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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 7, July (2014), pp. 65-73 © IAEME

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COMPARATIVE STUDY OF PARABOLIC

TROUGH CONCENTRATORS

Santosh Chandra Anand, Dr. Ajeet Kumar Rai, Vivek Sachan

MED, SSET, Sam Higginbottom Institute of Agriculture Technology and Sciences,

Allahabad (U.P.), India

ABSTRACT

In the present work two parabolic trough concentrator system of different rim angle and

different reflector aperture area is designed, fabricated, and evaluated, and operated for generate hot

water .There one system is 45° rim angle and next is of 90° rim angles, here reflector aperture area of

45° rim angle has operatically 20% more than 90° rim angle system, but remaining all other features

are same for both the 90° and 45° system. On operation of the system we gets nearly same efficiency

for both the system thus we observe that for all same equal features except of reflector aperture area,

the efficiency got 20+% more for 90° rim angle compare to 45° rim angle. The Supporting stand of

concentrator is made of mild steel & reflector is made of acrylic sheet with a rim angle of 45 and 90

degree and aperture area of 2.20 m square and 1.84 m square with a concentration ratio of 10.00 and

08.30. Both the receiver tube has made of aluminum metal material. The thermal performance of the

PTC was determined based on ASHRAE 93-1986 (RA 91) .The maximum instantaneous thermal

efficiency separately both system is obtained closely to 58.32% – 59.24 % range. The total cost for

each separate system is calculated Rs 4500 Indian.

Keywords: Reflector Aperture Area, Rim Angle, Concentration Ratio.

INTRODUCTION

The Improper use of fossil fuels has led to negative imbalance in the natural environment so

need of using both non-renewable and renewable energy resources were taken to be the main aim

and the utilization objective outlined the need to use energy efficiently. Power plants and

domestically uses parabolic trough collectors to concentrate the direct solar radiation onto a tubular

receiver to generate hot water and water steam for Boiler and turbines for Industrial and domestic

and Buildings uses also. Parabolic trough power plants are participates in the biggest part of the total

installed concentrating solar power technology. Hence other technologies systems like Fresnel power

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plants, solar tower power plants and dish/ Sterling systems are also using but parabolic trough power

plants provide over 90% of the capacity of concentrating solar power plant technology that is in

operation or under construction up to September 2010. In the projected additional capacity more than

70% are constituted by parabolic trough power plants. PTC mainly consists of a cylindrical parabolic

reflector and a metal tube receiver at its focal plane. The receiver is black coated for heat loss

resistant and much absorb heat radiation purpose black paint coated at the outside surface covered by

concentrator and rotated about one axis to track the sun`s directional motion. The sun tracking

system has two types, one is single tracking and another is double tracking system. In PTC

concentrated heat is transferred through the absorber tube to working fluid for required purpose.

The aperture diameter, rim angle, reflector property and absorber size and shape is defines the PTC.

The absorber tube is made of aluminum for quick heat, low cost; reduce weight and protection from

salt decomposition and corrosion on outer surface of tube. Hence it is difficult to curve a very large,

mirror strips so used in the shape of parabolic cylinder. Reflectors are made of anodized, aluminum

Mylar or curved silvered glass. The concentration ratio for a cylindrical.

Absorber tube varies from 5 to 30+. The Concentration ratios for composite system can be

theoretically very high with the imaging concentrators of precise optical elements and continuous

automatic tracking system is in the range of 10 to 40 000. The reflector and absorber with collector

are fixed on the frame structure. The major energy losses from a concentrator-receiver assembly for

normal incidence rays deflected away from concentrating plane, losses during reflection from

reflecting surface and convection loss from the receiver to surrounding.

PTC can be oriented in these three directions, East-west, north-south or polar directions. The

East-west, north-south is simple to assemble and have higher incidence angle cosine losses. The

polar configuration intercepts more solar radiation per unit area as compared to other modes.

DESIGN CONSIDERATIONS

In the present PTC has following innovative characteristics; easy constructible, strong and

stable in structure, light in weight and low in cost. There we have used for reflecting mirror two

Acrylic Mirror Sheets of the size 0.92 meter wide, 1.22 meter long and 4 mm thick has placed

longitudinally in each system. The parabolic profile is determined by the shape of the ribs and the

width of the sheets. The weight of sheets is rest on said ribs. The total aperture area is for 45° is

2.208m² and for 90° included collectors shadow is 1.840 m², but the reflector’s total area is equal for

both the systems.

PTC DESIGN PARAMETERS

The PTC dimensions and designing parameters has determined by these considerations.

For these two separate systems of 90° and 45° rim angle of parabola these two

considerations has determined. The first is aperture of the parabola of 1.22m wide acrylic sheet is

determined by ‘Wa’ and second is focal length ‘�’ of parabola. And D₀ is the outer diameter of the

receiver tube.

Aperture of parabola ‘Wa’

Wa � ��

��

� ��

� ��


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Focal length of parabola ‘�’

� � ��

� �

Geometrical concentration ratio ‘C’ for tubular receive

� � � ! "#

We have taken D₀= 0.03 m

The three parameters rim angle, aperture width and focal length are determine the cross-

section of PTC. ψ is expressed as a function of the ratio of the aperture width to the focal length.

tan ψ=�/%�

�&'(��

)�*� )��

The function of the rim angle is expressed as the ratio of the aperture width to the focal length

that is = a/f, Here � is the focal length, i.e. the distance between the vertex of the parabola and the focal

point. Equation of parabola on x axis:

- � . /�

�0

Table 1: Specifications of PTC

FABRICATION OF PTC

The PTC parts has constructed according to given below process and descriptions.

1-Frame structure for PTC system holding and support

For holding and support the receiver and reflector a frame structure of made by mild steel

Iron has assembled with use of arc welding, drilling and nut-bolt joint. The reflector support frame

portion was adjustable to horizontal axis for automatic or manual sun tracking. The single Sun

tracking and double Sun tracking structure is designs according structure frame manner.

1 Rim angle( φr ) 45° 90°

2 Focal length ( �) 0.51m 0.25m

3 Aperture width ( Wa) 1.20m 1.04m

4 Diameter of receiver tube (D₀) 0.0384m 0.0384m

5 Length of Parabola (L) 1.84m 1.84m

6 Effective Aperture Area (Aa) 1.56m 1.30m

7 Concentration Ratio (C) 9.952 8.625

8 Reflectivity of collector (ρ) 0.7 0.7

9 Absorptivity of receiver tube (α) 0.6 0.6

10 Transitivity of receiver tube (τ) 0.6 0.6

11 Intercept Factor (γ) 0.72 0.72


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2- Parabolic Trough or Receiver-Absorber assembly

Commonly parabolic trough has four parameters are used to determine, in this system, trough

length (1.84m), focal length (25cm, 51cm), aperture width (104cm, 120cm) and rim angle

ψ (90°, 45°).

Rim angle is angle between optical axis and line between focal point and rim angle. Focal

length is distance between focal point and vertex. Aperture width is distance between one rim to

other rim in one parabola.

Rim angle has an effect on the concentration ratio and on the total irradiance per meter

absorber tube in W/m. If the rim angle is very big then the way of reflected radiation from outer part

of mirror is very long and the beam spread is very big, reducing, hence, the concentration ratio.

Hence a mirror with a smaller rim angle and the same aperture width would permit a higher

concentration ratio.

At 45° rim angle the highest concentration ratio is reaches and on increasing rim angle the

Sun image is widening and then concentration ratio is decreasing capture to 45° rim angle, and when

below the 45° rim angle now Sun image is more widening compare to first condition and then C.R. is

much more decreases.

In higher rim angle the absorber tube is nearer allocated the mirror then radiation beam is

spread and reduces C.R. The shadow of tubular also reduces mirror receiver area, aperture and C.R.

also. The absorber distance is larger in very large rim angle as well as very small rim angle.

The more weight carries the radiation aberration due to mirror slope error. Mirror is determined as at

a Sun position it captures the radiation beams.

EXPERIMENTAL SETUP

In this experimental setup of parabolic trough concentrator we have used a 30 liter storage

source tank for supply the working fluid to receiver tube. The storage source tank level is 0.25m

higher than the receiver tube’s maximum height and the connecting tube is flexible and 0.75m long

to rotate the receiver movement and direction for Sun tracking. Our system was oriented in north-

south to capture maximum isolation. The system was able for manual tracking.

Figure 1: Experimental Setup

In the system collector performance over the day is quit uneven and is reduces in the hours

after sunrise and before sunset. So to reduce this problem we have enlarged 0.50m length of receiver

tube at inlet and outlet side in the focal plane.


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We have applied adjustable flow control valve arrangement at collector end to variance the

different, constant mass-flow rate with drop-wise flow. Our flow rate was 2 liter/3 liter/4 liter per

hour droplet-wise constant mass-flow rate condition 8 hours day observations.

We have fitted the thermocouple sensors to observe the temperature at point of receiver’s

in/mid/out points, source storage tank, collector storage, reflector sheet’s front/back sides and

ambient temperature pick out.

We have used solarimeter to measure the solar radiation intensity on mirror sheet and

receiver tube. For measuring wind velocity we used anemometer.

PERFORMANCE OF PARABOLIC TROUGH COLLECTOR

In this system estimated performance are the solar field efficiency and useful heat output

from solar effected area under different operating conditions for characterizing the performance.

Thermal Efficiency In thermal efficiency, it is affected by thermal losses. Thermal losses depend on the

temperature difference between the heat transfer medium and the surrounding air. Heat loss from a

warmer surface to ambient air is because of convection and thermal radiations.

Qconv = h.A.∆T

Qrad = Ԑ.ρ.A.(T4

amb-T4

abs)

Qcond = (1111/b).A. ∆T

Here b is insulation thickness.

Collector overall efficiency 2c is defined as the ratio between the useful output Qu by collector to

global irradiance I on Aa

2c=34 5

*Ib

For collector useful output Qu

Qu=mCp(T0-Tc)=AaIb 2₀-Aabs*Ul(Tabs-Ta)

Optical efficiency

Optical efficiency 2₀ is defined as total amount of radiation absorbed on absorber tube outer

surface to the amount of direct normal radiation incident on aperture area. If incident radiation is

normal to the aperture is (6=0°) then the optical efficiency 2₀=ρ.(τ.α).γ

RESULT AND DISCUSSION

Number of observations has taken on both the system in the month of May 2014, in solar lab

ground of SHIATS Allahabad, Uttar Pradesh, India.

The observation data of both 90° rim angle and 45° rim angle PTC has taken and calculated

the thermal efficiency at separately particular day. The maximum thermal efficiency has got 58% for

45° rim angle and 59% for 90° rim angle systems.


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Fig 2: Variation of solar intensity with respect to time of a day on a particular day in the month of

may (23/05/2014)

Figure 3: Variation of wind velocity with respect to time of a day in the month of May

at a particular day (23/05/2014)

Figure 4: Variation of temperature with respect to time of a day of receiver tube at in/mid/end point

at a particular day (23/05/2014)


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Figure 5: Variation of temperature rise (inlet-outlet) difference with respect to time of a day of on a

particular day 23/05/2014

Figure 6: Variation of thermal efficiency with respect to time of a day, at mass flow rate of 2

liter/hour on a particular day in the month of May (21/05/2014)




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The result shows that the thermal efficiency of both PTC at different mass flow rate of drop

wise constant continue flow of working fluid (Water).

The result has shows that for same size mirror sheet at rim angle 45° and 90°, the thermal

efficiency has closely equals. Hence in 90° rim angle PTC has active Sun radiation receiving active

aperture area of reflector sheet receiver is 20% less due to aperture width and absorber tube is closely

to reflector sheet appears the shadow on mirror sheet. Thus 90° rim angle PTC system is 20% more

efficient than 45° rim angle PTC system for same equal area of receiver mirror sheet and absorber

tube system.

CONCLUSION

From the observations, calculated data and calculations in relation with analysis and

discussion, this research investigation can be calculated that fabricated PTC has maximum thermal

efficiency has got 58% for 45° rim angle system and 59% for 90° rim angle system. The result

concludes that 90° rim angle PTC has 20% more efficient compare to 45° rim angle PTC system.

And 90° degree rim angle system is permits easy protection with flat glass mirror reflector sheet.

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