Thermal Performance Of A Spiral Ground Heat Exchanger In Sand/bentonite Blend

The renewable geothermal energy with huge reserves and environmental protection has been widely used in recent years.At present,shallow geothermal heating(refrigerating)technology has been basically mature which mainly uses heat pump technology to obtain(release)energy into shallow soil,thereby achieving heating or cooling of buildings.The thermal performance of spiral heat exchanger in sand-based backfill material in the heating and cooling mode was investigated in terms of Reynolds numbers,operation modes and bentonite and graphite additives.The variation of thermal conductivity for sand/bentonite blend with different water content and bentonite addition ratio was addressed.The effect of bentonite on the heat and moisture diffusion of sand was also investigated.The thermal conductivity of different samples was measured by the thermal needle probe method.The results showed that the thermal conductivity of the sand/bentonite blends increased first and then decreased with the increase of water contents.While,the thermal conductivity decreased with rising the blending ratios of bentonite.In addition,the average thermal conductivities of the mixtures with 5%and 10%graphite were about 9%and 13%higher than that of the sand/bentonite blend.The water holding capacity for the sand/bentonite mixtures with 5%blending ratio increased by about 14%compared with that of the sand individual.In the heat and moisture diffusion experiment,it was found that the 5%bentonite additive promoted the heat transfer of sand.The final distribution of water content for sand and sand/bentonite blend in the cooling and heating modes had the peak value.At the maximum change of moisture content,the change of moisture content in the sand was 2%and 147%higher than that in the sand/5%bentonite mixture in the heating and cooling modes,respectively,which indicated that the inhibition effect of bentonite on sand moisture migration in the cooling mode was more significant than that in the heating mode.The average values of apparent thermal diffusivity of sand and sand/5% bentonite mixture in the cooling mode were 1.6×10-6 m2 s-1 and 2.1 ×10-6 m2 s-1,respectively.The apparent thermal diffusivity of the mixture was 31%higher than that of sand.By determining the soil hydraulic and the relationship between the thermal conductivity and water content of the model,the heat and moisture diffusion of sand can be predicted more accurately.The calculation results showed that the moisture diffusion content increase with increasing of the heat source temperature.The range of sand moisture diffusion increased as the initial moisture content increased.When the heat source action time was 48 h,the moisture diffusion range of sand was 174%larger than that when the heat source action time was 6 h,which indicated that the range of sand moisture diffusion increased with the increase of the heat source action time.In the experiment of heat transfer performance of ground heat exchanger,in the cooling mode,the surface heat transfer coefficients of the water between the Reynolds numbers of 2000 and 4500,ranged from 225 to 610 W m-2 K-1.The average surface heat transfer coefficient in the heating mode was 11 percent higher than that in the cooling mode between the Reynolds numbers of 2000 and 4000.The dimensionless surface heat transfer correlations of the water in the heating and cooling modes were determined,Nu=0.109Re0.512 and Nu=0.0004Re1.187,respectively.In the cooling mode,when backfill material was sand/5%bentonite/5%graphite blend,the integrated evaluation factor was 20%higher than that when backfill material was sand.The results showed that the higher the thermal conductivity of the backfill material,the better the heat transfer of the ground heat exchanger.In the heating mode,after 10 h operation,the average heat flow rate,which was averaged only in operation time,were 102,129,163 and 246 W,respectively for the continuous operation mode,intermittent operation mode with 4 h on,1 h off;2.5 h on,2.5 h off and 1 h on,4 h off,indicating that the intermittent operation can gradually restore soil temperature,thereby enhancing the heat transfer process.The research results of this paper have certain guiding significance for the research of ground source heat pump.