Development of composite low temperature source heat pump hot water unit

The sewage heat pump project at the Tanzhou Sewage Treatment Plant in Miyun County showcases an innovative approach to utilizing untreated urban wastewater as a renewable energy source for heating, cooling, and hot water supply. By integrating advanced technology and equipment from Northern Europe, the system effectively extracts low-grade thermal energy from the wastewater through heat exchangers and heat pump units, transforming it into high-quality energy for building use. This sustainable solution not only reduces reliance on traditional fossil fuels but also significantly lowers environmental impact. Keywords: Sewage heat exchanger, Heat pump, Energy efficiency, Environmental protection **Project Overview** Located along the scenic Chaobai River, the Tanzhou Sewage Treatment Plant serves as the sole centralized facility in Miyun County, processing nearly 24,000 cubic meters of wastewater daily. The consistent temperature of the sewage—ranging between 13°C and 15°C—makes it an ideal heat source. The plant includes approximately 10,000 square meters of buildings, including offices, factories, and garages, all of which are heated using raw sewage during winter and cooled during summer. Additionally, the system provides domestic hot water, making it a multi-functional and efficient energy solution. Originally, the site used coal-fired boilers, which contributed to air pollution. To align with Beijing’s environmental goals, the heating system was upgraded to a sewage-source heat pump system. The project involved retrofitting both office spaces and industrial equipment. Office areas were equipped with fan coil units, while the plant facilities installed high-performance steel radiators. **System Principle and Design** During winter, the sewage temperature ranges from 12°C to 15°C, and after passing through the heat exchanger, it drops to around 7°C to 10°C. The system uses this wastewater as a low-temperature heat source for the water-source heat pump, which then heats the circulating water to 50°C/45°C for heating purposes. In summer, the sewage temperature rises to 14°C–18°C, and the heat is rejected through the system, providing cooling with a supply temperature of 7°C–12°C. The heat pump also recovers heat for domestic hot water, ensuring year-round functionality. The system design includes separate water distribution manifolds for each floor, with seasonal switching of valves to adapt to heating or cooling needs. The heating and cooling parameters are set according to standard conditions, with indoor temperatures ranging from 18°C–24°C in winter and 22°C–26°C in summer. **Load Calculation and System Requirements** Based on an estimated heating load of 60 W/m², the total design area of 10,000 m² results in a heating load of 600 kW. For cooling, the load is calculated at 80 W/m² for 70% of the building, totaling 560 kW. The system requires 80 t/h of water in winter and 120 t/h in summer, with a 5°C temperature difference. Given the sewage flow rate of 1,000 t/h, the system has ample capacity to meet these demands. **Challenges and Innovations** One of the main challenges is the use of untreated sewage, which contains suspended solids, sediment, and varying pH levels that can cause corrosion, scaling, and clogging in the heat exchanger. To address this, the project employed advanced materials and surface treatments, sourced from Finland, Sweden, and Denmark, and assembled in Malaysia. After three years of operation, the heat exchanger showed no signs of corrosion or fouling, proving its suitability for local wastewater conditions. A dual-grid system was also implemented to prevent large and small particles from entering the heat exchanger. Coarse and fine screens remove debris before the wastewater reaches the heat exchanger, ensuring smooth and uninterrupted operation. **Performance Data and Economic Analysis** From January 10, 2003, the system operated successfully, even during one of the coldest winters in recent years. Over two months, detailed data was collected every two hours, including electricity consumption, temperatures, and pressures. The total electricity usage over 20 days was 45,300 kWh, with an average COP (Coefficient of Performance) of 3.81. At a cost of 0.5 yuan per kWh, the total cost was 22,650 yuan, averaging 1,132.5 yuan per day. Considering a 120-day heating season and adjusting for weather conditions, the annual cost per square meter came to approximately 13.59 yuan, including domestic hot water. Compared to traditional heating methods, this system offers significant economic benefits. **Project Advantages** - **Environmental Protection:** Eliminates the need for coal, gas, or oil, reducing CO₂ emissions and air pollutants. No waste, wastewater, or dust is produced. - **Energy Efficiency:** Operates 20% more efficiently than conventional air conditioning systems due to stable sewage temperatures. - **Cost Savings:** Combines heating, cooling, and hot water into one system, replacing multiple devices and reducing overall costs. - **Stability:** Sewage temperature remains relatively constant, ensuring reliable and efficient operation without issues like defrosting in winter. - **Automation:** Features four compressors that automatically adjust based on load, improving efficiency and ease of management. **Conclusion** The sewage-source heat pump system at Tanzhou Sewage Treatment Plant represents a groundbreaking application of wastewater energy for heating and cooling. It not only supports environmental sustainability but also sets a new standard for energy-efficient building systems. The success of this project in Miyun County highlights the potential for similar initiatives across China, promoting cleaner, greener, and more sustainable urban development.

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