Enhancing tubular solar still productivity using composite aluminum/copper/sand sensible energy storage tubes

https://doi.org/10.1016/j.solmat.2020.110882Get rights and content

Highlights

  • Novel composite sensible heat storage tubes are proposed for tubular solar stills.

  • Proposed device enhances freshwater productivity and efficiency by 24.1% and 20.1%.

  • Proposed device was able to produce 4.9 L/m2day of freshwater with $0.0087 CPL.

Abstract

Water scarcity is a big challenge especially for developing countries and desert areas like Saudi Arabia. Using solar energy for freshwater production is the way forward for future development of the water industry. The present study developed a new composite sensible heat storage tubes (CSHSTs) to improve the freshwater productivity of a tubular solar still activated by a parabolic concentrator solar tracking system. 12-CSHSTs were inserted inside the trough of the tubular solar still forming heterogeneous cavities for the saline water. Each CSHST consists of three main solid components (black painted aluminum tube, copper wire and silica sand). The experimental work was conducted under the climatic conditions of Ha'il city in Saudi Arabia. Results showed enhancements by 24.05% and 20.06% in the freshwater yield and thermal daily efficiency, respectively. The developed device produced 4.9 L/m2day with 13% lower freshwater production cost of $0.0087 per liter. Inserting the 12-CSHSTs inside the trough with the heterogeneous cavities increased the contact area with the saline water. This novel configuration enhanced significantly the heat transfer mechanism. The developed device showed higher performance compared to the previous studies in the literature.

Introduction

Solar still is a simple and common method for solar water desalination. Basin type solar still is the common and the oldest solar still type. It has simple design as a rectangular basin covered with a tilted glass plate. The basin is made from any black painted low cost material and sealed with wood or fiberglass. The conventional basin solar still has low productivity. However, with its development conducted over long time journey, the productivity of the basin solar still has been significantly improved. Many research efforts have been conducted for long time with this type leading to important achievements. In addition, some important researches introduced various techniques and geometries for the developed basin type solar still have been recently published. Many techniques were used to increase the productivity of the basin solar still such as wick with corrugated absorber and nanofluids [1,2], trays and trays with reflectors and nano-PCM [3,4], TiO2 Nano layer [5], rotating drum and rotating discs [6,7], vertical and inclined fins [8]. Recent theoretical and numerical modeling of the basin solar still were conducted [9,10]. On the other hand, water can be also extracted from humid air using solar stills, which works well in high humidity regions and hard with desert places (low humidity and productivity) [[11], [12], [13], [14]]. However, the productivity of water extraction from air is considerably lower than that from the conventional solar stills working with saline water resources. Although, the tubular solar still (TSS) is more efficient than the basin type solar still (SS), the development of the TSS goes slower than the basin type SS. In all cases, the cost per liter (CPL) of the freshwater production is an important evaluating parameter measuring the effectiveness of the solar still performance. The CPL of TSS ranges between $0.0046 and $0.0212 [15,16]. TSS can be directly activated using parabolic concentrator with solar tracking (PCST) [17] or compound parabolic concentrator (CPC) [18]. The TSS passed through many development studies in terms of its geometry, performance and cost analysis. In 2013, Fukuhara and Ahsan [19] proposed a lightweight and low cost TSS device. Their results showed high freshwater productivity (6.2 L/m2day). In 2016, Arunkumar et al. [16] studied the performance of the CPC-TSS. Their results showed an increase in the freshwater productivity by 33.7% compared to the TSS without CPC. Also in 2016, Arunkumar and Kabeel [17] tested the effect of using paraffin wax as a phase change material (PCM) inside the trough of the CPC-TSS. A half-cylindrical jacket was formed inside the trough of the CPC-TSS and filled with 0.45 kg of paraffin wax. Using PCM with CPC-TSS enhances its yield by 8.4%. In 2017, Elashmawy [18] developed and tested a parabolic concentrator solar tracking system PCST augmented with the TSS (PCST-TSS). His results showed a great increase in the water productivity due to the efficient concentration of the sunrays. Despite of the higher loss of the heat associated with increasing heat supply to the TSS, the productivity was increased by 676% for the same tube length. In addition, the cost added due to the PCST was compensated by the larger yield produced. The CPL of the PCST-TSS was reduced by 45% compared with TSS alone. In 2019, Elashmawy [20] experimentally studied the effect of tube thickness on the PCST-TSS performance. His results showed an increase in the efficiency and the yield by 13.35% and 21%, respectively, when tube thickness reduced from 5 mm to 3 mm. Moreover, reducing tube thickness enhanced the CPL by 37.5% due to the lower cost of the 3 mm tube and higher device productivity. Also in 2019, Kabeel et al. [21] experimentally studied the effect of TSS tube surface cooling and saline water depth on the TSS productivity. In their work, 2 L/h and 0.005 m were determined as optimum water-cooling flow rate and saline water depth. They reported that the TSS yield and efficiency were 5.85 L/m2day and 55%, respectively, at the optimum conditions. In 2020, Elashmawy [22] used 3 kg of gravel (locally available) inside the trough as a sensible heat storage material. Gravel is widely available and almost has no additional cost to the PCST-TSS. He reported that using gravel enables PCST-TSS to produce more freshwater (14.2%) with higher thermal daily efficiency (13.9%). Moreover, the CPL is reduced by 12% reaching a very competitive level ($0.0088). El-Said et al. [23] used a vibrating steel screen system to enhance heat transfer conditions by eliminating water surface tension. Their results showed an enhancement of the device performance by 34%. Kabeel et al. [24] proposed new trough geometry of two concentric cylinders to improve CPC-TSS performance. They reported that the effect of changing the gap between the two trough cylinders was evaluated. Maximum productivity and efficiency were achieved at 20 mm gap (saline water layer) recording an enhancement of 91% and 61% respectively. Kabeel et al. [25] developed the device of the two trough cylinders by filling the inner half cylinder with paraffin wax (PCM) and nano-graphite forming hybrid storage materials. Their results showed an overall enhancement of the CPC-TSS performance. They reported that the productivity and efficiency of the proposed device are 9.7 L/m2 and 45% respectively compared with 5.9 L/m2 and 38% for the conventional device. Kabeel et al. [26] proposed a composite heat storage material of a nano-graphene oxide particles mixed with paraffin wax (NPCM). In their study, a rectangular trough was used. They reported that using NPCM increased water evaporation rate by 41.3% and daily efficiency by 116.5%. Sharshir et al. [27] introduced a useful mini review of the various techniques used to improve the performance of the TSS. Recently, Kabeel et al. [28] introduced a comprehensive review of the TSS design developments with performance and economic evaluation.

Development of TSS with sensible heat storage materials (SHSM) is going very slowly with little number of publications. Reviewing of some related studies of the conventional basin solar still with SHSMs may help for showing a clear picture about the benefits of using SHSM. In 1989, Akash et al. [29] enhanced the performance of the basin type solar still of double slope by using different SHSMs. They reported that the black dye showed the best performance compared to black ink and rubber. Also the black dye was able to enhance SS yield and efficiency by 60%. In 2001, Nafey et al. [30] tested single basin SS with gravel and black rubber. Their results showed water productivity enhancement by 20% with gravel and black rubber. In 2002, Naim et al. [31] developed a charcoal wick material to enhance solar still performance. They reported that using the developed wick increased the productivity by 15%. In 2008, Sakthivel and Shanmugasundaram [32] proposed a black granite gravel to enhance the performance of the basin type SS. They reported a yield enhancement of 16.4%. In 2010, Murugavel et al. [33] experimentally studied the effect of using different SHSMs on the performance of the basin SS with double-slop glass covers. They reported that the quartzite rock with 5 mm saline water depth produced the maximum freshwater yield (2.09 L/m2day). In 2013, Priya and Mahadi [34] compared the productivity of the solar still with black dye and ink. They reported an increase of the freshwater productivity by 65% and 35% respectively. Shanmugan [35] compared the performance of basin SS supported by three different SHSMs. He reported that SS with concrete stones produced 5.4 L/m2day freshwater and 5.82 L/m2day hot water for heating systems. Also concrete stone showed 17% higher efficiency compared with the two other SHSMs (pebbles and granite). In 2014, Arjunan et al. [36] experimentally studied the performance of SS with gravel, pebbles, blue metal stone and paraffin wax. They reported that black granite gravel showed the highest performance among the others with 9.7% and 19% enhancement for freshwater yield and efficiency respectively. In 2016, Samuel et al. [37] used sponge materials as low cost SHSMs to enhance the performance of the basin SS. Their results showed 22.73% enhancement of the freshwater productivity. In 2017, Deshmukh and Thombre [38] investigated the performance of the basin SS when operated under different depths of SHSMs (sand with oil). Their results showed 6% enhancement of freshwater productivity with lower depths of the SHSMs. In 2018, Kabeel et al. [39] tested jute cloth knitted with sand to enhance the performance of the basin SS. Their results showed 18% increase of freshwater productivity reaching 5.9 L/m2 of water yield. Kabeel et al. [40] proposed and tested graphite as a SHSM to enhance the performance of the basin SS. Their results showed 77.47% freshwater yield enhancement with relatively high thermal efficiency (60.22%). In 2019, Sahoo and Subudhi [41] used jute cloth as SHSM inside a basin SS with reflectors. Their results showed 72.18% and 41.51% enhancements of freshwater yield and efficiency, respectively. Nasri et al. [49] proposed low cost SHSMs with basin SS. They reported that gravel gives higher freshwater yield (5 L/m2day) than black polyethylene (4.48 L/m2day) and sand (3.84 L/m2day). Recently, Saravanan et al. [50] investigated the effect of using kanchey marbles as a SHSM on the performance of basin SS with double slope glass cover. Their results showed 16.32% higher efficiency compared to the conventional SS. The literature survey of the SS with SHSMs showed that, using SHSMs with basin type solar stills has positive effect on the device performance. SHSMs enhanced basin SS productivity and efficiency (as maximum achieved results) by 77.47% [40] and 41.51% [41] respectively. Comparing with TSS, the maximum achieved enhancements of yield and efficiency were 14.2% and 13.9% respectively. The lower achievement of using SHSMs with TSS compared to basin SS is due to the tight little space available inside the TSS trough compared with the large space inside the basin of the conventional SS. This geometry constraint leads to lower amount of SHSMs inside the TSS. However, the water production cost of TSS with SHSMs is lower, compared with the conventional SS.

Thus the present study developed a novel composite sensible heat storage tubes (12-CSHSTs) with heterogeneous configuration inside the saline water of the TSS trough. This new concept is expected to increase the contact area with the saline water leading to a significant enhancement of heat transfer conditions. This configuration has not been tested for the TSS before. The developed 12-CSHSTs is expected to improve the TSS performance with lower CPL due to low cost materials.

Section snippets

Theoretical analysis

Theoretical analysis is conducted under some assumed conditions as follows:

  • -

    Heat capacities of some materials such as saline water and transparent tube are ignored.

  • -

    Temperature distributions of the saline water and tube material are assumed homogeneous.

  • -Device has no leakage.

Setup design and configuration

Fig. 2 shows the schematic illustration diagram the proposed device with the twelve incorporated composite sensible heat storage tubes (12-CSHSTs). And Fig. 3 shows a photograph of the test-rig with the PCST. Each CSHST consists of a cylindrical aluminum tube filled with sand and copper wire as shown in Fig. 2. Table 1 shows the characteristics of the different materials used in the proposed CSHSTs. The 12-CSHSTs were placed inside the half-cylindrical trough allowing saline water to get in

Experimental results

Experimental work was conducted outdoor on the real life field in June 29th, 2020, Ha'il city, Saudi Arabia. The experiment started at sunrise (5:30 a.m.) and ended slightly before sunset (7 p.m.) where no yield was observed. The trough of the PCST-TSS was filled with the 12-CSHSTs stacked as shown in Fig. 2, Fig. 3. Only one liter of saline water was used to fill the spaces between the12-CSHSTs to avoid saline water spillage outside the trough. The orientation of the PCST with respect to the

Conclusions and recommendations

The present study examined a new heat storage composite system concept to enhance the performance of the PCST-TSS. It consists of an aluminum tubes filled with a copper wire at the center and filled with sand that termed as a composite sensible heat storage tubes (CSHSTs). After using 12 of the CSHSTs within the PCST-TSS the following conclusions can be given:

  • Using the CSHSTs within the PCST-TSS increases the freshwater yield and thermal efficiency by 24.05% and of 20.06%, respectively,

CRediT authorship contribution statement

Mohamed Elashmawy: Project administration, Funding acquisition, Supervision, Methodology, Investigation, Conceptualization, Writing - original draft, preparation. Mohamed M.Z. Ahmed: Conceptualization, Writing - original draft, Data curation, Resources, Investigation, Visualization, Writing - review & editing.

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

This research has been funded by Scientific Research Deanship at University of Ha'il - Saudi Arabia through project number RG - 0191324.

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