Development of composite low temperature source heat pump hot water unit

**Abstract:** The sewage heat pump project at the Tanzhou Sewage Treatment Plant in Miyun County introduces advanced technology and equipment from Northern Europe, utilizing untreated urban sewage as a source for a water-source heat pump system. This innovative approach extracts low-grade energy from wastewater through heat exchangers and heat pump units, converting it into high-quality heating, cooling, and hot water supply. The system not only improves energy efficiency but also reduces environmental impact by eliminating the need for fossil fuels. **Keywords:** Sewage heat exchanger, Heat pump, Economy **1. Project Overview** Located along the beautiful Chaobai River, the Tanzhou Sewage Treatment Plant in Miyun County is the sole centralized facility responsible for treating 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. These structures are heated using untreated sewage during winter and cooled using the same source in summer. Additionally, the system provides domestic hot water, making it a versatile and efficient solution. This project was initiated to replace coal-fired boilers that contributed to air pollution. As part of Beijing’s environmental protection efforts, the heating system was upgraded to use a sewage-source heat pump system. The transformation included retrofitting office spaces with cooling fan coils and installing high-performance steel string radiators in the plant area. **2. System Principle and Design** In winter, the sewage temperature ranges from 12°C to 15°C, while the effluent temperature drops to 7°C to 10°C after passing through the heat exchanger. The system extracts this heat as a low-temperature source for the water-source heat pump, which then heats the circulation system to supply water at 50°C and return at 45°C. During summer, the sewage temperature rises to 14°C–18°C, and after heat exchange, it reaches 19°C–23°C. The system uses this as a cooling source, with the heat pump providing chilled water at 7°C–12°C. The heat pump also recovers heat to ensure domestic hot water supply throughout the year. Water distribution to individual buildings is managed via a manifold, and the system switches between heating and cooling modes using valve controls. **3. Design Parameters** - Winter outdoor temperature: -12°C - Indoor design temperature (winter): 18°C–24°C - Summer outdoor temperature: 33.2°C (wet bulb: 27.3°C) - Indoor design temperature (summer): 22°C–26°C Load calculations: - Heating load: 60 W/m² for 10,000 m², totaling 600 kW - Cooling load: 80 W/m² for 7,000 m², totaling 560 kW Water requirements: - Winter: 80 t/h with a 5°C temperature difference - Summer: 120 t/h with a 5°C temperature difference The sewage flow rate of 1,000 t/h exceeds the system’s demand, ensuring reliable operation. **4. Key Challenges and Solutions** One of the main challenges was designing a heat exchanger capable of handling raw sewage, which contains suspended solids, sediment, and has a corrosive nature. The viscosity of the wastewater was found to be about 40 times higher than that of clean water, posing significant risks of corrosion, scaling, and clogging. To address this, advanced materials and surface treatments were used, 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 Chinese wastewater conditions. A two-stage screening system was implemented to prevent large particles from entering the heat exchanger. Coarse and fine grids intercepted debris, reducing the risk of blockage and ensuring smooth operation. **5. Measured Data Analysis** The system began operating on January 10, 2003, and successfully endured one of the coldest winters in Beijing. Over a period of two months, staff recorded data every two hours, including electricity usage, temperatures, and pressures. From February 6 to February 26, 2004, the following data was collected: - Sewage temperature: 11–17°C - Evaporator inlet: 10–16°C - Evaporator outlet: 6.5–14°C - Condenser inlet: 37.5–44°C - Condenser outlet: 40–46.9°C - Domestic hot water: 50–55°C Electricity consumption over 20 days was 45,300 kWh, with an average COP of 3.81. At an electricity cost of 0.5 yuan/kWh, the total cost was 22,650 yuan, averaging 1,132.5 yuan per day. Considering a 120-day heating season and a 0.6 adjustment factor, the cost per square meter was 13.59 yuan, significantly lower than traditional heating methods. **6. Project Advantages** - **Environmental Protection:** Eliminates the need for coal, gas, or oil, reducing CO₂ emissions and air pollutants. - **Energy Saving:** Operates 20% more efficiently than conventional systems due to stable sewage temperatures. - **Cost-Efficient:** Combines heating, cooling, and hot water supply, replacing multiple systems. - **Stable Performance:** Sewage temperature remains relatively constant, improving system reliability. - **High Automation:** Features four compressors that automatically adjust based on load, reducing maintenance and increasing efficiency. **7. Conclusion** The sewage-source heat pump system represents a sustainable and efficient solution for heating and cooling. By utilizing untreated sewage, the system promotes energy conservation and environmental protection. Its successful implementation in Miyun County marks a significant advancement in China’s adoption of wastewater reuse technologies. This project not only supports the development of smoke-free and eco-friendly cities but also sets a precedent for future green infrastructure projects.

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