Elsevier

Solar Energy

Volume 207, 1 September 2020, Pages 10-21
Solar Energy

Experimental investigation of the effect of using water and ethanol as working fluid on the performance of pyramid-shaped solar still integrated with heat pipe solar collector

https://doi.org/10.1016/j.solener.2020.06.032Get rights and content

Highlights

  • Investigated the effect of using heat pipe solar collector integrated with pyramid-shaped solar still.

  • Conducted the effect of using water and ethanol as working fluid.

  • An optimal filling ratio was 40%.

  • The maximum hourly and accumulated yield were 1235 and 6970 ml/m2, respectively.

  • The cost of produced fresh water was 0.0137 $/l.

Abstract

In this paper, a modified pyramid-shaped solar still (MPSS) including a conventional pyramid-shaped solar still (CPSS) integrated with a heat pipe solar collector (HPSC) was fabricated and experimentally assessed in the outdoor conditions of Mashhad, Iran (latitude of 36°18', longitude of 59°34'). Two different working fluids (water and ethanol) were used at three different filling ratios (FRs). Results showed that using the HPSC increased the temperature difference between the glass cover and the basin water. Therefore, using the HPSC augmented the hourly and accumulated yield. The accumulated yield was escalated to 6970 and 3300 ml/m2 for the MPSS (with water as the working fluid at the FR of 40%) and the CPSS, respectively. Moreover, the HPSC increased exergy, although the instantaneous efficiency was rather invariant. Based on the experimental data the values of the total internal heat transfer coefficient have been calculated. Furthermore, the best performance of the MPSS was obtained with water as the working fluid at the FR of 40% whereas ethanol at the FR of 40% also showed a very good performance. Finally, the economic analysis revealed that the MPSS was economically reasonable and an estimated cost for 1 L of distillate was 0.0137 $ for the proposed MPSS.

Introduction

Although potable water is vital to the survival of all living creatures, its availability has been dramatically reduced due to population growth, poor management, increasing pollutions. Water purification is one of the approaches of compensating for water shortages whereby various methods, designs, and technologies have been proposed (Tiwari and Sahota, 2017). For instance, solar desalination (generating pure water using solar energy) is categorized as one of the simplest and most cost-effective emerging methods. Therefore, various types of passive and active solar distillation systems have been designed so far (Singh et al., 2019) where there has been a diverse range of configurations and designs for basin-type solar stills (Durkaieswaran and Murugavel, 2015). Despite the highly diverse designs, basin-type solar stills are still faced with many challenges and constraints. Among the different proposed designs, pyramid-shaped solar stills (PSS) are one of the best designs to lift the restrictions on conventional basin-type solar stills due to the independence from positioning their inclined surface to the direct sun radiation (Dwivedi and Tiwari, 2009), reduction in the effect of shading of sidewall on water surface (Sathyamurthy et al., 2014), larger condensing area (Yadav and Sudhakar, 2015), and lower cost of distilled water obtained (Kabeel et al., 2010). In order to improve the performance of PSSs, investigators have focused on water depth, glass cover angle and using jute wick. Some of these studies can be summed up as follow.

Hamdan et al. (Hamdan et al., 1999) made a theoretical and experimental comparison of the performance of three different designs of conventional pyramid-shaped solar still (CPSS), namely single basin, double basin, and triple basin, concluding that the maximum daily efficiency of 44% was observed for the triple basin design. Al-Hinai et al. (Al-Hinai et al., 2002) predicted the productivity of a CPSS in different climates using a mathematical model. Based on their results, one can reach an average annual solar still yield of 4.15 kg/(m2day) by optimal selection of water depth, cover tilt angle, and insulation thickness. Fath et al. (Fath et al., 2003) compared a CPSS and a single-slope solar still by conducting an analytical study. According to their yearly performance results, the single-slope still was slightly more efficient and economical than the CPSS. Furthermore, Eze and Ojike (Eze and Ojike, 2012) carried out a comparative evaluation of two basin-type passive solar still, including rectangular-shaped and pyramid-shaped type. They reported that the rectangular-shaped solar still offered an average efficiency of 36.8% while the pyramid-shaped type yielded an efficiency of 28.9%. Hence, the rectangular-shaped solar still outperformed the pyramid-shaped type in terms of being used for people living in coastal areas in particular. However, contrary to the already-mentioned studies, Arunkumar et al. (Arunkumar et al., 2012) compared the performance of seven solar still designs, including a CPSS and a double basin solar still, and reported that the value of distillate output from the CPSS was higher than that of the double basin solar still. Moreover, Wassouf et al. (Wassouf et al., 2011) designed, constructed, and tested a two prototype solar still, namely a 0.2 m2 poly vinyl chloride (PVC) pyramid-shaped still and a 0.6 m2 triangular-prism PVC solar still, to reduce the high capital cost associated with the still. The average cost of water over an estimated useful life of 4 years was 0.046 and 0.063 $/l for the pyramid-shaped and triangular prism stills, respectively. Kabeel (Kabeel, 2009) reported that using jute wick in a concave-shaped basin increased the daily productivity of CPSS since jute wick heightened the amount of absorbed solar radiation and enhanced the evaporation surface area due to the capillary effect. Mahian and Kianifar (Mahian and Kianifar, 2011) and Kianifar et al. (Kianifar et al., 2012) (using a mathematical model and experimental tests, respectively) reported that the use of a low-cost fan with negligible power consumption inside a CPSS is an effective and economical way of enhancing the evaporation rate and thus freshwater production. Moreover, Taamneh and Taamneh (Taamneh and Taamneh, 2012) experimentally studied the effect of forced convection on the performance of a CPSS. They showed that approximately 25% increase in freshwater production for a forced convective system (with fan) in compare to free convection. Kalaivani and Radhakrishnan (Kalaivani and Radhakrishnan, 2013) built a CPSS to investigate its internal and external heat transfer coefficients and thermophysical properties. They revealed that water temperature plays a key role in determining the amount of the distillate output. The distillate water productivity of their still was around 2.8 L per day. Al-hassan and Algarni (Al-hassan and Algarni, 2013) fabricated three CPSSs to assess the effect of water depth on the performance of solar stills. Based on their experimental results, lower amounts of saline water lead to more enhanced performance and, in turn, a higher yield of solar still. Sathyamurthy et al. (Sathyamurthy et al., 2014) examined the effective factors in the performance of a CPSS and concluded that water temperature, the temperature difference between water and inner glass cover, and convective and evaporative heat transfer coefficients decrease during the sunshine hours enlarging in water depth. Furthermore, based on their results, the production rate of distillate output is heightened at higher values of wind speed. Sathyamurthy et al. (Sathyamurthy et al., 2014) reported that the use of paraffin wax as phase change materials (PCM) —acting as a latent heat storage subsystem— increases the distillate output of a triangular pyramid-shaped solar still by 20%. Prakash et al. (Prakash et al., 2016) designed and built a CPSS wick-type solar still and compared the solar stills with/without wick material in the basin as the evaporating surface, concluding that the use of wick material increases the distillate yield by 18%. Kabeel et al. (Kabeel et al., 2016) investigated the effects of glass cover angle on the performance of a CPSS. Their experimental results revealed that the accumulated distillate water productivity offered by the CPSS decreases at higher glass cover angles above the latitude angle. Moreover, the maximum accumulated distillate water productivity of the CPSS is obtained when the glass cover angle equates to the latitude angle.

Solar collectors have been used as an excellent method to optimize and enhance the performance of conventional solar stills. Tiwari and Sahota (Tiwari and Sahota, 2017) provided a useful classification of different varieties of solar collectors that can be integrated with a solar still.

Flat plate solar collectors (FPSCs) are designed to collect solar energy and transform it into thermal energy by using water as a working fluid (Tiwari et al., 2016). Lawrence and Tiwari (Lawrence and Tiwari, 1990) reported that enhancing productivity was achieved by coupling the single basin solar still to an FPSC through a parallel tube heat exchanger placed inside the water in the basin. Rai and Tiwari (Rai and Tiwari, 1983) evaluated the performance of single basin solar still coupled to an FPSC. The results confirmed that the use of the FPSC causes significant enhancement in average daily production (up to 24%). Tripathi and Tiwari (Tripathi and Tiwari, 2006) have obtained similar results by a still coupled with two FPSCs of 4 m2 effective area. Also, they investigated the water deepness effect on the convective heat transfer coefficient. Tiwari et al. (Tiwari et al., 2009) performed a study on solar still integrated with an FPSC. The water production of the active solar still has been found to be about 3.08 L, which is greater than that of the passive solar still (1.14 L).

Evacuated tubular solar collectors (ETSCs) were introduced to collect solar energy for thermal applications. They can be used for indirect space heating and crop-drying through a heat exchanger, and the pre-heating of working fluid for lower-capacity power generation (Tiwari et al., 2016).

Using double slope solar still integrated with N identical ETSCs, Singh and Tiwari (Singh and Tiwari, 2017) analyzed variation in annual production at different depths of water and mass flow rate. They have concluded that the value of annual energy is higher by 6.85% for double slope solar still integrated with N identical ETSCs than similar single slope set up. Kumar et al. (Kumar et al., in press) also performed an analysis on a single slope solar desalination unit coupled with N identical ETSCs for increasing the average daily energy output.

Heat pipe solar collectors (HPSCs) have been able to significantly overcome the restrictions of FPSCs and ETSCs and, in turn, have cut the attention of many researchers in various applications including water heating (Esen and Esen, 2005, Porras-Prieto et al., 2014), solar cooking (Esen, 2004), space heating (Albanese et al., 2012, Robinson et al., 2013), and power generation (Manikandan and Kaushik, 2016). HPSCs comprise two major parts, namely heat pipe and glass evacuated tube. A glass evacuated tube is a sealed tube made of glass and includes two inner and outer tubes. The inner tube acts as the absorbing surface while the outer tube plays the role of transparent glass to facilitate the solar radiation transfer. The gap between the two glass surfaces has been insulated to prevent or minimize heat loss. Also, when several glass surfaces are positioned in a parallel arrangement, an ETSC is formed indeed.

A limited number of studies have been addressed the effect of HPSCs on the enhancement of the efficiency of solar energy-based desalination systems (Mamouri et al., 2014, Mosleh et al., 2015, Alwaer and Gryzagoridis, Tanaka et al., 2005, Chong et al., 2014, Huang et al., 2015, Behnam and Shafii, 2016). For instance, Mamouri et al. (Mamouri et al., 2014) investigated the performance of HPSC in conventional basin solar still. The performance of their proposed stiller was better than that of other conventional basin-type solar stills by far, the value of freshwater production rate was about 1 kg/(m2h).

As seen, there are many studies in the literature on the theoretical and experimental investigation of the performance of PSSs and there is just one study on using HPSC in basin-type solar still. Although working fluid and filling ratio are two effective parameters in the performance of heat pipes (Ersöz, 2016, Kabeel et al., 2017, Jahanbakhsh et al., 2015), no previous study has investigated the effect of two former factors on the efficiency of HPSC-integrated solar systems. Hence, this study aims to examine the effect of working fluid and filling ratio (FR) on the performance of conventional basin-type solar stills. In this respect, a modified pyramid-shaped solar still (MPSS) including a CPSS integrated with a HPSC (consisting of 5 thermosyphon heat pipes and 5 glass evacuated tubes) has been built. The temperature variations of the glass cover and the basin water, the hourly and accumulated yield, and the exergy and instantaneous efficiency have been assessed for two different working fluids (water and ethanol) at different FRs (30%, 40%, and 50%).

Section snippets

Test site meteorological environment

The CPSS and MPSS were tested under Mashhad in a typical summer month. Mashhad is located in Iran (latitude of 36°18', longitude of 59°34') 999 m above the sea level. The city has a cold semi-arid climate. The probability of a sunny day in March to early July is 90 percent. The longest period of heat stress occurs at 15:30 which begins in May, lasting to September for about 120 days (Esmaili and Ghalhari, 2014). Moreover, the average solar irradiation in Mashhad in June and July is about

Comparison between the CPSS and MPSS

Fig. 5 depicts the temperature variations of the basin water and glass cover on the MPSS and CPSS. As can be seen, the temperature of the basin water has been always higher than that of the glass cover for both the MPSS and CPSS (with the maximum value of 59.2 and 71.1 °C for the CPSS and MPSS, respectively). This is because the glass cover had heat transfer with the surrounding environment. The temperature difference between the glass cover and the basin water is one of the most central

Conclusions

To date conventional solar stills have not been embraced due to low yields. This study sought to address developing a new design still at affordable cost for saline water desalination in rural and coastal areas with more yield compared to the conventional stills. Therefore, a pyramid-shaped solar still with an effective area of 0.25 m2 coupled to a HPSC, including five thermosyphon heat pipes and five glass evacuated tubes, was studied experimentally. Moreover, the performance of the still was

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors would like to thank Eng. Pejman Nabinia for technical supporting this project. Eng. Alireza Esmaeeli is very much appreciated for his financial support for doing research work.

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