factors affecting the performance of glass metal seal of ... · concentrating solar collectors...

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 13, Number 8 (2018) pp. 81-92

© Research India Publications. http://www.ripublication.com

81

Factors Affecting the Performance of Glass–Metal Seal of Solar Receiver

Tubes: A Review

Vinod Kumar Verma

Research Scholar, Department of Mechanical Engineering,

Rajasthan Technical University, Rawat Bhata Road

Kota, Rajasthan, India

and

Assistant Professor, Department of Mechanical Engineering,

Government Engineering College, NH-8 Badliya choraha

Ajmer, Rajasthan, India

B.Tripathi

Associate Professor, Department of Mechanical Engineering,



K.B. Rana




Rahul Chhibber


Indian Institute of Technology, NH 65 Nagaur Road

Karwar, Jodhpur, Rajasthan, India

Abstract:

World is heading towards renewable sources of energy with

increase in the population. Solar energy is one of the major

renewable energy sources which has tremendous scope

because of direct and large solar flux abundance, as well as

large open fields to harness the direct sun rays. Generally

solar energy is harnessed by PV panels and various kinds of

concentrating type receivers. In the concentrating type solar

receiver tubes, main cause of failure is leakage at the

junction of glass to metal joint (seal) due to high difference

in coefficient of thermal expansion of glass tube and steel

bellows. These seals are also subjected to high residual

stresses and repeated thermal cycling during its operation.

For making glass to metal seal, various materials and

procedures are under investigation to overcome this failure

and increase the life and efficiency of tubes. The effects of

various factors on the performance of glass–metal seal of

solar receiver tubes were comprehensively reviewed

throughout this article.

Keywords: Glass–metal seal; Solar receiver tube;

Ceramics; Renewable energy.

Introduction:

Nowadays the world is facing problem of energy crises

because of rapid global populace boom, as energy

production rate is lagging the energy demand. By 2040 it is

expected to see a rise of 40% from now, in the total energy

demand. The power production is mainly dependent on

conventional fossil based fuels, which are estimated to

significantly deplete after 70 years [2]. Today, managing

and halting climate changes is a big challenge produced by

the overexploitation of conventional resources [3].

Renewable energy is attractive and environmentally friendly

option for reducing fossil fuel based power generation. In

the field of renewable energy, solar energy is one of the

viable and promising aspects for sustainable development. It

is expected that solar energy can supply approximately 45%

of the world energy demand by 2050 [4].

Presently solar energy is mainly harvested by using

photovoltaic (PV) panels and various kinds of solar

collectors (i.e., concentrating and non-concentrating type).

Concentrating solar collectors receive the solar radiations

and convert them into thermal or electric power. Parabolic

trough collector (PTC) are proven as the world’s most

matured and field tested concentrated solar collector

technology which can be used for harnessing of solar energy

for large scale. One of the most mature and field tested

Concentrating type solar collector technologies for large

scale harnessing of solar energy is parabolic trough collector

(PTC) [5-8]. In any parabolic trough solar power system,

receiver tube is the key component, which typically

accounts 30% of the total cost of solar field [6, 7, 9-12]. The

parabolic trough receiver tube typically consists of metal

tube (absorber) with a solar selective coating and glass



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envelops the surrounding of absorber tube to form an

annular space between the absorber tube and the glass as

shown in Fig. 1.

Figure 1: Parabolic trough receiver tube [13]

With parabolic trough, solar radiations are concentrated and

reflected towards the absorber tube through which heat

transfer fluid is flowing. It increases the temperature of heat

transfer fluid which finally converts the solar radiation

energy into useful thermal energy [6, 7, 9-12]. The purpose

of glass tube is to minimize the radiative and convective

heat losses. In the space between the metal pipe and glass

tube, high pressure vacuum tight environment is maintained

with the help of glass-metal transitional elements and metal

bellows.

The function of vacuum tight enclosure is to reduce heat

losses at high operating temperatures, protect thin film

coating of receiver tube from oxidation and accommodate

for the difference in thermal expansion in between glass

envelope and metal pipe [14-15] Appropriate distance

between glass envelop and absorber tube is kept to avoid

large deflections from the hot absorber tube which might

cause of breakages in the glass envelope [16]. Dimensional

stability at elevated temperatures and chemical inertness at

high and low temperatures makes glass as one of the best

suitable materials in sealing [17]. Other than PTC tubes,

application of glass-metal seals are widely visible in the

field of solid oxide fuel cells, electronic component

packages, semiconductors, vacuum tubes applications in

electronic feed through [18-40].

In the PTC tube, joining of glass and metal is a big

challenge for researchers because of differences in physico-

chemical properties of glass and metal. Breakage of glass-

metal seal is the major issue and adhesive is not an

appropriate solution for glass-metal joining due to its lower

stability at higher temperatures and low-stress application

[13]. Hence failure or degradation of receiver tubes is the

biggest cost factor in PTC power plant [43].

Thousands of solar parabolic trough receivers are installed

around the globe without the quality control monitoring and

standardization [44]. Many factors need to be considered for

designing and developing the high quality glass-metal seals

if adequate component of lifetime is to be achieved. For the

past several years, researchers have been focusing on the

designing and development of efficient glass-metal seal by

considering the important influencing factors.

The objective of current study is to present an exhaustive

review of various factors affecting the performance of

glass–metal seal of solar receiver tubes.

2 Factors affecting the life and integrity of glass metal

seal

It is recognized that a wide variety of metals and alloys can

be well bond with glass under suitable conditions which led

to the development of many useful products and

applications [45]. With newer glass ceramic materials, many

superior products can be developed along with many

advance applications [45-46]. The major challenge is ability

to tailor the thermal expansion characteristics of glass and

metal to attain a good integrity of glass metal seals over a

longer period of time. It make glass-ceramics as an ideal

candidate for coating applications and sealing applications

where compatible thermal expansions are essentials. In last

few years, there has been a dramatic revival of the interest in

both glass and glass ceramic to meal seals [45, 47-49].

Substantial work is in progress to enhance the performance

characteristics of sealants under the extraordinary working

conditions of vacuum, high temperature and thermal cycling

at various environmental conditions.

Donald et al. [50] did a comparative study between sealing

to multicomponent alloys and pure metals, shows that alloy

systems are more likely for deleterious reactions,

particularly for Fe or Cr metallic species because of the

influence of additional metal species in the reaction one,

these helping to promote these reactions. Through careful

control of the initial glass composition or processing

conditions, deleterious reactions can be minimized.

2.1 Oxide Layer

Direct bonding is not possible in glass to metal seal due to

high difference in properties of glass and metal; hence an

oxide layer should be developed on the surface of metal

which act as a transitional interface [51] from glass

properties to metal properties. Therefore the overall

properties of the glass bond depend on the properties of the

metal oxide layer [52-54]. For topography characterization

of surfaces, roughness factor is widely used. Acid etched or

passivated titanium surfaces can also enhance the bonding

of oxidation layer in sealing [55]. Roughness increased

using vegetable oil on stainless steel substrates are also

under investigation [56].

Series of alloy were studied by some researchers [57-59]

which show that poor mechanical strength is obtained by

various metal oxides and mixture of metal oxides that are

developed on the surface of most metals and alloys.

AISI 304 type Austenitic stainless steel has FCC structure,

which bears high coefficient of thermal expansion while

making glass to metal seals. AISI 430 type Ferritic stainless

steel have BCC structure which goes under transformation

to FCC at around 850⁰ C [51]. This transformation changes

the thermal expansion coefficient which may leads to cracks

while cooling of glass metal seal. Titanium can be used to

obtain fully stabilized ferritic structure. Stable nitrides and

carbides can be formed by tiding up carbon and nitrogen

with titanium [60]. Hence the stability of ferritic structure

can be increased by decreasing the measure of nitrogen and

carbon interstitials in the compound.

A study of Mantel [61] indicates that 20% Cr ferritic

stainless steel possesses a suitable thermal expansion



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coefficient for sealing to a soft glass. The chromium affects

the thermal expansion coefficient of the metal. Thermal

treatment under wet hydrogen air leads to a theory of oxide

film formation. According to this theory, double oxide layer

is formed with thermal treatment. Outer layer is formed with

spinel type MnCr2O4 with titanium oxide which dissolves in

the glass. Mechanical cohesion is ensured with the inner

layer of chromia scale. At the metal-oxide interface, silicon

contained in the steel gets segregated and thus decrease the

adhesion property of metal oxide consequently metal–glass

seal.

Most of the researchers hold that preferable metallic oxide

should be FeO or Fe2O4 but never Fe2O3 when Fe is present

in the metallic alloy [62].

By peroxidation of metal surface, interfacial chemical bonds

are usually formed which ease the transition from

ionic/covalent oxide bonding in the glass to metallic

bonding. Extensive summaries and technological supports

were discussed by Kohl [63] and Pask [64] in their studies.

2.2 Wettability

Wettability reflects contact capacities between two sorts of

material. Characterization of wettability of liquid over solid

can express in terms of contact angle that the fluid makes on

solid surface as shown in Fig. 2.

Figure 2: Drop size analysis to measure contact angle

If this angle of contact is an acute angle, it shows

nonwetting behavior and when this angle of contact is an

obtuse angle, solid is wet by the liquid. Generally

nonwetting behavior is shown between glass and metal but

on introducing metallic oxide film on the surface of metal,

wettability becomes very fine.

To achieve strong glass to metal bonding at elevated

temperature, high wettability and fusion of glass to metal

surface is required. When oxidized glass reacts with metal

oxide layer, a strong bond is formed [65, 66]. Following

criterion should be fulfilled to achieve better quality of glass

metal seal [67]:

Glass should wet and hold fast to the surface of metal.

At the glass metal seal interface, ideal oxide layer

thickness must exist.

Metal should not have thermal critical points (allotropic

transformations) inside the extended elevated

temperatures which will reach in glass to metal seal and

least temperature arrived in the service.

Glass should not re-boil when heated for making the

seal nor should be metal give off gases.

Glass metal sealing must be vacuum tight.

In our previously published study [13] wettability effect of

borosilicate glass on different metals was studied and

resulted that wettability of borosilicate glass over SS304 is

maximum as compared to copper and low carbon steel (LCS

1018) substrates. Due to waviness on the surface of metal,

borosilicate glass flowability decreases. Copper oxides are

less thermal stable which erode during the sealing process,

hence low wettability of glass achieved with copper. Due to

no contact angle of glass on LCS 1018, very poor wettability

achieved, thus glass detached easily from the surface of LCS

1018 after sealing.

2.3 Materials and thermal expansion coefficient

Glass Metal sealing should have high mechanical strength as

well as hermeticity at metal glass bond. For a successful

product, extreme performances are expected from the

component materials which are produced by utilizing

modern technology. Ability to resist weather over a long

period of time, can withstand low and high temperature

cycles, mechanical loading cycles, etc. are the one part of

this success whereas, low product cost, low maintenance

cost, low scrap at the time of manufacturing and high

reliability in the hands of customer during service are also

essential features [68].

Closer the coefficient of thermal expansion (CTE) of metal

and glass, lower will be the limit of stresses developed in

glass. A craving to utilize stainless steel in glass metal seal

led to the development of a family of phosphate glasses [69]

who’s CTE (1.78x10 –5/⁰ C at 26⁰ to 350⁰ C) nominally

matched to 304 stainless steel.

CTE remains constant for a single phase of metal whereas

the case is different for glass. Above annealing temperature,

CTE of glass increases and becomes unimportant on arrival

of softening temperature. If the difference in CTE of glass

and metal remains in the range of ±10% [70] from the range

of ambient temperature to the glass annealing temperature,

almost consistent expansion curves of glass and metal is

obtained and the stresses in the glass limits under its tensile

strength.

With experimental results and calculation, Chambers et al.

[71] concludes that elastically computed stresses are not

always conservative. While expressing and predicting the

presence of stresses, effect of structural relaxation is

extremely important but significantly differs from cursory

elastic predictions and capable of producing fractures in

glass.

A material combination within a CTE range of 5.0 x 10-6/K

is used to develop matched glass to metal seal in request to

guarantee vacuum solidness for the lifetime of the collector.

[43].

2.4 Design

In glass to metal seals, there are inevitable residual thermal

stresses generated, not only by the difference in CTE



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between glass and metal but also due to sealing geometry

[72-73]. These residual stresses produce negative effect to

the seal strength. For designing of glass-metal seal,

generally three approaches are pursued which are,

compressive sealing, bonded sealing and compliant bonded

sealing [19].

To reduce the end loss in the receivers, short and

recompressed bellows are used. Structural parameters were

compared among different designs of receiver tube (Himin

PTR–2011, Solel UVAC3 and Schott 2008 PTR–70)

Table 1: Structural parameters comparison of analyzed

receivers [74,75]

3 Factors effecting the efficiency and performance of

solar receiver tube

Failure or degradation of glass to metal sealing of heat

collecting element (HCE) is considered as the only largest

cost factor in 9 parabolic trough solar power plants

commissioned in USA.

Investigation in parabolic trough solar power technology by

[76] concludes that the operating performances for existing

power plants with parabolic trough are robust and excellent

in commercial power industry. Substantial technology

progress has been realized since the last commercial

parabolic trough plant was build. Future parabolic trough

pants are likely to be more competitive along with

economical thermal storage system.

3.1 Thermal Characterization and Stress Concentration

To design and optimize the receiver structure of a parabolic

trough for less failure, temperature distribution of solar

receiver is also required to identify and analyze [77]. During

the formation of glass metal seals, interfacial reactions of

glass and metal species may leads to undesirable elements

and internal stresses which may affect the behavior and life

of seal components. For elevated temperature systems, those

reactions which happen under dangerous working states

should be considered [22, 50].

Study of overall heat loss, end loss and thermal emittance of

the coating are the essentials for evaluating thermal

characterization. It helps to indicate the thermal and

economic performance of the parabolic trough power plant.

Annual thermal loss varies from 10% to 15% of the incident

irradiation over the collector [78, 79]. The energy balance

in the parabolic trough collector is expressed as follows [80-

81]

Qcoll = Qabs – Qloss = Qabs – Qg,loss – Qend,loss

Where Qcoll is the total possible collector output, calculated

by the increase in enthalpy in heat transfer fluid flowing

through receiver; Qabs is the absorbed thermal energy; Qloss

is the overall heat loss power and equal to the sum of Qg,loss,

that is heat loss from the glass envelop and Qend,loss from the

receivers ends.

[16]. Examining the overall heat loss (Table 2), end loss and

coating thermal emittance of receiver to evaluate its thermal

characterization. The heat losses rapidly increase with the

increase of absorber temperature. Deviation in temperature

is observed while using end coil heater was significantly

lower than those without the end coil heaters with absorber

temperature of 400⁰ C. The thermal emittance first

decreased and then significantly increases with the increase

in absorber temperature. A higher emittance at a lower

temperature may be caused by the ratio of the end heat loss

in the total heat loss being too high.

Table 2: Comparison of heat loss for different

configurations of receiver at 250ºC [82]

There is 30-40% failure of the HCEs at Solar Electric

Generating System (SEGS) VI-IX over a decade of

operation due to thermal stresses in glass metal seals which

causes fracture of the glass tube, vacuum loss, and

degradation of selective coating in the presence of air [83].

For repairing the tubes, whole plant loop has to be shut

down and thus results in the reduction of system efficiency.

Sealing joints can crack and tube leakage occurs when stress

value surpasses the glass intension. Even in short time it is

not a crack but gradually it produces microcrack and with

stress function, chronic leakage may occur. On the collision

or vibration of these tubes, microcrack expands and spreads

rapidly to damage the tubes.

Due to high difference in coefficient of thermal expansion of

glass and metal, and high operational temperature difference

i.e. metal pipe reaches about 400⁰ C, and the glass tube

reaches upto 100⁰ C [84] metal folding bellows are used [5].

Lupfert et al. [85] performs various kinds of thermal tests

for characterizing the thermal properties of two available

parabolic trough receivers. The main problem for

quasiequilibrium test is the lost power determination, for the

bench scale test. The main advantage of the presented test is

that they include the receiver ends and bellows in the

measurement. Investigation concludes that irrespective of

the different specifications in emissivity of two products,

there were no major differences found in the energy balance

at elevated temperature, under a range of uncertainty of

several different measuring approaches.



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3.2 Coating

Currently available coatings are limiting the operating

temperature upto 400⁰ C above which stability and

performance reduces drastically reduce. To increase the

operating temperature of solar field from 400ºC to 500ºC,

better selective coatings are necessary which possess low

thermal emittance and high solar absorptance at 500ºC thus

reduce the cost of solar electricity. For high temperatures

solar fields, present coatings are neither stable nor giving

desirable performance. Future coatings should not only

stable in evacuated environment but also in air at the time of

vacuum is breached [86]. Some recent designs are planned

to reach the working temperature of 600ºC, which will

increase the performance by 5 to 10% to attain the market

requirement. Such systems are expected to work for 20 to 25

years. Improved coatings have more than 95% absorbance

and emittance of 10% at 400ºC [87].

Solar absorber surfaces must have high reflectance (ρ ≈1) at

λ≥2μm and low reflectance (ρ ≈0) at wavelengths (λ) ≤2 μm

for efficient photothermal conversion. For application in

parabolic trough, spectrally selective surface must be

thermally stable above the temperature range of 500⁰ C,

ideally in air, bears a greater value then 0.96 for solar

absorptance and lower values then 0.07 for thermal

emittance at 400ºC [88].

Heat Collection Element (HCE) use an evacuated receiver

tube of stainless steel coated with cermet (highly absorbing

metal dielectric composites consists of fine metal particles

in ceramic matrix or a dielectric), enveloped with Anti

reflected coated Pyrex glass and a conventional glass metal

seal. Original HCE design used a Mo-Al2O3 cermet solar

coating deposited by radiofrequency planar sputtering with

good optical properties of α = 0.96 and εM (350°C) = 0.16

(where εM is the εmeasured at 350ºC) for 350ºC < T <500ºC

in the vacuum Lanxner and Elgat [89]. Universal Vacuum

Collector (UVAC) employed with a multilayer of Al2O3

based cermet, without the use of Mo, with an improved AR

coating with 0.10 emittance and 96% absorptance at 400ºC

which is also stable in air and humidity at higher

temperatures Mahoney [90]. Further a good literature by

Kennedy [86] is available for medium to high temperature

absorber coatings, out of which some are durable for above

500ºC operations. Different transition metals, specially

formed from refractory metals of group IVA, VA and VIA

along with their binary and ternary compounds are

suggested for the application in high temperature operations.

Cuomo et al. [91], states that Titanium, hafnium, zirconium

metal oxide, boride, nitride, carbide and silicide are few of

the materials with highest melting points in nature, where

HfC have a melting points of 3316ºC. These materials are

very hard, high resistance to wear, oxidation and [92, 93].

For higher photo thermal conversion efficiency then

surfaces normally using a homogeneous cermet layer or a

graded seal, a double cermet film structure has been

developed [94].

Kennedy and Price [8] used Computer aided design software

to modeled optically a solar selective coating with α= 0.959

and ε = 0.061 at 400ºC composed a material stable at higher

temperatures. Optical properties of the deposited layer are

measure, extracted and applied the constants to the model.

Calibration of material and thickness and uniformity are

verified and also check the stoichiometry and optical

properties along with morphology study of individual layer.

The thermal and durability of commercial prototypes solar

selective coatings (Solel, Schott) and NREL’s prototypes are

determined by high temperature exposure in inert gas oven.

Results has been analyzed and used to improve advance

solar selective coatings.

AR coating is done on both the surfaces of outer glass tube

to reduce Fresnel reflective losses from the glass surfaces

which maximizes the solar transmittance Lei et.al (2007 -1).

Highly efficient selective coating had high solar absorptance

of 95% and low thermal emittance, 10% at 400⁰ C [16].

3.3 Vacuum level and residual gasses

At any existing operational solar plant, the required vacuum

conditions in annulus may be compromised by glass

breakage, bellow and seal breaking, getter decomposition

and hydrogen penetration (Price et al., 2006). Metallic

compound getters are used in vacuum annulus to absorb

hydrogen and other gases that permeate over time [83]. By

NREL, UVAC-3 [75] and Schott PTR70 [74] receivers were

tested with indoor steady state equilibrium method in the

condition of annulus vacuum. By DLR, similar indoor test

stand was established. A research team from Southeast

University and Sanle Electronic Group jointly worked

together to develop a suitable high temperature vacuum

receiver for China’s future parabolic trough projects since

early 2000s. When the crucial technologies of high vacuum

glass to metal seal is achieved [95] and selective coatings

for high performance are developed, a new kind of high

temperature receiver specialized for power generation has

been developed.

Wang et al. [76] investigated a one dimension non vacuum

receiver heat transfer model for four different heat transfer

mechanism in non–vacuum annulus. Given model fits

accurately to predict receiver’s real heat loss from

laboratorial tests and once when vacuum fails, receiver’s

heat loss shows remarkable effects on changing the

environmental conditions. Performance of selective coating

could be roughly evaluated by combining experimental data

with one dimension model. To identify the real heat loss in

combination with the influence of sunlight, various

environmental conditions and at different vacuum, field test

with parabolic solar collector and high pressure organic oil

(quasi steady state test) was conducted in the parabolic

trough solar demonstration project at Sanle Science and

Technology park.

Residual gas in the annulus leads to many negative effects

such as coating degradation, decrease in system efficiency,

environmental sensitivity enhancement.

Lie et al. [96] uses ANSYS finite element software and use

photoelastic technique to measure the residual stresses

distribution in glass metal seals with tubular geometry.

Simulation results are agreed with the measured tensile

strength of glass to metal joint on material tensile testing

machine. Near the glass to metal interface, circumferential

residual stresses are compressive whereas axial stresses are

tensile on the outer surface and compressive on the inner

surface of glass tube. Critical stress concentration occurs



86

near the glass metal interface zone and hence it should be

avoided from heat shocks and nonuniform temperature

distributions. Probability of seal fracture can be reduced and

seal configuration can be improved by employing rounded

end metal ring instead of blunt or think edge ring. Residual

stress decreases with the increase in depth of metal

embedded into the glass and leads to increase in sealing

strength. For glass to metal seal in receiver tube, the optimal

depth should be less than 8mm.

Lei et al. [43] suggested that if the difference of CTE

between glass and metal is less than 7%, good sealed

element is obtained. With the increase in glass tube

thickness, value of residual stress drops and seal strength

has increase. It is also recommended to have thick end glass

tube and length should be more then x0 or π/4βg

4 Modeling and testing methods to analyze the integrity

and performance

There are various factors on which the performance and

efficiency of solar collectors depends. To analyses these

factors through experimental model, a huge amount of

money and time is required. Hence by using various

software platforms, thermal models are developed easily

with a low cost and time. Since 1970s, with the help of

thermal models, various studies are conducted to calculate

the expected solar field size, to study the collector

degradation pattern, various thermal losses and its effect on

efficiency and performance of collector can be done. By

trailing various fluid flow rates, strategies can be develop to

enhance the performances [97]

Many researchers had carried their work in the field of

thermal modeling such as Dudley et al. [79], Forristall [80].

A wide study of Padilla et al. [98] and Kalogirou [99]

proves and validates the thermal models with the results

obtained by experimental setup.

Out of various studies done on one dimensional steady state,

an analytical model was presented by Dudley et al. [79] for

a parabolic trough solar collector LS-2, SEGS. Results are

collected on efficiency and thermal losses of collector by

performing several experiments on rotating test platform of

AZTRAK, which is situate at Sandia National Laboratories,

shows good agreement with the theoretical data. A much

more detailed study with Engineering Equation Solver

(EES) about one and two dimensional heat transfer model is

proposed by Forristall [80] which is applicable for long and

short receivers. This EES is capable in calculating thermal

performance of parabolic trough solar collector/receiver

along with many geometric parameters, various material

properties, different kinds of fluid flow and variable

operating conditions. For the study of thermal radiation

losses at glass envelop and steel absorber, a comprehensive

radiative analysis has to be done. Such study for parabolic

trough solar receiver/collector was reported by Padilla et al.

[98] where they divide the mass of glass envelope and steel

absorber in various segments and use energy balance

applied through one dimensional numerical heat transfer

analysis.

Due to the geometrical position of solar receiver tube and

collector, the amount of solar flux received towards the

collector side and other side of the receiver tube is different.

Similarly the amount of radiation falls over the main

receiver tube is much higher and concentrated then the one

falls at inactive ends such as steel bellows and glass metal

seal. Due to this fact the temperature in the complete fluid

line is different at various points. Many researchers studied

this variation in temperature and focused their study on

those properties which are temperature dependent and

produce non-uniformity in the flow line [100-109]. To

analyses the fluid flow dynamics and non-uniformity of heat

transfer in the application of parabolic trough solar receiver,

various software based models were presented. Some of

them used numerical methods in computational fluid

dynamics (CFD) in three dimensional non-uniform studies

[9,100,106-119]. Depending upon the input variables and

complexity of three dimensional non-uniform CFD models

prepared on particle swarm optimization (PSO) or genetic

algorithm (GA) time taken to complete the intelligent

optimization varies from few days to week and sometimes

up to a few months.

Many researchers worked to increase the heat transfer in

receivers and reduce the thermal emission from glass

envelops. Reddy et al. [120-125] works in the same domain

and suggested many porous inclusions that are inserted at

one end of the inner absorber, which ultimately increase the

heat transfer in receiver. Infra-Red (IR) reflecting layer is

also used to cover the glass tube which helps to suppress the

thermal emission. In the study of Grena [123], irradiated

half of the glass is covered with IR reflecting layer that

reduces the thermal emissions. In the initial stage, optical

and thermal simulations, two simplified halve models were

derived from liner Fresnel solar collector [124]

Half insulated theory is further investigated numerically for

conduction and convection heat loses by Ansary and

Zeitoun [125]. They investigate about a receiver tube which

is half insulated air filled annulus, which also carried a heat

resistant thermal insulation material.

Various studies has been carried out to change the type of

flow and producing vortex inside the tube. A three

dimensional numerical study has been presented by Cheng

et al. [126, 127] in which they coupled heat transfer

enhancement and turbulent flow by placing vortex

generators at the sides of absorber tube. In other study of

one dimensional non uniform or uniform thermal model,

two ends were kept at different solar radiation and heat

transfer modes. Study concludes the use of thermal model

for quick computing, performance analysis on the basis of

fluid flow, heat transfer and materials properties.

Software modeling gives a large freedom to researchers in

the analytical approach. For evacuated and non-evacuated

tubes, efficiency of PTC was predicted by Padilla et al. [98]

with the use of analytical heat transfer model. Validations of

software models must be verified with the results from

experimental setup. In the study of Pope and Schimmel



87

[128] analytical model was validated with the data obtained

from SNL collector test facilities.

5. Conclusions:

Design and manufacturing of glass to metal seal is still a big

challenge in front of researchers. Breakage of seal and

vacuum breech are the major issues which are affecting the

life and performance of solar fields. A limited amount of

research in terms of literature, reports, books, patents,

industrial investigations etc. is available for various glass-

metal seal applications. A detailed review was done to cover

all the parameters which affect the integrity of seal, directly

or indirectly. Based on those reviews following conclusions

are drawn:

More materials and alloys are required to investigate

on micro and Nano level to achieve a better coefficient

of thermal expansion of glass and metal and thus

reduce the level of residual stresses.

Various factors affecting the oxide layer properties

such as exposure time, surface finish, atmospheric

conditions etc. should be carefully observed and tailor

to achieve optimum results.

Wettability factor should be analyzed on micro level

and combinations of positive results should also be

experimented in a combined manner to attain better

results.

Vast study on thin film coating will also help us to

achieve better results with limited resources and setup

for receiver tubes and glass envelop.

Advance getters should be investigated and employed

to overcome the residual glasses in annulus.

Continuous vacuum with the help of vacuum pumps is

also one the possible solution.

Researchers haven’t got much success in this field;

hence many facts need to be addressed in future

research to improve the life and performance of glass-

metal seal in solar receiver tube. Further R&D

improvements will definitely reduce the production

cost and time to setup, and investigate the results on a

larger scale by additional help of software and

computing tools.

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