Ground source heat pump system with buried pipe

1. Principle and characteristics of ground-source heat pump system with underground pipe.

The way for ground source heat pump system to obtain shallow geothermal energy is to use underground heat exchange system. Its working principle is that heat transfer medium (mainly water or glycol) circulates in closed vertical or horizontal ground pipes, and uses the temperature difference between the heat transfer medium and underground rock and groundwater to exchange heat, so as to achieve the purpose of utilizing shallow geothermal energy, and then heating and cooling buildings through heat pump technology. See 3- 1 1 and figure 3 for the working schematic diagram.

Figure 3- 1 1 Working Principle Diagram of Ground Source Heat Pump in Summer

Fig. 3- 12 working principle diagram of ground source heat pump in winter

In addition to all the characteristics of ground source heat pump, ground source heat pump also has the following remarkable characteristics:

(1) The project needs to drill a large number of holes according to the cold and hot load, run closed circulating pipes with certain strength, corrosion resistance and good heat transfer performance, and then connect all circulating pipes to enter the computer room and the main engine.

(2) Ground-source heat pump system dissipates heat (absorbs heat) with underground rock and groundwater through conduction, which is different from ground-source heat pump system which mainly dissipates heat through convection, and its heat exchange efficiency is lower than that of ground-source heat pump system.

(3) Compared with the traditional air conditioning system, the main disadvantage of ground source heat pump system is that the initial investment of ground source heat exchanger is higher than that of ground source heat pump system, which is also one of the main reasons that hinder the development of ground source heat pump system.

(4) Compared with the ground source heat pump system, the ground heat exchanger occupies a larger area than the ground source heat pump system. This is another important reason that hinders the development of ground source heat pump system in densely populated and densely built areas. Underground heat exchangers are generally arranged below green spaces, roads, parking lots, squares and school playgrounds. And under the foundation of the building and in the pile foundation of the building.

(5) Compared with the ground-source heat pump system, the ground-source heat pump system with ground pipe has less influence on the underground space environment because it does not take water from the ground, and the program is simpler than that of the ground-source heat pump system.

(6) It has the characteristics of environmental protection, high efficiency and energy saving, low operating cost, multi-purpose for one machine, mature technology, wide application range (in principle, it is suitable for any stratum and building), and there is no need to extract groundwater, so it has broad application prospects in the future.

(7) From the hydrogeological point of view, the water abundance of porous groundwater in loose layer is mainly affected by the particle size of aquifer. The larger the particle size, the greater the porosity, the better the water abundance and the stronger the permeability of the formation. Therefore, underground water source heat pump and ground source heat pump projects have certain complementary requirements for hydrogeological conditions, that is, areas that are not suitable for groundwater are often suitable for ground source heat pumps. Taking Beijing as an example, ground source heat pumps are mainly distributed in Haidian and Fengtai districts in the middle and upper parts of alluvial fans of Yongding River, while ground source heat pumps are mainly distributed in Shunyi, Changping, Chaoyang and Haidian mountainous areas, that is, the vast areas in the middle and lower parts of alluvial fans of Wenyu River, Yongding River and Chaobai River.

2. The composition and basic situation of ground source heat pump system are introduced.

Ground source heat pump system is similar to ground source heat pump system, and consists of three parts: ground source heat exchange system, computer room system and terminal system. From a professional and technical point of view, the design and construction of terminal system belongs to HVAC specialty; The computer room system is mainly composed of host computer, electrical automatic control system and water flow control system, and its core is heat pump unit technology; The design and construction of underground heat exchange system belongs to geology and hydrogeology, and must be completed by professional departments with geological exploration and sinking construction qualifications. Therefore, the core of the ground source heat pump system is actually a new comprehensive, environmental protection and energy-saving technology supported by separate HVAC technology, heat pump unit technology and geological survey technology, which is multidisciplinary and organically composed.

According to the different buried modes of underground pipelines, the underground heat exchanger system can be divided into horizontal underground heat exchanger and vertical underground heat exchanger, as shown in Figure 3- 13 and Figure 3- 14. Horizontal buried pipes are trenches dug underground, with a depth of 1.5 ~ 2.5m, and 2, 4 or 6 heat exchange plastic pipes are buried in each trench. Because the horizontal buried pipe occupies a larger area than the vertical buried pipe and its efficiency is lower than that of the vertical buried pipe, most of the ground source heat pump systems built in China adopt the vertical buried pipe system.

Fig. 3- 13 horizontal buried tube heat exchanger

Figure 3- 14 Vertical Buried Tube Heat Exchanger

The buried depth of vertical buried pipe system is generally between 50 ~ 150 m, and the buried depth of most boreholes is about 100m, and the diameter of boreholes is generally between120 ~150 mm. Most boreholes are drilled in the Quaternary loose layer, and a few projects are drilled in bedrock, such as Shanshui Yijia Villa in Changping District, Beijing. Backfilling is used to fill between drilling hole and buried pipe. Backfilling methods mainly include virgin slurry backfill, medium sand backfill, plain soil backfill and cement mortar backfill. The material of buried pipelines is mainly HDPE pipes, most of which are φ32mm in diameter.

According to the number of U-tubes vertically buried in the heat exchange hole, the system can be divided into single U-tube system and double U-tube system, as shown in Figure 3- 15 and Figure 3- 16. The heat exchange mode between buried pipe and surrounding rock and soil is conduction heat dissipation or heat absorption. In order to avoid mutual interference between heat exchange holes and save land, the design spacing of buried pipe holes is generally 4 ~ 6 m; According to different design requirements, the circulating liquid (heat exchange medium) in the buried pipe can be water or antifreeze.

Figure 3- 15 Single U-shaped vertical ground source heat pump heat exchange system

Figure 3- 16 Double U-shaped vertical ground source heat pump heat exchange system

3. The core technology of ground source heat pump system-the analysis of heat exchange capacity of single hole.

In the process of popularizing ground source heat pump technology, due to the complexity and variability of geological and hydrogeological conditions in different regions, especially the difference of groundwater table depth and groundwater seepage velocity, the thermal conductivity of rock (soil) layers and the heat exchange capacity of buried pipes per linear meter in different regions are very different. Underground heat exchange systems that can be successfully applied in one area are often not applicable in another area. Even in the same area, the project site is located in the upper, middle and lower reaches of river alluvial fans. Therefore, like ground source heat pump system, geological exploration technology is still the core of ground source heat pump system technology and the key to the successful application of shallow geothermal energy development and utilization projects.

Underground heat exchanger is the core of ground source heat pump technology, which consists of many underground pipe holes, U-shaped pipes and horizontal pipes. Under a certain cold and heat load, if there are too many holes in the buried pipe, the heat exchange capacity of a single hole can't reach the optimal heat exchange capacity, which means that the initial investment of the project is too large, the occupied area is also large, and the circulating pump at the end of the buried side is also large, which reduces the operation economy; If the number of underground pipe holes is small, and the heat exchange capacity of a single hole cannot meet the load requirements, it means that the outlet temperature of circulating liquid will be lower and lower in winter, resulting in "end cooling" phenomenon, and the outlet temperature will be higher and higher in summer, resulting in "end heating" phenomenon, which will reduce the energy efficiency ratio of main engine operation, even lead to the shutdown protection of main engine, and the system will not run, and the result will eventually affect the economy and stability of the system.

Whether the design of ground heat exchanger is reasonable or not determines the economy and operation reliability of ground source heat pump system. Therefore, the analysis of heat transfer capacity of single hole is the core of the design of ground heat exchanger. The method of enhancing heat transfer of underground heat exchanger is basically the same as that of traditional heat exchanger, that is, increasing heat transfer temperature difference, increasing heat transfer area and reducing heat transfer resistance.

The change of heat transfer temperature difference is limited by formation temperature, circulating liquid temperature and heat pump host parameters. The formation temperature is constant in all areas and cannot be changed. The temperature of circulating liquid is the outlet temperature of evaporator or condenser, which is controlled by the performance and parameters of main engine. Too high or too low outlet temperature will reduce the energy efficiency ratio of the main engine and affect the economy of the system.

Increasing the heat exchange area is actually increasing the length of the ground heat exchanger, which means increasing the initial investment of the project and increasing the floor space. Too long ground heat exchanger will not improve the economy of the system, but will reduce the economy of ground source heat pump engineering.

Therefore, the main method to strengthen the heat transfer of ground heat exchanger is to reduce the heat transfer resistance. The heat transfer process between circulating liquid and underground rock mass and groundwater is controlled by the following two factors: first, underground heat exchanger; The second is the heat transfer performance of rock and soil and groundwater. In the process of engineering practice, the space area involved is usually divided into buried pipe in the hole, backfill part and rock and soil part outside the hole by the hole wall. The heat transfer outside the hole consists of two parts: one is the thermal resistance of the rock and soil layer from the hole wall to the undisturbed remote medium at the end, which mainly depends on the thermal conductivity of the rock and soil; Second, the additional thermal resistance caused by the mutual interference of temperature fields between buried pipes mainly depends on the arrangement and spacing of buried pipes and the balance of heat release and heat release. The thermal resistance of heat transfer in borehole is mainly composed of thermal resistance in pipe and thermal resistance of filler outside pipe, which is easy to be controlled by engineering measures and can increase the heat transfer capacity of single hole.

1) external thermal resistance of borehole

The thermal conductivity and thermal diffusivity of rock and soil are very important to the design of ground heat exchanger, which determines the length, layout and spacing of ground heat exchanger and the occupied area. The thermal conductivity of rock and soil indicates the thermal conductivity through the earth. Thermal diffusivity is a measure of the earth's ability to transfer and store heat. The water content of rock and soil has great influence on the thermal conductivity and thermal diffusivity of rock and soil. In summer, the temperature of circulating liquid in underground pipeline heat exchanger is higher than that of rock and soil, which leads to the decrease of water diffusion around underground pipeline, the drying of rock and soil, the decrease of its thermal conductivity and the formation of thermal instability. When designing the length of heat exchanger, special attention should be paid to areas with scarce groundwater or deep burial.

During the operation of ground heat exchanger, the temperature field of rock and soil around ground heat exchanger will change. With the increase of ground temperature change and the expansion of area, the heat transfer between adjacent ground heat exchangers will be affected. The increase of heat transfer resistance and the decrease of heat transfer caused by the change of ground temperature are called variable temperature resistance. If the heat absorbed or released by the ground heat exchanger from the rock and soil is unbalanced within one year, it will cause the accumulation of excess heat (cold energy), cause the change of underground constant temperature and lead to the increase of thermal resistance.

Groundwater seepage has a very important influence on the heat transfer capacity of buried pipes. Because of the large thermal capacity of groundwater, it absorbs or releases a lot of heat. In the case of groundwater seepage, heat or cold energy is easily taken away by flowing groundwater, forming another heat flow channel, which greatly reduces the heat transfer resistance. Even in areas with unbalanced cooling and heating loads, groundwater flow will weaken the influence of "thermal resistance".

2) Thermal resistance in borehole

The thermal resistance of borehole is mainly controlled by the heat transfer performance of buried pipe and backfill soil. Buried pipes should be made of plastic pipes and fittings with good chemical stability, certain strength (mainly considering the pressure of circulating liquid on buried pipes when buried pipes are deep), corrosion resistance, high thermal conductivity and small flow resistance. At the current technical and economic level, polyethylene pipes (PE pipes) are mostly used in existing projects, which is the result of comprehensive consideration of the above requirements.

Under the current technical and economic level, choosing suitable backfill soil is the most suitable means to reduce investment and improve system operation economy for most ground source heat pump projects. Backfill is between the buried pipe and the hole wall, and its purpose is to enhance the heat exchange ability between the buried pipe and the surrounding rock and soil, at the same time, to prevent surface water from infiltrating into the ground through drilling holes and polluting groundwater, and to avoid cross-contamination between groundwater in different aquifers. The selection of backfill materials and correct backfill construction are of great significance to ensure the performance of ground heat exchanger. Using backfill materials with poor thermal conductivity will significantly increase the thermal resistance in the borehole, which will lead to the increase of the total length of the borehole under the same conditions, which also means the increase of the initial investment and operating cost of the system.

According to the Technical Code for Ground Source Heat Pump Engineering (GB50366—2005), "Grouting backfill material should be mixed slurry of bentonite and fine sand (or cement) or special backfill material; When the local buried heat exchanger is located in dense or hard rock and soil, cement-based materials should be used for grouting and backfilling; Backfill materials and their proportions shall meet the design requirements ". The author suggests that coarse sand and gravel should be used for backfilling below the groundwater level, and cement mortar should be used for backfilling above the groundwater level. The reason for this is the following：

(1) Backfilling with coarse sand (D2 ~ 4 mm, with good roundness) in the borehole area below the groundwater level will make full use of the characteristics of large heat capacity and good fluidity of groundwater, take away the generated heat or cold energy as soon as possible, and form a convection (absorption) heat channel. Because of the risk of cross-contamination of groundwater, it should be used cautiously in areas with stratified groundwater pollution;

(2) In the drilling area above the groundwater level, the backfill soil must be dense and complete, completely isolated from the contact between air and buried pipelines, and completely avoided the air from mixing into the backfill soil. The above requirements will be achieved by backfilling with cement mortar. More importantly, cement mortar backfilling has good thermal conductivity, economy and sufficient durability.

4. Technical requirements for design and construction of ground source heat pump system

The design and construction of ground source heat pump system should strictly abide by the Technical Code for Ground Source Heat Pump Engineering (GB50366—2005). According to years of experience in construction and operation monitoring of ground source heat pump projects, the following points should be noted:

(1) If the site conditions permit, the underground heat exchanger should be built as close as possible to the control room, so as to save the circulating power on the underground side to the greatest extent and improve the efficiency ratio of the system. According to the investigation, during the summer operation of a source heat pump project in Changping District, Beijing, the power consumption of circulating pump (including terminal circulating pump) accounts for 40% ~ 50% of the total power consumption, which is obviously higher than the normal value. The reason is that the construction site of ground source heat exchanger is far from the computer room and the power of circulating pump is too large.

(2) If possible, it is best to run the cooling season after the completion of the ground source heat pump project, in order to ensure the operation effect in winter and prevent the risk of freezing of circulating liquid (if it is water).

(3) Groundwater has a very important influence on the heat transfer capacity of underground pipe holes. However, in general, in areas with fast groundwater seepage, aquifer particles are large, and underground pipe hole construction is difficult, which increases the construction cost. Therefore, the relationship between construction cost and heat exchange should be considered comprehensively.

(4) When the buildings are scattered and the site conditions permit, the distributed computer room should be adopted, which is beneficial to improve the economy of the project.

(5) The depth of the underground pipe hole is about100m m. Once the ground source heat pump system is built and put into operation, it needs to occupy the underground space permanently (the area below 2m), which will have an impact on regional planning (such as subway lines) and pipeline layout.

(6) When backfilling, it is necessary to backfill with a shovel, and the speed should not be too fast to prevent the backfilling from being false due to too fast backfilling. Backfilling with trolleys and vehicles is prohibited.

(7) In the operation stage of the project, we should pay close attention to and record the supply and return water temperature of the main engine and the power consumption of the main engine and the circulating pump, so as to lay a foundation for scientific analysis of the operation of the project.

(8) Because the test time is limited (generally about 65,438+00 days) and the influence of "temperature-dependent thermal resistance" is not considered, the thermophysical results often cannot fully reflect the operation of a heating or cooling season. It is suggested that the design should refer to the experience of existing projects in the same area and hydrogeological conditions.

(9) The influence of "variable temperature resistance" and engineering economy should be comprehensively considered in the layout of buried pipe holes.

(10) In the engineering design of ground source heat pump, it is necessary to ensure the hydraulic balance of buried pipe holes in various places and ensure that the flow velocity in each circulating pipe is basically the same.

(1 1) The velocity in the buried pipe should be accurately calculated. Excessive flow velocity will not increase heat exchange, but will reduce the economy of the project. If the flow velocity is too small, the heat transfer capacity of a single hole will decrease.

Because there are a large number of underground drilling rigs in ground source heat pump engineering, the cost of underground drilling rigs is often the main determinant of initial investment. It is suggested that the exploration test hole must be constructed in the stage of engineering demonstration, so as to grasp the construction difficulty and cost of the project and lay the foundation for the project budget. According to the Technical Code for Ground Source Heat Pump Engineering (GB50366-2005), before the scheme design of ground source heat pump system, the site conditions of the project and the shallow geothermal energy resources should also be investigated. Before the scheme design of ground source heat pump system, the geotechnical geological conditions of the project location should be investigated, including:

(1) geotechnical layer structure;

(2) Thermal properties of rock and soil;

(3) Geotechnical temperature;

(4) Buried depth, water temperature, water quality and distribution of underground static water level;

(5) the direction and speed of groundwater runoff;

(6) Thickness of frozen soil.