\makeatletter
\@ifundefined{HCode}
{\documentclass[screen,CRCHIM,Unicode,Thematic]{cedram}
\newenvironment{noXML}{}{}
\def\tsup#1{$^{{#1}}$}
\def\tsub#1{$_{{#1}}$}
\def\ndash{\text{--}}
\usepackage[T1]{fontenc}
\def\thead{\noalign{\relax}\hline}
\def\tbody{\noalign{\relax}}
\def\endthead{\noalign{\relax}\hline}
\def\tabnote#1{\vskip4pt\parbox{.95\linewidth}{#1}}
\def\xsection#1{} 
\usepackage{amssymb}
\RequirePackage{etoolbox}
\def\jobid{crchim20240198}
\newenvironment{inftab}{}{}
%\graphicspath{{/tmp/\jobid_figs/web/}}
\graphicspath{{./figures/}}
\newcounter{runlevel}
\let\MakeYrStrItalic\relax
\csdef{Seqnsplit}{\\}
\def\refinput#1{}
\def\back#1{}
\def\dollar{\$}
\def\ubreak{\break}
\let\Lbreak\break
\def\botline{\\\hline}
\def\mn{\phantom{$-$}}
\def\0{\phantom{0}}
\usepackage{multirow} 
\def\nrow#1{\@tempcnta #1\relax%
\advance\@tempcnta by 1\relax%
\xdef\lenrow{\the\@tempcnta}}
\def\morerows#1#2{\nrow{#1}\multirow{\lenrow}{*}{#2}}
\def\xmorerows#1#2{#2}
\gdef\inftabbreak{\end{tabular}
\end{inftab}\newpage
\begin{inftab}
\begin{tabular}{@{}ll}
\tbody}
\usepackage{hyperref}
\usepackage{textcomp}
\allowdisplaybreaks
\makeatletter
\g@addto@macro{\UrlBreaks}{\UrlOrds}
\gappto{\UrlBreaks}{\UrlOrds}
\makeatother
\DOI{10.5802/crchim.346}
\datereceived{2024-03-15}
\daterevised{2024-07-07}
\datererevised{2024-09-12}
\dateaccepted{2024-09-24}
\ItHasTeXPublished
}
{\documentclass[crchim]{article}
\usepackage[T1]{fontenc}
\def\CDRdoi{10.5802/crchim.346}
\let\inftabbreak\relax
\def\xmorerows#1#2{\morerows{#1}{#2}}
\let\newline\break
\let\ubreak\relax
\makeatletter
}
\makeatother

\usepackage{upgreek}

%\copyeditor{RB}

\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\makeatletter
\def\TITREspecial{\relax}
\def\cdr@specialtitle@english{Materials and Energy Valorization of Biomass and Waste : The Path for Sustainability and Circular Economy Promotion}
\def\cdr@specialtitle@french{Valorisation \'energ\'etique et mati\`ere de la biomasse et des d\'echets : la voie de la durabilit\'e et de la promotion de l'\'economie circulaire}
\makeatother

\title{Performance augmentation of triangular solar still using
sensible energy storage materials: thermo-exergo-economic and
sustainability analyses}

\alttitle{Augmentation des performances d'un distillateur solaire
triangulaire \`{a} l'aide de mat\'{e}riaux sensibles au stockage
d'\'{e}nergie : analyses thermo-exergo-\'{e}conomiques et de
durabilit\'{e}}

\author{\firstname{Naveen} \lastname{Sharma}\CDRorcid{0000-0002-3326-1800}\IsCorresp}
\address{Department of Mechanical Engineering, Netaji Subhas
University of Technology, New Delhi-110078, India}
\email{naveen.sharma@nsut.ac.in}
\email[N. Sharma]{sharma.naveen28@yahoo.com}

\author{\firstname{Ashutosh} \lastname{Patel}}
\addressSameAs{1}{Department of Mechanical Engineering, Netaji Subhas
University of Technology, New Delhi-110078, India}

\author{\firstname{Raman} \lastname{Deep}\CDRorcid{0009-0005-2311-8508}}
\addressSameAs{1}{Department of Mechanical Engineering, Netaji Subhas
University of Technology, New Delhi-110078, India}

\author{\firstname{Shivam} \lastname{Gautam}}
\addressSameAs{1}{Department of Mechanical Engineering, Netaji Subhas
University of Technology, New Delhi-110078, India}

\shortrunauthors

%\thanks{}

\keywords{\kwd{Heat storage material}
\kwd{Desalination}
\kwd{Sustainability analysis}
\kwd{Thermo-exergo-economic analysis}
\kwd{Triangular solar still}}

\altkeywords{\kwd{Mat\'{e}riau de stockage de chaleur}
\kwd{Dessalement}
\kwd{Analyse de durabilit\'{e}}
\kwd{Analyse thermo-exergo-\'{e}conomique}
\kwd{Distillateur solaire triangulaire}}

\begin{abstract}
People in remote areas often struggle with limited access to potable
water. This experimental research aims to augment the performance of a
triangular solar still (TSS) by incorporating sensible heat storage
materials (SHSMs) for sustainable freshwater production. Four different
configurations---standard TSS, TSS with bricks (TSS-B), TSS with
paper napkins wrapped around the brick (TSS-NB), and TSS with sand bed
(TSS-S)---were tested under the climatic conditions of New Delhi
(28\textdegree{} 35\tralicstex{\textquotesingle}{$'$} N, 76\textdegree{} 54\tralicstex{\textquotesingle}{$'$} E), India, during October 2023.
The performance of TSS was assessed based on energy, exergy, economic,
and environmental parameters. From experimental results, it was
observed that the daily yield of the TSS-S was 3.12 L/m\textsuperscript{2}, which is about 81.40\% higher than that of TSS.
Additionally, the daily yield increased by around 34.86\% for TSS-B and
about 28.23\% for TSS-NB than TSS. Incorporating SHSMs into the TSS
led to significant enhancements in energy efficiency (19.15\% to
75.12\%) and exergy efficiency (7.41\% to 36.44\%) compared to TSS,
solidifying it as a viable choice for freshwater production. The
economic assessment revealed that employing SHSMs resulted in a
reduction in the cost per liter of freshwater generation by up to
42.86\% (for TSS-S) and a corresponding decrease in the payback period
by up to 44.33\% (for TSS-S) compared to TSS. Furthermore, the
exergetic sustainability of the TSS was explored using improvement
potential and sustainability index parameters. Moreover, the TDS and pH
values of the yield produced from TSS, with and without SHSMs, adhere
to water quality standards.
\end{abstract}

\begin{altabstract}
Les habitants des r\'{e}gions isol\'{e}es sont souvent confront\'{e}s
\`{a} un acc\`{e}s limit\'{e} \`{a} l'eau potable. Cette recherche
exp\'{e}rimentale vise \`{a} augmenter les performances d'un
distillateur solaire triangulaire (TSS) en incorporant des
mat\'{e}riaux sensibles au stockage de chaleur (SHSM) pour une
production durable d'eau douce. Quatre configurations diff\'{e}rentes :
le TSS standard, le TSS avec des briques (TSS-B), le TSS avec des
serviettes en papier enroul\'{e}es autour de la brique (TSS-NB) et le
TSS avec un lit de sable (TSS-S), ont \'{e}t\'{e} test\'{e}es dans les
conditions climatiques de New Delhi (28\textdegree{} 35\tralicstex{\textquotesingle}{$'$} N, 
76\textdegree{} 54\tralicstex{\textquotesingle}{$'$} E), en Inde, au cours du mois d'octobre 2023. Les performances du
TSS ont \'{e}t\'{e} \'{e}valu\'{e}es sur la base de param\`{e}tres
\'{e}nerg\'{e}tiques, exerg\'{e}tiques, \'{e}conomiques et
environnementaux. Les r\'{e}sultats exp\'{e}rimentaux ont montr\'{e}
que le rendement journalier du TSS-S a \'{e}t\'{e} \'{e}valu\'{e} \`{a}
3,12 L/m\textsuperscript{2}, soit 81,40\% de plus que
celui du TSS. En outre, en comparaison avec le TSS, ce rendement
journalier a augment\'{e} d'environ 34,86\% et de 28,23\% pour le
TSS-B et le TSS-NB, respectivement. Par ailleurs, par rapport au TSS
l'incorporation des SHSMs a conduit \`{a} des am\'{e}liorations
significatives de l'efficacit\'{e} \'{e}nerg\'{e}tique (de 19,15\%
\`{a} 75,12\%) et exerg\'{e}tique (de 7,41
\% \`{a} 36,44\%), le consolidant comme un choix viable pour la
production d'eau douce. L'\'{e}valuation \'{e}conomique a
r\'{e}v\'{e}l\'{e} que par rapport \`{a} TSS,
l'utilisation de SHSMs (configuration TSS-S) a permis de
r\'{e}duire le co\^{u}t de la production du litre d'eau douce
jusqu'\`{a} 42,86\% et une diminution de la p\'{e}riode de retour sur
investissement de 44,33\%. En outre, la durabilit\'{e} exerg\'{e}tique
du TSS a \'{e}t\'{e} explor\'{e}e en utilisant les param\`{e}tres du
potentiel d'am\'{e}lioration et de l'indice de durabilit\'{e}. De plus,
les valeurs de teneur en sels dissous et de pH de l'eau obtenue \`{a}
partir du TSS, avec et sans SHSMs, sont conformes aux normes de la
qualit\'{e} des eaux.
\end{altabstract}

\maketitle

\vspace*{5pt}

\twocolumngrid

\end{noXML}

\section{Introduction}\label{sec1}
\vspace*{-4pt}

Every living organism, including humans, plants, and animals, relies on
water for its existence. Mother Earth's most abundant resource is
water, which can be found in  oceans, glaciers, and groundwater.
However, the water available in these sources is not in  pure form;
it contains some impurities, such as salt and pollutants. The amount of
fresh water suitable for human consumption is less than 1\%. Apart from
drinking, there are many other uses for water in fields including agriculture
and industry~\cite{1}. 

The current rise in population, rapid industrialization, and changing
human lifestyles contribute to an intensified demand for fresh water,
upsetting the natural balance of water supply. A report by UNICEF and the
World Health Organization (WHO) states that in the twentieth century, 
more than 2.2 billion people
lack access to safe drinking water worldwide~\cite{1}.

Several methods are used for desalination, for instance, membrane
distillation~\cite{2}, reverse osmosis process, multi-stage flash,
electrolysis, adsorbents for the treatment of water~\cite{3,4}, and
dehumidification and humidification~\cite{5}. Desalination processes
currently rely on fossil fuels or electric energy, contributing to
global warming in the long run. Since India enjoys a tropical climate,
with approximately 250 to 330 sunny days each year, solar
energy finds versatile applications such as electricity
generation~\cite{6}, water purification~\cite{7}, air and water
heating~\cite{8}, and cooking and drying~\cite{9}.

Solar desalination stands out as the optimal approach for freshwater
production due to its simple design, utilization of eco-friendly
energy, and suitability for remote locations~\cite{10}. However, solar
stills (SSs) continue to demonstrate a limited level of productivity.
Numerous studies have been published to augment the productivity of SSs
by modifying their basic designs, including single slope, double slope,
multi-effect, drum SS, and hemispherical SS~\cite{11, 12}. In a recent
systematic review, Bait~\cite{13} compared triangular pyramids,
weir-like structures, spherical designs, and hemispherical SSs. The
study focused on design modifications, heat transport fluids, sensible
heat storage materials (SHSMs), collector integration, and cooling
processes. It was concluded that hemispherical, weir-like, and
pyramidal SSs outperformed other types.

Younis et~al.~\cite{14} explored recent developments in SSs, including
single-slope, double-slope, condenser-based, and hybrid designs. Key
findings revealed that incorporating materials such as charcoal,
matting, sponges, dyes, wicks, porous or energy-storing materials,
black rubber, and floating absorber sheets improved basin absorption.
Additionally, using inclined external flat-plate reflectors, combined
stills, condensers, and solar collectors significantly enhanced
productivity while minimizing heat loss.

{In an experimental study by Ramzt et~al.~\cite{15}, natural and
artificial materials, like black loofah, regular loofah, fine steel
wool, and steel wool pads (SWPs), were used to enhance SS performance and
productivity. Steel wool pads achieved the highest yield of 
4.384~L/m$^{2}$ and the highest thermal efficiency at 32.74\%. The cost 
per liter (CPL) was lowest with SWPs at 0.0034 \dollar/L/m$^{2}$.
Exergo-economic analysis indicated that SWPs were the most
promising modification for improving SS performance.

In a research endeavor, Deshmukh and Thombre~\cite{16} conducted
experiments involving a single-slope SS employing sand and Servotherm
medium oil working as SHSMs. Their findings indicated that a reduced
depth of the SHSM resulted in the highest distilled water production
when compared to traditional stills. Interestingly, the productivity
continued to rise due to SHSM even during nighttime.

Abdelgaleel et~al.~\cite{17} explored the productivity of SSs using
iron hemispherical pieces (IHPs) and SWPs. The
findings indicated that productivity increases by 35\% when there are
120 IHPs. Moreover, a substantial 28\% increase in productivity was
achieved with the addition of SWPs.


Kumar et~al.~\cite{18} examined the performance of an integrated 
triangular solar still (TSS) at
various water depths. The study's observations indicated that the TSS
achieved maximum yield at 0.02~m water depth. The integration of the
inclined SS resulted in an increased inlet water temperature, leading
to the production of maximum distilled water (7.52~kg/m$^{2}$). In
their research on a double-basin SS, Modi et~al.~\cite{19} explored the
correlation among water depth, evaporation rate, and freshwater yield.
The authors noted that the daily distilled production was higher for
lower water depths, specifically 2.02 and 1.79~L/m$^{2}$ with water
depths of 0.01 m and 0.04 m, respectively, in the lower basin. 

{Furthermore, Kabeel et~al.~\cite{20} studied the impact of water depth
(0.02--0.06~m) on the freshwater yield of standalone conventional,
inclined, and integrated conventional and inclined SSs. The
findings revealed a significant 46.23\% increase in yield with the
integrated SS. Moreover, it was noted that the maximum accumulated
distilled water was observed when the water depth was maintained at
0.02~m. In a related study, Manokar et~al.~\cite{21} explored the impact
of water depth and insulation on pyramidal acrylic SSs. The daily yield
of the SS experienced a 28\% \mbox{enhancement} at a water\break depth of 0.01~m.
}

Physical parameters play a pivotal role in augmenting the productivity
of SSs. Studies by Azooz and Younis~\cite{22} and Kabeel
et~al.~\cite{23} investigated the effect of  glass cover inclination
on the distillate output of single-slope and pyramidal SSs,
respectively. Both studies demonstrated that higher inclinations
resulted in lower productivity.

Feilizadeh et~al.~\cite{24} inspected the impact of dimensional
parameters on the effectiveness of a single-slope, single-basin SS. The
authors revealed that higher specific height resulted in lower
productivity, with the optimal height for the still being around 0.10~m.
Additionally, maintaining a width-to-length ratio of about 0.4 led to
maximum productivity. In another study conducted by Jamil and
Akhtar~\cite{25} in an indoor setting, the impact of specific height
(varying from 0.266 to 0.341~m in increment of 0.025~m) on a
conventional SS was investigated. Their results revealed a noteworthy
50\% increase in productivity when the specific height was reduced from
0.341 m to 0.266~m.\looseness=-1

Recently, Bait et~al.~\cite{26} developed a new design of SS that embeds
a simple distiller into a cylindrical solar collector shape. This
collector raises the basin water temperature by 31\% and 40\% for
passive and active solar stills, respectively, resulting in increased
yield and energy efficiency.

Furthermore, modifying the absorbing area represents another approach
to increase the effectiveness of SSs. Appadurai and
Velmurugan~\cite{27} scrutinized the impact of using fin-type solar
ponds on the performance of SS. Their observations revealed that the
use of fins increased the area available for heat transfer, causing a
substantial 45.5\% increase in daily distilled water production.
Additionally, their study on fins in a solar pond showed a
productivity increase of approximately 27.6\%. In an analogous manner,
El-Naggar et~al.~\cite{28} observed the effect of fins on SSs and noted
a significant 55.37\% increase in daily distilled water output.
Moreover, Sharma et~al.~\cite{29} inspected the enhancement of
freshwater production in pyramid-shaped SSs by incorporating copper
tube fins spaced at 9 cm and 14~cm. The study indicated a 60\%
improvement in yield with fins at a spacing of 9 cm and a 33\%
improvement at 14~cm compared to an SS  without fins. Tuly
et~al.~\cite{30} considered the combined influence of phase change
material (PCM), nanoparticles, and fins in a double-slope SS. Their
findings indicated a substantial increase in productivity as well as
energy and exergy efficiencies. Specifically, the modified experimental
setup demonstrated a noteworthy 20.1\% and 25\% increase in energy and
exergy efficiencies, respectively. In a recent study, Panchal
et~al.~\cite{31} made improvements to the single-slope SS by
incorporating perforated fins, pebbles, and an evacuated tube solar
collector. Experimental results demonstrated remarkable improvements:
productivity increased by 244.1\%, thermal efficiency rose by 2.32\%,
and the CPL decreased by 10\%. Furthermore, the modified SS
achieved nearly 2.44 times the CO$_{2}$ mitigation value than the
standard configuration.

Employing porous and heat storage materials represents an alternative
approach to augment SS performance. Kabeel and Abdelgaied~\cite{32}
incorporated PCM to augment the distillate output of a single-basin SS,
reporting a substantial improvement. The daily yield increased by
67.18\%. Shalaby et~al.~\cite{33} introduced a v-corrugated absorber SS
incorporating PCM and explored the impact of providing a wick material
over the absorber plate. By employing PCM, the daily performance showed
a notable 12\% improvement compared to the v-corrugated basin alone.
The authors noted a slight decrease in daytime productivity when using
PCM, coupled with a significant escalation in overnight yield.
Specifically, the diurnal yield of the SS with PCM was reported to be
11.7\% higher than that of the v-corrugated still with PCM and wick
material. 

Abdel-Aziz et~al.~\cite{34} aimed to enhance the effectiveness of an SS
by using paraffin wax as a PCM and a solar-powered electric heater. The
experimental findings indicated that activating the heater within the
PCM resulted in a daily yield increase of 2.38, 2.66, and 3.1 times
during spring and 2.2, 2.39, and 2.67 times during summer, compared to
the conventional SS. The highest daily production occurred in both
seasons (spring and summer) when the temperature of the paraffin wax
was maintained at 65~\textdegree C.

Additionally, Shoeibi et~al.~\cite{35} aimed to improve the operational
effectiveness of solar desalination systems by using mirrors, waste
thermoelectric generators, and black iron fragments as SHSMs. During
the study, two thermoelectric generators were attached to the basin to
harness electrical energy within the absorber plate. The results
indicated that conventional SS, 
SS-TEG, SS-TEG-WI, and SS-TEG-WI-M produced
fresh water at rates of 0.64~L/m$^{2}$, 0.63~L/m$^{2}$, 0.652~L/m$^{2}$,
and 0.796~L/m$^{2}$, respectively.\looseness=-1
}

In an experimental investigation, Hemmatian et~al.~\cite{36} used
pulsating heat pipes, evacuated solar collectors, and PCM to enhance
yield during low solar irradiation. The system, which employs paraffin
wax as the PCM, produces 2248~mL/m$^{2}$, resulting in a 40\% increase
in daily freshwater production compared to 
conventional SS. Additionally, the
system reduces CO$_{2}$, SO$_{2}$, and NO emissions and exhibits energy
efficiencies of 19.4\% for solar stills using heat pipes (SSHP) and
20.3\% for solar stills employing pulsating heat pipes (SSPHP). The
exergy--economic factors for conventional SS, SSHP, and SSPHP are 
1.27 kWh/\dollar, 1.56~kWh/\dollar, and 1.62~kWh/\dollar, respectively.
\looseness=-1

Shoeibi et~al.~\cite{37} tested a modified single-slope SS
integrated with a photovoltaic panel, heat pipes, and a thermoelectric
generator to improve performance and efficiency. The daily water
productivity values for conventional SS, solar still with a wick structure (SS-WT), solar
still with heat pipes (SSHP), and solar still with heat pipes and a
wick structure were approximately 0.748~L/m$^{2}$, 
0.832~L/m$^{2}$, 1.058~L/m$^{2}$, and 1.162~L/m$^{2}$, respectively. Omidi
et~al.~\cite{38} investigated a hybrid system that mitigates pollution
by utilizing a parabolic-trough heat pipe solar collector. The study
found that water flow rate, length of the porous medium, and the number
of heat pipes significantly affect energy efficiency. The system
achieved its highest recorded daily output of 4.940~L/day.

Furthermore, Shanmugan et~al.~\cite{39} examined the theoretical and
practical performance of SS by employing wick materials in conjunction
with a PCM and nanoparticles. The findings indicated that the yields of
wick materials in an SS with PCM and nanoparticles were 7.460~L/m$^{2}$
and 4.120~L/m$^{2}$, respectively. In a study by El et al~\cite{40},
the traditional hemispherical solar still was investigated with
a TiO$_{2}$ nanocoated tank and glass cooling. The daily energy
efficiency values for a coated basin at concentrations of 0.1\%, 0.2\%, and
0.3\% with glass cooling are 40.34\%, 46\%, and 51\%, respectively.
Shoeibi et~al.~\cite{41} investigated the effectiveness of SSs that
utilize Al$_{2}$O$_{3}$ nanofluid film cooling to enhance
yield. The results indicated that double-slope SSs with nanofluid film
cooling increased yield by 4.8\% compared to tubular desalination.
Dual-slope solar desalination with nanofluid film cooling provides 
CPL at 0.0362 \dollar/L.

Kabeel et~al.~\cite{42} used jute cloth wrapped around sand for heat
storage. Their findings showed a 25\% improvement in efficiency,
accompanied by a freshwater yield of 5.9~L/m$^{2}$ when utilizing jute
cloth. Pal et~al.~\cite{43} created a thermal model for a modified
multi-wick double-slope SS. The researchers observed peak values for
instantaneous energy and exergy efficiencies, reaching 35\% and 3.83\%
at 5:00~p.m., respectively. Additionally, the annual reduction in
CO$_{2}$ emissions was determined to be 7.82~tons with jute wicks and
8.69 tons with black cotton wicks, at 0.01\,m water depth. Yousef
et~al.~\cite{44} explored the energy, exergy, economic and
\mbox{enviroeconomic} aspects of single-slope SS using various absorbing
materials. Their findings revealed that the SS with steel wool fibers
exhibited a 25\% increase in  daily water yield, while the SS with
hollow cylindrical pin fins showed a 16\% enhancement.

Manokar et~al.~\cite{45} conducted an experimental investigation on a
modified single-slope SS incorporating base heating and glass cooling.
Experimental findings revealed notable improvements, with a 24.03\%
increase in yield, a 25\% enhancement in energy efficiency, and an
18.9\% boost in exergy efficiencies when employing glass cooling.
Additionally, the application of the base heating method resulted in
even more substantial improvements, with a 50.11\% increase in yield,
along with 50.66\% and 59.61\% enhancements in energy and exergy
efficiencies, respectively.

Elashmawy~\cite{46} examined the influence of small-sized gravel (3
kg) as SHSMs on tubular SSs. The study demonstrated a significant
improvement in efficiency and daily productivity, with increases of
36.34\% and 4.51~L/m$^{2}$, respectively, against the scenario
without gravel. Furthermore, Kabeel et~al.~\cite{47} delved into the
performance of tubular SSs featuring v-corrugated basin plates with
wick material. The findings showcased a substantial daily average
efficiency boost of 46.86\% with the incorporation of wick material.
Shoeibi et~al.~\cite{48} examined the use of porous materials in solar
water desalination systems and reported that activated carbon increases
energy efficiency by 94.14\%, whereas black steel wool fibers and
aluminum fins increase output by 42.3\% and 20.9\%, respectively.

In a separate investigation, El et~al.~\cite{49} explored productivity
enhancements in a traditional SS by \mbox{incorporating} sandbags as solar
radiation absorbers, functioning as SHSMs. The results indicated a
noteworthy productivity increase of approximately 34.57\%.
Dhivagar~\cite{50} evaluated the influence of magnetic powder as an
SHSM. Specifically, black iron oxide magnetic powder was employed to
augment the productivity of SS. The outcomes revealed a remarkable 32\%
enhancement in the effectiveness of the magnetic-powder-equipped SS,
with significant enhancements observed in both exergy and energy.

Mevada et~al.~\cite{51} conducted experimental research on the impact of
SHSM on performance, along with an analysis of energy and exergy
efficiency. The incorporation of SHSM increased the rate of distillate
water output. The energy and exergy efficiency were measured as 43.29\%
and 12.55\%, respectively, surpassing the values achieved by
conventional SSs. In an experimental study, Bait~\cite{52} compared the
performance of conventional and modified SSs and showed that the annual
yield for the modified SS is approximately 405.04~kg/m$^{2}$, while
the conventional SS yields around 549.77~kg/m$^{2}$. The payback
period for the conventional SS was approximately 7.7~years,
whereas the modified SS had a payback period of approximately
21 years.

Shoeibi et~al.~\cite{53} explored a novel configuration for solar
desalination that incorporated thermoelectric elements on both the cold
and hot sides. During the experiment, ambient air flowed through a
cooling tank and a water heater block connected to the hot side. The
results revealed significant improvements in freshwater yield (79.4\%),
energy efficiency (11.2\%), and exergy efficiency (45.7\%).

For the purpose of improving freshwater yield and enhancing
environmental sustainability, Dhivagar et~al.~\cite{54} examined mussel
shell biomaterial as a porous medium and SHSM in a SS. Mussel
shells absorb solar radiation and store solar thermal energy, raising
the water's temperature. It was found that 
SS with mussel shell biomaterial had superior energy
efficiency (10.3\%) and exergy efficiency (9\%), and reduction in CO$_{2}$
emissions (11.1\%) and CPL (10.9\%) to conventional SS.

Recently, Mahala and Sharma~\cite{55} proposed an innovative design
of pyramid SS with HSMs that results in significant improvement of
daily yield (84\%), energy efficiency (81.1\%), and carbon credits earned
(76\%) along with substantial reduction in CPL of yield
produced (29.2\%) and payback period (29\%) when compared with
conventional SS.

\begin{figure*}
\vspace*{-1pt}
\includegraphics{fig01}
\vspace*{-2pt}
\caption{\label{fig1}Photograph and schematic of the experimental test rig.}
\end{figure*}

\begin{figure*}
\includegraphics{fig02}
\vspace*{-2pt}
\caption{\label{fig2}(a) Schematic and (b) images of studied
configurations.}
\vspace*{-2pt}
\end{figure*}


The pertinent literature indicates that SSs exhibit low
productivity, making their commercialization challenging in the
domestic sector. Consequently, various active and passive techniques
have been \mbox{employed} to enhance the yield produced and energy efficiency
of SSs. Despite extensive literature, comparative studies on TSS using
bricks and wicking materials as SHSMs
are lacking. Additionally, the use of sand as a bed is not adequately
covered. Taking a step forward, this study explores the use of SHSMs
to increase the performance of TSS. The goal is to ensure a sustainable
supply of fresh water even during periods without direct sunlight.
Typically, SHSMs accumulate thermal energy during peak sunshine hours
and gradually release it when solar intensity decreases. This research
investigates and compares the effectiveness of TSS with different SHSM
configurations, including the standard TSS, TSS with bricks (TSS-B),
TSS with paper napkins wrapped around the brick (TSS-NB), and TSS with
a sand bed (TSS-S). The comparison considers performance parameters,
such as freshwater yield, energy efficiency, exergy efficiency, and
economic factors. Additionally, the key metrics like
``improvement potential'' and ``sustainability
index'' are employed to assess the exergetic sustainability of each
configuration. Furthermore, the study involves measuring and comparing
total dissolved solids (TDS) and pH levels in the raw water and the
produced yield. These findings enhance the existing benchmark dataset
for SS systems.


\section{Experimental details}\label{sec2}

The present experimental facility, depicted in \mbox{Figure~\ref{fig1}}, was
developed at NSUT West Campus, New Delhi. The experiments were carried
out between 8:00 a.m.\ and 7:00 p.m.\  during October 2023. The test rig
comprises a triangular wooden basin with an internal absorbing area of
0.43~m$^{2}$. Each side of the basin, with a height of 0.3 m, is
covered with galvanized iron sheet, acting as an absorber plate, with
black coating (absorptivity of 0.95) to augment the absorption of solar
radiation. The TSS basin is insulated from the sides and bottom by
using a 20 mm wooden box for decreasing the heat loss to the 
surroundings. In the basin, 7~L of saline water is filled. A water
level indicator is used to ensure a constant amount of water. A
drainage valve at the bottom facilitates the removal of contaminated
water and cleaning. A 3 mm thick triangular pyramidal glass cover
serves as a condensing cover. 


Solar radiation penetrates the glass cover, heating the basin water and
generating water vapor. These vapors condense on the inner side of
the glass cover due to its lower temperature. To collect the yield
output, a calibrated flask is utilized. 

\begin{table*}
\caption{\label{tab1}Thermophysical properties of the SHSMs}
%\tabcolsep=2pt
\begin{tabular}{lcccc}
\thead
Material  & Density (kg/m$^3$)  & 
\parbox[t]{2cm}{\centering Specific heat (J/(kg${\cdot}$K))}  & 
\parbox[t]{3.5cm}{\centering Thermal conductivity (W/(m${\cdot}$K))} & 
\parbox[t]{2cm}{\centering Emissivity coefficients}\vspace*{2pt} \\
\endthead
Bricks~\cite{56} & 1500--1800  & 840  & 0.9--1.2  & 0.74--0.93  \\
Sand~\cite{57} & 1600--1700 & 932--958 & 0.2--0.7~ & 0.7--0.9\0 \\
Paper napkin~\cite{58} & 0.62--1.26 & 1.52 & 0.08--0.18 & 0.6--0.93 
\botline
\end{tabular}
\end{table*}

The study investigates the performance of four distinct configurations
with and without heat storage, namely TSS, TSS-B, TSS-NB, and TSS-S,
illustrated in Figure~\ref{fig2}. Coarse sand and bricks are used as
heat storage materials. Given the critical role of SHSMs in  
SS productivity, a constant mass of approximately 15 kg was
maintained throughout the experiment. Due to its high thermal mass,
SHSM absorbs and retains heat during the day and radiates it out at
night. Productivity comparison is made only for days with almost
similar solar \mbox{radiation} intensity and at the constant amount of raw
water. Table~\ref{tab1} presents the thermophysical properties\break of the
SHSMs. 



Solar radiation is quantified using a solar power meter with an
accuracy of ${\pm}$5. K-type thermocouples are placed at different
positions to monitor temperatures including the basin, water, vapor,
condenser glass, and ambient. Hourly data recording is performed
manually using a digital temperature indicator. Hourly
measurements of the produced fresh water are made with a calibrated
flask. Additionally, pH and TDS meters are utilized to assess the
quality of both saline and purified water. Table~\ref{tab2} outlines
the precision, range, and typical uncertainty of the measurements
$(\text{uncertainty}=\text{precision}/\sqrt{3})$~\cite{55}.


\begin{table}
\caption{\label{tab2}Instruments with their uncertainty}
\tabcolsep=2.5pt
\begin{tabular}{ccc}
\thead
Tools & Range ${\pm}$ precision & Error \\
\endthead
\parbox[t]{3cm}{\raggedright\hangindent1pc Solar power meter (LX-107)$^{\mathrm{a}}$} & 0${-}$2000 ${\pm}$ 5 & \0${\pm}$2.88\0\0 \\
Collecting flask$^{\mathrm{b}}$ & 0${-}$1000 ${\pm}$ 50 & ${\pm}$28.87\0\0 \\
%\parbox[t]{2cm}{\raggedright\hangindent1pc 
Temperature display$^{\mathrm{c}}$
 & 0--1200 ${\pm}$ 1 & \0${\pm}$0.58\0\0 \\
Thermocouple$^{\mathrm{d}}$ & 0${-}$1200 ${\pm}$ 1 & \0${\pm}$0.58\0\0 \\
pH Meter & 0--14 ${\pm}$ 0.01 & \0${\pm}$0.0058 \\
TDS meter$^{\mathrm{e}}$ & 0 to 2000 ${\pm}$ 2\% & \0${\pm}$1.155\0
\botline
\end{tabular}
\tabnote{$^{\mathrm{a}}$ Solar radiation measured in W/m$^{2}$.}
\tabnote{$^{\mathrm{b}}$ Distillate output measured in mL.}
\tabnote{$^{\mathrm{c}}$ Temperature displayed in \textdegree C.}
\tabnote{$^{\mathrm{d}}$ Temperature measured in \textdegree C.}
\tabnote{$^{\mathrm{e}}$ TDS measured in ppm.}
\end{table}


Following the procedure adopted by Mahala and Sharma~\cite{59}, the
uncertainties in hourly yield and overall daily yield were determined
to be ${\sim}$1.5\% and ${\sim}$2.1\%, respectively. Additionally,
considering the uncertainties in solar radiation measurement, daily
yield, and temperature measurement, there was an associated error of
approximately ${\pm}$2.8\% in energy efficiency and ${\pm}$3.2\% in
exergy efficiency.

\section{Data reduction}\label{sec3}

\subsection{Energy analysis}\label{sec31}

The hourly basis thermal energy efficiency of the TSS is obtained
by~\cite{59}  
{\begin{eqnarray}
\eta _{\mathrm{en}}&=&\frac{\sum m_{\mathrm{w}}\times h_{\mathrm{fg}}}{A_{\mathrm{t}}\int I_{\mathrm{S}}
(t)\mathrm{d}t}\times 100\label{eq1} \Seqnsplit
h_{\mathrm{fg}}&=&2.4935\times 10^{6} \mathop{[}1-9.478 \times 10^{-4}\,  T_{\mathrm{w}}\nonumber\\
&&\quad +\,1.31\times 10^{-7}\,{T}_{\mathrm{w}}^{2}-4.79
\times 10^{-9}\,{T}_{\mathrm{w}}^{3}\mathop{]}\label{eq2}
\end{eqnarray}}\unskip

Here, $m_{\mathrm{w}}$ represents the distillate output (L/m$^{2}$), $I_{\mathrm{S}}$
denotes solar irradiation (W/m$^{2}$), $A_{\mathrm{t}}$ refers to the TSS area
(m$^{2}$), and $h_{\mathrm{fg}}$ indicates latent heat of evaporation in the case of
water (J/kg).

\subsection{Exergy analysis}\label{sec32}

The exergy efficiency for TSS is determined by~\cite{60} 
{\begin{equation}\label{eq3}
\eta _{\mathrm{ex}}=\frac{E_{x,\mathrm{out}}}{E_{x,\mathrm{in}}}
\end{equation}}\unskip
The output exergy and the exergy input of the TSS can be found from
Equations (\ref{eq4}) and (\ref{eq5}), respectively: 
{\begin{eqnarray}
\sum E_{x,\mathrm{in}}&=&I_{\mathrm{S}}\times A_{\mathrm{t}}\left[1-\frac{4}{3}
\left(\frac{T_{\mathrm{amb}}}{T_{\mathrm{sun}}}\right)+
\frac{1}{3}\left(\frac{T_{\mathrm{amb}}}{T_{\mathrm{sun}}}\right)^{4}\right]\label{eq4} \Seqnsplit
\sum E_{x,\mathrm{out}}&=&m_{\mathrm{w}}\times h_{\mathrm{fg}}\times 
\left[1-\left(\frac{T_{\mathrm{amb}}}{T_{\mathrm{w}}}\right)\right] \label{eq5}
\end{eqnarray}}\unskip
where $T_{\mathrm{sun}}$ is the sun temperature (6000 K) and
$T_{\mathrm{amb}}$ is the ambient temperature~\cite{37}.

\subsection{Economic analysis}\label{sec33}

The cost breakdown for each component of TSS across all studied
configurations, as depicted in Figure~\ref{fig2}, is detailed in
Table~\ref{tab3}. The economic analysis parameters and equations are
outlined below~\cite{59}.

Let $C_{\mathrm{in}}$ represent the initial cost spent in the construction of
TSS, with an annual interest rate ($i$) of 8\%, and $n$ denote the
lifespan of TSS ($n=$ 15 years). The capital recovery factor and annual
fixed cost ($C_{\mathrm{AFC}}$) are computed as follows:
{\begin{eqnarray}
&&\text{Capital recovery factor }=\frac
{i(1+i)^{n}}{(1+i)^{n}-1}\label{eq6}\Seqnsplit
&&C_{\mathrm{AFC}}=C_{\mathrm{in}}\times \text{Capital recovery factor}\label{eq7}
\end{eqnarray}}\unskip

Annual operational and maintenance cost $(C_{\mathrm{AOMC}})$, encompassing
expenses for regular cleaning, feed water filling, freshwater
collection, and scaling removal due to salt deposition inside the TSS,
is set at 15\% of $C_{\mathrm{AFC}}$:
{\begin{equation}
C_{\mathrm{AOMC}}=0.15\,(C_{\mathrm{AFC}})\label{eq8}
\end{equation}}\unskip
Consequently, the annual salvage cost ($C_{\mathrm{ASV}}$) is determined as
{\begin{equation}
C_{\mathrm{ASV}}=0.20\,C_{\mathrm{in}}\times \frac{i}{(1+i)^{n}-1}\label{eq9}
\end{equation}}\unskip
The total annual cost $(C_{\mathrm{TAC}})$ is computed by
{\begin{equation}
C_{\mathrm{TAC}}=C_{\mathrm{AFC}}+C_{\mathrm{AOMC}}-C_{\mathrm{ASV}}\label{eq10}
\end{equation}}\unskip
Annual freshwater yield ($M_{\mathrm{a}}$) in~L/m$^{2}$ is evaluated using
{\begin{equation}
M_{\mathrm{a}}=m_{\mathrm{w}}\times N_{\mathrm{d}}\label{eq11}
\end{equation}}\unskip
where $m_{\mathrm{w}}$ denotes the mean daily freshwater yield per unit area
and $N_{\mathrm{d}}$ indicates the number of sunny days (330 days for New
Delhi).

``Cost per liter'' of yield generated $(C_{\mathrm{CPL}})$ is estimated as
{\begin{equation}
C_{\mathrm{CPL}}=\frac{C_{\mathrm{TAC}}}{M_{\mathrm{a}}}\label{eq12}
\end{equation}}\unskip
Annual market cost of yield ($C_{\mathrm{AMA}}$) is calculated as 
{\begin{equation}
C_{\mathrm{AMA}}=M_{\mathrm{a}}\times \text{Market cost of water per liter }\label{eq13}
\end{equation}}\unskip
Net annual earnings $(C_{\mathrm{AE}})$ is determined by
{\begin{equation}
C_{\mathrm{AE}}=C_{\mathrm{AMA}}-C_{\mathrm{AOMC}}\label{eq14}
\end{equation}}\unskip
The payback period ($P_{\mathrm{p}}$), in days, for the TSS is calculated by 
{\begin{equation}
P_{\mathrm{p}}=\frac{C_{\mathrm{in}}}{C_{\mathrm{AE}}}\times 365\label{eq15}
\end{equation}}\unskip

\begin{table}
\caption{\label{tab3}Fabrication cost of each TSS (\dollar)\vspace*{3pt}}
\tabcolsep3.5pt\fontsize{9.8}{12}\selectfont
\begin{tabular}{ccccc}
\thead
Component of TSS & TSS & TSS-B & TSS-NB & TSS-S\vspace*{2pt} \\
\endthead
Glass cover & 17.96 & 17.96 & 17.96 & 17.96 \\
Absorber plate (GI) & 20.96 & 20.96 & 20.96 & 20.96 \\
Wooden frame & 26.35 & 26.35 & 26.35 & 26.35 \\
Collecting jar & \02.99 & \02.99 & \02.99 & \02.99 \\
{Aluminum frame} & \02.99 & \02.99 & \02.99 & \02.99 \\
Black paint & \02.99 & \02.99 & \02.99 & \02.99 \\
PVC pipe & \01.80 & \01.80 & \01.80 & \01.80 \\
Insulation  & \01.80 & \01.80 & \01.80 & \01.80 \\
\multicolumn{5}{l}{Heat storage material}\\
\quad Sand  & -- & -- &  -- &\01.79 \\
\quad Napkins  &  -- & -- & \00.23 & -- \\
\quad Bricks & -- & \02.99 &\02.99 & --\\  
{Total cost (\dollar)} & 77.84 & 80.84 & 81.08 & 79.64
\botline
\end{tabular}
\vspace*{7pt}
\end{table}

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}Ambient temperature and solar intensity variation
for different configurations.}
\vspace*{-3pt}
\end{figure*}


\subsection{Sustainability index}  
The efficiency with which a system utilizes its resources is called
sustainability index (SI). The following formula is used to determine the
sustainability index~\cite{61}.
{\begin{equation}
\mathrm{SI}=\frac{1}{1-\eta _{\mathrm{ex}}}\label{eq16}
\end{equation}}\unskip

\subsection{Improvement potential}  
Reducing irreversibility within a system contributes to an enhancement
in exergy efficiency. The subsequent equation is employed to calculate
the improvement potential (IP)~\cite{62}.
{\begin{equation}
\mathrm{IP}=(1-\eta _{\mathrm{ex}})^{2}\times E_{x,\mathrm{in}}\label{eq17}
\end{equation}}\unskip

\section{Results and discussion}\label{sec4}

Experiments for different configurations---TSS, TSS-B, TSS-NB, and
TSS-S---are carried out between 8:00~a.m.\ and 7:00 p.m. Experimental data
related with temperatures at strategic locations, along with
measurements of solar radiation intensity and distillate yields on an
hourly basis, are recorded manually. \mbox{Figure~\ref{fig3}} illustrates hourly
variation in solar intensity and ambient temperature for the studied
configurations in October 2023. As expected, solar radiation gradually
increases during the forenoon hours, reaching its peak value at 12:00
p.m., and gradually decreases in the afternoon, reaching its lowest point
at sunset around 6:00 p.m. Figure~\ref{fig3} also shows that the ambient
temperature follows a similar trend, rising concurrently with solar
radiation in the forenoon and declining in the afternoon. The average
solar intensity and ambient temperature measured are approximately 
285~W/m$^{2}$ and 36~\textdegree C, respectively. 

Notably, the variabilities in solar irradiance and ambient temperature
during the experimental days fall within ${\pm}$2.5\%. The average
wind speeds for the four days were 9.85 km/h, 8.45 km/h, 9.27 km/h, and
8.96 km/h, indicating that the fluctuations during the experimental
days remained within ${\pm}$5\%. Consequently, the obtained recorded
data for the four configurations can be equated to assess the
effectiveness of SHSMs in enhancing the performance of \unskip\break the TSS.

\begin{figure*}
\includegraphics{fig04}
\caption{\label{fig4}Hourly temperature variations within a solar still
for different configurations.}
\vspace*{4pt}
\end{figure*}

Figure~\ref{fig4} illustrates the hourly variation in temperature
associated with the studied configurations. For TSS, the peak
temperatures observed are 65~\textdegree C for water, 66~\textdegree C
for the basin, and 58~\textdegree C for the glass. The average
temperature dissimilarity between water and glass is 3~\textdegree C,
creating the necessary conditions for vapor condensation over the
inner glass surface. Due to the absence of SHSMs, water and basin
temperatures rise rapidly in the forenoon and decline rapidly in the
afternoon for TSS. In the case of TSS-B, the maximum temperatures
recorded are 58~\textdegree C for water, 62~\textdegree C for the
basin, and 57~\textdegree C for the glass. The use of bricks as SHSM
leads to lower temperatures compared to TSS during daytime, as the
bricks absorb a portion of solar radiation. In addition, the release of
the absorbed heat by the bricks in the evening hours results in a lower
temperature drop for TSS-B compared to TSS, leading to higher
productivity during the \unskip\break evening.

For TSS-NB, the highest temperatures are 58~\textdegree C for water,
59~\textdegree C for the basin, and 60~\textdegree C for the glass. The
presence of napkins wrapped over bricks reflects a considerable part of
solar intensity to the glass cover, increasing the glass temperature in
the forenoon. \mbox{Although} TSS-NB has a slightly higher glass temperature
compared to the other cases, it decreases as solar intensity declines
in the afternoon.

In the case of TSS-S, peak temperatures are 55~\textdegree C for water,
60~\textdegree C for the basin, and 54~\textdegree C for the glass.
TSS-S, with the highest mass of SHSM, absorbs a significant portion of
solar intensity initially and radiates it during the night.
Consequently, all temperatures in TSS-S remain lower compared to TSS.

\begin{figure}
\includegraphics{fig05}
\caption{\label{fig5}Variation in yield for different configurations.}
\vspace*{-4pt}
\end{figure}

Figure~\ref{fig5} illustrates the hourly variation in distillate
productivity for the investigated configurations. Typically, the hourly
yield from the SS is primarily \mbox{influenced} by solar intensity
and the temperature difference between water and glass.
Figure~\ref{fig5} shows that the yields of all examined cases closely
align in the beginning. As time advances, the yield production rate
diminishes during the forenoon hours due to the introduction of thermal
mass, specifically SHSMs. However, post 3:00 p.m., there is a shift in
productivity trends, with TSS-S exhibiting a higher yield. The solar
energy absorbed by bricks and sand in the early part of the day
elevates water temperature after 3:00~p.m., as depicted in
Figure~\ref{fig4}, consequently enhancing rates of evaporation and
condensation. 

{
Figure~\ref{fig6} depicts the variation in daily productivity across
all four configurations. During the daytime, TSS generates the highest
quantity of yield, specifically 1.56~L/m$^{2}$, followed by TSS-B with
1.51~L/m$^{2}$, TSS-NB with 1.32~L/m$^{2}$, and TSS-S with 
1.18~L/m$^{2}$. The rise in thermal mass due to SHSMs is linked to the
reduction in yield production rates in the daytime. This is attributed
to the increased absorption of solar energy by SHSMs. Evidently, the
yield output for TSS-NB is about 14\% lower than that of TSS-B owing to
higher glass cover temperature as can be seen in Figure~\ref{fig4}.\looseness=-1

\begin{figure}
\includegraphics{fig06}
{\vspace*{-2pt}}
\caption{\label{fig6}Cumulative yield comparison.}
{\vspace*{-2pt}}
\end{figure}

During the night, TSS-S exhibits the highest productivity of 
1.95~L/m$^{2}$, while TSS-NB, TSS-B, and TSS demonstrate productivities
of 0.97~L/m$^{2}$, 0.58~L/m$^{2}$, and 0.15~L/m$^{2}$, respectively.
The utilization of SHSMs results in a notable improvement in nighttime
productivity, ranging from 287\% to 1200\%.

A more in-depth examination of Figure~\ref{fig6} indicates that TSS-S
takes the lead by providing the highest overall yield, reaching
3.12~L/m$^{2}$, followed by TSS-NB at 2.29~L/m$^{2}$, TSS-B at
2.08~L/m$^{2}$, and TSS at 1.72~L/m$^{2}$. Notably, TSS-S exhibits
yield levels approximately 81.40\%, 50\%, and 36.24\% higher than those
of TSS, TSS-B, and TSS-NB, respectively. The higher thermal mass of
TSS-S allows it to store more radiation and elevate the evaporation
rate throughout the night, resulting in maximum yield during nighttime
hours. Evidently, napkins serve as evaporating agents for TSS-NB,
thereby increasing the evaporation rate during the night, and as the
glass temperature decreases in the evening, the yield rises. Overall,
TSS-NB demonstrates a yield that is 33\% and 10\% higher than that of
TSS and TSS-B, respectively. 
}


\begin{figure}
\includegraphics{fig07}
\caption{\label{fig7}Comparison of energy and exergy efficiency.}
\end{figure}

Conclusively, SHSMs boost heat transfer rates during the evening,
elevating water temperatures and consequently maximizing freshwater
yield through increased evaporation rates. Incorporating SHSMs into TSS
(TSS-S) results in a notable 81.40\% enhancement in freshwater yield
compared to TSS, establishing it as a promising choice for sustainable
freshwater production.

The comparison of the overall efficiency for all four
configurations---TSS, TSS-B, TSS-NB, and TSS-S---is depicted in
Figure~\ref{fig7}. The efficiency of TSS is determined by the distilled
water output and the \mbox{absorbed} solar irradiation. Figure~\ref{fig7}
illustrates that the efficiency of TSS-S surpasses that of TSS, TSS-B,
and TSS-NB. This is likely attributed to the higher water temperature
and lower glass temperature, resulting in an elevated evaporation rate
during the night and consequently yielding maximum output. The
efficiencies for TSS, TSS-B, TSS-NB, and TSS-S are 11.54\%, 13.75\%,
14.97\%, and 20.21\%, respectively. Remarkably, the efficiency of TSS-S
is approximately 75.12\% higher than that of TSS. 

The exergy, as calculated from Equation~(\ref{eq3}), is influenced by
both ambient and basin water temperatures. Figure~\ref{fig7} reveals
that TSS-S exhibits the highest exergy efficiency at 1.46\%, marking 
36.44\%, 26.95\%, and 25.86\% increases compared to TSS, TSS-B, and
TSS-NB, respectively. The higher basin water temperature during the
night in TSS-S generates more exergy. TSS-B and TSS-NB have
relatively higher basin temperatures than TSS, thereby resulting in
higher exergy efficacy of approximately 7.41\% and 8.67\%,
\mbox{respectively}. 


Furthermore, the economic analysis assesses the viability of the TSS.
The pricing of each TSS component, influenced by its local market
value, plays a significant role in the overall investment.
Table~\ref{tab4} presents the cost analyses for all considered TSS
configurations. 

\begin{figure}
\includegraphics{fig08}
\caption{\label{fig8}Variation in cost per liter of water and payback
period for different configurations.}
\end{figure}

\begin{table}
\caption{\label{tab4}Cost estimation corresponding to individual
configurations\vspace*{2pt}}
\begin{tabular}{ccccc}
\thead
\parbox[t]{2cm}{\raggedright Economics comparison} & {TSS} & {TSS-B} & {TSS-NB} & {TSS-S}\vspace*{2pt} \\
\endthead
$C_{\mathrm{in}}$, in \dollar & 77.84 & 80.84 & 81.08 & 79.64 \\
$C_{\mathrm{AFC}}$, in \dollar & \09.03 & \09.38 & \09.40 & \09.23 \\
$C_{\mathrm{AOMC}}$, in \dollar & \01.35 & \01.40 & \01.41 & \01.38 \\
$C_{\mathrm{ASV}}$, in \dollar & 15.57 & 16.17 & 16.22 & 15.93 \\
$C_{\mathrm{TAC}}$, in \dollar & \09.82 & 10.19 & 10.23 & 10.05 \\
$M_{\mathrm{a}}$, in~L/m$^{2}$ & ${\sim}$564 & ${\sim}$686 & ${\sim}$752 & ${\sim}$1029 \\
$P_{\mathrm{p}}$, in days & 212 & 181 & 165 & 118
\botline
\end{tabular}
\vspace*{1pt}
\end{table}

The effectiveness of studied SSs is evaluated by comparing the
CPL of freshwater generation and the payback period, as
illustrated in Figure~\ref{fig8}. The CPL of distillate yield
varies for different configurations: TSS (Rs.~1.4), TSS-B (Rs.~1.2),
TSS-NB (Rs.~1.1), and TSS-S (Rs.~0.8). Additionally, the corresponding
\mbox{payback} periods for TSS, TSS-B, TSS-NB, and TSS-S are 212~days, 181
days, 165 days, and 118 days, respectively.

In comparison to all investigated configurations, TSS-S emerged as the
most cost-effective desalination method, as it has the lowest CPL,
about 42.86\% lower than TSS. In addition, the TSS-S also \mbox{exhibited} the
least payback period, approximately 94~days lesser than TSS followed by
TSS-NB and TSS-B. In summary, the desalination system incorporating
SHSMs exhibited enhanced economic performance, whether augmented with
sand or bricks (with and without napkins) within the system.

Figure~\ref{fig9} illustrates the IP and
SI  for all four configurations under study. The
IP for investigated configurations ranges from 98.63 to 98.85. By
emphasizing IP, researchers can enhance process efficiency, reduce
waste, and maximize resource utilization. Increased productivity and
cost savings are the ultimate \mbox{outcomes} of such improvements. Notably,
the IP varies between 98.63 (lowest for TSS-S) and 98.85 (highest for
TSS), indicating that TSS offers greater room for improvement while
TSS-S offers the least. 

The SI for investigated configurations is within the range of 1.01 to
1.014. The SI can take any positive value ranging between 1 and
${\infty}$. A higher SI signifies a reduced environmental impact per
unit of production. It represents approaches that optimize renewable
energy utilization, minimize waste, and decrease energy consumption.
Clearly, the SI value for TSS-S is the highest (1.014). This indicates
that the TSS-S utilizes solar energy more effectively than other
configurations. 

\begin{figure}
\includegraphics{fig09}
\caption{\label{fig9}Improvement potential and sustainability indices
for different configurations.}
\end{figure}

\begin{table*}
\caption{\label{tab5}Comparison of water quality parameters}
\begin{tabular}{ccccccc}
\thead
\xmorerows{1}{Parameters}  & 
\morerows{1}{\parbox[t]{2cm}{\centering Before desalination}}  
& \multicolumn{4}{c}{After desalination} &
\morerows{1}{\parbox[t]{2.6cm}{\centering WHO \newline recommendation}}
\\\cline{3-6}
& & TSS & TSS-B & TSS-NB & TSS-S & 
\vspace*{2pt}  \\
\endthead   
pH & 6.1 & 7.3 & 7.3 & 7.4 & 7.2 & 6.5--8.5 \\
TDS (mg/L)  & 1236 & 42 & 56 & 58 & 156 & 50--300
\botline
\end{tabular}
\end{table*}

\begin{table*}
\caption{\label{tab6}Performance comparison of this study with relevant existing literature}
\tabcolsep=2pt
\begin{tabular}{cccccc}
\thead
S. No.  & Authors & Type of solar still  & HSMs & 
\parbox[t]{3cm}{\centering Productivity improvement (\%)}  & CPL (\dollar)\vspace*{2pt} \\
\endthead
1  & \parbox[t]{2.5cm}{\centering Deshmukh and Thombre~\cite{16}} &
Single slope & \parbox[t]{3.5cm}{\centering Sand and Servotherm medium oil} & 
\06.25 & --\vspace*{2pt}\\
2  & Kumar et~al.~\cite{18} & \parbox[t]{3cm}{\centering  Triangular pyramid $+$ single slope} & 
Water & 79.05 & --\vspace*{2pt} \\
3  & Kabeel et~al.~\cite{42} & Single slope & Sandbag and jute cloth & 
25.24 & --\\
4  & Yousef et~al.~\cite{44} & Single slope & Sandbag & 35.24 & 0.04\0\0 \\
5  & El et~al.~\cite{49} & Single slope  & Sandbags & 34.57  & 2.31\0\0 \\
6  & Dhivagar~\cite{50} & Single slope &  Magnetic powder 
& 31.22 & 0.02\0\0\vspace*{2pt} \\
7  & Mahala and Sharma~\cite{55} & 
Square pyramid & Gravels \& PCM & 84.00 & 0.22\0\0\vspace*{2pt} \\
\xmorerows{2}{8}  & \xmorerows{2}{Current study} & 
\xmorerows{2}{Triangular pyramid} & TSS-B   & 36.24  & 0.0149\\
&&&TSS-NB  & 50.00 & 0.0136\\
&&& TSS-S & 81.41 & 0.0096
\botline
\end{tabular}
\end{table*}


As indicated in Table~\ref{tab5}, the distilled water yielded by TSS
exhibits a TDS concentration ranging from 42 to
156 mg/L and a pH level between 7.2 and 7.4. These values fall within
the recommended range established by the WHO, and therefore, 
the water is suitable for various household applications.


\section{Performance comparison with pertinent \mbox{literature}}\label{sec5}

The performance parameters, namely, the productivity improvement and
CPL of fresh water produced, are compared. The performance comparison is 
presented in Table~\ref{tab6},
with the available literature of SSs having different types of heat storage
materials for evaluating the performance of the investigated
configurations. The findings indicate that the yields in this study are
relatively better or comparable than those reported in previous
benchmarks in the field. Furthermore, the configurations investigated
in the present study are superior and commendable compared to others
due to their lower CPL.


\section{Conclusions}\label{sec6}

The current investigation assesses the performance of TSSs
based on  thermo-exergo-economic and sustainability analyses.
Based on the experimental outcomes, the following insights can be
drawn:
\begin{itemize}
\item The distilled water output in TSS is significantly influenced by
the mass of SHSMs and the area of solar radiation absorption.
Additionally, ambient temperature and glass temperature play crucial
roles in distilled water production.
\item The integration of SHSMs with TSS resulted in increased
freshwater yield compared to TSS alone. The enhancement in yield ranged
from 20.93\% to 81.39\% for TSS with SHSMs in comparison to TSS,
establishing it as a promising option for sustainable freshwater
production.
\item TSS-S exhibits a notable improvement of approximately 75.12\% in
energy efficiency, whereas TSS-NB and TSS-B demonstrate \mbox{enhancements} of
about 29.92\% and 16.07\%, \mbox{respectively}, compared to TSS.
\item Exergy efficiency increases by approximately 36.44\%, 8.41\%, and
7.47\% for TSS-S, TSS-NB, and TSS-B, respectively, in comparison to
TSS.
\item Economic assessment demonstrated a noteworthy decrease in CPL,
reaching up to 42.86\% (TSS-S), and a corresponding reduction in the
payback period by as much as 44.33\% (TSS-S) with the integration of
SHSM. This leads to the conclusion that incorporating SHSMs into TSS
is both a \mbox{cost-effective} and environmentally sustainable method for
producing fresh water.
\item The improvement potential and sustainability index for the
examined cases lie within the ranges of 98.63--98.85 and 1.01--1.014,
respectively.
\item The TDS and pH levels of the fresh water generated from TSS adhere
to the recommended ranges outlined by the WHO. Consequently, the
obtained fresh water is suitable for domestic applications.
\end{itemize}

The current study could be expanded by integrating PCM with SHSMs to
notably improve the performance of the solar desalination system.
Additionally, the efficiency of TSS could be further enhanced by
incorporating a pulsating heat pipe along with SHSMs. Exploring diverse
glass cover cooling techniques in conjunction with SHSMs could be seen
as a further scope of future research for improving the freshwater
generation from TSS.

{\vspace*{-2pt}}

\section*{Nomenclature}
\mbox{}
\begin{inftab}
\begin{tabular}{ll}
\tbody
${A}_{\mathrm{t}}$ & TSS basin area (m$^{2}$) \\
${C}_{\mathrm{AFC}}$ & Annual fixed cost \\
${C}_{\mathrm{AOMC}}$ & \parbox[t]{3cm}{\raggedright Annual operational and maintenance cost} \\
${C}_{\mathrm{ASV}}$ & Annual salvage cost \\
${C}_{\mathrm{CPL}}$ & Cost per liter \\
${C}_{\mathrm{in}}$ & Initial cost \\
${C}_{\mathrm{TAC}}$ & Total annual cost \\
${E}_{x,\mathrm{in}}$  & Exergy input (kWh) \\
${E}_{x,\mathrm{out}}$   &   Exergy input (kWh) \\
${h}_{\mathrm{fg}}$ & Latent heat of evaporation (J/kg) \\
${i}$ & Annual interest rate  \\
${I}_{\mathrm{S}}$ & Solar irradiation (W/m$^{2}$) \\
${M}_{\mathrm{a}}$ & Annual freshwater yield \\
${m}_{\mathrm{w}}$ & Yield (L/m$^{2}$)  \\
${n}$ &  Lifespan \\
${N}_{\mathrm{d}}$ & \parbox[t]{3cm}{\raggedright Number of sunny days (330 days for New Delhi)} \\
${P}_{\mathrm{p}}$ & Payback period \\
${T}_{\mathrm{amb}}$ & Ambient temperature (K) \\
${T}_{\mathrm{sun}}$ & Sun temperature (6000 K) \\
$\eta _{\mathrm{en}}$ & Thermal efficiency \\
$\eta _{\mathrm{ex}}$ & Exergy efficiency \\
\end{tabular}
\end{inftab}

{\vspace*{-2pt}}

\section*{Abbreviations}

\begin{inftab}
\begin{tabular}{ll}
\tbody
{IP} & Improvement potential \\
{PCM}  & Phase change material \\
\inftabbreak
{SI} & Sustainability Index \\
{SHSMs} & Sensible heat storage materials \\
{TSS}  & Triangular solar still \\
{TSS-B} & Triangular solar still with bricks \\
{TSS-NB} & \parbox[t]{5cm}{\raggedright Triangular solar still 
with paper napkins wrapped around the brick} \\
{TSS-S} & Triangular solar still with sand bed  
\end{tabular}
\end{inftab}

\section*{Declaration of interests} 
The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

\section*{Funding}
The research underpinning this publication was undertaken independently
and was not contingent upon any external financial endorsements or
grants. 

\section*{Acknowledgments}

The authors would like to thank the Department of Mechanical Engineering,
Netaji Subhas University of Technology, New Delhi, for giving them the
opportunity to use their resources and work in such a challenging
environment.

\back{}

\bibliographystyle{crchim}
\bibliography{crchim20240198}
\refinput{crchim20240198-reference.tex}

\end{document}
