Application of waste fly ash and graphite powder used as backfill materials in vertical ground heat exchangers

Backfill material plays a significant role in heat transfer between the ground heat exchangers (GHEs) and surrounding soil. Fly ash is the fine ash wasted from coal combustion in thermal power plants, which may cause a serious of problems including air, water and soil pollution. To fully utilize the waste resources and improve the thermal performance of backfill material, a method of using waste fly ash and graphite powder is proposed, and the feasibility of using them as backfill material is investigated via laboratory and field test, in which the economic aspect is taken into consideration. The results indicate that the highest thermal conductivity of thermally enhanced backfill material could be increased to 2.391 W/(m center dot K) via laboratory test, with a growth rate of 23.69 %, in which the mass ratio of graphite reaches 10 % meanwhile guaranteeing the workability of fresh mortar. Heat transfer rates could be enhanced by 27.22 % compared with traditional backfill material of natural sand via field test, corresponding to a highest heat transfer rate of 55.80 W/m. The economical value could be achieved through the lower cost of raw material and the reduction of drilling depth.