Layered backfill method of phase change backfill material of ground source heat pump system and application thereof

Abstract

The present application relates to the technical field of medium-deep geothermal energy development, in particular to a layered backfilling method of phase change backfilling material of a ground source heat pump system and application thereof, the method comprising the following steps: according to the drilling depth, the drilling is divided into a shallow section and a medium-deep section, and the shallow section is backfilled with heat insulation backfilling material; the medium-deep section is divided into temperature layers according to temperature ranges, and corresponding phase change backfilling materials are selected according to the temperature ranges of the temperature layers, and the phase change backfilling materials are backfilled in layers. By adopting the layered backfilling scheme of phase change backfilling material, the phase change latent heat effect of the phase change material can be utilized in the whole heat exchange section of the geothermal well according to the phase change temperature of the stratum. Based on the stratum capable of exerting the phase change effect, compared with the backfilling scheme of phase change backfilling material with a single phase change temperature, the layered backfilling scheme of phase change backfilling material can effectively improve the utilization efficiency of geothermal energy.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of medium-deep geothermal energy development, and particularly relates to a layered backfill method of a phase change backfill material of a ground source heat pump system and application thereof. BACKGROUND

[0002] Traditional ground source heat pump technology is extremely limited in ground heat extraction due to large land occupation, small heat exchange capacity and other shortcomings. The backfill material is an extremely important link for heat exchange between the ground source heat pump and the rock soil, and plays a crucial role in the efficient operation and reliable work of the ground source heat pump system.

[0003] For example, a deep well heat exchange heat pump system using phase change heat absorption of working medium disclosed in Chinese patent document CN 113883735 A (application number 202111155230.1) includes a buried pipe in the stratum, a backfill layer is arranged between the rock layer and the buried pipe, and the backfill layer is filled with special cement, so that the backfill layer has good thermal conductivity. At the same time, the effective use of geothermal resources is realized by using circulating working medium with phase change performance. However, the special cement, expanded soil, drilling cuttings and coarse sand are all traditional backfill materials. The mixture of the traditional backfill materials and other additives or high thermal conductivity materials such as graphite and steel slag has a large thermal influence radius and occupies a large area of buried pipe. Moreover, with the operation of the heat pump system, heat is continuously extracted and released from the ground, the soil temperature is continuously reduced, resulting in the "cold accumulation" phenomenon. The working performance of the heat exchanger of the heat pump system is continuously attenuated, the operating condition is continuously deteriorated, and it is difficult to extract heat. Although the system uses circulating working medium with phase change characteristics, the above problems still occur during operation.

[0004] When the phase change material is used for backfilling, the material phase change releases and absorbs a large amount of latent heat to alleviate the soil temperature change range around the ground source heat pump system, thereby avoiding the "cold accumulation" phenomenon and maintaining the stable operation of the ground source heat pump system. The thermal influence range can also be reduced, the buried pipe spacing can be shortened, and the system heat exchange efficiency can be improved.

[0005] But in the process of solving the above technical problems, the prior art mainly improves the phase change material itself. For example, Chinese patent document CN 115819021 A (application number 202211525088.X) discloses a backfill material and backfill method for vertical spiral buried pipe ground source heat pump. The backfill material includes backfill material one and backfill material two. The backfill material one includes sand and expanded soil. The backfill material two includes phase change material paraffin and expanded graphite. However, as described in detail in the specific embodiments, the same backfill material is filled in the well after the backfill material is prepared. For another example, Chinese patent document CN 114620976 A (application number 202210247306.1) discloses a phase change temperature control controllability low-strength material for heat supply pipeline backfilling and a preparation method thereof. The material specifically includes dredged sediment, composite phase change material, cement, and water. The composite phase change material is composed of rice husk ash and paraffin. By introducing the phase change material into the preparation of the controllability low-strength material, the material has phase change temperature control performance. However, similar to the aforementioned patent document, the phase change material disclosed in this patent is also a phase change material with a uniquely determined component composition. Similarly, the journal document "Research Progress of Backfill Materials for Drilled Wells in Ground Source Heat Pump Systems" (Zhang Wen, Hao Bin, et al. Drilling and Exploration Engineering, 2021, 48(8): 96-102.) analyzes the existing backfill materials from the aspects of thermal conductivity, mechanical properties, and engineering characteristics, and predicts the future development according to the existing problems. It is also believed that "numerical simulation analysis of phase change materials in advance will certainly play a significant role in the research and development of phase change materials and the future application of engineering." Therefore, it can be seen that after studying the existing backfill materials and combining the existing problems, the conclusion is also to improve the performance of the phase change material itself.

[0006] However, in the actual application process, the utilization depth of the medium-deep geothermal resource is usually between 300-3000m, which is different from the utilization depth of the shallow geothermal energy, which is usually within 300m. Scientific exploration shows that the temperature gradient of every 100m is about 3℃, so the temperature difference of every 300m is about 9℃. The existing backfilling method cannot meet the demand of the entire ground source heat pump system for releasing and absorbing latent heat at different depths when using a single phase change material for backfilling, so it is necessary to backfill the phase change material according to the soil / rock temperature stratification to ensure that the overall heat exchange effect of the ground source heat pump system is always in an optimal state. In addition, for the winter heating working condition, the return water temperature is usually between 15-20℃, and the average daily temperature in winter is 5℃. The maximum temperature of the shallow surface within 0-300m is about 14℃, which is much lower than the return water temperature. When a single phase change backfilling material is used for backfilling at the shallow surface depth, the heat loss of the working fluid will be reduced, and the system efficiency will be reduced, so it is necessary to consider the actual working condition to backfill the material in layers to ensure the efficient operation of the ground source heat pump system. SUMMARY

[0007] The purpose of the present application is to overcome the shortcomings of the prior art, provide a layered backfilling method of phase change backfilling material for ground source heat pump system, determine the layered backfilling scheme according to the drilling depth and the ground temperature distribution, select the corresponding phase change material according to the temperature range of each layer for layered backfilling, and ensure that the ground source heat pump system can operate stably, reliably and efficiently, and improve the heat exchange capacity.

[0008] To achieve the above technical effects, the present application adopts the following technical scheme:

[0009] A layered backfilling method of phase change backfilling material for ground source heat pump system, comprising the following steps:

[0010] According to the drilling depth, the drilling is divided into a shallow section and a medium-deep section, and the shallow section is backfilled with a heat insulation backfilling material; the medium-deep section is divided into temperature layers according to the temperature range, and the corresponding phase change backfilling material is selected according to the temperature range of each temperature layer, and the phase change backfilling material is backfilled in layers.

[0011] The advantage of the phase change backfill material over the common backfill material is that the phase change material mixed in the phase change backfill material can store or release a large amount of latent heat when the phase change material reaches the phase change temperature to maintain the soil temperature balance in the heat extraction area of the well, thereby avoiding the "cold accumulation" phenomenon of the common backfill material. However, when the soil temperature in the heat extraction area is significantly lower or higher than the phase change temperature, the phase change backfill material cannot utilize the latent heat, resulting in the same function as the common backfill material. Therefore, to fully utilize the beneficial effect of the phase change backfill material, the key is to make the whole heat exchange section of the medium-deep geothermal well reach the phase change temperature of the phase change material.

[0012] However, for medium-deep geothermal development, geothermal wells with a development depth of 1000-3000 m account for a large proportion. Therefore, for such geothermal wells, there is a temperature difference of 30-90 °C between the wellhead and the deepest heat exchange area. Determining a single phase change temperature based on the average formation temperature or the operating temperature will limit the phase change backfill material to a certain formation, which obviously cannot fully utilize the heat supply potential of the whole well. Using the layered backfill scheme of the phase change backfill material to determine the phase change temperature according to the formation temperature can make the whole heat exchange section of the geothermal well utilize the latent heat effect of the phase change material. Based on the formation that can utilize the phase change effect, compared with the single phase change temperature backfill scheme of the phase change backfill material, the layered backfill scheme of the phase change backfill material can effectively improve the utilization efficiency of geothermal energy.

[0013] Preferably, the shallow surface is a heat insulation layer, and the heat insulation layer is backfilled with a heat insulation backfill material. Further preferably, the heat insulation backfill material is selected from non-polluting thermal insulation materials such as foamed cement.

[0014] The shallow surface depth range is backfilled with a heat insulation backfill material to block the heat exchange between the low-temperature soil and the working fluid in the ground source heat pump system, thereby avoiding heat loss and improving the overall average temperature of the ground source heat pump system.

[0015] Preferably, one temperature layer is set in the medium-deep layer for every 9-27 °C temperature range.

[0016] According to the principle of temperature layer division, the more the temperature layers are divided, the closer the phase change temperature of the phase change backfill material in each temperature layer to the formation temperature, which can ensure that the phase change backfill material fully undergoes phase change to realize a stable energy storage and release process. However, from the economic and safety perspectives, the more the layers are divided, the more complex the proportioning process of the phase change backfill material, resulting in a significant increase in cost, and the composition of the phase change backfill material is also more complex, which is more likely to cause backfill layering due to the differences in structure, composition, and density of different backfill materials, thereby damaging the safety of the ground source heat pump system.

[0017] Further preferably, if the total temperature difference of the medium-deep layer cannot be divided by the temperature range of the temperature layer,

[0018] When the remainder is less than half of the temperature range, then the remaining temperature layer is incorporated into the adjacent temperature layer;

[0019] When the remainder is greater than or equal to half of the temperature range, then the remaining temperature layer is listed as an independent temperature layer.

[0020] Preferably, the phase change backfill material comprises ordinary backfill material and phase change material, and the mass ratio of the ordinary backfill material and the phase change material is 100:(5-10).

[0021] Further preferably, the ordinary backfill material comprises Portland cement, quartz sand, expansive soil, deaerated water, carbon fiber, defoaming agent and water reducing agent, and the mass ratio is 100:(200-250):(1-3):(50-60):1:(0.05-0.10):(0.2-2).

[0022] The defoaming agent is selected from one of silicone defoaming agent, polyether defoaming agent or silicon polyether defoaming agent; the water reducing agent is selected from one of naphthalene series water reducing agent or polycarboxylic acid series water reducing agent; the deaerated water is made by removing air in tap water, and by removing air in water, the generated air bubbles in the backfill material in the stratum are effectively reduced, and the heat exchange efficiency is improved.

[0023] By adding high thermal conductivity fiber material carbon fiber in the ordinary backfill material, the thermal conductivity, adhesion, bending strength and compressive strength and other properties of the backfill material are effectively improved, and at the same time, the voids or cracks between the layers of backfill material due to phase change deformation are avoided, and the heat exchange efficiency of the ground source heat pump system is ensured.

[0024] Further preferably, in the phase change backfill material corresponding to each temperature layer, the phase change temperature of the phase change material is the average temperature of the temperature layer ±3.0℃.

[0025] By controlling the phase change temperature of the phase change material of each temperature layer, the phase change backfill material can be well matched with the temperature layer, and the large amount of latent heat released or absorbed when each layer of phase change material changes phase is fully utilized, the amplitude of the soil temperature change around the ground source heat pump system is significantly reduced, the "cold accumulation" phenomenon is reduced, and the heat exchange capacity of the ground source heat pump system is improved.

[0026] Further preferably, the phase change material is microcapsule phase change material.

[0027] The microcapsule technology is used to completely coat the phase change material, avoid pollution of the soil caused by leakage and loss of the phase change material when absorbing or releasing latent heat and changing phase, and also reduce the volume change of the phase change material when changing phase, effectively control the influence of the phase change material on the backfill well.

[0028] Further preferably, the microcapsule phase change material comprises a wall material and a core material, and the particle size of the microcapsule phase change material is 1-100 microns.

[0029] Further preferably, the wall material is selected from at least one of melamine resin, urea-formaldehyde resin, melamine-formaldehyde resin and polyurethane, etc.

[0030] When the phase change materials of adjacent temperature layers have a large difference in composition, a large pore is generated between the phase change backfill materials of adjacent temperature layers due to a large difference in deformation of the phase change materials when phase change occurs, which damages the operation safety of the ground source heat pump system; when the phase change materials are of the same or similar types, the deformation generated during phase change is similar, thereby effectively reducing the influence on the backfill drillings and ensuring good heat exchange of the ground source heat pump system.

[0031] Further preferably, the microcapsule phase change material is externally provided with a metal packaging layer made of high-thermal-conductivity metals such as iron and copper.

[0032] By providing the metal packaging layer, the wall material of the phase change material can be effectively prevented from being corroded in the backfill drilling, thereby avoiding the leakage of the phase change material; the framework constraint effect provided by the metal packaging layer can expand the material selection range of the phase change material; when the phase change material undergoes phase change, the deformation generated is constrained by the metal packaging layer, thereby effectively controlling the pore size that may be generated between adjacent backfill layers. Meanwhile, the addition of high-thermal-conductivity metals can further improve the thermal conductivity of the phase change backfill material and further optimize the heat exchange performance of the ground source heat pump.

[0033] Preferably, the specific method of the layered backfill phase change backfill material is as follows:

[0034] The backfill is performed by using the stepwise lifting method, i.e., the backfill is performed at a distance of 10 m from the bottom of the well, the phase change backfill material starts to be pulled out when it reaches the mouth of the grouting pipe, the pulling-out height is 15 m each time, and the process is repeated until the backfill is completed; meanwhile, the pulling-out speed is controlled to fully discharge the air in the well and compactly backfill the phase change backfill material.

[0035] The application also provides the application of the above-mentioned layered backfill method, specifically the application in the well of a medium-deep ground source heat pump system.

[0036] Preferably, in the above-mentioned application, the medium-deep ground source heat pump system supplies heat in winter.

[0037] The application has the following beneficial effects:

[0038] 1. The layered backfill method of the phase change backfill material of the ground source heat pump system provided by the present application adopts a layered backfill scheme of the phase change backfill material, the shallow surface depth adopts the heat insulation backfill material, the low-temperature soil cannot exchange heat with the working fluid of the ground source heat pump system, thereby avoiding heat loss and improving the overall average temperature of the ground source heat pump system; the middle-deep rock-soil layer is backfilled with the phase change backfill material with a corresponding phase change temperature according to the temperature of each layer. In each temperature range, the latent heat released or absorbed by each layer of phase change material during phase change can be fully utilized, thereby significantly reducing the amplitude of the soil temperature change around the ground source heat pump system, reducing the generation of the "cold accumulation phenomenon", and improving the heat exchange performance of the ground source heat pump system.

[0039] 2. By adding high-thermal-conductivity fiber material carbon fiber in the common backfill material, the thermal conductivity, adhesion, bending strength and compressive strength and other properties of the backfill material are effectively improved, at the same time, the voids or cracks between the backfill materials of each layer due to phase change deformation are avoided, and the heat exchange efficiency of the ground source heat pump system is ensured; at the same time, the microcapsule technology is used to completely coat the phase change material, avoiding the leakage and loss of the phase change material during the phase change caused by the absorption or release of latent heat to pollute the soil, and also reducing the volume change of the phase change material during the phase change, effectively controlling the influence of the phase change material on the backfill well.

[0040] 3. The core material of the microcapsule phase change material is selected from the same or similar types of materials, and the deformation of the material during phase change is similar, thereby effectively reducing the influence on the backfill well and ensuring good heat exchange of the ground source heat pump system. BRIEF DESCRIPTION OF DRAWINGS

[0041] Figure 1 is a schematic diagram of the layered backfill of the phase change backfill material of the 2400m deep middle-deep layer ground source heat pump system provided by example 4;

[0042] Figure 2 is a structural schematic diagram of the metal packaging layer packaging the microcapsule phase change material provided by example 4;

[0043] Figure 3 is a data graph of the outlet water temperature in two working conditions in the experimental example;

[0044] Figure 4 is a solid-liquid phase diagram in two working conditions in the experimental example;

[0045] 1. Metal packaging layer; 2. Wall material; 3. Core material. DETAILED DESCRIPTION

[0046] The present application will be further described below in conjunction with examples and drawings.

[0047] Example 1:

[0048] A layered backfill method of a phase change backfill material of a ground source heat pump system, comprising the following steps:

[0049] According to the drilling depth, the drilling is divided into a shallow section and a medium-deep section, the shallow section is backfilled with a heat insulation backfill material; the medium-deep section is divided into temperature layers according to the temperature range, and the corresponding phase change backfill material is selected according to the temperature range of each temperature layer, the phase change backfill material is backfilled in layers, and the backfilling is performed by using a step-by-step lifting method, that is, the backfilling is performed at a distance of 10 m from the bottom of the well, the phase change backfill material is pulled out of the grouting pipe when the phase change backfill material reaches the grouting pipe, the pulling height is 15 m each time, and the process is repeated until the backfilling is completed, while the pulling speed is controlled to fully discharge the air in the well and compactly backfill the backfill material.

[0050] In the layered backfill method, the shallow surface is a heat insulation layer, the heat insulation layer is backfilled with a heat insulation backfill material, and the heat insulation backfill material is foamed cement; in this embodiment, one temperature layer is set every 9℃ in the medium-deep section from top to bottom.

[0051] The phase change backfill material includes ordinary backfill material and phase change material, and the mass ratio of the ordinary backfill material and the phase change material is 100:5, and the two are stirred uniformly and then backfilled; the ordinary backfill material includes Portland cement, quartz sand, swelling soil, degassing water, carbon fiber, defoaming agent and water reducing agent, and the mass ratio is 100:200:1:50:1:0.05:0.2.

[0052] In this embodiment, the carbon fiber is 3mm chopped carbon fiber produced by Toray New Materials (Guangdong) Co., Ltd.; the defoaming agent is YT-20A type silicone defoaming agent produced by Jinan Yutao Chemical Co., Ltd.; and the water reducing agent is FDN-C type naphthalene series water reducing agent produced by Shandong Wanshan Chemical Co., Ltd. The above-mentioned materials are only examples, and other types of carbon fiber, defoaming agent and water reducing agent can also be selected by those skilled in the art according to actual operation needs. Other materials such as Portland cement and quartz sand are all conventional commercially available products in the art, and will not be described here.

[0053] In the phase change backfill material corresponding to each temperature layer, the phase change temperature of the phase change material is the average temperature of the temperature layer ± 3.0℃.

[0054] The phase change material is a microcapsule phase change material, the microcapsule phase change material includes wall material and core material, the wall material is selected from melamine resin and urea-formaldehyde resin, and the mass ratio of the two is 1:1; and the core material is selected from paraffin. In this embodiment, the microcapsule phase change material can be prepared by the method provided in Example 4 of patent document CN 113583633 A, and the materials are all conventional commercially available materials, which can achieve good coating rate.

[0055] The advantage of the phase change backfill material compared to the ordinary backfill material is that the phase change material mixed in the phase change backfill material can store or release a large amount of latent heat when the phase change temperature is reached to maintain the soil temperature balance in the heat extraction area, thereby avoiding the "cold accumulation" phenomenon that occurs when the ordinary backfill material is used. When the temperature is significantly lower or higher than the phase change temperature, the phase change backfill material cannot utilize the latent heat of phase change, and its functional effect is equivalent to that of the ordinary backfill material. Therefore, to fully exert the beneficial effect of the phase change backfill material, the key is to make the entire heat exchange section of the medium-deep geothermal well reach the phase change temperature of the phase change material. However, for medium-deep geothermal development, geothermal wells with a development depth of 1000-3000 m account for a significant proportion. Therefore, for such geothermal wells, there is a temperature difference of 30-90°C between the initial heat exchange interval and the deepest heat exchange interval. Determining a single phase change temperature based on the average temperature of the stratum or the operating temperature will limit the phase change backfill material to a certain stratum, and it is obviously difficult to exert the heat supply potential of the entire geothermal well. By using the layered backfill scheme of the phase change backfill material, the phase change temperature can be determined according to the stratum temperature, so that the latent heat effect of the phase change material can be utilized in the entire heat exchange section of the geothermal well. Based on the stratum that can exert the phase change effect, compared to the single phase change temperature phase change backfill material backfill scheme, the layered backfill scheme of the phase change backfill material can effectively improve the utilization efficiency of geothermal energy.

[0056] Embodiment 2:

[0057] A layered backfill method of a phase change backfill material of a ground source heat pump system, comprising the following steps:

[0058] According to the drilling depth, the drilling is divided into a shallow section and a medium-deep section, and the shallow section is backfilled with a heat insulation backfill material; the medium-deep section is divided into temperature layers according to the temperature range, and the corresponding phase change backfill material is selected according to the temperature range of each temperature layer, the phase change backfill material is backfilled layer by layer, and the backfill is performed by using the step-by-step lifting method, that is, the backfill is performed at a distance of 10 m height from the bottom of the well, the phase change backfill material starts to be pulled out when it is over the grouting pipe, the pulling height is 15 m each time, and the process is repeated until the backfill is completed, while the pulling speed is controlled to fully discharge the air in the well, and the backfill material is tightly backfilled.

[0059] In the layered backfill method, the shallow surface is a heat insulation layer, and the heat insulation layer is backfilled with a heat insulation backfill material, and the heat insulation backfill material is selected from non-polluting heat insulation materials such as foamed cement; in this embodiment, one temperature layer is set every 27°C from top to bottom in the medium-deep section.

[0060] The phase change backfill material comprises common backfill material and phase change material, and the mass ratio of the common backfill material and the phase change material is 100:10; the common backfill material comprises Portland cement, quartz sand, expansive soil, deaerated water, carbon fiber, defoaming agent and water reducing agent, and the mass ratio is 100:250:3:60:1:0.10:2. In the embodiment, the carbon fiber, the defoaming agent and the water reducing agent are selected in the same manner as in the embodiment 1.

[0061] In the phase change material in the phase change backfill material corresponding to each temperature layer, the phase change temperature of the phase change material is the average temperature of the temperature layer ± 3.0℃.

[0062] The phase change material is a microcapsule phase change material, the microcapsule phase change material comprises wall material and core material, the wall material is made of melamine formaldehyde resin and polyurethane mixed at a mass ratio of 1:1; the core material is made of paraffin and fatty acid, and the mass ratio of the two is 2:1, and the fatty acid is made of capric acid and lauric acid at a mass ratio of 1:3.

[0063] In the embodiment, the melamine formaldehyde resin is selected from the high-quality melamine formaldehyde resin produced by Shandong Xindongneng Chemical Co., Ltd.; the polyurethane is selected from the water-soluble polyurethane produced by Shanghai Kaijin Chemical Co., Ltd.; the paraffin is selected from the paraffin particles produced by Zhengzhou Lixing Chemical Product Co., Ltd.; the capric acid is selected from the high-quality capric acid produced by Jiangsu Minglin Chemical Technology Co., Ltd.; and the lauric acid is selected from the high-quality lauric acid produced by Jinan Shunwang Chemical Co., Ltd. The above-mentioned materials are only examples, and other types of corresponding reagents can also be selected by those skilled in the art according to actual operation needs.

[0064] In the embodiment, the microcapsule phase change material is further provided with a metal packaging layer outside, the metal packaging layer is made of iron, the metal packaging layer is in the shape of a hollow cuboid, the size is 2cm*1cm*1cm, and the metal packaging layer is filled with the microcapsule phase change material. After mixing the metal packaging layer containing the microcapsule phase change material with the common backfill material, the backfill operation can be performed.

[0065] Embodiment 3:

[0066] An application of the layered backfill method provided in the embodiment 1 or the embodiment 2, in particular, an application in the drilling of a medium-depth ground source heat pump. The ground source heat pump system only supplies heat in winter.

[0067] Embodiment 4:

[0068] A layered backfill method of a phase change backfill material of a 2400m deep medium-depth ground source heat pump system, in the embodiment, one temperature layer is set every 9℃.

[0069] For winter heating operations, taking the average daily temperature of 5℃ as a baseline, and considering a 3℃ temperature gradient per 100m of ground temperature, the temperature of the rock and soil strata rises by 9℃ for every 300m of depth used for utilizing medium-deep geothermal resources. The return water temperature of the ground source heat pump system is 15-20℃.

[0070] Based on the drilling depth, the well is divided into shallow and medium-deep sections, and a phase change backfill material layered backfilling scheme is formulated.

[0071] As attached Figure 1 As shown, for shallow ground depths of 0-300m, the temperature of the soil and rock must be lower than that of the return water. To avoid heat loss of the working fluid, heat-insulating backfill material is used for depths of 0-300m.

[0072] Specifically, foamed cement is used as the insulation backfill material.

[0073] Specifically, the temperature range of soil and rock at a depth of 300-600m is 14-23℃. A mixture of 65% decanoic acid and 35% lauric acid can be used as a phase change material with a phase change temperature of 20.3℃ and a latent heat value of 128.6kJ / kg.

[0074] Specifically, the temperature range of soil and rock at a depth of 600-900m is 23-32℃. A mixture of 40% decanoic acid and 60% lauric acid can be used as a phase change material with a phase change temperature of 29.9℃ and a latent heat value of 126.9kJ / kg.

[0075] Specifically, the temperature range of the soil and rock at a depth of 900-1200m is 32-41℃. A mixture of 75.5% lauric acid and 24.5% stearic acid can be used as a phase change material with a phase change temperature of 37.0℃ and a latent heat value of 182.7kJ / kg.

[0076] Specifically, for soil and rock temperatures at depths of 1200-1500m, which range from 41-50℃, a mixture of 77% myristic acid and 23% stearic acid can be used as a phase change material. The phase change temperature is 46.4℃ and the latent heat value is 180.6kJ / kg.

[0077] Specifically, the temperature range of soil and rock at a depth of 1500-1800m is 50-59℃. A mixture of 40% paraffin and 60% myristic acid can be used as a phase change material with a phase change temperature of 57.5℃ and a latent heat value of 213.9kJ / kg.

[0078] Specifically, the temperature range of soil and rock at a depth of 1800-2100m is 59-68℃. Palmitic acid can be selected as a phase change material with a phase change temperature of 63.0℃ and a latent heat value of 213.9kJ / kg.

[0079] Specifically, the geothermal temperature range of 2100-2400m depth is 68-77℃, and stearic acid can be selected as the phase change material, the phase change temperature is 72.6℃, and the latent heat value is 183.8kJ / kg.

[0080] More specifically, to avoid leakage of the phase change material, all the above phase change materials are packaged and coated using microcapsule technology, the phase change material microcapsules are in powder form, and the particle size is 50μm.

[0081] More specifically, as shown in the accompanying drawings, a hollow rectangular steel packaging layer with a size of 2cm×1cm×1cm is made, and the above phase change material microcapsules are filled respectively to further package the phase change material and improve the thermal conductivity of the material. Figure 2

[0082] Specifically, the ordinary backfill material is prepared by mixing ordinary Portland cement, quartz sand, bentonite, deaerated water, carbon fiber, defoaming agent and water reducing agent, and the proportion is 100:200:1:50:1:0.08:0.1.

[0083] In this embodiment, the defoaming agent is an organic silicon defoaming agent, and the water reducing agent is a naphthalene series water reducing agent. The above selection is only for illustrative purposes, and those skilled in the art can also select other parameters and types of backfill materials according to actual needs.

[0084] Specifically, the phase change backfill material is prepared by fully stirring and mixing the ordinary backfill material and the phase change material packaged in the metal packaging layer, and the proportion is 100:5. The thermal conductivity, thermal diffusivity, specific heat, consistency, flexural strength and compressive strength of the phase change backfill material are measured to ensure that they meet the engineering quality standards.

[0085] Specifically, the backfill is performed by using the step-by-step lifting method, that is, the backfill is performed at a height of 10m from the bottom of the well, and the phase change backfill material starts to pull out the pipe when it reaches the pipe opening. The pulling height is 15m each time, and the process is repeated until the backfill is completed. The pulling speed is controlled to fully discharge the air in the well and compact the backfill material.

[0086] Specifically, the backfill height is determined according to the height of the grouting pipe, the grouting flow and the instrument detection, and each layer of the phase change backfill material is backfilled according to the layered backfill scheme of the phase change backfill material. The heat insulation backfill material layer is backfilled in the shallow surface until the grouting is completed.

[0087] Example 5:

[0088] The layered backfill method of the phase change backfill material of the 2400m deep and medium deep geothermal heat pump system, in this embodiment, one temperature layer is set every 27℃.

[0089] ​For the winter heating condition, the average daily temperature in winter is taken as 5℃, and the temperature gradient of the ground is 3℃ per 100m. Therefore, the temperature of the rock and soil increases by 27℃ per 900m, and the rock and soil temperature is divided into layers. The return water temperature of the ground source heat pump system is 15-20℃.

[0090] According to the drilling depth, the drilling is divided into a shallow section and a medium-deep section, and a layered backfill scheme of the phase change backfill material is developed.

[0091] For the depth of 0-300m of the shallow ground surface, the temperature of the rock and soil is less than the return water temperature, so as to avoid the heat loss of the working fluid, the heat-insulating backfill material is used for the depth of 0-300m.

[0092] Specifically, the heat-insulating backfill material uses foamed cement.

[0093] Specifically, the temperature of the rock and soil in the depth of 300-1200m ranges from 14-41℃, and a mixture of 40% capric acid and 60% lauric acid can be selected as the phase change material, the phase change temperature is 29.9℃, and the latent heat value is 126.9kJ / kg.

[0094] Specifically, the temperature of the rock and soil in the depth of 1200-2400m ranges from 41-77℃, and a mixture of 40% paraffin and 60% myristic acid can be selected as the phase change material, the phase change temperature is 57.5℃, and the latent heat value is 213.9kJ / kg.

[0095] More specifically, to avoid leakage of the phase change material, all the above phase change materials are packaged and coated using microcapsule technology, the phase change material microcapsules are in powder form, and the particle size is 50μm.

[0096] More specifically, a steel shell with a radius of 1.0cm is made, and the above phase change material microcapsules are filled to further package the phase change material and improve the thermal conductivity of the material.

[0097] Specifically, the ordinary backfill material is prepared by mixing ordinary Portland cement, quartz sand, bentonite, deaerated water, carbon fiber, defoaming agent, and water reducing agent, and the proportion is 100:250:1:60:1:0.05:2.

[0098] In this embodiment, the defoaming agent is an organic silicon defoaming agent, and the water reducing agent is a naphthalene series water reducing agent. The above selection is only for illustrative purposes, and other parameters and types of backfill materials can also be selected by those skilled in the art according to actual needs.

[0099] Specifically, the phase change backfill material is prepared by fully stirring and mixing the ordinary backfill material and the above packaged phase change material, and the proportion is 100:10. The thermal conductivity, thermal diffusivity, specific heat, consistency, flexural strength, and compressive strength of the phase change backfill material are measured to ensure that they meet the engineering quality standards.

[0100] Specifically, the backfilling is performed by using the stepwise lifting method, that is, the backfilling is performed at a height of 10 m from the well bottom, the phase change backfilling material starts to be pulled out when the phase change backfilling material reaches the mouth of the grouting pipe, the pulling height is 15 m each time, and the pulling is repeated until the backfilling is completed. The pulling speed is controlled to ensure that the air in the well is fully discharged, and the phase change backfilling material is compacted and backfilled.

[0101] Specifically, the backfilling height is determined according to the height of the grouting pipe, the grouting flow rate and instrument detection, each phase change backfilling material layer is backfilled according to the phase change backfilling material layering backfilling scheme, and the heat insulation backfilling material layer is backfilled in the shallow surface until the grouting is completed.

[0102] Experimental example:

[0103] The heat extraction process of the coaxial heat exchanger for 2400 m deep medium-deep geothermal energy under two working conditions is analyzed by using ANSYS software. Specifically, the working condition one is backfilled by using a single phase change material, and the working condition two is backfilled by using layered phase change materials. The environmental parameters are as follows: the water temperature at the well mouth is close to 0℃ and the water flow is smooth, the well mouth temperature is 5℃, and the well bottom temperature is 77℃. The phase change temperature of the single phase change backfilling material is set to 59℃. The layered phase change backfilling material is divided into four layers, and the phase change temperatures of the layers are set to 27℃, 41℃, 56℃ and 70℃ from top to bottom. The single phase change backfilling material and the layered phase change backfilling material are mixed with the ordinary backfilling material before backfilling, and the ratio of the phase change backfilling material to the ordinary backfilling material is 5:100. The composition of the ordinary backfilling material is the same as that in Example 4.

[0104] The inlet water flow rates of 0.4, 0.6 and 0.8 m·s -1 are studied under the two working conditions, the outlet water temperature is monitored, and the data statistics are shown in Figure 3 . It is found that the outlet water temperature of the working condition of the layered phase change material backfilling is at least 17% higher than that of the working condition of the single phase change material backfilling when the outlet water temperature is close to the steady state.

[0105] As shown in Figure 4 , the central part of the figure is the geothermal system pipeline, and the two sides are the phase change of the phase change material from solid to liquid in the backfilling material. By analyzing the solid-liquid phase diagrams of the two working conditions, it can be found that when the single phase change material backfilling is used (i.e. the left side of the solid-liquid phase diagram), the liquid phase ratio gradually decreases to 0 from the well bottom to the middle of the wellbore, which indicates that the latent heat of phase change in the part close to the well bottom of the geothermal well is utilized. When the layered phase change backfilling material is used for backfilling (i.e. the right side of the solid-liquid phase diagram), as shown in Figure 4 , the phase change occurs in each layer, which indicates that compared with the working condition of the single phase change material backfilling, more latent heat of phase change is utilized in the working condition of the layered phase change material backfilling, which makes the circulating water obtain more heat, and the outlet water temperature, i.e. the temperature rise of the circulating water in the well, is significantly improved.

Claims

1. A layered backfill method for a phase change backfill material for a ground source heat pump system, characterized by, The method comprises the following steps: According to the drilling depth, the drilling is divided into a shallow section and a medium-deep section, the shallow section is backfilled with a heat-insulating backfill material; the medium-deep section is divided into temperature layers according to temperature ranges, and corresponding phase-change backfill materials are selected according to the temperature ranges of the temperature layers, and the phase-change backfill materials are backfilled layer by layer; One temperature layer is set in the medium-deep section according to a temperature range of 9-27℃; The phase-change backfill material comprises a common backfill material and a phase-change material, and the mass ratio of the common backfill material to the phase-change material is 100:(5-10); In the phase-change backfill material corresponding to each temperature layer, the phase-change temperature of the phase-change material is the average temperature of the temperature layer ± 3.0℃.

2. The layered backfill method of claim 1, wherein, The shallow surface is a heat-insulating layer, and the heat-insulating layer is backfilled with a heat-insulating backfill material.

3. The layered backfill method of claim 2, wherein, The heat-insulating backfill material is foamed cement.

4. The layered backfill method according to claim 1, characterized in that, if the total temperature difference of the medium-deep section cannot be divided by the temperature range of the temperature layer, when the remainder is less than half of the temperature range, the remaining temperature layer is integrated into the adjacent temperature layer; when the remainder is greater than or equal to half of the temperature range, the remaining temperature layer is listed as an independent temperature layer.

5. The layered backfill method of claim 1, wherein, The common backfill material comprises Portland cement, quartz sand, expanded soil, degassing water, carbon fiber, defoaming agent and water reducing agent, and the mass ratio is 100:(200-250):(1-3):(50-60):1:(0.05-0.10):(0.2-2).

6. The layered backfill method of claim 1, wherein, The phase-change material is a microcapsule phase-change material, the microcapsule phase-change material comprises a wall material and a core material, and the particle size of the microcapsule phase-change material is 1-100μm.

7. The layered backfill method of claim 6, wherein, The wall material is selected from at least one of melamine resin, urea-formaldehyde resin, melamine-formaldehyde resin and polyurethane, and the core material is selected from at least one of paraffin or fatty acid.

8. The layered backfill method of claim 7, wherein, The fatty acid is selected from at least one of capric acid, lauric acid, stearic acid, myristic acid or palmitic acid.

9. The layered backfill method of claim 6, wherein, The microcapsule phase-change material is externally provided with a metal packaging layer made of iron or copper.

10. The layered backfill method according to any one of claims 1-9, in particular the application in the drilling of a medium-deep ground source heat pump system.

11. Use according to claim 10, wherein the compound is ###0002### The medium-deep ground source heat pump system supplies heat in winter.

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