98% of the land area stretches between a latitude of 20 N– 50 N, from subtropical zones in the south to the temperate zones (including warm-temperate and cool-temperate) in the north [12]. Five cities, Harbin, Beijing, Shanghai, Guangzhou and Kunming have been selected to represent the five climate zones, which have been presented in the ‘‘Thermal Design Code’’ [13] for buildings in China. These five climate zones are categorized into the Very Cold Zone, Cold Zone, Hot Summer and Cold Winter Zone, Hot Summer and Warm Winter Zone and Mild Zone (see Fig. 1). Harbin is located in the Very Cold Zone with a continental monsoon climate. The coldest month’s average air temperature is below minus 10 C. The hottest monthly average air temperature is 24.5 C. Beijing is located in the Cold Zone with a climate of the continental type, with cold and dry winters, due to the Siberian air masses that move southward across the Mongolian Plateau. The summers are hot owing to warm and humid monsoon winds from the southeast. Shanghai is located in the Hot Summer and Cold
2.2. Description of the Thermal Resistance Ventilation model It is rarely practical to perform a detailed thermal simulation procedure to assess the potential of natural ventilation at the architectural strategic design stage. However, it is very important for architects to evaluate the potential of natural ventilation in design by taking into account factors such as the glazing ratio, fabric U value, climate, thermal mass, the internal gain and occupancy pattern and ventilation profiles. Thermal and airflow interactions are characteristic of natural ventilation airflow systems. A simplified, coupled, thermal and airflow model has been developed by integrating the British Standard natural ventilation calculation method [2,14] for a single zone within a four-node thermal resistance network model, which is called the Thermal Resistance Ventilation (TRV) model [9]. The heat flow of the room has been modelled as a four-node, simplified, thermal resistance network model (TR Model), originally developed for LT Europe (Baker and Yao), and is an energy design tool requiring minimal data input [15,16]. The simplified model focuses on a single zone with parameters (such as glazing area, fabric U value, internal gains, lighting datum level, thermostat setting, occupancy profile, thermal mass type, etc) to replicate those chosen for the proposed design. The detailed description of the TRV model is presented in Ref. [9]. The integrated thermal and airflow model has been computerized using the Visual Basic language. The equations are listed in Appendix 1. The TRV model can generate detailed hourly data of an indoor thermal environment to evaluate the natural ventilation cooling potential of a given location and proposed building
Table 2 Mean monthly outdoor air temperature (source [17]) and acceptable comfort temperatures (italicized), C. City
Fig. 2. Acceptable operative temperature ranges for naturally conditioned spaces [19].
Harbin Beijing Shanghai Kunming Guangzhou
May
June
July
August
September
Taout
Tcom
Taout
Tcom
Taout
Tcom
Taout
Tcom
Taout
Tcom
16.5 23.1 20.8 17.0 26.5
22.9 25.0 24.2 23.1 26.0
22.2 25.7 24.2 20 27.2
24.7 25.8 25.3 24.0 26.2
24.5 27.3 29.7 20.9 28.2
25.4 26.3 27.0 24.3 26.5
21.7 25.8 27 20.5 29.1
24.5 25.8 26.2 24.2 26.8
15.8 21.2 24.9 19.4 27.8
22.7 24.4 25.5 23.8 26.4
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Table 3 U value of envelope in each zone (W/m2 K) [20,21].
External wall Glazing
Very Cold Zone (Harbin)
Cold Zone (Beijing)
Hot Summer and Cold Winter Zone (Shanghai)
Mild Zone (Kunming)
Hot Summer and Warm Winter Zone (Guangzhou)
0.4–0.52
0.82–1.16
1.0–1.5
1.5
1.5
2.5
4
3.2–4.7
6.0
6.0
information such as thermal mass, U values of envelopes and internal gains.
2.3. Input requirements For the building simulation and the evaluation of the potential of natural ventilation, basic building information is required, which is described below. 2.3.1. Climate data The climate data includes horizontal diffuse solar radiation, normal beam solar radiation, air temperature, wind speed and direction and humidity. This information is embedded within the programme.
(1) Daytime ventilation to cool the building interior and occupants by replacing or diluting warm indoor air with cool outdoor air. The approach can improve the occupants’ thermal comfort by increasing convective and evaporative heat transfer, and by decreasing the indoor air temperature. This scenario may be suitable for buildings where night ventilation is not achievable due to, for example, security reasons. (2) Nighttime ventilation to cool the building interior by precooling the thermal mass of the building fabric or a thermal storage system with cool nighttime outdoor air. This approach can reduce the peak air temperature, structural temperature and create a time lag of the peak temperature. This system is suitable for buildings where daytime ventilation is not advisable due to external air and noise pollution. (3) Combined daytime and nighttime ventilation. 2.5. Assessment index – natural ventilation cooling potential Indices for the natural ventilation cooling potential (NVCP) are proposed in this paper. The NVCP is defined as the ratio of the number of hours within the comfort zone over the total occupied hours. To assess natural ventilation cooling potential, it is necessary to obtain two kinds of information, which are the (1) acceptable comfort level, and (2) actual internal comfort temperatures.
2.4. Natural ventilation profiles
2.5.1. Acceptable comfort temperature ASHRAE Standard ‘‘thermal environmental conditions for human occupancy’’ 55-2004 [18,19] proposes the optional method for determining acceptable thermal comfort in a naturally conditioned space. It assumes that the thermal conditions of the space are regulated primarily by the occupants through the opening and closing of windows. Allowable indoor operative temperatures for spaces that meet these criteria may be determined from Fig. 2. This figure includes two sets of operative temperature limits – one for 80% acceptability and one for 90% acceptability. The 80% acceptability limits are for typical applications and will be used when other information is not available. In this paper, the ASHRAE Standard 55-2004, acceptable operative temperature ranges for naturally conditioned spaces, is used to assess the cooling potential of natural ventilation. The algorithm used to calculate the comfort temperature is as follows [18,19]:
Three scenarios are considered for the natural ventilation regimes.
Tcom ¼ 0:31Taout þ 17:8
2.3.2. User and building information This requires the information of occupancy pattern, the internal heat gain and the acceptable comfort temperature. Building information includes building orientation, room dimensions, glazing ratio, envelope U value and thermal mass type, such as heavy, medium or light. (In this paper, for generic office buildings in China, the medium weight construction is considered.) The building location, such as open flat country, country with scattered windbreaks, urban or city, needs to be identified. 2.3.3. Ventilation strategy This will include strategies such as daytime ventilation, nighttime ventilation and combined day and night ventilation.
(1)
Table 4 Cooling potential of natural ventilation of Harbin (%) (medium thermal mass). Profiles
Internal gain
Cross/single
May HS
Day only
Low gain Medium gain High gain
June LS
HS
July LS
September
Summer
HS
LS
HS
August LS
HS
LS
HS
LS
Cross Single Cross Single Cross Single
96 81 96 75 91 58
100 89 97 81 96 66
92 60 88 43 76 25
94 73 93 68 87 38
59 31 54 20 49 12
73 44 66 35 54 19
53 11 45 5 29 3
75 26 68 17 45 5
85 49 77 42 69 30
90 60 87 53 79 39
77 46 72 37 63 26
86 58 82 51 72 33
Night only
Low gain Medium gain High gain
Single Single Single
91 82 68
94 90 75
77 60 40
88 77 50
40 30 17
55 42 25
33 22 11
48 35 19
62 56 44
73 63 50
61 50 36
72 61 44
Day and night
Low gain
Cross Single Cross Single Cross Single
100 95 100 94 97 82
100 97 100 96 100 89
99 88 97 83 94 70
100 92 100 89 97 80
81 57 79 51 75 40
87 74 85 67 80 52
85 50 81 44 71 28
93 69 90 58 81 42
97 80 96 75 95 64
98 88 98 82 96 75
92 74 91 69 86 57
96 84 95 78 91 68
Medium gain High gain
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Table 5 Cooling potential of natural ventilation Beijing (%) (medium thermal mass). Profiles
Day only
Internal gain
Low gain Medium gain High gain
Cross/single
May
September
Summer
HS
LS
June HS
LS
July HS
LS
August HS
LS
HS
LS
HS
LS
Cross Single Cross Single Cross Single
77 53 71 42 59 29
89 66 83 43 70 40
40 27 37 19 31 13
53 32 46 28 37 17
35 9 24 6 15 3
48 17 42 12 25 7
35 9 28 6 17 4
54 18 45 13 30 6
84 52 78 43 64 22
89 68 87 56 79 40
54 30 48 23 37 14
67 40 61 30 48 22
Night only
Low gain Medium gain High gain
Single Single Single
67 57 40
78 68 49
36 27 15
46 38 22
17 11 5
27 20 9
20 15 8
28 21 13
59 45 27
69 60 36
40 31 19
50 41 26
Day and night
Low gain
Cross Single Cross Single Cross Single
96 77 91 71 83 61
99 87 99 80 92 68
69 48 66 42 60 33
78 57 74 52 67 42
65 30 57 24 49 15
76 46 71 40 61 26
67 32 61 28 51 21
78 48 74 41 63 29
91 79 89 72 86 57
97 86 96 81 89 71
78 53 73 47 66 37
86 65 83 59 74 47
Medium gain High gain
where, Tcom ( C) is the acceptable comfort temperature, the indoor operative temperature is used as an index. Taout ( C) is the monthly average outdoor air temperature. According to Eq. (1) and the monthly average outdoor air temperature of each city, the acceptable comfort temperature levels have been worked out and listed in Table 2. 2.5.2. Indoor operative temperature The operative temperature is the average of the air temperature and the mean radiant temperature weighted, respectively, by the convective heat transfer coefficient and the linearized radiant heat transfer coefficient for an occupant [18]. For occupants engaged in near-sedentary physical activity (with metabolic rates between 1.0 met and 1.4 met), not in direct sunlight, and not exposed air velocity greater than 0.2 m/s, the relationship can be approximated with acceptable accuracy by [19]:
To ¼ ðTa þ Tr Þ=2
(2)
where, To is the operative temperature ( C), Ta is the indoor air temperature ( C) and Tr is the mean radiant temperature ( C). The TRV model can generate the indoor air temperature and internal surface temperature on an hourly basis, therefore the operative temperature can be calculated using Eq. (2).
3. Natural ventilation cooling potential in the Chinese climate context Based on the discussion in Section 2, to assess the NVCP of office buildings in China, the indices of High Standard NVCP (NVCPHS) and Low Standard NVCP (NVCPLS) have been proposed, of which the thermal comfort meets 90% and 80% acceptability, respectively. The summer season is assumed to be from the 1st May to the 30th of September. The TRV natural ventilation assessment program has been used to carry out the dynamic simulations to generate robust internal temperatures in order to investigate the cooling potential of natural ventilation in the offices in the five climate zones in relation to the parameters of the ventilation profiles and regimes, internal gains, thermal mass and transmittance, etc. 3.1. Basic information Office rooms in China can be grouped into two types: one is the high standard, fully central air-conditioned, cell office whilst the other is the traditional office room equipped with a split room airconditioner, which will operate when needed. A typical office room is selected for this study, of which the required information for thermal simulation is as follows: Room dimension is 3.6 m in length, 5.4 m in depth and 3.0 m in height.
Table 6 Cooling potential of natural ventilation of Shanghai (%) (medium thermal mass). Profiles
Internal gain
Cross/single
May HS
Day only
Low gain Medium gain High gain
June LS
July
August
September
HS
LS
HS
LS
HS
LS
HS
LS
Summer HS
LS
Cross Single Cross Single Cross Single
93 75 88 67 81 43
99 86 96 78 89 61
45 28 38 27 30 24
63 32 53 29 40 25
20 5 14 2 9 1
31 8 25 7 18 3
10 0 6 0 4 0
23 2 15 1 8 0
44 29 40 25 30 15
58 36 51 30 40 24
42 27 37 24 31 17
55 33 48 29 39 23
Night only
Low gain Medium gain High gain
Single Single Single
79 71 45
87 79 58
33 30 25
41 35 31
7 4 2
15 9 4
2 2 0
6 4 2
33 27 16
41 34 25
31 27 18
38 32 24
Day and night
Low gain
Cross Single Cross Single Cross Single
98 87 96 82 91 73
100 95 99 90 96 82
70 40 67 35 56 31
81 56 79 47 69 37
40 15 37 11 30 8
58 27 51 21 40 13
36 7 32 5 24 3
44 17 42 12 37 6
67 43 65 37 55 32
77 58 74 49 67 37
62 38 59 34 51 29
72 51 69 44 62 35
Medium gain High gain
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Table 7 Cooling potential of natural ventilation of Kunming (%) (medium thermal mass). Profiles
Day only
Internal gain
Low gain Medium gain High gain
Cross/single
May
June
July
August
September
Summer
HS
LS
HS
LS
HS
LS
HS
LS
HS
LS
HS
LS
Cross Single Cross Single Cross Single
93 80 91 68 81 46
96 92 96 84 91 62
94 78 90 67 79 48
97 91 96 84 89 61
95 82 92 72 84 57
97 94 96 87 92 68
91 81 87 76 81 60
95 88 92 82 86 72
92 81 89 72 83 60
97 88 95 83 89 69
93 80 90 71 82 54
96 91 95 84 89 66
Night only
Low gain Medium gain High gain
Single Single Single
83 70 49
92 84 60
82 72 51
93 83 62
83 75 57
93 83 69
87 79 61
91 87 71
84 76 64
90 84 71
84 74 56
92 84 67
Day and night
Low gain
Cross Single Cross Single Cross Single
97 91 96 86 93 69
99 96 98 93 96 80
97 91 96 87 91 70
99 97 98 93 96 84
97 93 96 86 93 73
99 97 98 94 96 85
96 90 94 86 90 76
98 94 98 91 93 84
97 89 96 86 92 76
100 94 98 90 97 85
97 91 96 86 92 73
99 96 98 92 96 84
Medium gain High gain
Orientation is south–north. Glazing ratio in the south wall is 0.35 and 0.25 in the north wall. Occupied period is from 08:00 to 18:00. Thermal mass is assumed as medium (some heavy structures, e.g. concrete slab with wooden floor or light-weight concrete walls). Internal shading devices are applied to the southern window in summer. The office room is located in a city. The office is assumed to be medium gain (25 W/m2). The internal gain has been classified as low gain with 15 W/m2, medium gain with 25 W/m2 and high gain with 35 W/m2. Table 3 lists the U value of the building envelopes in each zone according to the Chinese design regulations [20,21]. 3.2. Results’ analysis Fifteen cases of different ventilation profiles and types in conjunction with different internal gains for the five cities have been simulated using the TRV model. The results of the high standard and low standard cooling potential of each case are listed in Tables 4–8. The monthly detailed results are very useful for
architects in designing natural ventilation for their buildings. From the simulation results we can summarise the following: (1) The natural ventilation cooling potential (NVCP) is greatly influenced by the combined effect of the local climate, the thermal properties of the building, ventilation types and profiles. The climate with the lower monthly mean external air temperature has the higher NVCP. For example, among these five cities, Kunming and Harbin have lower monthly mean external air temperatures than other cities and therefore they have higher NVCPs. However, the building’s thermal properties also have a significant impact. For example, the monthly mean external air temperatures in both Harbin and Kunming are about the same, but the NVCP in Kunming is greater than that in Harbin. This is because the insulation level of the external envelope (wall and window) in Harbin is much higher than in Kunming due to the Chinese building design regulations. (2) Internal heat gain has a significant impact on NVCP and should be considered in the strategic design stage. (3) In the Very Cold Zone, Day and Night cross-ventilation is sufficient for summer cooling in most cases. In the hottest months, July and August, the single-sided ventilation has less cooling potential. Thus Day only or Night only natural ventilation is not sufficient for office cooling in July and August. Therefore,
Table 8 Cooling potential of natural ventilation of Guangzhou (%) (medium thermal mass). Profiles
Day only
Internal gain
Low gain Medium gain High gain
Cross/single
May
September
Summer
HS
LS
June HS
LS
July HS
LS
August HS
LS
HS
LS
HS
LS
Cross Single Cross Single Cross Single
71 37 62 22 47 6
87 57 81 43 63 18
39 10 30 6 12 2
62 19 50 13 32 6
34 11 31 5 19 1
45 28 42 17 31 5
14 1 8 1 3 0
30 5 24 3 11 1
5 0 3 0 1 0
16 2 11 1 5 0
33 12 27 7 16 2
48 22 42 15 28 6
Night only
Low gain Medium gain High gain
Single Single Single
38 23 10
60 41 21
14 8 3
26 18 7
15 9 3
27 18 9
4 3 1
11 8 3
2 1 1
5 4 2
15 9 4
26 18 8
Day and night
Low gain
Cross Single Cross Single Cross Single
88 58 81 47 68 27
95 71 92 62 83 44
64 24 57 19 45 10
82 44 75 31 62 20
47 28 44 20 36 11
55 37 53 33 47 20
38 11 31 8 23 4
61 21 53 17 40 10
25 4 21 3 14 2
42 11 36 8 28 5
52 25 47 19 37 11
67 37 62 30 52 20
Medium gain High gain
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36
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Temperature Distribution during the office hours in July
34
Temperature °C
32
NVCPLS=80%
30 28
NVCPHS=63%
26 24 22 20 18
8 13 18 12 17 11 16 10 15 9 14 8 13 18 12 17 11 16 10 15 9 14
Time (hour) Fig. 3. Temperature distribution of Beiqijia Low-Energy Demonstration Building in July.
(5)
(6)
(7)
(8)
36
to the residential building studied by Graca [8] because Beijing has a greater day–night external temperature difference than Shanghai. 4. Case study A case study for a natural ventilation strategic design has been conducted for a Ministry of Construction Demonstration Project in Beijing. The exemplar low energy exhibition, conference and administrative headquarters for the Future House Exposition Site are in Beiqijia on the 5th Ring Road. The whole construction area of the building is about 7000 m2. The building has a north–south orientation. The building information is similar to the typical office building described above. The only difference is the U value of the building fabric. As this is an energy efficiency demonstration office building, a higher insulation level compared to the building regulations is proposed. The U value of the external wall is 0.35 W/m2 K instead of the standard one of 0.82 W/m K and the U value of the windows is 2.0 W/m2 K instead of standard one of 4 W/m2 K in order to achieve overall energy efficiency. An advanced shading device is proposed in this design. The internal gain is assumed to be in the medium range. Reviewing the results in Table 5, we can see that the NVCP in Beijing in summer is between 37 and 78% for High Standard and 47–86% for Low Standard. The initial indicators of NVCP demonstrate that there is a great potential to apply natural
Temperature Distribution during the office hours in August
34 32
Temperature °C
(4)
nighttime ventilation should be considered in the Cold Zone for a natural ventilation building when it is applicable. In the Cold Zone and the Hot Summer and Cold Winter Zone, natural ventilation is not sufficient for summer space cooling. A hybrid ventilation system is recommended for offices in this zone in order to use natural ventilation to provide space cooling in the high NVCP period and only operate air-conditioners when the natural ventilation cannot meet the spacecooling requirements. In the mild zone, full natural ventilation for an office building is highly recommended. For the office with a high internal gain, Day and Night cross-ventilation has higher efficiency than the single-sided one. In the Hot Summer and Warmer Winter Zone, natural ventilation cannot meet the space-cooling requirements, so mechanical cooling or air conditioning is desirable. The cooling potential of hybrid natural ventilation should be further studied for different control strategies. This issue is not discussed in this paper. The Night only ventilation profile is not suitable for office buildings, particularly for the office with higher internal gains, because the internal gains released during the daytime cannot be exhausted immediately and may therefore cause discomfort. The natural ventilation cooling potential of office buildings in Beijing is slightly higher than for Shanghai. This result is similar
NVCPLS=81%
30 28 26
NVCPHS=66%
24 22 20 18
8 13 18 12 17 11 16 10 15 9 14 8 13 18 12 17 11 16 10 15 9 14
Time (hour) Fig. 4. Temperature distribution of Beiqijia Low-Energy Demonstration Building in August.
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ventilation in office buildings in Beijing. Based on the analysis from the research output in Section 3, the architectural design team (Short and Associates) was convinced that hybrid natural ventilation in this building could achieve low energy consumption. Simulations for this specific building have been run using the TRV model to assess the NVCP. The structure of the building, from inside to outside, is made of aerated concrete plus 100 polystyrene boards plus an air layer and cladding panels, therefore it has been classified as heavy mass. Based on the building information, a dynamic simulation has been run for this strategic design. Figs. 3 and 4 show the temperature distribution during office hours. The comfort temperature is 26.3 C in July and 25.8 C in August according to Table 2. Therefore the comfort range will be between 22.8 and 29.8 C with low standard and 23.8 and 28.8 C with high standard in July; 22.3 and 29.3 C with low standard and 23.3 and 28.3 C with high standard in August. From the statistics, we can see that the NVCP will be 80% with the low standard level and 63% with the high standard level in July, and 81% with the low standard level and 66% with the high standard level in August. The simulation results give confidence to the designer to implement a hybrid ventilation system (as opposed to a fully airconditioned solution) in this Low-Energy Demonstration Building design. In order to enhance the natural ventilation, stack ventilation is proposed and a further detailed energy simulation will be performed using Energy plus. This is not the intention of this paper.
5. Conclusion There is a significant impact of multiple factors of climate, building properties, occupancy and operation profiles on the
performance of natural ventilation. Strategic natural ventilation design should consider not only the climatic conditions but also multiple factors such as the building’s thermal characteristics, the ventilation type and profile and internal gains. Accordingly, for the natural ventilation strategic design, the coupled thermal and airflow thermal resistant ventilation (TRV) model has been used to assess the natural ventilation cooling potential (NVCP) in the five climate zones of China. The outputs from this research will be useful to architects involved in natural ventilation design in China. They can also be useful as natural ventilation strategic design references for similar weather and building condition cases in the other parts of the world. The method of using the TRV model to assess natural ventilation cooling potential (NVCP) is universal though the case studies of natural ventilation strategic design for office buildings have been carried out in the five climate zones of China. The Low-Energy Office Building Demonstration project is the showcase of the application of this robust method for architects to assess natural ventilation potential at the early design stage. The detailed natural ventilation system design should be further analysed using more advanced simulation tools in the detailed design stage.
Acknowledgements The authors wish to acknowledge the funding from the Asia Link ‘‘Centre for Sino-European Sustainable Building Design and Construction’’ project for the collaboration work and the support from the project ‘‘Key Technologies on Control and Improvement of Building Indoor Thermal Environment’’(2006BAJ02A09) funded by the Chinese Ministry of Science and Technology under the Chinese Key R&D National 11th Five-Year Plan Programme.
Auxiliary heating & cooling
ventilation Lighting gains T3 Deep node
T2
V
auxiliary T1
light
Surface node
Ta Ambient node
Room node pc Casual gains
Solar gains
solar
conduction
R. Yao et al. / Renewable Energy 34 (2009) 2697–2705
Appendix 1. Thermal resistance network model [9]
(i) Node Room
C1
dT1 ¼ a1 Fpc þ Fsolar þ Flight þ Fauxilary þ g12 ðT2 T1 Þ dt þ g1a ðTa T1 Þ ðA1Þ
(ii) Node Surface
C2
dT2 ¼ a2 Fpc þ Fsolar þ Flight þ Fauxilary þ g12 ðT1 T2 Þ dt ðA2Þ þ g23 ðT3 T2 Þ
(iii) Node Deep Mass
C3
dT3 ¼ g23 ðT2 T3 Þ þ g3a ðTa T3 Þ dt
g1a ¼ Aglaze Uglaze þ 0:3Vn
(A3) (A4)
References [1] Emmerich SJ, Stuart Doles W, Axley JW. Natural ventilation review and plan for design and analysis tools. National Institute of Standards and Technology; 2001. U.S. Department of Commerce. [2] CIBSE applied manual AM10: natural ventilation in non-domestic buildings. CIBSE, ISBN 0 900953772; 1997.
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[3] Lechner N. Heating, cooling, lighting: design methods for architects. 2nd ed. John Wiley & Sons, Inc; 2000. [4] Chen Q. Using computational tools to factor wind into architectural environment design. Energy Build 2004;36:1197–209. [5] Yang L, Zhang G, Li Y, Chen Y. Investigating potential of natural driving forces for ventilation in four cities in China. Build Environ 2005;40:738–46. [6] Axley JW, Emmerich SJ. A method to assess the suitability of a climate for natural ventilation of commercial buildings. In: Proceedings of indoor air 2002, Monterey, CA; 2002. [7] Ghiau C, Allard F. Assessing climatic suitability to natural ventilation by using global and satellite climate data. In: ROOMENT 2002, eight international conference on air distribution in rooms, Copenhagen, Denmark; 2002. [8] Graca G, Chen Q, Glicksman LR, Norford LK. Simulation of wind-driven ventilative cooling systems for an apartment building in Beijing and Shanghai. Energy Build 2002;34:1–11. [9] Yao R, Steemers K, Baker N. Strategic design and analysis method of natural ventilation for summer cooling. Build Serv Eng Res Technol 2005;26(No. 4). [10] Yao R, Steemers K, Li B. Climatic suitability analysis for natural ventilation cooling in office building in China. In: PLEA2005 – The 22nd conference on passive and low energy architecture, Beirut, Lebanon; 13–16 November 2005. [11] Yao R, Steemers K. Strategic design for natural ventilation in buildings. In: Proceedings of ninth international conference on air distribution in rooms, 5–8 September 2004, Coimbra, Portugal. [12] Lama Joseph C, Tsanga CL, Yangb L, Lia Danny HW. Weather data analysis and design implications for different climatic zones in China. Build Environ 2005;40:277–96. [13] Chinese National Standard GB 50176-93, Building design code for civil buildings. Beijing: Chinese Plan Publication House; 1993. [14] Allard Francis. Natural ventilation in buildings. James and James; 1998. [15] Baker N, Yao R. LT Europe – an integrated energy design tool. In: Design with the environment PLEA 2002, conference, proceedings Toulouse, France. PLEA; 2002. p. 669–73. [16] Yao R, Baker N, McEvoy M. A simplified method for referring the energy and overheating performance of window design. Archit Sci Rev 2003;46(3): 225–31. [17] Chinese Statistic Yearbook. Beijing, China: National Bureau of Standards; 2002. [18] ANSI/ASHRAE 55-2004. ASHRAE standard: thermal environmental conditions for human occupancy. Atlanta, USA: ASHRAE; 2004. ISBN 1041-2336. [19] de Dear R, Brager G. Thermal comfort in naturally ventilated buildings: revision to ASHRAE Standard 55. Energy Build 2002;34:549–61. [20] Chinese National Standard JGJ 26-95. Energy conservation design standard for new heating residential buildings. Beijing: Chinese Architectural Publication House; 1996. [21] Chinese National Standard JGJ 134-2001. Design standard for energy efficiency of residential buildings in the hot summer and cold winter zone. Beijing: Chinese Architectural Publication House; 2001.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Assessing the natural ventilation cooling potential of office buildings in different climate zones in China Runming Yao a, *, Baizhan Li b, Koen Steemers c, Alan Short c a
School of Construction Management and Engineering, the University of Reading, Whiteknights, PO Box 219, Reading RG6 6AW, UK The Faculty of Urban Construction and Environmental Engineering, Chongqing University, PR China c The Martin Centre for Architectural and Urban Studies, Department of Architecture, University of Cambridge, UK b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 July 2008 Accepted 8 May 2009 Available online 4 July 2009
This paper presents an investigation of the natural ventilation cooling potential (NVCP) of office buildings in the five generally recognised climate zones in China using the Thermal Resistance Ventilation (TRV) model, which is a simplified, coupled, thermal and airflow model. The acceptable operative temperature for naturally conditioned space supplied by the ASHARE Standard 55-2004 has been used for the comfort temperature setting. Dynamic simulations for a typical office room in the five representative cities, which are Harbin, Beijing, Shanghai, Kunming and Guangzhou, have been carried out. The study demonstrates that the NVCP depends on the multiple impacts of climate, the building’s thermal characteristics, internal gains, ventilation profiles and regimes. The work shows how the simplified method can be used to generate detailed, indoor, operative temperature data based on the various building conditions and control profiles which are used to investigate the NVCP at the strategic design stage. The simulation results presented in this paper can be used as a reference guideline for natural ventilation design in China. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Natural ventilation cooling potential (NVCP) Thermal Resistance Ventilation (TRV) model Climate zone Office Strategic design
1. Introduction Natural ventilation is often considered to be the most energy efficient and healthy solution for ventilating a building, with the potential to reduce the energy cost required for air conditioning. Adequate ventilation within a building is necessary to maintain the occupants’ health and comfort. Natural ventilation is one way of providing this, allowing the provision of a comfortable working environment coupled with the potential for low energy usage and low operating costs [1,2]. Ventilation, whether mechanical or natural, may be used for the following: Air quality control: to control building air quality by diluting internally generated air contaminants with cleaner outdoor air. Direct cooling: to directly cool building interiors and occupants by replacing or diluting warm indoor air with cool outdoor air to enhance the convective transport of heat and moisture, when conditions are favourable, thus lowering the indoor air temperature.
* Corresponding author. Tel.: þ44 118 3788606; fax: þ44 118 9313856. E-mail address: [email protected] (R. Yao). 0960-1481/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2009.05.015
Indirect night cooling: to directly cool building interiors by precooling the thermal mass of the building fabric or a thermal storage system with cool, nighttime, outdoor air. When applicable, natural ventilation can offset cooling energy consumption, the associated energy costs and CO2 emissions. However, potential cooling energy saving depends on the climate (temperature and wind), ventilation profiles and types, internal heat gains, the building’s thermal characteristics and human expectations of thermal comfort. In recent years, research on the climatic suitability and potential of natural ventilation in different climate zones has been conducted, for example, the potential of using natural ventilation in the United States for residential buildings [3] (cited from Ref. [4]). Emmerich proposed a method for evaluating the climatic suitability of a given location for direct ventilation cooling and complimentary nighttime ventilation cooling of a building’s thermal mass, and the climatic suitability statistics for 10 Californian locations has been demonstrated [1]. Axley and Ghiau have studied the methods of analysing climate suitability for natural ventilation, or more specifically the building’s effective pressure difference for natural ventilation [6,7] (cited from [5]). Yang [5] proposed a model of the potential of natural driving forces for natural ventilation in residential buildings in China. Graca studied wind-driven, ventilating,
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Nomenclature Aglaze C1 C2 C3 g12 g1a g23 g3a n V T1 T2 T3 Ta
a1 a2 Vauxiliary Vpc Vsolar Vlight
area of glazing, m2 thermal capacity of the room, which includes its contents, Wh/ C thermal capacity of the shallow surface material of the room, Wh/ C thermal capacity of deep material of the room, Wh/ C conductance between N1 and N2, W/ C conductance between N1 and node A (infinite capacity), W/ C conductance of the massive material, W/ C conductance between N3 and NA, W/ C air change rate, ac/hour room volume, m3 temperature at Node 1, C temperature at Node 2, C temperature at Node 3, C external air temperature, C proportion of the gain delivered to room node, 70% proportion of the gain delivered to surface node, 30% artificial lighting gain entered to room, W casual gain entered to room, W solar gain entered to room, W lighting gain entered to room, W
cooling systems for residential buildings in Shanghai and Beijing using BLAST and CFD programs[8]. To summarise, the research on the potential of natural ventilation has two main categories. (1) Calculation of the driving force
(wind-driven and stack effect) considering indoor and outdoor climate data, and (2) calculation of the internal air temperature using comprehensive, coupled, thermal and airflow simulation. The first category considers the factors of climate such as external air temperature, wind velocity, etc and the internal air temperature, but ignores the heat storage potential of the building itself. It usually calculates the cooling potential based on a fixed, constant, indoor air temperature. However, in a naturally ventilated building, the indoor air temperature fluctuates according to the outdoor air temperature. Therefore these kinds of methods may cause inaccurate estimations. The experimental work by Straaten shows that whenever the total area of the ventilation apertures in the building facade exceeds 20% of the total area of the facade, the uncoupled thermal and airflow approach is not adequate [8]. The second category combines climate information and building information to perform a detailed assessment of natural ventilation. Usually, the detailed thermal simulation program and the CFD program are applied to calculate simultaneously indoor and outdoor airflow, but this requires a powerful computer and is timeconsuming. This approach is not suitable for the strategic design stage. There is a great need for a robust method that allows architects to perform a strategic analysis of natural ventilation in the early stages of a design. To solve this problem, a simplified, coupled, thermal and airflow method has been developed by integrating the British Standard natural ventilation calculation method within a four-node thermal resistance network model, which is called the Thermal Resistance Ventilation (TRV) model [9]. It is very useful when assessing the potential of natural ventilation at the predesign stage with adequate accuracy. It takes into account not only the local climatological conditions, but also building type, ventilation and occupancy profiles. In addition, the expectation of thermal comfort varies from region to region, thus the adaptive thermal comfort concept has been applied in this paper.
Fig. 1. The climate zones for building thermal design in China [21].
R. Yao et al. / Renewable Energy 34 (2009) 2697–2705
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Table 1 Climatic characteristics of the representative cities, source [13,17]. Zone
City
Main index
Complementary index
Latitude
Annual wind speed, m/s
Very cold Cold Hot summer Cold winter Mild
Harbin Beijing Shanghai
NDAT5 145 days NDAT5 ¼ 90–145 days NDAT5 ¼ 0–90 days NDAT25 ¼ 40–110 days NDAT5 ¼ 0–90 days
39.56 31.0 25.51
3.6 2.83 3.0
25.01
2.5
Hot summer Warm winter
Guangzhou
ATCM ¼ 10 C ATCM ¼ 0–10 C ATCM ¼ 0–10 C ATHM ¼ 25–30 C ATCM ¼ 0–13 C ATHM ¼ 18–25 C ATCM 10 C ATHM ¼ 25–29 C
NDAT5 ¼ 100–200 days
25.01
2.2
Kunming
Note: ACTM ¼ average temperature in the coldest month; ATHM ¼ average temperature in the hottest month; and NDAT5 ¼ number of days that average temperature is below 5 C; NDAT5 ¼ number of days that average temperature is above 25 C.
The climate of China is extremely diverse and variable with a tropical climate in the south and a sub-arctic one in the north. The design codes are also different for the five thermal zones. Based on previous research [10,11], this paper assesses the natural ventilation cooling potential using the Thermal Resistance Ventilation (TRV) model for office buildings in the five climate zones in China. The method can perform a quick assessment and therefore is very useful for strategic natural ventilation design.
Winter Zone and has a North Asian hot monsoon climate, with four distinctive seasons. The city has a lot of sunshine. Kunming is located in the Mild Zone and its climate is characterized by the distinctive feature of only a slight variation in annual temperatures. Guangzhou is located in the Hot Summer and Warm Winter Zone with a subtropical monsoon climate. The climate characteristics of the five cities are listed in Table 1 [13]. The hourly meteorological data generated from the Meteonorm software package are used for climate data inputs in the program.
2. Research method 2.1. Climate zones in China China is a large country with an area of about 9.6 million km2. About 98% of the land area stretches between a latitude of 20 N– 50 N, from subtropical zones in the south to the temperate zones (including warm-temperate and cool-temperate) in the north [12]. Five cities, Harbin, Beijing, Shanghai, Guangzhou and Kunming have been selected to represent the five climate zones, which have been presented in the ‘‘Thermal Design Code’’ [13] for buildings in China. These five climate zones are categorized into the Very Cold Zone, Cold Zone, Hot Summer and Cold Winter Zone, Hot Summer and Warm Winter Zone and Mild Zone (see Fig. 1). Harbin is located in the Very Cold Zone with a continental monsoon climate. The coldest month’s average air temperature is below minus 10 C. The hottest monthly average air temperature is 24.5 C. Beijing is located in the Cold Zone with a climate of the continental type, with cold and dry winters, due to the Siberian air masses that move southward across the Mongolian Plateau. The summers are hot owing to warm and humid monsoon winds from the southeast. Shanghai is located in the Hot Summer and Cold
2.2. Description of the Thermal Resistance Ventilation model It is rarely practical to perform a detailed thermal simulation procedure to assess the potential of natural ventilation at the architectural strategic design stage. However, it is very important for architects to evaluate the potential of natural ventilation in design by taking into account factors such as the glazing ratio, fabric U value, climate, thermal mass, the internal gain and occupancy pattern and ventilation profiles. Thermal and airflow interactions are characteristic of natural ventilation airflow systems. A simplified, coupled, thermal and airflow model has been developed by integrating the British Standard natural ventilation calculation method [2,14] for a single zone within a four-node thermal resistance network model, which is called the Thermal Resistance Ventilation (TRV) model [9]. The heat flow of the room has been modelled as a four-node, simplified, thermal resistance network model (TR Model), originally developed for LT Europe (Baker and Yao), and is an energy design tool requiring minimal data input [15,16]. The simplified model focuses on a single zone with parameters (such as glazing area, fabric U value, internal gains, lighting datum level, thermostat setting, occupancy profile, thermal mass type, etc) to replicate those chosen for the proposed design. The detailed description of the TRV model is presented in Ref. [9]. The integrated thermal and airflow model has been computerized using the Visual Basic language. The equations are listed in Appendix 1. The TRV model can generate detailed hourly data of an indoor thermal environment to evaluate the natural ventilation cooling potential of a given location and proposed building
Table 2 Mean monthly outdoor air temperature (source [17]) and acceptable comfort temperatures (italicized), C. City
Fig. 2. Acceptable operative temperature ranges for naturally conditioned spaces [19].
Harbin Beijing Shanghai Kunming Guangzhou
May
June
July
August
September
Taout
Tcom
Taout
Tcom
Taout
Tcom
Taout
Tcom
Taout
Tcom
16.5 23.1 20.8 17.0 26.5
22.9 25.0 24.2 23.1 26.0
22.2 25.7 24.2 20 27.2
24.7 25.8 25.3 24.0 26.2
24.5 27.3 29.7 20.9 28.2
25.4 26.3 27.0 24.3 26.5
21.7 25.8 27 20.5 29.1
24.5 25.8 26.2 24.2 26.8
15.8 21.2 24.9 19.4 27.8
22.7 24.4 25.5 23.8 26.4
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Table 3 U value of envelope in each zone (W/m2 K) [20,21].
External wall Glazing
Very Cold Zone (Harbin)
Cold Zone (Beijing)
Hot Summer and Cold Winter Zone (Shanghai)
Mild Zone (Kunming)
Hot Summer and Warm Winter Zone (Guangzhou)
0.4–0.52
0.82–1.16
1.0–1.5
1.5
1.5
2.5
4
3.2–4.7
6.0
6.0
information such as thermal mass, U values of envelopes and internal gains.
2.3. Input requirements For the building simulation and the evaluation of the potential of natural ventilation, basic building information is required, which is described below. 2.3.1. Climate data The climate data includes horizontal diffuse solar radiation, normal beam solar radiation, air temperature, wind speed and direction and humidity. This information is embedded within the programme.
(1) Daytime ventilation to cool the building interior and occupants by replacing or diluting warm indoor air with cool outdoor air. The approach can improve the occupants’ thermal comfort by increasing convective and evaporative heat transfer, and by decreasing the indoor air temperature. This scenario may be suitable for buildings where night ventilation is not achievable due to, for example, security reasons. (2) Nighttime ventilation to cool the building interior by precooling the thermal mass of the building fabric or a thermal storage system with cool nighttime outdoor air. This approach can reduce the peak air temperature, structural temperature and create a time lag of the peak temperature. This system is suitable for buildings where daytime ventilation is not advisable due to external air and noise pollution. (3) Combined daytime and nighttime ventilation. 2.5. Assessment index – natural ventilation cooling potential Indices for the natural ventilation cooling potential (NVCP) are proposed in this paper. The NVCP is defined as the ratio of the number of hours within the comfort zone over the total occupied hours. To assess natural ventilation cooling potential, it is necessary to obtain two kinds of information, which are the (1) acceptable comfort level, and (2) actual internal comfort temperatures.
2.4. Natural ventilation profiles
2.5.1. Acceptable comfort temperature ASHRAE Standard ‘‘thermal environmental conditions for human occupancy’’ 55-2004 [18,19] proposes the optional method for determining acceptable thermal comfort in a naturally conditioned space. It assumes that the thermal conditions of the space are regulated primarily by the occupants through the opening and closing of windows. Allowable indoor operative temperatures for spaces that meet these criteria may be determined from Fig. 2. This figure includes two sets of operative temperature limits – one for 80% acceptability and one for 90% acceptability. The 80% acceptability limits are for typical applications and will be used when other information is not available. In this paper, the ASHRAE Standard 55-2004, acceptable operative temperature ranges for naturally conditioned spaces, is used to assess the cooling potential of natural ventilation. The algorithm used to calculate the comfort temperature is as follows [18,19]:
Three scenarios are considered for the natural ventilation regimes.
Tcom ¼ 0:31Taout þ 17:8
2.3.2. User and building information This requires the information of occupancy pattern, the internal heat gain and the acceptable comfort temperature. Building information includes building orientation, room dimensions, glazing ratio, envelope U value and thermal mass type, such as heavy, medium or light. (In this paper, for generic office buildings in China, the medium weight construction is considered.) The building location, such as open flat country, country with scattered windbreaks, urban or city, needs to be identified. 2.3.3. Ventilation strategy This will include strategies such as daytime ventilation, nighttime ventilation and combined day and night ventilation.
(1)
Table 4 Cooling potential of natural ventilation of Harbin (%) (medium thermal mass). Profiles
Internal gain
Cross/single
May HS
Day only
Low gain Medium gain High gain
June LS
HS
July LS
September
Summer
HS
LS
HS
August LS
HS
LS
HS
LS
Cross Single Cross Single Cross Single
96 81 96 75 91 58
100 89 97 81 96 66
92 60 88 43 76 25
94 73 93 68 87 38
59 31 54 20 49 12
73 44 66 35 54 19
53 11 45 5 29 3
75 26 68 17 45 5
85 49 77 42 69 30
90 60 87 53 79 39
77 46 72 37 63 26
86 58 82 51 72 33
Night only
Low gain Medium gain High gain
Single Single Single
91 82 68
94 90 75
77 60 40
88 77 50
40 30 17
55 42 25
33 22 11
48 35 19
62 56 44
73 63 50
61 50 36
72 61 44
Day and night
Low gain
Cross Single Cross Single Cross Single
100 95 100 94 97 82
100 97 100 96 100 89
99 88 97 83 94 70
100 92 100 89 97 80
81 57 79 51 75 40
87 74 85 67 80 52
85 50 81 44 71 28
93 69 90 58 81 42
97 80 96 75 95 64
98 88 98 82 96 75
92 74 91 69 86 57
96 84 95 78 91 68
Medium gain High gain
R. Yao et al. / Renewable Energy 34 (2009) 2697–2705
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Table 5 Cooling potential of natural ventilation Beijing (%) (medium thermal mass). Profiles
Day only
Internal gain
Low gain Medium gain High gain
Cross/single
May
September
Summer
HS
LS
June HS
LS
July HS
LS
August HS
LS
HS
LS
HS
LS
Cross Single Cross Single Cross Single
77 53 71 42 59 29
89 66 83 43 70 40
40 27 37 19 31 13
53 32 46 28 37 17
35 9 24 6 15 3
48 17 42 12 25 7
35 9 28 6 17 4
54 18 45 13 30 6
84 52 78 43 64 22
89 68 87 56 79 40
54 30 48 23 37 14
67 40 61 30 48 22
Night only
Low gain Medium gain High gain
Single Single Single
67 57 40
78 68 49
36 27 15
46 38 22
17 11 5
27 20 9
20 15 8
28 21 13
59 45 27
69 60 36
40 31 19
50 41 26
Day and night
Low gain
Cross Single Cross Single Cross Single
96 77 91 71 83 61
99 87 99 80 92 68
69 48 66 42 60 33
78 57 74 52 67 42
65 30 57 24 49 15
76 46 71 40 61 26
67 32 61 28 51 21
78 48 74 41 63 29
91 79 89 72 86 57
97 86 96 81 89 71
78 53 73 47 66 37
86 65 83 59 74 47
Medium gain High gain
where, Tcom ( C) is the acceptable comfort temperature, the indoor operative temperature is used as an index. Taout ( C) is the monthly average outdoor air temperature. According to Eq. (1) and the monthly average outdoor air temperature of each city, the acceptable comfort temperature levels have been worked out and listed in Table 2. 2.5.2. Indoor operative temperature The operative temperature is the average of the air temperature and the mean radiant temperature weighted, respectively, by the convective heat transfer coefficient and the linearized radiant heat transfer coefficient for an occupant [18]. For occupants engaged in near-sedentary physical activity (with metabolic rates between 1.0 met and 1.4 met), not in direct sunlight, and not exposed air velocity greater than 0.2 m/s, the relationship can be approximated with acceptable accuracy by [19]:
To ¼ ðTa þ Tr Þ=2
(2)
where, To is the operative temperature ( C), Ta is the indoor air temperature ( C) and Tr is the mean radiant temperature ( C). The TRV model can generate the indoor air temperature and internal surface temperature on an hourly basis, therefore the operative temperature can be calculated using Eq. (2).
3. Natural ventilation cooling potential in the Chinese climate context Based on the discussion in Section 2, to assess the NVCP of office buildings in China, the indices of High Standard NVCP (NVCPHS) and Low Standard NVCP (NVCPLS) have been proposed, of which the thermal comfort meets 90% and 80% acceptability, respectively. The summer season is assumed to be from the 1st May to the 30th of September. The TRV natural ventilation assessment program has been used to carry out the dynamic simulations to generate robust internal temperatures in order to investigate the cooling potential of natural ventilation in the offices in the five climate zones in relation to the parameters of the ventilation profiles and regimes, internal gains, thermal mass and transmittance, etc. 3.1. Basic information Office rooms in China can be grouped into two types: one is the high standard, fully central air-conditioned, cell office whilst the other is the traditional office room equipped with a split room airconditioner, which will operate when needed. A typical office room is selected for this study, of which the required information for thermal simulation is as follows: Room dimension is 3.6 m in length, 5.4 m in depth and 3.0 m in height.
Table 6 Cooling potential of natural ventilation of Shanghai (%) (medium thermal mass). Profiles
Internal gain
Cross/single
May HS
Day only
Low gain Medium gain High gain
June LS
July
August
September
HS
LS
HS
LS
HS
LS
HS
LS
Summer HS
LS
Cross Single Cross Single Cross Single
93 75 88 67 81 43
99 86 96 78 89 61
45 28 38 27 30 24
63 32 53 29 40 25
20 5 14 2 9 1
31 8 25 7 18 3
10 0 6 0 4 0
23 2 15 1 8 0
44 29 40 25 30 15
58 36 51 30 40 24
42 27 37 24 31 17
55 33 48 29 39 23
Night only
Low gain Medium gain High gain
Single Single Single
79 71 45
87 79 58
33 30 25
41 35 31
7 4 2
15 9 4
2 2 0
6 4 2
33 27 16
41 34 25
31 27 18
38 32 24
Day and night
Low gain
Cross Single Cross Single Cross Single
98 87 96 82 91 73
100 95 99 90 96 82
70 40 67 35 56 31
81 56 79 47 69 37
40 15 37 11 30 8
58 27 51 21 40 13
36 7 32 5 24 3
44 17 42 12 37 6
67 43 65 37 55 32
77 58 74 49 67 37
62 38 59 34 51 29
72 51 69 44 62 35
Medium gain High gain
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Table 7 Cooling potential of natural ventilation of Kunming (%) (medium thermal mass). Profiles
Day only
Internal gain
Low gain Medium gain High gain
Cross/single
May
June
July
August
September
Summer
HS
LS
HS
LS
HS
LS
HS
LS
HS
LS
HS
LS
Cross Single Cross Single Cross Single
93 80 91 68 81 46
96 92 96 84 91 62
94 78 90 67 79 48
97 91 96 84 89 61
95 82 92 72 84 57
97 94 96 87 92 68
91 81 87 76 81 60
95 88 92 82 86 72
92 81 89 72 83 60
97 88 95 83 89 69
93 80 90 71 82 54
96 91 95 84 89 66
Night only
Low gain Medium gain High gain
Single Single Single
83 70 49
92 84 60
82 72 51
93 83 62
83 75 57
93 83 69
87 79 61
91 87 71
84 76 64
90 84 71
84 74 56
92 84 67
Day and night
Low gain
Cross Single Cross Single Cross Single
97 91 96 86 93 69
99 96 98 93 96 80
97 91 96 87 91 70
99 97 98 93 96 84
97 93 96 86 93 73
99 97 98 94 96 85
96 90 94 86 90 76
98 94 98 91 93 84
97 89 96 86 92 76
100 94 98 90 97 85
97 91 96 86 92 73
99 96 98 92 96 84
Medium gain High gain
Orientation is south–north. Glazing ratio in the south wall is 0.35 and 0.25 in the north wall. Occupied period is from 08:00 to 18:00. Thermal mass is assumed as medium (some heavy structures, e.g. concrete slab with wooden floor or light-weight concrete walls). Internal shading devices are applied to the southern window in summer. The office room is located in a city. The office is assumed to be medium gain (25 W/m2). The internal gain has been classified as low gain with 15 W/m2, medium gain with 25 W/m2 and high gain with 35 W/m2. Table 3 lists the U value of the building envelopes in each zone according to the Chinese design regulations [20,21]. 3.2. Results’ analysis Fifteen cases of different ventilation profiles and types in conjunction with different internal gains for the five cities have been simulated using the TRV model. The results of the high standard and low standard cooling potential of each case are listed in Tables 4–8. The monthly detailed results are very useful for
architects in designing natural ventilation for their buildings. From the simulation results we can summarise the following: (1) The natural ventilation cooling potential (NVCP) is greatly influenced by the combined effect of the local climate, the thermal properties of the building, ventilation types and profiles. The climate with the lower monthly mean external air temperature has the higher NVCP. For example, among these five cities, Kunming and Harbin have lower monthly mean external air temperatures than other cities and therefore they have higher NVCPs. However, the building’s thermal properties also have a significant impact. For example, the monthly mean external air temperatures in both Harbin and Kunming are about the same, but the NVCP in Kunming is greater than that in Harbin. This is because the insulation level of the external envelope (wall and window) in Harbin is much higher than in Kunming due to the Chinese building design regulations. (2) Internal heat gain has a significant impact on NVCP and should be considered in the strategic design stage. (3) In the Very Cold Zone, Day and Night cross-ventilation is sufficient for summer cooling in most cases. In the hottest months, July and August, the single-sided ventilation has less cooling potential. Thus Day only or Night only natural ventilation is not sufficient for office cooling in July and August. Therefore,
Table 8 Cooling potential of natural ventilation of Guangzhou (%) (medium thermal mass). Profiles
Day only
Internal gain
Low gain Medium gain High gain
Cross/single
May
September
Summer
HS
LS
June HS
LS
July HS
LS
August HS
LS
HS
LS
HS
LS
Cross Single Cross Single Cross Single
71 37 62 22 47 6
87 57 81 43 63 18
39 10 30 6 12 2
62 19 50 13 32 6
34 11 31 5 19 1
45 28 42 17 31 5
14 1 8 1 3 0
30 5 24 3 11 1
5 0 3 0 1 0
16 2 11 1 5 0
33 12 27 7 16 2
48 22 42 15 28 6
Night only
Low gain Medium gain High gain
Single Single Single
38 23 10
60 41 21
14 8 3
26 18 7
15 9 3
27 18 9
4 3 1
11 8 3
2 1 1
5 4 2
15 9 4
26 18 8
Day and night
Low gain
Cross Single Cross Single Cross Single
88 58 81 47 68 27
95 71 92 62 83 44
64 24 57 19 45 10
82 44 75 31 62 20
47 28 44 20 36 11
55 37 53 33 47 20
38 11 31 8 23 4
61 21 53 17 40 10
25 4 21 3 14 2
42 11 36 8 28 5
52 25 47 19 37 11
67 37 62 30 52 20
Medium gain High gain
R. Yao et al. / Renewable Energy 34 (2009) 2697–2705
36
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Temperature Distribution during the office hours in July
34
Temperature °C
32
NVCPLS=80%
30 28
NVCPHS=63%
26 24 22 20 18
8 13 18 12 17 11 16 10 15 9 14 8 13 18 12 17 11 16 10 15 9 14
Time (hour) Fig. 3. Temperature distribution of Beiqijia Low-Energy Demonstration Building in July.
(5)
(6)
(7)
(8)
36
to the residential building studied by Graca [8] because Beijing has a greater day–night external temperature difference than Shanghai. 4. Case study A case study for a natural ventilation strategic design has been conducted for a Ministry of Construction Demonstration Project in Beijing. The exemplar low energy exhibition, conference and administrative headquarters for the Future House Exposition Site are in Beiqijia on the 5th Ring Road. The whole construction area of the building is about 7000 m2. The building has a north–south orientation. The building information is similar to the typical office building described above. The only difference is the U value of the building fabric. As this is an energy efficiency demonstration office building, a higher insulation level compared to the building regulations is proposed. The U value of the external wall is 0.35 W/m2 K instead of the standard one of 0.82 W/m K and the U value of the windows is 2.0 W/m2 K instead of standard one of 4 W/m2 K in order to achieve overall energy efficiency. An advanced shading device is proposed in this design. The internal gain is assumed to be in the medium range. Reviewing the results in Table 5, we can see that the NVCP in Beijing in summer is between 37 and 78% for High Standard and 47–86% for Low Standard. The initial indicators of NVCP demonstrate that there is a great potential to apply natural
Temperature Distribution during the office hours in August
34 32
Temperature °C
(4)
nighttime ventilation should be considered in the Cold Zone for a natural ventilation building when it is applicable. In the Cold Zone and the Hot Summer and Cold Winter Zone, natural ventilation is not sufficient for summer space cooling. A hybrid ventilation system is recommended for offices in this zone in order to use natural ventilation to provide space cooling in the high NVCP period and only operate air-conditioners when the natural ventilation cannot meet the spacecooling requirements. In the mild zone, full natural ventilation for an office building is highly recommended. For the office with a high internal gain, Day and Night cross-ventilation has higher efficiency than the single-sided one. In the Hot Summer and Warmer Winter Zone, natural ventilation cannot meet the space-cooling requirements, so mechanical cooling or air conditioning is desirable. The cooling potential of hybrid natural ventilation should be further studied for different control strategies. This issue is not discussed in this paper. The Night only ventilation profile is not suitable for office buildings, particularly for the office with higher internal gains, because the internal gains released during the daytime cannot be exhausted immediately and may therefore cause discomfort. The natural ventilation cooling potential of office buildings in Beijing is slightly higher than for Shanghai. This result is similar
NVCPLS=81%
30 28 26
NVCPHS=66%
24 22 20 18
8 13 18 12 17 11 16 10 15 9 14 8 13 18 12 17 11 16 10 15 9 14
Time (hour) Fig. 4. Temperature distribution of Beiqijia Low-Energy Demonstration Building in August.
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R. Yao et al. / Renewable Energy 34 (2009) 2697–2705
ventilation in office buildings in Beijing. Based on the analysis from the research output in Section 3, the architectural design team (Short and Associates) was convinced that hybrid natural ventilation in this building could achieve low energy consumption. Simulations for this specific building have been run using the TRV model to assess the NVCP. The structure of the building, from inside to outside, is made of aerated concrete plus 100 polystyrene boards plus an air layer and cladding panels, therefore it has been classified as heavy mass. Based on the building information, a dynamic simulation has been run for this strategic design. Figs. 3 and 4 show the temperature distribution during office hours. The comfort temperature is 26.3 C in July and 25.8 C in August according to Table 2. Therefore the comfort range will be between 22.8 and 29.8 C with low standard and 23.8 and 28.8 C with high standard in July; 22.3 and 29.3 C with low standard and 23.3 and 28.3 C with high standard in August. From the statistics, we can see that the NVCP will be 80% with the low standard level and 63% with the high standard level in July, and 81% with the low standard level and 66% with the high standard level in August. The simulation results give confidence to the designer to implement a hybrid ventilation system (as opposed to a fully airconditioned solution) in this Low-Energy Demonstration Building design. In order to enhance the natural ventilation, stack ventilation is proposed and a further detailed energy simulation will be performed using Energy plus. This is not the intention of this paper.
5. Conclusion There is a significant impact of multiple factors of climate, building properties, occupancy and operation profiles on the
performance of natural ventilation. Strategic natural ventilation design should consider not only the climatic conditions but also multiple factors such as the building’s thermal characteristics, the ventilation type and profile and internal gains. Accordingly, for the natural ventilation strategic design, the coupled thermal and airflow thermal resistant ventilation (TRV) model has been used to assess the natural ventilation cooling potential (NVCP) in the five climate zones of China. The outputs from this research will be useful to architects involved in natural ventilation design in China. They can also be useful as natural ventilation strategic design references for similar weather and building condition cases in the other parts of the world. The method of using the TRV model to assess natural ventilation cooling potential (NVCP) is universal though the case studies of natural ventilation strategic design for office buildings have been carried out in the five climate zones of China. The Low-Energy Office Building Demonstration project is the showcase of the application of this robust method for architects to assess natural ventilation potential at the early design stage. The detailed natural ventilation system design should be further analysed using more advanced simulation tools in the detailed design stage.
Acknowledgements The authors wish to acknowledge the funding from the Asia Link ‘‘Centre for Sino-European Sustainable Building Design and Construction’’ project for the collaboration work and the support from the project ‘‘Key Technologies on Control and Improvement of Building Indoor Thermal Environment’’(2006BAJ02A09) funded by the Chinese Ministry of Science and Technology under the Chinese Key R&D National 11th Five-Year Plan Programme.
Auxiliary heating & cooling
ventilation Lighting gains T3 Deep node
T2
V
auxiliary T1
light
Surface node
Ta Ambient node
Room node pc Casual gains
Solar gains
solar
conduction
R. Yao et al. / Renewable Energy 34 (2009) 2697–2705
Appendix 1. Thermal resistance network model [9]
(i) Node Room
C1
dT1 ¼ a1 Fpc þ Fsolar þ Flight þ Fauxilary þ g12 ðT2 T1 Þ dt þ g1a ðTa T1 Þ ðA1Þ
(ii) Node Surface
C2
dT2 ¼ a2 Fpc þ Fsolar þ Flight þ Fauxilary þ g12 ðT1 T2 Þ dt ðA2Þ þ g23 ðT3 T2 Þ
(iii) Node Deep Mass
C3
dT3 ¼ g23 ðT2 T3 Þ þ g3a ðTa T3 Þ dt
g1a ¼ Aglaze Uglaze þ 0:3Vn
(A3) (A4)
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