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PORTABLE SUN TRACKING PHOTOVOLTAIC POWER SYSTEM
A Project
Presented to the faculty of the Department of Electrical and Electronic Engineering
California State University, Sacramento
Submitted in partial satisfaction of
the requirements for the degree of
MASTER OF SCIENCE
in
Electrical and Electronic Engineering
by
Toan Anh Le
FALL
2013
© 2013
Toan Anh Le
ALL RIGHTS RESERVED
ii
PORTABLE SUN TRACKING PHOTOVOLTAIC POWER SYSTEM
A Project
by
Toan Anh Le
Approved by:
__________________________________, Committee Chair
Fethi Belkhouche
__________________________________, 2nd Reader
Preetham B. Kumar
____________________________
Date
iii
Student: Toan Anh Le
I certify that this student has met the requirements for format contained in the University
format manual, and that this project is suitable for shelving in the Library and credit is to
be awarded for the project.
__________________________, Graduate Coordinator
Preetham B. Kumar
Department of Electrical and Electronic Engineering
iv
___________________
Date
Abstract
of
PORTABLE SUN TRACKING PHOTOVOLTAIC POWER SYSTEM
by
Toan Anh Le
For many people who live in rural areas of Southeast Asia, such as Laos, Cambodia and Vietnam,
access to electric power is either too expensive or it is unavailable. This unfortunate condition
may be greatly mitigated, however, by the deployment of inexpensive and highly efficient
portable electric power generation systems. This project proposes the design of such a system by
exploiting current photovoltaic panel and battery technology. This portable power system is
designed to power small applications such as portable electronics and other low-power electrical
devices. To gain maximum power from solar panels, the panels will be mounted on a single axis
sun tracking device. A lead acid battery, for storing harvested energy, is selected because of its
availability and suitability. To charge the battery, a battery charging circuit will be used based on
good efficiency and a control algorithm. Overall, this project is intended to solve system
integration challenges with solar panel, battery, and charger in order to achieve the greatest power
efficiency from the solar panel and the maximum battery life while maintaining good overall
system portability and low cost. The final product is a working device that can be used to power
small electronic devices for a practical duration.
_______________________, Committee Chair
Fethi Belkhouche
_______________________
Date
v
ACKNOWLEDGEMENTS
I would like to take a moment to thank Dr. Fethi Belkhouche for his patience and
technical guidance for this project. Additionally, I would like to thank my work
colleagues, Udaya Natarajan and John Mackin, for providing valuable ideas and
feedbacks for this project report.
vi
TABLE OF CONTENTS
Page
Acknowledgements ………………………………………………………………………vi
List of Tables ……………………………………………………………………………. x
List of Figures …………………………………………………………………………... xi
Chapter
1
INTRODUCTION .......................................................................................................1
2
BACKGROUND .........................................................................................................3
3
2.1
Fundamental of Solar Power......................................................................... 3
2.2
Existing Tracking Technology...................................................................... 5
2.2.1
Single Axis Tracker ...................................................................................... 5
2.2.2
Dual Axis Tracker ......................................................................................... 6
2.3
Introduction to PV Battery Charging ............................................................ 6
2.4
Existing Portable Solar Power Implementation ............................................ 9
2.4.1
Portable Concentrating Solar Power Supplies ............................................ 10
2.4.2
Portable Solar Systems using a Step-up Power Converter ......................... 10
2.4.3
A Low-cost Photovoltaic Energy Harvesting Circuit for Portable Devices 11
PROJECT FOCUS AND DIFFERENTIATION .......................................................12
3.1
Optimization ............................................................................................... 12
3.2
Cost and Power Savings ............................................................................. 12
3.3
Application Usefulness ............................................................................... 13
vii
4
DESIGN AND INTEGRATION ...............................................................................14
4.1
Component Selection .................................................................................. 14
4.2
Single-Axis Sun Tracker ............................................................................. 14
4.2.1
Microcontroller ........................................................................................... 14
4.2.2
Motor/Actuator ........................................................................................... 15
4.2.3
Sensor .......................................................................................................... 16
4.2.4
Software Algorithm .................................................................................... 17
4.2.5
Power Optimization .................................................................................... 19
4.2.6
External Interrupt ........................................................................................ 20
4.3
5
Solar Power System .................................................................................... 21
4.3.1
Solar Panel .................................................................................................. 21
4.3.2
Battery ......................................................................................................... 24
4.3.3
Battery Charger ........................................................................................... 25
4.3.4
Inverter ........................................................................................................ 27
4.4
System Hardware Setup .............................................................................. 27
4.5
System Diagrams ........................................................................................ 30
SYSTEM ANALYSIS, TESTING, AND RESULTS ...............................................32
5.1
Power Consumption Analysis ..................................................................... 32
5.1.1
ArbotiX Board Power Measurements ......................................................... 32
5.1.2
Inverter Power Measurements .................................................................... 33
5.2
Functionality Test Results .......................................................................... 34
viii
5.2.1
External Interrupt Circuit Measurements .................................................... 34
5.2.2
Charging Data Analysis – Sunny Day – With Sun Track – Default Vout and
Vmppset .................................................................................................................... 36
6
5.2.3
Charging – Sunny Day – With Sun Track – Tuned Vout and Vmppset ..... 38
5.2.4
Charging Data Analysis - Sunny Day – No Sun Track .............................. 41
5.3
Future Improvements .................................................................................. 42
5.4
Bill of Materials .......................................................................................... 43
CONCLUSION ..........................................................................................................45
Appendix A. Single Axis Sun Tracker Code .............................................................47
Appendix B. Eval Board Schematic – bq24650EVM ...............................................53
Appendix C. Power Inverter Specifications ..............................................................54
Appendix D. Arbotix Board Schematic .....................................................................55
References ..................................................................................................................56
ix
LIST OF TABLES
Tables
Page
Table 1 ATMEGA644P Sleep Modes .............................................................................. 19
Table 2 Battery Specifications .......................................................................................... 24
Table 3 Typical Calculated Application Usage Time ....................................................... 25
Table 4 Power Measured for the ArbotiX Board .............................................................. 33
Table 5 Battery Charging Current and Voltage on Typical Sunny Day – Day 1 (Fall
2013) .................................................................................................................... 36
Table 6 Battery Charging Current and Voltage on Typical Sunny Day – Day 2 (Fall
2013) .................................................................................................................... 36
Table 7 Tuned Battery Charging Current and Voltage on Typical Sunny Day – Day 1
(Fall 2013) ........................................................................................................... 39
Table 8 Tuned Battery Charging Current and Voltage on Typical Sunny Day – Day 2
(Fall 2013) ........................................................................................................... 40
Table 9 Sunny Day Battery Charging Without Sun Tracking Enable .............................. 42
Table 10 Project Bill of Materials ..................................................................................... 44
x
LIST OF FIGURES
Figures
Page
Figure 1 Solar Cell Angle between Sunlight and Normal Plane ........................................ 4
Figure 2 Typical Vbat and Ibat when Charged Directly from PV Array – Power Source
Disconnected [6] .................................................................................................. 8
Figure 3 Typical Vbat and Ibat when Charged Directly from PV Array - Current
Regulated from Upper Voltage Set Point [6] ....................................................... 8
Figure 4 Typical Vbat and Ibat when Charged Directly from PV Array - Current
Regulated from Floating Voltage Set Point [6] ................................................... 9
Figure 5 ArbotiX Robocontroller Using ATMEGA644P [10] ......................................... 15
Figure 6 AX-12A Dynamixel Actuator ............................................................................ 16
Figure 7 Simple Light Sensor Circuit ............................................................................... 16
Figure 8 Sun Brightness Detect Algorithm – One Axis Sun Tracking ............................. 18
Figure 9 External Interrupt Circuit ................................................................................... 20
Figure 10 Expected Output Waveform ............................................................................. 21
Figure 11 20 Watt Mono-Crystalline Solar Panel............................................................. 22
Figure 12 Solar Panel I-V Characteristic .......................................................................... 23
Figure 13 Solar Panel Power Curve .................................................................................. 23
Figure 14 TI bq24650EVM Board .................................................................................... 26
Figure 15 System Front View ........................................................................................... 28
Figure 16 System Back View ........................................................................................... 29
xi
Figure 17 System Side View............................................................................................. 29
Figure 18 Main System Diagram ...................................................................................... 30
Figure 19 During Charge Cycle Diagram ......................................................................... 30
Figure 20 System Normal Operation Diagram ................................................................. 31
Figure 21 Power Measurement for ArbotiX Board .......................................................... 32
Figure 22 Power Measurement for the Inverter ................................................................ 34
Figure 23 External Interrupt Waveform – Rising Edge .................................................... 35
Figure 24 External Interrupt Waveform – Falling Edge ................................................... 35
Figure 25 Charging Curve Measured On Sunny Day ....................................................... 37
Figure 26 Tuned Charging Curve Measured On Sunny Day 1 ......................................... 40
Figure 27 Tuned Charging Curve Measured On Sunny Day 2 ......................................... 41
xii
1
Chapter 1
INTRODUCTION
According to the Rural Electrification data, in Southeast Asia alone, more than
160 million people do not have access to electricity. This number corresponds to 80
percent of those people who live in rural areas of Southeast Asia [1]. Consequently, the
most basic electric appliances, such as lighting by which to read, are unavailable to
millions of people throughout Southeast Asia.
A viable solution to the appalling lack of accessible electricity to these rural
homes may be found in the application of Distributed Generation (DG) systems. A
conventional power distribution grid employs larger, centralized power generation plants,
whereas a DG power distribution system employs smaller, distributed power generation
plants. DG systems are common in developing countries. However, the fuel for these
power generation systems is typically in the form of fossil fuels, limited natural resources
that produce carbon dioxide gas and deleterious particulate matter upon combustion [2].
Elevated levels of such environmental aerosol and carbon dioxide has been demonstrated
to adversely affect both human and environmental health. Furthermore, because carbonbased fuel is a limited resource, its availability and therefore its cost is destined to
become prohibitive.
In contrast to carbon-based fuels, renewable energy systems, such as solar, wind
and geothermal, offer a practical solution to the critical problems of diminishing fuel
supply and environmental pollution. And finally, the enhanced health and increased
2
productivity of benefiting societies would more than offset the financial costs associated
with development and deployment of renewable energy. The majority of the world’s
seven billion people, including many people of rural Southeast Asia, have a critical need
of efficient and affordable power. This long un-met need can finally be met with the
efficacy of specially-designed, distributed generation systems.
On the market today, there is already a great variety of portable solar power
generators. Section 2.4 discusses different implementations and the possibilities for
further improvements in this area. The proposed photovoltaic (PV) based power
generation in this report focuses on efficiency, cost and portability. The efficiency is
achieved by utilizing as much power from the solar panel as possible. This is where the
single axis sun tracker will come into the picture, as it will direct the solar panel to the
sun by using photoresistor sensors. Batteries will be used and efficiently maintained by
solar energy and a control circuit to protect the battery from over and under charging.
Because the main system components will be selected based on both cost and efficiency,
these characteristics will be carefully considered. System portability is also an important
consideration, which is dependent mainly upon battery size. The battery must be selected
for its weight, but at the same time it must provide enough power for at least one day of
constant use. This project is intended to exploit existing photovoltaic technology and
unique system components in order to produce the most efficient and low-cost portable
power generation system possible.
3
Chapter 2
BACKGROUND
This section will discuss the background and the fundamentals of solar
technology, and the advantages and disadvantages of using a tracking system with solar
panels. Different battery charging methods are discussed and analyzed.
2.1
Fundamental of Solar Power
Solar panels are composed of crystalline silicon. Each solar panel has multiple
solar cells that are connected in either series or parallel. When the solar cells are
connected in series, the output voltage is increased. Similarly, when the solar cells are
connected in parallel, the output current is increased. However, whether or not greater
efficiency is gained by a series arrangement or by a parallel arrangement will depend
upon its particular application.
Because solar cells are made of layered silicon, different doping elements are
observed to form the p-n junction. P-type or positive type will have positive charges that
contain extra holes. N-type or negative type will have negative chargers that contain
extra electrons. The border between p-type and n-type forms a neutral region that serves
as a barrier between the positive and negative types. When light shines onto the p-n
junction, the electron/hole pair is formed because of the photon travel frequency.
Because the p-n junction forms the neutral region due to the potential difference, the
electron cannot travel to the other side while the holes can. As a result, the electrons’
only route is to travel through the metal gate, which in turn adds to the load on the other
side of the junctions [3] [4].
4
Each solar panel is rated with an output voltage and output current. The amount
of current produced is directly correlated to the intensity of the sun’s rays that are
absorbed by the PV panel.
Normal
Sunlight
θ
Solar Cell Panel
Figure 1: Solar Cell Angle between Sunlight and Normal Plane
Sunlight shines onto the solar cell panel at an angle θ, which is the angle between
the sun’s rays and the normal line. This is called the angle of incidence, as illustrated in
Figure 1. The power generated by the solar panel can be calculated by this equation:
W = 𝐴 ∗ 𝛼 ∗ cos⁡(𝜃)
(1.0)
where:
A – limiting conversion factor
α – sunlight intensity
θ – angle between normal plane and sunray
From this equation, it can be seen that the maximum power generated by the solar
cell panel is produced when the sunray is perpendicular to the normal plane. This is
equivalent to an angle Ɵ of 0°. On the other side, the solar panel will produce zero power
if the sunlight is kept parallel to the panel, which corresponds to an angle Ɵ of 90°.
5
When using a fixed panel, there is significant power loss during the day because the panel
is not kept perpendicular to the sunlight. An automated tracking system, however, could
provide maximum power production by keeping the panel at a margin that is always
perpendicular to the sunlight [5].
2.2
Existing Tracking Technology
The PV panel must be kept perpendicular to sunlight in order to produce
maximum power. There are several tracking methods available. It is important to choose
the method that fits the application, which is dependent upon cost, accuracy and other
factors. Regardless of the tracking system selected, it must maximize power production.
2.2.1
Single Axis Tracker
This type of tracker can only rotate 360° horizontally. This means that it has only
one degree of freedom with one axis of rotation. Typically, the panels are mounted at an
angle with respect to the ground. There are several types of single axis trackers that can
be categorized into one of the following.

Horizontal Single Axis Tracker (HSAT)

Vertical Single Axis Tracker (VSAT)

Tilted Single Axis Tracker (TSAT)

Polar Aligned Single Axis Tracker (PASAT)
In general, PASAT or TSAT can increase solar radiation by as much as 10% [5].
6
2.2.2
Dual Axis Tracker
A dual axis tracker can rotate 360° and tilt from 0° to 180°. This is equivalent to
two axes of freedom and two axes of rotation. The gain in efficiency from a single axis
to a dual axis tracker is insignificant [5]. Because the dual axis tracker uses more motors,
the power consumption tends to be higher. In a typical application, the tracker should not
diminish return on power production due to current consumption. Because of these
shortcomings, the pros and cons of deploying a duel axis tracker must be carefully
considered.
There are two types of dual axis trackers.

Tip-Tilt Dual Axis Tracker (TTDAT)

Azimuth-Altitude Dual Axis Tracker (AADAT)
The difference between TTDAT and AADAT is in the action of the primary axis.
A TTDAT primary axis is horizontal to the ground, while a AADATA primary axis is
vertical to the ground. Between the two, AADAT has more deployments because it
produces greater power gain compared to the TTDAT [5].
2.3
Introduction to PV Battery Charging
In photovoltaic systems, deep-cycle batteries are used typically to store the
harvested energy. This allows the energy to be used as it is needed and not wasted. In
developing countries, the use of a battery or a battery bank can enable a constant and
dependable supply of power for a small household.
One of the most important, and oftentimes the most costly, components of a PVbased system is the battery or battery bank. Correctly charging a battery and obtaining
7
the maximum number of cycles from it can be challenging. There are several factors,
including environmental and charging, that greatly influence the life of deep-cycle
batteries. One of the most important aspects of battery charging is regulation of the
charge limits in different usage scenarios. If excess current is supplied, damage to the
battery will occur, which will result in reduced battery life and additional system cost.
Therefore, battery charging can be thought of as a control and stability problem. The role
of the control and stability algorithm is to regulate the current that is supplied to in order
to maximize the battery’s life and to avoid damaging the battery’s cells.
Batteries that are used in PV applications are capable of deep discharge cycles
and are typically valve regulated, lead-acid (VRLA). These types of batteries are highly
efficient and, while they are expensive in absolute terms, they can be inexpensive in
terms of total wattage output. Therefore, correct regulation of a battery’s charge and
discharge is important, because it can greatly extend the battery’s life and thereby reduce
its cost.
The most common charging scheme in a PV-based system is to connect the
current from the solar array directly to the battery or battery bank. There are three simple
cases for current regulation to the battery in this scheme.
For the first case, the battery voltage and current plot is illustrated in Figure 2.
Vbat and Ibat are the battery voltage and current, respectively.
8
Overcharge limit
Vbat
Ibat
Open-circuit
voltage
Lower limit
Full current from
PV array
Figure 2: Typical Vbat and Ibat when Charged Directly from PV Array – Power Source
Disconnected [6]
When the overcharge limit is reached, the battery will simply disconnect from the
power source. In this case, the power source is a solar array. The current that charges the
battery is always the maximum current from the PV array until the power source is
disconnected.
In the second case, illustrated in Figure 3, when the upper voltage set point is
reached, the current is simply regulated such that Vbat is operated in a flat line.
Voltage regulation
set point
Vbat
Lower limit
Ibat
Full current from
PV array
Figure 3: Typical Vbat and Ibat when Charged Directly from PV Array - Current Regulated from
Upper Voltage Set Point [6]
9
The third case, illustrated in Figure 4, is similar to the second case. However,
instead of the current being regulated at the upper voltage set point, it is regulated at the
floating voltage set point.
Voltage regulation
set point
Vbat
Floating voltage
Lower limit
Ibat
Full current from
PV array
Figure 4: Typical Vbat and Ibat when Charged Directly from PV Array - Current Regulated from
Floating Voltage Set Point [6]
There are three issues with each of these types of charging schemes. First, the
voltage regulation set point does not always relate to the battery 100% State of Charge
(SOC). Second, these charging schemes cannot charge multiple battery cells uniformly
in large battery arrays. And third, due to the variability of sun light during the day, PV
panels do not always output consistent amounts of voltage and current. As a
consequence of each of these issues, a battery’s life can be greatly diminished. As one
can see, improvement of these systems is still needed.
2.4
Existing Portable Solar Power Implementation
This section reviews three recent system deployments that are described in the
IEEE journal.
10
2.4.1
Portable Concentrating Solar Power Supplies
In Fraas et al. 2010 [7], a new solar panel technology was introduced. This
technology is called compact portable solar generator and can be operated at 12W, 36W
and 75W. The 12W panel is designed to be carried in one’s pocket, while the 36W panel
is designed to be the size of a notebook. The larger 75W panel uses a single axis sun
tracker and is designed to fit into a pack back. Overall, the portability of the solar panels
in this project is excellent because of their size and because they use advanced solar cell
technologies. However, it should be noted that the costs of these systems were not
mentioned. Additionally, the overall system components, such as battery capacity,
battery life and battery charger, were not discussed [7].
2.4.2
Portable Solar Systems using a Step-up Power Converter
In Gao et al. 2007 [8], the authors describe and validate the configuration of a
solar array designed to provide optimal power generation on cloudy days. This system is
based on parallel-configured solar panels, step-up power converter and a high-speed,
maximum power point tracker (MPPT). Because the amount of current produced by the
panel is directly proportional to the sunlight it receives, configuring the panel cells in
parallel would help to maximize the power produced by each of the cells. They used two
different solar cell arrangements of either series or parallel. The results showed that the
parallel configured panel provides greater power generation by a factor of two. The
MPPT algorithm that was implemented helped to maintain the output voltage from the
panel at its maximum level [8].
11
The authors designed parallel-configured panels in order to maximize power
output. However, the system could use further improvements. For example, the authors
used only a step-up power converter (boost converter). They did not use a step-down
power converter (buck converter). This means that if the voltage produced by the panel
is over the specifications of the battery charger, then under certain conditions the battery
charger may not perform at an optimal level. Additionally, battery specifications, sun
tracking and equipment costs were not discussed.
2.4.3
A Low-cost Photovoltaic Energy Harvesting Circuit for Portable Devices
In Chung et al. 2011 [9], a low-power battery charging circuit with constant
voltage MPPT algorithm is discussed. The design keeps system costs low by using a
minimum of circuit components. The targeted application is for charging the batteries of
small, portable electronics, such as a cellular phone battery. A polycrystalline solar panel
is used because of its greater power output and efficiency relative to an amorphous solar
panel. The implementation that was demonstrated improved overall efficiencies of
between 80% and 90% [9].
These systems are for charging only low-power, portable electronic devices where
the battery is 5V. Therefore, applications requiring greater voltages, such as lighting,
cannot be supported. Additionally, the designs do not account for sun tracking, so the
user must manually orient the solar panel in order to obtain the best possible light
intensity from the sun.
12
Chapter 3
PROJECT FOCUS AND DIFFERENTIATION
There are many implementations that are suitable for portable solar power
systems. The previous section provided an overview of some examples of the variety of
implementations. From the review, it can be observed that improvements can be made in
terms of cost, efficiency and usefulness. This section discusses the proposed project
solutions and improvements. The design section will provide details of the design’s
implementation.
3.1
Optimization
In this project, a single axis sun tracker is used to enhance the system for
achieving maximum power production. Because the single axis sun tracker itself will
consume power when it is powered by the battery under test, it is important to ensure that
the sun tracker does not consume more power than it enables.
A very effective method that is used to conserve power on the sun tracker is to
have the sun tracker power down into periodic “power saving mode.” When the sun
tracker is calibrated such that it is pointed directly at the sun, it can then go into power
saving mode. After a certain amount of time, an external interrupt can fire in order to
recalibrate the tracker. Because the sun tracker that is used in this project is supported by
an Atmel micro-controller, a power saving configuration can be implemented.
3.2
Cost and Power Savings
The overall cost of this design will be kept to approximatetly $400. This is
achieved by selecting components for both their efficiency and operating life. The power
13
optimization that is described in section 3.1 will contribute to overall cost reduction
because. The battery will produce 18 Ah which is enough for one day’s use of most
portable electronic devices. To charge it fully, it will require two days, given that the
solar panel produces one ampere of current. Compared to other projects mentioned in
section 2.4, the cost for this project is relatively inexpensive.
3.3
Application Usefulness
In certain regions of Africa, the duration of black out can be days. People are not
able to use electronic devices such as laptop and cellphone. If there is an emergency,
they cannot make the phone call. Rural regions in South East Asia, kids are still studying
using their oil based lamp. Camping without electricity is also not ideal. Small electrical
devices will need to be charged. In this project, the designs are geared toward the
applications mentioned earlier. Other applications are also possible.
14
Chapter 4
DESIGN AND INTEGRATION
This section discusses in detail about the design and integration of this project.
The implementations for the single axis sun tracker, battery and battery charger, and
inverter will be discussed.
4.1
Component Selection
The components are selected based on cost and efficiency. The components have
to fit into the goal of this project. They need to satisfy the following characteristics:
4.2

Low cost

High efficiency

One day or higher battery life for portable electronic usage

Buck and Boost charger type with MPPT
Single-Axis Sun Tracker
This section discusses the design and implementation for the single-axis sun
tracker.
4.2.1
Microcontroller
The arbotiX robocontroller using ATMEGA644P microcontroller is selected as
the main controller for the sun tracker. It provides the capability to implement the sun
tracker for this project. It includes support to drive three servos, eight analog and digital
inputs/outputs, and the freedom to implement a software algorithm [10]. Figure 5 shows
the PCB for the arbotiX along with it supported features.
15
1 - Analog port headers
2 - Left motor/encoder headers
3 - Dual motor driver, max current 1A
4 - Right motor/encoder headers
5 - I2C header
6 - ATMEGA644P
7 - Power selection header
8 - FTDI serial0/programming
9 - Digital port headers
10 - Reset Switch
11 - Serial1 header (also J1)
12 - Prototyping headers and user led
13 - XBEE socket
14 - In-system programming (ISP)
15 - 3 Bioloid headers
16 - Power terminals
Figure 5: ArbotiX Robocontroller Using ATMEGA644P [10]
4.2.2
Motor/Actuator
The servo actuator used on the arbotiX is Dynamixel AX-12A Bioloid. This is
the main motor used to rotate the panel 315°. It provides the capability to track speed,
temperature, shaft position, voltage and load. One of the features that will be heavily
used is the capability to read the current location of the actuator. This way, the history of
its location can be stored and later processed. This is an important feature for the
software algorithm. Figure 6 shows a picture of the actuator.
16
Figure 6: AX-12A Dynamixel Actuator [10]
4.2.3
Sensor
A photoresistor is used to detect the sun’s position. It is connected to the analog
input of the microcontroller. Figure 7 illustrates the circuit used in this project to sense
sunlight.
Figure 7: Simple Light Sensor Circuit
The 1k potentiometer is used to calibrate the sensitivity of the signal that is
generated by the photoresistor. With this setup, it is easy to connect to the arbotiX for the
input using the 5V, GND, and signal terminals.
17
4.2.4
Software Algorithm
The software that will be implemented will look for the brightest spot in an open
sky environment while at the same time optimizing system-wide power production.
Figure 8 outlines a simple algorithm. At first, when the system boots up, the actuator is
initialized and begins rotation from its current location to a known location, in this case,
it is the 0° location. Once the actuator reaches 0°, it will start to rotate to the maximum
location which is 315°. While rotating, the microcontroller will start reading the value
from the photoresistor and store the data in an array and the array index is the location for
that specific data. Once the actuator has reached the maximum location, the
microcontroller will search the array for the biggest value and use the array index to
rotate to that location. After the actuator has reached the targeted location, which should
be the bright location based on the value given by the photoresistor, the microcontroller
will put itself into sleep mode to save power. For every hour, an external interrupt is
triggered to bring the microcontroller back to its normal operating mode where it will recalibrate the solar panel to the current sun location. The difference between initialization
and recalibration is that in the initialization, the motor rotates 315° to find where the sun
is located. Whereas during recalibration, the motor is rotates about 180° only because the
solar panel current location does not deviate much from the sun location. Figure 8 shows
the block diagram of the algorithm. It is the main program flow for the software
implementation.
18

Power applied


Rotate motor 0 -315° for
initialization or rotate 0180° for recalibration
Read photo resistor
Store data in array
Find
brightest
area
Rotate motor
Find
brightest
location
from array
Reach
final
rotation
Initialize
motor
Brightest
location
Put system to
sleep
Trigger
external
interrupt
Recalibrate
system
Figure 8: Sun Brightness Detect Algorithm – One Axis Sun Tracking
19
4.2.5
Power Optimization
Because the arbotiX robocontroller with ATMEGA644P is used, power
optimization can be achieved. The goal of this project is to have the greatest possible
battery life coupled with the most efficient system configuration. The single axis sun
tracker will need to power from the main system battery. Therefore, the sun tracker
needs to consume the least power consumption as possible. Because the sun is not
moving very fast, when the solar panel is directly facing the sun after the algorithm from
Figure 8 has been executed, rather than keep pulling the data, it is wise to put the
microcontroller in sleep mode to save as much power as possible.
According to ATMEGA644P specifications, it has 6 sleep modes [11]. Table 1
shows the details of each sleep modes.
Table 1: ATMEGA644P Sleep Modes [11]
From Table 1, power-down sleep mode has the most power saving because most
of the system clocks are off. Thus, power-down sleep mode is selected as the sleep-mode
20
for the sun tracking system. To wake up the microcontroller from this state, an external
interrupt is used.
4.2.6
External Interrupt
The LTC6991 silicon oscillator is used to generate an external interrupt. The type
of oscillator from Linear Technology can generate signals with long time-periods, up to
9.54 hours. This is ideal for this project. The time to generate external interrupt is
scheduled for every 60 minutes so the sun tracking system can re-calibrate to the sun
brightest spot. Based on previous experiments, this is a typical time for re-calibration.
Figure 9 shows the proposed circuit using the LTC6991 chip.
The resistor and capacitor values chosen for the specific period of 120 minutes are
based on the LTC6991 specifications equations and TimerBlox Designer tool that can be
found on the vendor website.
Figure 9: External Interrupt Circuit [16]
21
Figure 10: Expected Output Waveform [16]
Figure 10 shows the expected output waveform. Reset is the active low signal.
The reset is kept high for normal operations. The period is 120 minutes, so half of the
pulse time duration is 60 minutes. Interrupting the microcontroller is edge sensitive. The
rising edge of the output from the LTC6991 will cause wake event. After 60 minutes
(half pulse period), the falling edge will cause similar wake event. This simulate wake
event every 1 hour. This is the expected behavior from this design.
The full one axis sun tracking software code is shown in Appendix A. It is
compiled in the Arduino environment and then sent to the microcontroller board.
Arduino is the main IDE that supports that arbotiX microcontroller board.
4.3
Solar Power System
The integration of the solar panel, battery charger, and battery is discussed in this
section.
4.3.1
Solar Panel
The solar panel vendor selected for this project is Instapark. The panel specifications
are:

Nominal wattage: 20 W
22

Maximum power voltage: 17.5 V

Maximum power current: 1.17 A

Open circuit voltage: 21.95 V
The specifications outlined above give an estimate of about two-days charge time
for an 18 Ah lead acid battery. The solar panel is made out of mono-crystalline, which is
also more efficient when compared to amorphous solar panels. A normal life usage for
amorphous panel is about 5-7 years while mono-crystalline solar panel can last more than
25 years while keeping its efficiency to 80% [12]. The panel is displayed in Figure 11.
Figure 11: 20-Watt, Mono Crystalline Solar Panel [12]
Based on the data given above from the vendor, the following I-V and power curve are
constructed at a certain light intensity level.
23
Figure 12: Solar Panel I-V Characteristic
Figure 13: Solar Panel Power Curve
24
From Figure 12 and Figure 13, it can be seen that the solar panel I-V characteristic is
non-linear. To get the most power, the solar panel needs to operate at the maximum of
the power curve. This corresponds to Vpm and Ipm of 17.5 V and 1.17 A, respectively.
4.3.2
Battery
A sealed, lead acid battery is used in this project. It is selected because of its
availability, low cost and high capacity. Table 2 lists the battery’s specifications.
Parameters
Nominal Voltage
Nominal Capacity
Approximate Weight
Internal Resistance
Shelf Life (77 F)
Temperature Dependency of Capacity
Values
12 V
18 Ah
11.4 lbs.
18 mΩ
3 months – 91%
6 months – 82 %
12 months – 64%
104 F – 102 %
77 F – 100%
32 F – 85%
5 F – 65%
Table 2: Battery Specifications
The battery used is a 12 V battery with 18 Ah capacities. This will provide
practical usage time. The battery physical dimension is 7.15x3.06x6.60 inch. It is a
relatively small battery with large capacity, which is ideal for portability. With the
selected solar panel, it will require approximately two days to fully charge the battery.
Table 3 provides example applications and their durations with full usage until the battery
is depleted.
25
Applications
60 W incandescent
Light bulb
14 W florescent light
bulb
Smartphone charging
Laptop charging
Consumption
(Amperage)
5
Duration
(Hours)
3.6
1.2
15
1
8
18
2
Table 3: Typical Calculated Application Usage Time
4.3.3
Battery Charger
The battery charger is the most important component in this project. It is
important to make sure that the battery is charged properly to ensure correct operation,
high efficiency, and greater battery life.
The proposed battery charger for this project is the TI bq24650. It is a switchmode battery charge controller with input voltage regulation. This means, it has the
capability to reduce the charge current when the input voltage falls below a certain
predetermined threshold. It is an important feature because the PV panel does not always
provide stable voltage. It also can boost the charge current when the solar panel cannot
provide enough current. This is a buck-boost type battery charger.
Constant-frequency, synchronous PWM controller with high-accuracy current and
voltage regulation, charge preconditioning, charge termination, and charge status
monitoring are the highlights of this battery charger. Different charging schemes can be
applied with the bq24650. This means that the battery can be charged in three phases:
preconditioning, constant current and voltage. When the current reaches one-tenth of the
fast charge rate, charging is discontinued. Additionally, this battery charger chip
26
provides a programmable, fast-charge timer for safety backup. The pre-charge timer is
fixed at 30 minutes. The bq24650 implements constant voltage algorithm, the simplest
maximum power point tracking (MPPT) to charge the battery at the maximum power
available from the solar panel.
Charging will automatically restart the charge cycle if the battery voltage falls
below an internal threshold. To save power, the bq24650 enters sleep mode when the
input voltage falls below the battery voltage.
Figure 14 below is the bq24650EVM evaluation board from TI. The schematic is
provided in Appendix B.
Figure 14: TI bq24650EVM Board [15]
From Appendix B, the battery voltage 𝑉𝑜𝑢𝑡 is configurable by changing the value
on R13 and R15. The TI bq24650 battery charger supports 2.1 V to 26 V batteries. For
this application, a 12 V battery is used; therefore, R13 and R15 are set to 499 kΩ and 100
kΩ, respectively, using the equation given by TI specifications:
27
𝑅13
𝑉𝑜𝑢𝑡 = 2.1𝑉 ∗ (1 + 𝑅15)
499⁡𝑘Ω
⁡⁡⁡⁡⁡⁡⁡⁡= 2.1𝑉 ∗ (1 +
) = 12.6⁡𝑉
100⁡𝑘⁡Ω
(2.0)
The maximum power point is set as follow:
𝑅17
𝑉𝑀𝑃𝑃𝑆𝐸𝑇 = 1.2𝑉 ∗ (1 + 𝑅19)
499⁡𝑘Ω
⁡⁡⁡⁡⁡⁡⁡⁡= 1.2𝑉 ∗ (1 +
) = 17.8⁡𝑉
36⁡𝑘⁡Ω
(3.0)
The above resistor values are being set for Vout of 12.6 V and Vmppset of 17.8 V by
default.
4.3.4
Inverter
A typical 500 watt inverter will be used in this project to convert DC battery
power to AC power. This is the simplest way to connect consumer electronics to the
system. See Appendix C for details on the inverter specifications.
4.4
System Hardware Setup
Portability is important in our design. The design is simple using plied wood to
construct the base. The top of the base has the actuator mounted which is then connected
to the solar panel. The photoresistor is mounted on the middle front of the panel. A
picture of the front design is shown in Figure 15.
28
photoresistor
st
1 level – solar panel
mount
2nd level – storage for
battery and inverter
Figure 15: System Front View
From the back, it is possible to see the Bioloid frame that is holding the panel at
45° with respect to the ground. The actuator and the arbotiX board are mounted together
to from the base for the solar panel. The arbotiX board is bolted down to the first-level
plywood for stability, because the solar panel already weights approximately 5 pounds.
During rotation, stability of the system is important. The second level of the system is
where the battery, battery charger, inverter and external interrupt circuit will be placed.
The back and side views of the system are show in Figures 16 and 17, respectively.
29
180°
180°
Bioloid Frame
Dynamixel AX-12A
Actuator
Figure 16: System Back View
Figure 17: System Side View
arbotiX board, using
Atmega644p Microcontroller
30
4.5
System Diagrams
+Vpower_sun_track
Sun
Tracking
Solar
Panel
20 W
Vin+
Vin-
Battery
Charger
TI bq24650
Vbatt+
Vbatt-
12 V Lead
Acid Battery
Vbatt+
Vbatt-
500 W
Inverter
Vout
-Vpower_sun_track
Figure 18: Main System Diagram
The main system diagram is shown in Figure 18. The sun tracking system is
operating at 12 V and is powered by the battery.
+Vpower_sun_track
Sun
Tracking
Solar
Panel
20 W
Vin+
Vin-
Battery
Charger
TI bq24650
Vbatt+
Vbatt-
12 V Lead
Acid Battery
Vbatt+
Vbatt-
500 W
Inverter
Vout
-Vpower_sun_track
Figure 19: During Charge Cycle Diagram
Figure 19 shows the system during charge cycle. The inverter is disconnected
from the system so all the current can be dedicated to charge the battery. The inverter
itself consumes about 350 mA under no load conditions. The output from the battery
charger on a typical sunny day is rated at about 1.1 A. Therefore, it is important to
isolate the inverter from the system to maintain peak charging current.
31
+Vpower_sun_track
Sun
Tracking
Solar
Panel
20 W
Vin+
Vin-
Battery
Charger
TI bq24650
Vbatt+
Vbatt-
12 V Lead
Acid Battery
Vbatt+
Vbatt-
500 W
Inverter
Vout
-Vpower_sun_track
Figure 20: System Normal Operation Diagram
Under normal operation, the sun tracking, solar panel, and battery charger are
isolated from the system. The battery is connected to the inverter to provide maximum
power to the user connected devices. This is illustrated in Figure 20.
32
Chapter 5
SYSTEM ANALYSIS, TESTING, AND RESULTS
5.1
Power Consumption Analysis
In this section, the two main components on the system are measured for power
consumption to make sure it does not weight down the current generated from the solar
panel to charge the battery.
5.1.1
ArbotiX Board Power Measurements
The arbotiX board with ATMEGA644p microcontroller and the Dynamixel AX-
12A are measured for power consumption. Because the microcontroller will be in active
mode during calibration and in sleep mode during stall, it is important to know how much
power it is consuming. The arbotiX board was connected to a DC power supply to
calculate the power consumption. Figure 21 below shows the setup diagram.
Current
meter
DC 12V
Volt
meter
arbotiX
board
Figure 21: Power Measurement for ArbotiX Board
A simple multi-meter is used to measure the power consumption of the arbotiX
board. The results are provided in Table 4.
33
Use Case
In idle mode
During calibration
In sleep mode
Power Measured
150 mA
300 mA
90 mA
Table 4: Power Measured for the ArbotiX Board
Idle mode occurs when the microcontroller is idle and the actuator is idle. The
torque is on at this time from the motor perspective. During sun tracking mode, the
actuator is calibrated to track the sun brightest position. The time to recalibrate is less
than 20 seconds (180° rotation). The time to initialize when the system is first powered
(315° rotation) is under one minute. Sleep mode takes place when the recalibration is
done and the microcontroller is put to sleep until an external interrupt occurs. From
Table 4, it can be seen that the sun tracking system does not consume much current.
During recalibration, it is only consuming 0.3 A and active for less than 1 minute at. The
rest of the time, the system is put to sleep and only consumes about 90 mA. This means
that the sun tracking system only consumes 9% of the current generated from the solar
panel.
5.1.2
Inverter Power Measurements
Using the same method as described above, the inverter is measured for power
consumption. This is illustrated in Figure 22. During the no-load phase, the inverter
consumes 350 mA. This is about 35% of the generated current of the solar panel. As a
result, the inverter is isolated from the system during battery charging phase. This is
illustrated in Figure 19. The inverter is powered on during normal operation only, as
shown in Figure 20.
34
Current
meter
DC 12V
Volt
meter
Inverter
Figure 22: Power Measurement for the Inverter
5.2
Functionality Test Results
This section analyzes the result of different scenarios and cases for functionality
and efficiency.
5.2.1
External Interrupt Circuit Measurements
The external interrupt circuit is designed for low power consumption using the
LTC6991. The circuit period is 120 minutes. The scope shot below shows a pulse
generated every 60 minutes. The rising edge of Figure 23 will trigger a wake event.
Similarly, the falling edge of Figure 24 will trigger another wake event. In total, for
every hour, a wake event is generated to wake the microcontroller out of sleep mode in
order to re-calibrate the solar panel. From the power consumption perspective, the
LTC6991 does not consume much current. From the measurement, it is consuming about
100 uA which is very small when compared to the entire system.
35
Figure 23: External Interrupt Waveform – Rising Edge
Figure 24: External Interrupt Waveform – Falling Edge
36
5.2.2
Charging Data Analysis – Sunny Day – With Sun Track – Default Vout and
Vmppset
It took 13 hours to fully charge the battery from depleted conditions. The
charging process was broken out into day 1 and day 2. Below is the data collected.
Time
Ibatt+
(mA)
Vbatt+
(V)
Pbatt+
(W)
Iin+
(mA)
Vin+
(V)
Pin+
(W)
Efficiency
(Pbatt+/Pin+)
8:00 AM
700
11.4
7.98
400
17.88
7.152
1.115771812
9:00 AM
900
11.82
10.638
550
17.89
9.8395
1.081152498
10:00 AM
1100
11.96
13.156
750
17.89
13.4175
0.980510527
11:00 AM
1020
12.11
12.3522
750
17.89
13.4175
0.920603689
12:00 PM
1030
12.22
12.5866
720
17.9
12.888
0.976613904
1:00 PM
950
12.28
11.666
710
17.9
12.709
0.917932174
2:00 PM
1000
12.4
12.4
750
17.9
13.425
0.923649907
3:00 PM
1000
12.47
12.47
730
17.91
13.0743
0.953779552
4:00 PM
800
12.52
10.016
650
17.9
11.635
0.860850881
5:00 PM
680
12.56
8.5408
500
18.1
9.05
0.943734807
6:00 PM
0
12.56
0
0
0
0
N/A
Table 5: Battery Charging Current and Voltage on Typical Sunny Day – Day 1 (Fall 2013)
Time
Ibatt+
(mA)
Vbatt+
(V)
Pbatt+
(W)
Iin+
(mA)
Vin+
(V)
Pin+
(W)
Efficiency
(Pbatt+/Pin+)
12:00 PM
400
12.55
5.02
280
20.55
5.754
0.872436566
1:00 PM
400
12.6
5.04
300
19.33
5.799
0.869115365
2:00 PM
400
12.6
5.04
280
19.53
5.4684
0.921658986
3:00 PM
0
12.62
0
0
19.5
0
N/A
Table 6: Battery Charging Current and Voltage on Typical Sunny Day – Day 2 (Fall 2013)
37
Figure 25: Charging Curve Measured On Sunny Day
Table 5 shows the data collected on Day 1 and Table 6 shows the data collected on
Day 2. Iin+ and Vin+ are the input current and voltage measured at the battery charger
input terminals, respectively. Together they represent how much power the battery
charger requested from the solar panel in order to provide regulated current at the output.
Ibatt+ and Vbatt+ are the battery charged current and voltage measured at the output of
the battery charger and battery terminal. Figure 25 shows the measured battery charging
curve. The horizontal axis on the bottom represents the time on day 1 and the horizontal
axis on the top represents the time on day 2. Left and right vertical axes represent the
current and voltage, respectively. The most interesting curve is the Ibatt+. The battery
charger started charging at 700 mA. After 2 hours, it increased the charged current to
38
about 1A based on sun ray intensity. Once the battery voltage has reached 12.5 V, the
charged current started to decrease. On day 1, due to the early sun set at around 5:45 PM,
the battery charger automatically turned off due to low illumination. On day 2, since the
battery is already at 12.5 V, the battery charger only charged at about 400 mA. From the
battery specifications, it is recommended to charge the battery at a lower current when
Vbatt+ is already passed 12 V in order to maintain the battery at good operating
condition. After Vbatt+ has reached 12.6 volt, the battery charger automatically turned
off according to the setting in equation 2.0 mentioned in the Battery Charger section.
Overall, the system reached about 90 percent efficiency when compared the power
generated from the solar panel to the power outputted from the battery charger.
Because Vout and Vmppset were not tuned properly based on the battery
specifications (see Figure 12 and Figure 13), the maximum charge current of 1.17 A was
not reach and the battery was not fully charged at the end of day. The battery only gives
a duration of one hour usage time on a 60 W light bulb. Next section shows the result
where Vout and Vmppset are tuned based on the battery characteristics.
5.2.3
Charging – Sunny Day – With Sun Track – Tuned Vout and Vmppset
In this section, Vout and Vmppset are tuned properly to efficiently charge the
battery. From the battery specifications, the battery charged float voltage is 13.6 V to
13.8 V. This represents the maximum charge voltage values that the charger has to reach.
From the equation,
𝑅13
𝑉𝑜𝑢𝑡 = 2.1𝑉 ∗ (1 + 𝑅15)
(4.0)
39
⁡⁡⁡⁡⁡⁡⁡⁡= 2.1𝑉 ∗ (1 +
549⁡𝑘Ω
) = 13.63⁡𝑉
100⁡𝑘⁡Ω
where R13 on the battery charger board is changed from 499k to 549k to give a Vout of
13.63 V.
Similarly, the Vmppset is tuned based on the maxima shown in Figure 13. From the
equation,
𝑅17
𝑉𝑀𝑃𝑃𝑆𝐸𝑇 = 1.2𝑉 ∗ (1 + 𝑅19)
487⁡𝑘Ω
⁡⁡⁡⁡⁡⁡⁡⁡= 1.2𝑉 ∗ (1 +
) = 17.433⁡𝑉
36⁡𝑘⁡Ω
(5.0)
where R17 is changed from 499k to 487k to give the Vmppset of 17.433 V.
From this configuration, the battery charger is at 99.6% of the Maximum Power Point
that is specified by the vendor.
Below is the charging data collected from depleted to full battery level with tuned
battery charger characteristics as described above.
Time
Ibatt+
(mA)
Vbatt+
(V)
Pbatt+
(W)
Iin+
(mA)
Vin+
(V)
Pin+
(W)
Efficiency
(Pbatt+/Pin+)
9:00 AM
800
11.73
9.384
580
17.53
10.1674
0.92294982
10:00 AM
970
12.28
11.9116
720
17.54
12.6288
0.943209173
11:00 AM
1020
12.38
12.6276
720
17.54
12.6288
0.999904979
12:00 PM
1000
12.49
12.49
730
17.54
12.8042
0.975461177
1:00 PM
930
12.63
11.7459
700
17.54
12.278
0.956662323
2:00 PM
920
12.68
11.6656
680
17.54
11.9272
0.978066939
3:00 PM
900
12.7
11.43
680
17.54
11.9272
0.95831377
4:00 PM
700
12.76
8.932
530
17.54
9.2962
0.960822702
5:00 PM
660
12.78
8.4348
520
17.54
9.1208
0.924787299
Table 7: Tuned Battery Charging Current and Voltage on Typical Sunny Day – Day 1 (Fall 2013)
40
Time
Ibatt+
(mA)
Vbatt+
(V)
Pbatt+
(W)
Iin+
(mA)
Vin+
(V)
Pin+
(W)
Efficiency
(Pbatt+/Pin+)
8:00 AM
660
12.37
8.1642
580
17.53
10.1674
0.802978146
9:00 AM
820
12.56
10.2992
600
17.53
10.518
0.979197566
10:00 AM
950
12.78
12.141
730
17.54
12.8042
0.948204495
11:00 AM
1050
13.2
13.86
800
17.53
14.024
0.988305762
12:00 PM
1000
13.44
13.44
830
17.54
14.5582
0.923191054
1:00 PM
910
13.54
12.3214
710
17.97
12.7587
0.965725348
2:00 PM
820
13.56
11.1192
650
18.02
11.713
0.949304192
3:00 PM
750
13.58
10.185
580
18.3
10.614
0.959581685
4:00 PM
650
13.6
8.84
500
18.3
9.15
0.966120219
5:00 PM
420
13.6
5.712
310
18.44
5.7164
0.999230285
6:00 PM
0
N/A
N/A
N/A
N/A
N/A
N/A
Table 8: Tuned Battery Charging Current and Voltage on Typical Sunny Day – Day 2 (Fall 2013)
Figure 26: Tuned Charging Curve Measured On Sunny Day 1
41
Figure 27: Tuned Charging Curve Measured On Sunny Day 2
After charging for two days, the final battery voltage reached 13.6 V. Afterwards,
the battery charger is turned off because Vout has reached 13.6 V. This is expected as
shown in equation 4.0 mentioned above. From Table 7 and Table 8, the efficiency
between the input power from the solar panel and the output power of the battery charger
is kept well above 90%. As a result of tuned Vout and Vmppset, the battery usage time is
changed from one hour (section 5.2.2) to two and a half hour based on a 60 W light bulb
running time.
5.2.4
Charging Data Analysis - Sunny Day – No Sun Track
The purpose of the sun tracking feature of the designed system is to improve the
performance of the battery charger by directing the solar panel to the sun brightest
42
position automatically. The battery charging data below is collected during a sunny day
in Fall 2013 without the use of the sun tracking feature.
Time
Ibatt+
(mA)
Vbatt+
(V)
Pbatt+
(W)
Iin+
(mA)
Vin+
(V)
Pin+
(W)
Efficiency
(Pbatt+/Pin+)
8:00 AM
870
11.7
10.179
610
17.53
10.6933
0.951904464
9:00 AM
940
11.98
11.2612
720
17.54
12.6288
0.891707842
10:00 AM
1000
12.08
12.08
740
17.54
12.9796
0.930691239
11:00 AM
1000
12.22
12.22
740
17.54
12.9796
0.941477395
12:00 PM
810
12.33
9.9873
600
17.54
10.524
0.949002281
1:00 PM
640
12.41
7.9424
480
17.54
8.4192
0.943367541
2:00 PM
410
12.41
5.0881
320
17.54
5.6128
0.906517246
3:00 PM
100
12.31
1.231
90
17.54
1.5786
0.77980489
4:00 PM
n/a
n/a
n/a
n/a
n/a
n/a
n/a
5:00 PM
n/a
n/a
n/a
n/a
n/a
n/a
n/a
6:00 PM
n/a
n/a
n/a
n/a
n/a
n/a
n/a
Table 9: Sunny Day Battery Charging Without Sun Tracking Enable
From Table 9, the data showed the battery charging current performed relatively
well up to 12 PM. After that, the charging current decreases dramatically when
compared to Table 5 and Table 7. This is because after 12 PM, the solar panel was not
positioned correctly relatively to the sun brightest position. Eventually, the sun and the
solar panel are parallel to each other, corresponding to an angle θ of 90° as specified in
equation 1.0 in section 2.1. As a result, the solar panel produced zero power. The
collected data proves that the single axis sun tracking improves the battery charging time
by as much as 2x.
5.3
Future Improvements
Some important improvements can be achieved. First, total cost savings can be
achieved by lowering the cost of the solar panel, battery, and battery charger. A kit was
43
used in this project due to time constrains. The battery charger can be designed and built
rather than purchased as a kit. Second, larger battery capacities and higher-wattage solar
panels are being considered for components of enhanced systems. While greater capacity
components may increase up-front costs, however the benefits of higher charged current,
faster charge time and longer battery life may outweight the increase in cost. Finally, the
constructed base can be improved by using a different material. For example, plexiglas
can be used to provide a more rigid and stronger foundation. This would contribute to
the system’s portability.
5.4
Bill of Materials
The original estimated cost for this project was approximately $300. The final,
actual cost is approximately $400. The cost increase is a result of the selected battery
charger is already 25% of the total cost. However, it is a worthy battery charger because
it provides the necessary buck-boost typed charger with constant voltage MPPT
algorithm. Even though the final cost of the system is $400, it is still well below the cost
of available portable solar power systems, which cost from $500 to $1000 [13].
Furthermore, many of these systems lack features that are provided by this system,
including sun tracking and DC to AC inverter. Table 10 lists the Bill of Materials for this
project.
44
Part
Solar Panel
Battery
Inverter
Arbotix Microcontroller
Actuator AX-12A
TI bq24650
JZ-02 Solar Mounting Kit
Arbotix mounting kit
Descriptions
20 Watt, Mono-crystalline
12 V, 18 Ah, VLAC Battery
500 W, 12 Vdc
Microcontroller for servo
DC motor
Battery charger
Mounting kit for solar panel
Mounting kit for arbotix board
Manufacture
Instapark
Interstate Batteries
Energizer
Arbotix
Dynamixel
Texas Instruments
Instapark
Arbotix
Price
$64.25
$39.99
$39.99
$40.00
$44.90
$99.00
$9.99
$20.00
Bioloid Frame F2
Photoresistor
100k Potentiometer
LTC6991
.1uF Cap
1 Mohm Res
170 Kohm Res
880 Kohm Res
Frame for holding solar panel
onto actuator
Frame for holding solar panel
onto actuator
A generic photoresistor
A generic potentiometer
Oscillator
Breadboard mounted
Breadboard mounted
Breadboard mounted
Breadboard mounted
SOT-23-6 Adapter SMT
PCB to DIP
SMT to DIP adapter
n/a
$1.99
17.5x20.5 inch plied wood
5.5x50 inch plied wood
Misc. jumper cable
Misc. screws, nuts, washers
Plied wood for based construction
System based stands
Cable connections
Misc. hardware
n/a
n/a
n/a
n/a
$15.00
$6.00
$5.00
$5.00
Bioloid Frame F4
Trossen Robotics
$1.75
Trossen Robotics
n/a
n/a
Linear Technology
n/a
n/a
n/a
n/a
$1.49
$1.00
$2.00
$1.57
$1.00
$1.00
$1.00
$1.00
Total
$402.92
Table 10: Project Bill of Materials
45
Chapter 6
CONCLUSION
The goal of this project is to design an inexpensive and portable solar power
system by utilizing a unique, single axis sun tracking device. This goal is achieved by
integrating five key components to create an exceptionally inexpensive and efficient
power-generating system: solar panel, sun tracker, battery, battery charger and inverter.
The single axis sun tracker is designed to use a low-power microcontroller with a
simple actuator and a photoresistor for the main light sensor. Optimization of the sun
tracker is designed to ensure that it consumes a minimum amount of energy. The
optimization is made possible by using an algorithm designed to employ a sleep mode
that enables a search and set routine to recalibrate the solar panel to the sun’s position at
hourly intervals. When activated, the microcontroller is put into a power saving mode
during non-search periods and then awakened at hourly intervals by way of an external
interrupt. Use of the sun tracking algorithm is estimated to save a tenth of a milliwatt
(mW) during each non-search period. Additionally, based on the battery charging results,
the sun tracking feature significantly improves the battery charging time, by as much as
100 percent.
The battery charger is tuned to provide the correct charge voltage. The overall
results achieve a 90% improvement in efficiency between the generated power from the
solar panel and the output of the battery charger. This is with a full-charge duration of
two sunny days. The average battery life when supporting a 60 watt load, from an
incandescent light bulb, is two and one-half hours. Ten hours of battery life can be
46
achieved by using a 14 watt fluorescent light bulb. With this system design, for example,
children in rural areas can enjoy the superior benefits of electric lighting by which to
study, people in Africa can charge their cell phones to make emergency calls during grid
black-outs, and campers can inexpensively charge their portable electronic devices. This
power generation efficiency is made possible by a generating system that costs less than
$400, an amount that is well below the cost of comparable systems available on the
market today.
47
Appendix A
Single Axis Sun Tracker Code
#include <ax12.h>
#include <BioloidController.h>
#include <Motors2.h>
#include <avr/sleep.h>
#include <avr/power.h>
#include <avr/interrupt.h>
Motors2 drive = Motors2();
BioloidController bioloid = BioloidController(1000000);
//analog pins
int sense = 0;
int button = 2;
int rotation[1024];
int recalib[256];
int recalib_loc[256];
int curPos = 0;
int rotate_bright_value = 0;
int rotate_bright_index = 0;
int recalCurPos = 0;
int recalBrightValue = 0;
int recalBrightIndex = 0;
int minimum = 0;
int recalMinimum = 0;
int i = 0;
int j = 0;
int temp_loc = 0;
int flag = 1;
void setup()
{
Serial.begin(9600);
pinMode(sense, INPUT);
pinMode(button, INPUT);
drive.init();
48
TorqueOn(1);
//setting up interrupt on digital pins for AVR board, these are
registers equivalent bits
//these settings are specific to the Atmel 644P that is used on
the arbotiX
PORTB |= 0x1E;
//Pin change for interrupt control register - enables the
interrupt vectors
//Port B uses pin change interrupt vector 1
PCICR |= (1 << PCIE1);
//Pin change mask registers decide which pins are enabled as
triggers
PCMSK1 |= 0x1E;
//enable the interrupt
interrupts();
delay(50);
}
void loop ()
{
if (flag == 1){
initialize_rotate();
searchBrightest_Rotate();
Serial.print("Done Setting Up Solar Panel for 1st time");
}
else{
startRecalib();
initialize_recalib();
Serial.print("Done re-calibrate solar panel");
}
delay(50);
sleepNow();
}
void searchBrightest_Rotate()
{
delay (500);
Serial.print("finding brightest location");
for (i=0; i<1023; i++){
SetPosition(1, i+5);
rotation[i]= analogRead(sense);
49
Serial.print("current brightness is: \n");
Serial.print(rotation[i]);
Serial.print("\n");
delay(5);
}
//find smallest value in array, the smallest value means the
brightest location
minimum = rotation[0];
for (j=1; j<1023; j++) {
if (rotation[j] < minimum){
minimum = rotation[j];
rotate_bright_value = minimum;
rotate_bright_index = j;
}
}
Serial.print("Brightest value is: \n");
Serial.print(rotate_bright_value);
Serial.print("\n");
Serial.print("Brightest location is: \n");
Serial.print(rotate_bright_index);
Serial.print("\n");
curPos = GetPosition(1);
delay(20);
while (curPos < rotate_bright_index){
SetPosition(1, curPos++);
delay(20);
Serial.print ("++++ \n");
}
while (curPos > rotate_bright_index){
SetPosition(1, curPos--);
delay(20);
Serial.print ("---- \n");
}
return;
}
void initialize_rotate ()
{
curPos = GetPosition(1);
Serial.print ("rotating to initial location");
while (curPos != 0){
curPos = curPos - 1;
SetPosition(1,curPos);
Serial.print ("***** \n");
delay(10);
}
50
return;
}
void initialize_recalib()
{
delay(50);
recalCurPos = GetPosition(1);
temp_loc = recalCurPos;
Serial.print("finding brightest location from curent
location");
for (i=0; i<255; i++){
SetPosition(1, temp_loc);
recalib[i]= analogRead(sense);
recalib_loc[i] = temp_loc; //location from current loc
temp_loc = temp_loc +1;
delay(20);
}
//find smallest value in array, the smallest value means the
brightest location
recalMinimum = recalib[0];
recalBrightValue = recalMinimum;
recalBrightIndex = recalib_loc[0];
for (j=1; j<255; j++) {
if (recalib[j] < recalMinimum){
recalMinimum = recalib[j];
recalBrightValue = recalMinimum;
recalBrightIndex = recalib_loc[j];
}
}
recalCurPos = GetPosition(1);
delay(20);
while (recalCurPos < recalBrightIndex){
SetPosition(1, recalCurPos++);
delay(20);
Serial.print ("++++ \n");
}
while (recalCurPos > recalBrightIndex){
SetPosition(1, recalCurPos--);
delay(20);
Serial.print ("---- \n");
51
}
return;
}
void startRecalib()
{
delay(50);
recalCurPos = GetPosition(1);
temp_loc = recalCurPos - 200;
Serial.print ("current position is \n");
Serial.print (recalCurPos);
Serial.print ("rotating a little more from current location to
recalibrate");
if (temp_loc >0){
while (recalCurPos > temp_loc){
recalCurPos = recalCurPos - 1;
SetPosition(1,recalCurPos);
Serial.print ("* \n");
delay(10);
}
Serial.print ("inside ***");
}
else{
while (recalCurPos > temp_loc){
recalCurPos = recalCurPos - 1;
SetPosition(1,recalCurPos);
Serial.print ("$ \n");
delay(10);
}
Serial.print ("inside $$$)");
}
return;
}
void sleepNow()
// put the arbotiX to sleep
{
Serial.println("****Entering Sleep Mode Power Down***");
delay(100);
set_sleep_mode(SLEEP_MODE_PWR_DOWN);
here
sleep_enable();
mcucr register
sleep_mode();
put to sleep
// sleep mode is set
// enables the sleep bit in the
// here the device is actually
52
sleep_disable();
// disable sleep after waking up
}
ISR(PCINT1_vect){
// this code will be called anytime an external interrupt occur
on digital pins
flag=0; //reset flag to zero
}
53
Appendix B
Eval Board Schematic - bq24650EVM
54
Appendix C
Power Inverter Specifications
Specifications
Output continuous Watt
Peak Power
Rated Input DC
Input Voltage Range
Rated Frequency
Rated Output AC
No Load Current Consumption
Output Wave Form
Operating Temperature
Cooling System
Power Factor
Ripple and Noise (Peak to Peak)
Weight
Description
500 W
1 kW
12 V, 54 A
10.5 to 15.5 V
60 Hz
105-125 Vac, 4.17 A
<.3A DC
Modified Sine Wave
0 to 40 C
Fan Cooling, over temperature
.65 Min
<250 mV
1.98 lbs
55
Appendix D
ArbotiX Board Schematic
56
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57
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