Cost analysis of different solar still configurations

Abstract

The enhancement of the productivity of the solar desalination system, in a certain location, could be attained by a proper modification in the system design. Therefore, different design configurations could be found in literatures. However, the increase in the system productivity with high system cost may increase also the average annual cost of the distillate. Cost analysis of different design configurations of solar desalination units is essential to evaluate the benefit of modification from the economical point of view. The main objective of this work is to estimate the water production cost for different types of solar stills. In this paper 17 design configurations are considered. Systems with higher and lower values of productivity are considered in this investigation. A simplified model for cost analysis is applied in this study. The results show that, the best average and maximum daily productivity are obtained from solar stills of single-slope and pyramid-shaped. The higher average annual productivity for a solar still is about 1533 l/m² using pyramid-shaped while the lower average annual productivity is about of 250 l/m² using modified solar stills with sun tracking. The lowest cost of distilled water obtained from the pyramid-shaped solar still is estimated as 0.0135 $/l while highest cost from the modified solar stills with sun tracking is estimated as 0.23 $/l.

Introduction

Desalination technologies have been used for about a century in land-based plants and on ships to provide water for a crew. The regular use of desalination technologies accelerated after World War II, as the demand for fresh water in arid countries. The cost for desalination has been decreasing rapidly, especially in recent years with the introduction of efficient, more cost effective technologies. For solar distillation systems, sunlight has the advantage of zero fuel cost but it requires more space (for its collection) and generally more costly equipment. In principle, the water from a solar still should be quite pure. The slow desalination process allows only pure water to evaporate from the basin and collect on the cover, leaving all particulate contaminants behind. A solar still is a simple device, which can be used to convert saline, brackish water into drinking water. Solar stills use exactly the same processes, which in nature generate rainfall, namely evaporation and condensation. Its function is very simple a transparent cover encloses a pan of saline water.

Desalination of brackish or seawater represents a consolidated system to resolve the water emergency. The main drawbacks to this solution however are high-energy consumption and high cost. Therefore, it is imperative to evaluate the possibility of using local renewable resources to desalt water. Since high cost is the greatest hindrance to solar energy application. This paper presents water desalination technologies using solar energy. It is objected to estimate the water production cost for different types of solar distillation units. Review of some different configurations of solar distillation system will be followed by the economical analysis and comparison between these types.

Single basin solar still is a popular solar device used for converting available brackish or waste water into potable water. Because of its lower productivity, it is not widely used. Numerous modifications are investigated to improve the productivity of the still. Solar stills of desalination plant have low thermal efficiency and productivity. This could be improved by various passive and active methods. For examples, Fath et al. [1] presented an analytical study as well as thermal and economic comparisons between two solar still configurations: the pyramid and the single-slope.

Samee et al. [2] presented the design and performance of a simple single basin solar still. A schematic diagram of the designed solar still is shown in Fig. 1. The average daily output of solar still based on data of 8 days in July 2004 was found to be 1.7 L/day for basin area of 0.54 m². Efficiency of the still was calculated as 30.65% with a maximum hourly output of 0.339 L/h at 1300 h.

Kumar and Tiwari [3] presented the life cycle cost analysis of the single-slope passive and hybrid photovoltaic (PV/T) active solar stills, based on the annual performance at 0.05 m water depth as shown in Fig. 2.

The effect of coupling a flat-plate solar collector on the productivity of solar stills (Fig. 3) carried out by Badran and Tahaineh [4]. It was found that coupling of a solar collector with a still has increased the productivity by 36%.

The modified unit (Fig. 4) includes a solar parabolic trough (solar energy concentrator) with focal pipe and simple heat exchanger (serpentine) was studied by Abdel-Rehim and Lasheen [5]. Oil is flowing through the focal pipe and serpentine to heat water in the still. The results show that fresh water productivity was increased by an average of 18%, due to the modification.

A sun-tracking system deployed for enhancing the single-slop solar still productivity by Abdallah and Badran [6]. A computerized sun-tracking device was used for rotating the solar still with the movement of the sun. A comparison between fixed and sun tracked solar stills showed that the use of sun-tracking increased the productivity for around 22%, due to the increase of overall efficiency by 2%. It can be concluded that the sun tracking is more effective than fixed system and it is capable of enhancing the productivity.

Al-Hinai et al. [7] performed a parametric study on a conventional pyramid double-sloped single basin solar still (Fig. 5) under climatic conditions of Sultanate of Oman at the Gulf region. They reported that under optimum design conditions, the still tends to give an average annual solar yield of approximately 4 l/m² day.

Badran et al. [8] studied the performance of a solar still augmented with a flat-plate collector (Fig. 6). They concluded that the mass of distilled water production increased by 231% in the case of tap water as a feed and by 52% in the case of salt water as a feed.

The productivity of the single basin solar still augmented by integrating fins at the basin plate by Velmurugan et al. [9]. To enhance the productivity of solar still, it modified with fin, black rubber, sand, pebble and sponges (Fig. 7). It found that the evaporation rate increased by about 53% when fins were integrated at the basin plate.

Velmurugan et al. [10] compared the performance of ordinary single basin solar still and wick type still (Fig. 8). To enhance evaporation of the still basin water, fins and sponges integrated at the basin of the still. It found that 29.6% productivity increased, when wick type solar still used, 15.3% productivity increased when sponges used and 45.5% increased when fins used.

A simple transportable hemispherical solar still (Fig. 9) designed and fabricated by Ismail [11] and its performance experimentally evaluated under outdoors of Dhahran climatic conditions. It found that over the hours of experimental testing through daytime, the daily-distilled water output from the still ranged from 2.8 to 5.7 l/m² day.

Velmurugan et al. [12] studied a stepped still with two different depth of trays. The basin plate contains twenty-five trays with 10 mm depth and twenty-five trays with 5 mm depth. Theoretical and experimental analyses made for fin type, sponge type, and combination of fin and sponge type stepped solar still. It found that when the fin and sponge type stepped used, the average daily water production has been found to be 80% higher than ordinary single basin solar still.

Sun-tracking systems used by some researchers to enhance the desalination production. Abdallah et al. [13] studied the performance of a traditional single-slope solar still through three design modifications addition of internal reflecting mirrors on all interior sides of still, using step-wise water basin instead of flat basin, and coupling the solar still with a sun-tracking system (Fig. 10). The inclusion of internal mirrors improved the system thermal performance up to 30%, while step-wise basin enhanced the performance up to 180% and finally the coupling of the step-wise basin with sun-tracking system gave the highest thermal performance with an average of 380%.

Sadineni et al. [14] studied weir-type inclined solar (Fig. 11). A weir-type solar still proposed to recover rejected water from the water purifying systems for solar hydrogen production. The results showed that the productivity of the weir-type still is approximately 20% higher.

A SP (solar pond) is a thermal solar collector that includes its own storage system. A SP collects solar energy by absorbing direct and diffuse sunlight. Therefore, Velmurugan and Srithar [15] presented a comparison between theoretical and experimental analysis of a mini SP assisted solar still. In a mini SP, experiments conducted for different salinity (Fig. 12). They discussed the effect of sponge cubes in the still, effect of integrating mini SP with the still and combination of both. It found that the average daily production of solar still increased considerably, when it integrated with a mini SP.

In an attempt to improve the daily productivity of the single effect solar stills, a single-slope single basin solar still integrated with a SSP (shallow solar pond) (Fig. 13) studied by El-Sebaii et al. [16]. The resulted show that the annual average values of the daily productivity and efficiency of the still with the SSP found to be higher than those obtained without the SSP by 52.36% and 43.80%, respectively.

The effect of adding a passive condenser on the performance of the single-slope, basin type solar still studied by few authors. Analysis of a parallel single and double glass solar still with separate condenser (Fig. 14) studied by El-Bahi and Inan [17] with minimum inclination (4°) investigated experimentally. The results show that the solar still operated without a condenser yields decrease about 70% compared with a condenser use.

Previous works suggest several improvements on single-slope such as, solar collector, solar concentrator, sun tracking, pyramid-shaped, pyramid with collector, fin type, wick and fin type, transportable hemispherical stepped, fins and sponges stepped, sun tracking, a weir-type, sponge and pond, a SSP, separate condenser are used to increase the productivity solar stills. Table 1 shows the different review types and area of solar stills, climatic condition and daylight hours.