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COMPARISON BETWEEN ASTRONOMICAL AND LIGHT

SENSOR FEEDBACK SUN-TRACKING ALGORITHMS

October 2010

D. Gomes* | L. Pina** | J. Martins***

* Faculdade de Ciências e Tecnologia, FCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal.

{davidcoimbragomes@gmail.com}

**WS-Energia, Taguspark Edf Tecnologia II, Pavilhão 46,2740-257 Porto Salvo, Portugal.

{pina@ws-energia.com }

*** CTS, Uninova, Dep.ª de Eng.ª Electrotécnica, Faculdade de Ciências e Tecnologia, FCT, Universidade Nova de

Lisboa, 2829-516 Caparica, Portugal. {jf.martins@fct.unl.pt}

D. Gomes* | L. Pina** | J. Martins***

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Structure of the presentation

• Company introduction

• Why track the Sun?

• How to track the Sun?

• High precision sensor

• Light sensor

• Astronomical sensor

• Comparison between sensors

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What we do

Core Business

High-Efficiency Photovoltaic Technology

WS Energia develops, designs, manufactures and

commercializes solar trackers, optics for solar

concentrators and robotic controllers for solar

trackers.

Mission

To be the number one in creating leading edge technology and know-how in the solar

energy sector.

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Values

We are

leaders in introducing reliable PV innovation

leaders in customization and client benefits

Who we are

July 2010

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Our products

Photovoltaic Concentrators

Monthly capacity of 1.000 units

Photovoltaic Trackers

Monthly capacity of 5.000 units

Robotic control for solar systems

Monthly capacity of 10.000 units

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Where we are

WS ENERGIA WORLDWIDE

2.000 trackers and concentrators

WS ENERGIA IN PORTUGAL

> 300 instalations

Company that installed the largest number of photovoltaic concentration systems in the world (by Photon 2009)

Until July de 2010

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Why track the Sun?

Fig.1: Comparison between a dual-axis tracking system and a static one.

The performance of a photovoltaic application depends essentially on the amount of solar

energy captured by the cells.

Dual-axis tracking systems can improve the amount of energy collected in about 40% when

compared to static systems.

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How to track the Sun?

Most Common ways to track the sun are:

• The astronomical calendar approach

Uses time and geographical coordinates

• The light sensor-based approach

Uses photo-sensitive devices like:

» photoresistors

» photodiodes

» Phototransistors

• Machine vision

Uses arrays of photo-sensitive devices like CCD cameras.

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A CCD camera Genius eFace 1300 was used.

Solar filter

Fig.2 a): CCD camera used Fig.2 b): Precision Sensor

The use of a solar filter is crucial to avoid damaging and the saturation of the camera.

High precision sensor

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This sensor provides the tracking error in both axis, in degrees.

Where the system

is pointing to

The center of the

SunFig.3: Illustration of the tracking error in both axes.

High precision sensor

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Light-sensor

Works by comparison.

Fig.4: Illustration of the sensor.

• The devices E and W are compared to track the sun in the horizontal axis.

• The devices N and S are compared to track the sun in the vertical axis.

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Which photosensitive device shall be used?

Several tests were made in order to choose it:

- Light direction.

- Light intensity.

- Electric design.

Fig.5 a): Picture of the test. Fig.5 b): Assembly of the three devices.

Light-sensor

Phototransistor Photoresistor Photodiode

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Fig.6: Comparison of the three devices with a 1KΩ resistor.

Which photosensitive device shall be used?

Light-sensor

• Photodiode (red)

» Response is too oscilatory

• Photoresistor (blue)

» Good response

• Phototransistor (green)

» Response is too abrupt

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Which photosensitive device shall be used?

Light-sensor

• Photodiode (red)

» Response is too oscilatory

• Photoresistor (blue)

» Good response

• Phototransistor (green)

» Response is too abrupt

Fig.6: Comparison of the three devices with a 1KΩ resistor.

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Light-sensor

1) The light-sensor uses four photoresistors

2) The PIC compares the readings

3) The information is sent to the tracking controller

Fig.7 a): Scheme of the sensor Fig.7 b): Picture of the sensor

Photoresistors

Integrated Circuit

Communication with

actuator controller

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Fig.11 : Error propagation chain

Fig.10: Assembly error

Light-sensor

Error propagation chain

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Astronomical sensor

Fig.8 : Error propagation chain

Error propagation chain

Fig.9 b) : Gravity sensor error

Fig.9 a) : Assembly errors

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True Sun position

Light sensor

Astronomical

approach

Fig.12 : Finding the sun’s path

Sun tracking with light sensor ( East – West axis )

Comparison between astronomical and light sensor

approaches

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Fig.13 : Finding the sun’s path

Comparison between astronomical and light sensor

approaches

Sun tracking with light sensor ( North – South axis )

True Sun position

Light sensor

Astronomical

approach

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Fig.14 : Sun tracking in a cloudy day with a light sensor

Comparison between astronomical and light sensor

approaches

In a cloudy day, the light sensor approach is not robust

True Sun position

Tracker position

Detected Sun’s

position

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Conclusions

• Light sensor:

Very accurate;

Simple and economic way to track the sun;

Installation misalignments do not influence tracking accuracy;

Not robust in a cloudy day.

• Astronomical approach:

The accuracy does not depend on the weather;

Many sources of error;

Installation misalignments influence tracking accuracy;

No feed-back.

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