Performance evaluation of U-tube borehole heat exchangers employing nanofluids as the heat carrier fluid

The present study aims to investigate comprehensively the performance of various nanofluids in single U-tube borehole heat exchangers (BHEs). Seven common nanoparticles with the volume fraction ranging from 0.1% to 2.0% are selected to be evaluated as the heat carrier fluid. Firstly, a comparative techno-economic analysis is performed in order to highlight the merits and drawbacks of each nanofluid. Then, a sensitivity analysis is performed to optimise the decrement percentage of BHE thermal resistance. Finally, by means of the linear regression of numerical results obtained for different nanofluids, simple equations are proposed allowing evaluation of the outlet fluid temperature for nanofluids. The obtained results indicate that Ag- and Cu-based nanofluids are characterised by the highest heat transfer enhancement, although this improvement is at penalty of a higher pressure drop and up to 31% higher required pumping power. On the other hand, SiO2- and TiO2-water nanofluids are the worst cases in terms of thermal performance, but at the same time, they are characterised by the lowest pressure drop. The optimum decrement percentage of thermal resistance yielded in presence of Cu-water nanofluid is equal to 4.31%. Furtheremore, it is shown that employing nanofluids for the purpose of BHE length reduction is not a promising choice. Economic analysis revealed that the cost of electrical energy for nanofluids due to the higher energy consumption of pump is negligible in comparison with the capital cost of nanoparticles. The SiO2 nanoparticles with a capital cost ranging from 5.8 to 17.5 €/m is the cheapest nanoparticle to employ, unlike the Ag nanoparticles.