The effect of employing nanofluid on reducing the bore length of a vertical ground-source heat pump Hamid Narei, Roghayeh Ghasempour ⇑ , Younes Noorollahi Faculty of New Sciences and Technologies, University of Tehran, Tehran, Iran article info Article history: Received 18 April 2016 Received in revised form 26 June 2016 Accepted 30 June 2016 Available online 7 July 2016 Keywords: Nanofluid Ground-source heat pump (GSHP) Ground heat exchanger (GHE) Multi-objective optimization Flower Pollination Algorithm (FPA) abstract In our era ground-source heat pumps are known as energy-efficient air conditioning systems. However, their high initial costs are a major obstacle to the widespread use of such systems. In this study, the effects of using Al 2 O 3 /water nanofluid as heat transfer fluid on reducing the bore length of a ground heat exchanger in a vertical ground-source heat pump are examined. For this purpose, the effective thermal conductivity and the effective viscosity of the nanofluid, which play prominent roles in the convective heat transfer, are optimized via using multi-objective Flower Pollination Algorithm. In this algorithm, the logic of the second version of non-dominated sorting is utilized to deal with the two objectives of this optimization. Then some of the best possible combinations of the thermal conductivity and the viscosity of the nanofluid, extracted from the obtained Pareto front, were used to compute the required bore length. The results were compared with the required bore length calculated using pure water as the heat transfer fluid in the ground heat exchanger. The comparison demonstrated that employing Al 2 O 3 /water nanofluid instead of water as heat transfer liquid reduced less than 1.3% of the bore length. Furthermore, investigating the reason of this low reduction in the bore length revealed that grout has the most potential to reduce the bore length among the heat transfer fluid, tubes, and grout. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction The necessity for reducing the emission of greenhouse gases, future worries on energy shortage, and recent moves towards envi- ronment sustainability have led to numerous efforts to lower energy consumption in buildings, which contribute to approxi- mately 40% of the total energy demand in US [1] and Europe [2] and 36% of the total greenhouse gases emission in Europe [2]. In buildings the majority of the energy demand is assigned to air con- ditioning systems [3]. Hence, an energy-efficient air condition sys- tem can decrease energy consumption dramatically. Based on ground energy sources, ground-source heat pumps (GSHPs) have been considered as energy-efficient air conditioning systems which benefit from the earth’s relatively constant temperature to provide heating, and/or cooling, and/or hot water [4]. The U.S. Environment Protection Agency (EPA) has claimed that GSHPs are the most energy-efficient and environmentally friendly air con- ditioning technology [5]. Vertical borehole ground heat exchangers (GHEs) are the most applied systems in GSHPs [6]. High initial and installation costs of vertical GHEs, however, are a formidable obstacle to the widespread application of GSHPs. A reason for the high costs of vertical GHEs is the required deep borehole, which may be ascribed to the inherently poor thermal conductivity of conventional heat transfer fluids (HTFs) (water or antifreeze/water mixture). Using nanofluids, suspension of nanosized particles in fluids, is considered as a promising way to overcome the low thermal con- ductivity of conventional HTFs. The addition of nanoparticles changes the thermo-physical properties of the base fluid, such as thermal conductivity and viscosity. Since the first report of anoma- lous thermal conductivity enhancement of nanofluids [7], numer- ous experimental and analytical researches [8–11] have been conducted on thermal conductivity of nanofluids. Experimental results have demonstrated that many factors, e.g. nanoparticle vol- ume fraction and size, affect the thermal conductivity of nanofluids [12]. Most of the experimental researches have shown that the thermal conductivity of nanofluids increases with the increment of particle volume fraction [8,9] and decrement of nanoparticle size [10,11]. Lee et al. [13] compiled the results of a large number of works in the thermal conductivity of nanofluids in an exhaustive review. Viscosity is another thermo-physical property that plays a prominent part in heat transfer systems due to its effect on con- vective heat transfer coefficient and the pumping power [14]. In spite of its importance, few researches have been conducted on http://dx.doi.org/10.1016/j.enconman.2016.06.079 0196-8904/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: ghasempour.r@ut.ac.ir (R. Ghasempour). Energy Conversion and Management 123 (2016) 581–591 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman