Experimental Performance Analysis of a Solar Desalination System Modified with Natural Dolomite Powder Integrated Latent Heat Thermal Storage Unit

Abstract

Solar desalination systems are effective and sustainable applications that are utilized to obtain potable water from saline or contaminated water. In this research, three solar desalination systems, including a conventional system, a phase change material (PCM)-based thermal energy storage unit (TESU), and a natural dolomite powder integrated PCM-based TESU, were structured and experimentally investigated. The developed solar desalination systems were analyzed simultaneously and the findings were discussed in detail. According to the empirically obtained outcomes, utilizing PCM-based TESUs and dolomite-powder-embedded PCM-based TESUs increased daily cumulative productivity by 10.15% and 17.70%, respectively, in comparison to the conventional distiller. Employing dolomite powder increased the energy and exergy efficiencies of the conventional distiller from 15.91% to 18.28% and from 1.26% to 1.78%, respectively. Moreover, environmental metrics such as global warming potential and the sustainability index of the developed solar desalination systems were analyzed within the scope of this work.

1. Introduction

Global energy demand is rising day by day due to the growing world population and developing industrial activities [1,2,3]. Utilizing fossil resources to meet the required demand has had serious negative impacts including greenhouse gas emissions, adverse climate events, and elevation of temperatures [4,5]. In this regard, renewable and sustainable energy alternatives should be utilized in order to meet required energy demand. The use of clean energy sources also helps to meet sustainable development goals [6,7]. Solar energy is one of the clean alternatives that can be used to generate electrical and thermal energies instead of fossil-based sources [8,9,10,11].

In addition to energy, one of the other crucial global needs is a continuous supply of freshwater [12]. The freshwater resources of the world are limited, and sustainable desalination solutions need to be developed in order to meet global freshwater demand, especially in arid areas and regions with water shortages [13,14]. In the last few years, desalination techniques employing solar radiation have been receiving attention from researchers because of their cost-effectiveness and easy application [15,16,17]. Various types of solar desalination systems (SDSs) have been designed and investigated in the past few years [18,19]. Basically, SDSs capture water evaporation from the basin, which condenses on the transparent cover above the system.

In the scientific literature, researchers have contributed to the efficiency enhancement of solar desalination systems by investigating different auxiliary heating devices. Al-harahsheh et al. [20] investigated an SDS modified with a solar water collector and a heat storage component. In the designed system, saline water was circulated over the solar collector to increase the water’s temperature. By applying this method, productivity was significantly increased. In another study, an electric heater was used in a double-slope SDS [21]. By using an external heating device, overall productivity was enhanced from 5.78 to 6.72 kg/m²/day. Alwan et al. [22] investigated a single-slope SDS with a rotating cylinder and an external solar collector. Utilizing the mentioned modifications significantly improved the overall yield of the SDS. Moreover, different basin materials were applied to the SDSs in some studies. El-Said et al. [23] utilized wire mesh in a tubular SDS with harmonic motion. The yield of the modified SDS was 34% higher than that of the conventional SDS. Younes et al. [24] analyzed wick SDSs with corrugated and semi-spherical structures and compared their performance to that of the conventional SDS. The thermal efficiency of the conventional and modified SDSs was 34% and in the range of 55.5–57%, respectively. In another study, Fathy et al. [25] integrated a parabolic collector with an SDS with a double-slope structure. Utilizing the tracked parabolic collector increased the daily yield from 4.31 to 10.93 kg/m². There were also some studies on the testing of various geometrical modifications in solar stills [26,27]. Moustafa et al. [28] analyzed a tubular solar distiller modified with an electric heater powered by a solar panel. The developed distiller was compared to a conventional tube-type distiller. According to the findings, a 32.17% improvement of accumulated productivity was obtained by using the mentioned modification. Alawee et al. [29] investigated a pyramid-shaped SDS modified with rotating components and auxiliary heaters. By applying the mentioned modifications, overall yield increased from 3000 to 7300 mL/m². Toosi et al. [30] used a stepped SDS with phase change materials (PCMs) and an external condensing component. Their findings showed that the combined utilization of the modifications significantly improved the overall distillate. Agrawal and Singh [31] studied a double-slope SDS with PCM and steel wool. By adjusting the water depth and applying the modifications, energy efficiency improved by 41%.

Thermal energy storage units (TESUs) are also widely utilized as a performance upgrading approach for both solar-thermal and other types of thermal systems [32,33,34,35]. They can also be applied successfully to SDSs [36]. In a study [37], pin-shaped fins and PCM were utilized to modify the TESU of an SDS. Accumulated productivity was improved by 17% in comparison to a conventional SDS. Vigneswaran et al. [38] analyzed an SDS with different TESUs. According to their experimentally obtained findings, the exergy efficiency of the SDS with multiple PCMs was 3.92% higher in comparison to that of the conventional SDS. Abu-Arabi et al. [39] developed an SDS with a TESU and a solar collector. Applying these modifications enhanced the yield of the SDS approximately 1.8 times. Cheng et al. [40] designed and tested an SDS with a shape-stabilized PCM. With the use of a PCM, the productivity of the SDS increased from 42% to 53%. Thermal conductivity values of PCMs can be upgraded by integrating nano-sized materials [41,42,43]. There are some works available in the literature that analyzed SDSs with nano- and micro-enhanced PCMs [44,45]. Kumar et al. [46] considered an SDS with a silica nano-enhanced TESU. They analyzed three different single-slope SDSs including a conventional, PCM-modified, and nano-PCM modified types. Adding TESU and nano-enhanced TESU improved the yield by approximately 51% and 67%, respectively, in comparison to the conventional SDS. Abdullah et al. [47] applied different types of modifications to an SDS including a corrugated absorbing surface, nano-embedded TESU, and solar-panel-driven auxiliary heater. Combined usage of the modifications upgraded the overall yield of the SDS by 180%.

In addition to the above-mentioned thermal performance enhancement methods, a few works utilized natural minerals to improve the thermal conductivity of TESUs [48,49]. In recent years, integration of micro- and nano-sized natural minerals in thermal systems has become a popular research field. Some researchers considered different natural minerals and rocks, such as kaolin [50,51], limestone [52], zeolite [53], dolomite [49], mafic rock, felsic rock, sandstone, conglomerate, and serpentinite [54], for use in thermal applications.

In contrast to other studies, natural dolomite powder was utilized to improve the thermal performance of a TESU in a solar desalination application. The main goal of this research was to analyze the effects of utilizing a dolomite-powder-embedded TESU on the energetic-exergetic and freshwater production performance of an SDS. In other words, the major goal of this study was to investigate the potential use of a natural mineral powder integrated TESU in an SDS. In this regard, three different SDSs, including a conventional and a modified TESU, and a dolomite-integrated TESU modified type have were developed, fabricated, and tested simultaneously. Moreover, energy-exergy and environmental analyses were performed considering the experimentally obtained findings. The main steps of the current research are presented in Figure 1.

2. Materials and Methods

2.1. Preparation of Dolomite-Powder-Integrated TESU

Three types of solar desalination systems were structured and tested in this study, two of which were modified with a TESU. In this context, two basins were produced by using an aluminum sheet 0.1 cm thick. The dimensions of the produced basins were 45 × 35 × 2.5 cm. Rubitherm Technologies GmbH RT42 paraffin wax was utilized as the PCM. Heat storage capacity, specific heat capacity, and thermal conductivity values of the employed PCMs were 165 kJ/kg, 2 kJ/kg.K, and 0.2 W/m.K, respectively. Moreover, the density value of the PCM for the solid and liquid phases was 0.88 and 0.76 kg/L, respectively. The first TESU contained only paraffin wax. In the first stage of the preparation process, the PCM was melted at a constant temperature (60 °C) on a heat source, and the produced basin was filled with the melted PCM considering the volume expansion value indicated by the manufacturer (12.5%). In total, both basins were filled 85% with PCM to avoid leakages. The second TESU was modified with natural dolomite powder. The SEM image and XRD pattern of the dolomite powder are presented in Figure 2 and Figure 3, respectively. Dolomite is grouped in the sedimentary rocks and minerals class and is one of the highest thermally conductive natural minerals [55]. It is also a highly accessible natural mineral in our specific test region [56]. The average particle size of the powder was 582 nm. The PCM was melted in the initial stage and dolomite powder was mixed with the melted PCM at a concentration of 2% (wt/wt). The attained mixture was stirred at 5 rpm for one hour with a mechanical mixer. After that, the mixture was sonicated in the liquid phase for 3 h to obtain a homogenous mixture. Preparation techniques were employed and the concentration ratio of the dolomite powder was selected in consideration of similar studies in scientific literature to avoid leakages and sedimentation of the utilized powder [57,58,59,60].

2.2. Experimental Setup

As stated before, the experimental part of this work involved simultaneous analysis of three different solar stills. The first solar desalination system (SDS) was produced as a conventional type. The second system (SDS-TESU) was modified with a paraffin-based thermal energy storage component. The third system (SDS-DTESU) was upgraded with a natural dolomite powder enhanced thermal energy storage unit (TESU). Both TESUs were placed under the base plate of the SDSs. A display of the experimental setup is shown in Figure 4, as well as the dimensions of the developed SDSs. Transparent surfaces including the top cover, north wall, and side walls of the SDSs were manufactured by using a 0.4 cm thick glass with 92% transmissivity. Other parts (frames and basin plates) were made from a 0.1 cm thick aluminum sheet. The bottoms and sides of the SDSs were thermally insulated.

2.3. Experimental Procedure

The performance test was conducted in Manisa, Turkey. The experimental procedure was performed in the summer of 2021. The coordinates of the specific test location were 38°36′38.28″ N and 27°22′53.76″ E. The developed SDSs were tested simultaneously to observe the effects of developed modifications on the energetic and exergetic performance. In this regard, each SDS was filled with 9.5 L of saline water, which corresponded to a water depth of about 6 cm. Temperature measurements were performed every 10 s by using thermocouples with ±0.5 °C accuracy. Measured temperature values were recorded using dataloggers with ±0.3 °C accuracy. Other metrics (water mass, solar radiation, and wind velocity) were measured in one hour intervals. The accuracy values of the utilized digital balance, solarimeter, and anemometer were ±0.02 g, ±10 W/m², and ±3%, respectively. Specifications of the measurement apparatus are shown in Figure 4. The desalination test was started at 08:00 a.m. and terminated at 17:00 p.m. Moreover, feed water was added manually each hour.

3. Theoretical Calculations

3.1. Energy-Exergy Analysis

The ratio of hourly energy of the freshwater to hourly energy inflow is known as the hourly energy efficiency of a solar desalination system, and can be found using the following equation [61]:

$${\eta }_{thr}=\frac{m_w·h_{fg}}{A_{SDS}·I·3600}$$

(1)

Exergy is an indicator of the capability of energy to perform work, and can be calculated considering the second law of thermodynamics. The output exergy of a solar distillation device can be found using the following equation [62]:

$${\dot{Ex}}_{ou}=\frac{{\dot{m}}_w·h_{fg}}{3600}.\left[1-\left(\frac{T_{am}+273.15}{T_w+273.15}\right)\right]$$

(2)

In Equations (1) and (2), h_fg is the latent heat of the evaporated water, which can be calculated by employing the following expressions [63,64]:

$$h_{fg}=2.4935·\left({10}^6-947.79·T_{wt}+0.13132·T_{wt}{}^2-0.0047974·T_{wt}{}^3\right),T_{wt}<343\mathrm{K}$$

(3)

$$h_{fg}=3.1615·\left({10}^6-761.6·T_{wt}\right),T_{wt}>343\mathrm{K}$$

(4)

The input exergy of a solar distiller can be computed by employing Equation (5) [65]:

$${\dot{Ex}}_{in}=A_{SDS}·I·\left[1-\frac{4}{3}\left(\frac{T_{am}+273.15}{T_{sn}}\right)+\frac{1}{3}{\left(\frac{T_{am}+273.15}{T_{sn}}\right)}^4\right]$$

(5)

The exergy efficiency of a solar distiller can be calculated as [66,67]:

$${\eta }_{exg}=\frac{{\dot{Ex}}_{ou}}{{\dot{Ex}}_{in}}$$

(6)

The empirical uncertainty expression is given as [68,69]:

$$W_R={\left[{\left(\frac{\partial R}{\partial x_1}{w}_1\right)}^2+{\left(\frac{\partial R}{\partial x_2}{w}_2\right)}^2+\cdots +{\left(\frac{\partial R}{\partial x_n}{w}_n\right)}^2\right]}^{1/2}$$

(7)

The obtained uncertainty metrics for temperature, water mass, and solar radiation were ±0.55 °C, ±0.58 g, and ±16.34 W/m², respectively. The obtained uncertainties are in line with those obtained similar studies on SDSs and other solar-thermal systems [70,71,72].

3.2. Environmental Impact Assessment

This type of investigation is generally conducted to define the influence of the fabricated SDS on the environment. In this regard, embodied energy (EE) for the employed component in the fabrication stage should be found. This metric indicates the amount of energy spent to produce the product from raw form to the end form. In this study, embodied energies were determined for each material considering similar studies. Some of the EE values were found by applying unit conversions. In addition, the embodied energy of the employed dolomite powder was not available in the academic literature. In this context, some assumptions were made. The EE value of the raw form of dolomite was taken from a relevant study [73]. Then, the energy consumption of the ball milling process was calculated. A Retsch PM100 ball mill was utilized in the milling stage of the dolomite powder (energy consumption: 1.25 kWh) [74]. Khanlari [75] produced alumina powder, which was employed in a heat transfer enhancement process. In the given study, ball milling was performed for 8 h. A ball mill’s grinding capacity depends on the volume of the grinding chamber. In the present work, a 500 mL grinding chamber volume was selected. Half of the chamber’s volume was occupied by grinding balls. Consequently, 10 ball milling periods (overall: 80 h) were required to obtain 1 kg of dolomite powder. Table 1 presents the total embodied energies of each component for manufacturing SDSs.

The mitigated CO₂ and NO emissions of an SDS during its lifetime can be expressed as [76,77]:

$$NetC{O}_{2}emissionmitigated=\frac{\left(Annualproductivity·h_{fg}\right)-EE}{1000}·1.58\frac{\mathrm{kg}}{\mathrm{kWh}}$$

(8)

$$NetNOemissionmitigated=\frac{\left(Annualproductivity·h_{fg}\right)-EE}{1000}·0.0046\frac{\mathrm{kg}}{\mathrm{kWh}}$$

(9)

In these calculations, 118 yearly sunshine days were recorded due to the climatic conditions of the test location [78]. The global warming potential of the SDS could be calculated as [76,79]:

$$GWP=\frac{\left(C{O}_{2}emissionfactor·GWPofC{O}_2\right)+\left(NOemissionfactor·GWPofN{O}_x\right){{\displaystyle \sum }}_{LS=1}^{LS=10}EE}{Totaldistilledfreshwaterinthelifespan}$$

(10)

The GWPs of NO_x and CO₂ were 33.0 and 1.0 kg CO₂ eq./kg, respectively, in the calculations. In addition, the lifespan of the SDS was recorded at 10 years [76].

The sustainability index is an important performance indicator of solar distillers and can be expressed as [80]:

$$SI=\frac{1}{\left(1-{\eta }_{exg}\right)}$$

(11)

Table 1. Embodied energy values of SDSs.

Material	Ref.	EE (kWh/kg)	Quantity (kg)	Overall Embodied Energy (kWh)
				SDS	SDS-TESU	SDS-DTESU
Aluminum sheet	[81]	55.28	1.60	88.44	88.44	88.44
PVC pipe	[82]	19.39	0.05	0.97	0.97	0.97
Glass	[83,84]	4.20	1.20	5.04	5.04	5.04
Fittings	[81]	8.89	0.10	0.89	0.89	0.89
Black paint	[85]	25.11	0.10	2.51	2.51	2.51
Thermal insulation	[86]	39.88	0.25	9.97	9.97	9.97
Paraffin wax	[87]	24.11	2.91	0	70.16	70.16
Dolomite powder	[73,74,75]	110.16	0.06	0	0	6.61
Overall embodied energy				107.82	177.98	184.59

Table 1. Embodied energy values of SDSs.

Material	Ref.	EE (kWh/kg)	Quantity (kg)	Overall Embodied Energy (kWh)
				SDS	SDS-TESU	SDS-DTESU
Aluminum sheet	[81]	55.28	1.60	88.44	88.44	88.44
PVC pipe	[82]	19.39	0.05	0.97	0.97	0.97
Glass	[83,84]	4.20	1.20	5.04	5.04	5.04
Fittings	[81]	8.89	0.10	0.89	0.89	0.89
Black paint	[85]	25.11	0.10	2.51	2.51	2.51
Thermal insulation	[86]	39.88	0.25	9.97	9.97	9.97
Paraffin wax	[87]	24.11	2.91	0	70.16	70.16
Dolomite powder	[73,74,75]	110.16	0.06	0	0	6.61
Overall embodied energy				107.82	177.98	184.59

4. Results and Discussion

In this section, the experimental findings of the analyzed SDSs are presented and discussed. The changes in the environmental conditions and water temperatures of the SDSs are illustrated in Figure 5. The average ambient temperature and solar radiation values were 30.19 °C and 855 W/m², respectively. In addition, the mean water temperatures for SDS, SDS-TESU, and SDS-DTESU were 51.68, 53.68, and 56.69 °C, respectively. Utilizing TESU and DTESU increased the water temperature values by 3.89% and 9.69%, respectively. In another study, a nano powder-embedded TESU was applied to an SDS, and compared to a conventional SDS [46]. They observed water temperature trends similar to those in the current study. In another study, Kabeel and Abdelgaied [88] simultaneously investigated two different SDSs to observe the influence of employing a TESU on the performance of an SDS. They attained water temperature behaviors similar to those the current research.

Figure 6 presents the variation in the hourly productivity of the SDSs via time. The average hourly productivities of SDS, SDS-TESU and SDS-DTESU were 0.38, 0.42, and 0.45 kg/m², respectively. Utilizing a dolomite-powder-applied TESU significantly improved the hourly yields. The difference between productivity values was seen more clearly in the afternoon when solar radiation values were lower. As known, SDSs receive solar different radiation different amounts at different times of day. Therefore, some heterogeneous distributions of temperature were found on the absorber surfaces. Employing TESUs at the basin side of the SDSs helped to obtain a more homogenous temperature distribution.

Figure 7 shows the change in the accumulated hourly productivities of the three SDSs over time. The accumulated hourly productivities for conventional (SDS), TESU-integrated (SDS-TESU) and dolomite powder-embedded TESU-integrated (SDS-DTESU) were 3.84, 4.23, and 4.52 kg/m², respectively. As shown, using dolomite-powder-embedded TESU increased the overall yield by 17.70% in comparison to the conventional SDS. In a study by Hassan [63], various types of SDSs were developed and tested. By applying different types of modifications, the accumulated yield was improved by approximately 2.3 to 4 kg/m² in winter climatic conditions. In a different study, the accumulated productivity was increased by approximately 20% by adjusting the condensation mechanism [89]. A simple TESU modification was applied to the SDSs to improve the performance without using any additional auxiliary heating devices.

Within the scope of this research, energy-exergy analyses of the developed SDSs were performed. The obtained mean energy efficiencies for SDS, SDS-TESU, and SDS-DTESU were 15.91%, 17.37%, and 18.28%, respectively. In a prior study, PCMs were integrated into the SDSs and thermal efficiency improved by approximately 38% [90]. In another study, the energy efficiency of an SDS was improved by about 30by using different basin materials [91]. Moreover, the changes in the exergy efficiency values of the tested SDSs via time are presented in Figure 8. The average exergy efficiencies for SDS, SDS-TESU and SDS-DTESU were 1.26%, 1.52%, and 1.78%, respectively. The maximum instantaneous exergy efficiency was 2.94% for SDS-DTESU at 14:00. Utilizing dolomite-integrated TESU in an SDS improved energy and exergy efficiencies by 14.89% and 41.26%, respectively, in comparison to the conventional type. Kianifar et al. [92] analyzed pyramid-shaped SDS and achieved an exergy yield between 2.1% and 3.3%. In another study, four different SDS configurations were tested, and the exergy efficiency value was between 0.48% and 9.00% [76].

Environmental impact assessments are an important aspect of analyzing solar-thermal applications. In this work, the global warming potential (GWP) and the sustainability index (SI) of the developed SDSs were calculated. As known, GWP is directly associated with the embodied energy of a specific device. The GWP potential for SDS, SDS-TESU, and SDS-DTESU was 0.24, 0.38, and 0.39 CO₂ eq./kg, respectively. Moreover, the average SI values for SDS, SDS-TESU, and SDS-DTESU were 1.014, 1.017, and 1.020, respectively. Utilizing natural dolomite powder significantly improved the effectiveness of the SDS, as mentioned before. However, the developed system can be considered pilot-scale, and the embodied energy of the PCM may consume a large part of the overall embodied energy of the system. In this regard, the GWP values of the modified SDSs were higher than those of the conventional SDS. Similar values were obtained in a study that analyzed SDSs with various configurations [76]. In the study, four different SDS were experimentally tested, and GWP and SI values were between 0.12 and 0.32 CO₂ eq./kg and 1.02 and 1.08, respectively.

A comparison of the findings with those obtained in similar studies that analyzed SDSs is given in

Table 2. Comparison of our findings with those of similar studies that analyzed SDSs.

Ref.	Geometry	Bottom Area (m2)	Modification	Energy Efficiency (%)	Yield Increase (%)
[63]	Single- and double-slope	1.5	Parabolic trough collector	22.39–32.59	90.40
[93]	Single-slope	0.25	Photovoltaic panel and external condenser	27.17	39.49
[94]	Cuboid	-	Thermoelectric module and parabolic collector	13	-
[95]	Pyramid	1	Blackened wick	50.3	17.7
[70]	Single-slope	1	Finned wick	55	23
[96]	Single-slope	0.25	Wick pile of jute cloth	29.4	23.7
[97]	Double-slope	1	External condenser, fins, TESU and wick	39.7	22.3
[98]	Tubular	-	PCM and reflector	-	8.2
This study	Single-slope	0.157	Conventional	15.91	-
	Single-slope	0.157	TESU	17.37	10.15
	Single-slope	0.157	Dolomite powder-integrated TESU	18.28	17.70

. The results of the current SDSs are in agreement with those obtained in similar research. The performance of the SDSs was upgraded in this study without utilizing complex geometries and additional heating devices. Moreover, the SDSs in this study were designed with single-slope geometry, one of the basic shapes that is employed in SDSs. The main reason for this was to observe the effects of employing the developed modifications on the performance of a simple-structured SDS. In this study, the analyzed SDSs were developed as prototypes and designed TESUs were attached directly to the SDSs by utilizing strong impermeable materials. SDSs can be designed as detachable structures for easier maintenance for domestic and industrial utilization.

5. Conclusions

In this study, the impacts of integrating PCM-based and natural dolomite powder embedded PCM-based thermal storage units on the performance of a solar still were analyzed. The performance of the system was improved by using a natural mineral powder. In this context, three different solar distillers were developed and analyzed experimentally. The major findings of the survey are:

• Mean energy and exergy efficiencies were increased by using a dolomite-integrated thermal storage unit from 15.91% to 18.28% and from 1.26% to 1.78%, respectively, in comparison to the conventional solar distiller.
• Applying a dolomite-powder-integrated storage unit increased overall cumulative freshwater yield by 17.70% compared to that of the conventional distiller.
• GWP and SI values were within the range of 0.24–0.39 CO₂ eq./kg and 1.014–1.020, respectively.

The findings from this study show the successful used of dolomite powder in the TESU of an SDS. In future studies, the developed modifications can be utilized in SDSs with high-efficient geometries such as stepped, double-slope, and pyramid-shaped desalination systems for commercial usage. Moreover, different types of natural minerals can be integrated into the TESU in an SDS system to analyze the performance increase.