Theoretical modeling of a glass-cooled solar still incorporating PCM and coupled to flat plate solar collector

https://doi.org/10.1016/j.est.2020.101372Get rights and content

Highlights

  • Theoretical modeling of glass cooled solar still (SC) unit having solar collector (SCC) and PCM (SCCP) was performed.

  • The effects of controllable and uncontrollable parameters on the unit efficiency and performance were investigated.

  • The addition of external collector and PCM increased the productivity by a factor of 2.4.

  • The optimum basin water: PCM mass ratio was found to be 2:1.

  • Among sodium acetate trihydrate (SAT), sodium thiosulfate pentahydrate, and paraffin wax PCMs, SAT was found the best.

Abstract

The performance of a modified solar still was investigated considering the following cases: solar still with glass cooling (SC), SC connected to an external collector (SCC), and SCC with phase change material (SCCP). Three different PCMs were used; sodium acetate trihydrate, sodium thiosulfate pentahydrate, and paraffin wax. The effect of various parameters; controllable (hot water circulation rate, PCM mass, and cooling water flowrate) and uncontrollable parameters (solar irradiation, ambient temperature, and wind speed) were investigated. Increasing the solar irradiation from 200 to 700 W/m2 increased the productivity from 0.9 to 3.4, 2.35 to 10, and 3 to 11.9 ml/min for the SC, SCC, and SCCP (with SAT as PCM) systems, respectively. The addition of external collector and PCM increased the productivity 2.4 times. Increasing the coolant mass flow rate from 0 to 10 kg/s, increased the productivity from 1 to 2.14, 1 to 6.65, and 1 to 7.5 ml/min for the SC, SCC, and SCCP systems, respectively. Additionally, increasing the hot water circulation rate of external collector from 0 to 0.1 kg/s increased the productivity from 2.4 to 6 and from 4 to 7.4 ml/min for SCC and SCCP system, respectively. The optimum mass ratio of basin water: PCM was found to be 2:1, while the best PCM type was found to be the one with the highest melting point and latent heat of fusion. The 24-h operation for a typical May month weather data in Jordan, revealed that the productivity improved 1.8 times for SCC and 2.3 times for SCCP-SAT compared with SC system.

Introduction

Water is a critical and pivotal component for living existence and living beings on earth. From all water available only 2.6% is considered freshwater. This small amount of freshwater is convenient to support life and vegetation on earth. Universally, the demand for freshwater is rapidly increasing, which present challenges in the coming centuries [1]. Water-borne diseases come from the use of contaminated water. Clean water availability on earth is in continuous decline due to water contamination caused by the manufacturing processes and industrial enterprises [2]. It is expected to experience about 60% water shortage in the year 2025, because of the world population rise [3]. According to World Health Organization [4], the permissible limit of salinity in water is 500 ppm and for particular cases up to 1000 ppm, while water available on earth has salinity up to 10,000 ppm. Seawater normally has salinity in the range of 35,000–45,000 ppm in the form of total dissolved salts. To obtain fresh water from saline water present on the earth, thermal or membrane desalination processes have been applied economically and easily. In particular, solar desalination has received special attention since it utilizes a renewable energy source. The conventional solar still system has been improved through the years. The improvements aimed to increase its productivity and efficiency. The improvements could be categorized into three fields: (a) improvements by changing the solar still geometry such as double-slope solar still [5], [6], [7], [8], multi-basin solar still [9], stepped solar still [10], [11], [12], [13], hemispherical solar still [14], and pyramid solar still [15], [16], [17], (b) improvements by enhancing the productivity with additional solar unit(s) connected to the still such as solar still coupled with flat-plate collector [18], [19], [20], [21], parabolic trough collector [22], solar dish concentrator improved [23], and/or with separate condenser [24, 25], (c) improvements by adding energy storage materials such as phase change material (PCM) [26] and nano-particles [27].

The improvement of conventional solar still by coupling with solar collector has been studied by many researchers. Badran and Al-Tahainesh [18] presented the effect of coupling a flat plate solar collector on the solar still productivity and found that the productivity increased by 36%; The highest output was for the least water depth in the basin (2 cm). Rajesh et al. [19] carried out work on single basin solar still connected to a conventional flat plate collector to study the effect of the solar still. The unit was operated with and without coupling flat plate collector. The production was noted between 8 a.m. to 5 p.m. sunlight through 25 days and found 40% enhancement in the yield observed with this system. Arslan et al. [20] performed experiments on various solar stills such as circular box, rectangular box and single tube solar stills coupled with a solar collector. It was inferred that circular box solar still gives a better efficiency compared to a single tube or rectangular box due to the lower heat losses from the circular box due to the reduced heat transfer surface area. Rajaseenivasan et al. [21] integrated flat plate collector with modified solar still having black gravel and jute cloth to enhance the evaporation rate and heat capacity of the still which increases the distillate yield of about 60%.

The improvement of the solar still by using PCM has been also studied. El-Sebaii et al. [26] studied the thermal performance of single slope basin solar still with and without stearic acid as a PCM, on typical summer and winter days in Jeddah. A mathematical model for the proposed still was formulated to investigate the performance of the still with a different PCM masses. A productivity of 9.0 kg/m2.day with an efficiency of 85.3% was obtained compared to 5.0 kg/m2.day when the still was operated without the PCM on a summer day. The used PCM was found more effective at lower masses of basin water on winter season. It was found that the PCM increased the evaporative heat transfer by 27% and the convective heat transfer coefficient was doubled. The same results were obtained by Dashtban et al. [28]. Radhwan [29] showed that using PCM beneath the basin enhanced both the productivity and efficiency of the still. The basin was placed on a slab filled with a layer of paraffin wax as PCM that acts as a latent heat thermal energy storage system (LHTESS).

Another approach of increasing the solar still productivity performed by enhancing the condensation process. This can be done by the cooling of the glass cover of the solar still or by integrating a condenser. Mousa and Abu-Arabi [24] designed and tested basin solar still enhanced by an external solar collector and double glass cover with cooling. The results showed that the production rate is proportional to the solar irradiation, ambient temperature, and cooling water flow rate. The productivity obtained was about 4 l/m2.day during a typical summer day in the northern part of Jordan. Al-Nimr et al. [25] studied the performance of a solar still integrated with a finned-condenser with PV module submerged in the basin of the still. The addition of the condenser improved the still efficiency two times higher than the conventional solar still.

From the introduced review, it can be noted that the proposed improvements either increase the supplied heat without enhancing the condensation process or enhance the condensation without increasing in the supplied heat. Other researches enhance both supplied heat (with solar collector and PCM) and condensation process but the PCM tubes aren't directly submerged in the basin of the still [30]. So, this work aimed to explore the performance of the solar still with enhancing the supplied heat by a flat-plate solar collector and enhancing the condensation process by cooling the inner glass cover of the still. In addition, the solar still system involves PCM directly immersed in the basin of the still.

This paper reports improvements on primary theoretical study in the performance of the solar still system proposed by Abu-Arabi, Al-harahsheh [31]. The major differnces between the present work and the earlier work are: (a) the PCM is directly immerged in the basin of the still, (b) different types of PCM with different masses have been studied, and (c) larger solar collector is used. In this paper, Section 2 present a detailed description of the system. Section 3 present a steady-state thermodynamic model of the system. Finally, Section 4 discusses the obtained results from the simulation processes by the Engineering Equation Solver (EES) program.

Section snippets

System description

The studied system consists of a conventional solar still coupled with a flat plate solar collector, cooling unit of the glass cover, and with PCM tubes immersed in the basin of the still (Fig. 1). The system includes two pumps; one to circulate the heating water (a variable speed pump of 28, 44 and 63 W) and one to circulate the cooling water (rating of 250 W). The cooling unit and the pumps were powered by using PV panels (4 × 325 kW). The function of the solar collector is to heat a

Mathematical model

The following assumptions are made before applying energy balance for each component of the system:

  • 1

    The system operates under steady state conditions since the time period considered in the simulation is one minute.

  • 2

    The sides and the bottom of the solar basin are well insulated except the outer glass cover.

  • 3

    Water mass in the still basin is constant.

  • 4

    Water temperature in the basin is uniform.

  • 5

    Lumped system for the PCM since the Biot number < 1, therefore, the PCM temperature is assumed to be uniform.

Results and discussion

This work aims to explain how both the uncontrollable and controllable parameters affect the productivity of the system in a theoretical steady-state approach. Eqs. (1)–(52) presented in the theoretical model have been solved using Engineering Equation Solver (EES). The concept of the simulation process is presented in Figure S2. The effect of each parameter was simulated using EES for three different structures namely:

  • (1)

    Solar still with inner glass surface cooling (SC)

  • (2)

    Solar still connected with

Conclusions

Three modifications on a solar still were investigated theoretically in this study, which are: solar still with glass cooling (SC), SC plus connected to an external collector (SCC), and SCC plus PCM in steel pipes immersed in water basin (SCCP). The effect of some controllable parameters such as the amount and type of PCM, hot water circulation rate between the solar still and the external collector, and cooling water flow rate on the performance of the units were investigated. The effect of

CRediT authorship contribution statement

Mousa Abu-Arabi: Conceptualization, Methodology, Writing - original draft, Writing - review & editing. Mohammad Al-harahsheh: Conceptualization, Investigation, Resources, Data curation, Writing - original draft, Writing - review & editing, Supervision, Project administration, Funding acquisition. Maysam Ahmad: Methodology, Software, Investigation, Writing - original draft. Hasan Mousa: Methodology, 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

The authors would like to thank Jordan University of Science and Technology for funding this work (Project No 86/2018).

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