Progress and latest developments of evacuated tube solar collectors

M.A. Sabiha a, R. Saidur b,n, Saad Mekhilef c, Omid Mahian d

a Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysiab Center of Research Excellence in Renewable Energy (CoRE-RE), King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabiac Power Electronics and Renewable Energy Research Laboratory (PEARL), Department of Electrical Engineering, University of Malaya,50603 Kuala Lumpur, Malaysiad Department of Mechanical Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran

a r t i c l e i n f o

Article history:Received 12 May 2014Received in revised form12 May 2015Accepted 7 July 2015

Keywords:Solar energyEvacuated tube solar collectorEfficiencyWorking fluidsChallenges

a b s t r a c t

Solar energy is the most available, environmental friendly energy source and renewable to sustain thegrowing energy demand. Solar energy is captured by solar collectors and an evacuated solar collector isthe most efficient and convenient collector among various kinds of solar collectors. In this paper, acomprehensive literature on why evacuated collector is preferable, types of evacuated collectors, theirstructure, applications and challenges have been reviewed. Latest up to date literature based on journalarticles, web materials, reports, conference proceedings and thesis have been compiled and reported.Applications of evacuated solar collectors in water heating, heat engines, air conditioning, swimmingpool heating, solar cooker, steam generation and solar drying for residential and industrial sectors havebeen summarized and presented. Collector efficiency of different types of evacuated collectors and theirperformance based on different working fluids have been reported as well. Based on the availableliterature, it has been found that an evacuated tube collector has higher efficiency than the othercollector. An evacuated tube collector is also very efficient to be used at higher operating temperature.There are few challenges that have been identified and need to be addressed carefully before installingan evacuated tube solar collector. However, after critically analyzing the available literature, authors havepresented some future recommendations to overcome the barriers and for enhanced performance of anevacuated tube solar collector.

& 2015 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10392. Evacuated tube solar collector (ETSC). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1039

2.1. Why an evacuated tube solar collector (ETSC) is preferable? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10392.2. Types of ETSC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1040

2.2.1. Single walled glass evacuated tube. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10402.2.2. Dewar tube. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1041

2.3. Mathematical modeling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10412.4. Applications of ETSC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1043

2.4.1. Domestic applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10432.4.2. Industrial applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1046

3. Challenges of using evacuated tube solar collectors (ETSCs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10503.1. Cost and maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10503.2. Fragility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10503.3. Snow removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10503.4. Overheating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1050

4. Economic consideration on the usage of ETSCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10505. Performance based on working fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10516. Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1051

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.2015.07.0161364-0321/& 2015 Elsevier Ltd. All rights reserved.

n Corresponding author. Tel.: þ966 13 860 4628; fax: þ966 13 860 7312.E-mail addresses: saidur@kfupm.edu.sa, saidur912@yahoo.com (R. Saidur).

Renewable and Sustainable Energy Reviews 51 (2015) 1038–1054

7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1051Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1052References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1052

1. Introduction

The most available source of renewable energy on earth is solarenergy as the earth receives millions of watts of energy everydaycoming from solar radiation. However, only a fraction of it in theform of day lighting and photosynthesis is used by the naturalworld, one third is reflected back into space and the rest isabsorbed by land, oceans and clouds. Thus, it is very reasonableto collect solar energy and utilize it efficiently to generate electricpower, heat and also for cooling purposes in a viable way. Theeffect of using solar energy on the environment for a variety ofapplications is minimal as it produces no harmful pollutants.Besides environmental consciousness, dwindling of traditionalenergy sources marks solar energy as the appropriate energysource to meet the increasing demand of energy worldwide.Researchers have investigated and developed technologies onhow to harvest solar energy to serve human beings and are stillconsidering new technologies to maximize the collection andutilization of solar energy [1].

There are particular challenges in the effective collection andstorage of solar energy though it is free for taking. As solarradiation is only available during daytime, the energy must becollected in an efficient manner to make use of most of thedaylight hours and then must be stored. Solar thermal collectorsare the existing components to capture solar radiation which isthen turned to thermal energy and transferred to a working fluidsubsequently. Therefore, solar collectors are the main and mostcritical components of any solar system [2].

There are basically two types of collectors, stationary andtracking [3] (Fig. 1). Different collector configurations can help toobtain a large range of temperature for example, 20–80 1C is theoperating temperature range of a flat plate collector (FPC) [4] and50–200 1C is for an evacuated tube solar collector (ETSC) [5,6]. Themost productive and mostly used solar collectors are FPCs butthese collectors have comparatively low efficiency and outlettemperatures. FPC is popular due to its low maintenance costand simple design.

However, FPC has two major drawbacks:

i. convection heat loss through glass cover from collectorplate and

ii. absence of sun tracking.

ETSCs have considerably lower cost and heat loss than thestandard FPCs [7,8]. On the other hand, an ETSC overcomes boththese drawbacks due to the presence of vacuum in annular spacebetween two concentric glass tubes, which eliminates sun trackingby its tubular design. Conventional FPCs are mainly designed forsunny and warm climates. Their performance reduces during cold,windy and cloudy days and they are greatly influenced by theweather as moisture and condensation cause early erosion ofinternal materials which might cause system failure. In contrast,ETSCs have outstanding thermal performance, easy transportabil-ity and expedient installation. In addition, ETSCs are suitable forunfavorable climates [9,10].

This paper presents a review of previous studies on ETSC, theirapplications, and suitability in solar thermal engineering systems.The former studies on ETSC mainly related to their suitability andperformance in various applications. Therefore, this review mainly

investigates the performance of ETSC for domestic and industrialapplications, factors that influence the collector efficiency, chal-lenges of using this collector as well as economic considerationregarding the usage of this collector. Some suggestions are alsomade for future research in this field. There is no review on ETSCtill now and thus this is the first systematic review paper on recentdevelopments of ETSC and their applications according to theauthors' opinion. Finally, it is the authors' hope that this reviewwill be useful to find more about ETSC, their applications, andchallenges and the future recommendations will help in futureresearch work.

2. Evacuated tube solar collector (ETSC)

A variety of technologies exist to capture solar radiation, but ofparticular interest of authors is evacuated tube technology.Numerous authors [3,11,12] have noted that ETSCs have muchgreater efficiencies than the common FPC, especially at lowtemperature and isolation. For instance, Ayompe et al. [13] con-ducted a field study to compare the performance of an FPC and aheat pipe ETSC for domestic water heating system. With similarenvironmental conditions, the collector efficiencies were found tobe 46.1% and 60.7% and the system efficiencies were found to be37.9% and 50.3% for FPC and heat pipe ETSC, respectively.

An ETC is made of parallel evacuated glass pipes. Each evac-uated pipe consists of two tubes, one is inner and the other isouter tube (Figs. 2 and 3). The inner tube is coated with selectivecoating while the outer tube is transparent. Light rays passthrough the transparent outer tube and are absorbed by the innertube. Both the inner and outer tubes have minimal reflectionproperties. The inner tube gets heated while the sunlight passesthrough the outer tube and to keep the heat inside the inner tube,a vacuum is created which allows the solar radiation to go throughbut does not allow the heat to transfer. In order to create thevacuum, the two tubes are fused together on top and the existingair is pumped out. Thus the heat stays inside the inner pipes andcollects solar radiation efficiently. Therefore, an ETSC is the mostefficient solar thermal collector [12].

An ETSC, unlike an FPC, can work under any weather conditionswhile it provides acceptable heat efficiency.

2.1. Why an evacuated tube solar collector (ETSC) is preferable?

According to many researchers [3,11,12] ETSCs have muchmore higher efficiencies than FPCs. ETSCs can collect both directand diffuse radiations. Besides excellent thermal performances,ETSCs have convenient installation and easy transportabilty.

Applications like desalination of sea water, air conditioning,building heating, refrigeration, and industrial heating requirehigher temperature and the performance of an ETSC is better thanan FPC for high temperature operations [15]. ETSCs are also able tooperate other higher temperature applications such as instanta-neous gas heater, boost element integrated single solar tanksystem, and boost tank incorporated solar pre-heaters [16].

Mangal et al. [17] mentioned that the peak energy output isprovided by an FPC only at mid-day when the sun is perpendicularto the surface of the collector whereas the evacuated solar tubesare able to track sun passively throughout the day as for cylindrical

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shape of evacuated tube. The incident angle of sunlight on thecylindrical tubes is at 90 1C throughout the day; hence the peakabsorption is always for an ETSC. It was also noted that the ETSC isless affected by low temperature and wind because of the vacuumenvelop between the inner and outer tubes of evacuated pipe. Thevacuum is formed to reduce convective and conductive heat lossby evacuating the air inside the interior tube of the ETSC.

They also reported that the maintenance of an ETSC is easy andinexpensive. If a tube is damaged or broken, the system does notleak or stop working, the collector still operates at lower efficiency.In case of evacuated collector, without shutting down the wholesystem, it is possible to replace the damaged tube whereas for theFPC if the collector is damaged, the entire system needed to beshut down to replace the collector. Thus FPCs have much higherrepair and maintenance cost than ETSCs.

To achieve the same heating performance as FPC, ETSC can beused as well. Authors mentioned that approximately 250–300 lwater storage tank is required for a standard household with 4–5members and the hot water needed for the household during thesummer and other seasons (large amount is needed in otherseasons) can be provided by 30 evacuated tubes. The same outputcan be produced using an FPC in summer but the hot water ismainly needed in other seasons. ETSCs are able to heat water allthe year round, even during overcast conditions as the tubes haveexcellent isolative properties and highly efficient absorption ofsolar radiation. Therefore, the average output of ETSCs over anentire year is 25–40% higher than FPCs per net m2.

Shriram Green Tech is a division of Shriram Industries Limited,a very well-known industry for marketing solar energy basedefficient equipment and most economical solution provider, givingservice for 115 years in India. In their website (accessed on 14April, 2014) comparisons are presented between ETSC and FPC.According to their website, ETSCs have much lower convection andconvecting losses than FPC and the emissivity is lower for anevacuated collector whereas for an FPC, emissivity is higher. AnETSC is able to generate heat quickly and the heat loss in thetubes is insignificant during daytime whereas for an FPC, heat

generation is slow and the heat loss in the collector and tank ishigh due to convection during daytime. Grouting of evacuatedcollectors is not required but grouting is required for FPCs. There isno limitation about the placement of the collector unlike oldertechnology such as FPCs. They also reported that the performanceof an ETC is satisfactory even in extreme cold condition such as�18 1C whereas an FPC will be damaged at high altitude due tofreezing of water [18] (Fig. 4).

2.2. Types of ETSC

According to Gao et al. [20] available types of evacuated tubesolar collectors can be categorized into two groups; one is thesingle-walled glass evacuated tube and the other is the Dewartube. There are many variations of the two basic types; forinstance, heat extraction can be through a U-pipe, heat pipe ordirect liquid contact.

2.2.1. Single walled glass evacuated tubeThe single-walled glass evacuated tube is popular in Europe.

Badar et al. [21] studied the thermal performance of an individualsingle walled evacuated tube with direct flow type coaxial pipingbased on analytical steady state model.

Kim et al. [22] investigated the thermal performance of an ETSCwith four different shaped absorbers both experimentally andnumerically. Four different shapes are: finned tube (Model I), tubewelded inside a circular fin (Model II), U tube welded on a copperplate (Model III) and U tube welded inside a rectangular duct(Model IV) as illustrated in Fig. 5.

Firstly, by considering only the beam radiation, the perfor-mance of a single collector tube was observed and it was foundthat the incidence angle has great influence on the collectorefficiency. Model III had the highest efficiency with small inci-dence angle but the efficiency of model II became higher thanmodel III with the increment of incidence angle. The incidenceangle has negligible impacts on collector performance while

Nomenclature

ETSC evacuated tube solar collectorFPC flat plate collectorCPC compound parabolic collectorSHC solar heating and cooling programIEA international energy agencyWGETSC water in glass evacuated tube solar collectorUPETSC U pipe evacuated tube solar collectorPLC Programmable Logic ControllerSWH solar water heating systemDSWH direct solar water heating systemCOP coefficient of performancePCM phase change materialHPA heat pipe absorberDFA direct flow absorberSAHP solar assisted heat pumpDFR diffuse flat reflectorHP-ETC heat pipe evacuated tube collectorc0 constantc1 constant (W m�2 k�1)c2 constant (W m�2 k�1)τ transmittanceα absorptanceQ heat rate (W)

QL thermal loss (W)Qu net heat energy absorbed by working fluid (W)S solar energy absorbed by selective absorbing

coating (W)D outer diameter of absorber tube (m)L the length of absorber tube (m)Ac surface area of collector (m2)G solar irradiation (W/m2)

Cp specific heat at constant pressure (j/kg 1C)_m mass flow rate (kg/s)Tout fluid outlet temperature (1C)Tin fluid inlet temperature (1C)Tm mean temperature of heat transfer fluid (1C)Ta ambient temperature (1C)FR collector heat removal factorUl overall loss coefficient (W m�2 k�1)Ut the edge loss coefficient of the header tube

(W m�2 k�1)Ue the loss coefficient from absorber tube to the ambient

(W m�2 k�1)Kθ incident angle modifiera incident angle modifier constantφ nanoparticles volume fraction (%)η collector efficiency

M.A. Sabiha et al. / Renewable and Sustainable Energy Reviews 51 (2015) 1038–10541040

considering the diffuse radiation and the shadow effects andmodel III is found to have the best performance for all ranges ofthe incidence angle.

A prototype of solar water heating system with looped heatpipe single walled evacuated tube was designed and both experi-mental and theoretical research have been carried out by Zhaoet al. [23] (Fig. 6).

Nkwetta et al. [24] demonstrated a solar collector whichcombines single walled evacuated tubes, heat pipe and an internalor external concentrator for improving output temperatures.

2.2.2. Dewar tubeDewar tube consists of inner and outer tubes which are made

of borosilicate glass and selective absorbance is used to coat theoutside wall of the inner tube to collect solar energy. The heat lossis reduced in by evacuating the layer between the inner and outertubes. Tang et al. [25] investigated on dewar tubes and mentionedthat the cheap price of dewar water in glass evacuated tube solarcollector (WGETSC) makes it popular than dewar tube with U pipeevacuated tube (UPETSC) with heat pipe. Tian [26] investigated thethermal performance of dewar ETSC with an inserted U pipe. Yanet al. [27] studied about the unsteady state efficiency of the dewartube solar collector having heat pipe inserted. Xu et al. [28] testedthe thermal performance of dewar tube solar collector undervarious dynamic conditions and they used air as the heat transferfluid. Kim et al. [29] investigated the performance of dewar tubewhere the inner tube was filled with coaxial fluid and the outertube was filled with an antifreeze solution and a one dimensionalmathematical model was established.

2.3. Mathematical modeling

There are two different procedures to measure the efficiency ofsolar thermal collectors: steady state test method and quasidynamic test method [30]. The boundary conditions for solarirradiation, ambient temperature and the inlet temperature ofthe collectors are maintained constant during steady state testmethod and for quasi dynamic test, the boundary conditions arefree to vary. In both the techniques, solar energy is the source of

Fig. 4. IEA SHC worldwide report 2012 [19].

Fig. 3. Evacuated tube solar collector.

Fig. 2. Evacuated tube [14].

Fig. 1. Types of solar collectors [3].

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heat in the solar collectors; therefore, irradiation is the inputpower which is received and absorbed by the collector and thentransferred to a working fluid. Thermal losses occur in the heattransfer process involved in an ETSC. Heat transfer can occurthrough conduction, convection and radiation. To perform heatbalance, heat transfer processes need to be included.

Thermal loss, QL can be expressed as

QL ¼ S�Qu ð1ÞThe useful heat which is delivered by a solar collector is thedifference between the energy absorbed by the working fluid andthe heat losses from the surface to the surroundings.

Qu ¼ S�QL ð2ÞThe thermal performance of solar collector under steady stateconditions can be calculated as follows:

Qu=Ac ¼ FR ταð ÞG�FRUL Tm�Tað Þ ð3ÞFrom Eq. (3), it is observed that the thermal performance of solarcollector depends on the intensity of the sunlight striking thecollector surface, the temperature of the surrounding environmentand the absorber plate and its optical and thermal performancerepresented by the values of ταð Þ and UL respectively. The trans-mittance (τ) of the glass cover and absorptance (α) of the absorberplate depend on the incidence angle of the collector and according

to literature the product of the transmittance and absorptance (τα)is approximately 0.836 [31–35].

Useful energy can also be expressed using Eq. (4) as

Qu ¼ _mCp Tout�Tinð Þ ð4Þwhere Cp is the specific heat of water; in case of nanofluids specificheat can be calculated using Eq. (5) [36]

Cp;nf ¼ ϕCp;npþð1�ϕÞCp;bf ð5Þwhere the subscripts nf, np and bf are for nanofluid, nanoparticleand base fluid, respectively.

The thermal efficiency of an ETSC can be measured by both Eqs.(7) and (8) [30,37].

Efficiency; η¼ Qu=AcG ð6Þ

Therefore; η¼ _mCp Tout�Tinð Þ=AcG ð7ÞEq. (7) gives the efficiency of evacuated collector with the knownvalue of fluid mass flow rate and the measured value of fluid inletand outlet temperature.

Another way of getting efficiency is by calculating the netoutput power by considering the heat losses shown in Eq. (8).

η¼ FR ταð Þ�FRUL Tm�Tað Þ=G ð8ÞThe heat loss coefficient UL is a function of the ambient

temperature and the temperature of the absorber plate but in

Fig. 5. Cross-section of (a) Model I, (b) Model II, (c) Model III and (d) Model IV [22].

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reality it is not a constant. The collector heat removal factor is afunction of flow rate which is considered due to the fact that theaverage fluid temperature and the average absorber temperature arenot same and the difference between these two temperatures is 0 to 1.The efficiency of collector depends on the heat loss coefficient (UL) andthe design of the absorber plate of the collector. Therefore, theapproach to obtain (FRUL) is

FRUL ¼ c1þc2 Tm�Tað Þ ð9ÞBy combining Eqs. (8) and (9),

η¼ FR ταð Þ�c1Tm�Tað Þ

G�c2

Tm�Tað ÞG

2

ð10Þ

However, Eq. (10) is only applicable to calculate the efficiency whensun strikes the collector perpendicularly but in reality sun is notalways perpendicular to the collector. Only at mid-day sun is perpen-dicular to the collector but at morning and afternoon sun strikes thecollector with a different angle. An incidence angle modifier (IAM) isthe solution to get the performance for different incident angles whichcan be described by the following equation [14]:

Kθ ¼ 1� tan θ=2� �a

; θ¼ π=3 ð11Þ

An incidence angle modifier is actually the incidence angle modifier forbeam radiation (Kθb) and incidence angle for diffuse radiation (Kθd).

Incidence angle modifier;Kθ ¼ KθbþKθd ð12ÞThe collector efficiency can be modified with the incidence anglemodifier which is expressed in Eq. (13) [3].

η¼ FR ταð ÞKθ�c1Tm�Tað Þ

G�c2

Tm�Tað Þ2G

ð13Þ

By combining Eqs. (12) and (13)

η¼ FR ταð ÞKθbþFR ταð ÞKθd�c1Tm�Tað Þ

G�c2

Tm�Tað ÞG

2

ð14Þ

2.4. Applications of ETSC

ETSCs are getting popular day by day for their uniqueness asthey are able to gather energy from the sun all day long at lowangles due to their tubular shape. Many researchers have doneresearches on ETSCs which can be used for heating or coolingpurposes in industries like drug and pharmaceutical, textile, paper,and leather and also for swimming pool, residential houses, boilerhouse, hospitals, hotels and nursing home. The use of ETSC can bediscussed in two sections which are domestic and industrialapplications. Table 4 includes the summary of previous work onapplications of ETSC (Fig. 7).

2.4.1. Domestic applicationsAn ETSC is a mature technology for domestic applications as it

can operate over a wide range of temperatures from medium tohigh according to the requirement. Fig. 8 demonstrates theapplications of an ETSC for domestic purposes.

2.4.1.1. Solar hot water. Since the last decade, the world market israpidly growing for solar water heaters which results in largescale developments of improved quality products by various newtechnologies. A Solar water heater is a device for heating water byusing solar energy to produce steam for domestic and industrialpurposes. Solar energy comes from the sun in infinite amount asthe form of solar radiation which falls on absorbing surface and

Fig. 6. Heat pipe evacuated tube collector [23].

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then gets converted into heat which is used for water heating.When evacuated tube collectors are used to heat water, it is calledevacuated tube solar water heater. There are various types of solarwater heaters such as flat plate solar water heater, concent-rated solar water heater and evacuated tube solar water heater.A concentrated solar water heater is used for very hightemperature water or steam and the flat plate solar waterheaters are getting replaced by evacuated water heaters due totheir numerous advantages [17]. ETSCs have been the coreattraction of modern development in the solar water heatermarket as the manufacturing cost is comparatively lower andETSCs have better performance than FPCs particularly forhigh temperature operations. Morrison et al. reported signifi-cant developments of solar water heaters using ETSCs whicheventually include 65% of 6.5 million m2/year in China [38].

Tang [25] studied the impact of different tilt angles on theperformance of solar water heaters with water in glass ETSC. Forthe experimental purpose, two sets of water in glass evacuated

tube solar water heater were constructed which were identical buthad two different tilt angles, one inclined at 221 and the other at461 from the horizon. It was reported that the heat removal to thewater storage tank from solar tubes is not influenced by collectortilt angle but the daily solar heat gain of the system and dailyradiation are significantly influenced by collector’s tilt angle. Thethermal efficiency of a solar water heater does not depend on theclimatic conditions as the evacuated tube has lower heat loss tothe ambient air from solar tubes. Therefore, the collectors shouldbe inclined at such an angle which gives the maximum annualsolar radiation in order to maximize the heat gain of solar waterheaters annually.

Budihardjo et al. [15] studied about the long term performanceof water in glass evacuated tube solar water heater both experi-mentally and numerically. They investigated the natural circula-tion flow rate through the evacuated tubes, tank heat losscoefficient and the collector efficiency of the solar water heater.They used 21 evacuated tubes in the collector which has fluid in

Fig. 8. Application of evacuated tube solar collector for domestic purposes [42].

Fig. 7. Graph of efficiency (η) and temperature (T) ranges of various types of collectors [18].

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direct contact with the glass tubes. The inlet and outlet tempera-tures were measured under steady state conditions to determinethe useful energy from the collector and a sun tracking frame wasmounted under constant radiation to determine the collectorefficiency. The optical efficiency was found to be 0.58 using alinear regression. They also reported that the collector designwhich includes tube aspect ratio, reflector curvature, collectorinclination, operating temperature of the collector and the radia-tion intensity has effects on the rate of natural circulation. Byconsidering each component, finally the results revealed that pre-heater system with evacuated collector is capable of 45% annualsaving in Sydney.

Gao et al. [20] experimentally investigated the effects ofthermal flow and mass rate on forced circulation solar hot watersystem. For the system two types of ETSCs namely water in glassand U pipe evacuated collectors were used. A comparison wasmade in terms of energy performance between WGETSC andUPETSC. From the comparison, UPETSC has 25–35% higher energystorage than WGETSC. The energy storage and also pump opera-tions are influenced by the flow rate and fluid thermal mass. Tooperate the pump in a stable condition and to take energy fromthe collector adequately and timely, an appropriate mass flow rateis important. It may be noted that the performance of energycollection will be reduced for higher flow rate.

Morrison et al. [38] investigated the features of solar heaterswith WGETSC based on the circulation rate through single endedtubes experimentally. To develop a numerical model of the heattransfer and the fluid flow inside a single evacuated tube, it wasassumed that there is no contact between the neighboring tubes inthe collector array. It was investigated that the circulation flowrate through the tubes has been significantly influenced by thetank temperature and also the radiation intensity falling onto theabsorber whereas the circulation flow rate is influenced by theinlet conditions of the tubes.

In another study, Budihardjo and Morrison [39] consideredoptical and heat loss characteristics to investigate the performanceof water in glass evacuated tube solar heaters. The domestic waterheating system was compared with FPCs and the performance of2 panel flat plate arrays was found to be higher than 30 evacuatedtube arrays.

Ayompe and Duffy [40] considered a heat pipe evacuated tubecollector (HP-ETC) to study the thermal performance of solarwater heating system. Experiment data was obtained over 1 yearperiod from a forced circulation solar water heating system with3 m2 HP-ETC installation on a rooftop in the Focas Institute inDublin, Ireland. To mimic the domestic hot water system, anautomated hot water draw off system was developed whichcomprises of electrical fittings, contactors, Programmable LogicController (PLC), solenoid valve, thermostat, relays and impulseflow meters. Water was used as the working fluid in the systemand the maximum outlet temperature of water was recorded as70.31 while 59.51 was noted at the bottom of the hot water tank.From the experimental investigation, it was revealed that the heatpipe ETCs are more efficient than FPCs of a solar water heatingsystem.

Arefin et al. [41] investigated the characteristics and theperformance of different types of ETSCs for solar water heatingsystems throughout the year. Besides determining the maximumoperating temperature for the solar water heater, they alsodetermined its feasibility by calculating the payback time. Theyalso reported that all glass evacuated tubes are the cheapest andsimplest and the heat loss is less than heat pipe collectors as theglass tube collectors are directly connected with the tank. Rela-tively small area is required for the system as the tank is mountedover the collectors and less time is required for water to becomehot due to thermosiphon process. They found the operating

temperature of the system to be 50 1C which is good enough fordomestic purposes and their cost analysis shows that the solarwater heater using an ETSC is more cost effective than the electricwater heater. Table 1 summarizes the previous studies on the ETSCused for solar water heating system.

2.4.1.2. Air conditioning. Nowadays researchers are investigatingenvironmental friendly technologies for air conditioning asproducing electrical energy causes some pollution. Mehta andRane [43] investigated the liquid desiccant based air conditioningsystem which is adaptable to solar energy, a pollution freerenewable energy source. The solar radiation is highly availablein summer when the demand of air conditioning is also higherwhich makes it logical to use solar energy source for air condi-tioning. They developed a novel approach of using an ETSC withheat pipes as regenerator for a liquid desiccant based solarcollector. They tested the collector at 100 1C to generate satu-rated steam which offers 51–60% efficiency for average 9 h. Theaverage thermal COP of 0.82 was achieved as there is no heat lossto air and the power consumption was less than 40 W because oflow pressure drop and flow rate of liquid desiccant collector. Toincrease COP by regenerating further, a liquid desiccant in lowtemperature stage which is possible by the latent heat produced inthe ETSC was introduced. The collector efficiency increased up to44.7% and the power output of distilled water up to 5.14 kg/hwhile regenerating liquid desiccant at 117 1C with 719 W/m2

global radiation.Another experiment was done by Morthy [44] on the perfor-

mance of solar air conditioning system using HP-ETC. From hisexperiment, it was concluded that to power the air conditioningsystem, the solar system is capable of producing adequate energy.The efficiency of heat pipe evacuated tube varies from 26% to 51%and the overall system has efficiency from 27% to 48%. Using solarair conditioning system with evacuated tube is very economical aszero energy cost is provided by the solar powered chilled watersystem. Besides, solar air conditioning system is a possible solutionto overcome environmental pollution.

2.4.1.3. Swimming pool. Sakhrieh et al. [45] conducted anexperiment on five types of solar collectors which are copperthermosyphon with black coating (Type I), copper collectors withblue coating (Type II), Copper solar collector (Type III), Aluminumsolar collectors (Type IV) and ETCs (Type V) for heating aswimming pool based on overall performance, competence anddependability. The aim of the experimental study was to replacethe heating system of the swimming pool at Hashemite Universityof Jordan by a more efficient and cost effective solar heatingsystem. From the experimental investigation, it was found that anETSC has the highest efficiency. However, the lowest paybackperiod of 1.5 years was found to be for aluminum collectors and forevacuated collectors, the payback period is found to be 1.9 years asshown in Table 2. Even though aluminum collectors have thelowest payback period, it is not suggested to use aluminumcollectors for large applications like heating a swimming pool.

Considering the efficiency, payback period and other perfor-mances, they recommended that evacuated solar collector is thebest collector compared to other collectors to be used in theheating system of the swimming pool.

2.4.1.4. Solar cooker. Sharma et al. [46] investigated the thermalperformance of a solar cooker based on ETSC with phase changematerial (PCM) storage unit. The prototype in Fig. 9 was designedin two separate parts, one for energy collection and the other onefor cooking.

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During sunshine hours, PCM stores solar energy which is usedfor cooking purpose at evening or night time. Different loads andloading times were used to conduct cooking at noon and eveningand the evening cooking was not affected by noon cooking ratherit was found to be faster as the heat in PCM storage was used. Thesolar cooker with an ETSC is expensive but it is able to providehigh temperatures up to 130 1C and allow users to cook in aconventional kitchen with shade at evening and at non-sunshining hours. The designed cooker has good prospective not onlyin Japan but also in other regions with good sun shine.

Kumar et al. [47] designed a solar pressure cooker based on ETSChaving two separate parts for solar energy collection and both theparts are attached with a heat exchanger. Besides experimentalinvestigation, they also developed a simulation model to determinethe performance of the cooker under various climatic circumstances.The designed solar pressure cooker was able to achieve temperaturesup to 120 1C which is much higher than the pressure cooker based onan FPC. Therefore, the solar pressure cooker with ETSC is reported tobe of high potential for community applications in Delhi.

2.4.2. Industrial applicationsFor industrial use, a higher temperature is required compared

to domestic applications. An ETSC is capable of generating

Table 2Efficiency and payback period of different types of solar collectors.

Parameters Copperthermosyphonwith blackcoating (Type I)

Coppercollectorswith bluecoating (TypeII)

Coppersolarcollector(Type III)

Aluminumsolarcollectors(Type IV)

ETCs(TypeV)

Efficiency(at2.00 pm)

80.1 77 71.3 78.7 93.5

Paybackperiod(years)

2.2 2.0 1.9 1.5 1.9

Table 3Performance of stationary collectors for solar drying.

Stationary collectors Temperature range (1C) Efficiency

500 W/m2 1000 W/m2

FPCs 30–80 0.71–0.75 0.72–0.75ETCs 50–200 0.44–0.82 0.62–0.82CPCs 60–240 0.45–0.73 0.58–0.72

Table 1Summary of previous researches on evacuated tube solar collectors used in water heaters.

Author Type ofinvestigation

Working fluid Types of ETSC Findings

Hazami et al. [64] Experimental Water ETSC – ETSC DSWH (84%) has higher solar fraction (i.e. in average) than FPCDSWH (68%) annually.

– ETSC can generate approximately 9% more energy than FPC.

Geo et al. [20] Experimental Antifreeze fluid(40% glycol byvolume)

WGETSC and UPETSC – In average, the thermal efficiency of WGETSC is less than UPETSC.– Comparing UPETSC and WGETSC with the same efficiency curves,

WGETSC achieves energy storage of 25–35% less than UPETSC.

Ayompe and Duffy [40] Analytical Water HP-ETSC – To operate solar water heating system, HP-ETCs are more efficientthan their FPCs.

Ma et al.[65] Analytical Water Glass evacuated tube(with U shaped absorbertube)

– Outlet temperature 38 1C, efficiency 59%, collector efficiencyincreases within synthetic conductance.

– Heat losses reduced.– Heat extraction is high.– The challenge is to preserve vacuum environment.

Yamaguchi et al. [66] Experimental R744 CO2 UPETSC – Collector efficiency 66%– Heat recovery¼65%

Rittidech et al. [67] Experimental R-134a Circular tube collector – Collector efficiency 76%.– Non-corrosive.– No freezing issues.– The challenge is to maintain vacuum environment.– Another problem is the release of non-condensable gases.

Morrison et al. [38] Experimental andnumericalsimulation

Water WGETSC – Storage tank temperature and the intensity of the radiation whichfall onto the absorber surface influence the circulation flow ratethrough the tubes.

– Circulation rate is strongly influenced by the inlet conditions ofthe tube.

Shah and Furbo[14] Water Double glass tube withtubular absorber

– Outlet temperature is 32 1C, efficiency 60–70%.– Suitable for high latitudes.– Heat transfer coefficient is higher.– The performance decreases because of shadow effects.

Morrison et al., 2004 [11] Numerical Water Water in glass

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temperature up to 200 1C [6]. Therefore, ETSC can be used forindustrial applications.

2.4.2.1. Heat engines. Madduri et al. [48] studied a commercialevacuated tube solar hot water system which was used in athermodynamic engine as a thermal power source. According tothem, it is important to use concentrators to achieve highefficiency solar thermal conversion to a heat engine from acommercial evacuated tube system which supplies input thermalpowers at temperatures of 180–220 1C. It was concluded that athigher temperatures, the concentrated evacuated tube is veryefficient to convert incident solar radiation to thermal power.The mechanical output power per unit of installed collector area isalso increased by this system from a heat engine.

2.4.2.2. Solar drying. The most common technique to preserveagricultural products is sun drying but this procedure has manydrawbacks and weaknesses since products can be spoiled by dust,wind, rain and moisture or loss of products due to animals andbirds. Decomposition can cause deterioration in the harvestedcrops, fungi and insect also can attack the products, etc. Besides,the process of sun drying requires large area to spread the

harvested crops which is time consuming and labor demanding.Therefore, to process the agricultural products such as vegetablesand fruits with zero energy costs in hygienic, clean and standardconditions, solar drying technology is an alternative solution. Forsmall scale food processing industries or for agricultural purposes,solar dryer technology is convenient, environment friendly, andreliable to produce hygienic and good quality food products as thistechnology requires less area, saves time, energy and labor costsand also improves the product quality [49]. Sharma et al. alsomentioned that the solar drying system is faster, healthier,cheaper, more hygienic and efficient than traditional dryingsystems.

Lamnatou et al. [50] investigated the thermodynamic perfor-mance analysis of a solar dryer which was used to dry apples,carrots and apricots using an evacuated tube air collector. As thereare many disadvantages of traditional sun drying, many research-ers have done research and applied solar dryers to dry agriculturalproducts but most of them using an FPC. They mentioned thatdryers with ETSCs are special of type which have significantadvantages over FPCs, higher efficiency is one of them. Theirexperimental results showed that the warm outlet air of the ETSCis able to attain the suitable temperature level for drying agricul-tural products. They also reported that the solar dryer with ETSC

Flow Meter

Data logger

PC

Solar irradiance at 20degree inclined and

horizontal surface

Evacuated tubesolar collector

Relief Valve

PCM storage unit

Cooking vessel

PCM temperatures

Flow ratevalve

Collector inlet and outlet

temperature

Water source

Fig. 9. Prototype of solar cooker based on ETSC [46].

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Table 4Summary of previous work on applications of evacuated tube solar collectors.

Author Type ofinvestigation

Working fluid Types of ETSC Objective Results

Nkwetta et al. [24] Experimental Water – HP-ETC– Pipe

absorber(EICPC-HPA)or directflowabsorber(EICPC-DFA)

Experimental characterization of the‘concentrator augmented solar collectorarray’, a component of solar collector.

– Concentrator augmented solar collectorarrays proved to be more economicalfor providing heating and coolingdemands due to the reduced number ofHP-ETCs.

– Reduction in reflector size and relatedreflector losses.

Kumar et al. [68] Experimental Air One endedevacuated tubes

Production of hot air without using anyintermediate fluid to produce hot air atdifferent air flow rates and efficiency.

– Performance of ETC significantlyincreases with the use of reflector.

– -Efficiency, outlet temperature andtemperature difference also increasewith the use of reflector.

Xu et al.[28] Experimental Air All glass ETC Testing the thermal performance of all glassETC under dynamic outdoor conditions.

– The energy balance analysis revealedthat the proposed dynamic method(steady state) is effective to solve theoperational limitations of steady statetest caused by uncontrollable weatherconditions.

Madduri et al. [48] Experimental ETC To achieve high efficiency at highertemperature (180–220 1C) usingconcentrated evacuated tube system.

– Thermal efficiency is 35%– Mechanical efficiency is 12%.– At higher temperatures incident solar

radiation is efficiently converted tothermal power and the mechanicaloutput power increases.

Caglar and Yamal [69] Theoreticalandexperimental

Water ETSC Designing a solar assisted heat pump (SAHP)with ETC for gaining higher efficiency.

– Efficiency of ETSC varies from 0.728to 0.807.

– From cost analysis, electricityconsumption has reduced by 19–45% forspace heating application by using thedesigned SAHP with ETC.

Nkwetta et al. [34] Experimental Water Concentratedand non-concentrated HP-ETC

Investigating the performance of HP-ETCcompared to a concentrated evacuated tubesingle-sided coated HP absorber for mediumtemperature applications.

– Comparing to the non-concentrated HP-ETC, the concentrated HP-ETC showedan improvement of 30% in overallaverage outlet and inlet fluidtemperature differential and 25.42%total daily energy collection.

Lamnatou et al. [50] Experimental Air Evacuated tubeair collector

Thermodynamic performance investigationof a solar dryer using an evacuated tube aircollector

– Suitable for solar drying applicationswithout preheating the outlet air.

– Proposed model showed very goodcorrelation coefficients for all theproducts (apple, carrots and apricots)used for testing purpose.

– Can be used for drying products oflarger quantities in both industrial andagricultural sectors.

Kumar et al. [47] Experimental Water ETSC Investigation of thermal performance of acommunity type solar pressure cookerbased on ETSC.

– ETSC based system supply heat athigher temperature at about 120 1C.

– On a clear sunny day, the system is ableto provide cooking facilities on severalbatches.

– Potential for community basedapplications.

Tang et al. [25] Experimental Water Glass ETSC Investigating the impacts of the collector tiltangle on daily thermal conversion and waterflow characteristics inside solar tubes.

– Tilt angle has noteworthy influence ondaily collectible radiation and heat gain.

– Tilt angle has minor influence onthermal conversion efficiency and heatremoval from solar tubes to water

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Table 4 (continued )

Author Type ofinvestigation

Working fluid Types of ETSC Objective Results

storage tank.– Evacuated tube has lower heat loss and

thus thermal efficiency of SWHs wasindependent of the climatic conditions.

Sharma and Diaz [4] Numerical Material ETSC based onminichannels

To study about the thermal performance ofsolar collector based on minichannels.

– Under identical operating conditions,ETC based on minichannels has higherefficiency than a similar ETC.

Liang et al. [70] Theoreticalandexperimental

Water Two layered glassU-tube evacuatedtube

Investigating thermal performance of afilled type and the copper fin evacuated tubewith a U tube.

– Satisfactory thermal performance isachieved by finned type evacuated tubewith U tube.

– Thermal performance is 12% higher forfilled type evacuated tube consideringthe component heat transmissionis 100.

Hayek et al. [30] Experimental Water WGETSC and HP-ETC

The eastern coast of the Mediterranean wasconsidered for overall performance of solarcollectors under local weather conditions.

– HP-ETC has 15-20% higher efficiencythan WGETSC.

– HP-ETC has better design than WGETSC.

Zambolin and Del Col[71]

Experimental Mixture of waterand propyleneglycol (to preventfreezing duringwinter)

FPC and ETC withCPC

1. Comparing FPC and ETSC in term of steadystate and quasi dynamic test methods.2.Comparing the daily energy performance ofFPC and ETSC.

– For a larger range of operatingconditions, ETSC found to have higherefficiency than FPC based on daily tests.

Gao and Ge [72] Experimental Water All glass ETSC Using all glass ETSC for heating purpose. – Average collector efficiency ranged from50% to 54% on daily basis.

– Collector efficiency varies from 51% to55% in winter.

Budihardjo andMorrison [39]

Experimentalsimulation

Water WGETSC Comparing the performance of WGETSC andFPCs in a range of places.

– The performance of a 2 panel FPC ishigher than an ETC with 30 tubeevacuated tube for domestic waterheating in Sydney.

– The performance of ETC is not sensitiveto the tank size.

Hayek [73] Numerical Water WGETSC Investigating the features and design andoverall performance of WGETSC.

– Better performance of standard water-in-glass tube under high heat inputs.

– ETCs can perform well under cloudyconditions.

Tang et al.[74] Numerical Single tube of all-glass ETSC

Developing a mathematical model tocalculate irradiation daily on single tube ofall-glass ETSCs based on solar geometry,knowledge of two dimensional radiationtransfers.

– Collector type, central distance betweentubes, size of solar tubes, tilt andazimuth angles, use of diffuse flatreflector (DFR) effect the annualradiation collection.

– Use of DFR can significantly improve thecollectors’ energy collection.

– To maximize annual energy collection,all glass ETSC should be fixed with a tiltangle which is smaller than the sitelatitude.

Kim et al. [75] Numericalandexperimental

Water Evacuated CPCsolar collector

Investigation and improvement of thermalperformance of evacuated CPC collectorwith a cylindrical absorber

– Comparing to stationary CPC solarcollector, tracking CPC solar collectorhas about 14.9% higher efficiency.

– Solar radiation and incident angle botheffect the efficiency of the collector.

Sharma et al. [46] Experimental Water ETSC Investigating the thermal performance of aprototype solar cooker based on an ETSCwith phase change material (PCM) storageunit.

– During summer months in Japan, thesolar cooker is capable of cooking twice(noon and evening) in a single day.

– Evening cooking was not disturbed bynoon cooking rather evening cookingwas faster because of PCM heat storage.

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has the capacity to dry agricultural products in larger quantitiesand thus it can be used in industrial sector.

Pirasteh et al. [51] investigated the performance of stationarycollectors (FPCs, CPCs and ETSCs) for agricultural and industrialproducts drying, presented in Table 3. Industries like textile,cement, clay brick production, wood and timber, waste watertreatment and dairy etc. can use solar drying in order to decreasefossil fuel consumption and to become more economical andenvironmental friendly.

Fadhel et al. [52] designed, fabricated and tested a solar assistedchemical heat pump dryer and investigated the performance ofthe system. An ETSC is used as the main component for the systemwhereas the other components are storage tank, dryer chamberand solid gas chemical heat pump unit. The climatological condi-tions of Malaysia were considered for the performance investiga-tion of solar chemical heat pump dryer. The experimental resultshows an efficiency of 74% of ETSC whereas the simulation givesthe efficiency of 80%. Chemical heat pump dryer with ETSCs issuitable for industrial drying purposes for instance drying textileproducts.

Fudholi et al. [53] also mentioned about chemical heat pumpdryer with evacuated tube for drying agricultural and marineproducts.

2.4.2.3. Steam generation. An ETSC can be used for applica-tions requiring high temperature such as steam cooking, boilers,laundry etc. as this is known as the best alternative thermaltechnology for generating high temperature up to 200 1C [5].Vendan et al. [54] studied on the design of an ETSC for hightemperature steam generation for the applications of steamcooking, boilers, laundry, etc.

3. Challenges of using evacuated tube solar collectors (ETSCs)

3.1. Cost and maintenance

Morisson et al. [38] mentioned that the world market of solarwater heater with ETSCs is significantly expanding due to the lowcost manufacturing process of tubular solar collectors. Accordingto China industry in 2001, about 65% of 6.5 million m2/year solarwater heaters are double glass evacuated tubular solar waterheaters.

According to Mangal [17], evacuated tubes are strong and longlasting. In case if any tube is broken, it just requires replacing thebroken tube which is cheap whereas for an FPC it is expensive asthe whole collector is needed to be replaced.

Arefin et al. [41] did the cost analysis for a solar water heaterand compared with the cost of an electric heater. They reportedthat the lifetime of a solar water heater is 30 years whereas thelifetime for an electric water heater is only 5 years. Therefore,almost every 5 years, the electric heater is needed to be replacedwhich is expensive. On the other hand after installation, solarwater heater does not require any maintenance cost and thus it ismore beneficial and cost effective to use solar water heater insteadof electric water heater.

Tang et al. [25] also mentioned that the manufacturing costs ofevacuated tubes are decreasing recently. Budihardjo and Morrison[39] mentioned that the water in glass, an ETSC, is the mostly usedcollector among other evacuated collectors in solar water heaterdue to its simple construction and low manufacturing costs.Shukla et al. [55] mentioned in the review of recent advances inthe solar water heating system that the performance of an ETSC isbetter than mostly used FPC due to its ability to produce hightemperature but ETSCs are not widely used because of its highinitial cost.

As ETSCs have natural frost protection, without any damageETSCs can be used in sub-zero temperatures whereas antifreezesystems need to be installed for flat plate panels under sametemperatures which is complicated and expensive. Regular repla-cement is required for the glycol used in flat plate systems as it cancause damage by freezing. The glycol needs to be replaced everyfew years (3 years). Therefore, it is an ongoing cost. In addition,leaking might happen while replacing glycol or due to damage offlat panel by storms which is an added risk.

3.2. Fragility

Evacuated tubes are made of two layers of annealed borosili-cate glass and the glasses which are made of annealed glass aremuch more fragile than tempered glass. Because of fragility, glasstubes can be shattered easily due to small hail, jostling or poorhandling. Therefore, extra care must be taken while transportingor handling ETSCs.

3.3. Snow removal

ETCs do not shed snow as the collector surface is not alwayswarm, the tubes are insulator in nature and the collector surface isirregular which lets the snow stick on tubes for a long time. As theglass tubes are fragile, it is not possible to scrape off theaccumulated snow which might make the system ineffective.

3.4. Overheating

One of the characteristics of ETSCs is that they produce hightemperature and get much hotter than other collectors. Therefore,ETSCs are not recommended for domestic solar water heating orfor solar space heating system as the high temperature can causesignificant problem when it exceeds boiling point of water; rather,it is recommended for commercial applications. For domestic use,it is essential to keep the temperature below 100 1C whichcontinuously requires ample load on the system; otherwiseweaknesses will be exposed in the material of evacuated tubedue to overheating and eventually the vacuum will be lost.

4. Economic consideration on the usage of ETSCs

ETSCs have not provided any real competition to FPCs in thepast years though ETSCs have been commercially available formore than 20 years. Recently, there has been a major expansion ofthe evacuated tube solar water heater market in China, Europe,and Japan as a result of globally growing industries of evacuatedtube collectors. This review has attempted to identify the eco-nomic advantages of ETSC over FPC by comparing the initial andmaintenance cost as well as the payback period.

From Table 5, considering the initial cost of installation, main-tenance and operational cost, and the payback period of thesystem, it can be observed that the SWH system using an ETSCis more cost effective than an FPC. However, this scenario is forChina. A recent study by Pappis et al. [56] was conducted in Brazilregarding the economic and environmental comparison betweenFPC and ETSC. They concluded that the ETSCs are the best choicefrom the environmental perspective due to least impact generatedduring the manufacturing process. However, an FPC is preferablefrom the economic point of view as greater investment is requiredfor an ETSC system.

Installing an ETSC is expensive in some countries but it is anadvanced technology at competitive price which requires very lessmaintenance afterwards. Mostly used FPC is an old technologywith higher price and it requires high maintenance as well.

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Therefore for the long term, an ETSC is more economical and costeffective than an FTC. Recent huge production and large scaleimplementation of evacuated tubes have proven this technology asmatured with 3 years payback period without any subsidies.

5. Performance based on working fluids

To improve the efficiency of solar collectors, researchers havemainlyfocused on several structural changes such as changing the structure ofsolar collectors or changing the coating to improve absorptivity butfrom the literature, only few studies focused on changing the workingfluid in order to improve the collectors' efficiency [57]. From recentstudies, it is found that theworking fluid can influence the performanceof solar collector significantly. Water, oil, and air are the most commonworking fluids used in solar energy system but the thermal conductivityof these fluids is relatively low [58]. Recently, researchers are investigat-ing on other working fluids such as nanofluids rather thanwater and airto improve the collector's efficiency. Nanofluids consist of base liquidand nanomaterials that have enhanced thermophysical properties suchas higher thermal conductivity, thermal diffusivity and convective heattransfer coefficients [59,60]. Besides improving the effectiveness of heattransfer, nanofluids also improve optical properties, transmittance aswell as extinction coefficient of solar collectors. By using nanomaterials,the efficiency of an FPC has increased up to 10% and the incidentradiation is found to be 9 times higher than a conventional FPC. For adirect absorption solar collector, the efficiency increased up to 10% usingnanofluids [61]. Table 6 represents the summary of the previous studiesregarding the performance of ETSCs based on different working fluids.

6. Future work

i. One of the drawbacks of ETSC is that the collector tubes arevery fragile and easy to be damaged. To overcome thisdrawback, research can be carried out on improving thestructure of evacuated collector to make their body harder.For example, nanotechnology can be used to build a harderand powerful evacuated collector.

ii. Evacuated tubes are made of annealed glass which is much morefragile than tempered glass and the material mostly used isborosilicate glass. Experiments can be done on materials ofglasses used in evacuated collector to have better efficiency.

iii. Grooved tubes which have spirally running grooves in innersurface can be used instead of usual tubes inside the collectorto improve the efficiency. The heat transfer coefficient ofgrooved tube is said to be 2–3 times higher than plan tubewith same specification.

iv. The effectiveness of heat transfer is directly related to theworking fluids of the collector to absorb the heat energy fromthe absorber plate. From the literature, ETSCs have beencommercially available for more than 20 years and water isbeing used as the working fluid which has several hundredtimes low thermal conductivity than working fluids with

metal or metal oxide [55]. Based on comprehensive studies, ithas been also realized that very few studies were conductedon ETSCs using nanofluids. As the evacuated collectors havebetter performance in producing high temperature due tominimal convection and radiation losses, using nanofluids inETSC is expected to raise the efficiency significantly.

v. Solar collectors are basically of two types namely stationaryand tracking, ETSCs are of stationary type. For stationary typesolar collectors sun tracker can be used to track the maximumsunlight throughout the day. Though the cylindrical shape ofthe ETSCs helps to track the sun passively throughout thewhole day but it is not able to absorb the maximum sunlightas the solar panel is positioned with a fixed angle. Solartracker is able to orient the collector along the direction of thesunlight and ensures the absorption of maximum sunlightthroughout the day by adjusting its orientation according tothe sun [62]. It is not essential to use a sun tracker but indifferent geographical conditions it can boost the collectorenergy from 10 to 100% [63]. It is expected that the use ofsolar tracker in ETSC panels will maximize the performanceefficiency especially for industrial or large scale uses.

vi. It is found from the previous studies that the use of nano-fluids in solar collectors reduces CO2 emissions and alsoannual electricity cost [58]. As it is expected that the effi-ciency of ETSCs will increase by using nanofluids, an eco-nomic analysis can be done to find the payback period of anETSC with different types of nanofluids and the also annualelectricity savings.

vii. The working fluids inside evacuated pipe move at a slowspeed as the fluid boundary layer is close to the pipe wall.Therefore, the heat transfer coefficient in heat exchangers islimited. The thickness of the boundary layer can be reducedby creating a turbulent flow which requires turbulators to beplaced inside the pipes and thereby the heat transfer coeffi-cient can be increased.

viii. For industrial applications, a hybrid system can be developedto minimize the evacuated collector area and to improve theoverall efficiency of the system by combining ETSCs withconcentrating collector. To achieve high temperature, concen-trating collectors use mirrors and lenses by concentratingsunlight of a large area onto a small area.

7. Conclusion

This paper presents an overview of recent studies on ETSCs andrevealed that this collector has great potential in industrial, residentialand agricultural sectors. ETSCs have been used for water heaters, solarcooker, air conditioning, heating swimming pool, drying agriculturalproducts and for many other purposes. An ETSC is highly recom-mended for higher temperature applications as they can gain highertemperatures easily and are able to preserve heat even when theoutside weather is cold. For the countries with good sunshine, an ETSC

Table 5Comparisons of economic performance between ETSC and FPC.

Author Location ofstudy

Type ofcollector

Tank volume(L)

Initial cost($)

Maintenance and operationalcost

Payback period (Compared to electricheaters) (years)

Saxena andSrivastava [76]

China ETSC 100 377 Negligible 4.41

Allen et al. [77] China FPC 110 4823-5627 Negligible 17Han et al. [78] China ETSC 200 330 $3.2/year 0.7-3.0Gastli [79] China FPC 200 2000 $20/year 19.7Koroneos and Nanaki[80]

China FPC 200 $1911 approximately 1–4% of theinvestment cost

5

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shows outstanding efficiency and the countries with cold weatherETSCs are very cost effective with excellent efficiency because of theirfreezing protection characteristics. The performance of an ETSC usingvarious working fluids is also presented in this paper and it was foundafter analyzing the available literatures that the ETSC performs muchbetter with nanofluids as working fluid rather than conventionalworking fluids such as water and air. Some recommendations aremade on future research. It is expected that it will be very useful forenergy producing industries as well as for research organizations.

Acknowledgments

The authors would like to acknowledge the Ministry of HigherEducation Malaysia (MOHE) for financial support. This work

was supported by UM-MOHE High Impact Research GrantScheme (HIRG) (Project no: UM.C/HIR/MOHE/ENG/40).

References

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Table 6Performance of evacuated tube collectors based on working fluids.

Author Type ofinvestigation

Working fluid Types of ETSC Performance

Hussainet al. [81]

Experimental Water based Ag andZrO2 nanofluids

ETSC consists of 20 evacuated tubes – The evacuated collector performs better using both Ag and ZrO2

nanofluids with higher nanoparticle concentration (5 vol%).– The performance of the collector is same as water for nanofluids lower

concentration of nanoparticles (1 vol%).

Hasan [82] ExperimentalandTheoretical

Nanofluid (Water–Al2O3).

Well instrumented collector consists of16 evacuated tubes

– The efficiency will increase 7.08% with using flat plate reflector, and16.9% with using curved plate reflector.

– The volume concentration of Al2O3 is proportional to ETC performance,efficiency will enhance 28.4% with 1% of Al2O3and 6.8% with 0.6% ofAl2O3, for 0.3% of Al2O3 does not make sensible enhancement.

Liu et al.[83]

Experimental Water based CuOnanofluid

Special open thermosyphon andevacuated tubular solar air collectorcombined with CPC

– Using nanofluid, the maximum value of collecting efficiency of openthermosyphon has an increment of 6.6%.

– The mean value of collecting efficiency of open thermosyphon has anincrement of 12.4%.

Gao et al.[20]

Experimental Antifreeze fluid (40%glycol by volume)

Water in glass and U pipe ETSC – The average thermal efficiency of WGETSC is less than UPETSC.– Comparing UPETSC and WGETSC with the same efficiency curves,

WGETSC achieves energy storage of 25–35% less than UPETSC.

MahendranandSharma[84]

Experimental TiO2 nanofluid ETSC – Compared to water, 2.0% TiO2 nanofluids increases the efficiency of ETCby 42.5%.

– The efficiency of collector shows greater enhancement at low volumeflow rate and concentration of nanofluids compared to its base fluidwhich was water.

Chouguleet al. [85]

Experimental Carbon nanotubenanofluid

Evacuated heat pipe and FPC – The performance of collector using nanofluid is better.– The average collector efficiencies at 31.51 are 25% and 45% for water

and nanofluid respectively– The average collector efficiencies at 501 are 36% and 61% for water and

nanofluid respectively.– The maximum instantaneous efficiency obtained by using nanofluid is

69% at 501 tilt angle.– Solar heat pipe collector (overall efficiency 25–69%) gives better

performance over conventional FPC (overall efficiency 12–20%)

Lu et al. [86] Experimental Deionized water andwater-based CuOnanofluids

Evacuated tubular solar air collector – The CuO nanoparticles have the potential to increase evaporation heattransfer coefficient by about 30%.

– The wall temperature of the open thermosyphon decreases due to theuse of the CuO nanofluid.

Zhang andYamagu-chi [57]

Experimental CO2 ETSC – Compared to water as working fluid, the supercritical CO2 has muchmore higher efficiency.

– The collector efficiency is above 60% using supercritical CO2 asworking fluid.

M.A. Sabiha et al. / Renewable and Sustainable Energy Reviews 51 (2015) 1038–10541052

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