Труды Всемирного конгресса Международного общества солнечной энергии

Подождите немного. Документ загружается.

ON THE ANTI-FREEZING OF FLAT-PLATE COLLECTORS IN SDHWS UNDER

CLIMATIC CONDITIONS OF YUNNAN, CHINA

Runsheng Tang, Zhimin LiˈZhiguo Sun, Yamei Yu, Hao Zhong

Solar Energy Research Institute, Yunnan Normal University

Kunming 650092, China

kingtang@public.km.yn.cn

ABSTRACT

Solar domestic hot water systems (referred to as SDHWS

in the paper) with flat-plate collectors have been widely

installed in residential and office buildings for the water

heating due to their reliable performance, low cost and

minimal need for maintenance. However, the substantial

fear of the damaging due to the freezing of collectors has

apparently hampered their popularization in the areas with

cold winters, even in the areas with mild winters. Just like a

water pipe, one approach to the anti-freezing of collectors

is to make the water inside the collectors flow. For a

thermosyphonic SDHWS, this can be achieved by

appropriate measures which make the systems produce

thermosyphonic or reverse circulation. The object of this

study is to investigate the temperature depression of

collector absorbers at nights and the feasibility to employ

reverse flow of thermosyphonic SDHWS for anti-freezing

under climatic conditions of Yunnan, China.

1. INTRODUCTION

In the past two decades, thermosyphonic SDHWS have

been widely installed in residential and office buildings for

water heating, and a number of theoretical and

experimental studies on its performance have been

conducted. In recent years, the substantial fear of damage

due to the freezing of collectors has apparently hampered

their popularization in the areas with cold winters, even in

the areas with mild winters. The commonly used methods

for the anti-freezing of collectors are: using evacuated tubes

in place of flat-plate collectors, using automatic drainage

devices for emptying water inside the collectors when the

water is close to freezing, and using antifreeze liquid as the

working medium of the system. However, these approaches

would either increase the cost or lower the thermal

performance of SDHWS.

Yunnan Province of China, with area of 0.34M square

kilometers and population of 43M, located at Yun-Gui

Plateau (longitude in 97.5-106

E and latitude in 21-29

N),

is a typically tropical and subtropical climate. Northern

high mountains of Yunnan function as natural obstacles to

the invading cold air currents from the northern part of the

country during winters. Apart from a small fraction of

Yunnan’s region with high mountains, such as Diqing and

part of Lijiang and Lujiang, air temperatures below 0 ć

are rarely observed in the winters. Yunnan has abundant

solar resource due to the clear sky resulting from the high

altitude. 90% of the areas of Yunnan have annual radiation

above 5000MJ/m

and annual sunshine duration above

2000 h, and all of Yunnan is climatically characterized by

rainy summers and warm dry winters [1, 2], thus here is an

ideal region for the solar thermal utilization.

Researches and development on solar energy technology in

Yunnan started at the early of 1970s. By the persistent work

of 30 years, the application of solar water heaters has been

rapidly expanded, and solar water heating has become a

new industry with annual sale of 0.5Mm

, which accounts

for one eighth of the total production of solar collectors in

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

703

the country. To date, there are more than 200 manufacturers

and companies in the business of solar hot water heaters in

Yunnan. The total installed solar water heater area has

reached 3Mm

, and the popularization of solar water

heaters for the water heating has reached 31% in cities and

8.6% in rural area, much higher than 3% of national

average level. Among the installed solar water heaters,

flat-plate collectors accounted for 74%, and evacuated tube

heaters accounted for about 20%.

Before 1997, almost all the installed solar water heaters

were flat-plate collectors. The cold temperature of –2ć

in Kunming on January 11 of 1998 caused many flat-plate

collectors to be damaged due to freezing. Since then,

all-glass evacuated tube solar heaters promptly entered into

solar market of Yunnan, due to the fear of freeze damage to

flat-plate collectors. In recent years, many approaches to

the anti-freezing of flat-plate collectors have been tested.

These approaches included: increasing pipe diameter of

absorbers, the use of inflatable absorbing pipes, such as

diamond shaped pipes, and rubber piping fittings.

The air temperature at night is the lowest in a day, and the

freezing-damage of a flat-plate collector is usually taken

place at night. The object of this study is to investigate the

temperature depression of absorber of flat-plate collectors

at nights and the possibility to employ reverse flow of

thermosyphonic SDHWS for anti-freezing under climatic

conditions of Yunnan, China.

2. EXPERIMENTAL INVESTIGATION OF TEMPER-

ATURE DEPRESSION OF ABSORBERS AT NIGHTS

To examine the effects of glazing and absorbing coating on

the temperature of collector absorbers, two solar water

heaters with each consisting of two flat-plate collectors

(one is glazed and the other is unglazed) was tested at a

fishing garden in the northern suburb of Kunming from 9 to

22 of January, 2006 (see Fig.1). One of the solar heater has

an aluminium absorber printed with matte black, the other

has a copper-aluminium composite absorber with

anode-oxidized coating. A valve is equipped at the upper

connecting pipe of both systems to control the operating

mode of both systems.

The experimental results showed that, under stagnant state

(the reverse flow was not allowed by closing the valve

equipped at the connecting pipe of systems ), the

temperatures of all collector absorbers were 6̚8ć and

about 1ć lower than the ambient temperature at clear

nights and overcast nights, respectively, and the maximum

temperature depression was appeared in between 5:00-7:00

A.M, indicating that the possibility of freeze-damage for

collectors during this period was high. Experimental results

also indicated that glazing and coatings had insignificant

effects on the stagnant temperature depression of a

collector absorber, but the weather conditions had

considerable effects. These results implied that the collector

may be damaged by freezing at clear nights even when the

air temperature is above 0ć, such as 2-3ć, and the

possibility of the freeze-damage at clear nights is much

higher than that at overcast nights(even snow nights )under

the climatic conditions of Yunnan.

Fig. 1: Photo of tested solar water heaters at the

experimental site.

Experimental results showed that, when the valve equipped

at the upper connecting pipe of both systems was opened

and reveres flow was allowed, the absorber temperatures

were stable all the night, and slightly lower than the

ambient temperature before midnight and 3-4ć higher

than the ambient temperature at 3:00-6:00AM for both

clear and overcast nights only if the water temperature

inside tanks was kept 30ć above the ambient temperature.

This result implies that the reveres flow resulted from

keeping water temperature inside storage tanks at certain

level above the ambient temperature is very effective for

the anti-freezing in the region with mild winters.

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

704

3. FREEZE PROTECTION PRINCIPLE OF COLL-

ECTORS IN A THERMOSYPHONIC SDHWS BY

REVERSE FLOW

Thermosyphonic SDHWS exposes all elements of the

system to the ambient air, hence freezing weather can cause

the water pipes and absorbers to freeze and burst. However,

pipe-freeze is not an issue under climatic condition of

Yunnan except some place in Lijiang and Xianggelila.

Theoretical studies on the issue of collector-freeze are

considerably complex because the collector-freeze is

influenced by many factors such as climatic conditions,

pattern of hot water use, insulation of systems, diameter of

pipes as well structure of systems. To date and to our

knowledge, systematic theoretical and experimental studies

on the issue of collector-freeze in a thermosyphonic

SDHWS are rarely reported in literature.

As well known, collectors or pipes are less likely to freeze

in a system where draws are regularly or continuously

made because the water in the pipes, which could be close

to freezing, is refreshed with warm water from the hot

water tank. The warmer water could also melt previously

formed ice and can supply energy needed for the exposed

collectors or pipes to keep the temperature of water in the

pipes above freezing temperature. Hence, one way to

inhibit the freezing of water inside the collectors is to

make the water inside the collectors circulate along the

loop formed by a thermosyphonic SDHWS. Although this

would lead to the reduction in thermal efficiency of the

systems, the freeze protection penalty resulting from the

circulation of warm water from storage tank into

collectors is small in the mild climate, as indicated by

Bruce and Charles [3].

During the night, the collector acts as a thermal emitter, and

this causes reveres thermosiphoning all the night.

Theoretically, the revere flow increases with the increase in

water temperature inside the storage tank for a given

thermosyphonic SDHWS. Thus, if the heat from the hot

water tank by the reverse flow is enough to offset the heat

loss of collectors for the duration of the night, the freezing

of collectors can be avoided. This means that the freezing

of collectors can be avoided only if the water temperature

inside the tank is kept above certain level.

4. INVESTIGATION ON FREEZE PROTECTION OF

COLLECTORS

Jiayuan residential zone of Kunming, built in 1999, have

1108 residential houses, and all of the houses were

equipped with 4m

thermosyphonic SDHWS on October of

1999. The collectors installed in Jiayuan were all-copper

absorber with absorbing pipes shaped as diamond from

tubes of 12 mm in diameter and 0.6 mm in thickness, and

the storage tanks were cylinder with 230 liters in volume.

These 1108 sets of SDHWS were installed by five solar

companies. A heavy snow event occurred on January 5 of

2000 with the lowest air temperature being -2ć, such

weather went on 2 days followed by consecutive shining

days. 341 panels of all 2216 panels were damaged by

freeze at clear night on January 7. Investigation on the

event found that: no collector was damaged in the systems

on 671 unoccupied houses, 7 collectors of 112 systems

(installed in 4 buildings) with an insulated down connecting

pipe was damaged. This is in part because the water

temperature in the storage tank of the systems for

unoccupied houses must be higher, as compared to others,

due to no draws being made before the event, indicating

that keeping water in the storage tank at high temperature

can effectively inhibit the freezing of collectors.

A successful example of the anti-freezing of collectors by

the use of auxiliary electric heaters is 12 thermosyphonic

solar systems installed in four residential buildings of

Yunnan Tobacco Company. Each of these systems,

providing the hot water for 10 families, consisted of 18

collectors, and the collectors of the system were flat-plate

copper absorber collector with collecting area of 2m

and

12mm diameter of the absorber pipes. On November 6 of

1998, about one third of the collectors were damaged by a

heavy frost with lowest air temperature being -1ć. After

then, all collectors of 12 systems were replaced by

collectors of copper-aluminum composite absorber with

diamond shaped absorbing pipes. Meanwhile, the tank of

each system was equipped with 30 KW electric heaters.

The auxiliary electric heaters in each system are controlled

such that they always keep water temperature at 45-60ć

from 18:00 to 0:00 (in fact, almost no draws are made after

0:00) and from 5:30 to 8:00. To date, no collector of these

12 systems has been damaged by freezing.

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

705

In 2002, 12 sets of 4m

SDHWS with down connecting

pipe wrapped by an electric heating tape (rated power of

100W) were installed at Lijiang of Yunnan where the air

temperature below -5ć is often observed at the winter

nights. The electric heating tape are controlled to maintain

the water temperature inside the down connecting pipe

between 15-25ć. Two winter’s operation indicated that the

use of electric heating tape is an effective measure for the

anti-freezing of collectors under such climatic conditions,

with annual assumption of electricity less than 10KWh.

5. SUMMARY

Experimental results showed that the stagnant temperature

of collector absorbers was 6-8ć and 1ć lower than the

ambient temperature at clear nights and overcast nights,

respectively. If the reverse flow is allowed, the absorber

temperatures were stable and even higher than the ambient

temperature only if the water temperature inside the tank

was kept at certain level. Investigation also indicated that

freeze-damage is usually taken place at clear nights after a

snow event or a sudden drop of air temperature due to the

cold air invasion form the northern China and can be avoid

by reverse thermosiphoning by keeping water temperature

inside the storage tank at certain level under the climatic

conditions of Yunnan.

6. ACKNOWLEDGEMENTS

This work was conducted under the financial support from

Yunnan Normal University.

7. REFERENCES

(1) Zhimin Li, Runsheng Tang, Chaofeng Xia, Huilong

Luo and Hao Zhong. Towards green rural energy in

Yunnan, China. Renewable Energy (in press).

(2) Chaofeng Xiao, Huilong Luo, Runsheng Tang and Hao

Zhong. Solar thermal utilization in China. Renewable

Energy 2004; 29/9:1549-1556.

(3) Bruce AW and Charles SB. Freeze protection for

flat-plate collector using heating. Solar Energy 1977,

19:745-746.

PERFORMANCE COMPARISONS OF DISH TYPE SOLAR CONCENTRATOR WITH

MIRROR ARRANGEMENTS AND RECEIVER SHAPES

Joo Hyun Seo

Graduate School, Department of Mechanical Engineering,

Inha University, Inchon, Korea

senseseo@hotmail.com

Dae Sung Ma

Graduate School, Department of Mechanical Engineering,

Inha University, Inchon, Korea

Yong Kim

Graduate School, Department of Mechanical Engineering,

Inha University, Inchon, Korea

Tae Beom Seo

Department of Mechanical Engineering,

Inha University, Inchon, Korea

Yong Heack Kang

Korea Institute of Energy Research

Daejeon, Korea

ABSTRACT

The performance comparisons of dish type solar

concentrators are numerically investigated. The dish type

solar concentrator considered in this paper consists of a

receiver and multi-faceted mirrors. In order to investigate

the performance comparisons of dish type solar

concentrators, six different mirror arrangements and four

different receivers are considered. A parabolic-shaped

perfect mirror of which diameter is 1.40 m is considered as

the reference for the mirror arrangements. The other mirror

arrangements consist of twelve identical parabolic-shaped

mirror facets of which diameter are 0.405 m. Their total

collecting areas, which are 1.545 ΃, are the same. Four

different solar receiver shapes are a conical, a dome, a

cylindrical, and a unicorn type.

In order to investigate the thermal performance of the dish

type solar concentrator, the radiative heat loss in the

receiver should be calculated. For calculation, the net

radiation method and the Monte-Carlo method are used.

Also, because the thermal performance of the dish type

solar concentrator can vary as the receiver surface

temperature, the various surface temperatures are

considered. Based on the calculation, the unicorn type has

the best performance in receiver shapes and the STAR has

the best performance in mirror arrangements except the

perfect mirror.

1. INTRODUCTION

The solar power plant system is considered to be one of the

best price competitiveness in renewable energy for

alternative sources of fossil fuels. To generate the

electricity using the solar energy, the high concentrating

system is essential. A dish type solar concentrator is one of

the high concentrating system.[1]

The dish type solar

concentrator consists of mirror facets which are

parabolic-shaped and a receiver which are situated at the

focal region of mirrors. The dish type solar concentrator

equipped extraneous solar tracking system can obtain high

temperature more than 1000ଇ, thus the dish type solar

concentrator is applied to new material synthesis or solar

electricity generation. Because the performance of the dish

type solar concentrator is affected by the receiver shape and

the mirror arrangement, designs about these are the most

important part in the dish type solar concentrator design. In

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

707

order to analyze the performance variation with the receiver

shape and the mirror arrangement, six different mirror

arrangements and four different receivers are proposed. Six

different mirror arrangements are perfect mirror,

2AND4INLINE, 2TOP, INLINE, STAGGERED and STAR.

Four different receiver shapes are a dome, a conical, a

cylindrical and a unicorn type.[2] In addition, the receiver

surface temperature variation is considered because the

effect of that on the system performance is important.

2. MODELING

In order to compare the performance of the dish type solar

concentrator with mirror arrangements, a perfect mirror is

decided as the reference of the mirror arrangements. Fig. 1

shows a dish type concentrator consisted of the perfect

mirror and a receiver. The aperture diameter and the focal

length of the perfect mirror are 1.4m and 1.5m, respectively.

The mirror surface is parabolic-shaped. The receiver is

placed at the focal region of the perfect mirror. As shown in

Fig. 2, five mirror arrangements which have the reflecting

area equal to the perfect mirror are considered. Each dish

type solar concentrator system consists the same 12 faceted

mirrors, which are parabolic-shaped, and a receiver. The

diameter and the focal length of faceted mirrors are 0.405m

and 1.5m, respectively.[2] The receiver is positioned at the

focal region of the faceted mirrors.

In order to analyze the performance of the dish type solar

concentrator with receiver shapes, four receiver shapes are

proposed as shown in Fig. 3. The width, height and

aperture diameter of each receiver are determined 0.16m,

0.17m, and 0.15, respectively.[3]

For analyzing the radiation heat transfer of the dish type

solar concentrator, TracePro, which is based on the Monte

Carlo method, is used. This method involves the use of a

random number generator to model the statistical processes

of photon emission, non-specular reflection, and absorption.

Photons are distributed uniformly at the aperture of mirror.

The paths of individual photon bundle are traced through

the optical system using geometry, and a tally made of the

ultimate fate of the bundles. Ray tracing is repeated until

photon bundle is absorbed in the receiver surface or come

out the receiver cavity.

Fig. 1: Mirror array (Case ĉ: Perfect mirror)[2].

Fig. 2: Mirror arrays (Case Ċ̚VI)[2].

Fig. 3:

Receiver shapes (Receiver 1̚4)[3].

In order to determine the system efficiency of the dish type

solar concentrator, the optical efficiency and the receiver

efficiency must be considered. The optical efficiency is

calculated as Eq. (1).[4]

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

708

Optical efficiency (1)

where, P

is the concentrated solar radiation intercepted by

the receiver aperture, P

mirror

is the solar radiation which is

incident to the mirror aperture. The total hemispherical

reflectivity of the mirror is assumed to be 0.9.

The receiver efficiency is determined by calculating the

heat loss of the receiver. The receiver heat loss may be

classified as the conduction, convection and radiation heat

loss. However, in this study, the only radiative heat loss is

considered and classified into the receiver surface

reflection and emission. Radiative heat loss due to surface

reflection is determined as Eq. (2).[3]

(2)

RL IN

−

where, N

is the number of absorbed photon bundles at the

inner surface of the receiver, N is the number of entered

photon bundles into the receiver. The total hemispherical

absorptivity of the receiver is assumed to be 0.9. In order to

calculate the radiative heat loss due to surface emission, the

net radiation model is used as shown in Eq. (3).[3]

[ 1 ] [ ] (3)

ij j ij ij bj

Fq FE

εε

−−

⎛⎞

−− = −

⎜⎟

⎝⎠

∑∑

where, the inner surface of the receiver is assumed to be

diffuse and gray. The total hemispherical emissivity of the

receiver is same to the above total hemispherical

absorptivity and the inner surface temperature of the

receiver is varied as 400, 500, and 600ଇ.

3. RESULTS AND CONSIDERATION

Fig. 4 shows the optical efficiency with mirror

arrangements. As shown in Fig. 4, Case ช has the best

optical efficiency with 90% in the multi-faceted mirror

arrangements and Case ญ shows the worst optical

efficiency with 81% in all cases. The difference between

Case I and Case IV is approximately 9%. Based on this

result, the performance improvement of the dish type solar

concentrator is possible by optimizing the mirror

arrangement.

Fig. 4: Comparisons of the optical efficiency of dish type

solar concentrator with mirror arrangements.

Fig. 5 shows the radiative heat loss due to surface reflection

with the mirror arrangements and receiver shapes. This

figure shows that Case ช- Receiver 3 combination has the

most radiative heat loss due to surface reflection with

66.09W and Case ญ- Receiver 2 combination has the least

radiative heat loss due to surface reflection with 19.51W.

On the other hand, Receiver 4 has the least radiative heat

loss due to surface in the receiver shapes.

Fig. 5: Comparisons of radiative heat loss in receiver due to

surface reflection.

The radiative heat loss due to surface emission is shown in

Fig. 6. This figure shows that as the inner surface

temperature of the receiver increases, the radiative heat loss

due to surface emission increases. Also, Receiver 3 has the

most radiative heat loss due to surface emission. This is

because the inner area of Receiver 3 is the largest in all

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

709

receiver shapes. On the other hand, Receiver 4 has the least

radiative heat loss due to surface emission because the

inner area is the smallest. Consequently, this result shows

that the radiative heat loss due to surface emission is in

proportion to the inner surface temperature and inner

surface of the receiver.

Fig. 6: Comparisons of the radiative heat loss in the

receiver due to surface emission.

Fig. 7 shows the receiver efficiency with mirror

arrangements and receiver shapes. The inner surface

temperature of the receiver is assumed to be 500ć. The

receiver efficiency is defined as Eq. (4).[2]

Fig. 7: Comparisons of the receiver efficiency with mirror

arrangements.

Receiver efficiency (4)

−

where, P

is the concentrated solar radiation intercepted by

the receiver aperture, P

loss

is the total radiative heat loss

from the receiver. Case ฌ- Receiver 1 combination has the

best receiver efficiency with 80.5% and Case ช- Receiver

3 combination has the worst receiver efficiency with 63.2%.

Therefore, the effect of the receiver efficiency on the

system performance is larger than that of the optical

efficiency.

Fig. 8 shows the system efficiency with mirror

arrangements and receiver shapes. The system efficiency is

determined as Eq. (5).[2]

Fig. 8: Comparisons of the dish type solar concentrator

system efficiency.

System efficiency

Optical efficiency Receiver efficiency (5)=×

As shown in Fig. 8, Case ช- Receiver 4 combination has

the best system efficiency with 71.5% and Case ญ -

Receiver 3 combination has the worst system efficiency

with 51.2%. These results show that the performance of

dish type solar concentrator is affected by mirror

arrangements and receiver shapes

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

710

4. CONCLUSION

The performance of the dish type solar concentrator is

numerically analyzed and the following conclusions are

derived from this study.

(1) Case ฌ has the best optical efficiency in multi-faceted

mirror arrangements except to case ช(perfect mirror).

(2) Case ช- Receiver 3 combination has the most radiative

heat loss due to surface reflection with 66.09W.

(3) The radiative heat loss due to surface emission increases

as the inner area and surface temperature of the receiver

increase. Receiver 4 has the least radiative heat loss due to

surface emission

(4) The best optimal system is the combination of Case

(perfect mirror) and ĉ Receiver 4(unicorn type) in this

paper.

5. REFERENCES

(1) Kang, Y. H., “Flux Distribution of the Dish

Concentrator”, Journal of the Korean Solar Energy

Society, 1999.

(2) Thomas, R. M., “Analysis and Design of Two Stretched

- Membrane Parabolic Dish Concentrators”, Journal of

Solar Energy Engineering, 1991, Vol. 113, pp. 180-187.

(3) Ryu, S. Y., “An Analysis of Heat Losses from a

Receiver for a Dish-Type Solar Energy Collecting

System”, Inha University, M. D. Thesis, 1999.

(4) Goodman, J. H., “A Spherical Dish Concentrator for

Process Heat”, American Solar Energy Society 1994

Annual Conference SOLAR 94, 1994.

STUDY ON TROUGH RECEIVER FOR LINEAR CONCENTRATING

SOLAR COLLECTOR

Hui Zhai, Yanjun Dai, Jingyi Wu, Ruzhu Wang

Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University

Shanghai 200240, China

zhaihui@sjtu.edu.cn, yjdai@sjtu.edu.cn, jywu@sjtu.edu.cn, rzwang@sjtu.edu.cn

ABSTRACT

In this paper, a trough black body cavity receiver for linear

concentrating collector, which has potential application at

moderate working temperatures (near 100 ) or higher ć

temperature (above 180 )ć , has been studied. Four

different shapes of the receiver have been proposed and

tested. The optical efficiencies of the four receivers are

simulated by using light tracking method. The thermal

performances of the cavity receivers are tested under

temperature levels, 90ć and 150ć, where the

concentrated solar radiation is simulated by electric heating

film. Both optical and thermal analysis illustrates that the

triangle trough receiver is the best choice. To reduce the

thermal losses, three kinds of glass cover are fixed on the

cavity receiver. The thermal losses of the three different

receivers with glass cover have been tested, when the inlet

temperature varies from 35 to 95 . ććThe experimental

results indicate that the glass cover can reduce the thermal

losses effectively. The semicircular cavity is tested in an

linear concentrating solar collector, in which the sunlight is

concentrated by a linear Fresnel lens(aperture width is 0.4m

and optical efficiency is 0.7) and scatters on the

semicircular cavity wall, absorbed by the copper tube and

transmitted to working fluid. The highest average

efficiency can reach 43% when the inlet temperature of the

working fluid is 90 .ć It is found from experimental results

that the efficiency of this designed cavity receiver is a

little lower than that of evacuated tube, but has good

potential in application because it is cheap and can be

manufactured easily.

1. INTRODUCTION

Linear concentrating solar thermal technologies offer a

promising method for large scale use of solar energy. These

solar collector systems which have linear concentrator

produce energy used for power, heating or cooling. Now

most popular receiver adopted in parabolic trough solar

collector is evacuated tube [1, 2]. It has tremendous

advantage in eliminating convection losses, but the high

cost and leakage between glass and metal tube are its

shortcomings. If the collector is used to provide heat with

temperature under 200ć, the cavity receiver may be a

better choice for its low cost and easy manufacture. This

kind of block body cavity receiver has been widely used in

point focus collector for collection of solar energy at

temperatures greater than 400ć [3-5]. Several papers have

also investigated the utilization of trough cavity receiver in

linear concentrating solar collectors [6, 7], the most popular

structure of receiver is a trough cylinder which top part is

interceptive. The media flows in the annular channel [6] or

several copper tubes [7] in the cavity room. To decrease the

thermal loss, the aperture area may be covered with pyrex

glass [7]. If the reflector aperture width is 2.18m and beam

solar radiation is 900W/m

, the thermal efficiency is larger

than 40% when the average value of inlet and outlet

temperature is under 200ć.

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

Подождите немного. Документ загружается.