PROCEEDINGS OF ECOS 2012 - THE 25TH INTERNATIONAL CONFERENCE ON

EEEEFFICIENCY, CCCCOST, OOOOPTIMIZATION, SSSSIMULATION AND ENVIRONMENTAL IMPACT OF ENERGY SYSTEMS

JUNE 26-29, 2012, PERUGIA, ITALY

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A Novel Non-Tracking Solar Collector for High Temperature Application

Wattana Ratismith and Anusorn Inthongkhum

Energy Research Institute, Chulalongkorn University,10330, Bangkok, Thailand,

wattana@eri.chula.ac.th and anusornaun@hotmail.com

Abstract: A parabolic trough solar collector is improved the efficiency by a novel design of compound parabolic trough solar collector where the aim is three-fold. Firstly, one aim is to achieve day-long collection efficiency without the need for mechanical tracking of the sun. Secondly, the collector must be designed to operate efficiently under diffuse solar irradiation as experienced for example in rainforest climate. Thirdly, one seeks to achieve as a high an output temperature as possible. Newly developed system consists of multiple compound parabolic troughs facing the sun at different angles. The salient feature of this design is that the system can collect the sunlight energy at every angle without any moving parts at the same time can receive the diffused light, the maximum efficiency of the collector is 32% and has an ability to achieve high output temperature, the maximum temperature at header of evacuated tube is 235 degrees Celsius, and is therefore suitable for high temperature application such as industrial uses or cooling application. Keywords:

solar energy, compound parabolic trough, non-tracking solar collector.

1. Introduction

A parabolic trough is a type of solar thermal energy collector which is generally used in solar power

plants. The solar collector is constructed as a long parabolic trough with a tube running its length at

the focal point. Sunlight is reflected by the trough and concentrated on the tube filled with synthetic

oil, which heats to 300-400 degrees Celsius [1-5]. The trough is usually aligned on a north-south

axis, and rotated to track the sun as it moves across the sky each day. Therefore it seems

unavoidable that there needs to be a tracking system that follows the position of the sun.

The disadvantage of the parabolic trough solar collector is that concentrating systems require sun

tracking to maintain sunlight focus at the collector. The tracking system increases the cost,

complexity and the maintenance cost due to the moving parts. This type of solar collector is not

preferred in a small residential house. Another problem is an inability to provide power in diffused

light conditions, which is due to the fact that the power output from concentrating systems drops in

cloudy conditions. As Thailand has a tropical rainforest climate, which causes the ratio of diffused

solar radiation to global solar radiation to be rather high (in the range of 31% to 58%) [8], one faces

a serious problem in utilizing such a solar collector to collect solar energy, especially in rainforest

climate.

A parabolic trough solar collector is improved the efficiency by a novel design of compound

parabolic trough solar collector which does not contain a solar tracking system and has an ability to

collect diffused sunlight by using compound parabolic troughs facing the sun at different angles [6-

7]. The non-tracking parabolic trough solar collectors were presented in ref. [8-20]. The advantage

of this design is that there are no moving parts in the system, which leads to reductions in the cost

and maintenance. This collector yields higher temperatures than flat plate solar collector and could

be used in the residential house, the maximum temperature at header of evacuated tube is 235

degrees Celsius, and is therefore suitable for high temperature application such as industrial uses or

cooling application.

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2. The Model

In order to design and develop the non-tracking solar collector, the mathematical model of

reflection of compound trough is calculated. Let the shape of a parabolic trough be described by the

curve y = f(x) on the x-y plane in Fig. 1. The law of reflection states that the angle of incidence θ is

equal to the angle of reflection relative to the tangent of the curve y = f(x) at any point (x,y). The

slope of this tangent line at point (x,y) is denote by mt = df(x)/dx, the slope of the incident ray by m0

and the slope of the reflected ray by m1.

θ θ

mt

m0

m1

y = f(x)

(x ,y )0 0

(x ,y )1 1

(x,y)

Fig. 1. The reflection of a light ray by a curve y = f(x). θ is represented an angle of incidence and

an angle of reflection. mt, m0 and m1 are slope of a tangent line, an incident ray and a reflected ray

respectively.

From trigonometry [5], the relationship between the angle θ between two lines and their relative

slopes mt, m0 and m1 is given as

1

1

0

0

11tan

mm

mm

mm

mm

t

t

t

t

+

−=

+

−=θ , (1)

which yields a slope of the first reflected ray 1m as

( )

12

2

0

2

001

−−

+−=

mmm

mmmmm

tt

tt. (2)

Similarly, the ith reflected rays can be calculated by using the relation

it

ti

it

it

mm

mm

mm

mm

+

−=

+

−

−

−

11 1

1 , (3)

where i are integers. From Eq. 1 and Eq. 2, the reflection of a parabolic trough can be simulated as

shown in Fig. 2.

Fig. 2. The reflection of parabolic trough solar collector at incident angle of 75 degrees where

blue and orange lines are incident and 1st reflected rays respectively. The circle is the position of

the focus point.

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For the incident angle of 75 degrees, the conventional parabolic trough in Fig. 2 cannot receive the

reflected rays. Therefore it needs solar tracking system to maintain sunlight at the focus point. The

parabolic trough solar collector is designed to have an ability to achieve day-long collection

efficiency without the need for mechanical tracking of the sun by using 3 compound parabolic

troughs facing the sun at different angles. Using Eq.(1-3), the reflection of non-tracking solar

collector at various time are shown in Fig. 3.

Fig. 3. The reflection of three-compound parabolic trough solar collector where blue, orange,

green and yellow lines are incident, 1st reflected, 2

nd reflected and 3

rd reflected rays respectively.

The circle in each trough is the position of evacuated tube.

The 3-compound parabolic trough shows that it has an ability to receive the sunlight at various time.

For 12.00 a.m., the solar collector can collect all reflected rays, the reflected rays in the middle

trough are concentrated at the lowest position of the tube and for both side of the middle trough, the

reflected rays are concentrated on the higher position inside the tube. When the time changes, the

reflected rays move up and down inside a tube. For this principle, this collector can collect the

sunlight in any time. However there are some ray losses when the time changes especially after 3.00

p.m. which could be ignored because of very low solar power.

The collector is designed to have an ability to collect diffused light. In Fig. 4, compound parabolic

trough can receive the incident rays in the period of 80 degrees. This implies that this collector has a

probability to collect incident rays from sunlight in both direct and diffused light in the period of 80

degrees at the same time while a conventional parabolic trough can collect the incident rays which

are nearly perpendicular to the trough. Although a parabolic trough could provide a high

concentration, the parabolic trough could not work effectively under diffused light conditions. The

experimental results have shown that the efficiency of the new design of solar collector is higher

than parabolic trough under diffuse solar irradiation as shown in Fig.10 and Fig. 11.

12.00 am

1.00 pm

2.00 pm

3.00 pm

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Fig. 4. The reflection of light rays at various angles of the incident rays. This design has an ability

to collect incident rays in the period of 80 degrees while the conventional parabolic trough can

receive the incident rays in the period of 10 degrees.

In this paper, SUNDA vacuum tubes (SEIDO1) are used to receive the concentrated light

from the trough. This tube is composed of flat plate absorber as shown in Fig. 5.

Fig. 5. The method to place an evacuated tube with flat plate absorber in compound parabolic

trough.

From Fig. 5, the flat plate absorber which is placed horizontally can receive reflected rays better

than the flat plate absorber which is placed vertically and cross shape absorber can collect all rays

but there are no cross shape absorber product at the moment. For this reason, flat plate absorber is

considered to place horizontally in each trough.

10

80

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3. Experiment

The solar collector in Fig. 3 has been invented consisting of three compound parabolic troughs

made of stainless sheets, oriented at different angles. The solar collector has an overall width of 1 m

and a length of 1.9 m, and the evacuated tubes (SUNDA vacuum tube, (SEIDO1)) are placed along

its axis. These evacuated tubes are connected to a manifold header pipe and connected with the

pump to feed the oil. The flow rate is set at 5 lpm. The collectors are fixed on Earth and aligned

along the north-south direction as shown in fig (6-7).

Fig. 6. The novel non-tracking solar collector has an overall width of 1 m and a length of 1.9 m.

Fig. 7. Diagram of test arrangement.

The experiment was performed in Bangkok, Thailand. The data was taken during the period of 9.00

a.m. to 4.00 p.m. on the 10th, 11

th, 12

th ,13

th and 14

th January 2012, The sky was not very clear

which lead the solar power is not smooth in any time. The diagram of test arrangement is shown in

fig. 7.

When the evacuated tubes absorb the sunlight from troughs, the heat from the tubes is transferred to

hot oil which flows in the system. The energy of the system can be calculated by [21]

N

E S

W

466 - 6

( ) ( )inoutC TTCmtQ −= &

, (4)

where t represents time, m& and C are flow rate and the specific heat of the thermal oil respectively.

The efficiency of the system in any time is

( )in

C

Q

Qt =η , (5)

where inQ is the solar power. The evacuated tube is placed in the trough and measured the

temperature at the header. The maximum temperature at heat pipe is 235 degrees Celsius as shown

in Fig. 8 and the maximum temperature of hot oil is 180 degrees Celsius for 0.5 litres of oil as

shown in Fig. 9.

08:00 10:00 12:00 14:00 16:000

200

400

600

800

1000

1200

Time

So

larE

ner

gyHW�m

2L

08:00 10:00 12:00 14:00 16:00

0

50

100

150

200

250

300

Time

Tem

per

atureHCL

Fig. 8. The maximum temperature at the header of evacuated tube plotted against time from 8.00

a.m. to 5.00 p.m. on the 12th

December 2011.

11:00 12:00 13:00 14:00 15:00 16:000

200

400

600

800

1000

Time

Sola

rEner

gyHW�m

2L

11:00 12:00 13:00 14:00 15:00 16:000

50

100

150

200

Time

Tem

per

atureHCL

Fig. 9. The hot oil temperature plotted against time from 9.00 a.m. to 4.00 p.m. on the 14th

November 2011. The maximum temperature is 180 degrees Celsius for 0.5 litres of oil.

From the experiment, the solar power on the 11th ,12

th ,13

th and 14

th of January 2012 in Bangkok

had been collected and its average is shown in Fig. 10. The results show that the efficiency of the

new-design solar collector at any time is fairly constant, which is similar to the parabolic trough

with solar tracking system, while the efficiency of a conventional parabolic trough at any time

distributes like a Gaussian curve having its maximum at around 11.30 a.m. as shown in Fig.11. The

466 - 7

three-compound parabolic trough solar collector yields higher temperature than flat plate or

evacuated tube solar collector. The average efficiency of solar collector is 25-32% .

10:00 12 :00 14 :00 16 :000

200

400

600

800

1000

Time

SolarPower)W)m2

)

10 :00 12 :00 14 :00 16 :000

10

20

30

40

50

Time

Efficiency)))

Fig. 10. The average solar power and efficiency of 3-compound parabolic trough plotted against

times in the period of 9.00 a.m. to 4.00 p.m. on the 10th

, 11th

,12th

,13th

and 14th

January 2012 in

Bangkok.

09:00 10:00 11:00 12:00 13:00 14:000

5

10

15

20

25

30

Time

Eff

icie

ncyH%L

Fig. 11. The parabolic trough in Fig. 4 has been invented. The average efficiency of parabolic

trough plotted against time from 9.00 a.m. to 2.00 p.m. on the 4th, 6th and 8th January 2010[9]

4.Conclusions

The new-design of solar collector has an ability to collect the sunlight at every angle, similar to the

parabolic trough with a solar tracking system. This solar collector has an ability to receive the

diffused light, and this make it suitable for using in all kinds of climate. There are no moving parts

in the system, which results in the reductions in the cost of the system, the cost of maintenance and

complexity. This collector needs only 3 evacuated tubes while SUNDA collector (SEIDO1) needs 8

tubes at the same area. This collector yields higher temperatures than flat plate or evacuated tube

solar collector. The maximum temperature at heat pipe is 235 C and oil temperature is 180 C. It is,

therefore, suitable for high temperature application such as industrial uses or cooling application.

466 - 8

5.Acknowledgements

The authors would like to thank the National Research University Project of CHE, the

Ratchaphiseksomphot Endoment Fund (Project No. EN1180I), 2-V research program of National

Research Council of Thailand (NRCT) and Energy Research Institute of Chulalongkorn University

for the financial supports. We also would like to thank Mr. Narong Amornpitakpunt, AMP

METALWORKS [Thailand] Co.,Ltd for his help for inventing the 1st and 2nd prototype of solar

collector.

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