Труды Всемирного конгресса Международного общества солнечной энергии

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

582

Fig. 3: The location of the interference order parameter for

m = 1 and m = 0.5 for Ni-NiO composite with f =

0.18 and f = 0.30 at the specified film thickness d =

120nm. Showing 4nd, λ as function of wavelength

illustrates the impact of dispersion.

the higher nickel content. Figure 4 shows the corresponding

reflectance spectrum indicating the correlation between

refractive index dispersion and reflectance transition

behavior. A refractive index with a negative slope would

have been even more favourable. The optimized nickel

content in the base-layer is 0.30, the steepness of transition

will be limited by the properties of chosen components.

Fig. 4: The corresponding reflectance spectrum with f =

0.18 and f = 0.30 at the specified film thickness d =

120nm.

3.3 Optimized Optical Requirement

As seen above, high intrinsic absorption cannot be

combined with a steep transition between the solar and the

thermal wavelength range in the case of Ni-NiO as coating

material. We can clearly see that the intrinsic optical

constants really set a limit to the sharpness in the

reflectance for this specific cermet, i.e. it’s limited by the

specified component itself.

A systematic study of the optical performance of different

metals and dielectrics to form composites reveals that free

electron metal in combination with low refractive index

dielectric shows a optical behavior closer to ideal

requirement. Composite of Al/AlN is one example, the

dispersion of n and k for composite Al/AlN is shows in

figure 5. The optimized base layer contains aluminium

content of 0.27 which is lower than the optimized nickel

content of 0.30 due to the electron density is high in

aluminium. For this aluminium content, k is large enough

to have intrinsic absorption, moreover, the dispersions of

both n and k decreases with increasing wavelength around

transition wavelength. Thus the transition of reflectance

is steeper than that for Ni-NiO. It is very good composite

coating material used in vacuum tube collectors. Study

shows that metal nickel (stable in air) can combine with

dielectric of lower n than that of NiO (n=2.3 at the most of

solar wavelength range) resulting in improved optical

performance. Studied Ni-Al

composite provides such

example[10].

Fig. 5: Dispersion of n and k for Al-AlN of aluminium

volume fraction 0.27 calculated according to

Bruggeman effective medium model.

4. CONCLUSIONS

The study reveals two main conclusions for three-layer

solar absorbing coatings. First, solar absorption should

mainly depend on the intrinsic (characterized by extinction

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

583

coefficient k), but less depend on interferance induced

absorption (characterized by refractive index n). Thus the

metallic content in the composite should be large enough to

give a reasonable extinction coefficient, but not beyond the

metal-nonmetal transition point. Second, the dispersion in

refractive index and extinction coefficient determines how

steep the reflectance transition is. Thus, the ideal dispersion

for refractive index should decrease with increasing

wavelength in the transition wavelength region. So does the

extinction coefficient. For Ni-NiO, the dispersion of

corresponding n and k at nickel content of 0.3 shown in

Figures 1 and 2 deviates from the ideal requirement, so the

transition of reflectance can not be very steep. By

comparison with the behavior of Al-AlN, nickel can

combines low refractive index dielectric to improve its

optical performace such as for Ni-Al

5. REFERENCES

(1) H.G. Graighead, R. Bartynski, R.A. Buhrman, L.

Wojcik, A.J. Sievers, “Metal/insulator composite

selective absorbers”, Solar Energy Materials, 1 (1979)

105-124.

(2) S. Zhao, E. Avendano, K. Gelin, J. Lu, E. Wäckelgård,

“Optimization of an industrial DC magnetron

sputtering process for graded composition solar thermal

absorbing layer”, Solar Energy Materials & Solar Cells,

90 (2006) 308-328.

(3) D.A.G. Bruggeman, “Berechnung verschiedener

physikalischer konstanten von heterogenen substanzen”,

Ann. Phys. 24 (1935) 636-664.

(4) S. Zhao, E. Wäckelgård, “Optimization of solar

absorbing three-layer coatings”, Solar Energy

Materials & Solar Cells, 90 (2006) 243-261.

(5) O.S. Heavens, “Thin Film Optics” in Optical

Properties of Thin Solid Films, 1954, Dover

publications, Inc., New York, pp46-50.

(6) D.B. Kushev, N.N. Zheleva, Y. Demakopoulou and D.

Siapkas, “A new method for the determination of the

thickness, the optical constants and the relaxation time

of weakly absorbing semiconducting thin films”,

Infrared Physics, 26 (1986) 385-393.

(7) David W. Lynch and W.R. Hunter, “Optical constants

of metal”, in Handbook of Optical Constants of Solids,

edited by Edward D. Palik, Academic Press, Inc., 1985,

pp313-323.

(8) N. Tsuda, K. Nasu, A. Yanase, K. Siratori, “Electronic

Conduction in Oxide”, Springer-Verlag, New York,

pp178.

(9) G. A. Niklasson and C. G. Granqvist, “Optical

properties and solar selectivity of co-evaporated

Co-Al

composite films”, J. Appl. Phys. 55 (1984)

3382-3410.

(10) T.K. Boström, E. Wäckelgård, G. Westin,

“Anti-reflection coatings for solution-chemically

derived nickel-alumina solar absorbers”, Solar Energy

Materials & Solar Cells, 84 (2004) 183-191.

A SOLAR WATER SYSTEM ACTING AS THE BUILDING MATERIALS

Zhu Li

School of Architecture

Tianjin University

Weijin Road No.92, Nankai District

Tianjin 300072, China

Ren Jianbo

School of Chemical Engineering and Technology

Tianjin University

Weijin Road No.92, Nankai District

Tianjin 300072, China

rjb.2008@yahoo.com.cn

Wang Yiping

School of Chemical Engineering and Technology

Tianjin University

Weijin Road No.92, Nankai District

Tianjin 300072, China

Kong Jianguo

Tianjin Cuipinghu Science Park

Tianjin 301908, China

ABSTRACT

A new type of solar water system in the form of building

material is designed, and applied to an office building in

Meijiang eco-district located in Tianjin City, China. In this

project, 60m

wall type and 30m

solar roof collectors are

used to supply hot water directly and also to be an

evaporator for an auxiliary heat pump with a power of

4.5kW. The energy rejected by the condenser contributes to

load requirements through a refrigerant (R22)-to-water heat

exchanger immersed in a 3m

hot water storage tank.

Experimental results show that, the daily-averaged

collector efficiency ranged from 40 to 50% and form 60%

to 70% in winter and summer, respectively.

1. INTRODUCTION

In recent years, China has became the top one as the

manufacturer or holder of solar water system in the world,

and a lot of scholars home and abroad have carried out a

series of studies on solar water system [1-3]. Most existing

solar water systems were directly installed on the roof or

wall, which consequently severely affect the appearance of

the building, even the civic landscape. When solar water

system is integrated into the envelope of the building, came

into being the solar integrated building. The method not

only abates the damager of buildings’ envelopment, even

becomes beautiful scenery in the city.

At present, the construction technology of building

integrated solar collector is the combination of the collector

and the building materials in the course of construction and

the gaps between collectors, collectors and building

materials need to dispose. The disadvantages of the above

are: the collector can never take place of the building

materials when was used, and the building materials are

wasted; the collector efficiency of the solar collector was

low, because of combination of the collector and the

building materials [4-6]. Concerning the above mentioned

issues, a new type solar water system was designed: in the

system, the solar collector is a section of the building

construction, and that has functions of collecting heat and

building materials; the solar collector is also the evaporator

of a heat pump, and improved the collector efficiency of the

system. This paper will introduce its design plan and its real

performance during winter operation in detail.

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

585

2. PROJECT DESCRIPTION

The solar water system is applied to an office building in

Meijiang eco-district located in Tianjin City, China, which

can offer demand of domestic heat water for office workers

in the building. The system is composed of solar collector

acting as the building materials, compressor unit, fluid

refrigerant transmission pump, heat storage water tank,

tubular heat exchanger, automatic control system and other

necessary auxiliary parts. Copper pipes as the core

component of collector are riveted in the neck with

aluminums fins; to improve the heat transfer efficiency,

aluminums sheet-nets is laid outside copper pipes; an

automatic control system is used to switch heat pump mode

and heat pipe mode; refrigerant absorbs heat from solar

radiation and ambient air, and then the vapor enters the

tubular heat exchangers and releases heat to tap water in the

heat storage water tank. In the paper, the solar water system

was built as the curved roof and southern facade, the

system is not only beautiful in structure, but also has the

advantages of high heat-collecting efficiency, convenience

installation, long service life, and less maintenance.

3. DESIGN DETAILS

The solar collector systems include solar wall collector and

solar roof collector. The illumination area of the solar wall

collector is 60m

and the solar roof collector is 30m

, the

total collector area is 90m

3.1 Solar Wall Collector

The solar wall collector relies on an arc shape building

southern facade. This section will introduce the

construction technology and tube arrangement in detail.

3.1.1 Structure Design

Copper pipe of diameter 8mm as the core component of

collector are riveted in the aluminums neck with the fins.

Figure 1 shows the profile of the aluminum neck with the

fins. The construction technology of the solar wall collector

is described using installation sequences from inside to

outside as shown in Figure 2: (1) the thermal insulation

polystyrene board, which is slot in the outside, was glued to

exterior wall outside surface; (2) copper tubes are lay in the

aluminums neck with the fins, and both were put in the

slots of the thermal insulation board; (3) the aluminum net

sheet was put on the copper tubes; (4) Stucco was paved as

the finish.

Fig. 1: Profile of the aluminums neck with the fins.

Fig. 2: Profile of the solar wall collector.

3.1.2 Construction Design

Primarily thermal calculation indicates heat transfer

properties of the solar collector will be the best when the

distance between adjacent copper tubes is 200mm. Figure 3

shows tube arrangement of the solar wall collector. Figure 4

shows final exterior of the solar wall collector.

3.2 Solar Roof Collector

The solar roof collector is finished with a fan-shape. This

section will introduce the construction technology and tube

arrangement in detail.

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

586

Fig. 3: Tube arrangement of the solar wall collector.

Fig. 4: Prototype of the solar wall collector.

3.2.1 Construction Scheme

The solar roof collector is based on the parapet wall

through the following procedures: (1) keeling on the

parapet wall; (2) fixing metal plate on the keel; (3) copper

tubes are lay in the aluminum neck with the fins and both

were fixed on the metal plate; (4) In order to reduce the

heat loss of the system, PC panel was covered on the metal

plate. The conventional building integrated solar collector

and the new solar roof collector are respectively shown in

Figure 5 and Figure 6. Promotion in building integration

can be obviously seen.

Fig. 5: Prototype of the conventional solar roof collector.

Fig. 6: Prototype of the new solar roof collector.

3.2.2 Tube Arrangement

Figure 7 shows the tube arrangement of the solar roof

collector. The distance between adjacent copper tubes is

200mm.

Fig. 7: Tube arrangement of the solar roof collector.

4. OPERATION MODE

The automatic control system include temperature

transmitter, pressure transducer, programmable control

system and industry-control microcomputer etc.

4.1 Solar Water System

Figure 8 shows the schematic diagram of the solar water

system. When solar irradiance and ambient temperature are

lower (for example, cloudy or winter days,) the system

starts two compressors. To enhance the system performance,

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

587

a compressor was stopped with increased solar irradiance

and ambient temperature (for example, two seasons of

spring and autumn). With solar irradiance and ambient

temperature further increasing, two compressors were

stopped, and the heat pipe model was run.

collector

vapor-liquid

equilibrium

rebound valve

corner valve

expansion valve

mirror

drierstrainers

hot water

storage tank

compressors

liquid storage tank

medium pump

liquidometer

circulating water pump

shower nozzle

rebound valve

corner valve

solenoid valve

corner valve

solenoid valve

Fig. 8: Schematic diagram of the solar water system.

4.2 Heat Storage Water Tank

The tube heat exchanger was set in the heat storage water

tank, high temperature and high pressure vapor of working

medium (R22) from the evaporator of the system exchange

heat with tap water in the heat storage water tank via the

tube heat exchanger. The water in the heat storage water

tank was used, when the temperature of the water in the

heat storage water tank reaches the set point.

Electromagnetic valve was installed in the heat storage

water tank for automatic watering. Turning on

electromagnetic valve for filling water to the heat storage

water tank, when the water level is below a certain level;

turning off electromagnetic valve, when the water in the

heat storage water tank is full. A high sensitivity liquid

level gauge was installed on the left side of the heat storage

water tank for overflow.

4.3 Circulating Water Pump

A small flow circulating water pump was inserted into the

long connecting pipe between heat storage water tank and

shower nozzles for compensating heat loss to ambient, and

accordingly keep the water temperature of shower nozzle

constant.

5. EXPERIMENT RESULTS AND ANALYSIS

5.1 Performance Analysis in Winter

The system was tested under various weather conditions

from 15 December 2006 to 31 January 2007, with the

averaged ambient temperature ranging from -4 to 5ć and

the solar radiance form 100 to 450W/m

. The temperature

of the water in the hot water storage tank was in the range

between 10 and 45 .ćć

The variations of the daily-averaged collector efficiency of

the system with date are shown in Fig 9.

Fig. 9: Variation of collector efficiency in winter.

Experimental results show that, the collector efficiency

ranged from 40 to 50% in winter.

5.2 Performance Analysis in Summer

The system was tested under various weather conditions

from 15 June 2007 to 31 July 2007, with the averaged

ambient temperature ranging from 27 to 34ć and the solar

radiance form 700 to 1000W/m

. The temperature of the

summer in the hot water storage tank was in the range

between 20ć and 45ć.

The variations of the daily-averaged collector efficiency of

the system with date are shown in Fig. 10.

Experimental results show that, the collector efficiency

ranged from 60 to 70% in summer.

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

588

6-29 7-1 7-3 7-5 7-7

0.60

0.62

0.64

0.66

0.68

0.70

Collector efficiency

Date

Fig. 10: Variation of collector efficiency in summer.

6. CONCLUSIONS

In the present study, a solar water system acting as the

building materials was designed and evaluated. During the

winter of 2006 and the summer of 2007, a series of

experimental studies were carried out on the system.

Experimental results show that, the daily-averaged

collector efficiency ranged from 40 to 50% and form 60%

to 70% in winter and summer, respectively.

A successful experiment of combination of solar heat

pipe-heat pump water system with building integration was

made by the system. Therefore, the system will impel

development of integrative solar architecture technology.

7. REFRENCES

(1) B. J. Huang, J. P. Chyng, “Performance characteristics

of integral type solar-assisted heat pump”, Solar Energy.

2001 (6):403~414

(2) P. Sporn, E. R. Ambrose, “The heat pump and solar

energy”, In: Proceedings of the World Symposium on

Applied Solar Energy Phoenix, Ariz, November 1~5.

(3) M. N. A. Hawlader, S. K. Chou, M. Z. Ullah, “The

performance of a solar assisted heat pump water

heating system”, Applied Thermal Engineering.

2001(21):1049~1065

(4) X. D. Fang, B. Y. Chen, Z. Y. Zheng, “A study of the

influence of solar radiation on the thermal performance

of evaporators of heat pump systems”, Applied Thermal

Engineering 2003(23):1551~1557

(5) T. N. Anderson, G. L. Morrison, M. Behnia, “An

experimental evaluation of an air -source and a

solar-boosted heat pump water heater”, ANZSES

Annual Conference Melboume, 2003, 593~599

(6) Y. H. Kuang, K. Sumathy, R. Z. Wang, “Study on a

direct-expansion solar assisted heat pump water heating

system”, International Journal of Energy Research

2003(27):531~548

AN IMAGING COMPOUNDING PARABOLIC CONCENTRATOR

He Kaiyan

Beijing Institute of Technology, Beijing, 100081, China

Guangxi University, Nanning, 530004, China

Zheng Hongfei

Beijing Institute of Technology, Beijing, 100081, China

Liu Yixin

Guangxi University, Nanning, 530004, China

Chen Ziqian

Beijing Institute of Technology, Beijing, 100081, China

Guangxi University, Nanning, 530004, China

ABSTRACT

An imaging CPC is developed. This new concentrator has

three remarkable characteristics: First, focus of the

concentrator is located outside the concave of the compound

parabolic concentrator while the focus of the paraboloid is

located inside. This characteristic is in favor of the

installation and maintenance of the energy receiver. It is also

beneficial to the disposal of the heat preservation for the

received solar energy. Secondly, when it is combined to the

conventional paraboloid it can changes a big beam of parallel

light into a small beam of parallel light with bigger density of

energy. Thus, the multiplication of energy flow density can

be realized. Finally, the direction of its emerging rays can be

adjusted in a very big solid angle (bigger than 2π).This

characteristic is very favor of the utilization of the

conducting light technology such as for the synchro indoor

or underground illumination in the daytime.

1. INTRODUCTION

There are two kinds of solar energy cocentrators, one is

non-imaging type, another imaging type. A Rabl and R

Winston’s CPC is the most popular used one among the

non-imaging types recently and the paraboloid is the most

popular used one among the imaging types. The focus of

the paraboloid is located inside the concave of the

paraboloid. This focus’s position sometimes is not

convenient for use. This paper develops a new type of

imaging concentrator. Actually it is an imaging

compounding parabolic concentrator ( or it is called as

imaging CPC). It’s notable characteristic is that it’s focus is

moved outside the concave of the concentrator. There isn’t

any shielding object inside the concave of the concentrator.

Support racks and receivers can be installed outside the

concave, which is especially in favor of the disposal of the

heat preservation and transfer, and is also benefit to the

sunlight transmission.

2. PRINCIPLE

Two parabolas with their apertures upward, as showed in

Figure 1, are symmetrically placed on two sides of the y

axis respectively. Followings are their equations:

Fig. 1: Schematic of the principle of the concentrator.

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

590

()

yxl

ˈ˄ 0p > ˅ (1)

()

yxl

=−

ˈ˄ 0p > ˅ (2)

where

is the focal parameter of the parabola. F and

F are their foci respectively. l is the distance between

F (or F ) and y axis. Use the straight line AB, which

parallel to the x axis, to truncate the two parabolas. The

position of AB must meet two requirements:

(a) as showed in Figure 1, its position must be above the

F (or F ), that is, the distance between it and x axis must

larger than

2p ,or 2yp> , 2yp> .

(b) The length of segment AB is just the half of the distance

between

F and F , that is,

BFF l==, (or 2EO OG l==, 2xl= ).

Let straight line AE and BG be perpendicular to x axis. So,

the four segments DA ˈ AE ˈ CB ˈ BG compose a

compounding planar graph. When revolute the

compounding graph around the y axis, a three dimensional

concentrator is made, if translate it along vertical direction

of the plane it belongs to, a two dimensional concentrator is

made.

The operation principle of the concentrator is explained

basing on the figure 1 as following: Parallel light rays strike

on the inside surface of the concentrator paralleling to the y

axis. The parabolic segment BC should have reflected

parallel light rays to its focus

F , actually, because of the

existence of the flat mirror AE, the light is focused on the

point F that is just on the symmetrical axis-y axis. The

requirement (a) mentioned above can guarantee the exit

aperture AB being above the focus

F and F so that the

flat mirror AE can receive the light rays and reflect them to

F. The requirement (b) let the mirror image focus F just

being on central axis-y axis. For the left half of the figure 1,

the same result has been gained. F is the focus of the

overall device. It is totally located in the outside of the

concentrator because only AN is the necessary part of the

flat mirror.

3. THE CONCENTRATOR DESIGN

3.1 The Distance Between Two Foci

Basing on the two requirements mentioned above, the

distance between two foci

FF can be found. According

to the requirement (b), for point B,

2xl= . Substitute

x into equation (1), then,

()

yxl

=+=

22 8

⎛⎞

⎜⎟

⎝⎠

According to the requirement (a),

2yp> , that is,

, therefore,

lp>

. So

FF l p=>

(3)

(or

, 0p > and 0l > )

3.2 The Length of the Flat Mirror

Figure 1 shows that after reflected by BC ray 1 strikes at

point N on the flat mirror AE. N is the lowest reflect point

among all by the rays. So AN is the required minimum

length of the flat mirror. Use h stand for it,

Nh= .

Basing on the straight line equations of

FB and AE,

y -the Y-coordinate of point N can be found as

EANhy y≥==−

93 3

88343

llplp

pp p

== − − = −

(4)

4. THE ANALYSIS OF THE PERFORMANCE

4.1 The Ratio of Entrance Aperture to Exit Aperture

Figure 1 indicates that the ray 2 is the farthest one from the

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

591

central axis who can go through out the exit aperture AB

and finally reaches the focus point F. As long as the

intersection point C, which the straight line

FA and the

parabolic segment CB share, is found, the radius of the

entrance can be found. Using straight line equations of

FA and equation (2), the coordination of C are:

lp lp

⎛⎞

=− + − +

⎜⎟

⎝⎠

()

19 9

24 4

yxl

lp lp

pl l

⎛⎞

⎜⎟

=−+−+

⎜⎟

⎝⎠

The ratio of entrance aperture to exit aperture is

59 2

2 (5)

lp lp

lll

ppp

lll

⎛⎞

⎜⎟

=−+ − +

⎜⎟

⎝⎠

⎛⎞

⎜⎟

=−+− +

⎜⎟

⎝⎠

β is the concentration ratio of entrance to exit. β is a

monotonic decreasing function with respect to the

parameter

2p , as showed in Fig. 2.

0 0.3 0.6 0.9 1.2 1.5

Fig. 2: The curves of α and

β with pl.

4.2 The Ratio of

Width to Height

The ratio of width to height is an important factor that

effects cost. Take the parabolic segment into consideration

only, it can be found as:

99 9

44 4

pp pp

ll ll

ppp

lll

⎛⎞

⎜⎟

−+ − +

⎜⎟

⎝⎠

⎛⎞

⎜⎟

−+ − + −

⎜⎟

⎝⎠

(6)

α is a monotonic increasing function with respect to the

parameter

2p , as showed in Fig. 2.

It is obvious that the specific value

2p is the only factor

that effects α and

β . We can define it as an eigen

parameter of the concentrator. Its value range is:

(7)

5. APPLICATION

As showed in Fig. 3, a paraboloid is installed near the exit

end of the concentrator. Its focus is just on the

concentrator’s focus F. Its direction of the central axis can

be at any angle with the central axis of the concentrator.

Thus the direction of the emerging rays which parallel to its

central axis can be adjustable. So the whole device is a

parallel light energy flow density multiplier with adjustable

direction of emerging rays. According to the direction of

Fig. 3: Combined with the paraboloid.

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

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