In the year 1994, Table 2 illustrates the distribution of total heat production across different heat pumping stations. The table shows how the heat generation was divided among various plants, with each station contributing to a specific district heating network. **Table 2: Distribution of Total Heat Production in 1994 at Different Heat Pumping Stations** | Pumping Station | Pipe Network | Heat Production (GWh) | |------------------|------------------|------------------------| | Värtan | Central | 2600 | | Hässelby | North-Western | 1100 | | Harrmarby | Southern | 800 | | Högdalen | Southern | 1200 | | **Total** | | **5700** | The regional cooling network and the heating network operate independently, with the cooling system still under development 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 launched its new district cooling system to cool the city center. A unique feature of this project is that most of the energy used for cooling comes from seawater in the Baltic Sea. The refrigeration unit is located 4 km away from the city center, near the existing Värtan heat pump station. This station has four heat pumps, each with a capacity of 25 MW, powered by seawater. The heat generated is sent into the district heating network. Two water intakes are used—one on the surface and one at a depth of 20 meters. Cold water is produced by drawing low-temperature seawater through these intakes, which is then passed through six plate heat exchangers to cool the water supplied to the district cooling network. The heat exchanger plates are made of titanium to withstand the corrosive nature of seawater. The cold water leaving the station is maintained at 6°C or lower, while the return water temperature varies between 16°C during high load and lower during low load. The maximum design load for the regional cooling system is 60 MW. After passing through the heat exchangers, the heated seawater is either released back into the sea or re-circulated depending on the operational mode. When seawater temperatures are insufficient, the water entering the heat exchanger is first cooled by the heat pump. Cold water travels through a 4 km long, 800 mm diameter pipeline to supply the city. **Figure 5: Schematic diagram of the Stockholm district cooling system** **Figure 5: Principal diagram for district cooling in Stockholm** Stockholm experiences relatively stable coastal weather, with an annual average temperature of +7°C. In July, the temperature reaches +18°C, and in January, it drops to -3°C. During normal winters, the surface of the seawater freezes for about two months, while the surface temperature in July can reach up to 20°C. Located at the mouth of Lake Mälaren, the largest lake in Sweden, Stockholm benefits from using seawater for cooling. The average salinity of the Baltic Sea around Stockholm is 0.6%. Freshwater flows over the denser seawater, creating a surface current that draws in lower-salinity water, forming a countercurrent that brings cold groundwater into the district cooling system. It takes approximately three months for this water to travel from the archipelago to the city. The water reaching the intake in July had left the archipelago in May, when ice was melting, resulting in very low temperatures. Natural convection causes variations in both surface and bottom water temperatures throughout the year. **Figure 6: Värtan seawater temperature** **Figure 6: Temperature at Värtan** As shown in Figure 6, neither surface nor bottom seawater is consistently cold enough to serve as the sole cooling source for the network. Therefore, the existing heat pumps are used to generate the required cooling capacity. These heat pumps are essential to the project and determine the location of the refrigeration station. 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, rising oil prices and cheaper electricity led to increased interest in heat pumps. Between 1984 and 1986, one of the world's largest seawater-based heat pump systems with a heating capacity of 180 MW was installed at the Värtan Ropsten Heating Station. Six units from the Swiss company AXIMA (Model Unitop® 50FY) were used. Initially, all units operated with refrigerant R22. A continuous seal oil system was implemented to prevent refrigerant loss and ensure uninterrupted operation. By 2003, the first heat pump was retrofitted with R134a after the deadline for phasing out R22. **Table 3: Technical Data** | Stand-alone heating capacity | 30 MW | |------------------------------|-------| | Stand-alone power consumption | 8 MW | | Evaporation temperature / condensation temperature | -3°C / +82°C | | Sea water inlet / outlet temperature | +2.5°C / +0.5°C | | Water temperature / return water temperature | +57°C / +80°C | | Adjust ability | 10–100% | **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 intakes)** **Figure 8: Machine room building for the 6 heat pump units. Two sea water intake pipes are connected to each unit.** To maintain a low temperature drop, a large volume of seawater is used as a heat source. During summer, warm surface water is utilized, while in winter, the inlet water is drawn from 15 meters deep, where the temperature remains around +3°C. High-powered seawater pumps supply water to two thin-film evaporators. Each heat pump unit contains two such evaporators, allowing a thin, stable water film to pass over the heat exchanger plates with minimal contact time. This enables the system to operate at very low temperature differences, enhancing reliability. A Siemens PLC control system manages local operations and controls the entire Värtan district heating station. The system has a payback period of approximately 3 years. 4. Summary China has over 110,000 kilometers of coastline, with many islands and peninsulas. Many major cities are located along the coast. Coastal cities in China are developing rapidly, with dense building structures and high demands for environmental protection and energy-saving technologies. They also have decades of experience in central heating and pipe network design. In northern China, such as the Yellow Sea and Bohai Sea areas, the lowest sea surface temperature in February is mostly above 2°C, which meets the operating conditions for heat pumps. Similar to Sweden, the COP of heat pumps in these regions can reach about 3. Deeper water reduces installation costs. In summer, at a depth of 35 meters, seawater temperatures are typically around 12–14°C. Due to the influence of cold water masses near the Shandong Peninsula, isotherms are more intense, leading to lower seawater temperatures. The South China Sea exhibits tropical deep-sea characteristics, with an average annual surface temperature of 28.6°C, except for the northern coast. The temperature difference between north and south is generally 4°C, and in summer, it is only 2°C. Deep-sea temperatures can be as low as 2.36°C with minimal seasonal variation. Cities like Dalian and Qingdao in northern China share similar climates with northern Europe and have favorable conditions for utilizing seawater resources. By combining local geography with heat pump technology, large-scale development can bring significant economic and social benefits. In Dalian, seawater and wastewater source heat pumps can be combined with large district cooling systems. In Beijing, wastewater sources can be paired with large heat pumps and heat recovery systems. In Shanghai, seawater can be used to support large heat pumps and regional cooling. In Guangzhou, the Pearl River water can be used for district cooling. In short, the development of district cooling must consider local geography, climate, and water temperature conditions, and implement large-scale solutions tailored to the region.

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