Thermal efficiency improvement of a solar desalination process by parabolic trough collector Open Access

ABREVIATIONS

MENA

Middle East and North Africa

UDES

Unité de Développements des Equipements Solaire

WHO

World Health Organization

UNICEF

United Nations International Children's Emergency Fund

ENR

Renewable Energies

CFD

Computational Fluid Dynamic

CSP

Concentrated Solar Power

CPC

Cylindrical Parabolic Collector

TES

Thermal Energy Storage

SYMBOLS

A

Area (m²)

e

Thickness (m)

M

Mass (kg)

V

Velocity (m/s)

Vf

Heat transfer fluid velocity (m/s)

Re

Reynolds number

T

Temperature (K)

K

Thermal conductivity (W/m.K)

L

Latent heat of water vaporization (J/kg)

C_p

Specific heat [C_w] (J/kg.K)

h

Heat transfer coefficient (W/m².K)

Q

Heat flux (W/m²)

Total heat transfer rate

I

Incident solar intensity (W/m²)

α’

Fraction of solar flux absorbed

ɛ

Emissivity

P

Partial saturated vapor pressure (N/m²)

Pr

Hourly yield (l)

t

Time (s)

C_f

Geometric concentration factor

SUBSCRIPTS

a

Ambient

g

Glace

i

Inner

o

Outer

w

Water

c

Convection

e

Evaporation

r

Radiation

f

Heat transfer fluid

D

Distiller basin

S

Sand

0

Initial

abs

Absorber

exch

Exchanger

ref

Reflector

max

Maximum

avg

Average

Nexus water/energy security is one of the main global challenges in impact terms on the next future generation. Today, 70% of water is used for agriculture. To feed 9.7 billion people by 2050, agricultural production and water share must increase by 60% and 15%, respectively (Eau 2019). A significant decrease in quality and quantity of available freshwater has harmful effects on human health and economic activity (The Global Risks Report 2016). According to a report published by WHO and UNICEF (UNICEF 2017), three out of ten people (3/10) worldwide do not have access to safe and easily accessible water in their homes; while the Kyoto Protocol in 1997 aimed to engage the international community to limit its production of greenhouse gases. Based on these two realities it appears that renewable energies (ENR) may offer the best alternative solution to guarantee a socio-energy, geopolitical and climatic security.

Desalination of seawater and brackish water using solar energy is generating growing interest in some developing countries. The solar desalination technique through the greenhouse effect is based on evaporation–condensation process and dates back to the year 1872 when it was used for the first time on a large scale in the north Chilean desert (1872). Since then, the principle has not changed, although improvements have been obtained in the form of used construction materials and methods of work (Khedim et al. 2004). The concrete still is a basic reference in greenhouse solar distillation (Tiwari & Lawrence 1991; Ranjan & Kaushik 2013). The main advantages of the greenhouse effect desalination are the water quality produced, and the possibility of integrating different solar thermal energy sources, namely concentrated solar power (CSP). There are four types of CSP technologies; parabolic collectors, fresnel mirrors, solar tours and cylindrical parabolic collectors (CPC), with the earliest in use being the cylindrical parabolic collector. Given that solar distillation has been shown to be technically feasible, simple and profitable to operate (Ranjan & Kaushik 2013), several studies have been carried out in this area to further improve the yield and quality of the distilled water produced (i.e. distilled water quality) as well as to evaluate the importance and utility of the hybrid solar still system.

Bagheri et al. (2020) investigated two types of desalination process, an indirect use of solar energy where solar panels and CPC were used for further water heating in the basin, and a direct use of energy generated from solar plates directly in the basin by thermal elements which produced more desalinated water. Tiwari et al. (2003) used a continuous water flow over the glass cover coupled with a solar collector to produce more freshwater. In order to overcome this problem, many active solar stills have been developed (Tiwari & Tiwari 2008); the PV-Therm collector (Gaur & Tiwari 2010), reflecting mirror (Fang et al. 2018), CPC (Prasad & Tiwari 1996; Ullah & Kang 2019), Flat plat solar collector and TES (El-Sebaii et al. 2011) have been used to improve the performance of the traditional solar still. Bellatreche et al. (2016) studied the influence of the geometry, seawater mass and TES on the quantity and quality of freshwater. TES improves the freshwater production, due to the continuation of the heat transfer process overnight. Khalifa & Hamood (2009) carried an experimental investigation to validate and enhance the correlation of three parameters of a solar distiller in which the solar still with the higher brine depth has a larger nocturnal production. Voropoulos et al. (2003) proposed a model ‘input-output’ method for the description of the solar still coupled with solar collectors.

Considering the solar energy potential of the Middle East and North Africa (MENA) region and particularly Algeria, this study considered the use of the parabolic trough solar collector technology coupled with thermal energy storage (TES) by sand, in order to improve the thermal efficiency of the desalination greenhouse process. Sand is an excellent thermal accumulator, it does not degrade and never loses its thermal characteristics during storage and destocking cycles. Sand is a non-polluting material and presents no end-of-life problem, it costs almost nothing. This thermal process is the key to resolving future water problems in the regions with the lowest water content, namely arid and isolated sites. The aim of this study was to assess the heat amount effect delivered by the solar concentrator to the TES system integrated into the solar still. The sensitive storage system was defined by an isolated enclosure equipped with a tubular exchanger trapped in the sand (storage material). This thermal storage will allow estimation of the distillation continuity process. Thus, the distilled water amount produced by the system will be optimized. The hybrid still was compared with conventional concrete still.

The system is equipped with a solar still consisting of a tank filled with seawater to be treated, stagnant within a tank with a selective absorbent layer. The solar still operates as a hybrid system comprising: the CPC for energy storage and a photovoltaic panel for power to run the circulators. The solar still has a basin area of 1 m² and the reservoir is topped with side walls of an optimized height and covered with a glass inclined at an angle of 20° to fine tune solar radiation transmission and allow an easy flow of water droplets. Figure 1 shows the schematic diagram of the system. The water volume was adjusted to occupy a selected thickness (Bellatreche et al. 2016). The mass of seawater is heated by the greenhouse effect in parallel with a heat exchanger connected to a CPC collector containing a heat transfer fluid to concentrate and accelerate the heating process. The heat exchanger is integrated into a sand storage bed to extend the distillation process in the absence of the sunlight.

The choice of sand as the storage material is based on the published study by Taamneh & Al-Shyyab (2016). They experimentally investigated the effect of natural Jordanian zeolite as a TES material on the thermal efficiency of a pyramid-shaped solar still. The results showed that the productivity of the solar still can be considerably improved through adding zeolite beneath the water basin. They found that the daily freshwater productivity of the solar still with zeolite increased by about 43%. The obtained experimental results were confirmed with CFD simulation results. It was demonstrated that the CFD results are in good agreement with the experimental results by Taamneh (2016). The storage material type choice (quartz sand) is made based on these thermal characteristics and its availability. The quartz sand thermal aptitude is mainly determined by its SiO₂ content, the higher the content the more the sand behaves favorably against thermal loads. In general, the reference values for the SiO₂ content are between 94 and 99% (Foundry Lexicon 2003). The selected sand rich in quartz has an average content of 98%, found in abundance in the southern regions of Algeria (Grand Sahara). For example, the sand chemical composition of the Bechar–Taghit region is defined by 98.7% of SiO₂ and the remainder include CaCO₃, Fe₂O₃, MgO, Al₂O₃, TiO₂ (Anas Boussaa et al. 2018).

The thermal model elaborated by (Gaur & Tiwari 2010) was used to determine the heat and mass transfer for the theoretical study of the hybrid solar still with CPC and TES (Figure 2).

The integral equations of Gaur &Tiwari (2010) were used for the first and second zone. However, the equations of the 3rd, 4th and 5th zones are modified according to the TES integration, with the sand through the heat transfer fluid which flows in the exchanger. The model of Gaur & Tiwari (2010) does not have a storage zone, and the energy given by the plant solar collectors directly heats the seawater (convective transfer). This configuration is totally different compared to the present work (energy supplied to the water basin is conductive in nature).

The thermal balance of each zone is described by the following equations:

The radiation values and the wind velocity were recorded in the weather station of UDES Bou-Ismail. The heat transfer fluid used in this analysis is ethylene glycol with an evaporation temperature of T_ev = 471 K. The physical and chemical characteristics of materials utilized are given in Table 1.

The evaporation mass M_ew is calculated directly from Equation (7). The freshwater mass harvested is defined in Equation (15).

The CFD Ansys Fluent code was used for the present simulation. It allowed modeling of fluid mechanics mass and heat transfer problems, based on the finite volume method. Two equations are used in this work concerning Flow and Energy in unsteady laminar mode. The relationship between velocity and pressure corrections (pressure–velocity coupling) to enforce mass conservation was effectuated by the ‘SIMPLE’ algorithm. The second order was employed for the spatial pressure discretization. The 3D thermal analysis consists of two parts:

• (1)

  The simulation of CPC absorber.

• (2)

  The heat transfer between the sand storage bed and copper tubular exchanger (Figure 3).

The CPC absorber is a copper tube with L_abs = 1.4 m and R_abs = 0.04 m, with an 870,000 structured mesh, tight at the walls. The heat transfer fluid inlet velocity (ethylene glycol) into the absorber is set at Vf_abs = 0.5 m/s.

The solar irradiative flux received by the absorber consists of two parts, direct solar radiation flux, and flux concentrated by the parabolic trough collector with a geometric concentration factor C_f = 21.9, defined as a ratio of the reflector opening area A_ref to that of the absorber A_abs.

It is considered that the solar radiation is constant during the same hour and that the heat transfer fluid is incompressible (Figure 3(a)). Depending on the computing capacity, and geometry symmetry consideration, a numerical computational volume specific to the storage system was adopted, in order to determine the sand temperature. The considered volume dimensions are (16 × 16 × 32) 10⁻³ m, i.e. 8.2 × 10⁻⁶ m³, with a copper exchanger tube of a radius R_exch = 0.007 m and a thickness of e_exch = 0.001 m. The exchanger with a length of 7.2 m is embedded in a sandbox which is in contact with the solar still copper base. The heat transfer fluid circulation velocity in the exchanger is Vf_exch = 2.5 m/s. The exchanger inlet temperature is the average CPC absorber outlet temperature, which is considered constant along the exchanger, during a time interval of one hour (depending on the solar radiation during the day) (Figure 3(b)).

The developed numerical model is validated against the concrete solar still experimental results in square geometry carried out at the Bou-Ismail (Algeria) site in May 2014 (Bellatreche et al. 2016). Table 2 presents the comparison of the daily distillation between the concrete still experimental and the theoretical results for the developed model as a function of the seawater volume. The model absolute average precision is estimated at 8.7%. However, the minimum default error is −7%. The accepted error for the different models proposed in the specialized literature is about 5–10% (Tiwari et al. 2003; Shalaby et al. 2016; Mahmoud et al. 2019). The present model is included in the tolerated error interval.

The average error 8.7% is explained by the following assumptions which neglect in the theoretical model:

• The pressure losses in the pipes.

• The salt deposits at the still tank bottom and the variation in the seawater density to be distilled.

• Heat transfer at the solar still side walls.

• The distillate losses at the maximum production hours by the gravity effect.

• The stratification in water mass and the added seawater temperature.

• Wind velocity effect (The experimental wind velocity values are not the same for the seawater volumes studied).

The relative error of daily distillate yield between experimantal and theoretical result (M_w0 = 35 kg) is −3% (default error) after the wind velocity effect introduction V_wind = 4.16 m/s (Equation (1)). The error has decreased from −10% to −3% considering the wind velocity (Equation (1)) (Figure 4). The wind favors the solar still glass cooling by forced convection, this cooling accelerates the condensation process at the interior glass level by increasing the temperature gradient between the two medium (fluid-wall interaction).

The increase in the seawater volume to be distilled implies a decrease in the freshwater production (Gaur & Tiwari 2010), due to the seawater warming process slowness. However, for low volumes the acceleration of the evaporation process induces an increase in salt concentration and deposits at the distiller basin base, reducing the solar radiation absorption by the black coating.

According to the specific experimental results to the distiller geometry and the climatic conditions for a several days, it was found that the optimum mass to be distilled corresponds to M_w0 = 35 kg (Bellatreche et al. 2016). Based on the freshwater yield produced and its variation depending on the initial seawater volume, the concordance criterion between theoretical and experimental results was imposed. The theoretical model behavior is in good agreement with the experimental values for the same weather conditions.

The CFD simulation is the first step in this study. It allows the quantification of heat flux Q_cpc captured by the CPC at the absorber tube level and transferred to the storage material (quartz sand) through the copper exchanger buried in the sandbox (Figure 5).

The temperatures at the outlet absorber tube T_cpc and of the storage material (quartz sand) T_sand are determined numerically by CFD simulation. These temperatures vary according to the daily solar radiation. The maximum day solar radiation is recorded at 12 p.m., I = 845 W/m², with a relatively constant ambient temperature T_a/avg = 291 K. The temperatures T_cpc and T_sand continue to increase until reaching their maximum values at 7 p.m. T_cpc/max = 311 K and T_sand/max = 303.8 K, just before sunset, despite the decrease in solar radiation after 12 p.m. However, a difference of −19% between the two temperatures at the same hour was observed (Figure 5).

The ethylene glycol velocity (T_f0 = 294 K) at the heat exchanger Vf_exch = 2.5 m/s and the absorber tube Vf_abs = 0.5 m/s relative to the Reynolds numbers Re_exch = 1,966 and Re_abs = 2,247 respectively correspond the laminar flow regime (Firdaous et al. 2015). However, at higher temperatures (T_f = 313 K) and for the same velocity, Reynolds numbers recorded are Re_exch = 3,885 and Re_abs = 4,440. These values define a pre-turbulent transition regime (Firdaous et al. 2015), promoting heat exchange and minimizing pressure losses, contrary to the developed turbulence regime. The temperature has a direct influence on the heat transfer fluid viscosity variation, which is clearly observed on the flow regime (+97.6% on the Reynolds number). The flow regime influence on the heat transfer coefficient h is represented in an intrinsic manner via the heat flux Q_cpc.

The CFD approach enables the determination of heat transfer fluid temperatures T_abs and the exchanger temperatures T_exch, from the daily solar radiation concentrated by the collector (Figure 6). These results are used in the hybrid still energy balance to determine the influence of Q_cpc on the distilled water production Pr.

The flux yielded by the sand to the solar still reaches its maximum value at 5 p.m. with ϕ_cpc = 75 W/m². The CPC flux daily evolution as a function of the temperature T_cpc is defined by a Gaussian curve. The total daily flow received by the still is Σϕ_cpc = 592 W/m² (Figure 6). The storage of the energy produced by the parabolic trough collector and additional flux strongly depends on the system weather and initial conditions.

This section deals with the dual influence of the solar distiller basin material (concrete or copper) and the CPC heat flux transferred to the seawater mass, on the daily freshwater production Pr (Equations (7) and (15)) for a single day. Using tarnished copper instead of concrete increases the daily water production from 3.4 l to 5.1 l, it presents +50% of the production Pr. However, the combination of copper plus CPC thermal flux offers a production gain of 3.1 l (3.4 l–6.5 l). This improvement correspond a +91% of the traditional concrete solar still production for the same meteorological conditions (ambient temperature, solar irradiation and wind velocity) (Figure 7).

These results can be explained on the one hand by the copper super thermal conductivity and the black coating with high solar radiation absorption and on the other hand, by the additional flux Q_cpc (Equations (4) and (5)) transmitted via the sandbox to the solar still, accelerating the evaporation process by increasing the seawater temperature T_w (Equation (6)). The influence of extra thermal flux on the daily yield improvement has been illustrated in the specialized literature by several studies (Table 3).

The seawater temperature during the day changes according to the daily solar radiation. It has the same appearance for the three types of solar still (concrete, copper and hybrid). The maximum temperature values T_w are recorded between 12 p.m. and 4 p.m. (Figure 8), with T_w.cpc/max = 312.3 K. Its average temperature T_w during the same period is T_w.cpc/avg = 310.6 K, which represents an increase of +26.6% compared to the concrete solar still (Table 4). Effectively, the variation in seawater temperature is closely linked to solar radiation during the day. Solar radiation is responsible for the greenhouse effect, the principal mechanism in the evaporative-condensation distillation process. This thermal source is both direct or principal (radiation received directly at the distiller level) and indirect or secondary (radiation absorbed by the heat transfer fluid at the CPC level). Part of this secondary source allows the seawater temperature of the hybrid still to be higher by +26% compared to the conventional concrete still. The other part is stored at sand level. A more detailed study will allow quantification of storage and destocking over several days of operation.

The hourly yield becomes maximal at 1 p.m. due to high water temperature which increases the value of evaporative heat transfer coefficient. The maximum daily yield value is related to the hybrid solar distiller T_w-cpc/max = 1.14 l. However, the average hourly yield value recorded between 12 p.m. and 4 p.m. is 0.98 l/hr (ΣPr_w = 3.92 l which represents 60%). This value represents a growth of +151.3% compared to the quantity produced by the concrete solar still (Table 4). Between 9 a.m. and 12 p.m., the amount hourly yield value is ΣPr_w = 2.28 l (35%). Between 4 p.m. and 8 p.m., the amount hourly yield value is ΣPr_w = 0.32 l (5%) (Figure 9).

In the period between 12 p.m. and 4 p.m., the radiative energy flux absorbed by the copper is greater than that absorbed by the concrete, due to the material thermal effusivity (611 J/K·m²·s^1/2 for copper and 23.6 J/K·m²·s^1/2 for concrete), with a ratio of 23 times. The copper low thermal inertia is compensated by the TES introduction.

Following the modifications to a traditional solar greenhouse concrete still defined by additional thermal CPC flux, TES (quartz sand) and distiller basin material (tarnished copper) for a seawater mass of 35 kg, the main findings and perspective are:

• The additional CPC system and TES have a positive effect on the solar distiller performance +91%. However the TES influence has not a significant impact during the experiment first phases, for this a more detailed study will allow quantification of storage and destocking over the alternation day/night and for several operation days.

• It is important to properly size the storage tank (heat transfer fluid) in order to ensure the heat source alternation.

• The choice of the additional thermal flux process is strongly dependent on the basin solar still material to improve the freshwater daily yield production.

• The hybrid solar still daily efficiency is proportional to the solar radiation. It is more gainful on the socio-economic plan for desert regions (Sahara).

All relevant data are included in the paper or its Supplementary Information.