The ground source heat pump system absorbs the energy of the earth, including natural energy such as soil, well water, lakes, rivers, etc., absorbs energy from the nature of the earth in winter, releases energy to the nature of the earth in summer, and then supplies heat to the building by the heat pump unit. A new air-conditioning system that can be heated and refrigerated by using renewable energy, which is energy-efficient and pollution-free, and can be widely used in public buildings, houses, schools, hospitals and other buildings. Ground source heat pump technology is a revolutionary new technology emerging in the field of building central air conditioning. In the comprehensive office building project of the Dingxi City Center Sub-branch, the People's Bank of China is committed to the technology introduction, product development and marketing of the ground source heat pump project, and has made some useful explorations.
1. Project Overview
The Dingxi City Center Sub-branch Comprehensive Office Building Project is located on the east side of Xihuan Road in Anding District. It is a frame structure with a north-south trend. The main building is an 8-story hat on the ground and the podium is 2 stories. The building area is 7912m2.
Ground source heat pump system has three forms: 1 ground pipe heat exchanger ground source heat pump (also known as soil heat exchanger ground source heat pump); 2 ground water source source heat pump; 3 surface water source heat pump. According to the characteristics of insufficient groundwater and surface water in Dingxi City, the project uses a buried heat exchanger ground source heat pump system. Ground-source heat exchanger The ground source heat pump is divided into two types: horizontal buried pipe and vertical buried pipe. In view of the limitation of the site, a vertical heat pipe ground heat pump system with small footprint is selected.
There are two types of circuits for fluid flow in the buried pipe heat exchanger: series and parallel. The U-tube is connected in parallel with the same heat exchanger. The parallel system has smaller pipe diameter, lower pipe cost, and is arranged in the same way. Program. When the flow between each parallel loop is balanced, the heat transfer energy is the same, and its pressure drop characteristic is beneficial to improve the system capability.
According to the requirements of "Technical Specifications for Ground Source Heat Pump System" (GB50366-2005), according to the site survey results, the horizontal spacing of U-tube vertical wells is 4.5m, the drilling hole depth is 120m; the designed hole depth is 100m, the aperture is 100m. 200mm, the number of holes is designed to be 138, and the hole is placed as far as possible around the machine room.
2. Engineering investigation
The condition of the project site and the conditions of shallow geothermal energy resources are the basis and prerequisites for the application of the ground source heat pump system and the choice of which form of ground source heat pump system. Its management work mainly does the following:
(1) "Technical Specifications for Ground Source Heat Pump System Engineering"
(GB50366-2005) stipulates that before the design of the ground source heat pump system, the site survey should be conducted and the shallow geothermal energy resources should be investigated. The engineering survey shall be undertaken by a professional team with qualifications for surveying. After the completion of the engineering survey, an engineering survey report shall be prepared and recommendations shall be made for the availability of resources.
(2) The survey of the site conditions is mainly based on data collection. The radius of the survey area should be larger than the proposed heat exchange area of ​​100-200 m. The survey should include the following contents: (1) site planning area shape and slope, ie topography; 2 floor space and distribution of existing buildings and planned buildings; 3 woodland vegetation, ponds, drainage ditch, overhead power lines And the distribution of telecommunications cables; 4 the distribution and depth of existing and planned underground pipelines and underground structures in the site; 5 the location, type structure, depth, stratum profile and water level, water output, water temperature and In terms of water quality, it is also necessary to understand the use of the well, the method of mining, the annual water consumption and the change of the water level.
(3) Geotechnical geological conditions can be investigated by reference to "Geotechnical Investigation Specifications" (GB50021) and "Water Supply and Hydrogeology Survey Specifications" (GB50027). When the vertical buried heat exchanger is used, the underground heat transfer system is surveyed by drilling. The drilling plan should be determined according to the size of the site. The drilling hole depth is at least 5m deeper than the borehole. The survey of buried pipe heat transfer system shall include: 1 the structure of the rock layer; 2 the thermal properties of the rock and soil; 3 the temperature of the rock mass; 4 the static water level, water temperature and water quality distribution; 5 the groundwater runoff direction and velocity; Layer thickness.
(4) Thermal properties of rock and soil refer to the thermophysical parameters of geotechnical soil, including thermal conductivity, density and specific heat of rock and soil. The method of measurement may be laboratory method or on-site measurement.
1 Laboratory method: The geotechnical samples of different depths of the exploration holes are measured, and the geothermal parameters of the exploration holes are calculated by their depth-weighted average.
2 On-site measurement method: On-site testing of rock and soil should be carried out after the test buried pipe condition is stable. According to the depth of the buried pipe, the test should generally be carried out 72 hours after the installation of the test pipe, and the test results of the two drill holes and the two drill holes are taken as the arithmetic mean.
3. Vertical buried tube heat transfer system design
The vertical buried heat exchange system uses the underground heat exchange coil to exchange heat with the soil, and uses the earth as a low-temperature heat source and heat-dissipating place for the heat pump unit. It consists of an underground coil and a conventional water source heat pump unit. When heating in winter, the soil acts as a low-temperature heat source for the heat pump unit, and the heat in the soil is supplied to the room through the underground coil. When the summer is cooled, the soil acts as a heat-discharging place and cools the room. The load and the energy consumed by the compressor are discharged into the soil through the underground heat exchange coil. That is to say, the soil stores heat in summer and re-releases heat in winter, so that the earth's heat is basically stable. The following management tasks are mainly done during the design phase:
(1) Before the design of the buried pipe heat exchange system, the feasibility and economy of its implementation should be evaluated based on the results of the engineering survey. The engineering design shall be undertaken by a professional organization with design qualification. After the engineering design is completed, the relevant departments shall review and approve the drawings.
(2) The system design should carry out the annual dynamic load calculation. The minimum calculation period should be one year. During the calculation period, the total heat release of the system should be balanced with its total heat absorption. Because the annual cold and heat load balance is out of balance, the temperature of the rock and soil in the buried pipe area will continue to increase or decrease, which will affect the heat transfer performance of the buried heat exchanger and reduce the operating efficiency of the system.
(3) The design and calculation of the buried pipe heat exchange system is unique to the design of the ground source heat pump system. In order to ensure that the design meets the requirements and meets the requirements for use, it is usually necessary to design the thermal properties of the geotechnical and soil materials and the thermal properties of the backfill. Factors such as the characteristics of the heat medium are calculated based on the measured data. Considering the particularity and complexity of the calculation, special software should be used. When designing and calculating, the loop header should not be included in the length of the heat exchanger.
(4) The buried depth of the vertical buried heat exchanger shall be determined by calculation, and the minimum is preferably greater than 20m; the borehole diameter shall not be less than 0.11m; in order to avoid heat exchange short circuit, the drilling spacing shall meet the heat transfer requirements, determined by calculation, That is, when the rock body absorbs and releases heat, it should take a small value; otherwise, it should take a large value, and the spacing should be 3-6 m. The project takes 4.5m, the hole diameter is 200mm, and the hole depth is 100m; the depth of the horizontal connecting pipe should be 0.6m below the frozen soil layer, and the distance from the ground should not be less than 1.5m. In order to ensure timely system exhaust and enhanced heat transfer, the fluid in the pipe should maintain a turbulent flow regime. Recommended flow rate for pipeline: Double U-shaped buried pipe should not be less than 0.4m/s; single U-shaped buried pipe should not be less than 0.6m/s.
(5) Both ends of the buried pipe loop should be connected to the supply and return water loop headers separately, and should be arranged in the same way; the number of pipe loops connected to each supply and return water loop header should be equal, supply, The spacing of the return water loop header should not be less than 0.6m. In order to reduce the impact of water wells and outdoor drainage facilities, the heat exchanger installation location should be away from the water wells and outdoor drainage facilities; in order to shorten the length of the supply and return water headers, it should be close to the equipment room or set around the machine room.
(6) In order to increase the safety and reliability of the system, an automatic liquid filling and leakage alarm system shall be provided; in order to facilitate the filling of the system, a flushing pipe is generally reserved on the water separator or the water collector; The liquid return pipe shall be equipped with closed expansion tanks, charging and discharging facilities, pressure gauges, thermometers and other basic instruments and components; in areas where antifreeze is required, antifreeze protection devices shall be provided; to prevent system blockage, the buried heat exchange system shall be suitable. Set backwashing system, the flushing flow rate should be 2 times of the working flow; the hydraulic calculation should be carried out according to the hydraulic characteristics of the actually selected heat transfer medium during design, and the variable flow design should be adopted to save electricity consumption; it should be determined according to the geological characteristics. For the filler formulation, the thermal conductivity of the backfill should not be lower than the thermal conductivity of the rock mass outside the borehole.
4. Construction quality management of vertical buried pipe heat exchange system
Before the construction of the vertical buried heat exchange system, it is necessary to understand the functions of the underground pipelines and other underground structures in the buried pipeline site and their exact location, and carry out the site cleaning and leveling; the engineering survey data and design documents of the buried pipeline area should be provided. And the construction drawings approved and approved, and completed the construction organization design, submitted for review and approval, and carried out the drawing review and technical delivery work.
The system construction shall be completed by a professional team with such engineering construction qualifications. The professional construction personnel shall hold the professional qualification certificate; the construction plan shall clearly indicate the location of the drilling well and the entrance to the building and the machine room. The location of all underground utility equipment within the planned construction land area shall be indicated; the plan shall be reviewed and the actual deviations shall be agreed before approval; before the construction begins, the construction preparation shall be completed and the contractor shall obtain and work. All start-up permits for the project.
The quality of the buried pipe is very important for the vertical buried pipe heat exchange system, and the pipe protection work should be strictly checked and done. The buried pipes and fittings entering the site shall be strictly examined and examined, and inspected one by one, and air testing shall be carried out for leak testing; materials shall not be exposed to sunlight for a long time during storage, and should be handled with care during handling. Belts, slings or slings shall be loaded and unloaded, and shall not be thrown or towed along the ground; polyethylene pipes shall comply with the requirements of "Polyethylene (PE) pipes for water supply" (GB/T13663); polybutene pipes shall comply with "hot and cold water" Use the requirements of polybutene (PB) piping system (GB/T19473·2).
4.1 Connection requirements for vertical buried pipes
(1) Buried pipes should be connected by hot melt or electric fusion. The connection of polyethylene (PE) pipelines shall comply with the relevant provisions of the current national standard "Technical Regulations for Buried Polyethylene Water Supply Pipeline Engineering" (CJJ101).
(2) For U-shaped elbow joints of vertical buried heat exchangers, it is advisable to use shaped U-shaped elbow finished parts, and it is not advisable to use straight pipes to make elbows.
(3) The length of the pair of vertical U-tubes of the buried heat exchanger should meet the requirements of the connection with the loop header after the insertion of the borehole. The two open ends of the good U-tube should be timely. seal.
4.2 Hydraulic test of vertical buried pipe heat exchange system
The hydrostatic test of the vertical buried heat exchange system shall be carried out in four steps as follows.
(1) The first hydraulic pressure test shall be carried out before the vertical buried heat exchanger is inserted into the borehole. Stabilized under test pressure for 15 min, the pressure drop after voltage regulation is not more than 3%, no leakage is qualified; after sealing, insert the borehole under pressure, and keep pressure for 1 h after grouting.
(2) After the vertical buried heat exchanger and the loop header are assembled, a second hydraulic test is performed before backfilling. Under the test pressure, the voltage is regulated for at least 30 minutes, the pressure drop after voltage regulation is not more than 3% and no leakage is qualified; each test pressure should be witnessed and recorded.
(3) After the loop header is connected with the machine room and the water collector, the third water pressure test should be carried out before backfilling. Under the test pressure, it is stable for at least 2 h, and no leakage is acceptable.
(4) After the vertical buried heat exchange system is completely installed, and the flushing, draining and backfilling are completed, the fourth hydraulic pressure test is carried out. Under the test pressure, the voltage is regulated for at least 12h, and the pressure drop after voltage regulation is not more than 3%; the water pressure test is slowly boosted by a manual pump, and it is observed and inspected at any time during the boosting process, and no leakage is acceptable. The water pressure test is not replaced by the air pressure test.
The U-tube installation of the heat exchanger shall be carried out immediately after the drilling is drilled and the wall of the hole is solidified and the experience is accepted. When the hole wall is not strong or there are holes, caves, etc., it is difficult to form holes. In the process of the lower pipe, the U-shaped pipe should be filled with water, and a spring card or fixed branch should be used every 2~4m. The card method separates the two tubes of the U-tube to improve the heat exchange effect.
System flushing is a necessary step to ensure the reliable operation of the vertical buried heat exchange system. Before the vertical buried heat exchanger is installed, the buried heat exchanger and the loop header are assembled and the whole system is installed. Upon completion, the piping system should be flushed to prevent blockage.
4.3 Grouting backfill of vertical buried heat exchanger
After the U-tube of the vertical buried heat exchanger is installed, it should be immediately filled and backfilled to seal the aquifer; the appropriate slurry can be used to strengthen the thermal contact between the soil and the heat exchanger to prevent pollutants from falling down from the ground. Leakage and prevention of water movement between aquifers are mandatory.
(1) Grouting is the process of using a mud pump to pump a mixed slurry pump into a wellbore through a grouting pipe. The pump pressure of the mud pump is sufficient to return the mud at the bottom of the hole to the surface. When the density of the upper mud is equal to the density of the material, the filling process is considered to be completed.
(2) The consistency of grouting should be ensured during grouting. The grouting pipe should be gradually extracted from the vertical hole to be grouted according to the speed of mechanical grouting, so that the grouting liquid can be sealed from bottom to top to ensure the grouting is dense. , no cavity; otherwise it will reduce the heat transfer effect and affect the quality of the project.
(3) The vertical buried tube heat exchanger grouting back filler should be a mixture of bentonite and fine sand or cement, or other special grouting materials. The proportion of bentonite should be 4% to 6%.
(4) If the heat exchanger is expected to operate in a very dense or hard rock or rock below the permafrost, it is better to use cement based grouting because the pore water will freeze and expand, which will damage the bentonite. Grouting the material, and the forces generated can cause the pipe to be squeezed and throttled.
(5) In order to minimize the preparation time between each batch of grouting ingredients, a large-capacity grouting mixer/separating storage tank should be used; a PVC pipe with a diameter of not less than 30 mm should be used as a conduit, and the diameter of the pipe should be selected. To reduce the frictional resistance loss and connect it to the U-tube heat exchanger before placing it down into the vertical hole.
(6) When the buried pipe exceeds 40m, the grouting backfill should be carried out after the surrounding drilling is completed. The purpose is to drill the adjacent U-shaped pipe once the hole is inclined, which is convenient for replacement.
(7) When the outdoor ambient temperature is lower than 0 °C, the physical and mechanical properties of the plastic buried pipe will be reduced, which will easily cause damage to the buried pipe. Therefore, when the outdoor ambient temperature is lower than 0 °C, it is not suitable to carry out the construction of the buried pipe heat exchanger; if construction is required, it should adopt reliable insulation and antifreeze measures according to the winter construction requirements.
(8) In order to ensure the quality of construction, quality management still needs to consider the following points:
1 supervise the operation of the grout to ensure that the grout is thoroughly mixed in the correct proportion and is sufficiently viscous to be filled into the shaft hole by a pump;
2 Grouting construction should have spare grouting pipes, hoses and spare equipment that can be easily used on the construction site to ensure the continuity of grouting;
3 positive displacement pump (spiral or piston type) is most suitable for filling the grout down into the shaft hole;
4 suction pipe with inner diameter of 80~100mm and discharge pipe with inner diameter of 30~50mm can meet the requirements;
5 cement base grouting should be composed of pure cement and pulverized powder with a mass ratio of 5%;
6 water and cement take a mass ratio of 0.55 to 0.6, which can meet the requirements.
5. During the installation of the quality management system for system inspection, operation, commissioning and acceptance, on-site inspection shall be carried out and an inspection report shall be provided. The contents of the inspection shall comply with the following provisions: 1 The materials such as pipes and fittings shall comply with the current national standards; 2 the position and depth of the borehole, the diameter and wall thickness and length of the buried pipe shall meet the design requirements; The proportion should meet the design requirements, the water pressure test should be qualified; 4 the flow of each loop should be balanced, and should meet the design requirements; 5 the characteristics and concentration of antifreeze and preservative should meet the design requirements; 6 circulating water flow and temperature difference between inlet and outlet All should meet the design requirements.
The heat exchange system mostly uses polyethylene (PE) pipe, which is a kind of thermoplastic material. Unlike the steel pipe, the material itself has the characteristics of creep and stress relaxation under pressure, so the tester should understand the pressure drop more comprehensively. 1 The hydrostatic test should not be less than 1.5 times the working pressure of the pipeline, and the test pressure should not be lower than 0.8 MPa; the hydraulic test should not be replaced by the air pressure test. 2 The test shall be carried out in two stages: pre-test and main test. In the first test step, the water pressure in the pressure test pipe shall be reduced to atmospheric pressure for 60 min, during which time the air shall not enter the pipeline; The water pressure inside rises to the test pressure and is regulated for 30 min. If there is pressure drop, water injection pressure can be added, but it should not be higher than the test pressure, and no leakage is observed. Step 3, stop the water injection pressure for 60 minutes, the pressure drop is not If the test pressure exceeds 70%, the work in the pre-test phase ends. Main test phase: After the end of the pre-test phase, the pipe drain water is quickly depressurized, and the pressure drop is 10% to 15% of the test pressure. During the period, the amount of water discharged by the pressure reduction should be accurately measured, set to △V(L). When ΔV(L)≤△Vma(xL), the residual pressure of the pipeline should be recorded every 3 minutes and recorded for 30 minutes; When the residual pressure of the pipeline rises within 30 minutes, the hydraulic pressure test ends and the test result is qualified. If the residual pressure of the pipeline does not rise within 30 minutes, the pressure is continuously reduced for 60 minutes, and the pressure drop does not exceed 0.02 MPa for the entire 90 minutes. The test results were also qualified.
The inspection of the backfilling process shall be carried out in synchronism with the installation of the vertical borehole heat exchanger. The contents of the backfilling process include backfill ratio, mixing procedure, grouting and sealing inspection.
The vertical operation of the ground source heat pump system should be carried out before commissioning, and the overall operation, commissioning and acceptance should be carried out.
(1) The ground source heat pump system should be commissioned and adjusted before the overall operation and commissioning. The main contents include: 1 The technical data of pressure, temperature and flow of the system should meet the requirements of relevant technical documents; 2 The continuous operation of the system should reach normal and stable; the pressure of the pump and the current of the pump motor should not fluctuate greatly. 3 The operation of various automatic measuring and detecting components and actuators should be normal, and meet the requirements of the building equipment automation system for monitoring and controlling the measured parameters; 4 control and testing equipment should be able to communicate normally with the system's testing components and actuators. The status parameters of the system should be able to display normally. Equipment interlocking, automatic adjustment and automatic protection should be able to operate normally. 5 The commissioning report should include all the test data of the preparation record before commissioning, the hydraulic balance unit and the system test run.
(2) The overall operation and commissioning of the ground source heat pump system shall comply with the following requirements: 1 The overall operation and commissioning plan shall be formulated before the overall operation and commissioning, and reported to the professional supervision engineer for examination and approval; 2 Water source heat pump system unit commissioning The water system and the wind system should be balanced before commissioning to determine the total circulating flow of the system. The flow of each branch and the flow rate of each end equipment meet the design requirements. 3 After the hydraulic balance adjustment is completed, the test run of the water source heat pump system unit should be carried out and the operation should be completed. Record, the operation data should meet the technical requirements of the equipment; 4 After the test run of the water source heat pump unit is normal, the system should be tested for 24 hours and fill in the operation record; 5 the ground source heat pump system should be commissioned in winter and summer, and commissioned. The result should meet the design requirements. After the commissioning is completed, the commissioning report and operating procedures shall be prepared and submitted to Party A for confirmation and archiving.
(3) Before the underground heat pump system is buried in the vertical, the operation test should be carried out in winter and summer, and the measured performance of the system should be evaluated. Operational tests include measurements of indoor air parameters and system operating energy consumption. System operating energy consumption includes the energy consumption of all water source heat pump units, pumps and end equipment.
(4) The acceptance of the ground source heat pump system for vertical buried pipes shall comply with the provisions of the “Technical Specifications for Ground Source Heat Pump Systems†(GB50366-2005), and shall also comply with the relevant national standards.
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