Application of seawater heat pump system in Stockholm and its prospect in China (2)
Table 2 Distribution of Total Heat Production in 1994 at Different Heat Pumping Stations Table 2 The total heat production in 1994 divided on production plants The regional cooling network and heating network are independent and the other is a pipeline, which is still under construction as shown in Figure 4. Figure 4 Schematic diagram of the district cooling transport network in central Stockholm 3.2 Stockholm sea water district cooling system In May 1995, Stockholm Energi started to cool the center of Stockholm (the Swedish capital) with its new district cooling system and various process processes. The unique aspect of this project is that most of the energy used to cool the system comes from seawater in the Baltic Sea. The refrigeration unit is located near the existing Värtan heat pump station 4 km from the city. The heat pump station has four heat pumps, each with a capacity of 25MW and energy from sea water. The heat generated at the heat pump station is sent to the district heating network. The heat pump station has two water intakes, one on the surface and the other on the sea floor at a depth of 20m. The cold is generated by aspirating low temperature seawater at the intake, sending it to a heat pump, and then passing through six plate heat exchangers to cool the water supplied to the district cooling network. In order to withstand corrosive, salty seawater, the heat exchanger plate consists of titanium. The temperature of the cold water leaving the pumping station is 6 ° C or lower and the return water coming back from the pipe network is 16 ° C at higher load and lower at lower load. Regional cooling system design maximum load is 60MW. After passing through the heat exchanger, the heated seawater is released into the sea or back into the heat pump, depending on the current mode of operation. In the operating phase where seawater temperature is not sufficient, the water entering the heat exchanger is first cooled to a suitable temperature by the heat pump. Figure 5. The prepared cold water enters the city from the cold station through a 4km long, 800mm diameter pipeline. Figure 5 Schematic diagram of the Stockholm district cooling system Figure 5 Principal diagram for district cooling in Stockholm Coastal climate changes little between winter and summer in Stockholm. The annual average temperature is +7 ℃, July is +18 ℃, January is -3 ℃. In normal winter, the surface of seawater is frozen for two months, and the surface temperature of seawater in July reaches 20 ℃. Stockholm, on the shores of the Baltic Sea, at the mouth of Lake Malaren, the largest lake in Sweden, offers the advantage of using seawater for cooling. The average salinity of the Baltic Sea water in the area around Stockholm is 0.6%. The inflow of freshwater is lower than the density of seawater, so freshwater floats above the seawater and across the islands into the Baltic Sea. This surface current draws some of the lower saltwater and attempts to displace the sea, creating a countercurrent flow into the waters surrounding Stockholm. It is this countercurrent that conveys low-temperature groundwater to the watershed of the district cooling system. It takes about three months to flow across the islands to Stockholm. The water arriving at the refrigeration station water intake in July left the surface of the archipelago in May. The water temperature in the archipelago in May was very low, because the ice melted at that time. Natural convection conditions cause the bottom water temperature and surface water temperature to vary over the year. Figure 6. This figure also shows the temperature required for the refrigeration system. The ultimate guarantee to the user's water temperature does not exceed 6 ℃. During summer high-load phases, increase the capacity of the network by lowering the water temperature. [10] Figure 6 Värtan seawater temperature Figure 6 Temperature at Värtan As shown in Figure 6, either surface or bottom seawater is not low enough throughout the fall to be used as the only cold source to meet the needs of the cooling network, so the existing heat pump was then used to produce the required Cooling capacity. These heat pumps are necessary for this project and determine the location of the refrigeration station. [10] 3.3 Värtan Ropsten - the world's largest seawater heat pump heating station The Värtan Ropsten district heating station provides about 60% of the total energy input to the Central Network. In the early 1980s, oil prices rose, electricity prices were cheap, and people were more interested in heat pumps. Between 1984 and 1986, one of the world's largest seawater-based heat pumps with a heating capacity of 180 MW was installed at the Värtan Ropsten Heating Station . A total of six units from the Swiss company AXIMA refrigeration centrifugal pump unit (Model Unitop ® 50FY). Initially, all units were operated with refrigerant R22. A continuous operation of the seal oil system to prevent the operation of the refrigerant loss and heat pump unit to stop running. After the deadline for stopping the use of refrigerant R22 was established, in 2003 the first heat pump refrigerant was replaced with R134a. Table 3 Technical Data Table 3 Technical Data Figure 7 Unitop ® 50FY heat pump unit outline installed a total of six such units Figure 7 General view of a heat pump unit type Unitop ® 50FY. 6 of these units are installed. Figure 8 Place heat pump unit room Each heat pump unit connected to two seawater intake Figure 8 Machine room building for the 6 heat pump units. Two sea water intake pipes are connected to each unit. In order to maintain a low temperature drop, a large amount of sea water is used as heat source. Summer use the surface of the warm water. In winter, the inlet 15m deep, the water temperature is fixed at about +3 ℃. A high-powered seawater pump supplies seawater to two thin film evaporators. Two such evaporators are installed in each heat pump unit. A thin, stable water film passes through the surface of the heat exchanger's flat plate and has a short contact time. For this reason, thin-film evaporators can operate at very low temperature differences, increasing the reliability of system operation. The Siemens PLC control system is used for local control and management of the heat pump unit as well as control of the entire Värtan district heating station. [11] The system payback period is about 3 years [8] . 4 summary There are over 110,000 kilometers of coastline in our country, with numerous islands and peninsulas. Many big cities are located on the coast. At present, the coastal cities in China develop rapidly and the buildings are densely distributed. They have great demands on environmental protection technologies and energy-saving technologies. Moreover, they have accumulated decades of experience in central heating and have completed the design and management of pipe networks experience. China's Yellow Sea and Bohai Sea areas in February the lowest temperature of the sea surface temperature most of the region in more than 2 ℃, the temperature can meet the heat pump operating conditions, the same in Sweden under the condition of the heat pump COP value can reach about 3, and water depth The reduction can greatly reduce pipe installation costs. In the summer, at a depth of 35 meters, seawater temperature is mostly around 12-14 ° C. Affected by cold water mass near Shandong Peninsula, more intensive isotherms lead to lower seawater. The distribution of South China Sea water temperature has obvious characteristics of tropical deep sea. The average annual surface water temperature, with the exception of the northern coast, most of the 28.6 ℃. South China Sea water temperature difference between north and south is generally 4 ℃, summer is smaller, only 2 ℃. Deep water temperature as low as 2.36 ℃, few seasonal changes [12] . Some cities in northern China, such as Dalian and Qingdao, are very close to the climate in northern Europe and have very convenient conditions for the utilization of seawater resources. If according to local geographical conditions, combined with heat pump technology, large-scale development as a whole, it brings huge economic and social benefits. In Dalian, the seawater source and sewage source heat pump can be used to combine large heat pumps with district cooling. In Beijing, wastewater sources are used to combine large heat pumps with district cooling and heat recovery technologies. In Shanghai, seawater sources can be used to convert large Heat pump and regional cooling combined; Guangzhou can use the Pearl River water for district cooling. In short, the development of regional refrigeration must be combined with the local geography, climate and water temperature conditions, and take large-scale roads according to local conditions. The structure of the traditional wipers, there are inevitably multiple pressure points, already cannot adapt to modern car windshield this mainstream surface design, usually, use three months after they leave the car appeared different degree: scrape wash not ideal, water mark excessive noise, friction, thus seriously affect reason such as driving snow days safe! Without bone wipers system with unique intelligent adaptive spring steel shell fragments, dispersed homogeneously pressure, make each part blade force balance, avoid the disadvantages of the traditional windshield wipers. 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