Thermal analysis of extensive green roofs combined with night ventilation for space cooling

Accepted Manuscript Title: Thermal analysis of extensive green roofs combined with night ventilation for space cooling Author: Lin Jiang Mingfang Tang PII: DOI: Reference:

S0378-7788(16)31412-8 https://doi.org/doi:10.1016/j.enbuild.2017.09.080 ENB 7999

To appear in:

ENB

Received date: Revised date: Accepted date:

2-11-2016 18-9-2017 25-9-2017

Please cite this article as: Lin Jiang, Mingfang Tang, Thermal analysis of extensive green roofs combined with night ventilation for space cooling, (2017), https://doi.org/10.1016/j.enbuild.2017.09.080 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Highlights 

Experimental analysis of green roofs combined with night ventilation is explored.

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79% heat gain can be reduced and 6h heat gain hours can be shortened during a

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day. Air organization for night ventilation plays an important role in cool storage.



Correlations between climate factors and cooling reduction are presented.



Cooling energy saving and operating hours for HVAC reduced are discussed.

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Page 1 of 35

Lin Jiang, Mingfang Tang∗

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Thermal analysis of extensive green roofs combined with night ventilation for space cooling

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Faculty of Architecture and Urban Planning, Chongqing University, Chongqing 400045, P.R. China

Abstract

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Green roofs and night ventilation are well-known passive techniques for energy saving during cooling period. This study refers to the analysis of thermal properties and energy performance of the combination of green roof and night

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ventilation. Firstly, a field experiment was conducted to compare the thermal performance of green roof and bare roof when combining with night ventilation. Three series experiments were carried out to analyze cooling effect of green

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roof with and without night ventilation on summer sunny day and another ex-

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periment with natural ventilation all day long on summer rainy day was also monitored. Then, data from the field study were used to validate the green roof

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and night ventilation model incorporated within a building energy simulation program. This model was then used to estimate the energy saving for an office building in three typical climates. Results show that combining green roofs and night ventilation can significantly reduce the indoor air temperature and heat gains on sunny day but have no appreciable effect on rainy day. Because the cooling potential of green roof and night ventilation are strongly depends on the climate and thermal mass, a simple equation to assess the air flow rate of night ventilation when combined with green roof was proposed. Furthermore, the correlation analysis between weather factors and cooling effect of green roofs as well as the air organization mode for night ventilation are also discussed. ∗ Corresponding

author.Tel.: +86 138 8329 2626. Email address: [email protected] (Mingfang Tang)

Preprint submitted to Journal of Energy and Buildings

September 18, 2017

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Keywords: Green roofs, Night ventilation, Energy saving, Passive cooling,

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Cool storage

1. Introduction

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As a rapidly growing developing country, China is dealing with lots of prob-

lems: the fastest urban growth, environmental deterioration and energy short-

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ages. In 2009, the building sector was responsible for 25% of China’s total primary energy consumption and 18% of the overall China’s GHG emissions [1]. Recently, the building energy consumption in China surpassed the US, and it is

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expected to increase significantly in the next decades, pushed by the demand of new residential buildings [2]. Of the various thermal loads, one from the build-

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ing roof accounts for about 20–40% in China [3], therefore, it is very important to improve the building roof thermal performance. Green roofs, also known as “living roofs ”, “eco-roofs”, “roof gardens”, is an

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efficient method to reduce indoor air temperature and energy consumption[4], which is a living vegetation system installed and grown on a building roof.

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Green roofs can reduce the heat flux through a building envelope, as the plant foliage shades the roof and absorbs part of the thermal energy by photosynthesis,

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the soil substrate acts as an insulation layer with a high thermal capacity and low thermal transmittance, and plants provide transpirational cooling [5, 6]. Then the cooled surface of green roofs, in turn, reduces the heat transferred into the building [7]. Besides, green roofs has many other benefits, such as

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decrease of water runoff [8, 9], mitigation of urban heat island effect (UHI)[10] [11, 12], reduction of CO2 [13] and sound [14, 15], enhancing internal membranes durability [16, 17], providing wildlife habitats for many species [7], and aesthetics [18]. Nowadays, green roofs have been widely used, especially in European, North-American and also some Asian countries [19]. Green roofs is typically

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divided into two categories, including intensive and extensive green roofs [19, 20, 3, 21, 22]. Of the two types, extensive green roofs is most common around the world due to building weight restrictions, costs and maintenance.

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Night ventilation is another traditional and low-cost passive technique that can significantly improve thermal comfort and reduce electricity demand [23]. This technique, which introduces the outdoor cool air to pass through the build-

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ing at night, cools down the indoor air and the building structure and prevents

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overheating the buildings in the following day [24, 25]. In fact, the building structure (walls, partitions, floors and ceilings) acts as a heat storage in the

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daytime and releases the absorbed heat in the nighttime, when the cooling effect of natural ventilation is leading [26]. Field study on real buildings showed that the use of night ventilation in buildings can reduce 20 – 25% of their airconditioning demand or, when air-conditioning is not used, it can reduce peak

‰ [27, 28, 29].

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indoor temperature by up to 3

Based on current literature energy saving of green roofs is still the main driving force for which they are promoted and adopted. Santamouris et al.

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[30] undertook a experimental investigation and simulation analysis of a green roof system installed in a nursery school building in Athens. The simulations

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revealed that 12–87% cooling energy consumption were saved by using green roof

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in summer but it has no influence during winter. Coma et al. [31] conducted a experimental research for more than one year in Mediterranean continental

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climate. Results show that both extensive green roof cubicles can save 16.7% and 2.2% energy consumption in summer, 6.1% and 11.1% in winter. Yang et al. [32] found that the green roofs can reduce 15.2% cooling energy demand than the conventional one in a typical sub-tropical climatic. A simulation conducted in

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Mily through EnergyPlus software, the use of an extensive green roof provided with an irrigation system leads to a reduction by about 10% in the annual primary energy needs for cooling and heating [33]. However, the cooling and heating effect of green roof strongly depends on

the climates as well as the characteristics of plants and roof structure. Getter et 55

al. [34] conducted a field study in a Midwestern U.S. climate during four seasons for a year. They found that green roof reduced heat flux through the building envelope by an average 67% during summer while in winter the reduction was only 13%. Peak temperature differences between gravel and green roof were 3

Page 4 of 35

larger in summer than other seasons (sometimes by as much as 20 60

‰). In winter,

temperatures measured at the top of the insulation layer were found to be more

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variable for roofs without snow cover than with snow cover. In autumn and

spring, temperatures variation between green roofs and gravel roof was similar.

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A comparison was conducted to qualify the energy performance of intensive,

semi-intensive and extensive green roofs in Mediterranean climate by Silva et al.[35]. According to their findings, the three green roof types lead to similar

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heating energy needs but extensive green roof solution shows higher cooling energy needs than semi-intensive and intensive ones, of 2.8 and 5.9 times more.

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The green roof simulation module has been successfully implemented in the EnergyPlus building energy simulation program by Sailor [36]. He found that 70

building energy consumption varies significantly in response to variations in

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growing media depth, irrigation, and vegetation density (LAI). Besides, green roofs energy savings are more relevant for low thermal insulated roofs. In Costanzo’s study [33], the performance of the sample building was

zones, the sensible heat fluxes released by the roof to the outdoor environ-

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assessed through dynamic simulations in three different typical Italian climatic

ment were cut down in each city when using both green roofs (from 42% to

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75%, depending on the climate). Niachou [37] conducted field experiment and simulation using TRNSYS software in a hotel situated in Loutraki region, the experimental data showed that green roof in non-insulated buildings could re-

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duce 10

‰ on the exterior surface compared with the bare roof, while there was

no significant temperature variations between the external surfaces of insulated buildings with and without the implementation of green roof. And the simulation results showed the energy savings up to 48% for non-insulated, 7% for moderate insulated and less than 2% for high-insulated cases were estimated.

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Similar results were obtained by many other reserchers [31], [17], [35], [5] [38], [39], [40], [30]. So in this study, there is no insulation for green roofs. Although there are lots of research studies have been conducted to evaluate the performance and benefits of green roofs and ventilation separately, combing them together is rare. Most researches have been carried out in no ventilation 4

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performance, which is often occurs when using HVAC system. Yang He’s experimental data showed that the indoor air temperature at night was about

‰ higher for green roof than common roof when the windows and doors

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2.5

of both rooms were locked[41]. Because the cool outdoor air temperature and

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sky long wave radiation at night make outer surface of the common roof be cooled quicker and easier than green roof. However, it is often non-occupied

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at night in the office buildings and schools, or for some residential buildings, when the air temperature is cool at night, the people would like to open the window to get fresh air instead of using air-conditioners, so it is very suitable

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and energy-saving to adopt night ventilation. Therefore, combining green roof and night ventilation may be a good solution for energy conservation and indoor air condition improvement.

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The study of La Roche and Berardi[5] is most relevant here. They presented a variable insulation green roof system consisting of a plenum located between a green roof and the room underneath and sensor-operated fan that couples(or decouples) the green roof mass with the indoor environment. Experiments done

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on a few cells have been tested in a hot and dry climate over several years. Results show that the variable insulation system can adjust the thermal capacity

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of roof effectively both in summer and winter. In hot day night, when the fan of the variable insulation system was on, green roofs combined with night

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ventilation lead to more comfortable conditions inside the space, the variable insulation green roof behaved like the uninsulated green roof (cooler). While in cold days, when the fan was turned off, the variable insulation green roof behaved like the insulated roof (warmer). This result confirmed that turning the plenum fan on and off could be used both in summer and winter effectively.

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Another research provided an additional cooling strategy for buildings with green roofs when they remain too warm over night[42]. The water-to-air heat exchangers proved to cool the indoor air in the test cells by almost 10

the exterior temperatures were above 35

‰.

‰ when

In this study, field experiments were carried out at Chongqing University 120

to discuss the temperature reduction and heat flux of the extensive green roofs 5

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combined with night ventilation during the summer time. Then, data from the field study were used to validate the green roof and night ventilation model

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incorporated within a building energy simulation program. This model was

then used to estimate the energy saving for an office building in three typical

climates. The correlation analysis between weather factors and cooling effect of

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green roofs as well as the air organization mode for night ventilation are also

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discussed.

2. Materials and methodology

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2.1. Experimental setup

The extensive green roof was installed in April 2015 on the roof of Archi-

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tectural department hall on the Chongqing University campus in Chongqing (29.6N, 106.5E, southwest of China). Chongqing has a warm and humid climate (K¨ oppen Cwa/Cfa), its summers are long and hot, with highs of 33 –34

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‰ in July and August in the urban area, the extreme temperatures in summer has reached 43 ‰. Winters are short and somewhat mild. So the main energy

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consumption is for cooling in summer. The monthly average precipitation is 175.5mm from May to August.

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The thermal performance of the green roof was experimentally evaluated for

cooling periods in 2015 and 2016. Three series comparison experiments were

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carried out including natural night ventilation by opening vents, mechanical night ventilation by fans, and no night ventilation combined with green roof on sunny day. For investigating the climatic impact, another experiment with natural ventilation all day long on rainy day was also conducted (Fgi.1). The experimental setup consists of two house-like cells (Fig. 2) with identical

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internal volumes (1.3 × 1.0 × 0.9 m). One cell was covered by green roof laid above a 15 cm non-insulated concrete slab, and another one was bare concrete slab (15 cm thick) roof with no insulation, the U -value of the concrete slab is 0.62 W/m2 K. The roof is the only construction system that differs between the two cells. In order to evaluate the roof cooling effects, thermal insulation was

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Rainy day

Close

Close

a

b Mechanical night ventilation by fan

No night ventilation

d

Natural ventilation by opening vents

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Natural night ventilation by opening vents

Close

c

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Sunny day

Figure 1: Three series experiments including natural night ventilation by opening vents,

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mechanical night ventilation by fans and no night ventilation combined with green roof on sunny day and one experiment of natural ventilation with green roof on rainy day.

installed inside walls and ground except both roofs. The walls were constituted

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by polyethylene insulation, brick (24 × 11.5 × 5 cm) and mortar from inside to outside, the U -value of the concrete slab is 1.90 W/m2 K. The floor was covered

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with 5cm thick bricks and polyethylene insulation inside. Each cell has two vents (60 × 4 cm) on south and north. A 12 W fan was installed on a 16 × 155

16 cm opening at the bottom of south wall for each cell, which has two gears

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to switch the fan speed with air change rate of 12 and 30 ACH respectively.

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The ventilation system operated solely during night period from 9 pm to 8 am, cause the outdoor temperature was lower than indoor temperature during this

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period. 160

The extensive green roof was consisted of 9 greenery modules which were

connected together by buckles, each module was 50 × 50 × 6.5 cm (Fig. 3). The greenery module is combined plant layer, substrate layer, filtering membrane, drainage layer and root barrier together (Fig.4). The plant used in this experiment was Bryophyllum, which is a plant genus of the Crassulaceae family,

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about 30 – 150 cm high. The substrate was about 5 cm thick. 2.2. Instrumentation

Fig. 5 shows the location of sensors used to evaluate the thermal behavior during the experiments. The physical parameters measured including: the local meteorological data (air temperature, relative humidity, solar radiation, wind 170

speed); inner and outer surface temperature of concrete slab; leaf surface tem7

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Figure 2: Experimental cells on the roof of Architectural department hall.

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Figure 3: Green roof plant: Bryophyllum. Figure 4: Construction view of green roof module.

perature and soil temperature; heat fluxes of the roofs; solar reflectivity and transmissivity of the plant; wind speed of the vent. And in 2016 summer, soil volumetric water content were also measured. Equipment details and variables measured are summarized in Table 1. All data were recorded at 10 min intervals.

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3. Results

The experimental results allow evaluating and comparing the thermal be-

havior of two cells during summer time. Indoor air temperature, exterior and internal surface temperature of the concrete roof slab, heat flux through both roofs when using natural ventilation, mechanical ventilation with different air 180

change rate and no ventilation at night are compared between green roof and 8

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实验概况 Temperature sensor Heat flux sensor Wind speed sensor

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Canopy

Solar radiation sensor

Substrate Concrete slab

Soil moisture sensor

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Brick wall Vent Insulation layer

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Fan

Green roof

Bare roof

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Figure 5: Sensors location used to evaluate the thermal behavior of the studied cells.

Table 1: Instrumental specifications.

Onset weather station

Type

Variable

Accuracy

S-THB-M002

Temperature

±0.21

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Equipment

‰(0 ‰ to 50 ‰)

S-THB-M002

RH

±2.5% (10% - 90%)

S-WSB-M003

Wind speed

± 1.1m/s

S-LIB-M003

Solar radiation

±10 W/m2 or ±5%

Soil moisture

±0.033 m3 /m3 or ±3.3%

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S-SMD-M005

‰

T type

Temperature

±0.1

Heat flux sensor

HFM-215N

Heat flux

±3%

Onset Solar sensor

S-LIB-M003

Reflectivity of plant

±10 W/m2 or ±5%

S-LIB-M003

Transmissivity of plant

±10 W/m2 or ±5%

AP471 S2

Wind speed of the vent

±0.05 m/s

Onset Solar sensor

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Thermocouple

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Delta Wind speed sensor

bare roof. Pearsons correlation analysis is also presented to identify the relationship between weather factors and thermal performance of green roof in this section.

3.1. Natural night ventilation by opening vents on sunny day

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In this experiment, both cells’ vents on the south and north wall were open

from 21:00 to 8:00 over two sunny days (08/30/2015 – 08/31/2015), the outdoor air could go through the cells to cool indoor air and ceilings, the average air change rate was around 8 ACH. The outdoor weather condition is shown in Fig. 6, maximum value of global horizontal solar radiation was about 660 W/m2 ,

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and outdoor temperature oscillated between 23.7

‰ and 38.0 ‰. Fig. 7 shows

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1800

100

100

1.0

1600

90

90

0.9

1400

80

80

0.8

1200

70

70

0.7

1000

60

60

800

50

600

40

400

30

30

200

20

20

0

10

10

Absorptivity of Bryophyllum

Rate

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)

0

Temperature (

RH (%)

40

Transmissivity of Bryophyllum

0.6

0.5

0.47

0.4

0.38

0.3

0.2

0.15

0.1

0

0.0 0

5

10

Time

Outdoor temperature

15

20

25

30

Number of readings

Outdoor RH

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Solar radiation

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-200

50

00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00

2

Solar radiation (W/m )

Albedo of Bryophyllum

Figure 6: Outside climate conditions

Figure 7: Absorptivity, albedo and

(08/30/2015 – 08/31/2015).

transmissivity of Bryophyllum (08/30/2015 –

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08/31/2015).

absorptivity, albedo and transmissivity of Bryophyllum. The transmissivity is

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0.15, suggesting that the canopy of Bryophyllum blocks and absorbs most of the solar radiation, only 15% of solar radiation went through the canopy.

Fig. 8 shows the temperature comparison between green roof and bare roof

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3.1.1. Temperature distribution

with natural night ventilation (08/30/2015 – 08/31/2015). The exterior concrete slab surface temperature of bare roof is highest in the daytime, the peak

‰, which appears almost at the same time with outdoor

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temperature and leaf surface temperature, the rest of the measured points are

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delayed at various degrees. On account of the thermal storage character of substrate, the peaks of green roof are delayed more than bare roof, and the exterior concrete slab surface temperature of green roof is below 30 the maximum difference is 14

‰ in the whole day,

‰between the two roofs. Additionally, the indoor

temperature of green roof is significantly lower than bare roof, the maximum

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indoor temperature difference is 5.0 ifference is 3.0

‰, and the average indoor temperature d-

‰from 8:00 to 21:00 when vents were closed. When opening the

vents to operate natural night ventilation, the indoor temperature of both cells declines obviously, which gets highly close for about 3 hours later. For the green roof, when the vents closed from 8:00 to 21:00, the exterior 10

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concrete slab surface temperature of green roof is lowest, vertical temperature sequence is: exterior surface of roof slab< interior surface of roof slab < indoor

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air < leaf surface< outdoor air, indicating that the substrate cool the indoor space in daytime; when the night ventilation operated, the outdoor temperature

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is lowest, as the cool air from outside flowing into the cell, the indoor tempera-

ture is lower than interior concrete slab surface temperature. Therefore, green

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roof dominants the daytime thermal reduction, while the outdoor air is the main cooling source for indoor space in night ventilation period.

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40

35

30

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Temperature (

)

45

25

Natural ventilation

00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00

20

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Time Indoor temperature of green roof

Exterior concrete slab surface temperature of green roof

Indoor temperature of bare roof

Exterior concrete slab surface temperature of bare roof

Outdoor temperature

Internal concrete slab surface temperature of green roof

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Leafsurface temperature

Internal surface temperature of bare roof

Figure 8: Temperature comparison between green roof and bare roof with natural night

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ventilation (08/30/2015 – 08/31/2015).

For the bare roof, the exterior and interior concrete slab surface temperature

of bare roof is lower than green roof at night, and even lower than outdoor

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temperature during 3:00 – 8:00. That is because the cool air can flow upon the bare roof directly, the heat convection and sky long wave radiation makes the surface temperature lower. The indoor air temperature of bare roof fluctuates intensively, the variation of exterior concrete slab surface is up to 12 the variation of green roof is just 2

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‰. However,

‰. Thus it can be seen that green roof

provides better thermal protection against solar radiation and high outdoor temperature in the hot daytime, which makes the roof in a relatively steady state. However, the disadvantage is that, the green roof insulates the room at

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night, preventing the heat dissipating from the cell. So using night ventilation

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is a commendable solution to improve indoor environment at night. 3.1.2. Heat flux

Data on 08/31/2015 are selected for the heat flux analysis. Fig. 9 and Table

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2 show that there are two sections in green roof heat transfer process, and three

sections in bare roof heat transfer process in a whole day. To the green roof,

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there are: (1) heat gain period: 0:00 – 8:00 (8h), (2) heat loss period: 8:00 – 24:00 (16h). To the bare roof, there are: (1) heat gain period: 0:00 – 6:30

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(6.5h), (2) heat loss period, 6:30 – 15:30 (9h), (3) heat gain period: 15:30 – 24:00 (8.5h). It is notable that the heat transfers out of the green roof invariably from 8:00 to 24:00, while the heat transfers out of bare roof from 6:30 to 15:30, and

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then reverses at 15:30, which makes the heat gain period of bare roof 6h more than green roof. And this situation makes the indoor environment of bare roof worse than green roof, especially in the afternoon. In addition, as shown in

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Fig. 9 (a), the heat gain period of green roof is 0:00 – 8:00, during which the night ventilation is operated, and the interior surface temperature of green roof

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is slightly higher than indoor temperature, so the heat is transferred from green roof to indoor space, the total heat gain in this period is just 18.9 W/m2 , and the

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value of bare roof is 24.5 W/m2 . This is because the bare roof stored more heat in the daytime, the interior surface temperature of the roof slab is higher than outdoor air temperature, especially at the beginning when the night ventilation was operated. Table 2 also shows that green roof can reduce 75% heat gain

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compared with bare roof in a whole day. Table 2: Comparison of heat gain and loss with natural night ventilation (08/31/2015).

Green roof Date

8.31

Heat gain (W/m2 )

Heat loss (W/m2 )

Heat

Hours of heat gain

Hours of heat loss

Heat

gain/loss

Period

Hours (h)

Period

Hours (h)

reduction

18.9

208.8

0.1

0:00-08:00

8

08:00-24:00

16

75%

Heat gain (W/m2 )

Heat loss (W/m2 )

Heat

Hours of heat gain

Hours of heat loss

gain/loss

Period

Hours (h)

Period

Hours (h)

80.2

79.0

1.0

00:00-06:30

15

06:30-15:30

9

Bare roof Date

8.31

15:30-24:00

12

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-00:59 00:00 00:59 01:59 02:59 03:59 04:59 05:59 06:59 07:59 08:59 09:59 10:58 11:58 12:58 13:58 14:58 15:58 16:58 17:58 18:58 19:58 20:57 21:57 22:57 23:57 24:57 40

50

(b)

(a) 40

40

50

40 35

)

Heat loss 0

20

Heat gain

-10

-10

15

15 -20

Natural ventilation

Natural ventilation -30

00:00

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18:00

17:00

16:00

15:00

14:00

13:00

12:00

11:00

10:00

09:00

08:00

07:00

06:00

05:00

04:00

03:00

02:00

01:00

10

00:00

00:00

23:00

22:00

21:00

20:00

19:00

18:00

17:00

16:00

15:00

14:00

13:00

12:00

11:00

10:00

09:00

08:00

07:00

06:00

05:00

04:00

03:00

02:00

01:00

00:00

10

23:00

-20

-30

20

Heat gain

22:00

Heat gain

Temperature (

25

10

21:00

Heat loss 0

30 20

20:00

25

10

19:00

30 20

30

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)

2

Heat flux (W/m )

30

Temperature (

2

Heat flux (W/m )

35

Time

Time Internal concrete slab surface temperature of green roof

Outdoor temperature

Internal concrete slab surface temperature of bare roof

Outdoor temperature

Indoor temperature of green roof

Heat flux of green roof

Indoor temperature of bare roof

Heat flux of bare roof

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Figure 9: Comparison of heat flux transferred through roof surfaces with natural night ventilation on 08/31/2015.

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Moreover, the value of heat gain and loss of bare roof is nearly equal, the ratio of heat gain and loss is close to 1. On the contrary, heat loss of green roof is far more than heat gain, the ratio of heat gain and loss is just 0.1. It means, green

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roof plays an important role in reducing heat gain, especially in the daytime; although the heat transfer reversed when the night ventilation was operated, the outdoor cool air made the indoor air temperature below the interior roof

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surface temperature, which is positive for indoor thermal environment. 3.2. Mechanical night ventilation on sunny day

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In this field measurement, both cells’ vents on the north wall were open, 260

the south vents were closed, and the fans on south wall were operated from 21:00 to 8:00 over three sunny days (08/22/2015 – 08/24/2015). The outdoor meteorological condition is shown in Fig. 10. Maximum value of global horizontal solar radiation was about 700 W/m2 , outdoor air temperature ranged between 24.2

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‰ and 34.7 ‰. The air change rate was 30 ACH in 08/22/2015 –

08/23/2015, and 12 ACH on 08/24/2015. The absorptivity, albedo and transmissivity of Bryophyllum is shown in Fig. 11, only 20% of solar radiation passed the canopy. 3.2.1. Temperature distribution Fig. 12 shows the temperature comparison between green roof and bare

270

roof with mechanical night ventilation (08/22/2015 – 08/24/2015). It indicates 13

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100

100

1.0

1400

90

90

0.9

80

80

0.8

70

70

0.7

60

60 50

)

0.6

0.5

600

40

400

30

30

200

20

20

0.2

10

10

0.1

0

0

0.0

00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00

0

40

ip t

50

Transmissivity of Bryophyllum

0.42

0.4

0.38

0.3

0.20

0

5

10

15

Time Outdoor temperature

20

25

30

35

Number of readings

Outdoor RH

us

Solar radiation

cr

800

Rate

1000

Absorptivity of Bryophyllum Albedo of Bryophyllum

Temperature (

1200

RH (%)

1600

2

Solar radiation (W/m )

00:00 01:00 02:00 03:01 04:01 05:02 06:02 07:03 08:03 09:04 10:04 11:05 12:05 13:06 14:06 15:07 16:07 17:08 18:08 19:09 20:09 21:10 22:10 23:11 24:11 25:12 26:12 27:12 28:13 29:13 30:14 31:14 32:15 33:15 34:16 35:16 36:17 37:17 38:18 39:18 40:19 41:19 42:20 43:20 44:21 45:21 46:22 47:22 48:23 49:23 50:24 51:24 52:24 53:25 54:25 55:26 56:26 57:27 58:27 59:28 60:28 61:29 62:29 63:30 64:30 65:31 66:31 67:32 68:32 69:33 70:33 71:34 72:34

Figure 10: Outside climate conditions

Figure 11: Absorptivity, albedo and

(08/22/2015 – 08/24/2015).

transmissivity of Bryophyllum (08/22/2015 –

an

08/24/2015).

that, the variation tendency of mechanical night ventilation is similar to natural

temperature difference is up to 5.7 is 3.3

‰, the average indoor temperature difference

‰ from 8:00 to 21:00 when there is no ventilation.

While when the air

change rate reduced to 12 ACH, the maximum indoor temperature difference is

‰, the average indoor temperature difference is 2.6 ‰ from 8:00 to 21:00.

te

3.6

d

275

M

night ventilation. When the air change rate is 30 ACH, the maximum indoor

Therefore, increasing the air change rate when using mechanical ventilation at

Ac ce p

night will reduce the indoor temperature more efficiently in the following day. 3.2.2. Heat flux

280

As shown in Fig. 13 and Table 3, when the air change rate increases from

12 ACH to 30 ACH, the roof structure stored more cooling energy at night, and to both green roof and bare roof, the hours of heat gain is 1 hour less than 12 ACH. The green roof can reduce 75% and 79% heat gain with 12 ACH and 30 ACH respectively. Moreover, when air change rate increased, the heat gain and

285

loss increase simultaneously by both green roof and bare roof. Table 3 reports the ratio of heat gain and loss is always 0.1 for green roof as the air change rate increases, but the ratio is 0.9 and 1.2 respectively for bare roof. It shows that, increasing the air change rate can strengthen heat exchange, but having less influence to heat gain and loss ratio of green roof. This is due to the thermal 14

Page 15 of 35

45

ip t

35

30

25 30 ACH

12 ACH

Time

us

00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00

30 ACH

20

cr

Temperature (

)

40

Indoor temperature of green roof

Exterior concrete slab surface temperature of green roof

Indoor temperature of bare roof

Exterior concrete slab surface temperature of bare roof

Internal concrete slab surface temperature of green roof

Leafsurface temperature

Internal surface temperature of bare roof

an

Outdoor temperature

Figure 12: Temperature comparison between green roof and bare roof with mechanical night

290

M

ventilation (08/22/2015 – 08/24/2015).

stability of the green roof.

d

3.3. No night ventilation on sunny day

In order to compare the effect of night ventilation, no night ventilation

te

measurement was also conducted over two sunny days from 09/03/2015 to 09/04/2015. The meteorological conditions are shown in Fig.14. In Fig.15, the comparison of indoor temperature, internal roof surface temperature and heat

Ac ce p

295

flux through the inner roof surface are presented. It is worthy to be noticed that the indoor temperature and internal roof surface temperature is nearly equal when there is no night ventilation from 0:00 to 8:00. The heat gain at night is less than adopting night ventilation strategy for both green roof and bare roof.

300

Moreover, from 8:00 to 24:00, the indoor air temperature of green roof is always higher than interior surface temperature. Comparing the night ventilation strategy, as the night ventilation was operated, the indoor air temperature will decline, and lower than interior roof surface temperature later. Table 4 summarizes the difference between indoor and outdoor temperature

305

among no ventilation, natural ventilation and mechanical ventilation in nighttime. The average indoor and outdoor temperature difference of green roof is

15

Page 16 of 35

50

40

40

ip t

(b) 40

40

)

10

25

Heat loss 0

Heat gain

-10

-10 15

15

-20

Fan ventilation

12 ACH

00:00

23:00

22:00

21:00

20:00

18:00

17:00

12:00

11:00

10:00

09:00

08:00

07:00

06:00

05:00

04:00

03:00

00:00

10

40

Internal concrete slab surface temperature of bare roof

Outdoor temperature

Indoor temperature of bare roof

Heat flux of bare roof

50

(c)

(d) 40

40

20

10

25

Heat loss 0 20

Heat gain

30

20

an

30

Temperature (

2

2

)

30

Heat flux (W/m )

35

35

30

10

25

Heat loss

0

Heat gain

-10

40

)

Heat flux of green roof

Temperature (

Outdoor temperature

Indoor temperature of green roof

us

Time

Time Internal concrete slab surface temperature of green roof

50

20

Heat gain

-10

15 -20

15

-20

Fan ventilation

30 ACH

30 ACH

30 ACH

00:00

23:00

22:00

21:00

20:00

19:00

18:00

17:00

16:00

15:00

14:00

13:00

10

12:00

11:00

10:00

09:00

08:00

07:00

06:00

05:00

00:00

04:00

-30

00:00

23:00

22:00

21:00

20:00

19:00

18:00

17:00

16:00

15:00

14:00

13:00

12:00

11:00

10:00

09:00

08:00

07:00

06:00

05:00

04:00

03:00

02:00

01:00

00:00

10

03:00

30 ACH

02:00

Fan ventilation -30

01:00

Heat flux (W/m )

12 ACH

12 ACH

-30

00:00

23:00

22:00

21:00

20:00

19:00

18:00

17:00

16:00

15:00

14:00

13:00

12:00

11:00

10:00

09:00

08:00

07:00

06:00

05:00

04:00

03:00

02:00

01:00

00:00

10

02:00

12 ACH

01:00

Fan ventilation

16:00

-20

-30

20

Heat gain

cr

20

Heat gain

15:00

Heat loss 0

30

20

14:00

25

30

13:00

10

Temperature (

30 20

Temperature (

2

Heat flux (W/m )

)

35

2

Heat flux (W/m )

35 30

19:00

50

(a)

Outdoor temperature

Internal concrete slab surface temperature of bare roof

Outdoor temperature

Indoor temperature of green roof

Heat flux of green roof

Indoor temperature of bare roof

Heat flux of bare roof

M

Time Internal concrete slab surface temperature of green roof

Time

Figure 13: Comparison of heat flux transferred through roof surfaces with mechanical night

Ac ce p

te

d

ventilation (08/22/2015, 08/24/2015).

Table 3: Comparison of heat gain and loss with mechanical night ventilation (08/22/2015, 08/24/2015). Green roof ACH (h−1 )

12

30

Heat gain (W/m2 )

14.9

19.8

Heat loss (W/m2 )

155.5

Heat

Hours of heat gain

Hours of heat loss

Period

Hours (h)

Period

Hours (h)

01:00-8:00

8

00:00-01:00

16

75%

16

79%

gain/loss

0.1

Heat reduction

23:00-24:00

08:00-23:00

242.3

0.1

00:00-08:00

8

08:00-24:00

Hours of heat gain

Hours of heat loss

Period

Hours (h)

Period

Hours (h)

00:00-06:00

15

06:00-15:00

9

14

04:00-14:00

10

Bare roof ACH (h−1 )

Heat gain (W/m2 )

Heat loss (W/m2 )

Heat

12

60.0

63.8

0.9

gain/loss

15:00-24:00 30

94.4

81

1.2

00:00-04:00 14:00-24:00

16

Page 17 of 35

90

90

1400

80

80

1200

70

70

1000

60

60

800

50

600

40

400

30

30

200

20

20

0

10

10

)

100

1600

40

ip t

Temperature (

50

0

0

cr

00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00

-200

RH (%)

2

Solar radiation (W/m )

100

1800

Time

Solar radiation

Outdoor temperature

Outdoor RH

(b) 40

40

30 20

25

10

Heat loss 0 20

Heat gain

30

an

)

2

Heat flux (W/m )

30

Temperature (

35

30

25

10

Heat loss

0

20

Heat gain

Heat gain

-10

40

20

-10

15 -20

15

-20

10

00:00

23:00

22:00

21:00

20:00

19:00

18:00

17:00

16:00

15:00

14:00

13:00

10

12:00

11:00

10:00

09:00

08:00

07:00

06:00

05:00

04:00

03:00

02:00

00:00

00:00

23:00

22:00

21:00

20:00

19:00

-30

M

Time

18:00

17:00

16:00

15:00

14:00

13:00

12:00

11:00

10:00

09:00

08:00

07:00

06:00

05:00

04:00

03:00

02:00

01:00

00:00

-30

01:00

2

Heat flux (W/m )

35

)

50

(a)

Temperature (

40

50

us

Figure 14: Outside climate conditions (09/03/2015-09/04/2015).

Time

Internal concrete slab surface temperature of green roof

Outdoor temperature

Internal concrete slab surface temperature of bare roof

Outdoor temperature

Indoor temperature of green roof

Heat flux of green roof

Indoor temperature of bare roof

Heat flux of bare roof

(09/03/2015).

‰, 1.7 ‰, 0.5 ‰ respectively.

te

2.4

d

Figure 15: Comparison of heat flux transferred through roof surfaces with no night ventilation

So using night ventilation can significant-

Ac ce p

ly reduce indoor temperature at night. Furthermore, the average indoor and outdoor temperature difference of green roof with no night ventilation is larger

310

than bare roof, but smaller when adopting night ventilation. It indicates that when there is no night ventilation, the green roof acts as a insulation layer, it’s advantage for daytime but disadvantage for nighttime in summer. 3.4. Natural ventilation on rainy day As the former three experiments presented, the cooling ability is significantly

315

effective when green roof combined with night ventilation on summer sunny days. However, the performance of green roof is strongly depends on the climate [5, 43]. In order to study how the green roof works in rainy days, a measurement was conducted from 09/18/2015 to 09/20/2015. The meteorological conditions are shown in Fig.16. The solar input on summer rainy day has dropped to 14.9% 17

Page 18 of 35

Table 4: Comparison of difference between indoor and outdoor temperature among no ventilation, natural ventilation and mechanical ventilation in nighttime.

320

1.1

Min

1.0

0.3

0.1

Average

2.4

1.7

0.5

Max

3.8

2.6

2.6

Min

0.7

0.2

0.0

Average

2.3

2.1

0.8

ip t

Mechanical ventilation

2.1

us

Bare roof

Natural ventilation

3.9

cr

Green roof

No ventilation Max

of sunny days, the maximum value of global horizontal solar radiation was only 136 W/m2 , and the outdoor air temperature ranged between 19.7

‰ and 21.7

an

‰. Because of the low solar radiation and cool outdoor temperature, the vents on north and south wall were opened all the day. Fig.17 shows the temperature

325

M

comparison between green roof and bare roof with natural ventilation through out the day (09/18/2015 – 09/20/2015). It is interesting to notice that the temperatures of bare roof are much different from the sunny days, the external concrete slab surface temperature was between 20 and 21

‰on sunny days.

d

to 44

‰, which was up

And the indoor temperatures of the two cells tended

‰ lower

than bare roof in the daytime and nearly equal during the night. Therefore, on summer rainy days, no appreciable effect was obtained by green roof compared

Ac ce p

with bare roof due to the low solar radiation and rainfall cooling. 40

300

100

35

90 70 60

150

30 )

80

200

25 20

50

100

50

40

15

30

10

20

5

10

0

Temperature (

2

Solar radiation (W/m )

250

RH (%)

330

te

to converge, the average indoor temperature of green roof was 0.06

00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00

0

0

Time Solar radiation

Outdoor temperature

Outdoor RH

Figure 16: Outside climate conditions (09/18/2015-09/20/2015).

18

Page 19 of 35

ip t

21

20

cr

Temperature (

)

22

00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00

19

us

Time

Indoor temperature of green roof

Exterior concrete slab surface temperature of green roof

Indoor temperature of bare roof

Exterior concrete slab surface temperature of bare roof

Outdoor temperature

Internal concrete slab surface temperature of green roof

Leaf surface temperature

Internal surface temperature of bare roof

an

Figure 17: Temperature comparison between green roof and bare roof with natural ventilation all the day(09/18/2015 – 09/20/2015).

M

4. Discussion

4.1. Dimensioning of ventilation system for different building thermal envelopes and outdoor temperature

d

335

On the basis of the experiment, a mathematical model for the quantification

te

of the night ventilation as a function of the outdoor temperature swing for a specific location and the thermal envelope properties of a green roof building

Ac ce p

was developed. The thermal exchange in the experimental cell as a function of the massive air flow rate are given by Eq. (1), as shown below. Qvent = mc ˙ a (Tin − Tout )

(1)

where Qvent is the heat exchange with night ventilation (W), m ˙ is the mass

flow rate (Kg/s), ca is the air specific heat (J/Kg K),Tin is the test cell indoor air temperature (K), Tout is the outside air temperature (K). The test cell heat exchange is expressed by the Eq. (2): Qcell =

n X

Ui Ai (Tin − Tout ) =

i=1

n−1 X

Ui Ai (Tin − Tout ) − Qgr

(2)

i=1

where Ui is the heat transfer coefficient(W/m2 K), and Ai is envelope area (m2 ), 340

Qgr is the heat loss from the indoor space by green roof (W ), it is a positive 19

Page 20 of 35

value, the ”-” in front of Qgr means green roof makes the cell loss heat (according to experiment data, the heat flux of green roof in a day transfered from inside

ip t

to outside).

In a steady state, the heat gains of the test cell is equal to the heat exchange

mc ˙ a (Tin − Tout ) =

n−1 X

cr

with the night ventilation, namely Qcell = Qvent : Ui Ai (Tin − Tout ) − Qgr

Eq. (3) can also be described in such form below: n−1 X

Ui Ai (Tin − Tout )

an

mc ˙ a (Tin − Tout ) + Qgr =

us

i=1

(3)

(4)

i=1

345

That means combing night ventilation and green roof together is a positive

M

strategy to reduce the heat gain of the building envelope (except the green roof). And by using Eq.(3) we can dimension the night ventilation system, i.e.vents opening size or ventilation pipe length and diameter for different air flow rates.

the building envelop including green roof must be known.

te

350

d

The outdoor temperature for the specific location, and thermal properties of

Ac ce p

4.2. Transmissivity of Bryophyllum 1.0

Solar reflectivity

0.9

Solar transmissivity Solar absorptivity

0.8

0.7

Rate

0.6

0.5

0.4

0.3

0.2

0.1

0.0

August

September

October

Figure 18: Comparison of solar reflectivity, transmissivity and absorptivity of Bryophyllum form August to October.

As the plant Bryophyllum growing, the canopy got denser from August to October. It can be seen in Fig. 18 that the solar transmissivity of Bryophyllum 20

Page 21 of 35

is 0.15, 0.13, and 0.13, respectively in September, August and October. So 355

when adopting Bryophyllum as green roof plants can barrier more than 85%

ip t

solar radiation, only less than 15% solar radiation can go through the plants. It

also reports that the plants can absorb 50 – 67% of solar radiation and reflect

cr

30% – 37% of solar radiation. From August to October, the solar reflectivity of Bryophyllum reduced and the solar absorptivity rised as the plants growing. 4.3. Correlation analysis

us

360

This section aims to explore the correlation between meteorological factors

an

and soil water content with thermal performance parameters of both roofs under mechanical night ventilation condition, including foliage temperature Tf , air temperature with in the canopy Taf , soil temperature Ts , external surface temperature of structural layer Tes , internal surface temperature of structural

M

365

layer Tis , indoor air temperature Tin and heat flux through the roof qr . The method of Pearson correlation analysis is adopted.

d

As it shown in Table 5, solar radiation and outdoor temperature are both

370

te

positively correlated with all the temperatures other than heat flux through both roofs, while wind speed at night and outdoor humidity are just opposite. And soil moisture content is negatively correlated with temperatures and heat flux of

Ac ce p

the green roof. Solar radiation and outdoor temperature are highly correlated with foliage temperature, air temperature with in the canopy, external structure surface temperature and indoor temperature of bare roof, but less related with

375

external and internal roof surface temperature and indoor temperature of green roof. Suggesting that the thermal performance of green roof was less affected by solar radiation and outdoor temperature than bare roof due to the shading and cooling effect of plants. Correlations of wind speed at night is negative to all temperatures and posi-

380

tive to heat flux through both roofs. As the wind strengthened convection heat transfer both side of the roof, the external and internal surface temperature of the roof structure as well as the indoor temperature were decreased by increasing the wind speed at night. Because of the night ventilation, the indoor 21

Page 22 of 35

temperature is lower than internal surface temperature of roof structure, the 385

heat came in to the cell, so the correlations of roof heat flux is also positive.

ip t

The soil moisture content is highly correlated with external and internal roof structure surface temperature and heat flux of green roof, it means irrigation or

cr

precipitation can effectively reduce the green roof structure surface, and in turn

reducing the indoor temperature and leading to more heat be out of the indoor space.

us

390

Table 5: Correlation analysis under natural night ventilation ’s cooling effect. Green roof Tf

Taf

Ts

Solar radiation

0.798a

0.774a

0.642a

Wind speed (night)

-0.500a

-0.511a

-0.556a

0.974

0.982

a

Ourdoor humidity

-0.955

Soil moisture content a

a

a

-0.388

a

-0.966

a

-0.419

a

0.984

a

-0.979

a

-0.479

Bare roof

Tis

Tin

qr

Tes

Tis

Tin

qr

-0.351a

-0.163a

0.373a

-0.737a

0.981a

0.832a

0.507a

-0.810a

-0.465a

-0.481a

-0.533a

0.533a

-0.572a

-0.499a

-0.523a

0.504a

a

0.395

a

-0.403

M

Outdoor temperature

a

Tes

an

Factors

a

-0.603

a

0.574

a

-0.570

a

-0.605

a

0.908

a

-0.879

a

-0.540

a

-0.964

a

0.930

a

-0.772

a

0.779

a

a

0.684

a

a

0.951

a

-0.749a

-0.750

-0.693

-0.919

0.917a

–

–

–

–

Correlation is significant at the 0.01 level (2-tailed).

d

4.4. Air organization for night ventilation

te

Table 6 reports the comparison of indoor temperature difference between green roof and bare roof with no night ventilation, natural night ventilation 395

and mechanical night ventilation during 8:00 – 21:00 when all openings were

Ac ce p

closed. It shows that the average and maximum temperature difference of no ventilation is 1.9

‰ respectively.

‰, 3.3 ‰ and natural ventilation by vents is 3.0 ‰ and 5.0

However, when using mechanical ventilation, the average and

maximum temperature difference is 2.6

400

‰ and 3.0 ‰ respectively as the air

change rate is 12 ACH, it is even lower than natural ventilation with 8 ACH. While when the air change rate is up to 30 ACH by mechanical ventilation, the average and maximum temperature difference is 3.3

‰ and 5.7 ‰

which

is just slightly higher than natural ventilation. This is because the position of natural ventilation vents are close to the roof (Fig.1 (a)), the outside cool air

405

flows horizontally from one vent to the opposite vent. Nevertheless, shown as Fig.1 (b), the mechanical fan is at the bottom of south wall, initially this design aimed to have better indoor air mixture when fan operated. However, it seems 22

Page 23 of 35

the natural ventilation air flow mode is more effective for accumulating cooling energy into the roof at night and releasing it in the daytime. According to this phenomenon, a preferable air organization for night ventilation is leading the

ip t

410

air to flow closely to the roof ceiling with a slow air speed in order to store

cr

the cooling energy by roof construction rather than leading outside air to cool the whole room space. Additionally, if the air organization is applicable, using

415

us

natural ventilation can have a satisfying indoor temperature reduction effect, and there is no electric energy consumption for fan, so it is better for energy

an

saving.

Table 6: Comparison of indoor temperature difference between green roof and bare roof. No ventilation

ifference (

‰)

Maximum

temperature

‰)

12 ACH

30 ACH

3.0

2.6

3.3

3.3

3.6

5.7

5.0

d

difference (

1.9

Mechanical ventilation

8 ACH

M

Average temperature d-

Natural ventilation

te

4.5. Simulation of the energy performance This section aims to evaluate the energy saving potential of a green roof

Ac ce p

combined with night ventilation. The analysis has been carried out through nu420

merical simulations using EnergyPlus, which is one of the most advanced building energy simulation programs. The green roof model Ecoroof implemented in EnergyPlus program is based on the Fast All-season Soil Strength (FASST) model[44] developed and with some modifications introduced by Sailor[36]. 4.5.1. Validation of simulation model

425

Before the energy saving simulation, a model validation was carried out

using EnergyPlus and its thermal performance was simulated with the measured meteorological data on natural ventilation, mechanical ventilation (30 ACH) and no ventilation days. The physical properties of plant Bryophyllum in this case were used as input data for simulation. The green roof had a soil thickness of

430

0.05m, the vegetation had an LAI of 5, and leaf reflectivity of 0.3, leaf emissivity 23

Page 24 of 35

of 0.9, plant height of 0.50m, saturation volumetric moisture content of 0.25 (m3 /m3 ), residual volumetric moisture content of 0.01 (m3 /m3 ), and initial

ip t

volumetric moisture content of 0.2 (m3 /m3 ). Other parameters of the green

roof in the simulation we used the default data: leaf emissivity 0.95, minimum stomatal resistance 180 s/m.

cr

435

Green roof model validation was performed by comparing measured data in-

us

cluding hourly average vegetation temperature, internal roof structure surface temperature and indoor temperature to output temperatures of the simulation, as shown in Fig. 19. Table 7 summarizes the performance statistics for the experimental green roof: mean bias error (MBE), root mean square error (RMSE)

an

440

and standard deviation(SD). As shown in Fig. 19 and Table 7, the simulation results are higher than the experimental data, except the leaf temperature under no ventilation condition, all models generate mean bias errors ranging from -0.5

445

M

‰ to 2.1 ‰. It should be noticed that the measurements did not quantify all

necessary inputs for the Ecoroof model, such as LAI and stomatal resistances,

d

there are many degrees of freedom, that if optimized would allow further re-

te

duction in the bias error. Some differences could also be due to the shading of a 30-storey building on the east side of the cell, may leading the measured

Ac ce p

temperatures lower than simulation during the morning to noon(19) 450

According to this validation work, it is found that the computer simulation

program EnergyPlus with ecoroof and ventilation model perform adequately and is reliable for predicting the thermal performance of a green roof and night ventilation system in the energy saving simulation. Table 7: Temperature validation statistics for the experimental green roof.

‰) RMSE (‰) SD (‰) MBE (

Natural ventilation

Mechanical ventilation

No ventilation

Tf

Tis

Tin

Tf

Tis

Tin

Tf

Tis

Tin

1.0

1.6

0.7

1.0

1.5

0.2

-0.5

1.3

0.9

1.6

1.7

1.5

1.9

1.9

1.2

1.2

2.1

1.5

1.0

1.2

0.9

1.2

1.2

0.7

0.6

1.5

0.9

24

Page 25 of 35

50 Simulation

30 20

10

10

0

0

0

Time

(b) Internal roof surface temperature - natural ventilation

(c) Indoor temperature - natural ventilatoin

50 Simulation

Measurement

Measurement

Simulation

40

20

0

0

Time

(d) Vegetation temperature - mechanical ventilation

Time

50 Simulation

Measurement

0

0

0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 24:00

10

20 10

Time

Time

(g) Vegetation temperature - no ventilatoin

Measurement

30

an

20

0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 24:00

Temperature(

30

10

Simulation

40

Temperature(

40

20

(f) Indoor temperature - mechanical ventilation

50

Measurement

30

0

(e) Internal roof surface temperature-mechanical ventilation

40

20 10

Time

50

30

us

10

cr

30

10

0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 24:00

Temperature(

20

Measurement

0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 24:00

Temperature(

30

40

0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 24:00

40

Simulation

20 10

50 Simulation

30

Time

(a) Vegetation temperature - Natural ventilation

Measurement

ip t

Temperature(

Temperature(

20

0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 24:00

30

50

Temperature(

Simulation

40

Time

Temperature(

Measurement

40

0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 24:00

Temperature(

50

Measurement

Time

(i) Indoor temperature - no ventilatoin

M

(h) Inernal roof surface temperature - no ventilatoin

0

0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 24:00

Simulation

40

0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 24:00

50

Figure 19: Validation of EnergyPlus model using simulated and measured temperatures under

455

d

natural ventilation, mechanical ventilation and no ventilation condition.

4.5.2. Energy saving potential

te

The energy saving potential of the combination of green roof and night ventilation was evaluated for a one story rectangular (aspect ratio=1.8) office build-

Ac ce p

ing. The comparison was done between the green roof and traditional cool roof in three different representative climates: Chongqing (ASHRAE climate zone

460

3A), Beijing (ASHRAE climate zone 4A) and Guangzhou (ASHRAE climate zone 2A). The total floor area of this building is 518 m2 , and it is based on ASHRAE 90.1-2010 energy efficient building design standards and ASHRAE 62-2010 indoor air quality and ventilation standards. The buildings are modeled with typical occupancy, equipment, and thermostat schedules as given in

465

[45]. The night ventilation mode chose mechanical fan ventilation, the air change rate was 30 ACH. The control rule is when the outdoor temperature is lower than indoor temperature, the fan is on, and the HVAC is off, vice versa. With China Building Codes Design standard for energy efficiency of public buildings GB50189-2015 and GB50176-2015 Thermal design code for civil building, the

25

Page 26 of 35

470

heating and cooling set points during working hours were 18.0 and 26.0

‰,

30 Beijing Guangzhou

cr

20

15

us

10

5

0

June

July

August

an

Cooling energy reduction (%)

Chongqing

25

ip t

respectively.

Figure 20: Monthly cooling energy reduction associated with the green roof relative to the

M

cool roof in three different climates.

40

25

20

d

30

14h

te

Temperature (

)

35

8h

12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 00:00

15

Time

Air Temperature

outdoor air temperature

Ac ce p

Indoor

Figure 21: Night ventilation and HVAC operating hours on two typical days in Beijing.

Fig.20 shows the energy consumption in cooling period for an office building

with and without green roof in three different climates. The results reveal that combining green roof and night ventilation can significantly reduce the cooling

475

energy consumption, especially in Beijing, the reduction is up to 24.6% in June, probably because the diurnal temperature difference in Beijing is larger and the night temperatures is lower than the other two cities. In order to investigate the influence of night temperature to the HVAC operating hour, a simulation was done in Beijing on two typical days in June.

480

As shown in Fig. 21, the hours for night ventilation is strongly relative to the

26

Page 27 of 35

outdoor air temperature. In the first day, the diurnal temperature differences was 16.1

‰

‰ and the night mean temperature was 20.0 ‰(ranging from 17.1 to

ture differences was 16.6 from 20.4 to 30.0

‰ and the night mean temperature was 24.6 ‰(ranging

‰), the hours for ventilation is 8h. It can be concluded that,

cr

485

ip t

24.1 ), the hours for ventilation is 14h; in the second day, the diurnal tempera-

the night ventilation is more efficient for shorten the operating hours of HVAC

us

when the night temperature is lower.

5. Conclusions

490

an

This paper analyzes the thermal performance of integrating the green roof and night ventilation compared with bare roof in 4 series experiments. Tem-

M

perature distributions along the vertical direction of both roofs are presented, and heat transfer processes of both roofs are described in details. Based on the experimental data, a correlation analysis is carried out to identify the impact of

495

d

weather factors on cooling effect of green roofs. And air organization for night ventilation is also discussed. Then energy saving simulation is carried out in

te

three typical climates.

It is observed that combining green roof and night ventilation can significant-

Ac ce p

ly reduce the indoor temperature compared with no night ventilation strategy in daytime on summer sunny day. Night ventilation makes more cooling energy

500

stored in green roof and release it in the following day. Comparing the green

‰ and 3.3 ‰ on average when using fan ventilatioin in daytime, 5.0 ‰ and 3.0 ‰ roof and bare roof, the peak indoor air temperature can be reduced by 5.7

when using natural ventilation by opening vents. Integrating the green roofs and night ventilation can reduce 75-79% heat gain and shorten 6h heat gain hours

505

a day. However, on summer rainy days, no appreciable effect was obtained by green roof compared with bare roof due to the low solar radiation and rainfall cooling. Because the green roof acts as a insulation layer, it’s advantage for daytime but disadvantage for nighttime in summer. Combing night ventilation can solve

27

Page 28 of 35

510

this problem. In daytime, when there is no ventilation, green roof dominants the thermal reduction, while when night ventilation is operated, outdoor air is

ip t

the main cooling source for indoor space.

Besides, a simple equation to assess the air flow rate of night ventilation

515

cr

when combined with green roof was proposed.

We also found that the better energy saving mode of air organization for

us

night ventilation is leading the air flowing closely to the roof ceiling with a slow wind speed in order to store the cooling energy in roof construction rather than leading outside air cool the whole room space. The outdoor temperature

520

an

is the most correlative factor for cooling effect when combining the green roof and night ventilation together, followed by solar radiation, wind speed and soil moisture content.

M

The energy simulation shows that combing green roof and ventilation can save the cooling energy of 10.3 - 24.6% depending on the climates. And the night ventilation is more efficient for reducing the operating hours of HVAC when the night temperature is lower.

d

525

te

The presented data and analysis are expected to be helpful for understanding green roof combined with night ventilation on thermal performance. Due to the

Ac ce p

air organization for night ventilation remarkably influences the cooling effect, improvement for air organization system will be implemented. Furthermore,

530

the cooling effect of green roofs combined with night ventilation is influenced by many factors, such as weather conditions, plant characteristic, insulation, ventilation quantity, as well as thermal mass. Future studies will focus on the effect of plant characteristic and thermal mass by field measurement in a real school office building and simulations. And theoretical study will be carried out

535

in order to predict the energy saving.

6. Acknowledgement This project was founded by the National Natural Science Foundation of the Peoples Republic of China (Grant No. 51478059) and Project for fundamen-

28

Page 29 of 35

tal and Frontier Science of the Chongqing Science & Technology Commission 540

(Project No.cstc2014jcyjA90024). We express sincere thanks to Qiman Hu,

ip t

Kehua Li, Jinzhog Fang, Ke Xiong and Jiandong Ran for their assistance in

conducting the experiments. And the authors also appreciate the staffs’ help

cr

and sponsorship from Shanghai Zhonghui Ecological Technology Co., LTD.

545

us

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