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7-9 October 2009, Leuven, Belgium Geothermal Cooling Solution Research For Outdoor Cabinet Yuping Hong, Yuening Li, Jian Shi Huawei Technologies Co., Ltd.,Shenzhen,P.R.China Tel: +86-755-89652254 E-mail: hongyp@huawei.com Abstract- This paper introduces an ongoing research on geothermal cooling solution (GCS) for outdoor cabinets that are used to contain telecom equipment such as FTTX network, mobile network and etc. The principle of GCS is that the heat generated inside cabinet from telecom facilities is transferred to water by an air-water heat exchanger, and then water dissipates the heat to shallow underground soil by water-soil heat exchanger. A prototype of this GCS is built and tested in Shenzhen, China. Test result shows that the prototype meets the cooling requirement with at least 3 times cooling efficiency higher than that of traditional cooling solution. It is conclude that GCS is effective and valuable for practical application, and need further more investigation. Fig 1 Typical daily temperature curve of the atmosphere air and soil at difference depth (Shenzhen, China) Key word: geothermal cooling solution, outdoor cabinet I. INTRODUCTION With the fast development of broadband network, outdoor cabinets which contain telecom equipment like FTTX network are deployed more widely. Cooling plays an important role in design of these cabinets to maintain cabinet internal temperature at proper level; otherwise the equipment may fail to work. Traditional cooling technologies such as air-air heat exchanger, air conditioning utilize atmosphere as heat sink, which are limited for size, energy cost and acoustic noise. Advanced cooling solution with compact size, high energy efficiency and low noise, which meets environmental protection strategies as well, is preferred by the telecom operators. As we know, air temperature changes with season and time. In hot season, it is critical for traditional air cooling technologies to function due to high temperature of atmosphere. However soil temperature is more stable, the temperature impact of the air to the soil becomes weak with the increase of soil depth. Figure 1 shows that at the depth of 5m, the soil temperature is nearly stable in the whole year. The temperature curve in figure 1 is calculated with the theoretic formula from Reference [1]. Figure 2 shows that at about 0.8m depth, little will change with soil temperature in one day time, the temperature data is derived from Reference [2]. Soil temperature is higher than air temperature in winter and lower in summer. As summer is the worst case for telecom facilities cooling, soil provides an ideal “heat sink” for outdoor cabinet. In this paper, geothermal cooling is defined as cooling with soil. Fig 2 Typical temperature curve of soil in one sunny day (Shenzhen, China) [1] Recently, geothermal cooling is attracting a lot of attentions from telecom operators and equipment manufacturers. KPN reported research on cooling mobile shelter with vertical soil heat exchanger (VSHE), the VSHE is a kind of borehole cooling, and the drilling depth reaches 75m [3]. Huawei and Telecom Italia reported a joint research on cooling solution utilizing underground heat pipe. Test result shows that the cooling efficiency is much higher than that of traditional cooling solution [4] This paper introduces an ongoing research about new GCS for outdoor cabinet. The principle of this GCS is that the heat generated inside cabinet is transferred to water by an air water heat exchanger, and water dissipates the heat to shallow underground soil by water soil heat exchanger. The original proposal of this GCS is derived from traditional ground source heat pump (GSHP) technology, which is widely used in air conditioning system for buildings. The target of this research is to find a feasible, low cost and high reliable geothermal cooling solution for easy engineering design and practical application. ©EDA Publishing/THERMINIC 2009 13 ISBN: 978-2-35500-010-2
II. GCS DESIGN AND FIELD TEST 7-9 October 2009, Leuven, Belgium The GCS system mainly include following parts: airwater heat exchanger, water pump, flow meter and watersoil heat exchanger. The air-water heat exchanger is a tube-fin structure with fan help to circulate air between heat exchanger and equipment. It has two advantages compare with air-air heat exchanger or air conditioning. First, it is more compact and save space for the cabinet; second, it is good for noise insulation because the total unit is sealed in the cabinet, no outside vent is needed which will cause noise leakage. The water-soil exchanger is made of PE material tube which is anti-corrosive and high reliable and widely used in GSHP. The tube will be buried together with other electric cables used for the telecom equipment in shallow soil, which is important ways to control the total installation cost. The Water-soil exchanger has no pollution to environment as it is a closed loop system. Compared with traditional GSHP, GCS is much simpler as no compressor unit is applied. And the shallowly buried water soil tubes are easier to install compared with borehole. Borehole needs special drilling machine and takes more time and money. As the figure 3 shows, GCS have three main coupled heat transfer process. The first process is that air circulates through equipment chassis, cabinet and air-water heat exchanger of the GCS. Heat is transferred to water in the pipeline in this process. The second process is that water circulates through pump, flow meter, water-soil heat exchanger and air-water heat exchanger. Heat is transferred to soil in this process. The third process is soil dissipates heat to more distant soil and environmental air. This third process is more complicated because of transient changing climate and high thermal inertia property of soil. The GCS prototype is built and tested in Shenzhen. Shenzhen locate in south China where annual average temperature is 23℃, the highest temperature reach 37℃ in summer and last long time. It may represent severe working condition in the world, the testing can then be apply to many area like Europe, north Asia and etc. Figure 3 show detail the layout of the equipment inside the cabinet, GCS and the main temperature test point. The GCS is installed into a real typical outdoor telecom cabinet. The size of outdoor cabinet is 1550(L) x550(W) x1500(H) mm. GCS is set up in left side, while telecom equipment chassis is placed in the middle. The equipment chassis dissipates 750W, which is typical value in real case. Cooling requirement is to maintain maximum air temperature inside the cabinet below 70℃ with least noise and energy cost by cooling system. The Water-soil heat exchanger is made up of three layers tubes buried underground. The outer diameter of the tube is 20mm and inner diameter is 15.4mm. Depth of three layers is 1.2m, 1.8m and 2.4m. Tube length in every layer is about 20m. Fig 3 Profile of outdoor cabinet with GCS Thermocouples are placed at following points: two points for inlet and outlet water of water-soil exchanger or air-water exchanger, four points for inlet and outlet air temperature of outdoor cabinets, two points for chassis inlet and outlet air. Eight points for soil temperature in different depth (0.3m, 0.6m, 0.9m, 1.2m, 1.5m, 1.8m, 2.1m, 2.4m), two points for ambient air temperature. Test data are recorded by Data Acquisition System. Ⅲ. TEST RESULT AND DATA ANALYSIS Test starts with soil temperature investigation. During the test, the telecom equipment is not powered on. Figure 4 shows the soil temperature variation for eight days. The temperature of soil under 1.2m is almost stable and change little with air temperature. In 8 days, air temperature 21℃ varies from to 40 , while soil temperature at 0.6m depth varies only from 26 to 29 and only 1.2m depth. These temperature data will be used as reference temperature point in followed analysis. Figure 5 provides the comparison of daily mean soil temperature among the different layers and variation in ten days. Soil temperature decreases along with depths. There is about ℃ temperature difference between 1.2m and 2.1m. The variation rate of daily mean soil temperature under 1.2m depth is lower than 0.045 /day 1℃ 0.8~1℃ ©EDA Publishing/THERMINIC 2009 14 ISBN: 978-2-35500-010-2 ℃ ℃ changes at ℃ .
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