Ground source heat pump systems, particularly those with vertical buried heat exchangers, are known for their high efficiency and environmental benefits. However, the high cost of drilling often limits their widespread adoption. By integrating U-shaped heat exchange tubes into building pile foundations—such as precast pipe piles, bored piles, and underground diaphragm walls—the need for separate drilling is eliminated. This not only reduces construction costs but also makes better use of available space, offering a more sustainable and economical solution.
Keywords: ground source heat pump; underground heat exchanger; bored pile; prefabricated pile; nested installation
Ground source heat pump technology has gained significant traction due to its energy efficiency, environmental friendliness, and long-term cost savings. It is commonly used in air conditioning systems, either through vertical boreholes, surface water, or groundwater. Among these, the vertical buried tube system stands out for its compact design and high heat exchange efficiency. However, the high initial investment, largely driven by drilling costs, remains a major barrier to broader implementation.
One innovative approach is to nest the underground heat exchanger within the building's pile foundation. This involves embedding U-shaped heat transfer pipes inside precast pipe piles, cast-in-place piles, or diaphragm walls. This method eliminates the need for separate drilling, significantly lowering construction costs and reducing the footprint of the heat exchanger system. Additionally, because the piles are spaced apart, the thermal interference between adjacent U-tubes is minimal, leading to more stable and efficient operation. This technique is especially beneficial for buildings with limited green space and high floor area ratios.
1. Process Overview
In areas like Ningbo, where the soil consists of multiple layers such as clay, silt, and silty clay, deep foundations are typically required. These include precast pipe piles and bored piles, which are well-suited for the nesting process. The integration of heat exchange tubes into these structures allows for a seamless and cost-effective installation.
2. Installation of U-Tube in Precast Pipe Piles
Precast pipe piles, often made of reinforced concrete, can be designed with hollow cores to accommodate U-shaped heat exchange tubes. The diameter of the core varies depending on the pile size, and it can hold one or two U-tubes. Before installation, the U-tubes must be properly secured to prevent floating during grouting. The length of the tube should exceed the pile depth plus the cap height to ensure proper connection at the top.
The backfilling process, also known as grouting, plays a crucial role in enhancing heat transfer between the U-tube and the pile wall. High-pressure grouting ensures that the material fills all gaps, improving thermal conductivity and reducing resistance. The grouting must be done carefully to avoid large particles that could block the flow.
3. Installation of U-Tube in Bored Piles and Diaphragm Walls
Bored piles and diaphragm walls offer more flexibility in placing multiple U-tubes. These can be arranged in parallel or even connected in series within the pile body. The tubes are fixed using nylon ties and protected during the pouring process to prevent damage. After the pile is completed, the vertical pipes are connected horizontally to form a loop system that feeds into a manifold for the heat pump unit.
4. Project Examples
One example is an office building in China with a total area of 4,300 m². The project used 241 precast pipe piles, each equipped with double-U tubes. The system was connected to a main header, allowing the heat pump to operate efficiently. Soil analysis showed that the thermal conductivity of the surrounding soil was 1.6 W/m·K, while the backfill material had a higher value of 1.9 W/m·K, ensuring good performance.
Another example comes from Germany, where the Stadtwerke Bochum building utilized 104 embedded pipe piles, demonstrating the practicality and effectiveness of this method in real-world applications.
5. Conclusion
The integration of ground source heat exchangers with building piles offers a promising alternative to traditional drilling methods. It reduces costs, saves space, and improves system stability. However, due to the limited number of piles, this method may not fully support the entire heating and cooling load, requiring supplementary drilling in some cases. Successful implementation requires close coordination between civil engineering and HVAC professionals. As the technology advances, it is expected to become a key component of future green building designs.
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