In this section, we present the distribution of total heat production across different heat pumping stations in 1994. The data is summarized in Table 2, which shows the heat generation at each station connected to specific pipeline networks. **Table 2: Distribution of Total Heat Production in 1994 by Heat Pumping Station** | 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 and heating networks operate independently, with the cooling system still under development as illustrated in Figure 4.  ### 3.2 Stockholm Sea Water District Cooling System In May 1995, Stockholm Energi launched a new district cooling system in the city center, utilizing seawater from the Baltic Sea as its primary cooling source. This innovative project leverages the natural temperature differences in the sea to provide efficient cooling for buildings and industrial processes. The refrigeration unit is located 4 km away from the city center near the Värtan heat pump station. It consists of four heat pumps, each with a capacity of 25 MW, using seawater as a heat source. The heat generated is distributed through the district heating network, while the cold is used for the cooling system. The heat pump station has two water intakes: one on the surface and another at a depth of 20 meters. Seawater is drawn in, cooled through a series of plate heat exchangers made of titanium to resist corrosion, and then distributed through a 4 km-long pipeline with a diameter of 800 mm. The cold water leaving the pumping station is maintained below 6°C, while the return water reaches up to 16°C during peak loads. The maximum design load of the regional cooling system is 60 MW. After passing through the heat exchangers, the heated seawater is either released back into the sea or recirculated depending on the operational mode. In periods when seawater temperatures are not sufficient, the heat pump is used to pre-cool the incoming water before it enters the heat exchanger.  ### 3.3 Värtan Ropsten – The World’s Largest Seawater Heat Pump Heating Station The Värtan Ropsten district heating station provides approximately 60% of the energy input to the Central Network. In the early 1980s, due to rising oil prices and cheap electricity, there was a growing interest in heat pump technology. 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 Värtan Ropsten. Six units from the Swiss company AXIMA, model Unitop® 50FY, were installed. Initially, they operated with refrigerant R22, but after the phase-out of R22, the first unit was upgraded to R134a in 2003. **Table 3: Technical Data of the Heat Pump Units** | Parameter | Value | |-------------------------------|----------------------| | Stand-alone heating capacity | 30 MW | | Stand-alone power consumption | 8 MW | | Evaporation / condensation temp | -3°C / +82°C | | Sea water inlet / outlet temp | +2.5°C / +0.5°C | | Water / return water temp | +57°C / +80°C | | Adjust ability | 10–100% |  Each heat pump unit is connected to two seawater intake pipes. During summer, warm surface water is used, while in winter, water from a depth of 15 meters is drawn in, maintaining a stable temperature of around +3°C. High-powered seawater pumps supply the evaporators, where thin-film evaporation allows efficient operation even at low temperature differences. A Siemens PLC control system manages the entire process, ensuring smooth and reliable operation. The system has a payback period of about three years. ### 4 Summary China has over 110,000 kilometers of coastline, with many cities located along the coast. These urban centers have high demands for environmental protection and energy-efficient technologies. They also have extensive experience in centralized heating and pipeline network management. In China’s Yellow Sea and Bohai Sea regions, seawater temperatures in February often exceed 2°C, meeting the requirements for heat pump operations. Similar to Sweden, the coefficient of performance (COP) for heat pumps can reach about 3, and deeper waters reduce installation costs. In summer, at depths of 35 meters, seawater temperatures typically range between 12–14°C. The South China Sea exhibits tropical deep-sea characteristics, with an average annual surface temperature of 28.6°C, except in the northern areas. Cities like Dalian and Qingdao in northern China have climates similar to those in northern Europe, making them ideal for seawater resource utilization. Combining local geography with heat pump technology can bring significant economic and social benefits. For example, in Dalian, seawater and sewage-source heat pumps can be used in conjunction with district cooling systems. In Beijing, wastewater sources can support large-scale heat pump and district cooling projects. In Shanghai, seawater can be used to power large heat pumps and regional cooling systems. Guangzhou can utilize the Pearl River for district cooling. Overall, the development of regional cooling must be tailored to local geographic, climatic, and water temperature conditions to achieve optimal results.

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