| Shallow geothermal energy is a kind of green,environmental protection and sustainable low energy.At present,ground source heat pump is the main means of exploiting and utilizing shallow geothermal energy.Among them,the vertical buried pipe ground source heat pump is widely used in building refrigeration and heating because of its high heat exchange,small land area and other advantages.As the medium of heat exchange between U-shaped tube and surrounding rock,backfill material is very important to the normal operation of the whole heat pump system.Phase change materials are materials that change their physical form in response to the ambient temperature and maintain the same temperature.The process of phase transition is also accompanied by the storage and release of heat.The phase change material is used as an additive and mixed with backfilled raw sand to backfill the heat exchange hole.The main purpose is to use the latent heat generated in the process of phase transformation to improve the overall specific heat capacity of the backfill material in the hole,so as to improve the heat transfer effect of the heat pump system.In this paper,the thermal properties of phase change materials(PCM)in the backfilling of heat exchange holes were studied for the working modes of different thermal conductivity,multiple cycles and different start-stop ratios.The effects of phase change materials with different thermal conductivity on temperature fluctuation,heat transfer depth and heat influence range of heat transfer holes were studied by physical experiments and numerical simulations.The research results provide a new perspective and scientific support for the application of phase change materials in ground source heat pump backfill materials.The research content and main conclusions of this paper are as follows:(1)Based on previous studies,this paper found suitable phase change core material(CA/LA)and adsorption carrier(EG)by reviewing literature,and prepared phase change material(CA-La /EG)with high thermal conductivity in the laboratory,and then tested its thermal and physical properties,such as anti-erosion and anti-leakage.(2)A comparative study was conducted on the effect of phase change backfilled sand and ordinary backfilled raw sand in controlling temperature fluctuations inside and outside holes by building a thermal physics experiment platform.The experimental results showed that: In the heating stage,the temperature in the phase change group was lower than that in the ordinary group,with an average temperature difference of 2.84℃.In the recovery stage,the temperature difference was higher than that in the ordinary group,and at the end of the experiment,the temperature difference between the two was 0.7℃.(3)Through numerical simulation,the effects of backfill materials with different thermal conductivity and different proportions of phase change materials on the operating characteristics of the buried heat exchange tube and the temperature in the heat exchange hole were studied at 0.7 start-stop ratio and multiple cycles.Numerical simulation results show that the phase change material with low thermal conductivity cannot effectively improve the heat transfer efficiency of buried pipes.Because the thermal resistance of U-shaped pipe,backfill material and rock and soil layer is connected in series,if the heat storage capacity in the heat exchange hole is only increased,but the thermal conductivity rate is not improved or even reduced correspondingly,heat cannot be transferred layer by layer.And when the heat storage capacity of PCM and backfill sand is exhausted,it will lead to more serious "heat accumulation" problem.Capric acid-lauric acid/expanded graphite(CA-LA/EG)composite phase change material has the characteristics of high thermal conductivity and high heat capacity,which can timely diffuse the heat accepted in the heat exchange hole to the surrounding strata,reduce the temperature fluctuation in the hole during operation,and improve the heat exchange efficiency between the buried pipe and the surrounding rock and soil layer.Therefore,the phase change material with high heat capacity should be used together with the carrier or additive with high heat conductivity,and the heat in the heat exchange hole can be dissipated in time through the intermittent operation mode,so as to spare the heat storage space for the next operation cycle and continuously play the role of high heat capacity of PCM.(4)By adding expanded graphite to the backfilled raw sand to improve the comprehensive thermal conductivity of the backfill material,a thermal response test platform was built,and the difference of thermal influence between the ordinary backfilled raw sand and the heat-conducting enhanced backfilled sand was compared.The test results showed that: Using heat-conducting enhanced backfill sand(raw sand+ expanded graphite)can effectively improve the heat transfer depth of the heat exchange hole,in winter conditions,the heat transfer depth increased by 18.83%,and in summer conditions,increased by 19.14%.At the same time,the heat transfer ability between the heat exchange hole and the surrounding rock mass can be improved.Compared with the ordinary heat exchange hole,the ground temperature recovery time of the heat exchange hole with enhanced heat conduction is shortened by 7 hours and40 minutes.(5)By adding capric acid-lauric acid/expanded graphite(CA-LA/EG)high thermal conductivity composite material to the backfill raw sand to improve the overall specific heat capacity of the backfill material,and build a thermal response experiment platform,comparing the difference of thermal influence between the raw sand and phase change backfill sand,the test results show that: The use of high thermal conductivity composite phase change material as an additive of backfill material can effectively reduce the temperature fluctuation in the hole and increase the heat transfer of single hole depth.In summer conditions,the heat transfer of phase change hot hole depth is increased by about 21% compared with the ordinary heat transfer hole.In this study,capric acid-lauric acid/expanded graphite(CA-LA/EG)composite phase change materials with high thermal conductivity were developed to improve heat transfer and heat storage capacity of backfill materials.Numerical simulation software was used to analyze the influence of PCM with different dosage and thermal conductivity on the temperature in hole and heat transfer per unit well depth under the condition of long time and multi-cycle operation.In addition,the phase change material prepared in the laboratory is applied to the actual heat exchange hole backfill,and the thermal response test is carried out,and the research results that the heat conduction enhanced PCM is beneficial to the heat exchange hole are obtained,which has certain theoretical guiding significance for the application of phase change material in the ground source heat pump backfill. |
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