Труды Всемирного конгресса Международного общества солнечной энергии

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Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement

732

The 1-node collector model is characterized by the

differential equation 1.

'( ) ( ) ( )

()

(1)

CGFa a

τα ϑϑ ϑϑ

ϑϑ

=−−−−

−



Obviously the 1-node model does not distinguish between

absorber temperature and the temperature of the heat

transfer fluid. Therefore a 2-node-model has to be defined.

1.2 Two-Node-Model

The 2-node-model illustrated in Fig. 2 is used. In this case

the thermal capacity of the collector is subdivided into the

capacity of the absorber C

abs

and the fluid C

. The absorber

node is heated by the incident solar irradiance G depending

on the value of the transmittance-absorptance product (τα).

The heat losses of the absorber

loss



are based on the

temperature difference between the absorber and the

ambient. The heat transfer from absorber to fluid is

determined by the heat transfer capacity rate (UA) and the

temperature difference of the absorber and the fluid.

2-Nodes-Model

)(G τα

)(cpm

ϑϑ −



node 1

absorber

node 2

fluid

)()UA(

ϑϑ −



)(U)(Uq ϑϑϑϑ −+−=



)(G τα

)(cpm

ϑϑ −



node 1

absorber

node 2

fluid

)()UA(

ϑϑ −



)(U)(Uq ϑϑϑϑ −+−=



2-Node-Model

2-Nodes-Model

)(G τα

)(cpm

ϑϑ −



node 1

absorber

node 2

fluid

)()UA(

ϑϑ −



)(U)(Uq ϑϑϑϑ −+−=



)(G τα

)(cpm

ϑϑ −



node 1

absorber

node 2

fluid

)()UA(

ϑϑ −



)(U)(Uq ϑϑϑϑ −+−=



2-Node-Model

Fig. 2: Two-Node-Model.

Based on differential equation 2 and 3 the 2-node-model

has been implemented into a modified version of the

TRNSYS type 132.

infloutflpmflabsmfl

)())((

,,,,

ϑϑϑϑϑ −

−



(2)

()( )

() ( )

ϑϑ

τα ϑ ϑ

−

=− −

()

ϑϑ−−−

(3)

In the following a possible test procedure for the

determina-tion of the thermal performance of solar thermal

collectors using the 2-node model is introduced. Afterwards

the results are compared to the ones achieved with the same

collector but using the standardized test procedure

according to EN 12975.

2. DETERMINATION OF THE THERMAL PERFO-

RMANCE USING THE 2-NODE MODEL

The collector under investigation is a collector with

all-glass absorber tubes and copper U-pipes operated in

forced circulation mode.

In order to reduced the required testing time as much as

possible, the collector is only tested for one day under solar

irradiance. During this outdoor test the collector inlet

temperature is adjusted in such a way that the value

resulting mean fluid temperature is close to ambient

temperature. This test sequence is necessary to determine

the transmittance-absorbance-product (τα) and the incident

angle modifier for beam irradiance as well as diffuse

irradiance (K

(θ) and K

dfu

). The second test sequence is a

heat loss measurement (see figure 3). During this sequence

the collector is operated for about 2 hours without

irradiance at fluid temperatures of approx. 40°C, 60°C,

80°C und 100°C. This test sequence can be performed

either outdoors during night or indoors.

The collector parameters of the 2-node model are

determined using the same parameter identification tools

that are used for the 1-node model. Table 1 summarizes the

determined collector parameters.

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

733

Fig. 3: Collector temperatures during a heat loss

measurement.

TABLE 1: COLLECTOR PARA METERS (2-NODE

MODEL)

(τα) [-] 0.752

dfu

[-] 1.190

[W/(m²K)] 1.543

[W/(m²K²)] 0.013

abs

[J/(m²K)] 5220

[J/(m²K)] 17058

In order to compare the collector parameters achieved with

the 2-node model with the parameters obtained using the

EN 12975 test procedure the parameters (G(τα), U

and U

)

have to be converted to the parameters of the 1-node model

(η

, a

und a

). This was done using the method of

Rockendorf /3/. The resulting parameters are listed in table

2. Table 3 shows the collector parameters determined by

using the quasi-dynamic test procedure according to EN

12975.

TABLE 2: COLLECTOR PARAMETERS OF THE

1-NODE MODEL CALCULATED OUT OF THE

2-NODE MODEL PARAMETER

[-] a

[W/(m²K)] a

[W/(m²K²)]

0.706 1.463 0.012

Table 3 shows the collector parameters determined by using

the quasi-dynamic test procedure according to EN 12975.

All three parameters η

, a

and a

show a quite good

agreement.

The comparison of the power curves determined with the

1-node and the 2-node model is shown in Figure 4. The

maximum deviation, re-sulting from the slightly different

heat loss coefficient a

and a

, between the two curves is

4°%. This deviation is in the order of magnitude that can

also be observed if the same collector is tested twice

according to the same test method.

TABLE 3: COLLECTOR PARAMETER (1-NODE MODEL)

[-] 0.706

dfu

[-] 1.180

[W/(m²K)] 1.414

[W/(m²K²)] 0.016

eff

[J/(m²K)] 63080

100

200

300

400

500

600

700

800

900

1000

0 10203040506070

(

) [K]

collector output [W]

1-node 2-node

Fig. 4: Collector temperatures during a heat loss

measurement.

3. CONCLUSION

The 2-node model is, compared to the 1-node model, able

to distinguish between absorber and fluid temperature. This

feature enables the 2-node model to calculate the heat

transfer from the absorber to the fluid in case of a positive

collector output as well as from the fluid to the absorber in

case of a negative collector output.

Figure 5 illustrates this capability for a test sequence with

alternating collector output.

Using this ability a test procedure was introduced which is

able to reduce the testing time for the thermal performance

test significantly, e.g. from several days to one day and one

night, without a loss in accuracy.

Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement

734

Fig. 5: Measured and calculated collector output using the

2-node model.

4. NOMENCLATURE

A [m²] collector area

[W/(m²K)] heat loss coefficient

[W/(m²K²)] temperature dependent heat loss

coefficient

[J/(kgK)] specific heat capacity of the heat

transfer fluid

abs

[J/(m²K)] absorber heat capacity

[J/(m²K)] fluid heat capacity

F’ [-] collector efficiency factor

G [W/m²] hemispherical irradiance

(θ) [-] incident angle modifier for beam

irradiance

dfu

[-] incident angle modifier for diffuse

irradiance



[kg/s] mass flow rate of the heat transfer

fluid



[W/m²] heat flux from the absorber to the

heat transfer fluid

loss



[W/m²] heat losses a of the absorber

use



[W/m²] collector output

(UA) [W/K] heat transfer capacity rate between

absorber and fluid

[W/(m²K)] heat loss coefficient

[W/(m²K²)] temperature dependent heat loss

coefficient

abs

[°C] absorber temperature

amb

[°C] ambient temperature

fl,in

[°C] fluid inlet temperature

fl,m

[°C] mean fluid temperature

fl,out

[°C] fluid outlet temperature

(τα) [-] transmittance-absorbance product

5. REFERENCES

(1) ISO 9806-1:1994 Test methods for solar collectors

–

Part 1: Thermal Performance of glazed liquid heating

collectors including pressure drop, 1994

(2) EN 12975, Thermal solar systems and components

–

Solar Collectors, June 2006

(3) Rockendorf G., Schreitmüller K.R., Wetzel W. (1993):

Thermal collector test methods – A comparison of the

heat loss measurement and a measurement under

radiation

DEVELOPMENT OF A MOBILE, STAND-ALONE TEST FACILITY FOR SOLAR

THERMAL COLLECTORS AND SYSTEMS

Dominik Bestenlehner, Harald Drück, Stephan Fischer, Hans Müller-Steinhagen

Solar- und Wärmetechnik Stuttgart (SWT)

Pfaffenwaldring 6

70550 Stuttgart, Germany

E-Mail: bestenlehner@swt-technologie.de, drueck@swt-technologie.de

ABSTRACT

Testing of solar thermal systems and components requires

different standards and thus different test facilities. Usually

each facility is individually designed and constructed for

tests according to a specific test method. For performing

tests of solar thermal systems and collectors according to

the most popular standards, test laboratories or

manufactu-rers have to invest a huge amount of staff time,

resources and capital to set up the required test facilities.

A way to overcome this problem is the mobile, stand-alone

solar thermal collector and system test facility developed

by SWT-Technologie from Stuttgart, Germany, in

cooperation with the Institute of Thermodynamics and

Thermal Engineering of the University of Stuttgart.. The

complete test facility is installed inside a conventional 12

foot office container. This mobile container based test

facility is delivered to the site of the customer as a turn-key

product that can be put into operation within one single

day.

Even more, the mobile container based test facility is a

three-in-one test facility, since it is possible to perform tests

according to three different standards and test methods. The

performance of solar collectors can be tested according to

the European standard EN 12975 or the international

standard ISO 9806, respectively. The performance of solar

thermal systems can be determined according to the

international standards ISO 9459-2 (CSTG-method) and

ISO 9495-5 (DST-method).

This paper describes the mobile, container based solar

collector and system test facility in detail and reports the

experience gained so far.

1. INTRODUCTION

Solar thermal technology is a booming market and a wide

range of solar collectors and systems are produced by

numerous manufacturers all over the world. Testing of solar

thermal collectors and systems is required in order to asses

the thermal performance and the quality of these products.

Well established test procedures for solar collectors are

specified in the European Standard EN 12975 or ISO 9806,

and for solar thermal systems in ISO 9459-2

(CSTG-method) and ISO 9459-5 (DST-method).

In order to perform the tests specified in these standards,

each test laboratory or manufacturer requires appropriate

test facilities. Usually separate test facilities are used for

collector and system testing. Typically, these test facilities

are individually designed and installed at a specific

location.

2. THE TEST FACILITY

The requirements for performance testing of solar

collectors and thermal solar systems are different.

Therefore, it is common practice to use specifically

Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement

736

designed test facilities for testing these two categories of

solar thermal products. This results in at least two test

facilities, each containing a large number of measuring

equipment. Besides the fact that the set-up of the two test

facilities requires a very substantial investment it also

results in relatively high operational costs for the

maintenance of the two facilities and the calibration of all

the different sensors.

In order to decrease the number of test facilities and thus to

decrease the initial investment and the operational costs an

all-in-one test facility was developed by SWT. A further

requirement for this facility was some degree of mobility. It

is possible to dismantle the whole facility within a couple

of hours, load and ship it to any place and set it up again

within a short time. This mobility offers also the advantage

that the facility can be delivered as a ready to use turn-key

product to the customer and put in operation within one day.

Furthermore, the test facility is designed in such a way that

it can be operated independent from a fresh water or

cooling water net. Most importantly, the test facility

conforms to standards ISO 9459-2 and ISO 9459-5 for

system tests, and to the standard 12975-2 or ISO 9806 for

solar collectors.

2.1 System Tests According to ISO 9459-2 and ISO

9459-5

In part two and five of the standard series ISO 9459, two

possibilities for performance testing of domestic solar

thermal hot water systems are described.

With the CSTG (Complete System Testing Group) test

method standardized in ISO 9495-2, only solar thermal

systems without an integrated auxiliary heating element can

be tested. The CSTG method focuses only on sums of

energy. For the determination of the performance of a solar

hot water system according to the CSTG method, the solar

irradiance during the test day is summed up. In a second

step, the useful energy withdrawn from the system at the

end of the day is calculated based on measurements of inlet

and outlet temperatures and flow rate. Finally the

withdrawn daily energy is divided by the daily solar

irradiance. This test is performed for several days with

different irradiance values. Based on the results obtained in

this way the annual system performance can be calculated

for specific reference conditions.

During the test of the system according to the DST-method

(Dynamic System Test).standardized in ISO 9495-5 the

system is operated according to specified test conditions.

From the measured data recorded during this short-term test,

specific system parameters are determined by means of

parameter identification. Based on these parameters the

thermal performance of the thermal solar system can be

determined for specified reference conditions by means of

annual system simulations.

2.2 Collector Tests According to ISO 9806 / EN 12975

In order to determine the efficiency parameters of solar

thermal collectors according to ISO 9806 or EN 12975, two

different procedures can be used: the steady state method

and the quasi dynamic test method.

During the steady state test, all boundary conditions such as

solar radiance and ambient temperature must be constant.

After recording data points over a range of operating

conditions, the collector efficiency curve can be

determined.

During the quasi dynamic test the boundary conditions

must vary. Based on a series of measurements, specific

collector parameters are determined, as well. With the quasi

dynamic test method, additional parameters such as the

specific heat capacity of the collector and the incident angle

modifier coefficient can be determined in addition to the

efficiency curve.

2.3 Set Up of the Test Facility

One aim of the newly developed test facility is to combine

all three test methods in one test facility. This test facility

must be able to fulfil all requirements and qualifications

resulting from the above mentioned standards.

The housing of the test facility is a conventional 12 foot

office container. In this container the hydraulics for the

temperature unit are located as well as measuring

equipment and data logging instruments. In order to operate

the facility independent from a cooling network, a chiller

combined with a 600 litre cold water store is installed. With

the exception of the chiller, all components are located

inside the container. In figure 1, a schematic lay-out of the

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

737

major components is shown.

Fig. 1: Arrangement of major components (bird's view).

The facility is designed in such a way that it allows for

parallel testing of four collectors (according to EN 12975 /

ISO 9806) or four systems according to ISO 9495-2

(CSTG-method) or two systems according to ISO 9495-5

(DST-method)

To realise these different test configurations, the hydraulic

arrangement consists of one main loop that can be divided

into six smaller loops by using several valves. These six

loops are required to test two solar thermal systems according

to the DST-method at the same time. The hydraulics for

testing according to ISO 9459-2 and ISO 9806 / EN 12975

consist of only one test loop, which can be used to test four

systems (CSTG-method) or four collectors simultaneously.

In Figure 2, the lay-out of the complete hydraulic

arrangement is shown.

On the left side in the diagram the four connections for the

systems (ISO 9459-2, CSTG-method) or for the collectors

(EN 12975) are located (1-4). For the DST-method (ISO

9459-5), connections for two collectors (1-2) including the

two necessary connections to the solar collector loop

(DST-solar 1-2) are depicted. The middle part of the

hydraulic circuit is solely related to the DST-method, since

there are the connections for the thermal auxiliary heating

loops (DST-aux 1-2) and the storage tanks (DST-tap 1-2).

The first heat exchanger (HX1) in flow direction offers the

possibility to connect an optional external cooling grid. If

this is not available, the heat will be removed via the

second heat exchanger (HX2), which is on the secondary

side connected to the cold water tank. The cooling circuit

also supplies the air conditioning unit inside the container

with cold water. Behind the vertical line, which symbolizes

the wall of the container, the chiller is located.

Fig. 2: Circuit diagram of the complete hydraulic arrangement.

Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement

738

This chiller is positioned outside the container because this

allows a better supply of fresh air to the refrigerant

condenser and reduces the noise level inside the container.

Figure 3 shows the chiller connected to the container.

Fig. 3: Chiller connected to the container.

All hydraulically connections to the equipment being tested

are located outside the container and are realized with

conventional 1 inch screw fittings, countersunk into the

container walls. In figure 4 the connections to the collectors

and systems being tested are shown.

Fig. 4: Hydraulic connections to systems and collectors.

2.4 Durability and Reliability Testing

In addition to the equipment required for thermal

perfor-mance testing, the test facility is delivered with the

complete equipment required for durability and reliability

testing of solar thermal collectors according to EN 12975-2.

This comprises test facilities for outdoor exposure, external

and internal thermal shock, rain penetration, mechanical

load test and internal pressure test.

As an example for the additional capability, the standardised

rain penetration test facility is shown in Figure 5.

Fig. 5: The rain penetration test.

2.5 Patent

The mobile, stand-alone solar thermal test facility is

patented and listed under the number AZ 102007018251.3

at the German patent office. Major issues of this patent are

the mobile and stand-alone characteristics of the test

facility combined with the possibility to perform tests

according three different standards with a single test

facility.

3. EXPERIENCE GAINED TO-DATE

Up to now (summer 2007) two mobile test facilities have

been completed and one more is under construction. One of

the facilities was sold to the South African Bureau of

Standards (SABS) located in Pretoria, South Africa. This

test facility is shown in figure 6. There, establishing a test

centre for solar thermal equipment was part of a project

financed by the United Nations Development Program

(UNDP).

Fig. 6: The mobile, stand-alone test facility.

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

739

The South African Bureau of Standards is working with the

mobile, stand-alone test facility since the beginning of 2007

and has already tested several systems according to ISO

9459-2 (CSTG-method). Before shipping the test facility to

South Africa, two staff members of SABS were trained for

one week at SWT in Stuttgart, Germany. An additional

training programme took place on-site, after the test facility

had been set-up and commissioned in Pretoria.

The experiences gained with the mobile, stand-alone test

facility so far is very good. The initial operation was

performed together with an expert from SWT; it took only a

very short time period due to user-friendly set-up and

detailed instructions supplied with the test facility. The

facility was prepared for transport from Germany such that

only the wiring to the sensors outside of the container had

to be re-done. Up to now, the test facility operates without

notable problems and delivers reliable and accurate results.

4. CONCLUSIONS AND FURTHER PERSPECTIVES

The development and the set-up of a mobile, stand-alone

test facility based on a 12 foot office container was described

and the experience gained so far was reported.

For the future it is intended to develop a mobile,

stand-alone solar thermal test facility which, additionally,

allows for testing of hot water stores according to ENV

12977-3. Furthermore, it is intended to deliver turn-key test

facilities to several other test institutes, manufacturers and

universities.

5. REFERENCES

The European Standards (EN) and the International

Standards (ISO) mentioned above are available from:

www.beuth.de

6. ACKNOWLEDGEMENT

The development of the mobile test facility was supported

by the Deutsche Bundesstiftung Umwelt (DBU) under

grant number 22274 – 24/2. The authors gratefully

acknowledge this support and carry the full responsibility

for the content of this publication.

THE GLASS-TO-METAL SEALING PROCESS IN PARABOLIC TROUGH

SOLAR RECEIVERS

Dongqiang Lei, Zhifeng Wang

Institute of Electrical Engineering, CAS

P.O. Box 2703

Beijing 100080, China

Fengli Du

Himin Solar Energy Group Co. Ltd

De zhou, Shandong

ABSTRACT

This paper introduces the structure and components of the

receiver tube (HCE) in the parabolic trough solar thermal

power system, and presents the technologic difficulty of

achieving good glass-to-metal sealing during the process of

receiver tube manufacture. The glass-to-metal sealing of

HCE failure /degradation is the single largest cost factor in

the 9 parabolic trough solar power plants in the USA,

which also results in the system efficiency reduction. The

article also elaborates the principle of glass-to-metal

sealing and various demands on material for the sake of

fine sealing. Aiming to the current technique practice of

Kovar-to-glass sealing, technological process and optimal

processing parameters are put forward based on research

and experiments.

1. BACKGROUND

The solar receiver tube[1], also named heat collecting

element(HCE) in the Parabolic trough solar thermal power

system, is composed of a 70-mm outside diameter(O.D.)

stainless steel tube with a solar-selective absorber surface,

surrounded by an anti-reflection(AR) evacuated glass tube

with an 115-mm O.D. The individual HEC is about 4 m

long, a glass-metal transitional element arranged on a free

end of the glass tube. Expansion compensation is required

between the steel pipe and the glass-metal transitional

element to connect with each other, because of the different

thermal expansion coefficients of metal and glass and

because of the great difference in heating in operation in

which the metal pipe reaches about 400ć, and the glass

tube only 100ć [2]. This expansion compensation device

generally provided by a metal folding bellows [3].The outer

glass tube has an AR coating on both surfaces to reduce

Fresnel reflective losses from the glass surfaces, thus

maximizing the solar transmittance. Getters, which are

metallic compounds designed to absorb gas molecules, are

installed in the vacuum space to absorb hydrogen and other

gases that permeate into the vacuum annulus over time. A

diagram of an HCE is shown in Fig.1.

Fig. 1: Schematic of an HCE(source: SCHOTT Solar

Systems Ltd.).

It is the mainly technologic difficulty that achieving good

glass-to-metal sealing during the process of manufacturing

solar trough absorber tube. At president, there are only two

companies that can manufacture the HCE, one is Germanic

SCHOTT Solar Systems Ltd, and the other is Israeli Solel

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

741

Solar Systems Ltd.

Luz International Limited (Luz) is the world's leading

developer of parabolic trough solar electric systems. 9 large

commercial-scale solar power plants have been built by

Luz in the California Mojave Desert between 1984 and

1990. These plants, which continue to operate on a daily

basis, range in size from 14 to 80 MW and representing a

total of 354 MW of installed electric generating capacity.

The HCE is one of the most important elements to convert

the solar energy into thermal energy, and its thermal

performance decides the thermal efficiency of solar thermal

power station. The cost of receiver is about 20% of the total

costs in the solar power station [4]. From the operation and

maintenance statistics of the 9 parabolic trough solar

thermal power plants in the USA [5], there are 30-40%

failure of the HCEs at SEGS VI-IX for 9 to 11 years of

operation, because of thermal stresses in the glass-to-metal

seals joint, which cause vacuum loss, fracture of the glass

tube and solar selective coating degradation in air. It is not

economic to directly replace destroyed module, replacement

cost is about 1000$ / HCE (evacuated), because many of

them are welded together to build one loop. For repairing the

tubes, the whole loop has to be shut down. So the

glass-to-metal seal of HCE failure/degradation is the single

largest cost factor in the solar power system, which also

results in the system efficiency reduction.

2. INTRODUCTION

The glass-to-metal sealing of the HCE is a type of tubular

sealing, it requires not only certain mechanical strength, but

also excellent gas tightness under high vacuum condition.

Glass and metal are two kinds of materials which are

thoroughly different character with each other, if they are

required to make hermetic glass-to-metal seal, there are

several basic conditions to meet as follows:

2.1 Thermal Expansion Coefficients

The thermal expansion coefficient of metal is always

constant under the condition that there is no phase change

of metal. But the thermal expansion coefficient of glass will

increase when the temperature of glass exceeds the

annealing temperature, and it is not important when the

temperature arrives at the softening temperature, because

glass will become viscosity liquid and the thermal stresses

are able to automatically vanish, if the differences in

coefficients of expansion of the glass and metal do not

exceed ±10% [6] from the room temperature to the glass

annealing temperature range, the expansion curves of the

glass and metal are as far as possible consistent, the stresses

in the glass do not exceed its tensile strength.

If the expansion speeds and contraction speeds of the glass

and metal are dissimilar, both can produce the stresses in

the sealing body, when the stress value surpasses the

intension of the glass, the sealing joint will crack and

causes the evacuated tube leakage, even if it has not

cracked in the short time, but with the time going, it will

also gradually produce the microcrack and form the chronic

leakage as a result of stresses function. When the vacuum

tube is vibrated or collided, the microcrack can rapidly

spread and expand then the tube will damage.

2.2 Wettability

Wettability reflects contact abilities between two kinds of

material. In order to achieve fine hermetic sealing, it is very

important to let both the glass and metal have excellent

wetting ability each other. The glass to metal wettability is

the same as the principle of the wettability between a

droplet of the liquid on a horizontal solid surface, the

wettability of a solid by a liquid is characterized in terms of

the angle of contact that the liquid makes on the solid.

Depending on the type of surface and liquid, the droplet

may take a variety of shapes as illustrated in figure 2, The

condition θ>90°indicates that the solid is wet by the liquid,

and θ<90° indicates nonwetting, with the limits θ=0 and

θ=180° defining complete nonwetting and complete

wetting, respectively. The principle of wettability is the

very basic technology of the glass-to-metal seals. In general,

it is nonwetting between glass and pure metal, but when

there is a metallic oxide film on the surface of metal, the

wettability becomes very fine.

Fig. 2: A liquid droplet in equilibrium with a horizontal

surface.

Труды Всемирного конгресса Международного общества солнечной энергии - 2007. Том 2

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