A combined numerical and experimental study on graphene/ionic liquid nanoﬂuid based direct absorption solar collector Jian Liu, Zhuocheng Ye, Long Zhang, Xiaoming Fang n , Zhengguo Zhang nn Key Laboratory of Enhanced Heat Transfer and Energy Conservation, the Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China article info Article history: Received 5 September 2014 Received in revised form 23 December 2014 Accepted 12 January 2015 Keywords: Solar energy Direct absorption solar collector Ionic liquid Grapheme Receiver efﬁciency abstract Novel heat transfer ﬂuids with very low vapor pressure and high thermal stability are highly desirable for both high temperature direct solar collectors and concentrated solar collector. Herein a combined analytical and experimental study has been conducted on high temperature direct solar thermal collectors using graphene/ionic liquid nanoﬂuids as the absorbers. A one-dimensional transient heat transfer model has been used to predict the receiver temperature and efﬁciency with varying parameters such as solar and graphene concentration and receiver height. The results show that the experimental temperature is in good agreement with numerical results under the same conditions. Based on the model, it is shown that the receiver efﬁciency increases with the solar concentration and receiver height, but decreases with the graphene concentration. The receiver efﬁciency could be maintained 0.7 under the conditions of 0.0005 wt% of graphene in 5 cm receiver under 20  1000 W m 2 at 600 K. This work provided an important perspective to the graphene/ionic liquid nanoﬂuids for use as a kind of novel heat transfer ﬂuid in direct solar thermal collectors under concentrated solar incident radiation. & 2015 Elsevier B.V. All rights reserved. 1. Introduction Solar thermal utilization, one of the most practical and effective way in solar energy applications, is expected to solve the energy crisis without harm to the environment. In solar thermal systems, one major component is the solar collectors, which are special kind of heat exchangers that transform solar radiation energy to internal energy of the transport medium. Since their discovery in the early 1970s, direct absorption solar collectors (DACs) [1], where heat transfer ﬂuid (HTF) directly and volumetrically absorb solar incident radiation, have been demonstrated to be efﬁcient than surface rec- eivers owing to a reduction in temperature difference between the surface and the ﬂuid, consequently minimizing convective heat losses [2]. Moreover, increasing the operating temperature of HTF is crucial in solar thermal systems because higher temperature can signiﬁcantly reduce the electricity cost and improve the conversion efﬁciency of power generation. Consequently, a variety of concen- trated solar receivers such as parabolic troughs [3] and power tower [4], have been developed to increase the temperature of HTF. Such solar thermal technologies show great potential in solar thermal-to- electrical power conversion. This emerging technology gets stuck because its improvements require a new HTF that must have a very low vapor pressure at the hot operating temperature and combined with a high thermal stability, higher than 400 1C. Further, the piping layout of trough plants dictates that the ﬂuid not be allowed to freeze, which dictates the use of extensive insulation and heat tracing unless the ﬂuid has a freezing point near 0 1C [5]. Unfortunately, traditional HTFs have limitations: water [6], ethylene glycol [7], oil (Therminol VP-1) [8] are instable at high temperature and molten inorganic salts [5] have freezing points above 200 1C, limits the thermal conversion efﬁciency of the power cycle. Ionic liquids, composed of organic cations and organic or inorganic anions [9], have been demonstrated to have a wide range temperature of liquid [10]. For example, 1-hexyl-3-methylimidazolium tetraﬂuor- oborate ([HMIM]BF 4 ) has a freezing point even down to  80 1C and a decomposition temperature of 420 1C [11]. The remarkable property indicates that ionic liquids are better candidates of HTF in concen- trated solar thermal systems. Furthermore, the properties of ionic liquids can be tuned by altering cations and anions, making ionic liquids more attractive for thermal utilization [12]. The thermophysical properties of ionic liquids have been broadly reported at present [13–15], and the results conﬁrm ionic liquids are superior to traditional HTFs. However, the investigations on ionic liquids based solar collec- tors are rarely reported [16]. In DACs, the HTFs directly absorb solar incident radiation and storage to heat, the sunlight absorptivity of HTFs is important for photo-thermal conversion. Although most of ionic liquids have Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells http://dx.doi.org/10.1016/j.solmat.2015.01.013 0927-0248/& 2015 Elsevier B.V. All rights reserved. n Corresponding author: Tel.: þ86 20 8711 2997; fax: þ86 20 8711 3870. nn Corresponding author: Tel.: þ86 20 8711 2845; fax: þ86 20 8711 3870. E-mail addresses: cexmfang@scut.edu.cn (X. Fang), cezhang@scut.edu.cn (Z. Zhang). Solar Energy Materials & Solar Cells 136 (2015) 177–186