Selection of Ground Source Heat Pump System Types and Geological Condition Assessment
The selection of ground source heat pump system type depends directly on the geological conditions of the project site, the available land area, and local policy restrictions on groundwater resources. According to Section 3.1 of GB 50366 "Technical Code for Ground Source Heat Pump System Engineering," a site condition investigation must be conducted before implementation of a ground source heat pump system project, and shallow geothermal energy resources must be surveyed. The vertical ground heat exchanger system is the most commonly used form of ground source heat pump and is suitable for hotel projects in dense urban areas. Borehole depth is generally 80–200 m, borehole diameter 120–200 mm, and single U-tube (DN32) or double U-tube (DN25) high-density polyethylene (HDPE) heat exchange pipes are embedded in the boreholes. The design parameters of the vertical ground heat exchanger are determined based on soil thermal conductivity: typical soil thermal conductivity is 1.5–2.5 W/(m·K), heat extraction capacity is 50–80 W/m (single U-tube), and heat rejection capacity is 60–100 W/m. For a five-star hotel project, the number of boreholes for the vertical ground heat exchanger is relatively large. Taking a hotel with a cooling load of 4,000 kW as an example, approximately 150–250 boreholes are required (borehole depth 100–150 m), occupying about 2,000–4,000 m² (considering a 6×6 m spacing). The horizontal ground heat exchanger system is suitable for hotel projects with large idle land areas (such as golf courses or gardens), with pipe burial depth of 1.5–2.5 m and heat extraction capacity per unit pipe length of 10–30 W/m; however, it requires a large footprint and is not suitable for urban hotels with limited land. The groundwater-based ground source heat pump system uses groundwater as the heat exchange medium and offers high heat extraction efficiency (unit well yield can reach 50–100 m³/h), but reliable reinjection must be provided (reinjection rate not less than 100%), and local water resource management policies must be complied with. Geological condition assessment is the basis for ground source heat pump system design and requires investigation in the following aspects: (1) Thermal Response Test (TRT), with a test duration of not less than 48 hours, to obtain soil thermal conductivity, thermal diffusivity, and initial ground temperature data. (2) Hydrogeological investigation to determine groundwater level depth, aquifer thickness, permeability coefficient, and groundwater flow direction. (3) Ground temperature gradient testing to determine the temperature and depth of the subsurface constant-temperature layer. According to Article 3.2.2 of GB 50366, when designing vertical ground heat exchangers, specialized simulation software (such as GLHEPRO and Earth Energy Designer) shall be used based on the rock-soil thermal property test results to simulate heat exchanger performance over long-term operation of 10 years or more, ensuring long-term operational stability and reliability of the system. The Marriott MIDS standard requires that the ground source heat pump system adopt the vertical ground heat exchanger method, with borehole depth not less than 100 m, and that TRT testing be conducted to verify design parameters.
Design of Ground Source Heat Pump Tri-Generation System and Cooling/Heating Balance
The ground source heat pump tri-generation system (heating + cooling + domestic hot water) is the optimal form of ground source heat pump application for five-star hotels, as it can fully utilize the subsurface constant-temperature energy source to achieve integrated supply for multiple energy demands. According to Article 8.3.6 of GB 50736 "Code for Design of Heating Ventilation and Air Conditioning of Civil Buildings," buildings using ground source heat pump systems shall undergo annual cooling and heating load balance analysis, and the configuration of auxiliary cooling and heating sources shall be reasonably determined. The typical configuration of a five-star hotel ground source heat pump tri-generation system includes: ground source heat pump units (providing air-conditioning chilled/hot water and domestic hot water preheating), domestic hot water storage tanks, auxiliary heating devices (gas boilers or electric heaters), and cooling towers (for heat rejection peak shaving). The key to system design is the cooling and heating load balance analysis: the ratio of the hotel's annual cumulative cooling load to cumulative heating load is called the cooling/heating balance coefficient K. For five-star hotels in the Beijing area, the K value is approximately 1.5–2.0 (cooling load greater than heating load); in the Shanghai area, K ≈ 1.8–2.5; in the Guangzhou area, K ≈ 3.0–4.0. When the long-term heat rejection of a ground source heat pump system exceeds heat extraction, the subsurface temperature will rise year by year, and heat exchange efficiency will decline. According to the mandatory requirements of Article 4.3.5 of GB 50366 "Technical Code for Ground Source Heat Pump System Engineering," the total annual heat absorption and total heat rejection of a ground source heat pump system shall be basically balanced, and the imbalance rate should not exceed 10%. The main technical measures to address cooling/heating imbalance include: (1) Using the ground source heat pump system to produce domestic hot water to increase heat extraction: in summer, the heat rejected by heat pump units is used to produce free domestic hot water (it is estimated that 60%–80% of the rejected heat in summer can be recovered for domestic hot water production), reducing the heat discharged into the ground while saving domestic hot water heating costs. (2) Providing auxiliary cooling towers: when the subsurface temperature rises above a set value, the cooling tower is activated for auxiliary heat rejection to reduce the heat discharged into the ground. Cooling tower capacity is selected at 30%–50% of the peak heat rejection of the ground source heat pump system. Article 8.3.7 of GB 50736 permits the configuration of auxiliary heat dissipation equipment for ground source heat pump systems. (3) Providing auxiliary boilers: for hotels in cold regions with high winter heat extraction, gas boilers or electric boilers are provided as auxiliary heat sources to handle the peak load during the coldest periods. Auxiliary boiler capacity is determined by the difference between the design heating load and the heating capacity of the ground source heat pump. Operating strategies of the tri-generation system: In winter heating mode, the ground source heat pump units extract heat from the ground to provide 45/40°C hot water to air-conditioning terminals while producing 50–55°C domestic hot water. In summer cooling mode, the ground source heat pump units extract heat from the building and reject it into the ground while using condenser waste heat to produce free domestic hot water (heat recovery mode); the heat recovery amount can reach 15%–25% of the unit cooling capacity. In transition season mode, the ground source heat pump units are used solely to produce domestic hot water, or operation is carried out for subsurface temperature recovery. The Marriott MIDS standard requires that the domestic hot water supply temperature of the ground source heat pump tri-generation system be not less than 55°C and that auxiliary heating devices be provided.
Key design and construction technologies for borehole heat exchangers
The borehole heat exchanger is the core component of a ground-source heat pump system, and its design quality and construction level directly affect system performance and long-term operational stability. In accordance with the provisions of Section 4.3 and Section 5.4 of GB 50366 Technical Code for Ground-Source Heat Pump Systems, the design and construction of borehole heat exchangers shall strictly comply with the relevant technical requirements. Design technical points: (1) Borehole layout: borehole spacing is a key parameter affecting the long-term heat exchange performance of borehole heat exchangers. Square or triangular layouts are usually adopted, with spacing of 6×6 m or 5×5 m. According to TRT test results and 10-year operation simulations, when borehole spacing is less than 5 m, thermal interference between adjacent boreholes increases sharply, resulting in a 15%–25% drop in heat exchange efficiency. (2) Heat exchange pipe material: high-density polyethylene (HDPE) pipes are adopted, with nominal pressure PN1.6 MPa (suitable for borehole depths of 100–150 m). Common specifications are De25 (outer diameter 25 mm) double-U pipes and De32 single-U pipes. The heat exchange efficiency of double-U pipes is approximately 20%–30% higher than that of single-U pipes, but water resistance increases by approximately 50%. (3) Backfill material: the annular space between the borehole and the HDPE pipe must be filled with a dedicated backfill material (mixed slurry of bentonite + fine sand + cement), with thermal conductivity not less than 1.5 W/(m·K) and permeability coefficient not greater than 1×10⁻⁸ m/s. (4) Zoning: when the number of heat exchangers exceeds 100, they should preferably be divided into multiple zones (30–50 per zone), with each zone provided with an independent manifold, facilitating operation management and fault troubleshooting. (5) Venting and pressure stabilization: automatic air vents shall be installed at the highest point of the borehole heat exchanger system, and an expansion tank or pressure tank shall be provided for pressure stabilization. Key construction technologies: (1) Drilling: water-circulation rotary drilling rigs or percussion drills are adopted, with borehole verticality deviation not exceeding 1%. During drilling, formation profiles and groundwater level data shall be recorded in detail. (2) Pipe installation and pressure testing: before the HDPE pipe is lowered into the borehole, a hydrostatic test shall be conducted (test pressure 1.6 MPa, held for 30 minutes without leakage). Grouting and backfilling shall be carried out immediately after pipe installation, with the backfill slurry rising gradually from the bottom of the borehole to the borehole mouth. (3) Horizontal header connection: the HDPE pipes of each borehole are connected to the manifold within horizontal trenches, and the horizontal pipes shall be laid with a slope of not less than 0.3% to facilitate venting. Pipe connections adopt butt fusion welding (butt fusion welding temperature 200–210°C), and the weld strength shall not be less than 90% of the strength of the pipe body. (4) System flushing and commissioning: after installation of the borehole system, flushing shall be carried out, with flushing water flow not less than 1.2 times the design flow, until the outlet water runs clear. Then a system hydrostatic test shall be conducted (test pressure 1.0 MPa, held for 24 hours). Clause 5.5.3 of GB 50366 requires that after installation of the borehole system, hydrostatic testing and flushing shall be carried out and records shall be produced.
Operating strategies and benefit analysis of ground-source heat pump systems
The operating strategy of a ground-source heat pump system is directly related to system energy efficiency and the long-term cooling/heating balance. Through optimized operating strategy management, maximum system energy efficiency and minimum life-cycle operating costs can be achieved. In accordance with Clause 8.3.8 of GB 50736 Code for Design of Heating, Ventilation and Air Conditioning of Civil Buildings, the operation of ground-source heat pump systems shall follow the energy-saving priority principle and reasonably establish operating parameters and control strategies. Key operating strategy points: (1) Ground temperature monitoring: install 3–5 ground temperature monitoring boreholes within the borehole heat exchanger area, with temperature sensors (accuracy ±0.2°C), to monitor changes in the ground temperature field in real time. When the monitored ground temperature deviates from the initial ground temperature by more than 3°C, auxiliary cooling/heating sources shall be started for peak shaving to protect the underground thermal balance. (2) Chilled water supply temperature reset: dynamically adjust the chilled water supply setpoint of the ground-source heat pump units according to outdoor temperature and load demand (7–10°C in summer); for every 1°C increase in supply water temperature, the unit COP increases by approximately 3%–4%. (3) Domestic hot water priority control: in summer, give priority to using the heat recovery function of the heat pump units to produce domestic hot water, reducing heat rejected into the ground. The domestic hot water storage temperature is set at 55°C, and the heat recovery amount is adjusted according to demand. (4) Intermittent operation strategy: in transition seasons, an intermittent operation mode may be adopted (e.g., shutdown for 8 hours at night), utilizing the recovery capacity of ground temperature to improve system efficiency. Studies show that intermittent operation can increase heat exchange per unit borehole depth by 10%–15% compared with continuous operation. Economic benefit analysis: the initial investment of a ground-source heat pump system is usually 1.4–2.0 times that of a conventional central air-conditioning system (the additional investment for the borehole heat exchanger system is approximately RMB 300–500/m² of building area). Taking the above-mentioned Tianjin Marriott hotel project as an example: the initial investment of the ground-source heat pump system is approximately RMB 28 million (including RMB 12 million for borehole construction and RMB 16 million for heat pump units and plant room equipment), while the initial investment of the conventional electric cooling + gas boiler scheme is approximately RMB 14 million, a difference of RMB 14 million. Annual operating cost comparison: the annual operating cost of the ground-source heat pump scheme is approximately RMB 2.6 million, versus approximately RMB 4.8 million for the conventional scheme, saving RMB 2.2 million per year. The static investment payback period is approximately 6.4 years (including the depreciation factor of borehole heat exchanger facilities). Considering that the service life of a ground-source heat pump system is 25–30 years (borehole heat exchangers can last more than 50 years), the full life-cycle economic benefits are significant. Environmental benefits: compared with the conventional scheme, the ground-source heat pump system reduces CO₂ emissions by approximately 1,200–2,000 tons per year (depending on the local power emission factor), in line with the development direction of green buildings and carbon neutrality. The Marriott MIDS standard requires full life-cycle carbon emission accounting for ground-source heat pump systems and gives priority to adopting this technology in regions with a high share of renewable energy.