Theoretical investigation of solar desalination with solar still having phase change material and connected to a solar collector

Highlights

• A model for a solar still with PCM and connected to solar collector is developed.

• The model and experimental results are in good agreement.

• Sodium thiosulfate pentahydrate works well as PCM to supply energy at night time.

• Incorporating large PCM mass in the system reduces the productivity.

• Unit insulation and glass cover cooling increase productivity.

Abstract

A theoretical model is developed in this work to simulate a solar still connected to an external solar collector and incorporating Sodium Thiosulfate Pentahydrate as phase change material (PCM). Excellent agreement is obtained between the theoretical and experimental results for the unit productivity of fresh water and the basin water temperature profile. Sodium thiosulfate pentahydrate worked well as PCM to supply energy during night time for continuous water production and has improved the overall productivity. Incorporating large PCM mass in the system reduced the productivity; increasing the ratio of PCM mass to water mass from 10 to 100% reduced the productivity by up to 30%. While, incorporating large PCM mass in the system has kept the basin water temperature higher for longer time. This is useful if the unit is to be used for supplying potable water and hot water mainly after sunset and for heating purposes especially in cold places. Reducing the overall heat transfer coefficient from 10.4 to 2.6 W/m²·K can increase the productivity by >100%. The cooling water through the glass cover can have significant effect (37% increase) on the unit productivity of fresh water if its flow rate is increased from 0.01 to 0.1 kg/s.

Introduction

Water demand is increasing worldwide, while the available potable water is diminishing due to the water sources becoming saline and/or polluted or the only available source is saline water (i.e. seawater). Nowadays, countries that were deemed to have sufficient water are forced to treat some if not most of their water supplies due to these water problems. All water treatment methods require energy either as thermal and/or electrical. In remote areas where connection to the grid is normally not available, providing potable water from salty or polluted water becomes a challenge. However, most remote areas located near the sea and lack fresh water have abundant solar irradiation, which can be utilized to supply energy to solar desalination units.

Solar distillation is one of the most important water purification processes with solar still being the most common unit used in small capacity due to its low price and easy utilization. Several types of solar stills are used such as pit, cone, and domo solar stills; however, the basin still is the most common type. Basin stills have many different variations, but the two main categories of basin stills are having single or double sloped cover [1]. Solar stills can produce several liters of water per square meter of still area per day, with a reasonable capital cost investment. The average efficiency of the solar stills ranges between 30 and 60% with a long life time [2]. The use of solar still is not so popular due to its low productivity. Many works have been undertaken to improve the productivity of these stills. The productivity is mainly affected by the solar intensity [3], the ambient temperature, the glass cover material and its thickness [4], the velocity of wind [5,6] and the depth of water in the basin [7]. The recent work by Selvaraj and Natarajan [8] and Sharshir et al. [9] summarized the factors influencing the performance of solar sill. Under optimized operating, Rajamanickam and Ragupathy [10] obtained a maximum productivity of 3.1 L/m²·day using double slope solar still, while Abu-Arab et al. [11] reported a productivity of a single basin solar still to be <5 L/m²·day. This low efficiency is mainly due to the complete loss of water latent heat of condensation on the glass cover. To improve the performance of conventional solar stills, several improvements have been made, such as attaching solar collector [12] and the use of phase change material (PCM) [13,14] to store the solar energy during the day time and release the stored energy during night, hence keeping the water warm for longer hours.

Kalbasi et al. [15] modeled single and double effect solar stills and validated the modeling results experimentally. It was found that the separation of condensing surface and solar energy receiving surface will increase the daily productivity by 94% compared to conventional one. They also demonstrated that water depth in the basin has negative effect on the productivity, while increasing the temperature difference between the condensing surface and water in the basin will increase the productivity.

Rai and Tiwari [16] studied the performance of a solar still coupled with a flat plate solar energy collector. The study indicated that, the daily distillate production of a coupled single basin solar still is 24% higher than that of uncoupled still. Sathyamurthy et al. [17], and Badran and Al-Tahaineh [18] also studied the effect of coupling a solar collector to the solar still and found similar enhancement in the unit productivity. Other designs have been developed that use PCM to store the solar energy during the day time and to keep the solar stills running for 24 h. Storage system could be either sensible or latent heat. The latent heat energy storage systems have many advantages over the sensible systems including a large energy storage capacity per unit volume and almost constant temperature for charging and discharging [19]. Several research works have discussed the use of PCM as energy storage system for solar stills [5,14,[20], [21], [22], [23], [24], [25], [26]].

Sharshir et al. [9] incorporated paraffin wax as PCM to a weir-type cascade solar still and obtained an improvement of 31% on the daily productivity compared to that without PCM. Shalaby et al. [27] also used paraffin wax as PCM with a v-corrugated absorber single-basin still enhanced and obtained an enhancement of 12% on the productivity. Mousa and Gujarathi [14] theoretically analyzed the productivity of desalination units enhanced by PCM. It was found that the presence of PCM (melting point of 40 °C) have resulted in higher basin temperature after sunrise. They also concluded that choosing a PCM with higher melting point (40 to 50 °C) have led to higher productivity.

The previous work by Mousa and Abu Arabi [12] showed that basin solar still enhanced by an external solar collector cover with cooled glass cover improved the unit productivity. Further improvement of the same unit was carried out by Al-harahsheh et al. [13] who incorporated Sodium Thiosalfate Pentahydrate as PCM. A compartment attached was added to the basin bottom containing the PCM. The PCM were placed in plastic tubes having 2 mm thickness which were immersed in distilled water. This distilled water received heat from the water basin and is then transferred and stored in the PCM.

The objective of this study is to theoretically model the unit developed by Al-harahsheh et al. [13] incorporated with Sodium Thiosulfate Pentahydrate as PCM and attached to an external solar collector. To the authors' best knowledge modeling of such combination was not studied previously. The effect of operational parameters such as the cooling water flow rate, the water circulation flow rate through the solar collector, basin water depth, and ambient conditions on the system productivity is studied.