An experimental investigation on the effect of MWCNT-H 2 O nanoﬂuid on the efﬁciency of ﬂat-plate solar collectors Tooraj Youseﬁ a,⇑ , Farzad Veisy a , Ehsan Shojaeizadeh a , Sirus Zinadini b a Mechanical Engineering Department, Faculty of Engineering, Razi University, Kermanshah, Iran b Young Researchers Club, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran article info Article history: Received 30 April 2011 Received in revised form 4 September 2011 Accepted 21 January 2012 Available online 2 February 2012 Keywords: Flat-plate solar collector Collector testing ASHRAE standard MWCNT nanoﬂuid abstract In this study the effect of MWCNT nanoﬂuid, as absorbing medium, on the efﬁciency of a ﬂat-plate solar collector was investigated experimentally. The weight fraction of CNTs was 0.2% and 0.4%. The effect of Triton X-100 as a surfactant on the stability of nanoﬂuid was studied. The tests were performed in differ- ent mass ﬂow rates of nanoﬂuid from 0.0167 to 0.05 kg/s. ASHRAE standard was used to perform the tests. Results show that by increasing the weight fraction from 0.2% to 0.4%, there is a substantial increase in the efﬁciency. Also, using the surfactant causes an increase in the efﬁciency. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction Solar thermal energy is a very convenient source of heating and it is a technology that does not depend on scarce, ﬁnite energy re- sources. Solar collectors convert solar radiation into heat and transfer the heat to a medium. The solar energy can be used for heating or cooling systems. The most productive solar heaters are ﬂat-plate type, but these types have comparatively low efﬁciency and outlet temperatures. Beside the conventional methods to in- crease the solar collector efﬁciency, one of the most effective meth- ods is replacing the working ﬂuid, water, by high thermal conductivity ﬂuids. The ﬂuids with solid nanoparticles suspended called ‘‘nanoﬂuids’’. Nanoﬂuids are expected to present exceptional heat transfer properties compared with conventional heat transfer ﬂuids. Since the discovery of carbon nanotubes (CNTs) in 1991, car- bon-based nanomaterials have attracted great interest due to their unique physical, thermal, mechanical, and electrical properties [1]. Choi and Eastman [2] showed that the thermal conductivity of CNT–water nanoﬂuids was higher compared to that of their base liquids. According to the properties of the base liquid, geometry of CNT, and volume fraction, a broad range of enhancement has been reported in the literatures. For example with 1% volumetric fraction of MWCNT, the thermal conductivity of water was en- hanced by 40% [3]. CNTs always aggregate owing to very strong van der Waals interactions. One of the most important problems of CNTs in water is its dispersion. There are various methods to get homogenous dispersion of CNTs, such as chemical functionali- zation and physical interactions [4]. The former has been found effective but deteriorates the intrinsic properties of CNTs. Physical approaches using amphiphilic surfactants such as sodium dodecyl- benzene sulfonate (SDBS), sodium dodecyl sulfate (SDS) and Triton X-100 have been proven capable of debundling CNTs and stabiliz- ing individual tubes while maintaining MWNT intrinsic properties [5,6]. Recently some studies were reported about using nanoﬂuids in solar collectors. Natarajan and Sathish [7], has investigated the thermal conductivity enhancement of base ﬂuids using carbon nanotube (CNT), and suggested if these ﬂuids are used as a heat transport medium, it increases the efﬁciency of the conventional solar water heater. Tyagi et al. [8] has studied theoretically the capability of using a non concentrating direct absorption solar col- lector (DAC) and compared its performance with that of a conven- tional ﬂat-plate collector. In Tyagi research a nanoﬂuid that is a mixture of water and aluminum nanoparticles was used as the absorbing medium. According to the results of Tyagi’s study, the efﬁciency of a DAC using nanoﬂuid as the working ﬂuid is up to 10% higher than that of a ﬂat-plate collector. Otanicar and Golden [9] has studied environmental and economic inﬂuence of using nanoﬂuids to enhance solar collector efﬁciency in compare with conventional solar collectors. Otanicar et al. [10] has studied exper- imentally the effect of nanoﬂuids made from diversity of nanopar- ticles on the efﬁciency of the mciro-solar-thermal-collector. He presented an efﬁciency improvement up to 5% in solar thermal col- lectors by utilizing the nanoﬂuids as the absorption medium. 0894-1777/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermﬂusci.2012.01.025 ⇑ Corresponding author. Tel.: +98 21 88008744. E-mail address: info@tyouseﬁ.com (T. Youseﬁ). Experimental Thermal and Fluid Science 39 (2012) 207–212 Contents lists available at SciVerse ScienceDirect Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs