A Feasibility Study on the Application of Seawater-source Heat Pump in the Subway of Qingdao

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1 w y wz 10«2y 0~0 ( ) Journal of Korean Society of Urban Environment A Feasibility Study on the Application of Seawater-source Heat Pump in the Subway of Qingdao Youyuan Chen* ½ Yajun Han* ½ Yonggang Jia* ½ Ruixia Li** ½ Wenwei Li** ½ Yuanlin Zou** *Key Laboratory of Marine Environment and Ecology (Ocean University of China), Ministry of Education, Qingdao, Shandong, , China **Beijing Huaqing geothermal development Co., Ltd., Beijing, , China (Received 0 April 2010 : Accepted 0 May 2010) Abstract Coastal area where have free cold and heat source provided by a large amount of seawater, is ideal sites for the application of seawater-source heat pump (SWHP) system to provide cooling and heating. Due to SWHP system providing higher energy efciency and protect environment, it has gradually been applied especially to some governmental key projects in China. In the first phase of Metro Qingdao, eight subway s are close to the sea. It is convenient to pumping seawater as cooling source, so utilizing SWHP system cooling the subway s is planed. This paper discussed the feasibility of utilization of SWHP system, from two intake ways. Firstly, bedrock fracture structure on coastal zone was analyzed to study the feasibility of pumping deep fracture seawater for cooling source of SWHP. And the water inflow of pumping test shows that it is infeasible by pumping bedrock fissure water as cooling source. Secondly, the feasibility of pumping surface seawater is discussed. And surface seawater temperature is low enough as SWHP system cooling source. Then SWHP system operation ways are designed preliminarily, and several solutions are proposed to solve troubles which have happened during the use of SWHP system, such as the location of intake and outfall seawater system, seawater causticity and clean technology. The result indicates that Qingdao has great potential for applying surface seawater as heat pump system cooling source in Qingdao metro. Key Words : Seawater-source heat pump, Subway, Heating and cooling I. Introduction In China, energy consumption of buildings accounted for about 27.6% of total energy in 2001 and will increase up to 35% by 2020 (Li et al., 2007). As a part of building energy consumption, heating and airconditioning systems accounted for more than 50% of the total electrical energy consumption in buildings (Yao et al., 2005). So the application of cooling and heating systems which own higher energy efficiency and use renewable energy is the current urgent need for energy saving and the future trend of energy-saving buildings. Seawater-source heat pump (SWHP) system can convert the low-grade heat source of Corresponding author hanyajun1987@163.com seawater which is obtained from the sunlight and ground heat to high-grade heat source. Compared with coal-fired heating system and conventional airconditioning system, SWHP system have a higher coefficient of performance (COP) which is about 3 ~ 5 (Shu et al., 2010). Furthermore, SWHP systems offer competitive levels of comfort compared with standard technologies, reduce noise levels and visual contamination, saves greenhouse gas emissions and provide reasonable environmental safety, from a non-economical point of view. Due to these advantages, A larger number of SWHP systems have been applied in China such as Xiaoping project and Tuandao project (Li et al., 2007, Huo, Huo, 2009). And in the first phase of Metro Qingdao, the subway s need cooling in summer while these don t need heating in winter because there are large 1

2 2 Youyuan ChenÁYajun HanÁYonggang JiaÁRuixia LiÁWenwei LiÁYuanlin Zou number of heat produced by Metro cars and emitted by the heat storing in ground (Guo, 2007). Because seven subway s are close to the sea, there is large number of free seawater which can provide cooling source for SWHP system. Because of the favorable geographical conditions in Qingdao, in the process of SWHP application two ways of taking seawater are proposed, (1) Pumping deep fracture seawater; (2) Pumping surface seawater. By analyzing the factors of hydrogeological condition, seawater temperature, material corrosion and heat exchanger cleaning, the feasibility of the application of SWHP system in subway of Qingdao is analyzed. And two feasibility scheme of system operation are suggested. II. Study Area III. The Feasibility Study of Application of Swhp 1. Intake system According to the economic investigation, about 50% initial investment of SWHP system is used to intake/ outfall system, which includes pipes of taking and draining seawater, seawater pumps and water intake/ outfall unit (Van Ryzin et. al, 1991). Therefore, the cost of intake/outfall system is very key for the feasibility of SWHP. Using different ways to intake seawater, different water qualities and water temperature can be taken, which will largely affect the COP and the operation cost of SWHP system. Shown by geological exploration to the subway of Qingdao, 9 The first phase of Metro Qingdao includes 22 s which is shown by Fig. 1. The length of M3 is about 24.9 km. The subway line goes through municipal central region, two railway s, scenic spots of the southern city and commercial (NO.23 standards for Qingdao Olympic Sailing Center) districts of the northern city. There are 8 s which is Railway ~ Wusi square, and North railway closed coast. Especially, the first bathing beach is closest to coast, which the distance is less than 100 m. So the geographical factors make it favorable for the application of SWHP system. Qingdao shares a maritime climate. The average temperature in August which is the hottest month in Qingdao is about 27 o C. In this case, Qingdao subway requires the use of refrigeration air conditioning systems in summer. And the requirement of cooling load of closed coast s is shown by Table 1. Fig. 1. the sketch map of Qingdao subway project. Table 1. the requirement cooling load of closed coast s Number of Subway Station NO.1 NO.2 NO.3 NO.4 NO.5 NO.6 NO.7 NO.22 Name of Subway Station Railway Daxue road The first bathing beach Tiantai stadium Taipingjiao Zhanshan road Wusi square North railway Cooling load / kw

3 A Feasibility Study on the Application of Seawater-source Heat Pump in the Subway of Qingdao 3 Fig. 2. the location of faults and subway s. faults are distributed between No.1 and No.7. So bedrock fissure seawater may be abundant around fracture zones. In this case, two ways of taking seawater are being proposed, (1) Pumping deep fracture seawater, (2) Pumping surface seawater. Next the feasibility of SWHP systems will be discussed through two points Pumping deep fracture seawater Qingdao located New huaxia uplift zone with secondary tectonic unit-the northeast edge of Jiaonan uplift and southern South Central Jiaolai Depression. So bedrock fracture structure is fully developed in Qingdao which direction mainly is northeast. So less sediment, non-aquatic and cooler bedrock fissure seawater can be pumped through drilling deep wells, if the connectivity degree between faults and ocean is well. In this case, both the efficiency of SWHP system can be increased substantially and the initial investment of system can be saved. Because of directivity of structural fissure which is controlled by tectonic stress field, it is important to explore the direction of structural fissure for drilling to pump. In Fig. 2 the distribution of faults are showed and there are no faults structure around North railway. The largest fault zone was Cangkou fault zone (F12). And the pumping test has been done around Cangkou fault zone, where the depth and diameter of the test well are about 50 m and 90 mm respectively. The result shows water inflow was about 7.92 m 3 /d. The cooling load of Tiantai stadium is the least, which is about 1386 kw (showed by Table 1). Assumed the temperature difference between water supply and return is 5 o C and COP of SWHP system is 5, the cooling seawater requirement is about Fig. 3. The monthly average seawater temperature in south coast and Jiaozhou bay. m 3 /h. water permeability of bedrock is too small to satisfy SWHP system requirement. If we take this scheme, 730 wells are needed. It is not permitted through analyzing the initial investment. Cooling seawater inflow can not satisfied SWHP systems. Besides the project of Jiaozhou bay subsea tunnel is referred and water inflow of tunnel structural fissure is about 2 m 3 /h which is poorer than m 3 /h. In conclusion, the cost of drilling is too high because the bedrock is granite in Qingdao. So the propose of drilling deep wells to pump cooling seawater to provide cooling source for SWHP system is infeasible. However, the way of pumping deep bedrock fissure seawater can be applied to regions which have favorable geographical conditions for SWHP system Pumping surface seawater Seawater temperature is an important parameter for SWHP system, which will affect COP of system and the cost of operation. In Fig. 3, the monthly mean coastal surface seawater temperature of the south coast

4 4 Youyuan ChenÁYajun HanÁYonggang JiaÁRuixia LiÁWenwei LiÁYuanlin Zou seawater (the location is showed in Fig. 1) and Jiaozhou bay is shown. It can be seen that the hottest temperature both appear in August, the former is about 25.9 o C, and the later is about 25.5 o C. The temperature is within the suitable temperature range of heat pump unit. In summer, the temperature difference between seawater and air is about 4 o C~5 o C (Ma et al., 2004). The same time we refer to the project of Qingdao Olympic Sailing Center which locate by this project (showed as fig 1 NO 23) and have run for three years. Synthetically considering, the temperature in these two places are suitable to be used as cooling seawater of SWHP system. Then the feasibility analysis will be taken according to pumping surface seawater. Fig. 4. No.1 ~ No.7 SWHP system operation way (direct SWHP system). 2. SWHP system operation ways In Qingdao, the south coast region along the subway s is an important scenic spot. A lot of limitations of government policy are made to protect these heritages, so it's not suitable to build some pump rooms in the south coastal scenic spot. Meanwhile, it is only 898m from the first bathing beach to 5m depth contour. It is favorable to build a big pump house which provides the cooling load of seven s (No.1 ~No.7 s) around the first bathing beach. The direct SWHP system is selected. Seawater passes filter then can be directly taken to heat pump units (schematic layout is showed in Fig. 4) in this way the loss of energy can be reduced. The total cooling load is about kw. Supposing COP is 5 and temperature difference is 5 o C, the seawater requirement is about 2370 t/h. It is about 3462 m long from NO 22 to 5m depth contour. So it is favorable to build a separate pump house. Indirect SWHP system could be designed which can avoid heat pump units be corrupted. In this way, cold seawater can also be considered to be used as free cooling for partial-load operation during the transition seasons. In indirect SWHP system, seawater should transfer heat with plate heat exchanger after crossing filter, then enter heat pump units (schematic layout is showed in Fig. 5). Supposing the demand of total cooling load is about 1619kW, COP is 5 and temperature difference is 5 o C, the seawater requirement is about 340 t/h. Fig. 5. (NO22) SWHP system operation way (indirect SWHP system). 3. Discharging seawater The distance between intake and outfall is an important parameter. If the distance is too long, the initial investment will be too large. Whereas, if the distance is too short, the seawater short circuit may happen between taking and draining units. In that case, the cooling seawater temperature will become more and higher. COP will depress, and the SWHP system can not show the energy saving character. The distance can be calculated by the following formula (Zhou, 2005). 20Q L m 2 s Where L is the distance between intake and outfall (m); Q is cooling water consumption (m 3 /s). According to the function and protective target of maritime space, the seawater quality standard is Second Grade which stipulates the rising temperature can t be over 1 o C in summer (Huang et al., 1997). In this way, thermal effective radius (TER) has to be calculated. An assumption is made: the supply/return seawater temperature difference is 5 o C, the design seawater temperature is 27 o C and the cooling load is

5 A Feasibility Study on the Application of Seawater-source Heat Pump in the Subway of Qingdao 5 about kw (shown by Table 1), so the largest seawater demand is 0.67 m 3 /s and kw heat is discharged. The heat Synthetic transfer coefficient 10 ~ 100 W/ (m 2. o C) is taken (Jiang et al., 2006). In this case, the most maximum extent of cooling water affection is about 266 m 2. It s so small for the first bathing beach and won t produce huge harm. So we can neglect the effect of heat discharged by SWHP system. The seawater flows of Huiquan bay include wind current, tidal current and wave current. Wind current shows Northwest-southeast in summer. Flood tide shows Northwest-southeast and head tide shows Southeast-northwest. Wave current shows Northwestsoutheast (Li and Xu, 1992). In order to reduce the thermal effect, so outfall should be built in the northwest of bathing beach 4. SWHP system stabilization 4.1. Seawater corrosion and protection Sea water is a strong electrolyte. Many metal materials can be corrupted. Generally physical and chemical parameters of seawater which cause corrosion include salinity, chlorine, conductivity, ph, dissolved oxygen, temperature, flow rate and marine biology and so on. It is necessary to take some measures for SWHP system to avoid corrosion. Heat exchanger can not be protected by anticorrosion coating because it will reduce heat transfer efficiency of heat exchanger. So different anticorrosion measures should be taken for different parts of SWHP system. We can select titanium miniplate heat exchanger. And copper-nickel alloy can be selected for heat exchange tube in direct SWHP system. HDPE duct can be taken when pipe diameter < 600 mm and concrete pipe or steel pipe can be selected, when pipe diameter 600 mm. The same time, some anticorrosion measures should be also taken for pipe protection. E.g.: mechanical cleaning, anti-halobios adhesion material and catholic protection and so on Self-cleaning heat exchanger Taking a suitable filtering, the growth speed of fouling can be reduced. However, it is unavoidable, so it s necessary to study on effect of fouling on heat transfer performance of heat exchanger, then take some measures to clean heat exchanger. In indirect SWHP system, when seawater flows through plate heat exchanger, fouling will grow in plate heat exchanger. In direct SWHP system, seawater flows through heat pump units and fouling will grow in heat pump units too. So, different measures should be designed for different systems. In order to guarantee the continuous operation of heat pump unit, self-cleaning will be taken. The basic principles of self-cleaning include: Physical cleaning and Chemical cleaning For plate heat exchanger, CIP on-line chemical cleaning should be taken, because this system has obvious greater cleaning effect than physical backwashing. For direct SWHP system, tube nest cleaning system can be taken, because heat pump unit is unnecessary to stop to clean the evaporator and condenser. If physical cleaning method and chemical cleaning methods are taken, the heat units must be stopped (Chen et al., 2008). IV. Conclusions Cooler, Cleaner seawater can be got by pumping deep fracture seawater. Then the cost of filters and part cost of heat pump machine can be saved. Cooler seawater can provide more cold energy with more little seawater. However, seawater inflow is too little to satisfy SWHP system, so pumping deep fracture seawater in coastal can t be taken. The surface seawater temperature is low enough for heat pump units, so the way of pumping surface seawater can be taken. And the preliminarily systems architectures are designed as No.1 ~ No.7 s with direct SWHP system and NO22 with in direct SWHP system. Thinking about ocean current, intake should be designed on the northwest of the bay located the NO3, in order that the discharging heat is taken through intake units. The NO3 is next to the First Bathing beach. So some measures must be taken to avoid the thermal pollution. By calculating, the thermal impact area is about 266 m 2, it so small that can be neglected. Finally anticorrosion measures should be taken for different parts of SWHP system. e.g.: select titanium miniplate heat exchanger for plate heat exchanger,

6 6 Youyuan ChenÁYajun HanÁYonggang JiaÁRuixia LiÁWenwei LiÁYuanlin Zou Copper-nickel alloy for heat exchange tube in direct SWHP system and so on. And Self-cleaning system should be selected suitably. CIP on-line chemical cleaning should be taken in indirect SWHP system. Tube nest cleaning system should be taken in direct SWHP system. Acknowledgments The presented work was funded by the project of Qingdao subway. References 1. Li Z, Duan M, Shu H, Jiang S, Zhu Y., 2007, District cooling and heating with seawater as heat source and sink in Dalian, China, Renewable Energy, 32(15), pp Runming Yao, Baizhan Li, Koen Steemers, 2005, Energy policy and standard for built environment in China, Renewable Energy, 30(13), PP Shu Haiwen, Duanmu Lin, Li Xiangli, Zhu Yingxin, 2010, Energy-saving judgement of electric-driven seawater source heat pump distric heating system over boiler house district heating syst.em, Energy and Buildings, 42(01), pp Huo Shanglong, Huo Xiangxing, 2009, study on process design of large sewage/seawater heat pump s, Heating and Ventilation & Air-condition, 39(27), pp (in Chinese). 5. Guo Hongli, 2007, experiment study on subway waste heat-ground source hybrid heat pump, Harbin Institute of Technology University, Master s dissertation. 6. Van R, J.C. L, T.K., 1991, Air Conditioning With Deep Seawater: A Reliable, Cost Effective Technology, Ocean'91 Conference 1991 proceedings pp Ma J, Dai B, Zhang J., 2004, Reviews on use of subsurface seawater current as natural cold resources for local air-conditioning systems, Journal of North China Electric Power University, 31(06), pp Zhou J., 2005, Surface Water Intake Project, Chemical Industry Press. (in Chinese). 9. Huang Z et al., 1997, Sea water quality standard (GB ) (in Chinese). 10. Jiang S, Duan M, Wang S., 2006, advances in seawater thermal diffusion and new issues analysis, Energy Environmental Protection, 20(05), pp Li C, Xu H., 1992, preliminary exploration of bathing beach arrosion and protection in Qingdao Huiquan bay, Coastal Engineering, 11(01), pp Chen Y, Han C, Wang J, Zhang W, Shi L, Chen J., 2008, Experimental analysis on cleaning methods of heat pump units in surface water heat pump systems, Heating Ventilating & Air Conditioning, 38(12), pp (in Chinese).