Applied Energy 25 (1986) 1-8

Sensor System for Aligning a Single-Axis Tracker with Direct Solar Insolation

D. E. Prapas, B. Nor ton and S. D. Probert

Solar Energy Technology Centre, School of Mechanical Engineering, Cranfield Institute of Technology, Bedford MK43 0AL (Great Britain)

SUMMA R Y

The design of a device for providing azimuthal single-axis tracking of the Sun is described. The experimental testing of the device, when used in conjunction with a low concentration ratio parabolic-trough collector, is reported. The device developed allows such collectors to exploit more fully their potential for collecting significant amounts of diffuse insolation.

NOMENCLATURE

b Width of the top slot of the device (see Fig. 3) (m). D c Diameter of the cylinders enclosing the photo-resistors

R 2 and R~ (see (Fig. 3) (m). Dp Photo-resistor diameter (see Fig. 3) (m). K T Ratio of the intensities of the diffuse and the total

insolations. R o Resistance indicated in Fig. 5 (f~). R 1, R2, R' 1, R 2 Designations for the photo-resistors (see Fig. 3). sc Width of the elongated side slots (see Fig. 3) (m). s~ Width of gap between photo-resistors R 1 and R' x (see

Fig. 3) (m). W Length of the top slot of the device (see Fig. 3) (m). 0 w Acceptance angle of the narrow-band detection section

(see Fig. 3) (degrees). 1

Applied Energy 0306-2619/86/$03"50 O Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain

2 D.E. Prapas, B. Norton, S. D. Probert

INTRODUCTION

Low concentration ratio parabolic trough concentrating (PTC) collectors (i.e. those with the ratio of the areas of the absorber surface to the aperture of < 10) exhibit better efficiencies than flat-plate collectors for operating temperatures around 100°C, but they are not generally suitable for operating temperatures greater than about 150°C. The reflecting surface of such a system, compared with that of a compound parabolic concentrator (CPC), is manufactured more easily, because of its simpler geometry. However, unlike the CPC collector, it requires continuous tracking to follow the relative diurnal solar motion across the sky.

A low concentration ratio PTC collector possesses two advantages compared with a high concentration ratio system: (i) it can accept a significant amount of diffuse radiation; 1 and (ii) it does not require such precise tracking. These advantages are the main considerations which determine the design criteria for the tracking system of such a concentrat- ing collector. The tracking requirements for clear-sky insolation can be satisfied relatively easily. It is, however, more difficult to provide adequate tracking under relatively diffuse sky conditions. Such tracking is required to exploit fully the performance capabilities of this class of collectors. A simple tracking device, which complies with these design criteria, is described.

DESIGN

Typically the principal elements of a generic solar tracking system (see Fig. 1) are: (i) a sensor, which detects the relative position of the Sun; (ii) a control unit, including a differential amplifier, in which the signals generated by the sensor are processed; and (iii) a dc motor, activated by the control unit to rotate the collector troughs to the desired angle. via a mechanical tracking rod. The present study has concentrated on improving the design of one of these elements, namely the sensor. This device generates the control signals and, thus, influences significantly the tracking characteristics of the system.

The sun-tracker shown in Fig. 2, built into a commercially manufac- tured parabolic-trough collector, 2 was of a simple design, employing two photo-resistors. When the collector is misaligned with respect to the direct solar insolation, the photo-resistors are unequally exposed to

Sensor system for aligning a single-axis tracker 3

INPUT SIGNALS FROM MOTION OF-.~ I-COVER PHOTO-RESISTORS" ~' E[--O IRT- 1 REFLECTOR ~'~ ~ I__

Cr /F,XEOAB ORBERTOBE --O0,POT --MO AB E EF ECTOR / TO DE

,_ MOTOR

~ . \ \ DE MOTOR

Fig. I. Block diagram of a tracking system.

the Sun and so provide a correcting signal to the control unit for appropriately rotating the system. Though its alignment accuracy was adequate for wide-angle tracking, it was insufficient to keep the Sun's image within the acceptance angle I of the collector. The tracking accuracy was adequate under conditions when the ratio of instantaneous diffuse to total insolation, K T, was zero. For 0 < K T < 0.2, the tracking accuracy deteriorated progressively with increasing values of K T. The alignment was unacceptable when conditions were such that K T > 0.2.

A prototype "sun-sensor' has been developed (see Figs 3 or 4). Cadmium sulphide photo-resistors, with spectral responses similar approximately to that of the human eye, were employed to detect the insolation. Their resistances fall upon increasing the light intensity to which they are exposed accordingly to a logarithmic trend. The sun- sensor design combines two complementary tracking facilities, for wide

r SHADING SLAB

EAST

WEST

PHOTO'RESISTORS Fig. 2. Typical commercially available tracking device.

4 D.E . Prapas, B. Norton, S. D. Probert

b I I - - I I

/\ /

/ \

/ \

RiJ I L-!P-~R~ c'

FRONT ELEVATION

E.sT- L__B I I OpOc , = W E S T

Sc ' R)

i A

TOP VIEW

Fig. 3. Schematic diagram of the sun-sensor.

and narrow acceptance angles. The wide-angle detection is performed with photo-resistors R 2 and R (see Fig. 3), which are exposed to the Sun through the elongated slots BD and B'D' of the cylindrical enclosures C and C', respectively (see Fig. 3). When the collector troughs are in a position within the acceptance angle 0 w, of the narrow-angle tracking section, either or both of the two photo-resistors R 1 and R~ are exposed to direct solar insolation through the top slot AA'. These latter photo- resistors then take on the task of acting as signal providers for control of the tracking movement.

The complete sun-sensor assembly was mounted on top of one of the collector troughs as shown in Fig. 4. The collector frame was fixed in a south-facing position and its inclination was manually adjusted periodically (i.e. "-~ monthly) to follow the seasonal variation of the Sun's

Sensor system for aligning a single-axis tracker 5

- NARROW ACCEPTANCE TOP SLOT

x_ TRACKING ROD

Fig. 4. The sun-sensor in use.

TABLE 1 Specification of the Sun-sensor (see Fig. 3)

Item Symbol Value

Acceptance angle of narrow-band detection section 0 w 11.6 ° Width of elongated side slot s 3.0 mm Width of gap between the photo-resistors R~ and R' 1 sp 1.0mm Diameter of the cylinders enclosing R 2 and R' 2 D c 21.5 mm Diameter of the photo-resistors Dp 14.0 mm Length of the top slot W 60.0 mm" Width of the top slot b 3 mm Value of the resistance indicated in Fig. 5 R 0 47 f~

° This dimension could accommodate a variation of up to 24 ° in the declination of the Sun.

6 D.E. Prapas, B. Norton, S. D. Probert

TABLE 2 Variation of the Photo-resistor Resistance with the Intensity of Illumination

Subjective reference Illumination Resistance of (lux) photo-resistor

Bright sunlight 30 000 10 Fluorescent lighting 500 300 60 W electric light bulb at I m distance 50 2 400

declination angle. The specifications ofthe sun-sensor which are pertinent to its tracking performance are given in Tables 1 and 2.

The electrical circuit of the sun-sensor is illustrated in Fig. 5. The two pairs of photo-resistors R I - R 2 and R'I-R' 2 detect the Sun's presence in the relatively easterly and westerly direction's, respectively. Two electrical resistances of the same value, Ro, were introduced into the circuit in order to diminish the signals produced by the wide-angle detection photo-resistors. This configuration enabled a partial over-riding of the wide-angle detection section when the Sun fell within the acceptance angle for the narrow-angle tracking. This over-riding has been achieved by overwhelming the signal from the wide-angle photo-resistors and required no additional control device. For a full over-riding (i.e. when the Sun falls within the acceptance angle 0 w, the wide-angle signals are switched-off), additional control circuits are required. 3'4 However, the additional capital cost of the latter is seldom justified for a low concentration ratio PTC collector.

R~ R1 R~ R2

! CONTROL I ~

UNIT - - - -

O C MOTOR FOR I I DRIVIN{] THE TRACKING

ROD

Fig. 5. Electrical circuit for the sun-sensor.

Sensor system for aligning a single-axis tracker 7

O P E R A T I O N A L EVALUATION

The tracking accuracy of the sun-sensor was determined under both clear and relatively diffuse sky conditions. This accuracy of the tracking system was defined as the angular deviation within which it could not perform a correction to an existing misalignment error. Thus it represents the resolution of the optical accommodation of the tracking system. The commercially manufactured PTC collector used for this testing was allowed to operate only during the five middle hours of the day, because outside this period shading of the considered trough by its neighbouring troughs was experienced.

Under clear-sky conditions (i.e. with K r < 0-2), the tracking accuracy was found to be + 1.5 degrees. This is less than, and thus encompasses, the apparent acceptance angle of the particular PTC collector used, namely + 9.4 degrees. This value of the tracking accuracy refers to the overall tracking system: the sun-sensor still provided a correcting signal within the above tracking accuracy limits, but this signal was inadequate to activate the tracking mechanism. This was attributed to the response features exhibited by the rest of the system, namely the gain of the differential amplifier within the control unit and the kinematic characteristics of the motor-tracking rod-collector troughs assembly. However, no attempt was made to improve the design of these subsystems, as the tracking performance of the overall system was satisfactory for the particular collector under test.

No dependence of the tracking accuracy on the azimuthal position of the Sun was detected during the five middle hours of the day. However, such a dependence occurred with the originally fitted commer- cial tracking device. This was also observed with other tracking system designs. 5 Such inaccuracies are due to the dissimilar views of, and thus different radiation intensities received from, the celestial sphere and the ground for each of the photo-resistors (i.e. for simple two photo-resistor units) at different times of the day. The partial over-riding of the wide- angle detection section in the new design eliminates this tracking malfunction.

Under relatively diffuse sky conditions, the sun-sensor has exhibited consistently reliable performances. A tracking accuracy of + 4 degrees for a value of K x = 0.45 was achieved. It also achieved approximate tracking under almost totally diffuse sky conditions (i.e. for a value of K T close to unity) if a bright region existed in the sky. In the latter case,

8 D.E. Prapas, B. Norton. S. D. Probert

the tracking was determined from the actual appearance of the celestial sphere: the effect of 'misalignment' on the expectedly-poor thermal performance of the collector, under such insolation conditions, was insignificant.

CONCLUSIONS

The 'sun-sensor', a prototype device which provides azimuthal single- axis tracking for low concentration ratio'parabolic trough concentrating solar-energy collectors, has been both designed and tested. It provides adequate tracking even under relatively diffuse sky conditions. This is an important requirement for this class of tracking collectors. The simplicity of the sun-sensor makes it an easily-manufactured low-cost sun-tracking device.

A C K N O W L E D G E M E N T S

The authors are grateful to the Greek State-Scholarship Foundat ion for financial support for D. E. Prapas, M.Sc. A special debt of gratitude is due to Commercial Solar Energy Limited, Nottingham, Great Britain, for providing us with the parabolic trough concentrating collector used for the present tests.

REFERENCES

1. D. E. Prapas, B. Norton and S. D. Probert, Optics of parabolic trough solar-energy collectors with low concentration-ratios, Submitted for publica- tion, Solar Energy.

2. D. E. Prapas, B. Norton and S. D. Probert, Transient performance analysis of a parabolic-trough concentrating solar-energy collector, Submitted for publication, ASME J. Solar Energl' Eng.

3. P. J. He~sion and W. J. Bonwick, Experience with a sun-tracker system, Solar Energy, 32 (1984), pp. 3-1 I.

4. R. Zogbi and D. Laplaze, Design and construction of a sun tracker, Solar Energy, 32 (1984), pp. 369-72.

5. E. A. Farber, C. A. Morrison and H. A. lngley, A self-contained solar powered tracking device, ASME Paper 76-WA/HT-26, American Society of Mechanical Engineers, New York, USA (1976).