Abstract: This paper presents an in-depth review of the development and current status of water source heat pump technology, while identifying key challenges in its practical application. It introduces an innovative energy-efficient structure for water source heat pumps, which is expected to significantly enhance their performance and promote broader adoption in China. Keywords: water source heat pump; energy-saving; dual-evaporator; series-parallel configuration I. Introduction: The global rise in energy consumption and environmental pollution has become a critical challenge for societies worldwide. In response, countries are increasingly turning to sustainable technologies such as heat pumps. According to data from the IEA Heat Pump Center and the European Heat Pump Association (EHPA), Europe alone has over 4.5 million residential heat pumps, 1.5 million in the tertiary sector, and between 2.5 and 30,000 in industrial applications. EHPA aims to install at least 15 million residential heat pumps by 2010, which could save 100 TWh of energy annually and cut CO₂ emissions by 40 million tons. By 2002, more than one-third of new homes in Switzerland used heat pumps, and Japan had a 20% penetration rate in buildings. In contrast, China’s heat pump market began to grow rapidly after 1990, reaching 11.4 million units by 1997. With the growing restrictions on oil-fired boilers, heat pumps have found increasing opportunities in the market. The two main types of heat pumps are air-source and water-source (or ground-source). While air-source systems are highly affected by climate, water-source systems benefit from stable groundwater temperatures, making them more efficient and reliable. Their advantages include low energy consumption, use of renewable resources, no water depletion, and minimal environmental impact, aligning well with sustainable development goals. II. Current Status of Water Source Heat Pumps: Despite these benefits, the application of water-source heat pumps faces several challenges. One major issue is ensuring a consistent and stable water supply. Additionally, managing groundwater recharge and maximizing energy extraction from water during winter are critical concerns. Most projects currently rely on natural recharge, which depends on gravity. Maintaining balance in the water system is essential, requiring a ratio of 1:2 or 2:3 between extraction and recharge wells. This increases investment costs and reduces efficiency during partial load operations. Solving these issues while reducing costs and improving performance will be crucial for the future of water-source heat pumps. III. Energy-Saving Water Source Heat Pump Unit: To address the technical challenges associated with temperature fluctuations, a new design was developed. This unit features two small evaporators, each connected to one or more compressors, condensers, and expansion valves, forming independent refrigeration cycles. A valve system allows the two evaporators to operate in either parallel or series configurations depending on the season. During cooling mode, the valve opens, allowing return water from the air conditioning system to flow through both evaporators in parallel. Each evaporator operates at 12/7°C, ensuring optimal cooling capacity. In heating mode, the valve closes, and the groundwater flows through the evaporators in series, achieving a 10°C temperature drop. This configuration maintains high flow rates and heat transfer efficiency, even with reduced water flow, thereby minimizing groundwater usage without compromising performance. IV. Case Study: An office building with a total area of 4,600 m² was designed using this energy-efficient water source heat pump system. The cooling load required was 460 kW, and the heating load was 506 kW. The site had a single well producing 50–60 m³/h of water, with summer and winter temperatures of 16°C and 15°C, respectively. Four semi-hermetic piston compressors were used, paired with two evaporators and two condensers in separate systems. Under cooling conditions, the evaporators operated in parallel, with an inlet/outlet temperature of 12/7°C and an evaporation temperature of 2°C. The condensers also ran in parallel, with an inlet/outlet temperature of 16/26°C and a condensing temperature of 31°C. During heating, the evaporators were in series, with an inlet/outlet temperature of 15/9.5°C and 9.5/5°C, respectively. The condensers remained in parallel, with an inlet/outlet temperature of 40/45°C and a condensing temperature of 50°C. The results showed that the system provided 466 kW of cooling power with 89.5 kW of electrical input, achieving a COP of 5.2. For heating, the system delivered 511 kW of heat with 121.7 kW of power, resulting in a COP of 4.2. The water consumption was reduced by 20% in winter, while the heating capacity increased by about 10%. These improvements demonstrated the effectiveness of the new design in saving resources and reducing operational costs. V. Conclusion: This innovative structure ensures consistent heat transfer performance, regardless of whether the evaporator operates under a 5°C or 10°C temperature difference. The design fully utilizes the heat transfer area of the evaporator in both cooling and heating modes. Moreover, it reduces water consumption by 20%, increases heating capacity by 10%, and improves the COP by 7%. By optimizing the internal structure and adjusting the evaporator water system configuration, the system achieves greater efficiency, lower initial investment, and reduced operating costs. This not only enhances economic returns but also supports sustainable development, paving the way for wider adoption of water-source heat pumps in China. With continued innovation and implementation, this technology holds great promise for the future of green energy solutions.
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