NUMERICAL STUDY ON THE INFLUENCE OF NANOFLUIDS ON THE BOREHOLE HEAT EXCHANGER THERMAL RESISTANCE

Energy consumption in buildings covers approximatively 40% of the total energy demandand about 36% of the total greenhouse gases emission in Europe. The majority of this total energydemand can be ascribed to air conditioning systems, thus, more and more interest on energy-efficient airconditioning system has kept growing over the last years. In this context, air conditioning system basedon renewable energy sources could represent a viable alternative to meet building energy demand andto address climate change. Among several renewable energy sources technologically mature,nowadays the interest towards geothermal one is growing, due to its great potential to providesignificant energy saving compared to conventional systems. While the direct conversion ofgeothermal energy (high-enthalpy) into electricity is widely diffused, the use of geothermal sourcesin low energy heating-cooling systems (medium and low-enthalpy) employing ground source heatpumps is still limited by their high initial costs and long pay-back period. In the systems with verticalborehole heat exchanger, the majority of these costs are due to the high required borehole length,because of the low thermal conductivity of employed conventional heat carrier fluid.In order to make ground source heat pumps more economically competitive, the heat transfer betweenborehole wall and circulating fluid could be improved replacing conventional ethylene/water mixture bynanofluids, that are two phase systems comprising a carrier medium (liquid or gas) and very small (1 nm– 100 nm) dispersed nanoparticles. Although the heat transfer coefficient of nanofluids is much higherthan the common-base fluid, it should be taken into account that the thermo-physical properties ofnanofluids are strongly depend by the concentration of added nanoparticles: the highest convective heattransfer coefficient is reached when the nanoparticles volumetric concentration φ is between 0% and 1%;increasing the volumetric concentration above this range, the thermal conductivity of nanofluids remainsmore or less constant, while the viscosity increases a lot, causing a rise in a pressure drop without animprovement of heat transfer.In this paper, one-dimensional energy and momentum balances were used to carry out a numericalinvestigation on the use of nanofluids as heat carrier in BHE. Varying φ in the range reported above, theeffect of various nanoparticles (Ag, Cu, Al, Al2O3, CuO, Graphite, SiO2) on the borehole heat exchangerperformances was evaluated, choosing the best solution that ensures both the lowest increase in pressuredrop and the highest thermal borehole resistance reduction. Moreover, an economic analysis was carriedout, in order to analyse the influence of the use of nanlofluids on the costs of borehole heat exchangersystem. Moreover, a comparison with previous published results was done.