## 1 Introduction
Their outcomes illustrated that by using the nanofluids, thermal resistance of the system is suppressed, and the temperature gradient is reduced due to the promotion of the heat transfer. Also, using nanoparticle with smaller diameter or at higher concentration can potentially reduce the thermal resistance of the system. [PERSON] et al. [27] perused the influence of TiO\({}_{2}\) nanoparticle with average size of 21 nm on the heat-transfer mechanisms of alcohols in the HPs. They observed that by adding particles at 0.1% of volume fraction to the alcohol, the heat-transfer coefficient (HTC) can be enhanced by 10.6%. [PERSON] and [PERSON] [28] performed some experiments to study the efficacy of aluminum oxide/ H\({}_{2}\)O nanofluid on the HTC of a HP heat exchanger in the presence of butterfly tube inserts when the HP is at constant heat flux. The results illustrated that by adding nanoparticle to the system, a significant enhancement for Nusselt number was gained (-345%). [PERSON] et al. [29] carried out an experiment on CuO/ H\({}_{2}\)O nanofluid flowing through a tube to investigate the energy efficiency of a system working with HP. Also, an ANN model was expanded to investigate the thermal efficiency of the HP. It was identified the efficiency of the system can be improved with nanofluids. It was also understood that the heat transfer can be promoted up to 2.8 times, compared to the base fluid. They demonstrated that using a nanofluid can suppress the consumption of working fluid by 37% and can diminish the size of the system by 55%. [PERSON] et al. [11] carried a set of experimentsout, to peruse a HP working with ZrO\({}_{2}\)/acetone nanofluid. The setup was fabricated to simulate a solar collector. As expected, nanofluid diminishes the thermal resistance and gained HTC by 36.3%. [PERSON] et al. [6] experimentally analyzed the efficacy of TiO\({}_{2}\)-SiO\({}_{2}\)/water-EG nanofluid on the HTC of a HP. They found the heat-transfer efficiency can be improved by 254.4%, whereas the friction factor was also augmented by 76%, compared to the base fluid. [PERSON] et al. [11] fulfilled several experiments to study the efficacy of CNT/water nanofluid on HTC of a flat HP. They proved that the HTC of the system was improved by 40%, compared to water. HTC of pulsating HP (PHP) using 0.25-1.5 g/lit of GO/H\({}_{2}\)O nanofluids was experimentally studied by [PERSON] et al. [30]. Their results demonstrated that the thermal resistance of a HP can be diminished by 42%, while thermal conductivity and viscosity both increased when nanofluid was employed in the HP. [PERSON] et al. [31] modeled a HP including a double forward-facing steps where CuO/water and Al\({}_{2}\)O\({}_{3}\)/water were employed inside the HP. The results showed that Nusselt number can be augmented by raising the nanofluid's concentration, height of the step, and the fluid velocity. Also, they found that alumina/water nanofluid at vol.% = 4 has the highest HTC. Facing with the above literature, it can be stated that despite immense accomplished studies on the potential usage of the nanofluids in various thermal systems, less studies have been devoted to the solar thermal energy with the focus on the application of the nanofluids in smart cities, in which energy plays a key role in the municipal development. Renewable energy can open a new door towards the sustainability of the smart cities and nanofluid can contribute to this. Hence, in this study, a fundamental research is fulfilled to assess the feasible application of graphene nanoplatelets-n-pentane as the working fluid inside the HPs. N-pentane with the very high vapor pressure can be a fast-responding working fluid in the HP. Also, the presence of the graphene can improve the thermal characteristics of n-pentane.