Abstract: This paper provides a comprehensive overview of the development and current status of water source heat pump technology. It identifies key challenges in the practical application of such systems and introduces an energy-efficient design that enhances performance while reducing resource consumption. This innovative structure plays a crucial role in promoting the widespread adoption and sustainable growth of water source heat pumps in China. Keywords: water source heat pump, energy-saving, dual-evaporator, series-parallel configuration I. Introduction: Energy consumption and environmental pollution have become major global concerns, prompting increased interest in sustainable heating and cooling solutions. According to data from the IEA Heat Pump Center and the European Heat Pump Association (EHPA), Europe currently has about 4.5 million residential heat pumps, 1.5 million for the tertiary sector, and 2.5 to 30,000 for industrial use. EHPA aims to install at least 15 million residential heat pumps by 2010, which could save 100 TWh of energy annually and reduce CO₂ emissions by 40 million tons. In Switzerland, over one-third of new homes are equipped with heat pumps, and in Japan, the market share reaches 20%. In China, heat pump applications began to grow rapidly after 1990, reaching 11.4 million units by 1997, and continue to expand. The growing restrictions on oil-fired boilers have created significant opportunities for heat pump technologies. Heat pumps are generally categorized into air-source and water-source (or ground-source) types. While air-source systems are highly influenced by weather conditions, water-source systems benefit from stable groundwater temperatures, making them more efficient and reliable. Due to their low energy consumption, renewable energy utilization, no water depletion, and environmental friendliness, they align well with sustainability goals and have gained wide acceptance. II. Current Status of Water Source Heat Pumps: One of the main challenges in applying water source heat pumps is ensuring a consistent and reliable water supply. Additionally, managing groundwater recharge and maximizing energy extraction from the water source during winter remain critical issues. Most projects rely on natural recharge, which depends on gravity rather than pumping. Maintaining balance in the system is essential to ensure that extracted water is fully replenished. Typically, the ratio of extraction wells to recharge wells is around 1:2 or 2:3, which increases costs and reduces efficiency during partial load operation. The ability to reduce investment and operating costs while guaranteeing full recharge directly impacts the long-term success of these systems. Developing energy-efficient and water-saving heat pump units can significantly support the broader adoption of this technology in China. III. Energy-Saving Water Source Heat Pump Unit: To address technical challenges related to temperature differences and system efficiency, we designed a unit with two small evaporators connected in either series or parallel through valve control. Each evaporator operates within its own refrigeration cycle, including compressors, condensers, and expansion valves. During summer cooling mode, the valve allows parallel flow, enabling the return water from the air conditioning system to pass through both evaporators simultaneously. This setup ensures equal temperature drops across each evaporator (12/7°C). In winter heating mode, the valve closes, creating a series connection, allowing the groundwater to pass through both evaporators sequentially, resulting in a larger temperature drop (15/5°C). This design maintains consistent flow rates and heat transfer performance, effectively saving groundwater while maintaining efficiency. IV. Project Application Example: An office building covering 4,600 m² used this energy-saving water source heat pump system as a cold and heat source. The building required a cooling load of 460 kW and a heating load of 506 kW. With a single well providing 50–60 m³/h of water at 16°C in summer and 15°C in winter, four semi-hermetic piston compressors were selected, paired with two independent evaporator-condenser systems. Under cooling conditions, the evaporators operated in parallel, with inlet and outlet temperatures of 12/7°C and a refrigerant evaporation temperature of 2°C. The condensers also ran in parallel, with a condensing temperature of 31°C. In heating mode, the evaporators were connected in series, achieving a temperature drop of 10°C, while the condensers remained in parallel. The results showed a total cooling capacity of 466 kW with a COP of 5.2 and a heating capacity of 511 kW with a COP of 4.2. The system reduced water usage by 20% and improved heating capacity by 10%, proving the effectiveness of the design. V. Conclusion: This innovative structure ensures consistent heat transfer performance regardless of whether the evaporator temperature difference is 5°C in cooling or 10°C in heating. The evaporator's heat transfer area is fully utilized in both conditions. Additionally, the system improves heating capacity by about 10% and increases COP by approximately 7% under winter operating conditions. By optimizing the evaporator configuration and water system layout, the design reduces groundwater extraction, minimizes well numbers, and enhances overall efficiency. This not only lowers initial investment and operational costs but also brings significant economic and environmental benefits, accelerating the development and application of water source heat pumps in China.
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