Ground Source Heat Pump Underground Heat Exchanger and Pile Construction Nesting Process
Abstract: Vertical ground source heat pump systems offer high heat exchange efficiency, but their widespread use is limited due to the high cost of drilling. By fully utilizing building pile foundations and embedding U-shaped heat exchange tubes in prefabricated pipe piles, bored piles, and underground continuous walls, the drilling process can be avoided, significantly reducing the construction cost of underground heat exchangers and lowering the initial investment of the system.
Keywords: ground source heat pump; underground heat exchanger; bored pile; prefabricated pipe pile; nested process
Ground source heat pump systems have gained rapid popularity due to their high efficiency, environmental friendliness, and energy-saving performance, making them a leading choice for energy-efficient central air conditioning. These systems can be applied using vertical buried tubes, surface water, or groundwater. Among these, the vertical installation method offers higher heat exchange efficiency and requires less floor space for the underground heat exchanger. However, its application is constrained by the high initial investment, mainly due to the expensive drilling process.
Integrating the underground heat exchanger with the building’s pile foundation—by placing U-shaped heat exchange pipes inside precast pipe piles, concrete-filled piles, or underground continuous walls—can eliminate the need for separate drilling, reduce construction costs, and make more efficient use of available floor space. Additionally, the large spacing between piles minimizes thermal interference between U-tubes, resulting in more stable operating conditions for the heat exchanger. This technology opens up new possibilities for buildings with limited green areas and high floor area ratios, and it is expected to become a new model for vertical buried tube applications.
First, Process Introduction
In Ningbo, the vertical soil profile is generally divided into four layers: clay, silt, silty clay, and silt. The soft and thick soil layers require deep foundations for buildings. The common foundation types are precast pipe piles and bored piles, which are well-suited for the nesting of underground heat exchangers.
1. Laying U-Tube in Prefabricated Pipe Piles
Prefabricated pipe piles are typically made of reinforced concrete, wood, or steel. Hollow piles usually have diameters of 400mm or 550mm, with wall thicknesses of about 80mm. The inner cavity allows for the placement of one or two U-shaped heat exchange tubes, which helps reduce construction costs. Before installing the U-tube, it should be bundled with a grouting tube and secured to prevent floating. Protective materials like sacks are placed at the pile mouth to prevent wear and pressure drop during concrete pouring.
The length of the U-tube must be longer than the pile depth plus the cap height. After installation, backfilling (grouting) is performed to enhance heat transfer between the U-tube and the pile wall. Grout is injected from the bottom upward, ensuring tight compaction and minimal voids. The process is complete when the returned slurry density matches that of the grouting material.
2. Laying U-Tube in Bored Piles and Underground Continuous Walls
Bored piles are constructed by drilling holes, inserting reinforced cages, and pouring concrete. Diaphragm walls involve creating long narrow slots, hanging steel cages, and pouring concrete to form a continuous underground wall. Due to the larger diameter of bored piles, multiple U-tubes can be placed on the steel cage and arranged in parallel or series within the pile body. They are fixed with nylon cable ties and protected to avoid damage during pile head treatment.
For horizontal connections, vertical U-tubes are bent at the pile head, passed through the casing, and welded to the main manifold. Pressure tests are conducted to ensure no leaks. After all connections are completed, the horizontal pipes are covered with 15cm of sand or soil.
Second, Project Application Examples
This system uses U-shaped heat exchange tubes embedded in building pile foundations (including precast pipe piles, bored piles, and underground continuous walls). A number of U-tubes are assembled to form heat exchange circuits, connected to headers and separators, providing a cold and heat source for the building's air conditioning system.
Example 1:
An office building project covering 4300 m² uses a double-U hollow tube ground source heat exchanger system. It includes 241 precast pipe piles (400–500 mm in diameter, 41–45 m deep) with double-U tubes embedded. These are connected to a main water header, and the heat exchange occurs through the ground source heat pump unit.
Pile Foundation Overview:
The project uses prestressed concrete Φ400 and Φ500 pipe piles. The total number of piles is 243, with a column spacing of 9×9 meters.
Soil Properties and Backfill Thermal Parameters:
The soil layers from 0–50 m include clay, silt, and fine sand. The soil thermal conductivity is 1.6 W/m·K, and the backfill (saturated clay or expanded cement + clay) has a thermal conductivity of 1.9 W/m·K.
Other Thermal Parameters:
The average soil temperature is 19°C. To maintain high heat pump efficiency (EER ≥ 10 or COP ≥ 3.4), the heat pump operates between 8°C and 30°C.
Heat Exchange Calculation:
Annual heat absorption is 280 × 10ⶠBtu, and annual cooling is 1512 × 10ⶠBtu. Based on a 40-meter depth, the required total heat absorption length is 1440 m, and the cooling length is 7271 m. After applying a correction factor of 2.4, the total length becomes 17,450 m. This requires 218 piles, spaced 9×9 meters apart.
Example 2:
The Stadtwerke (Bochum) company office building in Germany used 104 sets of embedded pipe piles with U-tubes installed in a coupling manner.
Third, Conclusion
The nesting of underground heat exchangers with building piles offers significant advantages, including reduced drilling costs, lower initial investment, and more efficient use of space. While this technique is promising, it is often limited by the number of available piles, meaning that additional drilling may still be needed to supplement the system. The success of this method depends on close collaboration between civil engineering and HVAC professionals. As the technology advances, it is likely to become an increasingly popular solution for sustainable heating and cooling systems.
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