Труды Всемирного конгресса Международного общества солнечной энергии - 2007. Том 2
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Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement
532
f
QQ
Q
=
−
⋅ %100
(3)
Q
conv,net
represents the yearly heat demand of a conventional
(non-solar) domestic hot water system required in order to
cover the same load. It amounts to 3589 kWh/a and is
composed of the annual hot water demand of 2945 kWh/a
and the heat losses of the store of the conventional
reference system with 644 kWh/a.
Figure 4 illustrates the fractional energy savings f
sav
resulting from annual system simulations with the collector
efficiency parameters determined for the 15 collectors in
new state and after exposure.
Fig. 4: Fractional energy savings prior (new) and after
exposure.
Figure 5 shows the resulting relative decrease in the
fractional energy savings for the 15 collectors due to the
exposure. What is indicated on the Y-axis as “relative
change” means the percentage of the reduction in
comparison to the initial value of f
sav
prior to exposure.
Fig. 5: Relative change of the fractional energy savings f
sav
in percent.
The average reduction of the fractional energy savings is
about -2.4 % (relative) over the entire investigated three
year period. In order to point out this effect in more detail
two examples are presented in the following.
The subsequent calculations are based on the heat demand
of a conventional (non-solar) domestic hot water system of
3589 kWh/a (without taking into account boiler efficiency
and primary energy equivalent). With fractional energy
savings of, for example 55 % (which corresponds to a
saved heat quantity of 1974 kWh/a) a reduction of f
sav
to
53.7 % (which corresponds to a relative reduction of -2.4 %)
would mean that the auxiliary heater has to deliver 47 kWh
per year more conventionally produced heat due to ageing
of the solar collectors.
With regard to collector No. 9, that shows with -4.7 % the
highest deterioration, this means that 97 kWh of
conventionally produced heat are additionally required per
year due to ageing of the collector.
However, on the other hand half of the collectors show a
performance degradation that is below the mean value
of -2.4 % and therefore the increase of required
conventional heating energy for a system with these
collectors would be less than 50 kWh (i.e. less than 5 liters
of oil) per year.
4. CONCLUSIONS
The results of the investigation show that even after three
years of exposure with high temperature stress the thermal
efficiency of the collectors was not significantly reduced. In
this context it has to be considered that the collectors were
exposed outdoors to normal weather conditions but not
connected to a circulating heat transfer fluid that usually
cools down the collectors. This way of performing the
outdoor exposure leads to higher absorber temperatures and
therefore to an increased temperature stress, than it would
be the case under real operation conditions of the collectors
in a solar thermal system.
In order to quantify the ageing effects that would occur
after a certain operation period it is necessary to find a
relationship between “real” operation and exposure time as
well as conditions.
3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS
533
In order to obtain further knowledge about the ageing
behavior of solar collectors it is intended to repeat the
thermal efficiency tests with the investigated collectors
after approximately three or four years of additional
exposure.
5. NOMENCLATURE
A [m²] aperture area per collector unit
a
1
[W/(m²K)] heat transfer coefficient
a
2
[W/(m²K²)] temperature dependent heat
transfer coefficient
a
eff
[W/(m²K²)] effective heat transfer coefficient
f
sav
[%] fractional energy savings
G* [W/m²] hemispherical irradiance
Q
[W] power output
Q
aux,net
[kWh/a] auxiliary heat demand
Q
conv, net
[kWh/a] heat demand of a conventional
domestic hat water system
η
0
[-] conversion factor
T [K] temperature
6. REFERENCES
(1) Frei, U., Brunold, S., Häuselmann, T.; Langzeit-
Alterungsuntersuchungen an Abdeckung- smaterialien
für thermische Sonnenkollektoren; Institut für
Solartechnik Prüfung Forschung SPF, Rapperswil
(2) Final Project Report “QanKoll” (quantification of the
ageing behavior of solar thermal collectors) available in
German at: http://www.itw.uni-stuttgart.de/~www/
ITWHomepage/TZS/Lit_TZS.html
Note:
The results presented in this paper are obtained within the
project “QanKoll” (quantification of the ageing behavior of
solar thermal collectors). The Project QanKoll is financed
by the German Federal Ministry for the Environment,
Nature Conservation and Nuclear Safety under the contract
number 0329277A. The authors gratefully acknowledge
this support and carry the full responsibility for the content
of this publication.
IMPROVEMENTS OF MEASURING THE HEMISPHERICAL EMITTANCE
OF SELECTIVE COATING FOR ALL-GLASS
EVACUATED COLLECTOR TUBES
Zhou Xiaowen
Tsinghua Solar Ltd.,
Tsinghua University, Beijing 100084, China
xwzhou2003@yahoo.com.cn
Na Hongyue
Tsinghua Solar Ltd.,
Tsinghua University, Beijing 100084, China
Yin Zhiqiang
Department of Electronic Engineering
Tsinghua University, Beijing 100084, China
yinzq@tsinghua.edu.cn
ABSTRACT
The hemispherical emittance of the selective coating in
all-glass evacuated tubes should be no larger than 0.080 at
80ćf5ć in “All-glass evacuated collector tubes” of
China national standard. A novel equipment to determine
the hemispherical emittance of the selective coating in the
normal evacuated tubes was designed and set up. Some of
hemispherical emittances of Al-N/Al absorbing coatings in
all-glass evacuated tubes were measured.
1. INTRODUCTION
A novel equipment to determine the hemispherical
emittance of the selective coating in the normal evacuated
tubes was set up. The equipment is composed of three parts:
the test part, the controller and the record part (Fig. 1). The
equipment can reach a high accuracy by the novel type of
heater and adjuster.
2. EQUIPMENTS AND PRINCIPLES
An all-glass evacuated collector tube is a part of the test
part because of the high vacuum degree between the inner
and cover glass tube. The heater is divided into three parts
where the main heater in the middle and two auxiliary
Fig. 1: A novel equipment to determine the hemispherical
emittance of the selective coating in the normal
evacuated tubes.
heaters on the both sides (Fig. 2). The temperature of the
three heaters can be adjusted to be the same degree by the
adjuster within half an hour, so that there is only heat
irradiation through the selective coating on the inner glass
tube. The energy supplied by the power equaled to the heat
irradiation through the selective coating. The hemispherical
emittance of the selective coating can be calculated by
formula 1.
()
IU
A
TT
ε
σ
=
−
(1)
3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS
535
ε
h
, the hemispherical emittance of the selective coating
I, current of heater, A
U, voltage of heater, V
σ =5.67×10
-8
W/m
2
gK
4
, StefangBoltzmann constant
A
1
, the area of the determined surface, m
2
T
1
, temperature of heater at quasi-steady state, K
T
2
, temperature of the cooling water, K
Fig. 2: The test part.
A. inner glass tube with the selective coating˗ B. cover glass tube˗
C. vacuum jacket˗ D. jacket for cooling water˗ E. thermal couple and the
power˗ F. porcelain tube˗H
. main heater˗ HˈH . auxiliary heater.
3. ACCURACY EXPERIMENTS
A special vacuum tube with pure aluminium sputtering on
the outside of the inner glass tube was made. The
hemispherical emittance of aluminium surface at difference
temperature was compared in Table 1.
TABLE 1: HEMISPHERICAL EMITTANCE OF ALUM-
INIUM SURFACE AT DIFFERENT TEMPERATURE
T
1
˄ć˅
I
˄A˅
U
˄V˅
T
2
˄ć˅
A
1
˄m
2
˅
ε
h
80 0.033 8.4 17.0 0.024 0.024
150 0.058 14.7 17.0 0.024 0.025
300 0.138 32.6 17.2 0.024 0.033
The mean data were tested when the heater at quasi-steady
state in 20 minutes. The hemispherical emittance increases
when the temperature of the surface growth. Zerlaut.g.A
(1961) tested the ε
n
of pure aluminium which was polished
by oxidation aluminium and water is 0.022. Reynolds
(1963) determined the ε
h
of the unannealed aluminium foil
was 0.018. The ε
h
of aluminium that was not oxidation is
0.022 at 25 and 0.028 at 100ćć in some literatures. The
hemispherical emittance of pure aluminium which was
determined by the novel equipment agreed well with the
data published in former literature.
The relationship between the characteristic of radiation and
electronic of the aluminium can be calculated as
0.0348Tερ= , the ε
n
equals to 0.019, 0.025 and
0.034 separately at the temperature of 50 ,ć 150ć and
300ć.
4. EXPERIMENTAL RESULTS
An evacuated tube was especially made with Al-N/Al
absorbing surface sputtered on the outside of inner glass
tube and a gauge on the cover glass tube to determine the
vacuum degree in the vacuum jacket (fig. 3). The unsealed
tube connected direct to a high vacuum system.
Experimental study of hemispherical emittance with
various gas pressures in the vacuum jacket of an all-glass
evacuated collector tube can be carried out by controlling a
needle valve and regulating the electric heating power of
the heaters by the adjuster automatic.
Fig. 3: A special tube to determine the hemispherical
emittance in various vacuum degrees.
The hemispherical emittance of Al-N/Al surface on the
outside of the inner glass tube of an all-glass evacuated
collector tube has been measured at the temperature of 80ć
with various gas pressures between cover glass tube and
inner glass tube in the range of 1.9h10
-3
Pa to 9.6Pa (fig.4).
The hemispherical emittance of Al-N/Al absorbing surface
increases with the uptrend of the vacuum degree between
cover glass tube and inner glass tube. The hemispherical
emittance grow quickly when the vacuum degree is large
than 5×10
1
Pa.
Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement
536
( ( ( ( (
YDFXXPGHJUHH˄ 3 D˅
¦
i
Fig. 4: The hemispherical emittance of Al-N/Al surface
relative to various vacuum degrees (80ć).
The hemispherical emittance of the selective coating in
all-glass evacuated tubes should be no larger than 0.080 at
80ćf5ć in “All-glass evacuated collector tubes” of
China national standard. With the curve shown in fig.4, the
gas pressure between the inner and cover glass tube should
be 2.5×10
– 1
Pa~3.4×10
-1
Pa at the temperature of 80ć. Thus
the vacuum degree of a normal evacuated collector tube
can be calculated with the determination of the
hemispherical emittance of the tube.
5. CONCLUSION
The novel equipment to determine the hemispherical
emittance of the selective coating in the normal evacuated
tubes has a high accuracy according to comparing the
hemispherical emittance of pure aluminium with former
literature. The curve to describe the hemispherical
emittance of Al-N/Al surface relative to various vacuum
degrees (80 )ć was determined. The vacuum degree of a
normal evacuated collector tube can be calculated with the
determination of the hemispherical emittance of the tube.
6. ACKNOWLEDGEMENTS
The authors would like to thank Mr. Li Xuguang, Mr. Chen
Duanxue and Mr. Sun Haizhong for supply the special
tubes for experiments.
7. REFERENCE
(1) G. L. Harding and B. Window, “Free molecule thermal
conduction in concentric tubular solar collectors”, Solar
Energy Materials, 4(1981) 265-278.
(2) Wu Jiaqing, Yin Zhiqiang and Wang Dexiao, “The
determination of hemispherical emittances of selective
absorbing surfaces of evacuated collector tubes”,
ACTA ENERGIAE SOLARIS SINICA
, 2 (1989)
207-214.
(3) Gui Yuzong, Xue Zuqing, Zhou Xiaowen and Yin
Zhiqiang. Determination of emittance of selective
absorbing surfaces, SOLAR ENERGY
, Vol. 64,
pp.241-243, 1998.
PERFORMANCE STUDY ON SOLAR PV-THERMAL INTERNAL
CONCENTRATOR TUBE COLLECTOR
Jiang Xinian, Ge Hongchun, Gao Hanshan, Sang Shiyu, Zhou Xiaobo
Beijing Eurocon Solar Energy Tech. Co., Ltd.
Beijing 100083, China
ABSTRACT
One new internal concentrator PV-thermal tube collector is
presented. Crystal silicon cells are stuck on the absorber
strip of the metal pipe. The trough reflector within the glass
tube envelope concentrating the solar radiation onto the
cells and the absorber strip.
The solar radiation on the collector tube partly generates
electricity by the silicon cells. In the meantime the heat
collected is transferred through the strip-pipe. Thus we get
high combined PV-thermal performance.
The structure of the internal concentrator PV-thermal tube
collector is described. The electricity and thermal output
ratio; concentrator design; methods of transfer heat out are
discussed.
1. INTRODUCTION
The main cost of the PV system is the PV modules.
Concentrating PV system is one way to reduce the PV cost
by less PV modules used for the same power out put. If the
concentration ratio is larger than 10, the tracking apparatus
is required, the tracking apparatus will increase the cost and
it’s reliability is also to be solved.
PV thermal combined system has the advantages to
generate electricity and heat in same time, so the total solar
efficiency is higher.
A new non tracking solar PV-thermal internal concentrator
tube collector is developed
[1]
. As shown in Fig.1. a CPC
concentrator is placed within glass tube envelop, the
receiver is silicon cells stuck on the metal tube-fin. with
this kind of non tracking solar PV-thermal internal
concentrator tube collector both electricity and heat are
gained during operation.
Fig. 1: Cross section of non-tracking internal concentrator
PV-thermal tube.
1-absorber 2-glass tube 3-reflector 4- Si-cell
2. CONCENTRAOR DESIGN
Based on the theory of Nonimaging Concentrators
[2]
an
compound parabolic concentrator( CPC) special used for
half cylinder receiver is designed. The acceptance angle of
the solar direct radiation is 30°. The receiver is designed as
half cylinder shape. The flat plate of the cylinder surface
accept solar radiation for the solar cells, the up half
cylinder is used for the heat absorber. The CPC reflector is
Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement
538
inserted in to glass tube with diameter of 96mm, the half
cylinder receiver diameter is 25mm.
Table 1 shows the simulation result of the ratio of
gained rays on up side and down side of the receiver at
different incident angles. Because the up side is thermal
absorber, and down flat side is for cells. So the above
mentioned ratio is related to the thermal/electricity
performance ratio.
TABLE 1: RATIO OF GAINED RAY ON DOWNSIDE
AND UPSIDE OF RECEIVER AT DIFFERENT
INCIDENT ANGLES
UP SIDE(ˁ)
ˁ
DOWNSIDE( )
INCIDEN
ANGLE
0
reflect
1
reflect
1
reflect
2
reflect
≥3
reflect
0 26 24 50 0
5 27 25 48 0
10 27 24 24 22 3
15 28 32 18 18 4
20 28 40 14 12 6
25 28 48 9 10 5
30 29 50 9 7 5
Fig. 2 shows the 100 rays trace of concentrator at different
incident angles. It is observed the less incident angle the
more rays received on the down side surface which PV
cells are placed.
3. PV-THERMAL INTERNAL CONCENTRATOR TUBE
STRUCTURE
Heat absorber is made by copper tube welded onto metal
strip. Absorber up side is sputtered with selective coating.
The flat down side stick crystal silicon cells. Each cells are
series connected. The generated electricity is introduced out
side of the glass tube. The heat collected will be transferred
out of the glass tube by the medium circulating in the
copper tube.
0ºincident angles 5ºincident angles
10ºincident angles 15ºincident angles
20ºincident angles 30ºincident angles
Fig. 2: Ray trace of reflector at different incident angles.
Fig. 3: Cross section of the receiver.
1-copper tube 2-metal fin 3-thin insulation layer 4-Si-cell
For the purpose of keep the reflector in the designed shape,
one extruded aluminum is used as the support base of the
Miro 2 (made by Analod) high reflecting aluminum sheet.
It’s total reflection is 95%, diffuse reflection is less than 5%.
3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS
539
The glass tube is borosilicate glass with solar transmittance
of 90%. Fig. 4 shows the detailed structure how the
absorber is assembled in the glass envelop. One end of the
glass tube is vacuum sealed by metal cover. The generated
electricity is introduced out side of the glass tube through
wires. The heat collected will be transferred out of the glass
tube by the medium circulating in the copper tube or heat
pipe.
Fig. 4: PV-thermal concentrator tube.
1-absorber 2-mirror 3-suppor 4-glass tube 5-metal cover
6-copper tube 7-insulator 8-wire
4. SOLAR PV-THERMAL INTERNAL CONCENTRATOR
TUBE COLLECTOR
As shown in Fig. 5 eight PV-thermal internal concentrator
tubes are assembled as one collector panel. PV cells are
serious connected for the electric power output.
Fig. 5: PV-thermal internal concentrator tube collector.
1-PV-thermal tube 2-base support 3-manifold 4-casing 5-wire box
The absorber copper tubes are parallel connected to the
manifold. The thermal collector is in two types. If the
absorber tube is horizontal heat pipe, it’s condenser is dry
connected to the manifold by heat transfer block as shown
in Fig. 6. If the absorber tube is direct medium circulation
the copper tube is direct connected to the manifold as
shown in Fig. 7.
Fig. 6: Cross section of casing for heat pipe type
PV-thermal tube collector.
1-PV-thermal tube 2-condenser of horizontal heat pipe 3-metal block
4-manifold 5-case 6-insulation
Fig. 7: Cross section of casing for direct flow type
PV-thermal tube collector.
1-PV-thermal tube 2-coupling 3-tube in tube manifold 4-case
5-insulation
Solar PV-thermal internal concentrator tube is horizontal
placed in east west direction with the collector slope angle
same as the local altitude.
The solar PV-thermal internal concentrator tube collector
is suitable for the application as swimming pool heating
or domestic hot water heating when the temperature
required is lower than 60°C. In this range of temperature
the silicon cells efficiency is not strongly influenced by
the cell temperature.Fig. 8 shows the non-tracking solar
PV-thermal internal concentrator tube collector system is
Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement
540
under testing.
Fig. 8: Prototype of non-tracking solar PV-thermal internal
concentrator tube collector system.
5. CONCLUSION
Solar PV-thermal internal concentrator tube collector
combined the PV and thermal generation in one collector
tube. The cells are cooled by the heat transfer out. The total
solar energy efficiency is increased. The cost of PV module
is reduced by less cells used and no tracking used.
6. REFERENCES
(1)Jiang xinian. Solar PV-thermal internal concentrator
tube [P]. China Patent, No: 200610002978.7, Date:
2006 Feb 26.
(2)W.T.welford and R.Winston,The optics of Nonimaging
Concentrators:lighe and solar energy [M].Academic
Press New York San Francisco London,1978.
THERMOTROPIC RESIN SYSTEMS: RELATIONSHIPS BETWEEN FORMULATION
PARAMETERS, MATERIAL STRUCTURE AND OPTICAL PROPERTIES
Katharina Resch
Polymer Competence Center Leoben GmbH
Parkstrasse 11
Leoben 8700, Austria
resch@pccl.at
Gernot M. Wallner
University of Leoben
Franz-Josef-Strasse 18
Leoben 8700, Austria
wallner@unileoben.ac.at
ABSTRACT
This paper focuses on a comprehensive characterization of
various thermotropic resins under polymer physical aspects.
Numerous thermotropic layers were produced under
systematic variation of resin base, thermotropic additives
and additive concentration. A detailed investigation of
optical properties, switching temperature, switching
process and residual transmittance was performed with a
UV/Vis/NIR spectrophotometer. Switching temperatures
are compared with thermal transitions in the material
determined by Differential Scanning Calorimetry (DSC).
Whereas the different film types show a direct solar
transparency between 64 and 83% in the clear state, the
direct solar transmittance decreases to values of about 27%
to 80% above the switching temperature. In general the
thermotropic resins are characterized by a steep and rapid
switching process. The switching temperature can be
adapted by varying the additives. The comparison of films
thermal transitions with the switching performance reveals
a good correlation.
1. INTRODUCTION
Polymeric materials offer a significant cost-reduction
potential for solar thermal collectors and may thus benefit a
broader utilization of solar energy for low-temperature
heating purposes. Especially for domestic hot water
generation, where absorber temperatures of 80 to 90°C are
effective, cost-efficient plastics may be applied. However,
conventional solar thermal collectors reach stagnation
temperatures up to 200°C, which exceed the maximum
operating temperatures of cost-efficient plastics
(80-140°C). Thus, an adequate overheating protection is
required to prevent the polymers from irreversible
deformation and ageing. One possibility to control the
temperature of a flat plate collector is the use of
thermotropic glazings[1,2]. Thermotropic materials
change their light transmission behaviour from highly
transparent to light diffusing upon reaching a certain
threshold temperature[3]. By theoretical modeling of an
all- polymeric flat plate collector it was found that for
adequate overheating protection (i.e. stagnation
temperatures ranging from 80 to 140°C) switching
temperatures between 55 and 60°C as well as a residual
hemispherical solar transmittance of 25% to 60% in the
opaque state are effectual [1,2].
Among the different thermotropic systems especially
thermotropic resins that consist of thermotropic additives
dispersed in the matrix of a curable resin possess a high
potential for solar thermal applications. This motivated the
production of thermotropic resins exhibiting switching
characteristics required for overheating protection of solar
collectors for the present study. Various thermotropic layers
were manufactured under systematic variation of base resin
formulation as well as of additive type and concentration.
The overall objective of this investigation is to perform a
comprehensive characterization of these thermotropic
resins under polymer-physical aspects and to establish