solar collectors review

Performance enhancement of solar collectorsA review

Siddharth Suman, Mohd. Kaleem Khan n, Manabendra PathakDepartment of Mechanical Engineering, Indian Institute of Technology Patna, Patna 800013, India

a r t i c l e i n f o

Article history:Received 13 May 2014Received in revised form20 March 2015Accepted 23 April 2015

Keywords:NanouidsSolar selectiveCoatingsArticial roughnessHeat transferRibs

a b s t r a c t

Given rapid depletion of conventional energy sources and environmental degradation caused by their overexploitation, the renewable energy sources are believed to be the future. Technologies utilizing renewableenergy sources differ signicantly from one another, not only with regard to technical and economicaspects but also in relation to their reliability, maturity, and operational experience in utility scaleconditions. Technologies used to harness solar energy have emerged as the most promising and maturesince solar energy is abundant, freely available, and it has commercial potential too. This paper presents areview of advancements made in the eld of solar thermal technology with a focus on techniquesemployed for its performance enhancement. It also covers the description of different types of solarcollectors to facilitate the systematic understanding of solar thermal technology and the novel modica-tions realized in each category of solar collectors have been highlighted to promote the use of solar energyin routine activities. Performance enhancement techniques such as geometrical modications on theabsorber plate, use of solar selective coatings and nanouids have been given a special attention.

& 2015 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22. Solar collectors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1. Non-tracking or stationary collectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.1. Flat plate collectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.2. Hybrid photovoltaic/thermal (PV/T) collectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.3. Evacuated tube collectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1.4. Compound parabolic collectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2. Tracking collectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.2.1. One axis tracking collectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.2.2. Two axes tracking collectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3. Performance enhancement of solar collector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.1. Geometrical modications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.2. Solar selective coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.3. Nanouids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/rser

Renewable and Sustainable Energy Reviews

http://dx.doi.org/10.1016/j.rser.2015.04.0871364-0321/& 2015 Elsevier Ltd. All rights reserved.

Abbreviations: BNNT, boron nitride nanotube; CNT, carbon nanotube; CR, concentration ratio; CRS, central receiver system; CSP, concentrated solar power; DC, directcurrent; ETC, evacuated tube collector; ICS, integrated collector storage; LDR, light dependent resistor; MWNT, multi-wall nanotube; PCM, phase change material; PECVD,plasma-enhanced chemical vapor deposition; PV, photovoltaic; PV/T, photovoltaic-thermal; PVD, physical vapor deposition; SAH, solar air heater; SS/SS-N, stainless steel/stainless steel nitride; SWH, solar water heater; SWNT, single wall nanotube; TE, thermoelectric module; TPCT, two-phase closed thermosyphon

n Corresponding author. Tel.: 91 612 2552019; fax: 91 612 2277383.E-mail addresses: [email protected], [email protected] (Mohd.K. Khan).

Renewable and Sustainable Energy Reviews 49 (2015) 192210

1. Introduction

It is the age of machinesfor either necessity or luxury. Machinesneed energy to perform tasks. Meeting the ever-increasing demandof energy without degrading the environment has always beenconcern of scientic community. Generation of energy from limitedconventional sources has caused so much environmental degrada-tion that impact is visible in the form of pollution, acid rain, globalwarming etc. Thus, there is a crying need for producing green andclean energy from renewable sources. Among all the renewableenergy resources, the solar energy has emerged as one of the mostpromising renewable energy resource since it is abundant, freelyavailable, and it has commercial potential too. The conversion of solarenergy into different other forms is evident in nature, as shown inFig. 1. The solar energy is converted into chemical energy by theprocess of photosynthesis in green plants. The conversion of solarenergy into mechanical energy happens during the process ofevaporation from water bodies, and change in wind behavior. Inaddition, there are two broad ways of utilizing the solar energy forthe production of energy: (i) solarelectric conversion (convertingsolar energy directly into electrical energy using photovoltaic solarcell) and (ii) solarthermal conversion (converting solar energy intothermal energy using solar collector).

A lot of research works have been reported in the literature onsolarthermal systems. A few important review articles highlightingthese research works are mentioned in Table 1. Kalogirou [1]conducted an exhaustive review on different types of solar thermalcollectors and their applications. The various types of collectors werediscussed and presented with their optical, thermal and thermody-namic analysis. The applications of solar thermal systems in diverseareas of technology were illustrated to emphasize the need of its usewhenever possible. Barlev et al. [2] presented a review on concentrat-ing collectors, viz. parabolic trough collectors, heliostat eld collectors,linear Fresnel reector, parabolic dish collector etc. They suggestedthat concentrated solar power (CSP) technology could not only beused for electricity generation, but also for large range of otherapplications such as desalination of water, industrial heating andcooling, detoxication and disinfection of water etc. Other reviewarticles, listed in Table 1, are focused on any particular type of collectoror its specic application and have not discussed different techniquesemployed for the performance enhancement of solar collectors. Forexample, the methods like use of articial roughness, solar selectivecoating, and nanouids for enhancing the thermal performance ofsolar collectors have never been presented collectively.

This paper presents an overview of solarthermal technologywith an emphasis on methods adopted for enhancement of itsperformance. It also covers the description of different types ofsolar collectors to facilitate the systematic understanding of solarthermal technology and the novel modications realized in eachcategory of solar collectors have been highlighted to promote theuse of solar energy in routine activities.

2. Solar collectors

Solar collector is a device that collects thermal energy of solarinsolation by absorbing them. The thermal energy thus stored iscarried away by a owing uid and utilized for some specicpurposes. Fig. 2 shows the classication of solar collectors. The solarcollectors are broadly classied as non-tracking and tracking collec-tors. The non-tracking collectors are kept at rest and also known asxed or stationary collectors, whereas tracking collectors aredesigned to track the movement of sun so that the incoming solarradiations always fall perpendicular to them. The tracking solarcollectors are further classied as one axis tracking and two axestracking collectors. Non-tracking collectors are categorized as atplate, evacuated tube and compound parabolic collectors. Parabolictrough collector, cylindrical trough collector, and linear Fresnelreector fall under the category of single axis tracking systems,whereas central tower receiver, parabolic dish reector, and circularFresnel lens belong to dual axes tracking systems.

The collector that uses water as working uid is termed as solarwater heater (SWH), whereas the collector utilizing air as workinguid is called solar air heater (SAH). A solar water heating (SWH)system comprises of the solar collector as well as the storage tank.The solar water heating systems are further classied as passiveSWH systems (do not require external pumping agency and theow takes place due to thermo-syphonic action) and active SWHsystems (require pumping agency to circulate uid through them).An integrated collector storage system (ICS) has both the collectorand storage tank as single unit. Tang et al. [3,4] investigated effectsof temperature of water in the storage tank, structural andperformance parameters of a thermosyphon on domestic SWHat clear nights. It was suggested that vertical cylindrical tankinstead of horizontal cylindrical tank should be used for freezeprotection. It was also found that an absorber with solar selectivecoating prevents freezing at clear nights too.

Hossain et al. [5] reviewed on SWH collector and thermal energyperformance of circulating pipe, and summarized the ndingsabout the thermal performance of the at plate, concentrating,and other collectors of solar water heater with a mantle heatexchanger. They proposed an energy equation, which included aheat exchanger penalty factor. Shukla et al. [6] presented a reviewon the recent progress made in SWH technology. It was reportedthat the heat pump-based SWH could be a potential water heatingsystem in the regions where solar energy is sparse. The perfor-mance of such systems was found to be inuenced by the type ofrefrigerant employed. Shukla et al. [7] made a thorough survey onthe use of phase change material (PCM) in solar water heatingsystems or heaters (SWHs). It was concluded that only preliminarydesigns of PCM-based SWHs were available. An inbuilt thermalstorage could be an alternative to the present day solar waterheating system. Ogueke et al. [8] reviewed solar water heatingsystems for domestic as well as industrial applications. It wasconcluded that despite their higher efciency, active SWHs werenot as popular as passive SWHs. Jaisankar et al. [9] surveyed onvarious heat transfer enhancement techniques for increasing thethermal efciency of SWH. They suggested that an extensiveresearch is required on parallel ow solar collector, the shape ofFig. 1. Conversion of solar energy into other forms of energy.

S. Suman et al. / Renewable and Sustainable Energy Reviews 49 (2015) 192210 193

glass cover, and movement of ambience air over its surface for theoptimization of thermal performance of SWH.

The solar air heaters (SAHs), on the other hand, are classied assingle pass and double-pass SAHs. In single pass SAH, the air

passes through the heater once, whereas the air passes throughthe heater twice in double pass SAH. It ows over the absorberplate in the forward pass, and below the absorber plate in thereturn pass. The efciency of a double pass SAH is higher than the

Fig. 2. Classication of solar collectors.

Table 1Few important reviews on solar thermal systems.

Author(s) (year) System Findings

Kalogirou (2004) [1] Solar thermal collectors Feasibility of solar thermal collectors in a number of areas emphasized. Optical, thermal and thermodynamic analyses are presented.

Shukla et al. (2009) [7] Solar water heaters A phase change material with high latent heat and with large surface area for heat transfer can increase thermalperformance signicantly.

Ogueke et al. (2009) [8] Solar water heatingsystem

Active SWH system is more efcient than the passive SWH system.

ICS has a lower efciency than an ordinary passive SWH system.

Jaisankar et al. (2011)[9]

Solar water heaters The performance of parallel ow solar collectors is better than that of series ow collector. Research in parallelow collector is limited.

Barlev et al. (2011) [2] Concentrated collectors Presented the innovations and advancements in concentrated solar power technology.

Kumar and Rosen(2011) [17]

Photovoltaic-thermalcollectors

The overall efciency of PV/T collectors is higher than the sum of individual efciencies of photovoltaic andthermal collectors.

Ibrahim et al. (2011)[18]

Photovoltaic-thermalcollectors

Emphasized the need of improving PV efciency and its integration with building components.

Hossain et al. (2011) [5] Solar water heaters SWH with a siphon system achieves 18% higher efciency than that of a conventional system.

Chamoli et al. (2012)[16]

Double-pass solar airheater

No study reported on articially roughened double pass solar air heater. Maximum efciency is obtained if the channel depth and mass ow rate are same in both the upper and

lower ducts.

Tyagi et al. (2012) [15] Solar air heating system Energy storage systems are useful where the temperature difference between the day and night is high.

Shukla et al. (2013) [6] Solar water heatingsystem

Technical advancements in SWH systems.

Polymer at plate collectors are replacing metal at plate collectors.

Heat pump based solar water heating is useful in solar adverse regions.

Ho and Iverson (2014)[35]

Solar concentratingsystems

Plugging due to solidications and more efcient particle/uid heat exchangers are challenges to liquid and solidreceivers, respectively.

S. Suman et al. / Renewable and Sustainable Energy Reviews 49 (2015) 192210194

single pass heaters. The solar water heaters are more efcient thanthe solar air heaters because of higher convective heat transfercoefcient between the uid and the absorber plate. Some of therelevant studies on solar air heaters and their performanceenhancement have been mentioned in Section 3.1.

2.1. Non-tracking or stationary collectors

These collectors remain stationary irrespective of position ofthe sun in the sky. However, they are installed at a particular tiltand orientation angles, whose magnitudes are dependent uponthe geographical location (latitude), to maximize the harnessing ofsolar radiation, as shown in Fig. 3.

2.1.1. Flat plate collectorsA at plate collector, shown in Fig. 4, consists of a transparent

glass cover, an absorber plate with a parallel back plate. Dependingupon the type of uid, that is air or water, the ow passage isdesigned. For air as a working uid, the gap between the absorberplate and back plate is made passage for the ow of uid. Whenwater is used as a working uid, the copper tubes brazed on theabsorber plate are made ow passages. In this case, the back plate isnot required. The collector is insulated from the sides as well as fromthe bottom to further minimize the heat loss. A glass cover at the tophelps in reducing the convective as well as radiative heat loss fromthe absorber plate to the outside air. During prolonged usage, thedust settles on the glass cover of the collectors that affects theirperformance adversely. Ghazi et al. [10] summarized the effect ofdust settlement and various cleaning mechanisms deployed indifferent global zones. Cleaning is usually done by one of thefollowing methods: (a) washing by water jet, (b) blowing bycompressed air jet, (c) manual tilting of the collector, (d) use ofautomatic wiper, (e) use of super-hydrophilic nano-lm, and (f) useof electrostatic dust removal prevention method (counter balancingthe electric charge available in the dust).

The choice of collector dimensions (length and width) is sig-nicant and is dependent upon the type of application. Yeh and Lin

[11] carried out analytical and experimental studies to investigate theeffect of aspect ratio (length/width) on the efciency of at platecollector. It was found that the collector efciency increases withincrease in collector aspect ratio for constant collector area.

Few novel approaches in at plate collectors have beenreported in the literature. Ammari and Nimir [12] developed asolar water heater replacing metal absorber plate with tar coveringwater tubes. They found that the tar-based collector performedbetter than the conventional at plate collector in the late after-noon hours. Alvarez et al. [13] developed a at plate collector forsolar air heater in which absorber plate was replaced by an array ofrecyclable aluminum cans painted black. They found that max-imum thermal efciency achieved was 74%. The advantagesoffered by this novel collector are recycling of waste cans, beingcheaper, and providing a cleaner environment.

2.1.2. Hybrid photovoltaic/thermal (PV/T) collectorsA new generation of solar collectors, known as hybrid PV/T solar

systems, has recently been developed. Such systems serve both thepurposes (a) direct electricity generation using photovoltaic cells and(b) solar thermal conversion. One such system is shown in Fig. 5; itconsists of PV cells encapsulated on one side of an absorber plate,whereas there is provision for the ow of uid to be heated on theother side of the absorber plate. If the uid is water, the copper pipesare brazed on the other side. However, for air as working uid, achannel is provided between the absorber plate and the back plate; anarrangement already discussed in the description of solar air heaters. It

Fig. 4. Flat plate collector: (a) end view and (b) 3D view.

Fig. 5. Hybrid photovoltaic-thermal collector.Fig. 3. Orientation of stationary collectors.


should be noted that the efciency of PV cells decreases at hightemperatures. Hence, maintaining PV cells in a specic range oftemperature by removing the heat from them through a workinguid will result in their higher efciency, and the heat carried away byuid may also be utilized. Such systems can be employed forsimultaneous production of electricity and low temperature hot waterfor domestic purposes. Compared to PV systems, PV/T systems havemuch shorter economic payback period because of the integratedcooling arrangement of solar panel. Chow [14] carried out a meticu-lous review on hybrid PV/T collectors. It was mentioned that variouscongurations of PV/T were possible by changing the type of workinguids or coolants (air or water), type of collectors (at plate orconcentrating), material of photovoltaic cell (monocrystalline, poly-crystalline and amorphous silicon), number of glazing (single ordouble) and uid ow (above absorber, below absorber or counterow). It was also found that such collectors were in infancy stage andtheir commercial availability was very limited. Tyagi et al. [15]reviewed the research works on solar air heating systems and theirperformance for different applications. They concluded that at plateair heater produced low temperature hot air suitable for dryingagricultural products. They also suggested that hybrid photovoltaic/thermal (PV/T) type solar air heaters were found suitable for forcedconvective air heating with electricity generation. In addition, thethermal energy storage solar heaters using phase change materials(PCMs) were found to t for crop drying applications. Chamoli [16]carried out the extensive study of the research conducted on double-pass solar air heater with a focus on heat transfer enhancement,pressure drop and ow phenomenon. They observed that most of thestudies carried out so far were primarily focused on double pass solarair heater integrated with porous media and extended surfaces. Agood number of studies were also carried out with corrugatedabsorber. They suggested that there was a scope for investigationson double pass solar air heater having absorber plate articiallyroughened on the both sides. Kumar and Rosen [17] critically reviewedphotovoltaicthermal solar collectors exclusively for air heating andsuggested that PV/T air heater might be a promising technology forspace heating and drying applications. They reported that the hybridPV/T collectors are more efcient than each of the photovoltaic or solarthermal systems. However, the efforts should be made to improve itsefciency and design. Ibrahim et al. [18] summarized various types ofPV/T collector and its different designs along with their efciency.Systematic comparison of different designs features of PV/T collectorand their performance was also done. The highlight of their reviewwas the presentation of the future prospects of PV/T collectors inbuilding integration. They suggested that PV/T system could be mademore competitive by enhancing the efciency of PV and by developingbetter ways to integrate it with structural components of buildings.

In the direction of enhancing the efciency of PV/T system,Sardarabadi et al. [19] used cheap silica/water nanouid andperformed experimental investigation on it. After performingthermodynamic analysis, it was observed that silica/water nano-uid suspension substantially enhances the performance of a PV/Tsystem both energetically and exergetically. However, the cost ofstable nanouids is a concern. Recently, Zafar and Dincer [20]proposed a novel combined PV/Tfuel cell system, which can besimultaneously used for the generation of electricity directly, heatedair, puried water, and hydrogen. They even claimed that for large-scale operation or for longer durations, the water output can evencater the drinking water needs for residential and commercial travelpurposes. It will be exciting to witness the commercialization ofsuch concepts; however, cost is a big hurdle in this regard.

2.1.3. Evacuated tube collectorsThe evacuated tube collector (ETC) consists of a heat pipe kept

inside a glass enclosure, as shown in Fig. 6. The heat pipe uses liquidlike ethanol, methanol, water etc. to capture heat of solar insolationand this liquid transfers heat to some other working uid whileundergoing evaporationcondensation cycles. On receiving solar radia-tion, the liquid inside the heat pipe undergoes phase change and it isconverted into vapor, which rises toward the upper part of the heatpipe due to buoyancy. The vapor condenses back to liquid aftertransferring heat to the working uid in the heat exchanger sectionat the top. The liquid ows back to the bottom of heat pipe due togravity, and the cycle continues. The glass enclosure is evacuated tominimize the heat loss due to convection and to prevent climaticdegradation of its inner materials. Quite often, heat pipes are alsoevacuated to allow phase change of the inner liquid at low tempera-tures. Its efciency is not signicantly affected by change in incidenceangle of sun's radiation. This feature of ETC provides exibility to tubeorientation from 251 to 601 at the time of installation.

Fig. 6. Evacuated tube collector. Fig. 7. Compound parabolic collector: (a) 3D view and (b) 2D schematic.


Single tube ETCs are rarely used in practice. A number ofevacuated tubes are integrated and connected to a commonheader to achieve high coolant temperatures. The two types ofarrangements are commonly used for ETCs: H-type and T-typecollectors. It should be noted that T-type collectors collect slightlymore radiation compared to H-type collectors on an annual basis.Tang et al. [21] developed a mathematical model for calculatingdaily collectible radiation for given geometrical and structuralparameters of ETC. They recommended that for latitudes greaterthan 301, the tilt angle for T-type collector should be 101 less thanthe site latitude, and the same should be 201 less than the sitelatitude for H-type collector.

2.1.4. Compound parabolic collectorsThe compound parabolic collector (CPC) consists of a glass

cover, an absorber tube and two parabolic reecting surfaces, asshown in Fig. 7(a). The two parabolic reecting surfaces A and Bhave their focal points lying on each other, as shown in Fig. 7(b).The part of parabolic surfaces below focal points does notcontribute toward the convergence of solar radiations and, hence,the parts below the focal points are truncated. The absorber tube isplaced at the mid plane between the two focal points. If the angleof incidence is less than half of the acceptance angle, the solarradiation will pass through the receiver opening. If the angle ofincidence is greater than half of the acceptance angle, the solarradiation will ultimately be reected back to the ambiencethrough the upper opening (aperture). The orientation of theparabolic reectors should be such that the sun's position or angleof incidence has no effect on the performance of the collector.Thus, aligning the CPC's receiver or absorber tube along eastwestline eliminates the need of tracking throughout the day.

Nalwanga et al. [22] extended the use of CPC for water disinfec-tion using solar radiations. They tted 25 L borosilicate glass lledwith water infected by Escherichia coli bacteria on the CPC tube. Theexperimental results showed a complete bacterial disinfection withstrong dependence on exposure time and weather conditions. It wassuggested that CPC could be deployed for drinking water treatmentat household as well as at institutional level.

Guiqiang et al. [23] designed a novel lens-walled CPC with airgap. In this design, there is an air gap between the lens and thereector, which causes the total internal reection and minimizesthe optical losses due to specular reection. Consequently, theoptical efciency is signicantly improved.

2.2. Tracking collectors

The position of sun needs to be tracked for the maximumutilization of the incoming solar radiation. The intensity of solarradiation varies during the day as well as with seasons. Thetracking collectors can be divided into two categories: one axistracking and two-axes tracking collectors.

Mousazadeh et al. [24] reviewed the works on tracking sys-tems. It was mentioned that the use of a tracker in a solar collectorcan enhance the collected energy in the range of 10100%depending upon the time and geographical location of the site.The use of tracker increases the power consumption and it wasrecommended not to use trackers in low capacity solar collectors.

Ren et al. [25] discussed various methods used in alignment ofcollectors for maximum collectability of solar insolation. Thesemethods are broadly classied into two categories: mechanicalalignment, and optical alignment methods. Mechanical methodsinclude the use of inclinometers, the gauge block method, and thelinear displacement transducer method and are applicable for align-ing heliostats having several at mirrors. However, it was suggestedthat mechanical methods are not suitable for large scale heliostat

elds used in central tower receiver systems. Optical alignmentmethods, on the other hand, include photogrammetry, laser method,theoretical image overlay method, camera look-back method, fringereection method etc. These methods were found to be moreaccurate than their mechanical counterparts but are expensive.

2.2.1. One axis tracking collectorsThe collectors in this category are oriented along northsouth line

and the sun's position is tracked from east to west throughout theday. This category of collectors mainly comprises of parabolic troughcollector, cylindrical trough collector and linear Fresnel reector.

Kalogirou [26] developed one axis tracking system for collectorsunder this category. The system comprised of three light dependentresistors (LDRs), each serving a denite function. First LDR was usedfor collector orientation toward the sun, second LDR detected thepresence of cloud, and the third LDR was put for sensing whether itis day or night time. The signals of all these LDRs were fed to a DCmotor that rotated the collector through a gearbox.

2.2.1.1. Parabolic trough collectors. The parabolic trough collectorconsists of a parabolic reecting surface with an absorber tubeplaced along its focal line. The position of sun is tracked for normalincidence of solar radiations at any instant of time (Fig. 8). Garciaet al. [27] presented an overview of the existing parabolic-troughcollectors and their prototypes under development. The studysummarized the use of such collectors for energizing steam powercycles for electricity generation as the working uid temperature canbe of the order of 400 1C. Price et al. [28] claimed on the basis ofthorough review on parabolic trough collector that solar powergeneration using these collectors was the most reliable and robusttechnology. However, they emphasized that there was a need tobring down the cost.

The parabolic trough collector is usually employed for large-scale power generation. However, it can be also used for small-scale direct steam generation. The direct generation of steam usinga parabolic trough collector increases overall pressure drop, asthere is an additional acceleration pressure drop. Lobon andValenzuela [29] developed a model to predict the thermo-hydraulic performance of the parabolic trough collector in whichsaturated steam was generated directly. The numerical resultsshowed that the inlet pressure of working uid (water) was thedeciding factor for the feasibility of such systems. If the inletpressure was increased from 1 MPa to 2 MPa, the overall pressuredrop across the absorber was signicantly reduced for a givenoutlet pressure and temperature conditions. Based on this model,it was claimed that small parabolic trough collectors could be usedfor direct steam generation with an affordable pumping power.

Fig. 8. Parabolic trough collector.


Arasu and Sornakumar [30] used hand lay-up method formanufacturing low cost parabolic trough collector using reinforcedber glass. The collector conformed to ASHRAE standard 93 [31]

and its thermal efciency was found to be nearly 70%. Due to itslow cost, the authors recommended the methodology for batchproduction for all collector sizes.

2.2.1.2. Cylindrical trough collectors. The cylindrical troughcollectors are similar to parabolic collector with a difference thatthe rays converged in a focal plane rather than the focal line, asshown in Fig. 9. These types of collectors are rarely in practice.

2.2.1.3. Linear Fresnel reectors. The linear Fresnel reectors,shown in Fig. 10, are generally at mirrors used to concentratethe solar radiation on absorber tubes like in parabolic troughcollectors. Due to lower concentration factor than that of parabolictrough collector, the lower operating temperatures are normallyachieved in the working uid. Thus, such plants operate at a lowerefciency. Barlev et al. [2] studied the innovative work inconcentrated solar power extensively and found that linearFresnel reector is the cheapest and reliable for an operationaltemperature up to 300 1C.

Focus Fresnel reectors are used as an alternative to conven-tional tracking of trough collectors discussed in previous sections.Mills et al. [32] proposed a compact linear Fresnel reector systemin which there was an array of multiple receivers mounted onseparate towers. The proposed design was expected to produce95 MW of thermal energy and could be used as a preheater in aconventional thermal power plant.

2.2.2. Two axes tracking collectorsTwo axes tracking systems have two axes of rotation mutually

perpendicular to each other. Primary axis is one which is xed withrespect to the ground, whereas the secondary axis is positionedwith respect to the primary axis. The collectors are usually orientedparallel to the secondary axis. The two axes trackers are advanta-geous as they allow better utilization of solar radiations due to theircapability to track the sun both horizontally and vertically.

Sharan and Prateek [33] developed a low torque microprocessorbased two-axes tracking system. The design was based on thesolution of dynamic equations of planetary motion. They employeda stepper motor controlled by microprocessor. Use of microproces-sor lowered the cost of tracking system compared to sensor-basedtrackers. The system was tested for seasonal variations and theperformance was found to be accurate.

2.2.2.1. Central tower receivers. This type of arrangement isemployed in large-scale installations meant for power generation.The system has a central tower surrounded by a large number ofheliostats having individual two axes tracking systems, as shown inFig. 11. The solar energy is concentrated by the heliostat eld on a

Fig. 9. Difference between (a) parabolic and (b) cylindrical trough collectors.

Fig. 10. Linear Fresnel reector.

Fig. 11. Central tower receiver.

Fig. 12. Parabolic dish reector.


receiver mounted at the top of central tower. Water, molten salt,and pressurized air are the typical working uids for this type ofplants.

Barlev et al. [2] reported that heliostat eld collectors weregaining popularity rapidly as they could be used to generate veryhigh temperatures (up to 2000 1C). Behar et al. [34] summarized thestudies pertaining to the major components of central receiversystem (CRS). They suggested that CRS has the potential to replaceconventional thermal power plants. Ho and Iverson [35] exclusivelyconcentrated upon the studies of high-temperature central receiverdesigns for concentrating solar power. They discussed the classi-cation of receiver designs and key challenges associated with eachof them. They reported a revival of solid-particle approach inreceivers design due to promising corrosion and material interac-tion phenomenon. Nevertheless, it too has some technical chal-lenges associated like particle conveyance, attrition, transport etc.

2.2.2.2. Parabolic dish reectors. The parabolic dish collector, shownin Fig. 12, has a parabolic dish tted with two-axes tracking system.The rays are concentrated at a point where receiver is placed. Insidethe receiver, a heat exchanger is provided which allows the heat tobe transferred to the working uid. The arrangement looks like asatellite dish antenna. Eswaramoorthy and Shanmugam [36]developed a low-cost parabolic dish collector using a scrapsatellite dish antenna and studied thermal performance of thecollector. They found that for solar radiation intensity of 950 W/m2, the stagnation temperature inside the receiver was 450 1C. Thereceiver was made up of aluminum solar cooker whose thermal andoverall efciencies were found to be 60% and 40%, respectively.

Commercially available large parabolic dish collectors are costlydue to the requirement of very high precision in its manufacturing,and difculties in their transportation. Li and Dubowsky [37]presented a novel concept for designing and fabricating largeparabolic dish mirrors. They recommended the use of thin and atmetal petals having high reectivity assembled together with thehelp of cables to form a parabola. The proposed design has goodprospects to provide an alternative for expensive commerciallyavailable large dish collectors with an acceptable precision.

2.2.2.3. Circular Fresnel lens. A conventional convex lens is used toconcentrate light at a point. Extremely high temperatures areachieved at the point where rays are concentrated. This propertyof a lens is exploited in this type of collectors. The circular Fresnellens differs slightly from conventional lens with planar surfacemerged in between the convex side, as shown in Fig. 13. Thisarrangement allows capturing of more oblique light from a lightsource. Also, instead of a point, the rays are converged to a smallarea using circular Fresnel lens. It is light in weight even with largeaperture as they are capable of being manufactured in the form ofvery thin at sheet. Fresnel lens is generally used in lighthouse tobe visible over greater distances for the purpose of navigation.

Nia et al. [38] presented a design in which Fresnel lens was usedto generate thermal gradient across the absorber of thermoelectric(TE) module. From Peltier effect, the electromotive force is

generated and electricity is produced. For solar intensity of705.9 W/m2, an output power of approximately 1 W with 51.33%efciency was obtained. An array of Fresnel lenses was recom-mended for TE module with the use of intermediate uid.

3. Performance enhancement of solar collector

The efcient removal of heat from the solar collectors is theprime objective of any solar thermal system. The performance of asolar collector depends upon how much a working uid heatcarries away the heat from the collector. This section is dedicatedto the techniques used to enhance the performance of thecollectors discussed in the previous section. The literature onperformance enhancement of solar collectors can be classiedinto three main groups: (a) increasing the heat transfer coefcientbetween the absorber plate/tube and the working uid (i.e.,articially roughened absorber plates/tubes), (b) using specialtype of coatings on the absorber (i.e., solar selective coatings),and (c) increasing the thermal conductivity of the working uidusing nanoparticles (i.e., nanouids). These methods and therelevant studies have been described in the subsequent sections.

3.1. Geometrical modications

In principle, the heat transfer between a uid and a solid surfaceis increased by increasing the contact area (i.e. extended surfaces)and by creating turbulence, which promote mixing between thevarious uid layers. The nature of working uids actually governs thetype of geometrical modication to be made in the absorberassembly. If the working uid is air or gas, which has very lowconvective heat transfer coefcient, the extended surfaces/ns/cor-rugations are provided on the absorber plate. For water or liquid asthe working uid, twisted tapes/perforated tapes/wire coils, inserts/bafe plates, and internally nned tubes are provided to generateturbulence, which eventually increase heat transfer coefcient.However, the use of surface modications and turbulence promotersresults in an increased pressure drop, which ultimately increases theconsumption of pumping power. Any heat transfer enhancementtechnique is acceptable only if the gain in heat transfer rate is morethan the increase in pumping power. Different geometrical modica-tions are, thus, employed for air and water heaters.

Volumes of work are available for heat transfer enhancements insolar air heaters and solar water heaters. Some of the studiespertaining to air heaters are summarized in Table 2. Varun et al.[39] summarized the studies on the use of various roughnessgeometries and their other quantitative parameters. They discussedthe ow patterns of each shape along with Nusselt number andfriction factor correlations. Bhagoria et al. [40] experimentallyinvestigated transverse wedge shaped rib having relative height inrange of 0.0150.033 and varied the wedge angle from 81 to 151. Theribs yielded Nusselt number up to 2.4 times and the friction factorincreased up to 5.3 times in comparison to that of smooth duct. Themaximum heat transfer takes place for a relative roughness pitch ofabout 7.57 and at a wedge angle of 101. The correlations developedfor Nusselt number and friction factor are within the error limits of715% and 712%, respectively. Jaurker et al. [41] performedexperimental investigation on heat transfer and friction character-istics of a solar air heater using rib-grooved articial roughness tond optimized conditions for its performance. It is observed thatrib-grooved arrangement provides the best thermo-hydraulic per-formance and yields Nusselt number up to 2.7 times, while thefriction factor rises up to 3.6 times. The optimized groove positionto pitch ratio is determined to be 0.4, and the correlations devel-oped for the Nusselt number, and the friction factor are found to bewithin deviations of 2.73% and 3.16%, respectively. Lanjewar et al.Fig. 13. Circular Fresnel Lens: (a) 3D view and (b) ray diagram.


Table 2Heat transfer enhancement in air heaters.

Author(s)(Year)

Types of roughness Range of parameters Correlations Findings

Shape Conguration Heat transfer coefcients Friction factor

Bhagoria et al.(2002) [40]

Wedgeshaped rib

Re : 300018;000e=Dh 0:0150:033

p=e 60:171:0264op=eo12:12 8 3 15 3

W=H 5

Nur 1:89 104 Re 1:21 e=Dh 0:426 p=e 2:94 exp 0:71 ln p=e 2 h i

=10 0:018 exp 1:50 ln =10 2 h i

f r 12:44 Re 0:18 e=Dh 0:99 p=e 0:52 =10 0:49Maximum heat

transfer occurs fora relativeroughness pitch ofabout 7.57 and at awedge angle of 101

Jaurker et al.(2006) [41]

Rib-grooved Re : 300021;000e=D 0:01810:0363p=e 4:510:0g=p 0:30:7

Nu 0:002062Re0:936 e=D 0:349 p=e 3:318exp 0:868 ln p=e 2h i g=p 1:108 exp 2:486 ln g=p 21:406 ln g=p 3h i

f 0:001227 Re0:199 e=D 0:585 p=e 7:19exp 1:854 ln p=e 2h i g=p 0:645 exp 1:513 ln g=p 20:8662 ln g=p 3h i

Maximum heattransfer occurs fora relativeroughness pitch ofabout 6.0 and at agroove position topitch ratio of 0.4

Kumar andSaini (2009)[47]

Arc shaped Re : 600018;000e=D 0:0299; 0:0426p=e 10:0 30 3 ; 60 3

Analyzed using computational uid dynamics Renormalization-group k modelresults have goodagreement andmaximum overallenhancement ratiowas found 1.7

Karwa andChauhan(2010) [48]

Rectangularrib inv-downshape

Re : 107026;350e=Dh 0:020:07p=e 10w=e 2B=S 6 60 3

Theoretically analyzed Roughened ductsare suitable onlywhen mass owrate is low; at highow rate effectiveefciency is samefor bothroughened andsmooth duct

Lanjewar et al.(2011) [42]

W-shapedrib

Re : 230014;000e=Dh 0:018; 0:0225;0:02925 & 0:03375p=e 10W=H 8 30 3 ; 45 3 ; 60 3 & 75 3

Nur 0:0613 Re 0:9079 e=Dh 0:4487

=60 0:1331

exp 0:5307 ln =60 2 h if r 0:6182 Re 0:2254 e=Dh

0:4622=60 0:0817

exp 0:28 ln =60 2 h iMaximumenhancement ofNu and f due toarticialroughness is 2.36and 2.01 times,respectively, for601

f 2:32Re0:201 S=e 0:383 L=e 0:484 d=D 0:133

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Bhushan andSingh (2011)[43]

Circularprotrusions

Re : 400020;000

d=D 0:1470:367S=e 18:7537:5L=e 2537:5W=H 10e=D 0:03

Nu 2:1 1088Re1:452 S=e 12:94 L=e 99:2 d=D 3:9exp 10:4 log S=e 2h i exp 77:2 log L=e 2h i

exp 7:83 log d=D 2h iMaximumenhancement ofNu and f due toarticialroughness is3.8 and 2.2 times,respectively

Yadav et al.(2013) [44]

Circularprotrusionsin angulararc

Re : 360018;100e=D 0:0150:03p=e 1224W=H 11 45 3 75 3

Nu 0:154 Re 1:1017 p=e 0:38 e=D 0:521 =60 0:213 exp 2:023 ln =60 2 h i

f 7:207 Re 0:56 p=e 0:18 e=D 0:176 =60 0:038 exp 1:412 ln =60 2 h i

Heat transferenhancement andfriction factorattains maximumvalue fore=D 0:03 andp=e 12

Bhattacharyyaet al. (2013)[45]

Twistedtapes withintegraltransverseribs

c 0; 0:2; 0:4 & 0:6e=Dh 0:07692 & 0:1026

p=e 2:0437 & 5:6481Num 5:172 10:0749315Gz0:9416

2:57:9261 106 Sw:Pr0:565 2:655 2"

1:5363 1015 Reax:Ra 2:18i0:1

b=w 0:14 1 1exp 0:09818c : e=Dh

0:0658p=e 0:7925

! !

fRe Sw 17:355 22=Dh4=Dh 2

1106Sw2:67 1=7

1 1exp 0:0621c : e=Dh 0:0635

p=e 0:728

! Centre-clearedtwisted tapes withintegral transverseribs performsignicantly betterthan the individualtechnique usedalone

Kumar et al.(2013) [46]

V-shapedwith gap rib

Re : 200020000e=D 0:0220:043p=e 612W=w 110g=e 0:51:5Gg=Lv 0:240:80

30 3 75 3

Nu 8:532 103Re0:932 e=D 0:175 W=w 0:506exp 0:0753 ln W=w 2h i Gd=Lv 0:0348exp 0:0653 ln Gd=Lv

2h i g=e 0:0708exp 0:223 ln g=e 2h i =60 0:0239exp 0:1153 ln =60 2h i p=e 1:196 exp 0:2805 ln p=e 2h i

Nu 3:1934 Re0:3151 e=D 0:268 W=w 0:1132exp 0:0974 ln W=w 2h i Gd=Lv 0:0610exp 0:1065 ln Gd=Lv

2h i g=e 0:1769exp 0:6349 ln g=e 2h i =60 0:1553exp 0:1527 ln =60 2h i p=e 0:7941

exp 0:1486 ln p=e 2h i

Multi v-shapedwith gap rib ismore efcient thancontinuous multiv-shaped rib

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[42] used the W-shaped rib roughness inside the rectangular ductof a solar air heater and investigated the heat transfer along withfriction factor for different angles of attack of ow in the range of30751. It is observed that ribs in W-conguration are shorter thanribs of V-shaped conguration for same width of plate. Theboundary layer along relatively shorter W-ribs is thinner thanboundary layer along V-shape ribs, giving better thermo-hydraulicperformance for W-ribs than V-ribs. The best thermo-hydraulicperformance and the maximum enhancement of Nusselt numberand the friction factor are achieved at the angle of attack of 601. It isalso seen that rate of increase of Nusselt number with increasingReynolds number is lower than rate of increase of friction factorbecause reattachment of free shear layer does not occur and rate ofheat transfer enhancement is not proportional to that of frictionfactor at higher values of relative roughness height. Bhushan andSingh [43] performed experiments on solar air heater duct withcircular protrusions in order to determine the Nusselt and frictionfactor, which give qualitative idea of efciency of the solar airheater. They validated their experimental data for Nusselt numberand friction factor with DittusBoelter and modied Blasius

correlations, respectively. On comparison with smooth duct, theyobserved a maximum enhancement of 3.8 times and 2.2 times inNusselt number and friction factor, respectively. Yadav et al. [44]experimented with rectangular duct of solar air heater, which isroughened by circular protrusions oriented in angular arc. Thebenet of protrusion is that it does not add extra weight to thesolar heater. The maximum enhancement of Nusselt number andfriction factor, which occurs for arc angle value of 601, is found to be2.89 and 2.93, respectively. The numerical correlations developedfor Nusselt number and friction factor have been found to be withindeviations of 3.81% and 4.91%, respectively. Bhattacharyya et al. [45]performed heat transfer enhancement experiments in a circularduct solar air heater having a combination of integral transverseribs and centre-cleared twisted tape as articial roughness. Themajor breakthrough reported is that centre-cleared twisted tapesintegrated with transverse ribs gives signicantly better results thanthe individual enhancement technique used alone. Kumar et al. [46]conducted experiments on solar air heater having multi V-shapedwith gap ribs in the Reynolds number range of 200020,000 forangle of attack between 301 and 751. The value of Nusselt number

Table 3Heat transfer enhancement in water heaters.

Author(s) (Year) Structural modication Range of parameters System Test uid

Shape Material

Garcia et al. (2005)[49]

Wire coil Steel Re : 8090;000e=d 0:070:10d 18 mmp=d 1:172:68

p=e 1433

Heat exchanger Water, Waterpropyleneglycol

Jaisankar et al. (2009)[51]

Helical twisted tapes Copper L 1 mdi 11 mmdo 12:5 mmRe : 300023;000

H=D 36

Solar water heater (coppertube)

Water

Promvonge et al. (2012)[53]

Twin twisted tapes coupled with helicalribbed

Aluminum Re : 600060;000e=DH 0:06p=DH 0:27Y 2:179:39Coswirl with helical

Heat exchanger (double coppertube)

Water

Garcia et al. (2013) [50] Wire coil Steel m : 0:0110:047 kg=s Solar water heater (coppertube)

Water

Azmi et al. (2014) [52] Twisted tape Aluminum L 1:5 mdi 16 mmdo 19 mmRe : 800030;000 03%

H=D 015

Heat exchanger (copper tube) TiO2/Water nanouid

Nanan et al. (2014) [54] Perforated helical twisted-tapes Aluminum Re : 600020;000d=w 0:2; 0:4; 0:6s=w 1; 1:5; 2P=D 2y=w 3

e=D 0:03

Heat exchanger Water

Sandhu et al. (2014) [55] Twisted tape, wire coil and wire mesh Steel andcopper

Re : 2008000L 0:915 mdi 13:4 mm

do 16 mm

Solar water heater (coppertube)

Water


and friction factor was greater than that of duct with multi V-shaped without gap rib. This arrangement of articial roughnessexhibited an enhancement of Nusselt number up to 6.74 times forthe angle of attack of 601.

Kumar and Saini [47] used the computational uid dynamics toanalyze the performance of a solar air heater after incorporation ofarticial roughness in the form of thin circular wire. With increasein Reynolds number, Nusselt number is seen to increase, whereasthe friction factor decreases for all combinations of relative rough-ness height and relative arc angle. Karwa and Chauhan [48]theoretically worked on the evaluation of performance of a solarair heater having V-down discrete rectangular cross-sectionrepeated rib roughness. After analyzing the results, it was sug-gested that at low mass ow rate the roughened duct solar airheaters give signicantly better effective efciency while due toconsiderably large increase in pumping power at high mass owrate, it is not advisable to use roughened duct.

In water heaters, the heat transfer rate is increased by usingturbulence promoters such as twisted tape inserts, perforatedtwisted tape inserts, wire coil inserts, wire mesh etc. in the owpassage. Table 3 shows some recent studies on heat transferaugmentation in water heaters. Garcia et al. [49] used steel wirecoil inserts to evaluate the in-tube heat transfer coefcient usingwater and waterpropylene glycol as working uids. The testswere conducted for laminar-transition-turbulent regimes. Theyfound that the heat transfer rate increased by 200% for constantpumping power. It was also observed that the maximum enhance-ment occurred in transition regime. On the basis of these ndings,later Garcia et al. [50] implemented wire coil inserts in the atplate solar water heater. The tests were conducted for differentmass ow rates in the range 0.0110.047 kg/s. It was found thatthe wire-coil inserts increased thermal efciency of the collector.1431% increase in average thermal efciency was reported for thementioned range of mass ow rate. However, for higher mass owrates, the increase in heat transfer rate was not signicant.

Jaisankar et al. [51] performed experiments on helical twistedtape inserts with different twist ratios inside the copper tubes of atplate solar water heater to determine the heat transfer and pressuredrop characteristics. They used twisted tapes made up of copper.The Reynolds number was varied from 3000 to 23,000. It was foundthat with the increase in twist ratio, the swirl generation decreasedresulting in both heat transfer rate and pressure drop. The use oftwisted tapes reduced the collector area requirement by 824% for agiven efciency. Recently a similar study was conducted Azmi et al.[52] using aluminum made twisted tapes with a different test uid.The test uid was TiO2/water nanouid for concentrations rangingfrom 0% to 3%. It was found that the heat transfer coefcientincreased by a maximum of 23.2% at 1% concentration of nanopar-ticles. The increase in twist ratio caused a decrease in heat transfercoefcient. The thermo-hydraulic performance analyses showedthat maximum advantage of both nanouid and twisted tapeinserts occurred at twist ratio of 15 and concentration of 1%.Promvonge et al. [53] used twin twisted tapes inside a helical-ribbed tube of double tube heat exchanger. The direction of both

(tube and tapes) structural modications was designed to create aco-swirl of the uid. They found an improved overall performancewith the proposed design compared to individual modications.The highest thermal efciency was achieved for twist ratio of eight.

It has been seen that the use of various types of inserts increasesthe heat transfer as well as pressure drop. A modication in thetwisted tape by perforating it is expected to reduce the pressure dropand increase the turbulence further. Nanan et al. [54] experimentedwith perforated helical aluminum twisted tapes. It was found thatmaximum performance was achieved for 0.2 diameter ratio (d/w)and 2 pitch ratio (s/w) at the Reynolds number of 6000. Sandhu et al.[55] investigated three different inserts namely, twisted, wire coil,and wire mesh to evaluate the performance of solar water heater.The concentric wire coils were found to be the best among all otherinserts. The heat transfer coefcient increased by 110% at lowReynolds number and 460% at high Reynolds number.

3.2. Solar selective coatings

An efcient way to maximize the harnessing of solar insolationis to apply coatings of some specic materials on the absorbersurface. Coatings are broadly classied as non-selective coatings,and solar selective coatings, as shown in Fig. 14. The opticalproperties like reectivity, absorptivity, emissivity etc. of non-selective coatings are spectrally uniform, that implies opticalcharacteristics of such coatings are independent of wavelengthover a particular wavelength range. These coatings have poor solarselectivity and also they are thermally unstable at an elevatedtemperature resulting in poor absorber efciency. One of theexamples for non-selective coatings is ordinary black paint appliedon the absorbers surface. It increases both absorptivity andemissivity. In solar thermal application, a coating should have ahigh absorptivity but a low emissivity, so that it retains thetrapped thermal energy. This limits the applicability of non-selective coatings for solar thermal conversion technology. Thatis why researchers gave less attention to these coatings. Never-theless, recently Tulchinsky et al. [56] synthesized a new non-selective coating formed by thermal reaction of solgel titaniawith copper manganese spinel. This novel thermal coatingCu0.44Ti0.44Mn0.84Fe0.28O3 exhibits bixbyite structure and has anabsorptivity of greater than 95% in the visible range. This coatingcan be applied with ease either manually or by spray technique. Ithas potential to be employed for solar thermal conversion. How-ever, synthesis of some more new coatings with other similarcombination of materials needs to be examined for furtheroptimizing its optical properties.

Solar selective coatings, on the other hand, have differentabsorptivity and emissivity in different spectral regions. It meansthat the optical properties are spectrally dependent. From StefanBoltzmann law the energy emitted by a body is proportional tofourth power of its absolute temperature. Also, from Planck's law,the photonic energy of radiation is inversely proportional to itwavelength. This means a body at high temperature will emitthermal radiation at shorter wavelength and vice-versa. Thus, the

Fig. 14. Classication of solar coatings.


incoming solar radiation has shorter wavelength and thermalradiation emitted by the absorber surface will obviously havelonger wavelength. The solar selective coatings allow incomingsolar radiation to pass through it and block the emittance of longerwavelength thermal radiation. Thus, they help in capturing theradiative energy to achieve high temperatures. There are manytypes of coatings based on different absorption mechanisms suchas light trapping, particulate coatings, semiconductor-metalliclayers, multi-layer lms, quantum size effects, and intrinsicabsorption. Besides having a long-term thermal stability, these

coatings should have high absorptivity in the 0.32.5 mm spectralrange and low emissivity in the far infrared range (0.7 mmonwards) for given operating range of temperature [57]. Theoptical characteristic of a coating is dened in terms of solarselectivity, which is the ratio of solar absorptivity to emissivity ata given temperature. By improving the optical characteristic andmaking it thermally stable at high temperatures will eventuallyincrease the working uid temperature, thereby improving theoverall efciency of solar collectors. There are numerous processes,in principle, to achieve selective solar absorbing surfaces. The

Table 4Selective experimental studies on solar selective coatings.

Author(s) (Year) Material Technique used Conclusions

Substrate(s) Coating(s)

Abbas (2000) [58] Nickel platedcopper

Black chrome 0.96 0.12 Electrochemicaltreatment

Make solar collectors better efcient thermally by more than 30% Good thermal stability up to 300 1C

Schuler et al. (2000)[59]

Aluminum a-C:H/Ti 0.876 0.06 PVD/PECVD Selectivity as high as 14.4 is achieved without optimizing thickness Service life is predicted to be more than 25 years

Teixeira et al.(2001) [61]

Glass, Aluminumand Copper

CrCr2O3/MoAl2O3

0.880.94

0.150.04

Magnetronsputtering

Composite coatings offer excellent thermal stability Need to optimize the optical behavior of coatings by simulation

Farooq andHutchins (2002)[62,63]

Copper andaluminum

V-Al2O3 0.98 0.02 Magnetronsputtering

Number of layers were insignicant beyond a critical number Four layer PGSAC gives the best efciency

Cindrella (2007)[64]

Nickel-platedcopper

CoCd/NiCd 0.93 0.07 Electro-deposition

Selective coating is required essentially only for at plate collectors Low emittance coating required for unity concentration ratio systems

Shashikala et al.(2007) [65]

Nickel-platedaluminum alloy

Black NiCo 0.948 0.17 Electrochemicaltreatment

Nickel undercoat inuences the optical properties substantially Suitable for space applications too

Wazwaz et al.(2010) [66]

Aluminum alloy Ni 0.892 0.052 Electrochemicaltreatment

Average thermal efciency increases with increase in Nickel content Exists an optimum limit of nickel that gives the optimum efciency

Juang et al. (2010)[67]

Glass SS/SS-N 0.91 0.06 Magnetronsputtering

Fabricated high solar selectivity composite coating by the use of only onesputtering target having exibility to control thickness of layers

Du et al. (2011) [68] Silicon Ti0.5Al0.5N/Ti0.25Al0.75

0.945 0.04 Magnetronsputtering

Metallic character is exhibited by Ti0.5Al0.5N and resembles NaCl Semiconducting character is exhibited by Ti0.25Al0.75

Liu et al. (2012) [70] Copper andstainless steel

NbTiON/SiON

0.95 0.07 Magnetronsputtering

Oxidation and diffusion of copper degrades lm at high temperature Coating on SS exhibits better thermal stability above 500 1C

Nuru et al. (2012)[71]

Glass, silicon andcopper

AlxOy/Pt/AlxOy

0.94 0.01 Electron beam Dielectric/metal/dielectric coating exhibits high performance Increase in surface roughness of coating layers with temperature

Khamlich et al.(2013) [73]

Tantalum Cr/-Cr2O3 0.90 0.28 Aqueouschemicalgrowth

Optical properties are the function of annealing temperature More suitable for absorbers required for temperature below 600 1C

Liu et al. (2014) [74] Stainless steel CrAlO 0.924 0.21 Cathodic arc ionplating

Cr2Al nanograins intrinsic absorption property cause high absorptivity Excellent thermal stability, suitable for collectors at high temperature

Valleti et al. (2014)[75]

Copper andstainless steel

TiAlCrN/TiAlN/AlSiN

0.91 0.07 Cathodic arcPVD

Excellent thermal stability of coating at elevated temperature Substrate degrades at high temperature, need better alternate substrate

Cespedes et al.(2014) [76]

Silicon andstainless steel

MoSi3N4 0.926 0.017 Magnetronsputtering

Highest selectivity ratio at temperature above 600 1C Potential to be used as coating for CSP technology

Kumar et al. (2014)[77]

Copper CuOnanoparticles

0.84 0.06 Chemicaltreatment

Coating nanostructures varies with pH of reaction Multiple absorption takes place in porus structure of coating


challenge lies in developing coatings, which should not only becompatible with absorber surface but also economical and easy toproduce in bulk.

Table 4 shows the research on different types of solar selectivecoatings. Abbas [58] argued that black chrome or solchrome, whichis a metal-based coating, has high absorptivity and requires veryless maintenance. He conducted experimental evaluation on threecollector types viz. solchrome tig welded n and tube collector,solchrome omega soldered n and tube collector, and black paintedomega soldered n and tube collector. Solchrome coatings werefound to enhance the collector efciency by more than 30% even forlow temperature applications. Schuler et al. [59,60] developedtitanium containing amorphous hydrogenated carbon coating(a-C:H/Ti) and amorphous hydrogenated silicon carbon coating(a-Si:C:H/Ti) by combined PVD and PECVD process. Amorphoushydrogenated carbon coating was prepared by sequential deposi-tion of three layers viz. pure Ti, a-C: H/Ti and pure a-C:H on thesubstrate. The optical properties were reported to be a function ofvarious parameters and strongly dependent upon the quantity oftitanium. Without performing any optimization, the developed a-C:H/Ti coating yielded a solar selectivity of 14.4 at a temperature of100 1C. On conducting accelerated aging test with aluminum assubstrate, the service life period of coating was predicted to be 25years. Later they fused silicon in the preparation of coating andclaimed to signicantly enhance the lifetime stability, irrespective ofthe substrates, than former. Silicon posed challenge of fast degrada-tion under humid conditions but it can be well controlled bycontrolling the content of silicon. Nonetheless, this coating is a very

good candidate for vacuum collectors since they have long servicetime under high temperature. Teixeira et al. [61] produced aspectrally dependent multilayered coating having a thickness ofabout 300 nm based on metallic chromium and molybdenumembedded in the matrix of chromium oxide and aluminum oxide,respectively. The lms were manufactured by reactive DC magne-tron sputtering by keeping the oxygen supply in pulsated manner(oxygen ow was switched on and off) in order to produce multi-layered structure. Copper and glass were used as substrates. Theyestablished reactive sputtering as a potential process for theproduction of graded selective coating with the benet of control-ling the addition of metallic phases. However, they emphasized theuse of numerical simulation beforehand in order to optimize thecoating properties before its actual production. Farooq and Hutchins[62,63] developed a multilayer metal-dielectric graded index spec-trally dependent coating using co-sputtering technique. They per-formed computer simulation beforehand to understand the role ofvarious parameters like number of layers, thickness of each layers,best combination of metal-dielectric matrix etc. and to optimizethem. Experiments too were conducted to validate the computermodel ndings. Beyond a certain number of layers, the variation inoptical performance was found not to be considerable. Out of theinvestigated batch of coatings, four layer prime graded selectiveabsorber coating of V:Al2O3 gave the best result with solar absorp-tivity of 0.98 and emissivity of 0.02. Cindrella [64] critically analyzedthe effectiveness of selective coatings in solar thermal systems ofdifferent concentration ratios with respect to two composite solarselective coatings: cobaltcadmium and nickelcadmium. It wasfound that a coating with high value of absorptivity is essential forsolar thermal systems with higher concentration ratio (CR410)where as low emittance coatings should be taken for systems withunity concentration ratio. Shashikala et al. [65] investigated nickelcobalt coating keeping aluminum alloy with nickel undercoat assubstrate. Even though nickel undercoat was applied to resist thecorrosion, its thickness was found to inuence optical properties ofthe coating. The nickel undercoat thickness was optimized at57.5 mm. Hull cell experiments were used to optimize the electro-chemical process of coating preparation. The resultant selectivecoating was put under various tests such as humidity, thermalcycling, thermal stability etc. and it exhibited good environmentalstability making it suitable not only at earth's surface but also inspace. It has solar absorptivity and emissivity of 0.948 and 0.17,respectively. In a similar study, Wazwaz et al. [66] inspected theeffects of nickel content and alumina layer thickness in a selectivecoating on aluminum alloy substrates. Solar selectivity was found tobe proportional to amount of nickel. However, there existed a limitof 60 mg/cm2 of nickel content for the maximum selectivityand thermal efciency. Juang et al. [67] fabricated a stainless

Table 5Comparison of thermal conductivity of additives and base uids.

Material Thermal conductivity (W/m K)

Metallic solids Iron 83.5Aluminum 237Gold 318Copper 401Silver 428

Non-metallic solids Al2O3 40CuO 76.5Si 148SiC 270BNNT 200600CNT (MWCNT) 3000

(SWCNT) 6000

Base uids Water 0.613Ethylene glycol 0.253Engine oil 0.145

Fig. 15. Constituents of nanouid and their common examples.


steel/stainless steel nitride (SS/SS-N) cermet thin coating usingreactive sputtering. Based on ow variation of nitrogen gas, highmetal volume fraction and low metal volume fraction coatings wereproduced. The produced composite selective coating has solarabsorptance of 0.91 and low thermal emittance of 0.06 at 82 1C.Du et al. [68,69] developed and optimized a novel multilayerselective coating consisting Ti0.5Al0.5N and Ti0.25Al0.75N as absorberlayers and AlN as anti-reection layer. The optimization wasachieved using computer program TFCalc. The produced four-layered (AlN being dielectric) solar selective coating has a solarabsorptance of 0.91 and a low thermal emittance of 0.06 at 82 1C.The coating was found to be stable till approximately 500 1C in air.However, diffusion of oxygen causes degradation beyond 500 1C. Liuet al. [70] developed solar selective lm having NbTiON and SiONlayers on copper and stainless steel substrates using magnetronsputtering. The optimized thickness was found experimentally as150 nm and 70 nm for NbTiON and SiON layers, respectively. Thecoating exhibited better thermal stability on stainless steel sub-strate at an elevated temperature. Nuru et al. [71,72] prepared solarselective multilayer coating AlxOy/Pt/AlxOy on glass, silicon andcopper substrates using electron beam evaporator technique. Thiscoating was based on dielectric/metal/dielectric arrangement. BothAlxOy layers were dielectric in nature, whereas Pt layer was semi-transparent. Ellipsometric measurements and optical simulationwere used to optimize the parameters. An optimized coating havingthickness of 1370 deposited on copper demonstrated a solarselectivity of 15.6. While performing thermal stability test, therewas a sharp decline in the intensity of Pt grain in the temperaturerange of 300600 1C and it completely vanished forming CuO andCu2O at 700 1C. Khamlich et al. [73] produced chromium/alpha-chromium (III) oxide absorber coating on tantalum substrate using anovel and cost-effective approach named as aqueous chemicalgrowth method. This method is simple, cheap, and provides bettercontrol of parameters. Annealing done in the temperature range of400500 1C exhibited selectivity comparable to black chrome coat-ing. X-ray diffraction and attenuated total reection results estab-lished that such coatings are excellent for the application in systemswith temperature range below 600 1C. Liu et al. [74] developed afour layer coating based on chromiumaluminumoxygen by theprocess of cathodic arc ion plating. Different layers were obtainedby varying the oxygen concentration. On conducting tests onspectrophotometer and Raman spectroscopy to determine its opti-cal properties and thermal stability respectively, it was found that ithas solar absorptance of 0.924 and emittance of 0.021. This coatingcan be employed for solar thermal systems operating at tempera-ture above 600 1C. Valleti et al. [75] fabricated a multilayered nitridebased coating on copper and stainless steel using cathodic arcphysical vapor deposition. The coating has three layers namely,TiAlCrN as infrared reector, TiAlN as absorber, and AlSiN as anti-reective layer. On the one hand, the absorber layer TiAlN depositedat moderate nitrogen partial pressure with minimal Ti content

exhibits high absorptance properties, on the other hand infraredreector layer TiAlCrN deposited at high chromium content resultsin better absorptance properties. This multilayered selective coatingpresented a good thermal stability at high temperature with solarselectivity of 13. However, the substrates steel degradation wasobserved at elevated temperatures, leaving a scope of investigationfor a better alternative substrate. Cespedes et al. [76] depositedmultilayered selective coating using magnetron sputtering afteroptimizing it through optical simulations. It consists of silverinfrared reector, molybdenumsilicon nitride as absorber andsilicon nitride as anti-reective layer. This MoSi3N4 cermet-basedspectrally dependent coating demonstrated a high solar absorptiv-ity of 0.926 and a low thermal emissivity of 0.017 even attemperature as high as 600 1C; making it suitable for solar con-centrated power technology. Kumar et al. [77] recently preparedspectrally dependent coating at room temperature by chemicaloxidation of copper at various alkaline conditions. The nanostruc-ture of copper oxide layer was found to be dependent on pH of thepreparing solution. The porous nature of nanostructures causesmultiple absorptions. The coating possessed solar selectivity of 12,making it suitable for photo-thermal conversion.

3.3. Nanouids

Thermal properties of liquids play a decisive role in heating aswell as cooling applications in industrial processes. Thermal con-ductivity of a liquid is an important physical property that decides itsheat transfer performance. Conventional heat transfer uids likewater, oil, ethylene glycol etc. have inherently poor thermal con-ductivity, which makes them unt for applications where high heattransfer rate is required. There are continuous attempts to enhancethe inherently poor thermal conductivity of these conventional heattransfer uids using highly conducting solid additives. However, ifthe particulate size of solid additive is kept in order of micrometer orlarger, it has the drawbacks of particle sedimentation, corrosion ofcomponents of systems, particle clogging, excessive pressure dropetc. These problems get reduced signicantly if the particle size ofsolid phase is of order of nanometer, and the resulting uid is knownas nanouid. Nanouids are dilute liquid suspensions of nanoparti-cles with at least one of their principal dimensions smaller than100 nm [78]. The striking features which made nanoparticles suitablecandidates for suspension in uids are small size and a large surfacearea, less particle momentum, and high mobility. Due to very smallsizes and large specic surface areas of the nanoparticles, nanouidshave many superior properties like minimal clogging in owpassages, long-term stability, homogeneity, other than having anobvious high thermal conductivity; thermal conductivity of commonsolid additives and base uids is given in Table 5 [79,80]. To achieve astable nanouid, the particles should be dispersed in base uids withno or very little agglomeration. This can be done through variousmethods, including electrical, physical, or chemical. In the literature,

Table 6Classication of nanouids based on nanoparticles volume fraction.

Types of nanouid Shape of nanoparticles Volume fraction of nanoparticles ()

Dilute nanouid Spherical 0oo 0:001Tubular or rod-like (r length/diameter) 0oo1=r2

Semi-dilute nanouid Spherical 0:001oo 0:05Tubular or rod-like 1=r2oo1=r

Semi-concentrated nanouid Spherical 0:05oo 0:10Tubular or rod-like 1=roor

Concentrated nanouid Spherical 4 0:10Tubular or rod-like 41=r


the commonly used dispersion technique is ultrasonic or statorrotormethod [8185]. Sometimes stabilizing agents are also used duringdispersion and they are called surfactant or dispersants [85,86]. Thebase uids usually used for nanouids are water, ethylene glycol,transformer oil, and toluene. The nanoparticles that are used may bebroadly categorized into three groups: ceramic particles, pure metal-lic particles, and carbon nanotubes (CNTs). Different combinations ofthese base uids and nanoparticles uids give different nanouids.Thus, based on nanoparticles dispersed in base uids, there are threemajor types of nanouids, namely ceramic nanouids, metallicnanouids, and carbon nanotube nanouids, as shown in Fig. 15[79,87,88]. Moreover, the concentration of nanoparticles also decidesthe properties of nanouids; it means that the similar combination ofbase uid and nanoparticles in different concentrations will havedifferent properties. Based on the concentration of nanoparticles inthe base uid, nanouids are categorized as dilute, semi-dilute, semi-concentrated, and concentrated nanouids, more details are pro-vided in Table 6 [89].

Despite considerable research efforts and signicant progress inthe past decade, the fundamental understanding of nanouids islimited. This is indeed echoed through the signicant scattering ordisagreement of results reported in the literature. A few importantstudies related to various aspects of nanouids have been presentedin Table 7. Wang and Majumdar [80] summarized the heat transfercharacteristics of nanouids and reported that the eld is still ininfancy and its development faces challenges like the lack ofagreement between experimental results from different groups, theoften below par performance of suspensions, and lack of theoreticalunderstanding of the mechanisms of their heat enhancing character-istics. Trisaksri and Wongwises [90] critically reviewed the heattransfer characteristics of nanouids and concluded that the qualityof nanouids of enhancing thermal conductivity is dependent upon

type of base uid and nanoparticles, size and shape of nanoparticles,the particle volume fraction, pH value of nanouids and type ofparticle coating. The natural convective heat transfer of nanouids isdifferent from that of the common suspensions in that the particlesconcentration gradient is absent. The issue in exactly claiming aboutnanouids is that it is still not clear which is the best model to use forthe thermal conductivity of nanouids. It is predicted from theanalytical investigation that with increase in particle volume fractionand density of nanoparticles, the natural convective heat transfer ofnanouids should increase. The experimental ndings are in dis-agreement to the analytical results. Haddad et al. [91] studied theliterature on natural convective heat transfer of nanouids for bothsingle and two-phase models which is signicant for understandingsof solar water heating systems. Most of the numerical results haveclaimed of heat transfer enhancement by using nanouids but onexperimental investigation it was found that nanoparticles degradeheat transfer systematically. Since the experimental results aremostly obtained using one type of nanouid (Al2O3water), there isdire need of benchmark experiments for the validation of thenumerical results. Numerical studies trying to explain the observedanomalous enhancement in heat transfer should be based on two-phase approach as it considers slip velocity between the particle andbase uid, which plays an important role on the heat transferperformance of nanouids. Ahmed et al. [92] extensively studiedthe heat transfer augmentation technique using nanouids andobserved that presence of nanoparticles is more effective at thesmaller inclination angles. The variation in thermo-physical proper-ties of nanouids is caused by the amount of the agglomeration ofnanoparticles as well as the formation of compressed phase. Theeffective viscosities and effective thermal conductivities of nanouidare related to the aggregates of nanoparticles. They also found thatthe increase in pumping power as a result of large pressure drop and

Table 7Reviews on heat transfer augmentation using nanouids.

Author(s) Years Aspect of nanouidspresented

Remarks

Wang and Majumdar (2007)[80]

Heat transfercharacteristics

Investigation focused on thermal conductivity rather than heat transfer characteristics

Trisaksri and Wongwises(2007) [90]


No agreement over best model to use for the thermal conductivity of nanouids

Saidur et al. (2011) [95] Applications andchallenges

Exact mechanism of enhanced heat transfer for nanouids is still unclear

Haddad et al. (2012) [91] Natural convective heattransfer

Two phase ow model has more accuracy

Presence of nanoparticles deteriorates heat transfer systematically

Ahmed et al. (2012) [92] Heat transferaugmentation

Varying experimental results about the pressure drop associated to the suspension of nanoparticles in thebase uid

Chandrasekar et al. (2012)[93]

Forced convective heattransfer

Best combination of nanoparticle and base uid to be selected to optimize thermophysical properties

Sureshkumar et al. (2013)[94]


Hybrid nanouids are not more effective compared with the pure nanoparticles nanouid system

Mahian et al. (2013) [96] Applications in solarenergy

Need to nd the optimum volume fraction Vagaries in effect of particle size on the efciency of collectors

Nkurikiyimfura et al. (2013)[97]

Magnetic nanouid Solar radiation could be completely absorbed in a magnetic nanouid layer with about 10-mm thickness Studies regarding the parameter effects on the overall heat transfer coefcient and the uid ow within

the system still are needed

Kamyar et al. (2012) [98] Computational techniques Flow of nanouid in different geometries can be modeled using Lattice Boltzmann Method Two-phase model for a nanouid is more accurate


long-term uid settling combined with potential clogging of owpassages are demerits of nanouids. Chandrasekar et al. [93] com-prehensively studied the experimentally proposed mechanisms onthermophysical and forced convective heat transfer characteristics ofdifferent nanouids. They observed that increased thermal conduc-tivity of nanouid could be better understood by afrmations such asexhibition of Brownian motion by nanoparticles at higher tempera-tures or microstructural examination could provide better insight.Viscosity of nanouid decreases with increase in temperature,whereas it increase with decrease in particle size. Increase in particleconcentration increases viscosity but relation is not denite, it maybe linear or nonlinear. Classical mixing theory is sufcient to predictthe density of nanouids. Sureshkumar et al. [94] studied both thetheoretical and experimental investigation for the heat transfercharacteristics of nanouids. It is concluded that the reason forattaining higher heat transfer is due to higher wettability of nano-uids and increase in effective liquid conductance. On one hand thevolume fraction of nanoparticles increases the thermal efciency andon the other hand it reduces the thermal resistance signicantly incomparison to base uid. Saidur et al. [95] did a study on applicationsand challenges of nanouids and mentioned that the presence ofnanoparticles increases the absorption of incident radiation by morethan nine times over that of pure water. Nanouids stability and itsproduction cost are major factors that hinder the commercializationof nanouids. Mahian et al. [96] exclusively reviewed the applica-tions of nanouids in solar energy. It is found that some factors suchas adding surfactant to nanouid and a suitable selection of the pH ofnanouid are effective in the collector efciency. It is also imperativethat nanouids in different volume fractions should be tested to ndthe optimum volume fraction as higher volume fraction always is notthe best option. Nkurikiyimfura et al. [97] reviewed the advancementin heat transfer enhancement by magnetic nanouids and suggestedthat solar thermosyphons where the uid ow and heat transferprocess may be completely controlled by an external magnetic eldcould be the future design. Kamyar et al. [98] summarized thecomputational simulations performed to understand the behaviorof nanouids. Even though two-phase model gave good prediction,the need for better understanding the effect of Brownian motion onthermal behavior of nanouids was emphasized. A shift fromconvection heat transfer model to radiation heat transfer model orboiling heat transfer model for nanouids is a possibility.

Regardless of numerous investigations dedicated to the use ofnanouids for heat transfer enhancement in the last decade, there areno commercial nanouid solar collectors yet. However, Taylor et al.[99] developed a laboratory-scale nanouid dish collector and eval-uated its performance. On the basis of testing and evaluation of thiscollector, they claimed that nanouids have excellent potential forpower tower solar thermal power plants. Efciency improvement ofthe order of 510% is possible with a nanouid receiver. Ideally, theseenhancements could be realized with very little change in terms ofmaterials, system design, and initial capital investment to the entiresolar thermal system. If solar thermal power plants move to largerscale at sites having ideal solar insolation, nanouid receivers showeven more potential. They even performed statistical analysis forcapital gain and estimated that that a 100MW nanouid thermalplant operating in Tucson city of Arizona could add 3.5 million USdollar to the annual revenue. Some other studies on systems like heatpipe whose ndings can be implemented in solar thermal conversionhave been also performed. Naphon et al. [100] conducted the experi-ment on the copper heat pipe with titanium nanouid of diameter21 nm prepared using ultrasonic technique. With 0.10% volumetricconcentration of titanium nanoparticles, the efciency was found to be10.60% higher. Similar study in more extensive manner was conductedby Hung et al. [101] with alumina nanouids of 1030 nm particle size.Heat pipe length from 0.3 m to 0.6 m, tilt angle from 101 to 901,volume concentration ratio in the range of 2080% and heating power

2040W were varied to examine their effect and nd the optimalthermal performance parameters. For the heating rate of 40W, themaximum efciency was recorded for 0.45 m long heat pipe as 56.3%;however, the efciency was better with nanouid for all cases. It wasalso found that increasing the concentration of nanoparticles beyond acertain level does not enhance thermal efciency because of highwater adsorption. Noie et al. [102] studied the heat augmentationproperties of Al2O3H2O nanouid for a two-phase closed thermosy-phon (TPCT). Al2O3 with average particles diameter 20 nm weredispersed in distilled water using ultrasonic technique in volumefraction range of 1%, 1.5%, 2%, 2.5% and 3%. The efciency of TPCT wasbetter for all the concentrations of nanouid than that of pure water.This study established the heat enhancement properties of the Al2O3nanouid. Saidur et al. [103] evaluated the effect of nanouid proper-ties on direct absorption solar collectors. They noticed that eventhough particle size has no or minimal effect on optical properties,the size should be below 20 nm so that Rayleigh scattering happens.Once the desired optical properties are obtained, the volume fractionshould be kept minimal in order to eliminate clogging and instabilityin nanoparticles suspension. Youse et al. [104106] reported anextensive experimental investigation on the effect of two differentnanouids, namely Al2O3H2O and MWCNTH2O (multi-wall carbonnanotube), on the efciency of at plate solar collectors in literature.Collectors with absorption area 1.51 m2 and above 95% pure respectivenanoparticles with Triton X-100 as surfactant were taken for both thecases. For Al2O3 nanouid of 15 nm diameter, it was found that the0.2 wt% Al2O3 nanouid increases efciency by 28.3% for the ow rateof 3 l/min as well as use of surfactant enhances efciency by 15.63%maximum in comparison to Al2O3 nanouid without surfactant.However, 0.2 wt% MWCNT nanouid of 1030 nm diameter decreasesthe efciency without surfactant while increases with surfactant. Itwas observed that 0.4 wt% MWCNT increases efciency withoutsurfactant. On varying the pH of 0.2 wt% MWCNT nanouid, it wasseen that higher the difference between pH of nanouid and pH ofisoelectric point, higher is the efciency of the collector. The ndingsare signicantly different for these two different nanouids giving ascope for the trials to search for the best nanouids and optimize theparameters such as pH value, particle concentration, discharge rate etc.Recently, Sokhansefat et al. [107] developed a numerical model innite volume based commercial software for predicting the heattransfer characteristics of Al2O3/synthetic oil nanouid in a parabolictrough collector. It was established that there was a direct dependencyof heat transfer coefcient on the volumetric concentration of nano-particles. The model was validated with the experimental results withdeviation of less than 3.8%.

Although nanouids have opened the gate for extremely excitingpotential applications, there are few key issues associated with it andthey must receive greater attention in the future investigations. Thereis a need of in-depth experimental and theoretical researches tofacilitate the exact and systematic understanding of factors inuen-cing the performance of nanouids. Up to now, there is a lack ofagreement between experimental results from different groups aswell as experimental and analytical investigations are in disagree-ment. The detailed and precise characterizations of structure of thesuspensions may be the key to explain the discrepancy in theexperimental data. Increase in viscosity by the use of nanouids,which cause increase in pumping power, is also a concern. Long-termstability of the suspension in base uids in the practical conditionswhere there is possibility of varying and hi

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