Parametric study of the energy efficiency of the HDH desalination unit integrated with nanofluid-based solar collector

Transcription

1 Journal of Thermal Analysis and Calorimetry (2019) 135: ( (),-volV)( (),-volV) Parametric study of the energy efficiency of the HDH desalination unit integrated with nanofluid-based solar collector Kapil Garg 1 Vikrant Khullar 2 Sarit K Das 1 Himanshu Tyagi 1 Received: 1 June 2018 Accepted: 9 July 2018 Published online: 30 July 2018 Ó Akadémiai Kiadó, Budapest, Hungary 2018 Abstract Humidification dehumidification desalination (HDH) systems have been found to be ideal for treating seawater at a smaller scale It requires thermal energy to drive the processes inside the HDH system which can be supplied by a renewable source of energy (such as solar energy) For this purpose, a nanofluid-based direct absorption solar collector (DASC) can be used which has a relatively higher thermal efficiency, as compared to the conventional surface absorption-based solar thermal collector In this study, these two sub-systems HDH and DASC, are coupled through a heat exchanger In this paper, a numerical model has been prepared for DASC-based HDH system which aims to evaluate the energy efficiency of this combined system by calculating gained output ratio as a function of various parameters, related to the DASC, such as particle volume fraction (f v ), height (H) and length (L) of the collector, mass flow rate of nanofluid inside the collector ð _m nf Þ and amount of solar energy incident (q) on the collector The performance of the combined or integrated system has also been verified against the various parameters related to HDH system such as ratio of mass flow rate of the saline water to the dry air ð _m w = _m da Þ, effectiveness of the humidifier (e H ) and dehumidifier (e D ) and the bottom temperature ðt w1 Þ Also the results of the numerical model prepared in this study have been compared with those available in the literature Finally, the applicability and benefits of using nanofluids in various thermal desalination techniques have been presented Keywords Nanofluid Desalination Humidification Dehumidification Solar energy List of symbols C p Specific heat capacity (J kg -1 K -1 ) d p Diameter of nanoparticles (nm) f v Particle volume fraction (%) D _H Rate of total enthalpy change (kj) H Height of the collector (mm) h Convective heat transfer coefficient (W m -2 K -1 ) h a Specific enthalpy of moist air (kj kg -1 dry air) & Himanshu Tyagi himanshutyagi@iitrpracin Kapil Garg kapilgarg@iitrpracin Vikrant Khullar vikrantkhullar@thaparedu Sarit K Das skdas@iitrpracin 1 2 School of Mechanical, Materials and Energy Engineering, Indian Institute of Technology, Ropar, Rupnagar, Punjab , India Mechanical Engineering Department, Thapar Institute of Engineering and Technology, Patiala, Punjab , India h d Specific enthalpy of distillate (kj kg -1 ) h fg Enthalpy of vaporization (kj kg -1 ) h w Specific enthalpy of saline water (kj kg -1 ) K Radiative coefficients (m -1 ) k Thermal conductivity (W m -1 K -1 ) I Intensity (W m -2 lm -1 ) L Length of the collector (m) m Relative complex refractive index of nanofluid _m Mass flow rate (kg s -1 ) N Complex refractive index of nanoparticles n Index of refraction for basefluid Q Heat input (W) Q ak Absorption efficiency Q ek Extinction efficiency Q sk Scattering efficiency q Radiative flux (W m -2 ) S Solar weighted absorptivity T Temperature ( C) u Velocity of nanofluid in x-direction (m s -1 ) v Velocity of nanofluid in y-direction (m s -1 ) W Absolute humidity of air (kg vaporkg dry air)

2 1466 K Garg et al x y Direction along the length of the collector Direction along the thickness of the collector Greek symbols e Effectiveness j Index of absorption q Density (kg m -3 ) k Wavelength of irradiation v Particle size parameter Subscripts a Absorption ab Absorptivity amb Ambient b Brine bf Base fluid conv Convection D Dehumidifier da Dry air d Distillate or fresh water e Extinction H Humidifier HX Heat exchanger in Inlet of the collector i Inlet of humidifier or dehumidifier max Maximum nf Nanofluid np Nanoparticles out Outlet of collector o Outlet of humidifier or dehumidifier r Radiative s Scattering wb Wet bulb w Saline water Abbreviations CAOW Closed air open water DASC Direct absorption solar collector ED Electro dialysis FDM Finite difference method GOR Gained output ratio HDH Humidification dehumidification MEB Multi-effect boiling MSF Multistage flash RO Reverse osmosis SSF Single-stage flashing Introduction The number of fresh water resources on earth is limited, and moreover, they are unevenly distributed As a result, there is water scarcity in various regions of the world, affecting half of the world s population [1, 2] Wastewater treatment or water reuse and rain water harvesting could help to improve the crisis related to water shortage but increasing population, pollution of existing fresh water resources, changing environment and poor infrastructure for supplying water worsen the situation [3] In order to address the water scarcity problem, desalination plants are being installed which help in increasing the availability of fresh water Since almost 97% of the water on earth is saline in nature, which cannot be consumed directly, desalination provides the opportunity to ensure fresh water supply to every living being on earth [4] Removing salts from seawater requires huge amount of energy which can be supplied either in the form of high-grade energy or lowgrade energy Reverse osmosis (RO) and electro dialysis (ED) are some of the desalination processes which are driven by high-grade energy, whereas multistage flash (MSF), multi-effect boiling (MEB) and humidification dehumidification (HDH) desalination processes require low-grade energy (in the form of heat) Out of the various above-mentioned desalination methods, humidification dehumidification desalination (HDH) method has been found to be practically suitable for off-grid, remote locations which require water supply at a smaller scale (1 100 m 3 day -1 ), and for these regions installing largescale applications plants such as MSF, MEB and RO is not usually justifiable Also, HDH technology has several attractive features such as absence of very sophisticated components, lower maintenance requirements, ability to treat very high saline water (higher than seawater) and possibility of operating with relatively lower-temperature heat sources such as solar, geothermal and waste heat sources [5 7] Thus, its several attractive features motivate the researchers to explore the field of HDH desalination so that the lack of fresh water problem can be addressed Also, solar energy-based HDH desalination units can be installed in those areas where there is an abundance of solar energy and scarcity of fresh water which eliminates the use of fossil fuels for desalination Thus, it promises to provide clean water using clean energy at a smaller scale with ability to treat very high saline water HDH desalination technology is the improved version of solar still in which the heating, evaporation and condensation take place in three different components, due to which it has higher energy efficiency (due to improved latent heat recovery in the condenser) It uses a carrier gas such as air to transport the water vapor from evaporator to condenser, and the process is similar to rain cycle but occurs in a controlled fashion Narayan et al [8] have studied the various versions of the HDH cycle in which detailed thermodynamic analysis of these cycles has been carried out In their study, it has been concluded that the performance of the waterheated HDH cycle depends on the modified heat capacity

3 Parametric study of the energy efficiency of the HDH desalination unit integrated with 1467 ratio in the humidifier and dehumidifier, the effectiveness of the humidifier and dehumidifier, top and bottom temperatures and relative humidity of the air at the exit of the humidifier and dehumidifier Amer et al [9] have also studied the water-heated cycle theoretically and experimentally in which water flows in an open cycle, whereas air remains in closed cycle The study reports a maximum productivity of 58 L h -1 using wooden slates packing and with forced air circulation Sharqawy et al [10] have studied the similar system in which a comparison has been presented between water-heated cycle and air-heated cycle which provides the guidelines for designing the system Further, it has been reported that with increase in top temperature, the value of GOR for water-heated cycle reduces whereas for air-heated cycle its value increases The closed air open water (CAOW) water-heated cycle has also been studied using various types of solar collectors as a heat source to heat the saline water before it enters the humidifier Zubair et al [11] have studied the CAOW- HDH system using evacuated tube solar collector to heat the saline water The combined system is optimized and rate of fresh water production and cost of water (per liter) was evaluated for different locations The price of fresh water per liter was found to vary from $0032 to $0038 with respect to different locations Hermosillo et al [12] have also studied CAOW-HDH unit with evacuated tube solar collector, and it has been concluded that the system produces 50 70% more water per day as compared to the solar stills having same collector surface area Yildirim et al [13] studied a similar system but by using flat-plate solar collector to heat the saline water The system under consideration also contains a flat-plate doublepass solar air heater to heat the air before it passes through the humidifier The mathematical model was prepared to evaluate and compare the performance of the various configurations of the system such as only water heating, only air heating and both air and water heating against various influencing parameters such as air and feed flow rate, cooling water flow rate, etc Water heating system has been found to be important for clean water production In all the above-mentioned studies related to closed air open water (CAOW) water-heated HDH desalination system, saline water is heated directly inside the solar collector which operates on surface absorption-based phenomenon As seawater contains very high amounts of total dissolved salts in the range of 35,000 45,000 ppm, this high salinity can corrode and erode the heat transfer surface of the solar thermal collectors as seawater is aggressive at high temperature which will eventually bring down the performance of overall system Also, integrating heat exchanger to prevent the flow of saline water through the solar thermal collectors may decrease the overall energy efficiency of the system Moreover, these surface absorption-based solar thermal collectors are having high radiative losses at higher absorber plate temperature and having cyclic variations of high temperature causing damage to the material [14 19] Hence, the literature suggests that surface absorption-based solar thermal collectors may be replaced by the direct absorption solar collectors in which the working fluid (nanofluid) directly absorbs the sunlight while flowing through the collector The application of these collectors in the field of desalination is yet unexplored These types of collectors were reported to have relatively higher thermal efficiency as compared to surface absorption-based solar collectors under similar operating conditions [15] and could be used in the field of desalination But various researchers have studied the effect of using the nanofluids in solar thermal desalination systems where the nanofluids have been used as a working fluid in the conventional surface absorptionbased solar collectors Kabeel et al [20] have studied the single-stage flashing (SSF) desalination system which uses a nanofluid as the working fluid inside the flat-plate collector The gained output ratio of the system was reported to be 1058 El-Said et al [21] have also studied a nanofluid-based system in which an hybrid desalination system which integrates two stages of HDH desalination unit with single-stage flashing unit utilizes a flat-plate collector in which Al 2 O 3 H 2 O nanofluid has been used as a working fluid It has been concluded that the solar collector efficiency is affected by the nanoparticle volume fraction The same nanofluid has also been employed by Rashidi et al [22] to evaluate the productivity of single-slope solar still as a function of particle volume fraction of nanoparticles, and it has been reported that with the increase in volume fraction from 0 to 5%, the productivity of the system increases by 25% With the use of the same nanofluid (Al 2 O 3 H 2 O) in cascade solar still, the increase in hourly productivity is 22% [23] The efficiency of a thermal system is affected by the nanoparticle dispersion and nanoparticle suspension, and this efficiency can further be improved by employing a combination of nanofluid and inserts (baffles, twisted tape, vortex generators and wire coil inserts) [24, 25] Other than using nanofluids, the performance of the solar stills has been improved by implementing various structural changes such as inserting a reticular porous layer [26] which increases the productivity by 1735% and by portioning in solar still [27] due to which maximum enhancement in the productivity is 816% on a particular day The optimization of the size and position of the partitioning is also studied by response surface methodology [28] Entropy generation analysis of the single-slope solar still is performed for the design improvement [29], and it is indicated that the increase in glass and water temperature leads to increase in different types of entropy generation Exergy analysis for a double-slope

4 1468 K Garg et al solar still has been studied which is equipped with the thermoelectric heating modules, and maximum exergy efficiency has been reported to be about 25% In another study by Rashid et al [30] on the volume of fluid model for simulating vapor liquid phase change in solar still, 39% increase in the productivity has been achieved by the use of a sponge rubber porous layer having porosity of 04 in the basin of the solar still However, HDH desalination is the improved version of solar still as mentioned earlier and the use of nanofluids in the HDH technology should be examined in order to enhance the thermal performance of HDH desalination systems In the present study, nanofluid-based direct absorption solar collector (DASC) has been integrated with the one of the version (closed air open water) of the humidification dehumidification desalination (HDH) unit with the help of a counter-flow heat exchanger, thus replacing the surface absorption-based solar thermal collectors The heat transfer fluid used in these collectors is basically a suspension of nanoparticles of a particular material inside a base fluid (termed as nanofluid) The overall system performance has been quantifying in terms of gained output ratio (GOR) and is studied against the various influencing parameters inside the overall system such as particle volume fraction (f v ), height (H) and length (L) of the collector, incident solar flux (q), mass flow rate of the nanofluid ð _m nf Þ, ratio of mass flow rate of saline water to the dry air ð _m w = _m da Þ, effectiveness of the humidifier (e H ) and dehumidifier (e D ) and temperature of saline water at the inlet of the dehumidifier ðt w1 Þ which is called the bottom temperature Numerical model for direct absorption solar collector (DASC) and HDH unit is prepared and solved with the help of a computer program in MATLAB Validity of the proposed model is verified against the model available in the literature Finally, results of the various thermal desalination studies such as HDH, single-stage flashing (SSF) desalination and solar still desalination employing nanofluids as a working fluid have been presented to emphasize the benefits of using nanofluids in thermal desalination systems Direct absorption solar collector (DASC) The nanofluid-based DASC is simply a rectangular channel through which the nanofluid flows and is shown in Fig 2 The channel is covered with a glass cover to prevent the thermal losses (radiative and convective) to the surroundings The bottom surface of the channel is made adiabatic to prevent conductive losses The nanofluid enters the channel from one end and leaves the channel after getting heated by the solar energy The nanofluid is simply a fluid suspended with nano-sized particles of a particular material such as aluminum, copper, graphite, etc, which is able to absorb the solar energy incident on it due to scattering and absorption characteristics of these particles [31] Humidification dehumidification desalination (HDH) unit The HDH desalination unit shown in Fig 1 consists of a heater (counter-flow heat exchanger) in which saline water exchanges heat with the hot stream of nanofluid before it enters the humidifier at a desired temperature which is called the top temperature Upon entering the humidifier, the hot saline water is sprayed over the packing material inside the humidifier The cold air stream and saline water flows through the humidifier in a counter-flow manner As the air flows through the humidifier, it gets heated and its humidity content increases due to evaporation of the fresh water vapors from saline water This hot and humidified air then enters the dehumidifier where it flows over the cooling coils, gets cooled and dehumidified by exchanging heat with the cold saline water flowing inside the cooling coils As the air gets cooled and dehumidified, water vapors from the air gets condensed and distillate stream (fresh water) is obtained and air then again enters the humidifier as it remains in the closed cycle The sensible and latent heat released by the air is absorbed by the saline water entering the dehumidifier and gets preheated before entering the heat exchanger This heat recovery is important as it significantly influences the performance of the overall system System description The closed air open water (CAOW) humidification dehumidification desalination (HDH) system having water heating configuration integrated with nanofluid-based direct absorption solar collector (DASC) through a counterflow heat exchanger has been shown in Fig 1 The system consists of a solar collector and a HDH desalination system which consists of a counter-flow heat exchanger, a humidifier (evaporator) and a dehumidifier (condenser) Modeling of the system and details of the governing equations In order to evaluate the performance of the present system and study its behavior with respect to different parameters related to collector and HDH unit, a numerical model is prepared which has basically two parts

5 Parametric study of the energy efficiency of the HDH desalination unit integrated with 1469 Hot and Humidified Air m da, T ao m w, T w3 m w, T w2 Spray Nozzles cooling coils Heat Exchanger Dehumidifier Packing Material m nf, T out m nf, T in Humidifier Direct Absorption Solar Collector Cold Saline Water m w, T w1 Brine m b, T w4 Cold and Dehumidified Air m da, T ai m d, T d Pure Water Fig 1 Schematic of closed air open water (CAOW) water-heated humidification dehumidification desalination (HDH) system integrated with nanofluid-based direct absorption solar collector (DASC) with the help of a counter-flow heat exchanger k nf o 2 T ox 2 þ o2 T oy 2 oq r oy ¼ q nfc pnf u ot ox þ v ot oy ; ð1þ Graphite Nanoparticles Rectangular Channel x Incident Solar Radiation Glass Cover Base fluid H where u and v are the velocity components, k nf is the thermal conductivity, q nf is the density and C pnf is the specific heat capacity of nanofluid The irradiation or radiative flux (q r ) absorbed by the nanofluid gets converted into its internal energy and appears as the rise in temperature of the fluid and is defined by the following Eq (1a) [15, 33 35]: ZZ q r ¼ I k dkd ð1aþ y Fig 2 Schematic of nanofluid-based direct absorption solar collector used as a heat source for HDH desalination unit Modeling of direct absorption solar collector The first part of the model solves the governing differential equation for DASC which is an energy balance equation and is solved by using finite difference implicit method (FDM) in MATLAB This equation gives the temperature of the nanofluid at the exit of DASC (T out ) and is shown below as Eq (1) [32, 33]: L where I k is the spectral intensity of incident radiation and is obtained from [36], k is the wavelength of incident radiation and d is the solid angle subtended by the sun on earth The radiative flux or spectral intensity is absorbed by the nanofluid as it passes through the different layers of fluid, and this attenuation of intensity is governed by the simplified version of radiative transfer equation (RTE) which is given by Eq (1b) [15, 32, 35, 37]: oi k oy ¼ K ek nanofluid I k ; ð1bþ where K eknanofluid is the spectral extinction coefficient of nanofluid which is the sum of the spectral extinction coefficient for nanoparticles ðk eknp Þ and spectral absorption coefficient for base fluid ðk ekbf Þ which are represented by the following Eqs (1c) and (1d), respectively [15, 32, 34]:

6 1470 K Garg et al K eknp ¼ 3f vq ek ; ð1cþ d p K akbf ¼ 4pj bf k ; ð1dþ where f v is the particle volume fraction, Q ek is the sum of absorption (Q ak ) and scattering (Q sk ) efficiencies, d p is the diameter of nanoparticles and j bf is the index of absorption for base fluid The absorption and scattering efficiencies for nanoparticles are given by the following Eqs (1e) and (1f), respectively [15, 32, 34]: Q ak ¼ 4vIm 1 þ v2 m 2 1 m 4 þ 27m 2 þ m 2 þ 2 2m 2 ; ð1eþ þ 3 Q sk ¼ 8 3 v4 m2 2 1 m 2 þ 2 ; ð1fþ where v is the particle size parameter and is defined as v = pd p k, mis the relative complex refractive index of the nanofluid defined as m = N np n bf, N np is the complex refractive index of nanoparticles and n bf is the index of refraction of the base fluid In the present study, due to very small particle size parameter (v 1) and very small volume fraction (up to 005%), dependent scattering effects of nanoparticles are ignored and hence scattering is independent and allows the application of Rayleigh scattering theory [15, 32, 34] The above energy balance Eq (1) applied to DASC holds some assumptions such as the flow of nanofluid through DASC is steady state and 2-dimensional (plug flow), the bottom surface of the collector is highly insulating to heat transfer but transparent to solar radiation, radiative losses are neglected, and only convective heat transfer losses between the collector and surroundings is considered The following boundary conditions (BC) are required to solve the governing Eq (1) which are as follows [32, 33, 35, 38]: BC 1: The heat loss only due to convection is considered from the transparent top surface of the collector to the surroundings which can be represented by the following Eq (1g) [32, 35]: ot k nf oy ¼ h conv T ðx;0þ T amb ; ð1gþ y¼0 where h conv is the convective heat transfer coefficient and T amb is the ambient temperature BC 2: In order to prevent the conductive losses from the bottom surface of the collector, it is considered to be an adiabatic surface which is represented by the following Eq (1h) [32, 33, 35]: ot oy ¼ 0; ð1hþ y¼h where H represents the height of the collector BC 3: The third boundary condition is related to the temperature of the nanofluid at which it enters the collector (T in ) and is given by the following Eq (1i) [32, 33, 35]: T ð0;yþ ¼ T in : ð1iþ BC 4: In addition to the above-mentioned boundary conditions, another BC is required to solve Eq (1) which represents the incident solar energy or incident solar flux on the top surface of the collector (I ko ) and is represented by Eq (1j) [33, 35]: I kðx;0þ ¼ I ko : ð1jþ Modeling of HDH desalination unit Modeling of the CAOW water-heated HDH unit contains certain assumptions which are as follows All the processes of evaporation, heat transfer and condensation occur at steady state, there is no air leakage from the system, pumping or blower power is negligibly small in comparison with energy input inside the heat exchanger, and kinetic and potential energy terms are neglected while applying energy balance to the components [8] In the previous subsection, the numerical model prepared for DASC helps to calculate the temperature of nanofluid at the outlet of DASC This high-temperature nanofluid stream having mass flow rate, _m nf and temperature T out then enters the counter-flow heat exchanger in which it heats the preheated saline water from the dehumidifier with mass flow rate _m w and temperature T w2 to a desired temperature T w3 which is calculated by the following Eq (2): T w3 ¼ e HX _m nf C pnf ðt out T w2 Þ= _m w C psw þ Tw2 ; ð2þ where e HX is the effectiveness of the counter-flow heat exchanger and C pw is the specific heat capacity of the saline water The hot saline water stream then enters the humidifier column where it heats and humidifies the air flowing from the bottom of the humidifier with mass flow rate _m da (dry air mass flow rate), absolute humidity W i and dry bulb temperature T ai and exits the humidifier column at temperature T w4 and mass flow rate _m b (brine) The air exits the humidifier column with temperature T ao and absolute humidity W o, and the exchange of thermal energy and mass between the saline water and air is represented by the following Eq (3) [7, 8, 33, 39]: : _m w h w3 m b h w4 ¼ _m da ðh ao h ai Þ; ð3þ where h w3 is the specific enthalpy of saline water at the inlet of humidifier, h w4 is the specific enthalpy of brine at humidifier exit, h ai and h ao are the specific enthalpies of air at the humidifier inlet and exit, respectively The brine

7 Parametric study of the energy efficiency of the HDH desalination unit integrated with 1471 mass flow rate, _m b, can be calculated by the following Eq (4) [8, 39]: _m b ¼ _m da ðw o W i Þ: ð4þ The hot and humidified air after carrying fresh water vapors from saline water enters the dehumidifier and is cooled and condensed by the cold saline water flowing inside the cooling coils with mass flow rate _m w and temperature T w1 which gets preheated by the hot and humidified air and the energy balance between the saline water and air inside the dehumidifier is given by the following Eq (5) [8, 33]: _m w ðh w2 h w1 Þþ _m d h d ¼ _m da ðh ao h ai Þ; ð5þ where h w2 and h w1 are the specific enthalpies of saline water at the outlet and inlet of the dehumidifier, h d is the specific enthalpy of the distillate stream and _m d is the mass flow rate of the distillate stream (fresh water) which is given by the following Eq (6) [8, 33, 39]: _m d ¼ _m da ðw o W i Þ: ð6þ The enthalpy of the distillate stream can be evaluated by calculating its bulk temperature using the following polynomial function given by Eq (7) [40]: T d ¼ 0: T wbi þ 0: T wbo 0:007417T wbi T wbo 0:41913T wbi þ 1:0511T wbo þ 61:6186; ð7þ where T wbi and T wbo are the wet bulb temperatures of air at the inlet and outlet of the dehumidifier, respectively Also effectiveness of the humidifier and dehumidifier is defined to make a balance between the number of equations and unknowns in the model The effectiveness defined for these components (simultaneous heat and mass exchange devices) is analogous to the effectiveness defined for the heat exchangers and is based on the maximum change in total enthalpy rate change for the air and water streams flowing through the humidifier and dehumidifier The effectiveness of these components is defined as the actual change in total enthalpy rate (D _H) of any of the stream to the maximum possible change in total enthalpy rate (D _H max ) and is given by Eq (8) [41]: e ¼ D H: : ð8þ D _H max This definition of effectiveness can be written in terms of mass flow rate and specific enthalpy of dry air and saline water, for the following 2 cases (cases I and II) Case I, D _H max;w \ D _H max;a; e ¼ _m w;ih w;i _m w;o h w;o _m w;i h w;i _m w;o h ideal ; ð8aþ w;o where _m w;i and _m w;o are the mass flow rates of saline water at the inlet and outlet of the heat and mass exchange devices, h w,i and h w,o are the specific enthalpies of saline water at the inlet and outlet of heat and mass exchange devices and h ideal w;o is the ideal specific enthalpy of the saline water at the outlet of the device corresponding to the dry bulb temperature of the air at the inlet of the device Case II, D _H max;w [ D _H max;a; e ¼ _m daðh ao h ai Þ _m da ðh ideal a o h ai Þ ; ð8bþ where _m da is the mass flow rate of dry air flowing through the heat and mass exchange devices, h a,i and h a,o are the specific enthalpies of air at the inlet and outlet of heat and mass exchange devices and h ideal a;o is the ideal specific enthalpy of the air at the outlet of the device corresponding to the temperature of saline water at the inlet of the device The energy supplied in the heat exchanger by the hot nanofluid stream to the preheated saline water to heat it to the top temperature is given by Eq (9): Q input ¼ _m w ðh w3 h w2 Þ: ð9þ Finally, the energy efficiency of the overall system is evaluated by calculating the gained output ratio and is defined by the following Eq (10) [8, 33, 35]: GOR ¼ _m dh fg ; ð10þ Q input where h fg is the enthalpy of vaporization of the distillate obtained Numerical modeling of DASC-based HDH desalination unit In the present section, the numerical modeling of the direct absorption solar collector (DASC) and humidification dehumidification desalination (HDH) unit is discussed The equations are solved to obtain the energy efficiency of the overall system which is measured in the form of gained output ratio (GOR) The performance of the system is studied against the various parameters of the solar collector and HDH unit The governing differential equation, Eq (1), for DASC is solved using finite difference implicit method (FDM) The fluid domain inside DASC has been discretized in x- and y-directions resulting into the rectangular elements of uniform size on which the energy balance is applied, and the temperature field inside the collector is obtained The incident solar radiation is approximated to be spectrally distributed within the range of lm with a wavelength band gap of 01 lm The temperature of the fluid at collector exit is calculated as the

8 1472 K Garg et al average of the temperature values at all the nodes at x = L The nanofluid is considered to be made of graphite (C) nanoparticles suspended in the water (base fluid) The reason of choosing these particles is that they are cheap and having the highest solar absorptivity (S ab ) at a particular volume fraction (f v ) in comparison with metallic nanoparticles such as aluminum and copper, measured for a fixed nanofluid layer of thickness, H = 10 mm Moreover, at very less volume fraction say, f v = 004%, they are able to absorb almost 100% solar irradiance for the same thickness of nanofluid layer [33, 42] The thermo-physical properties of graphite nanofluid are approximated as the properties of base fluid due to very small particle volume fraction (f v ) taken in this study [43], and the optical properties of graphite nanoparticles are obtained from [44] The thermophysical properties of the base fluid (water) are obtained from [36] The number of nodes in x- and y-directions is chosen such that the error in the final solution (T out ) is less than 0005% The governing equations for HDH unit are also solved numerically with the help of a computer program prepared in MATLAB The solution converges when the absolute error in the solution is approximately less than 10-5 The properties of saline water are assumed to be equal to that of pure water for the ease of calculations [40] The air flowing through the HDH unit is assumed to be always saturated ( = 100%) Results and discussions The present section discusses the performance of the overall system by evaluating the energy efficiency The energy efficiency is the ratio of the product of mass flow rate of distillate ð _m d Þ and its latent enthalpy of vaporization (h fg ) to the heat input in the heat exchanger (Q input ) This ratio is also called the gained output ratio (GOR) The gained output ratio indicates the effectiveness of distillate obtained to the amount of heat recovery effected in the system [5], and it is studied against the various parameters of the collector which are mass flow rate of nanofluid ð _m nf Þ, height (H) and length (L) of the collector and particle volume fraction (f v ) of nanoparticles inside the base fluid Mass flow rate ratio ð _m w = _m da Þ, which is defined as the ratio of mass flow rate of saline water ( _m w ) to the dry air ( _m da ), effectiveness of the humidifier and dehumidifier (e H, e D ) and bottom temperature ðt w1 Þ are some parameters related to HDH unit which affect the gained output ratio Table 1 shows the values of the input variables used to evaluate the performance of the system Table 1 Values of the input variables used for the numerical study [8, 24, 26] Input variables Values Mass flow rate ratio, _m w = _m da 4 Effectiveness of the humidifier, e H 08 Effectiveness of the dehumidifier, e D 08 Mass flow rate of nanofluid, _m nf 02 kg s -1 Length of the collector, L 10 m Width of the collector, W 10 m Temperature of nanofluid at the inlet of DASC, T in 40 C Convection heat transfer coefficient, h conv 5Wm -2 K -1 Transmissivity of the glass cover, s 098 Effectiveness of heat exchanger, e HX 08 Ambient temperature, T amb 30 C Bottom temperature, T w1 30 C Incident solar flux, q 1000 W m -2 Diameter of graphite (C) nanoparticles, d p 10 nm Validation of the numerical model of DASC and HDH unit The numerical model prepared for DASC has been validated with the numerical and experimental studies on the similar system by Otanicar et al [38] The results of the validation in which the thermal efficiency of the DASC as a function of particle volume fraction has been evaluated are presented elsewhere [35] The numerical model prepared for closed air open water (CAOW) water-heated HDH unit in this study has been validated with the numerical study by Narayan et al [8] on a similar version of the HDH unit Figure 3 shows the results of the validation study which shows the variation of gained output ratio (GOR) as a function of mass flow rate ratio ð _m w = _m da Þ at different values of top temperature ðt w3 Þ obtained in the present study, and they are compared with the results obtained by Narayan et al [8] As can be seen from Fig 3, the results obtained in the present model are in close agreement with the results of Narayan et al [8] The initial conditions for the above study are taken as bottom temperature, T w1 ¼ 35 C, effectiveness of the humidifier, e H = 08 and effectiveness of the dehumidifier, e D =08 Variation of gained output ratio (GOR) as a function of the length of the collector (L) Figure 4 shows the variation in gained output ratio (GOR) when the length of the collector (L) is varied for different values of bottom temperature ðt w1 Þ It can be observed from Fig 4, at a fixed value of bottom temperature that as the length of the collector increases, gained output ratio

9 Parametric study of the energy efficiency of the HDH desalination unit integrated with 1473 T w3 = 80 C (Narayan et al) Tw3= 80 C (Current Model) Tw3 = 90 C (Narayan et al) T w3 = 90 C(Current Model) Fig 3 Variation of the gained output ratio as a function of mass flow rate ratio ð _m w = _m da Þ at different values of the top temperature ðt w3 Þ obtained in the present study and compared with the study by Narayan et al [8] T w1 30 C T w1 35 C DASC (T out ) due to which thermal energy input (Q input )in the heat exchanger increases resulting in the higher top temperature ðt w3 Þ When the top temperature increases, the temperature of the air at the humidifier outlet ðt ao Þ also increases (sensible energy) along with the increase in the humidity content of the air (W o ) as the air is able to carry more water vapor when its temperature is increased Hence, rate of evaporation of fresh water from saline water increases which ultimately results in the higher specific enthalpy of the air at the humidifier exit ðh ao Þ This hot and humidified air flows over the cooling coils in the dehumidifier preheating the saline water which initially was at a temperature T w1, gets cooled and looses water vapor which is obtained as a fresh water stream ð _m d Þ at the dehumidifier exit With increase in specific enthalpy of air at the humidifier exit ðh ao Þ, rate of heat transfer increases in the dehumidifier due to which rate of distillate increases (W o also increases) and specific enthalpy of saline water at the dehumidifier exit ðh w2 Þ also increases which leads to the increase in gained output ratio (GOR) due to higher rate of increase in useful energy (product of mass flow rate of distillate and its enthalpy of vaporization) as compared to heat input in the heat exchanger (Q input ) Further, there exists an optimum value of length for which the gained output ratio is maximum for all the different values of bottom temperature ðt w1 Þ when all other parameters considered in the study are kept constant, and after this optimum value of length, gained output ratio starts decreasing due to lower rate of increase of useful heat as compared to the heat input in the heat exchanger as shown in Fig 5 The optimum value of the length of the collector (L) for the bottom temperature ðt w1 Þ of 30 C is T w1 40 C Fig 4 Variation of gained output ratio (GOR) as a function of length of the nanofluid-based collector (L) at various different values of bottom temperature ðt w1 Þ for height of the collector, H = 4 mm and particle volume fraction, f v = 00125% (GOR) increases up to a certain value of length of the collector, attains maximum value and then starts decreasing At bottom temperature, T w1 ¼ 30 C, the increase in length from 4 to 6 m increases the system GOR only by 6428%, whereas at bottom temperature, T w1 ¼ 40 C, the increase in length from 3 m to 4 m increases the overall performance by 2 times When the length of the collector is increased, the nanofluid has to flow through the collector for a longer period of time and thus absorbs the greater amount of incident solar radiation which results in the higher temperature of the nanofluid stream at the exit of Fig 5 Comparison of the variation in the useful heat _m d h fg with the energy input in the heat exchanger (Q input ) when the length of the collector (L) is increased for bottom temperature, T w1 ¼ 30 C, height of the collector, H = 4 mm and particle volume fraction, f v = 00125%

10 1474 K Garg et al approximately 6 m, whereas at other two bottom temperature values it is 4 m, and beyond that the useful heat _m d h fg increases at a lower rate as compared to the heat input in the heat exchanger (Q input ) as can be seen from Fig 5; for example, with the increase in length from 6 to 10 m (666%) at bottom temperature, T w1 ¼ 30 C, the useful heat increases by only 14% whereas the increases in useful heat inside the heat exchanger is 72% With increase in the length of the collector, outlet temperature of nanofluid will always increase but not the performance of the system, and hence, it can be concluded that while keeping all other parameters fixed other than length, there will always exit an optimum value of T out at which the GOR of the system will be maximum Inherently, the top temperature ðt w3 Þ optimizes the performance of the system Variation of gained output ratio (GOR) as function of particle volume fraction (f v ) Figure 6 shows the variation of gained output ratio (GOR) as a function of particle volume fraction (f v ) The effect of particle volume fraction (f v ) on fresh water production rate ð _m d Þ is also shown It is shown that as the particle volume fraction (f v ) of nanoparticles inside the base fluid increases, gained output ratio (GOR) also increases but up to a certain amount and then starts decreasing reaching finally at a saturation level The increase in particle volume fraction (f v ) from 0001 to 0003% changes the value of GOR by only 5% As the amount of nanoparticles inside the basefluid increases, nanofluid becomes more opaque to solar irradiation due to increased amount scattering of radiation which makes it able to absorb the maximum amount of radiation Hence, the increased amount of absorption appears as the rise in temperature of the fluid at the collector outlet (T out ) As explained earlier, higher collector outlet temperature (T out ) leads to higher top temperature ðt w3 Þ due to which gained output ratio increases but up to a certain point In this case also while keeping all other parameters fixed, the increase in particle volume fraction beyond a certain amount leads to decrease in GOR due to increase of T out beyond the optimum value for which rate of increase of useful energy will be lower as compared to the heat input in the heat exchanger as explained earlier In this case, this optimum volume fraction is 0003%, and increase in volume fraction from 0003 to 005% decreases the value of GOR approximately by 345% Also at a fixed collector height (H), length (L), mass flow rate of nanofluid ð _m nf Þ and incident flux (q), there exits an optimum value of particle volume fraction at which the collector outlet temperature is maximum, and beyond this optimum value, solar irradiation get absorbed within the top layers of the nanofluid and is unable to reach the lower layers due to which collector outlet temperature starts decreasing It is observed that for higher volume fraction, performance of the system attains a saturation and starts decreasing on further increase in volume fraction (f v ) due to decrease in T out from optimum value at fixed values of operating parameters The rate of fresh water production ð _m d Þ increases as particle volume fraction (f v ) increases and then saturates after a certain volume fraction Higher volume fraction leads to higher collector outlet temperature up to a certain point, and as explained earlier, higher collector outlet temperature leads to increase in useful energy, and after a certain value of T out, rate of increase of useful energy starts decreasing due to which fresh water production starts decreasing The increase in volume fraction (f v ) from 0001 to 0003% increases the productivity by 36%, and it only increases by approximately 9% when the volume fraction changes from 0003 to 005% Variation of gained output ratio (GOR) as a function of height of the collector (H) Fig 6 Variation of gained output ratio (GOR) and fresh water production rate ð _m d Þ as a function of particle volume fraction (%) for height of the collector (H) = 4mm Figure 7 explains how the gained output ratio and fresh water production rate vary with the height of the collector The variation in GOR and production rate with height of the collector is similar to that in the previous analysis in which effect of particle volume fraction on system performance has been discussed As the height of the collector increases, GOR increases up to a certain point and then decreases When the height of the collector is increased from 055 to 1 mm, the GOR increases approximately by 74%, and upon further increase in the height from 1 to 5 mm, the GOR decreases by 33% Fresh water production rate ð _m d Þ increases by 8533% when the height is changed from 055 to 1 mm Rate of fresh water production

11 Parametric study of the energy efficiency of the HDH desalination unit integrated with 1475 Fig 7 Variation of gained output ratio (GOR) and fresh water production rate ð _m d Þ as a function of height of the collector (H) for particle volume fraction, f v = 00125% increases upon increase in height of the collector, and then, it shows asymptotic behavior with increasing collector height and may decrease on further increase in the height of the collector such as upon increasing the collector height from 1 to 5 mm, productivity increases only by 145% which represents the asymptotic nature of the variation of fresh water productivity ð _m d Þ with the height of the collector (H) When the height of the collector or thickness of the nanofluid layer (H) increases, more attenuation of the solar irradiation takes place due to which collector outlet temperature (T out ) increases When the height of the collector is increased beyond a certain point, T out starts decreasing as at a greater height of the fluid layer, solar radiation gets trapped within the top layers and layers below remain unheated due to which collector outlet temperature starts decreasing Hence, as explained earlier variation in T out decides how the variation in GOR and fresh water production rate takes place Variation of gained output ratio (GOR) as a function of mass flow rate ratio ð _m w = _m da ) at different values of solar irradiation (q) The variation of gained output ratio (GOR) as a function of mass flow rate ratio ð _m w = _m da Þ in HDH unit at different values of incident solar flux (q) is shown in Fig 8 It can be seen here that on keeping values of other parameters in the system fixed as done in the previous cases, there exists an optimum value of mass flow rate ratio at which the GOR peaks for every different value of incident radiation, and as the value of incident radiation increases, this optimum value of mass flow rate ratio shifts in the right with peak GOR value decreasing as the incident solar flux increases The increasing amount of solar flux (q) leads to higher Fig 8 Variation of gained output ratio (GOR) as function of mass flow rate ratio ð _m w = _m da Þ at different values of incident solar flux (q) with H = 4 mm and f v = 00125% Mass Fig 9 Variation of gained output ratio (GOR) as a function of mass flow rate of nanofluid ð _m nf Þ at different values of effectiveness of the humidifier and dehumidifier (e H, e D ) at height of the collector, H = 4 mm, particle volume fraction, f v = 00125%, T amb =25 C, T w1 ¼ 25 C and mass flow rate ratio ð _m w = _m da Þ =4 outlet temperature of nanofluid (T out ) due to increase in amount of irradiation absorbed by the fluid and as explained earlier that when collector outlet temperature exceeds beyond its optimum value, GOR starts decreasing At incident flux, q = 700 W m -2, the peak GOR value is 231 which occurs at a mass flow rate ratio ð _m w = _m da Þ of 4 At the same mass flow rate ratio of 4, the increase in incident flux (q) approximately by 30% decreases the GOR by 10% Also, at incident flux, q = 800 and 900 W m -2, the peak value of GOR occurs at the same mass flow rate H H H D D D

12 1476 K Garg et al ratio ( _m w = _m da = 45) and also the difference between these two peak values of GOR is less than 5% Variation of gained output ratio (GOR) due to variation in mass flow rate of nanofluid ð _m nf ) at different values of effectiveness of humidifier and dehumidifier (e H, e D ) Figure 9 shows the variation in gained output ratio as a function of mass flow rate of nanofluid ð _m nf Þ at different values of effectiveness of the humidifier and dehumidifier It is clear from Fig 9 that effectiveness of these components plays an important role in the performance of the overall system and gained output ratio increases with the increase in effectiveness of these devices as at higher effectiveness, heat and mass transfer increases, and hence, higher performance is obtained At the lowest mass flow rate of nanofluid, _m nf = 01 kg s -1, the increase in effectiveness by 125% increases the GOR of the overall system by more than 3 times It is also observed from Fig 9 that with increase in mass flow rate of nanofluid ð _m nf Þ, gained output ratio starts increasing, and after a certain value of mass flow rate of nanofluid, gained output ratio becomes almost constant The increase in mass flow rate value from 01 kgs to 05 kgs at the effectiveness value of humidifier and dehumidifier, (e H, e D ) = 08, leads to increase in the system GOR from 040 to 095, but at effectiveness value (e H, e D ) = 09, the increase in GOR is only from 179 to Table 2 List of various studies related to thermal energy-based desalination methods which employ nanofluid Sr no Authors Year Type of study Summary 1 Kabeel et al [20] 2 El-Said et al [21] 3 Gupta et al [45] 4 Mahian et al [46] 5 Rashidi et al [22] 6 Rashidi et al [23] 2014 The desalination system consists of the following components: (a) Single-stage flashing evaporation unit (b) Condenser (c) Flat-plate solar water heater (d) Heat exchanger 2016 The desalination system consists of the following components: (a) Two-stage humidification dehumidification unit (b) Single-stage flashing evaporation unit (c) Flat-plate solar collector which employs Al 2 O 3 H 2 O nanofluid as the working fluid inside the collector (d) Heat exchanger is used to transfer thermal energy absorbed by nanofluid to the saline water flowing through the desalination system 2017 The desalination system consists of a conventional solar still with some modifications in its design such as water flow over the glass (sprinkler attachment), white painted vertical walls of the solar still and nanoparticles are added inside the fluid in the basin 2017 The desalination system consists of the following components: (a) Tow flat-plate solar collectors (b) Single-slope solar still (c) Heat exchanger (d) Nanofluid storage tank 2018 The desalination system consists of a single-slope solar still which employs nanofluid as the working fluid 2018 The desalination system consists of a cascade (stepped) solar still with the height of right side 0285 m, height of left side 006 m and a length of 057 m Thermal properties of working fluid are enhanced by the addition of different concentrated nanoparticles The system is technically very efficient but is not economica The daily water production capacity of the unit was obtained as 1125 kg day -1, and maximum gained output ratio was reported to be 75 Nanoparticle volume fraction affects the system productivity and solar water heater efficiency The modified solar still is 54% higher efficient than conventional still The productivity is increased due to combined effect of all these modifications Using SiO 2 water nanofluid instead of water at high temperatures provides more enhancement in evaporation rates, whereas Cuwater provides at low temperatures Using nanoparticles of size less than 7 nm improves the system performance by less than 01% The productivity of the solar still increases with the increase in solid volume fraction of nanoparticles Upon increasing the solid volume fraction within the range of 0 5%, the productivity improves by 25% Numerical study is performed to investigate the effect of nanofluid on the system productivity The increase in volume fraction from 0 5% gives 22% higher hourly production