Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Experimental performance analysis of sensible heat storage in solar air collector with cherry pits/powder under the natural convection Mesut Abuşka a , Seyf Şevik b,* , Arif Kayapunar c a Department of Machine, Akhisar Vocational School, Manisa Celal Bayar University, Akhisar, Turkey b Department of Electric and Energy, Vocational School of Technical Sciences, Hitit University, Çorum, Turkey c Department of Mechanical Engineering, Manisa Celal Bayar University, Manisa, Turkey ARTICLE INFO Keywords: Solar air collector Sensible heat storage Cherry pits/powder Natural convection ABSTRACT It is important to enhance the availability of SACs either in cloudy weather or after sunset for space heating and agricultural drying process. In this context, this study aims to determine experimentally thermal performance of cherry pits as sensible heat storage material under natural convection conditions. In the frst collector (Type I), powdered cherry pits (32 kg) were used as heat storage material and cherry pits (24 kg) were used in the second (Type II). The third collector (Type III) with a fat absorber plate is for comparison. Although the maximum thermal efciencies were found 26.54% and 27.03% for Type I and Type II respectively, average thermal ef- ciency of Type I is higher than Type II. According to the test results, the inlet-outlet temperatures were equalized 46 min for the Type III, 200 min for Type II, and 270 min for Type I collector after sunset. As a result, it can be said that the use of cherry pits and powder as sensible heat storage material is promising. 1. Introduction Solar energy is abundant, time-dependent and environment friendly renewable energy. One of the most popular methods of utilizing solar energy is thermal application. Due to the heat transfer coefcient and the specifc heat of water, the water collectors are superior to the air collectors in terms of thermal efciency and heat storage character- istics. Thus, water heating collectors are common and air heater col- lectors are rare. Global installed capacity distribution of thermal col- lector type in use was spread into solar air heaters: 1.6 GWth - 0.4%, unglazed water heaters: 26 GWth - 6.3%, fat plate: 90.9 GWth - 22.1%, and vacuum tube heaters: 292 GWth – 71.1% [Mauthner et al., 2016]. Air collectors have a very small share of 0.4% in the total and it is obvious that the availability must be increased based on the heat sto- rage perspective. The fact that solar energy depends on time and has disadvantages such as cloudy weather conditions restricts its more ef- fcient use. The processes, such as heating and agricultural drying can be applied during the daytime only. The way to overcome it is to add heat storage capability to solar thermal collectors. In this way, solar energy will be stored in abundance throughout the daytime and will be used properly after sunset and at peak load hours. The natural convection is not much preferred by researchers due to the low convection heat transfer coefcient between the absorber plate and the fowing air through the channel. However, many researchers worked on performance of SACs under natural convection (Hatami and Bahadorinejad, 2008; Pakdaman et al., 2011; Al-Kayiem and Yassen, 2015). Hatami and Bahadorinejad (2008) presented a performance study on vertical fat plate SACs for six cases of air fow. They suggested Nusselt number correlations. The highest thermal efciency was found in SAC with three channels. Pakdaman et al. (2011) investigated the thermal performance of a fat-plate SAC has rectangular fns array under the natural convection. The enhancement about 20% was achieved by increasing the surface area almost 66%. Similarly, heat transfer rate increased when the tilt angle of the SAC is increased. The thermal efciency of designed SAC was found to be 11.4–18% ac- cording to the results of fve days under natural convection. Al-Kayiem and Yassen (2015) carried out an experimental study to predict the free convection heat transfer coefcient and to fnd the optimum angle of SAC. The optimum tilt angle was found to be 50° and thermal efciency was obtained between 13% and 33% under the natural convection. Researches and developments in the literature are generally focused on systems using packed beds that operate according to the principle of transfer of thermal energy and then reversing (Chauhan et al., 1996; Prasad et al., 2009; Agrawal et al., 2018). Mohanraj and Chandrasekar (2009) investigated for copra drying in a forced convection SAC com- bined with gravel mixed with aluminum scraps. Thermal performance of the copra dryer was found as 21%. Aboul-Enein et al. (2000) per- formed the numerical analysis of SAC with and without thermal storage https://doi.org/10.1016/j.solener.2018.09.080 Received 18 June 2018; Received in revised form 21 August 2018; Accepted 28 September 2018 * Corresponding author. E-mail address: seyfsvk@hotmail.com (S. Şevik). Solar Energy xxx (xxxx) xxx–xxx 0038-092X/ © 2018 Elsevier Ltd. All rights reserved. Please cite this article as: ABUSKA, M., Solar Energy, https://doi.org/10.1016/j.solener.2018.09.080