Energy performance analysis on telecommunication base station

Energy and Buildings 43 (2011) 315–325

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Energy performance analysis on telecommunication base station Rang Tu, Xiao-Hua Liu ∗ , Zhen Li, Yi Jiang Department of Building Science, Tsinghua University, Beijing 100084, PR China

a r t i c l e

i n f o

Article history: Received 1 July 2010 Received in revised form 30 August 2010 Accepted 21 September 2010 Keywords: Telecommunication base station Cooling load Envelope Energy consumption Air conditioning system

a b s t r a c t Telecommunication base station (TBS) has high indoor IT heat dissipation rate, and cooling load exists almost all year around. Energy consumption of air-conditioning system is 30–50% of the TBS entire energy. Envelope and heat pipe assisted air-conditioning system performances are investigated using annual hourly simulation software. In cold city Harbin, high insulation envelope is recommended to avoid heating requirement in winter; and in warm city Guangzhou, low insulation envelope is recommended to reduce the annual cooling load. Shading and roof ventilation have little impact on the reduction of cooling load. Simplified analysis method based on daily average steady values is proposed, which can reveal the main performance influencing factors and clearly direct the main approach in energy saving. The simplified method can accord well with numerical results and tested results available in literature. Contribution of each heat source can be clearly gained and analyzed, solar radiation does not possess large effect in TBS. Ideal thermal resistance with no heating or cooling requirement is then derived, envelope can be easily optimized and contribution of such kind of outdoor cooling source method using heat pipe can be easily obtained. © 2010 Elsevier B.V. All rights reserved.

1. Introduction With the development of civil economy and science, communication industry in China has jumped into a new era and coming with it is the increasing number and scale of telecommunication base stations (TBSs). Up till now, China has the largest communication network scale in the world, and 20 billion kWh electricity is consumed every year, one third of which is consumed by TBS [1]. In order to maintain TBS temperature within a safe range, air conditioning is needed almost all year around, which makes air conditioning system use 30–50% electricity of the entire TBS. The proper design of building envelope and air-conditioning system means a great energy saving potential in TBS, which has aroused more and more attention. There are two main ways to save energy, one is to reduce the load of air conditioning system by optimizing TBS envelope, and another is to improve the performance of air conditioning system. Demand for envelope is rather different when outdoor climate changes all year around. Envelope with good heat dissipation ability is good for the reduction of cooling load, when the outdoor temperature is lower than the indoor environment, but it also increases the possibility of heating. However in hot seasons, this kind of envelope will introduce more heat in, thus increase the cooling load. Nakao et al. [2] introduced a thermal control wall for TBS, which

∗ Corresponding author. Tel.: +86 10 6277 3772; fax: +86 10 6277 0544. E-mail address: [email protected] (X.-H. Liu). 0378-7788/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2010.09.019

can dissipate heat using a two-phase loop-type thermo-siphon system integrated inside the wall. The heat transfer coefficient of the wall was found to be one to ten times of the ordinary wall and the annual energy saving was estimated to be 20%. Zhang et al. [3] studied envelope design of TBS in Guangzhou city, using annual hourly simulation software DeST, finding that heat transfer coefficient of envelope and solar radiation mainly decide the heat transferred through envelope and in TBS where ventilation cannot be applied, the combined design of high heat transfer coefficient and low solar absorption wall and roof are also recommended methods. They also pointed out that ventilation was the most efficient method and energy saving potential will be decreased in the order of ventilation, heat transfer coefficient of envelope and adsorption rate of walls. The most commonly used air conditioning system in TBS is split air conditioner. Choi et al. [4] studied ways to improve the performance of split air conditioner. In order to make better use of natural cooling resource, some researchers studied ventilation heat exchange system especially for TBS [1]. They found that 49% energy saving potential can be achieved in Guangzhou and the payback period is two years. Rabie [5] gave an algorithm for the evaluation of air conditioning system capacity installed in a TBS, and verified its accuracy by taking a typical TBS in South Africa as an example [6]. To summarise, many researchers have made lots of valuable work to reduce the air-conditioning systems consumption in TBS through annual hour-by-hour simulation results or tested values. However, what are the main performance influencing factors in

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2. Numerical simulation Nomenclature specific heat capacity, J/(kg ◦ C) coefficient of performance solar adsorption ratio total envelope’s surface area, m2 air leakage flow rate through envelope, m3 /s heat transfer coefficient, W/(m2 ◦ C) cooling/heating load caused by air leakage through envelope, W heat transfer through envelope due to indoor and outdoor temperature difference, W indoor IT heat dissipation rate, W cooling/heating load of TBS, W solar radiation transferred through envelope, W solar radiation density, W/m2 thermal resistance, (m2 ◦ C)/W temperature, ◦ C solar radiation heat transfer rate heat convection coefficient, W/(m2 ◦ C) density, kg/m3

cp COP e F G K QA QEN QIT QL QS qs R T ˛s ˛w 

Subscripts a air i each surface of envelope o outdoor r room

TBS? Should TBS envelope be the same in the same city; is it affected by indoor IT dissipation rate? Does shading or roof ventilation work well as that in residential or commercial buildings? What is the energy saving potential of using outdoor cooling capacity? This study tries to answer these questions through a simple analysis method, which can clearly express the physical property of heat transfer performances in TBS. Firstly, numerical simulation results with different IT heat dissipation rates and envelopes will be analyzed. Two air-conditioning systems are adopted in the present study. One is conventional system and another is heat pipe assisted system which can use outdoor cooling capacity to cool indoor space. A simplified method based on daily average values is then proposed to clearly express the contribution of each heat source and give the efficient direction in energy saving. The concept of ideal heat resistance with no heating/cooling requirement is derived, which can clearly direct the selection of TBS envelope with different IT heat dissipation and places.

The information on current TBS in China is first investigated, and the results are summarized in Table 1. TBS is usually an unattached building with no windows, and the door is opened only for maintenance. The floor area is about 20 m2 . The total indoor IT heat dissipation is from 2 kW to 6 kW, and the envelope is 24 brick wall, 37 brick wall or 100 mm color plate. Fig. 1 shows some photos of TBS in China with brick wall or color plate envelopes. As indicated by Table 1, 100 mm color plate is adopted in TBS of Chengdu, Guangzhou, Panjin and Luoyang, however the outdoor climates are rather different. Are they the right choices? What kind of envelope is appropriate in the aspect of reducing annual electricity consumption of air conditioning system for TBS with different indoor IT heat dissipation rates and different outdoor climates? In this study, DeST software is adopted to simulate the annual hour-by-hour cooling/heating load of typical TBS. DeST software, developed by Tsinghua University, is a kind of building thermal environment and air conditioning system design simulation software based on state-space method [3,7]. It can be used to simulate the building’s hourly dynamic load, room temperature and evaluate the performance of various air conditioning systems. Fig. 2 gives the TBS model in DeST software, and the size is 4.6 m (length) × 4.6 m (width) × 3.6 m (height). The room temperature is controlled between 15 ◦ C and 28 ◦ C. Four typical envelopes will be analyzed in this study, which are 24 brick wall (240 mm claymortar + 20 mm lime mortar), 37 brick wall (370 mm claymortar + 20 mm lime mortar), 50 mm color plate (2 mm construction steel + 50 mm polystyrene foam + 2 mm construction steel) and 100 mm color plate (2 mm construction steel + 100 mm polystyrene foam + 2 mm construction steel). The heat transfer coefficients are 2.10 W/(m2 K), 1.57 W/(m2 K), 0.82 W/(m2 K) and 0.44 W/(m2 K), respectively. The higher the heat transfer coefficient is, the weaker the heat preservation quality will be. For brick wall type TBS, the aerated concrete insulation roof with the heat transfer coefficient of 0.812 W/(m2 ◦ C) is adopted. The indoor IT heat dissipation rates are set as 2 kW, 3 kW and 5 kW according to the investigated results shown in Table 1. Two cities are chosen: Harbin (cold climate) and Guangzhou (warm climate). The annual average temperature, the coldest month’s average temperature and the hottest month’s average temperature in Harbin are 4.1 ◦ C, −18.7 ◦ C and 22.9 ◦ C, respectively. However, the average temperature in Guangzhou changes to 22.2 ◦ C, 14.2 ◦ C and 28.8 ◦ C, respectively. Two envelops are simulated in Harbin, 37 brick wall and 100 mm color plate. Another two envelops with relatively poor heat transfer resistance are studied in Guangzhou, 24 brick wall and 50 mm color plate.

Table 1 Basic information of investigated TBS. No.

1 2 3 4 5 6 6 7 8 9 10 11

Area/m2

20 20 25 15 21 15 18 24 33.6 21.6 17.3 16

Envelope

100 mm color plate 100 mm color plate 24 brick wall 100 mm color plate 100 mm color plate 100 mm color plate 24 brick wall 100 mm color plate 37 brick wall 24 brick wall 100 mm color plate 24 brick wall

Indoor IT heat dissipation/kW

6.0 5.0 3.5 3.5 4.5 5.2 5.3 1.9 4.0 6.6 4.3 3.4

City

Chengdu Chengdu Guangzhou Guangzhou Guangzhou Kunming Langfang Panjin Panjin Luoyang Luoyang Jiaxing

Outdoor temperature/◦ C

Recommended envelope

Mean temperature at coldest month

Mean temperature at hottest month

Annual mean temperature

5.8 5.8 14.2 14.2 14.2 8.4 −3.8 −6.9 −6.9 1.5 1.5 5.2

25.8 25.8 28.8 28.8 28.8 20.1 26.4 24.4 24.4 27.0 27.0 28.2

16.6 16.6 22.2 22.2 22.2 15.4 12.6 9.5 9.5 17.6 17.6 17.0

24 brick wall 24 brick wall 24 brick wall 24 brick wall 24 brick wall 24 brick wall 24 brick wall 50 mm color plate 24 brick wall 24 brick wall 24 brick wall 24 brick wall

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Fig. 1. Photos of TBS: (a) TBS with color plate envelope; (b) TBS with brick wall envelop; and (c) indoor IT equipments.

Two types of air conditioning systems will be compared. One is conventional system: split air conditioner + electrical heater (if required), which is usually adopted in TBS in China at present. Another is heat pipe assisted system: split air conditioner + heat pipe + electrical heater (if required). Heat pipe can operate when outdoor temperature is cooler enough than the indoor environment, which uses outdoor cooling capacity to cool the indoor space with high efficiency (only fan consumes electricity, not compressor). Heat pipe consists of indoor heat exchanger and outdoor heat exchanger. Working fluid circulates between the indoor and outdoor heat exchangers, which evaporates in the indoor heat exchanger and condensates in the outdoor heat exchanger when outdoor air is cooler enough than indoor air. Heat pipe simulated in this study uses R22 as working fluid. R22 will work at 25.5 ◦ C and the corresponding pressure is 1058 kPa, when indoor and outdoor air is 28 ◦ C and 23 ◦ C, respectively. The required indoor (or outdoor) heat exchanger’s area of heat pipe to remove 2 kW heat to outdoor environment at 5 ◦ C temperature difference is about 70 m2 , and the size is 920 mm (L) × 460 mm (W) × 900 mm (H). The dimensions of outdoor heat exchanger are the same as those of indoor heat exchanger. The COP (coefficient of performance) of electrical heater is set as 1, and the COP of split air conditioner is 2.5. The heat pipe operates when indoor and outdoor temperature difference T is higher than 5 ◦ C. The COP of heat pipe varies with T: COP = 13, when T > 10 ◦ C; and COP = 3 + T, when T < 10 ◦ C. The simulation results by DeST software include annual hourly cooling/heating load, indoor temperature, and energy consumption of each component of the air-conditioning system. 3. Simulation results The room temperature and cooling or heating load of TBS with two kinds of envelope and three grades of IT heat dissipation (2 kW, 3 kW and 5 kW) are simulated in the cities of Harbin and Guangzhou, and the results will be given in this section.

3.1. Harbin Harbin is located in the severe cold region where summer is cool and short, and the indoor and outdoor temperature difference in winter can be as high as 40 ◦ C. So envelope with better heat preservation quality can reduce the heating demand in winter, but hamper heat dissipation through envelope in transition season which in return will enhance the cooling demand. The simulation results in Harbin are shown in Fig. 3. As indicated in the figure, TBS with 37 brick wall needs heating in winter, when indoor IT heat dissipation is 2 kW and 3 kW. If IT heat dissipation increases to 5 kW, cooling is needed all year around. But TBS with 100 mm color plate does not have heating load in winter, even when the IT heat dissipation is 2 kW. The annual cooling load of TBS with 100 mm color plate is much higher than that with 37 brick wall. And the annual average room temperature of TBS with 100 mm color plate is higher, due to its lower heat transfer coefficient. Fig. 4 presents annual electricity consumption of the two air conditioning systems at 2 kW, 3 kW and 5 kW IT heat dissipation with the two kinds of envelopes. It can be seen that: (1) Heat pipe assisted system consumes less energy compared with conventional system in the same condition. (2) The reduction of heating requirement in winter is the top priority, when indoor IT heat dissipation is low (2 kW). So 100 mm color plate shows better performance. (3) When IT heat dissipation is medium (3 kW), heating demand is reduced and cooling demand is increased. The selection of envelope relies mainly on air conditioning system: 37 brick wall shows better performance when conventional system is adopted and 100 mm color plate shows better performance when heat pipe assisted system is used. The reason can be explained as the high insulation of envelop will largely increase the annual cooling load, and the energy consumption to maintain the required indoor environment will be highly increased without high efficiency heat removal method. (4) When IT heat dissipation is high (5 kW), heat dissipation is the top priority and no heating is needed. Apparently, 37 brick wall shows better performance. The detailed energy consumption information of the airconditioning systems is listed in Table 2. It can be seen that at low IT heat dissipation (2 kW), heating accounts nearly 80% of the total energy consumption when the heat preservation property of envelope is poor. 3.2. Guangzhou

Fig. 2. Telecommunication base station model in DeST software: (a) plane view; and (b) three-dimensional view.

Guangzhou is located in the hot region, so envelopes with relatively poor insulation performance are studied. The simulation results are shown in Fig. 5, with no heating requirement during the whole year. The annual cooling requirement of TBS with 24 brick

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Table 2 Annual heating/cooling demand and energy consumption of each air conditioning unit in Harbin with 37 brick wall and 100 mm color plate. Envelope

37 brick wall

100 mm color plate

Indoor heat dissipation/kW

2 3 5 2 3 5

Annual heating demand/kWh

3023 806 0 0 0 0

Annual cooling demand/kWh

4033 8426 20,770 7756 15,848 33,112

Conventional system energy consumption/kWh

Heat pipe assisted system energy consumption/kWh

Heating

Heating

Heat pipe

Split air conditioner

Total

3023 806 0 0 0 0

230 519 1370 490 1063 2291

559 927 1661 754 1128 1877

3812 2251 3032 1244 2190 4167

3023 806 0 0 0 0

Split air conditioner 1613 3370 8308 3102 6339 13,245

Total 4636 4176 8308 3102 6339 13,245

R. Tu et al. / Energy and Buildings 43 (2011) 315–325

Electricity consumption(MWh)

14

wall is less than that with 50 mm color plate. But the cooling load will be higher in TBS with 24 brick wall when outdoor temperature is much higher (summer period) due to its larger heat transfer coefficient, but this situation only lasts for a short period. Taking a whole year into account, 24 brick wall is strongly recommended to insure better heat dissipation ability through TBS envelope. The result shows that the energy saving potential of heat pipe assisted air-conditioning system compared with conventional system (split air conditioner only) is not as obvious as that in Harbin. The reason is that heat pipe can operate only half year in Guangzhou. The longer the heat pipe can be operated, the higher the energy saving potential will be. According to the simulation results, poor insulation envelope is recommended in Guangzhou, such as 24 brick wall, and heat pipe assisted air-conditioning system can save 15–30% energy compared with conventional system.

13.24

12 Conventional system 10 8.31

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4.64 3.81

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0 37brick wall

100mm 37brick wall 100mm 37brick wall 100mm color plate color plate color plate

2000W

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Fig. 4. Annual energy consumption of different air conditioning systems in Harbin.

3.3. Effect of shading or roof ventilation on cooling load in TBS In residential buildings and commercial buildings in the south part of China, shading and roof ventilation are the commonly used

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Fig. 5. Daily average indoor and outdoor temperatures as well as cooling demand of telecommunication in Guangzhou at various indoor IT heat dissipation rates Q: (a) daily temperature when Q = 2 kW; (b) daily cooling demand when Q = 2 kW; (c) daily temperature when Q = 3 kW; (d) daily cooling demand when Q = 3 kW; (e) daily temperature when Q = 5 kW; and (f) daily cooling demand when Q = 5 kW.

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methods to reduce solar radiation and express better performance. Are these methods still appropriate for TBS? This section chooses Guangzhou as an example, other cities show similar results. The annual cooling demand and energy consumption of airconditioning system with and without shading and roof ventilation are simulated by DeST software. In the case of 2 kW indoor heat dissipation rate without shading or roof ventilation, the annual cooling demand is 11,115 kWh, the annual energy consumption to sustain required indoor environment is 4446 kWh and 3709 kWh for conventional system and heat pipe assisted system, respectively. In the same case with shading and roof ventilation, the annual cooling demand is 10,705 kWh, the annual energy consumption is 4282 kWh and 3593 kWh for conventional system and heat pipe assisted system, respectively. The energy saving potential with shading and roof ventilation, which is lower than 4%, is not obvious as that is in commercial or residential buildings, leading to a long payback period. Other cases show similar results. The reason will be analyzed in Section 5. 4. Simplified analysis method As discussed in Section 3.3, solar radiation is not a main performance influencing factor and has little impact on the system performance of TBS. So what is the main approach for energy saving? Is there a simple way to solve this problem apart from complex hour-by-hour simulation? These questions will be discussed in this section. It is known that, TBS is different from ordinary commercial buildings or residential buildings for huge and constant indoor IT heat dissipation all year around. Every building has heat inertia capacity, which delays and decreases the impact of outdoor changed climate on the indoor environment. Can the performance of TBS be analyzed in a relatively steady way, using daily 24 h average values? This idea may be suitable for TBS, due to the huge and constant indoor IT heat dissipation rate. This section will analyze the possibility of using the simplified daily average method, which can clearly express the main performance influencing factors and greatly reduce the simulation time. 4.1. Simplified method—daily average analysis

Fig. 6. Heat sources in telecommunication base station.

Envelope heat transfer coefficient K is obtained by considering roof and wall as a whole, which means K is different from the heat transfer coefficient of wall and the K values with different envelopes are 1.78 W/(m2 K), 1.39 W/(m2 K), 0.82 W/(m2 K) and 0.44 W/(m2 K) for 24 brick wall, 37 brick wall, 50 mm color plate and 100 mm color plate, respectively. QS can be obtained by Eq. (2) [8], where ˛s is the solar radiation heat transfer rate, and the values with different envelopes are 0.041, 0.032, 0.028 and 0.01 for 24 brick wall, 37 brick wall, 50 mm color plate and 100 mm color plate, respectively; e is the absorption rate of each surface, which is taken as 0.55 in this study; and ˛w is the heat convective coefficient of envelope’s outer surface, which is taken as 24 W/(m2 ◦ C). QS = ˛s × qs × F,

QL = QIT + KF(To − Tr ) + Ga a cp,a (To − Tr ) + QS

(1)

Ke ˛w

(2)

Surface with different orientations receive different amounts of solar radiation at the same time. qs can be obtained from Eq. (3), where qsi is the daily average solar radiation on each surface, and Fi is the corresponding surface area. qs =

There are four main heat resources in TBS as indicated in Fig. 6: indoor IT heat dissipation (QIT ), air leakage (QA ) and heat transfer through envelope (QEN ) as well as solar radiation transferred through envelope (QS ). The building load can be removed by electrical heater when heating is required, or by heat pipe and split air conditioner when cooling is required. The daily average cooling/heating load QL of TBS can be calculated by Eq. (1), where To and Tr are daily outdoor and indoor average temperatures respectively, QIT and QS are both daily average results.

where ˛s =

n (q F) i=1 si i  n

(3)

F i=1 i

The annual average values of qs of the simulated TBS in Harbin, Xining, Beijing, Shanghai, Kunming and Guangzhou are summarized in Table 3. No large difference exists among the cities. 4.2. Validation of the simplified method Two methods are adopted to validate the proposed simplified method, one is compared with the numerical results by DeST software, and another is compared with tested results available in literature. Fig. 7 shows the daily cooling demand calculated by both simplified method and DeST software in Harbin and Guangzhou

Table 3 Annual average qs of different cities (W/m2 ). City

West wall

South wall

East wall

North wall

Roof

Solar resource zonea

qs

Harbin Xining Beijing Shanghai Kunming Guangzhou

101.8 117.8 108.0 94.1 123.2 83.9

125.1 136.9 134.5 99.8 122.7 86.0

103.2 118.0 109.5 96.3 91.7 84.3

54.2 61.7 59.6 60.7 68.5 64.3

145.0 176.9 159.5 144.9 174.3 129.6

III II III III II III

107.9 125.1 116.6 101.5 119.1 91.7

a Zone division is according to annual solar radiation received by horizontal surface. Zone I: rich zone 6700 MJ/(m2 year); zone II: fairly rich zone 5400–6700 MJ/(m2 year); zone III: Ordinary zone 4200–5400 MJ/(m2 year); and zone IV: poor zone < 4200 MJ/(m2 year y).

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Fig. 7. Comparison of calculated results by simplified method with numerical results by DeST software at 5 kW indoor heat dissipation: (a) 37 brick wall in Harbin; (b) 100 mm color plate in Harbin; (c) 24 brick wall in Guangzhou; and (d) 50 mm color plate in Guangzhou.

with different building envelops. As indicated in the figure, the calculated results by the simplified method accord well with the numerical results. Zhang et al. [3] gave the test results of a TBS in Guangzhou for 44 h, from 3 pm January 8th to 1 pm January 10th. The base station size was 5.1 m (length) × 5.1 m (width) × 2.8 m (height). The indoor

IT heat dissipation rate was 4.35 kW. The heat resistances of wall and roof are 0.21(m2 ◦ C)/W and 1.38 (m2 ◦ C)/W, respectively. The convection heat transfer coefficient of inner side and outer side of the wall is taken as 3.5 W/(m2 ◦ C) and 24 W/(m2 ◦ C). The average outdoor ambient temperature was 12.8 ◦ C and indoor air temperature was 25.0 ◦ C. The average hourly cooling load during the tested

Fig. 8. Contribution of the main heat resources to the entire building load in Harbin telecommunication base station with 100 mm color plate at various indoor heat dissipation rates QIT : (a) variance with outdoor temperature when QIT = 2 kW; (b) variance with outdoor temperature when QIT = 5 kW; (c) typical day analysis when QIT = 2 kW; and typical day analysis when QIT = 5 kW.

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Fig. 9. Contribution of the main heat resources to the entire building load in Guangzhou telecommunication base station with 24 brick wall at various indoor heat dissipation QIT : (a) variance with outdoor temperature when QIT = 2 kW; (b) variance with outdoor temperature when QIT = 5 kW; (c) typical day analysis when QIT = 2 kW; and typical day analysis when QIT = 5 kW.

5. Discussion

3

2o

The validated simplified method will be adopted in this section to further analyze the TBS performance. The contribution of each heat source to entire cooling/heating load will be analyzed, and a concept of ideal heat resistance of envelop is then proposed for envelope selection in different places as well as with different indoor IT heat dissipation rates.

The proportion of solar radiation and air leakage will be further decreased when indoor IT heat dissipation increases. The ways to reduce solar radiation by shading or roof ventilation do not work well, due to the little effect of solar radiation on the building entire performance. As calculated by Eq. (2), envelope has a significant influence on QS . If better insulation envelope is adopted, QS will be lower, but the annual cooling load will be increased due to the difficulty of indoor large amount heat dissipated to outside. The annual performance of the typical TBS in Guangzhou with 2 kW IT heat dissipation is listed here. The annual QS is 2702 kWh and cooling demand is 12,398 kWh with 24 brick wall (poor insulation), and the values change to 1845 kWh and 15,363 kWh with 50 mm color plate wall (better insulation).

Ideal heat resistance (m • C/W)

44 h was about 3.3 kW [3] and the total cooling demand was around 147 kWh. As calculated by Eq. (1), heat transferred through envelope during the 44 h is 72.6 kWh, solar radiation transferred through envelope is 16.6 kWh and the total cooling demand is 135.5 kWh. The error between the predicted total cooling demand by the simplified method and the tested results is less than 8%. So the simplified method can be successfully used to analyze the cooling/heating demand of TBS with various envelopes.

5.1. Contribution of each heat source to the entire building load The contribution of each heat resource (as shown in Fig. 6) to the total building load can be separated clearly using the simplified daily average method, so that the efficient way to save energy can be obtained easily. The results in Harbin and Guangzhou at various indoor IT heat dissipation rates (2 kW and 5 kW) along with the outdoor daily average temperature are shown in Figs. 8 and 9, respectively. The typical days’ analysis in winter, transition season and summer are also shown in the figures. In both places, heat transferred through envelope is the main performance influencing factor, and heat dissipation by IT devices covers a large part. Solar radiation and air leakage cover small parts.

2kW/15ºC 3kW/15ºC 5kW/15ºC

2.5

2kW/28ºC 3kW/28ºC 5kW/28ºC

2 1.5 1 0.5 0 -0.5 -1 -25

-20

-15

-10

-5

0

5

10

15

20

25

30

o

Outdoor daily average temperature / C Fig. 10. Variance of ideal heat resistance with outdoor daily average temperature and indoor IT heat dissipation.

R. Tu et al. / Energy and Buildings 43 (2011) 315–325

323

Number of days

250 Harbin

200

Beijing

Shanghai

Kunming

Guangzhou

150 100 50 0 <-20

-20~-

-15~-

15

10

-10~-5

-5~0

0~5

5~10

10~15

15~20

20~23

>23

Temperature range Fig. 11. Outdoor daily average temperature distribution of several typical cities in China.

Fig. 12. Operation time of each air conditioning unit in heat pipe assisted air-conditioning system used in Harbin: (a) 37 brick wall envelope; and (b) 100 mm color plate.

37 brick wall 24 brick wall 24 brick wall 24 brick wall 24 brick wall 24 brick wall 24 brick wall 24 brick wall 24 brick wall 24 brick wall

IT dissipation ≥ 300 W/m

37 brick wall 37 brick wall 24 brick wall 24 brick wall 24 brick wall

IT dissipation = 200 W/m

100 mm color plate 100 mm color plate 24 brick wall 24 brick wall 24 brick wall Harbin Beijing Shanghai Kunming Guangzhou

IT dissipation = 100 W/m

The effect of solar radiation does not play important role as described in Section 5.1, and the values of qs does not vary much in different cities as shown in Table 3, so constant qs can be adopted to simplify the analysis. Therefore, Rideal mainly depends on the daily average temperature difference between indoor and outdoor environments, at certain IT heat dissipation QIT and TBS overall surface area. Effect of outdoor daily average temperature on the ideal thermal resistance Rideal is given in Fig. 10, with the indoor IT heat dissipations as 2 kW, 3 kW and 5 kW, respectively. Take the 2 kW IT heat dissipation results as an example, the two continuous lines express the indoor temperature sustaining as 28 ◦ C (upper indoor temperature limit) and 15 ◦ C (low limit). There is no heating/cooling requirement in TBS when building envelope’s resistance is between the two lines. However, different resistances of building envelope is required at various outdoor temperatures, and the real building’s envelope is hardly to meet all the time requirements. Outdoor daily average temperature distribution of several typical cities in China is given in Fig. 11. A suitable TBS envelope for a particular city for the purpose of making full use of natural cooling resource can be chosen, combining Figs. 10 and 11. Fig. 12 shows the results in Harbin with various indoor IT heat dissipation rates and different envelopes. The operation time of each air conditioning unit in heat pipe assisted air-conditioning system can be obtained as shown in Fig. 12. The major contradiction in the reduction of airconditioning system energy consumption and the function of each air-conditioning component can be clearly obtained. This gives the orientation and direction of choosing building envelope as well as air-conditioning system. Heating requirement in rather cold outdoor condition is the key point at low IT heat dissipation rate, so envelope with better insulation is then recommended. The heating/cooling demand as well as energy consumption of airconditioning system can be predicted by Eq. (4). During heating period, indoor temperature Tr equals to lower indoor temperature (15 ◦ C); during cooling period, Tr equals to higher indoor temperature limit (28 ◦ C). The outdoor daily average temperature at the beginning of heating or cooling can be obtained by the intersection points of ideal thermal resistances and selected real envelope’s resistance as indicated in Fig. 12. Using the ideal thermal resistance method and outdoor daily average temperature, the optimized envelope can be easily obtained. The recommended envelopes for the investigated base stations are also shown in Table 1. Poor insulation envelope should be chosen in hot climate. Take No. 7 TBS as an example, heat pipe can operate almost the whole year. The required cooling period with the original 100 mm color plate envelope is 361 days. If 50 mm color plate is selected, only 243 days need cooling. The envelope selection guidance for newly constructed base stations in Chinese typical cities is summarized in Table 4.

2

(5)

2

−[F(To − Tr ) + (e × qs )/˛w ] QIT + Ga a cp,a (To − Tr )

Table 4 Recommended building envelope for the new constructed TBS in typical cities.

Rideal =

100 mm color plate 37 brick wall 24 brick wall 24 brick wall 24 brick wall

The ideal thermal resistance Rideal is defined as the envelope heat resistance while the cooling/heating demand QL = 0. According to Eq. (4), Rideal can be expressed by:

100 mm color plate 100 mm color plate 24 brick wall 24 brick wall 24 brick wall

Recommended envelope with heat pipe assisted air-conditioning system

(4)

Recommended envelope with conventional air-conditioning system

F e (To − Tr ) + Ga a cp,a (To − Tr ) + qs F R ˛w R

City

QL = QIT +

IT dissipation = 200 W/m2

Eq. (1) can be written as Eq. (4), if we assume R = 1/K, where R is the overall heat resistance of TBS envelope.

IT dissipation = 100 W/m2

5.2. Ideal thermal resistance and its application

IT dissipation ≥ 300 W/m2

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6. Conclusions Annual hour-by-hour simulation software DeST is used in this study to analyze the TBS performance with different IT heat dissipation rates, envelopes and air conditioning systems. According to the fact that TBS has large and stable indoor heat dissipation, simplified analysis method based on daily average values is proposed. This method can reveal the main factors affecting TBS performance and guide the selection of envelope and air conditioning system. The main conclusions are as follows: (1) The selection of TBS envelope is influenced not only by outdoor climate but also by indoor IT heat dissipation rate and air conditioning mode. (2) Shading or roof ventilation do not work well in TBS as that in commercial or residential buildings, due to the fact that solar radiation only accounts for a small proportion of the IT heat dissipation. (3) Ideal envelope heat resistance with no heating/cooling demand is derived based on the daily-average steady analysis method, which can direct the envelope optimization. In severe cold city like Harbin, heat preservation is the chief consideration when IT heat dissipation is small, thus 100 mm color plate shows better performance. In southern city like Guangzhou, heat dissipation is the top priority, 24 brick wall is a better choice.

325

(4) Heat pipe assisted air-conditioning system is recommended in TBS, due to the efficiently utilization of outdoor cooling capacity. Heat pipe can be regarded as changeable heat resistance envelope, improving indoor heat dissipation to outdoor environment. So that greater energy saving potential can be achieved, compared with conventional air conditioning system. References [1] Y. Chen, Y.F. Zhang, Q.L. Meng, Study of ventilation cooling technology for telecommunication TBS in Guangzhou, Energy and Buildings 41 (2009) 738–744. [2] M. Nakao, K. Ohshima, H. Jitsukawa, Thermal control wall for telecommunication equipment rooms, in: Proceedings of the 10th International Telecommunications Energy Conference, San Diego, Canada, 1988. [3] Y.F. Zhang, Y. Chen, J. Wu, Q.L. Meng, Study on energy efficient envelope design for telecommunication TBS in Guangzhou, Energy and Buildings 40 (2008) 1895–1900. [4] J. Choi, J. Jeon, Y. Kim, Cooling performance of a hybrid refrigeration system designed for telecommunication equipment rooms, Applied Thermal Engineering 27 (11–12) (2007) 2026–2032. [5] N. Rabie, Modelling HVAC in a telecommunications environment, ENE 780, Final Report, Department of Electrical and Electronic Engineering, University of Pretoria, 2000. [6] N. Rabie, G.J. Delport, Energy management in a telecommunications environment with specific reference to HVAC, Building and Environment 37 (2002) 333–338. [7] T.Z. Hong, Y. Jiang, A new multizone model for the simulation of building thermal performance, Building and Environment 32 (1997) 123–128. [8] J.L. Zeng, Study on thermal performance adjustable building envelopes, PhD thesis, Department of Building Science, Tsinghua University, 2006.