Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Numerical investigation of the nanofluid effects on the heat extraction process of solar ponds in the transient step Mohamad Aramesh, Fathollah Pourfayaz ⁎ , Alibakhsh Kasaeian Department of Renewable Energies and Environment, Faculty of New Sciences and Technologies, University of Tehran, Tehran, Iran ARTICLE INFO Keywords: Solar pond Nanofluid Thermal performance Transient modeling Numerical simulation ABSTRACT In this paper, a modification of previous thermal modeling methods of solar ponds has been employed to si- mulate the heat extraction process by nanofluids in the transient step. A hypothetical rectangular solar pond with a cross section of 10 × 10 m 2 , and 2.7 m depth has been considered as the case study. The city of Tehran has been assumed to be the location of the pond. Using the synoptic data of the location, the heat storage process is modeled in August 2016. The model showed that after 24 days, the temperature of the lower convective zone (LCZ) reaches to 98.66 °C with 40.5093 GJ of stored thermal energy. At this point, the heat extraction process is modeled for a 48 h period after which this pond cannot provide enough thermal energy anymore. The thermal performance of heat removal for water and six different water-based nanofluids are investigated and compared theoretically. The nanofluids include Ag/water, Cu/water, CuO/water, Al 2 O 3 /water, SWCNT/water, and MWCNT/water. Also, different volume fractions between 0.1% and 5% are modeled. The amounts of the ex- tracted heat for 15 different concentrations of the selected nanoparticle types are calculated. Using the results, the threshold concentration has been determined for each type. Moreover, the heat extraction rate, the per- centage of the extracted heat, and the mean outlet temperature are investigated at the threshold concentrations. For all of these parameters, the SWCNT/water nanofluid has determined to have the best performance at its threshold concentration of 0.1%v/v. For the amount of the extracted heat, the model showed the minimum amount of 5.8141 GJ for water, and the maximum amount of 10.2892 GJ for the SWCNT/water nanofluid at its threshold concentrations. 1. Introduction Nowadays, many countries in the world are looking for sustainable sources of energy (Kasaeian et al., 2017b; Maleki et al., 2016). Many of the developed and developing countries are using the renewable en- ergies for this purpose (Yousefi et al., 2017). Salt gradient solar pond is counted as a reliable renewable energy source, especially for the low- grade thermal applications (Singh et al., 2011). Solar ponds can store the solar thermal energy, and this energy can be used to produce heat or electricity (Alcaraz et al., 2016). In the recent years, many scientists have studied the performance of the solar ponds (Abdullah et al., 2017; Assari et al., 2017; Bozkurt and Karakilcik, 2015b; Ganguly et al., 2017). One of the ways to improve the thermal performance of the solar energy technologies is employing nanofluids as the working fluid (Kasaeian et al., 2015, 2017a; Khanjari et al., 2016, 2017). However, there are few researches on the applications of the nanofluids in the solar ponds. In a review paper, Mahian et al. (2013) have investigated the different applications of nanofluids in the solar energy systems. For the solar ponds, they have proposed a setup to use a nanofluid for heat extraction from the pond. In the setup, nanofluid is stored in an external tank, and it flows through the bottom of the pond by a tube. However, no similar studies have been reported in their work. Al-Nimr and Al-Dafaie (2014) have presented a novel design for solar ponds without the salt gradient inside. In this design, the pond is composed of two layers. The top and the bottom layers are composed of mineral oil and Ag/water nanofluid, respectively. They have reported 216% more heat storage in the designed pond in comparison with an identical salt gradient solar pond. In another study, Ding et al. (2016) have designed and constructed a solar pond combined with thermo- electric cells for electric power generation. In their work, they have proposed that the nanofluids can be employed to enhance the perfor- mance of the thermoelectric generator by improving the heat exchanger efficiency. Also, in some other studies, utilizing nanofluids in the solar ponds has been only proposed and not investigated (Hemmat Esfe et al., 2015; Kashani et al., 2014; Srinivasacharya and Surender, 2014). To study the effects of employing the nanofluids, it is needed to simulate the heat extraction operation from the solar ponds. Most of the modeling http://dx.doi.org/10.1016/j.solener.2017.09.011 Received 15 June 2017; Received in revised form 21 August 2017; Accepted 1 September 2017 ⁎ Corresponding author. E-mail address: pourfayaz@ut.ac.ir (F. Pourfayaz). Solar Energy 157 (2017) 869–879 0038-092X/ © 2017 Elsevier Ltd. All rights reserved. MARK