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Low cost Sun Tracker for Solar PV powered Vaccine Refrigerator

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Pankaj Deshmukh[1] Electrical Engineering Profession

Contents

Vaccine Storage & Handling Toolkit, May 2014[edit]

This CDC guide tells us that the refrigerator we will be using for vaccine storage should not be dormitory type of refrigerator/freezer. The refrigerator we would use should be standalone refrigerator and maintain temperature between 35°F and 46°F (2°C and 8°C). Setting the average temperature around 40°F (5°C) and maintaining(stabilizing) it around that value is important. Freezing will damage the vaccines, so using standalone freezer which maintains temperatures between -58°F and +5°F (-50°C and -15°C) is recommended.

Medical Refrigeration[edit]

Medical refrigeration is essential to keep the medicines in good condition. Conventional refrigeration technique is not very good solution for medical refrigeration because conventional refrigerators don't maintain a uniform temperature in all different areas of the freezer or refrigerator. Without good refrigeration temperature(ideally should be 4°C) the medicines won't have potency to immunize or prevent the diseases. It will result in a life potentially lost.

Importance of maintaining cold chain cool[edit]

While transportation of medicine and vaccine, it is very important to keep the temperature cold between 2°C and 8°C with low electricity supply and in varying temperature conditions. Due to lack of good cooling system for vaccine and different whether during travel reports indicate that 75-100% of all vaccine shipments are compromised, immunization programs fail and countless lives are lost.

Storage temperature of water[edit]

Before and since the mechanical refrigeration technology was invented ice has been in common use for preventing food spoilage. For the perfect storage solution the water should be kept near +4°C to make sure living organisms and fish could stay alive. Density of water is the maximum at the temperature of +4°C and is equal to 999.9720 Kg/m3. As water has maximum density at 4°C of temperature it sits at the bottom of lake and the warmer or colder water then stays on top of it. Frozen ice is 0°C which floats on the layers of water having descending temperatures from 4°C to 0°C. If the water is warmer than 4°C the warmer water stays on top of water at the temperature of 4°C at the bottom.

Improving the cold chain in developing world[edit]

Vaccine is powerful health tool which saves around 3 million lives every year. Millions of dollars are spent by organizations like GAVI, UNICEF and Gates Foundation on vaccine programs every year. For all immunization and vaccine programs maintaining the cold chain is most important thing. All the vaccines except oral polio vaccine(OPV) should be kept between 2°C and 8°C. WHO has a set of standards known as Performance Quality Safety(PQS) that cold chain equipment should confirm to before it should be procured by NGOs and Ministries of Health. Technologies used in off-grid vaccine refrigeration includes absorption refrigeration, ice-lined mains powered refrigeration and solar powered vaccine refrigerators with battery storage.

Absorption technology has fuel supply problem if the fuel runs out then the vaccines become warmer because of high temperature. It also needs human intervention to check the temperature is maintained inside the storage compartment frequently, which is very inconvenient normal condition and worst in emergency. Ice-lined refrigeration is not suitable because it needs constant power supply. So, if power runs out storage compartment gets warmer. Another problem is different areas of the refrigerator do have different temperature in these kind of refrigerator. So, it needs just one compartments that maintains the temperature between 2°C and 8°C, doing that is not easy with these kind of refrigerators. Solar powered vaccine refrigerators which uses battery to store the power gets exposed to high temperature when battery fails.

True Energy Ltd. did a very good job by developing Sure Chill® vaccine refrigerator. There are two versions of these refrigerators. One is solar powered version and other is mains powered version. It requires only 4 hours of solar irradiation or mains power supply in a day to operate effectively at an ambient temperature of 46°C and 2.5 hours at 32°C. Moreover it can maintain internal temperature at favorable point for 247 long hours without power supply which is referred as holdover time(time for which compartment can maintain temperature below 10°C). It is energy efficient as it uses 1.00 - 1.15 kW/h per day at 43°C once internal energy store is fully charged. Temperature variation in the compartment is less than 0.5. It doesn't freeze vaccines or medicines so risk is eliminated.

The refrigerator works on patented technology which uses the principle that water is at its densest at 4°C. The water jacket around the storage compartment helps in maintaining the temperature around 4°C(typically between 2°C and 8°C). Water at 4°C stays at the bottom of water jacket and at the top the water freezes to ice which prevents the water down form freezing and provide perfect temperature condition to the compartment. So, no matter what are the external temperature the refrigerator maintains a good temperature in compartment for vaccine storage.

Rechargeable Lithium ion Battery and Charging[edit]

Lithium ion batteries provide twice the power than Nickel-cadmium and four times the lead acid batteries. The only problem is that these kind of batteries require voltage level, temperature and current monitoring circuits during charging and discharging for safety and protection from damage. So, depending on the application appropriate charge controller and monitoring device should be used or built by the hardware developer. Battery run time and charge time depends on the battery power pack, system output and input requirement.

Why use Li Batteries[edit]

All kind of fuel cells and batteries are electrochemical storage devices. Batteries replace mechanical power systems which uses fossil fuel. So by replacing mechanical power systems with batteries we can make the earth carbon dioxide foot print as less as possible. The only problem with using the batteries is its low energy output capacity as compared to mechanical systems. But Li batteries have higher energy storage capacity than other kind of batteries. Also research on increasing the energy density of Li batteries is actively going on using some novel methods discussed in this paper. With solid state Li-ion batteries based on either dry polymer or inorganic electrolytes and nano structuring of ion conducting polymer membrane the safety of the battery can be improved with high energy systems also. The cost of the batteries is dependent upon the material-synthesis process, its abundance and renewability. Also the raw material should be cost effective to process. The electrodes are produced using bio mimetic process which are cost effective. Li-ion battery technology is green and sustainable because of the fact that the electrodes made by the organics material which has low CO2 footprint while recycling can even further reduced in the future. Manufacturing Li-ion batteries with electrodes designed from nano-composites/nano-architectures by using rapid screening of new catalysts having special shape, structures and morphology, has succeeded in preparing electrodes that show 1000 mAhg-1 capacity which is 6-8 times more than today's electrodes and have less CO2 emission.

Maximizing capacity of battery pack by cell balancing[edit]

The capacity of the battery degrades over the time if the energy storage capacity of one of the battery is lower than the other. If the capacities of the batteries are matched then the battery pack can maintain maximum power. In battery pack the cell are connected in series and if the individual capacity of the batteries mismatch then overall storage capacity degrades over the time. Battery pack capacity could reduce by 13 - 18 %. To maintain the capacity of individual battery, the voltages across the batteries should be monitors and then if there is difference between then then a shunting circuit activates and tries to minimize the difference between charging and discharging voltages over different charging discharging cycles. A decent amount of cell balancing is anywhere from 10% - 20%. Using cell balancing technique improves the capacity of battery pack with the system run time and life of the battery pack.

Battery state-of-charge determination and their application[edit]

State-of-charge is another problem which should be taken care of with over voltage protection and deep discharge. Detecting state-of-charge gives us idea of how long do we have the device to stop working. The SOC is defined as "the ratio between the difference of the rated capacity and the net amount of charge discharged from a battery since the last full SOC on the one hand, and the rated capacity on the other hand". This means the state has reached full SOC when (according to DIN 43539)the battery current is not changing during 2 hour at constant charge voltage and constant temperature.

Methods of determining the state of charge
Discharge test under controlled condition: This is too time consuming for most of the application because it include consecutive recharge.

Ampere hour counting
This is the most common technique for calculating SOC. Maximum power can be maintained by balancing the battery current. If a starting point (SOC0) is given, the value of the current integral is a direct indicator for the SOC. Current loss calculation has to be taken into account while calculating Ampere hour counting. This method is low cost method and useful for PV applications.

Measurement of the electrolytes physical properties
Only vented lead-acid battery could use this method. The relationship between the change of acid density and the SOC can be used to determine the SOC. The density could be measured directly or indirectly by ion-concentration, conductivity, refractive index, viscosity, ultrasonic, etc. This method has some maintenance problems so its not a good idea to use it for PV application.

Open Circuit Voltage
This is usually used for VRLA batteries. Open circuit voltage relates in a linear manner to SOC. This measurement is done during the rest period. Rest period is when the battery is not charging or discharge, so it doesn't happen quite often in VRLA batteries and has to be combined with other SOC measurement techniques. But the problem is VRLA batteries also need monitoring of acid concentration and acid stratification which can generate inaccurate results. Other drawback is rest time could be so long that SOC monitoring could be almost meaningless.

Heuristic interpretation of measurement curves
There are several heuristic methods discussed in this paper but Artificial neural network method is the one which could be used with any type of battery and application given that training data for the net is available. So every time the error should be calculated between training data and measured data to know the battery SOC.

Impedance spectroscopy
This method compares the measured impedance with the Nyquist plot available for the lead-acid battery. The impedance depends on temperature and if the temperature is maintained in favorable condition then only this method is good to use. Because of this it is not a widely used method and not good for PV systems.

Internal resistance
The internal resistance is the voltage drop divided by the current change during the same (short) time interval which should be less than 10ms. If the interval is increased then impedance spectroscopy has to be used for good measurement which is not a good option for variable temperature environment application.

Kalman filters
A Kalman filter is an algorithm to estimate the inner states of any dynamic system by doing repeated measurements rather than relying on a single one. In our case, the battery is the dynamic system and one of the inner states is the SOC. This method is good for PV system if the measurements are done using appropriate circuit.

Solar photo voltaic charging of lithium-ion batteries[edit]

The high system efficiency can be achieved by directly charging the battery from the PV system with no intervening electronics, and matching the PV maximum power point voltage to the battery charging voltage at the desired maximum state of charge for the battery.

On site solar direct current charging
System Efficiency = PV efficiency x DC charging efficiency.

If more circuitry is added to the system then efficiency of new added circuit gets multiplied in the system efficiency equation which makes efficiency lower because the added circuit can't have 100% efficiency due to power usage.

Solar energy to battery charge conversion efficiency

in % Solar energy to battery charge conversion efficiency
Efficiency = (average voltage(V) x Charge increase(Ah))

(avg. solar irradiance(W m-2) x PV area(m2) x time interval(s) x 1/3600(sh-1))

Example Calculation

in % Charge Efficiency
Efficiency = 100% x 0.5 x (V2 + V1) x (Charge2 - Charge1)

([time between V1 and V2(s)]/3600s h-1 x PV area(m2) x time interval(s) x 1/3600(sh-1))

Lithium-ion capacitor[edit]

Lithium-ion capacitor

Specific energy 11–14 W·h/kg
Energy density 19–25 W·h/L
Specific power 160–2800 W/kg
Charge/discharge efficiency 95%
Self-discharge rate < 5%/month (temperature dependent)
Cycle durability >10,000
Nominal cell voltage 2.2–3.8 V

Lithium-ion capacitors can be used for solar power, wind power and other power generation projects.

Lithium-ion battery[edit]

Lithium-ion battery

Specific energy 100–265 W·h/kg or (0.36–0.95 MJ/kg)
Energy density 250–620 W·h/L or (0.90–2.23 MJ/L)
Specific power ~250-~340 W/kg
Charge/discharge efficiency 80 - 90%
Energy/consumer-price 2.5 W·h/US$
Self-discharge rate 8% at 21 °C, 15% at 40 °C, 31% at 60 °C(per month)
Cycle durability 400–1200 cycles
Nominal cell voltage NMC 3.6 / 3.7 V, LiFePO4 3.2 V

Solution for refrigeration[edit]

Sun Frost refrigerator Runs on either 12 volt or 24 volt DC. Uses fewer solar panels and batteries to purchase, transport, and install. It has rugged construction which means it is mechanical shock proof and has whether proof insulation. Cabinet is rust proof because it is made of all copper cooling system which prevents corrosion. Defrosting is easy, temperature control is excellent and compressor is only moving part. It runs only runs only 27% of the time in a hot 43° C (110° F) environment. Most efficient DC vaccine refrigerator in its size range. With no poser it could run for 9 days.

Sun Frost refrigeration cost

Price tag is $2,335.00. But it comes with full battery storage and solar panel. Installation help is also provided. Good thing is that it is approved by World Health Organization.

Sure chill ultimate tech solution

Specifications
MODEL BLF 100 AC (Mains) BLF 100 DC (Solar)
Size (cm) W: 89cm, D: 76cm, H: 185cm W: 89cm, D: 76cm, H: 185cm
Gross storage capacity (litres) 124L 124L
Vaccine storage capacity (litres) 99L 99L
Holdover time 43°C 250 hours(>10 days) 184 hours(>7 Days)
Holdover time 32°C 309 hours (>12 days)* 288 hours (12 days)*
Power consumption 43°C < 4 hours supply per day
32°C < 2½ hours supply per day
Solar array 200W to 370W depending on geographical location
Minimum rated ambient temperature 10°C 5°C
Refrigerant R134a
(CFC free)
R600a
(CFC/HFC free)
Insulation CFC/HFC free CFC/HFC free
PQS code E003/013(WHO) E003/019(WHO)

Sure chill is bigger than Sun Frost and holdover time is also greater than Sun Frost. But the cost is of Sure Chill is more than double as compared to Sun Frost $5,077. Though if we compare the volume then Sure Chill is more than 4 times the size of Sun Frost. So Sure Chill is more Efficient and cost efficient. But if we need small refrigerator then Sun Frost is a better choice.

Low cost vaccine refrigeration --[2]

Following are the normal refrigerators which are designed to maintain the temperature between 2℃ - 8℃ and use 60Watt - 70Watt power.

Specification
BT-5V48 MPC-5V48
2℃ - 8℃ 2℃ - 8℃
48/62 litre 48 litre
Pharmacutical Freezer/refrigerator Pharmacutical Freezer/refrigerator

Both these refrigerators cost around $300. Both of them do not run on DC power. They only perform good with constant supply. These type of refrigerators need to be monitored constantly, so both have high and low temperature alarm system. These require lot more power than Sure Chill or Sun Frost and are not certified by WHO for maintaining the cold chain.

Battery Pack solution[edit]

Most of the refrigerator run on 12V DC. So there are various 12V batteries which are available in the market with different storage capacities. mAh is the measure of capacity of the battery. 2000 mAh battery means the battery will discharge a 2A of current over an hour of time. According to the specific PV application the battery required will be having different capacity. Battery cost ranges anywhere from $15 to $100.

Lithium ion battery pack

The battery available on adafruit store gives 3.7 volt output with 2000mAh of capacity. It normally delivers 0.5 A of charge/discharge current and Charge cut off voltage for the battery is 4.2 V.

So there are plenty of options to select from in the market for choice of lithium-ion battery for low cost applications.

Hybrid Capacitor solution[edit]

Lanzar VCAP1200 Vector 12 Farad, 16 Volt Hybrid Capacitor is an example of the capacitor which could be used for power storage. The only thing one needs to keep in mind is super capacitors/hybrid capacitors can store less amount of power as compared to lithium-ion batteries or any other battery type. So power storing capacitors are only good for the application where one needs low power/small current and relatively fast discharge than lithium ion batteries. If someone still wants to use capacitors because of its compatibility to their application, but needs more power then number of capacitor/capacitor banks being used should be increased. Using capacitors is not cost efficient in PV systems. But if cost is not a problem then it is a good option.

Increase Solar Charging With An MPPT Power Tracking Charge Controller[edit]

Solar panels are used to track sunlight and heat and output power in terms of voltage and current. This is a webpage which explains why MPPT is essential to use in a battery charger. When we track sunlight using solar panel the heat and solar energy are trapped in solar cells and it gives rise to voltage and current. The amount of current and voltage we are getting depends on whether conditions also. Colder the whether more input voltage will be required which means that with the changing input voltage requirement the input and output impedance has to be matched to get the max output power. So MPPT does the same thing it matches the input and output resistance/impedance to get the maximum power output of solar panel which is given as input to the battery.

Lithium-ion capacitors can be used for solar power, wind power and other power generation projects.

Solar charge controllers for Lithium-ion, NiMH, NiCad[edit]

This charge controller is a general hardware solution which can be used as solar power charger for different kind of batteries. It has a MPPT tracker and can be programmed for different charging applications. All monitoring conditions can be monitors from remote location as it can communicate via serial channel. The same serial channel can be used to program the controller. As the micro controller has ADC and digital input/output port/ports it can used to measure battery charge termination measurement, capacity testing, battery quality testing, electrolysis, sensor control, etc. Typical programming fees are $500 to $2000 depending on complexity and test requirements for a particular application.

Input voltage Output Voltage Output Power
8-48 volts nominal, higher range possible 2 volts to 80 volts available 30 watts, 50 watts or higher depending on design

cost of controller

This is SEPIC(single ended primary inductor controller) topology DC/DC converter programmed to perform as a battery charger. It has low voltage cut-off and high voltage protection protection facilities on board. Single board costs $80. It has appropriate specifications for lead-acid battery chargers.

Solar tracking Concepts[edit]

Concepts on Solar Radiation

Declination Angle : The declination angle is the angle between the equator of the earth and a line drawn from the center of the earth to the center of the sun. The maximum declination angle is 23.45o on earth.The declination angle δ varies seasonally due to the tilt of the earth on its axis of rotation and the rotation of earth around the sun. The declination angle would be zero if the earth were not tilted. The declination angle varies between +23.45o to -23.45o. Only at equinoxes declination angle becomes zero.

Hour Angle : The hour angle is angular distance earth has rotated in a day. It is equal to 15o multiplied by the number of hours from local solar noon. Solar hour angle is zero when the sun is straight, negative before noon and positive after noon.

Solar Altitude(θZ) : The solar altitude is a vertical angle between the horizontal surface of earth and the line connecting to the sun. At sunrise and sunset the altitude is zero and is 90o when the sun is at zenith. Altitude relate to latitude of the site, declination angle and hour angle.

Solar Azimuth(θA) : The azimuth angle is the angle between the horizontal plane measured from true south or north. If someone lives in northern hemisphere and assuming zero azimuth to be south, azimuth will be negative before noon and positive after noon.

Single Axis and Dual Axis Tracker : Single axis tracker either tracks altitude or azimuth. Usually parabolic trough system tracks azimuth angle and rooftop system uses altitude tracking because lack of space. Other type of tracker is a dual axis tracker and tracks both azimuth and altitude angles of the sun.

Declination angle variation[edit]

For the calculation of the declination angle we have to consider that the earth is stationary and the sun is revolving around the earth. So for this type of calculation the co-ordinates have to be shifted under which the sun revolves the earth.

Changing Declination Angle

On summer solstice in the northern hemisphere the declination angle δ is at its maximum at 23.45o.

On spring equinox in northern hemisphere and autumn equinox in southern hemisphere declination angle δ is 0o.

On winter solstice in the northern hemisphere and summer solstice in the southern hemisphere declination angle δ is -23.45o.

Declination angle can be calculated by the equation :

δ = sin-1( sin(23.45o)sin((360/365)(d - 81)) )

where d is the day of the year with Jan 3 as d = 3. Power on the horizontal is maximum on day 81.[3]

Declination is zero at the equinoxes which come on March 22 and on September 22, positive during the northern hemisphere summer and negative during the northern hemisphere winter. The declination reaches a maximum of 23.45° on June 22 (summer solstice in the northern hemisphere) and a minimum of -23.45° on December 22 (winter solstice in the northern hemisphere).

Solar Tracking System Using Stepper Motor[edit]

This solar tracking system project was developed using ATMega16 microcontroller, LDR based maximum light intensity tracking and PV panel position control using Stepper Motors. Usually 30% to 60% increase in output power has been observed if the solar power system uses tracking as compared to the power system that doesn't use sunlight tracking. MPPT only tracks maximum output coming out of solar panel it can't help in getting more power input through out the day.

Light dependent resistors : There are two types of LDR CdS(cadmium sulfide) and GaAs(Gallium arsenide).LDR decreases its resistance as it is incident with light intensity. These sensors have saturation limit for light intensity. Beyond that light intensity LDR stops decreasing it resistance. Light intensity is measured in Lux, the illumination of sunlight is approximately 30,000 lux. Two such sensors had been used in this project to to track the sun light direction. If there is difference between the light intensity incident on both the resistors then the panel moves in the direction where it would get maximum light.

ATMEGA16 microcontroller : The micro controller has analog comparator inputs which can be used to compare the signals coming from the two LDRs and the output signal can be given to the stepper motor controller which generates logic sequence required for operation of stepper motor to be driven in particular direction.

Operation of the Solar Tracker : In normal day light condition the solar tracker will move from east to west. As 150o rotation is considered as maximum rotation, the micro controller will count 40 counts for 150o rotation and as soon as count becomes 40 the tracker stops rotating in west direction and rotates all the way to the 0o position. In bad whether condition, as the differential voltage doesn't go above the threshold, the micro controller is programmed to monitor this condition by checking the voltage every 1.5 minutes. Micro controller counts to 10 to make it 15 minutes and then rotates the panel through one step.

For protecting the PV panel surface from dust and avoiding the possible 50% loss of power because of dust the panels were fitted with brush rollers which would clean the surface twice during 24 hours every day. As this project uses single axis tracking, 20% increase in power was observed.

Motor selection for sun tracker[edit]

For solar tracking application the motors used has to perform good in all whether conditions and prevent any adverse effect like corrosion, wind loads and abrasive airborne particulate matter. Some of the modern motors also can communicate between them thereby reducing overall system cost and total cost of ownership (TCO).

Ac induction motors are not efficient and are very difficult to control at low speeds and that's why these motors are not used anymore after early solar tracking systems.

Stepper motors lose their economic benefits as we add more control components. Also there is an issue of motor binding because of very small air gap between stator and rotor if there is a temperature difference between them because of sunlight falling on some part of the motor. As the speed is limited there is problem with quickly moving panels in stowing position.

Permanent magnet brush dc motors (PMDC) have only one problem of brush or commutator wear. Its fast, controllable, efficient, and durable . Also good for stowing because wide range of speed.

Brushless dc (BLDC) motors have low TSO and maintenance free. No wear as it has no brushes for contacts. Highly efficient 85 - 90% and hits 3000 rpm which is advantageous in emergency stowing condition.

Various intelligent monitoring and control system can be integrated with BLDC for advanced, fast and efficient tracking systems using modern communication and networking methods.

Stowing the trackers in Winds[edit]

Solar trackers have to endure the effects of wind load in storms. The tracker structure may not have sufficient strength to withstand the elements, so its necessary to move and maintain a good stow position during winds.

Effects of Winds

In emergency high speed wind situation the solar panel boards can bend and result in less efficient or unusable system because of the different load condition on the edges of the solar panel board during the wind. Wind behavior tend to be varying depending on different fields where these panels are mounted.

The stow position

Theoretically effect of wind on a parallel plane is zero and the logical implementation of the board is in a horizontal position. But problem arises with this orientation because wind is not always horizontal. But because of wind at angle 20 to 30 degrees the dynamic pressure could be 105kg/m2 at 1.4 wind factor and 45kg/m2 at 0.6 wind factor on board's edges.

Panel weight and overall load

If the panel is placed as vertical as possible then panel weight becomes more equal on the surface and more endurable than in horizontal position.

Lower structure ensure the stability of the system with load but the tracking system should be able to go to more vertical position as possible. The solution for this is that the calculations must be performed keeping in mind the type of land , location, unique feature, unpredictable dynamics of wind, overall weight of system and structure design for a more stable system design which withstands the maximum wind condition.

Should we use ready made sun tracker for small application?[edit]

There are various micro controller based tracking solutions available in the market. Arduino Sun Hervester Shield V2 alone costs $55 which we have to use with arduino controller which costs around $25. Apart from this we need to buy motors and charger circuit or fabricate a charger circuit which would cost anywhere between $50 to $100 or more depending on particular application.[4] [5]

Obviously it doesn't make any sense to spend 100's of dollars if it is a small and simple application. If the system is more complex and multi tracker based system then using the above products could be sensible. Its always better to find some low cost alternative solution with simple electronic design with less complex circuit design which uses low power and is more efficient to use with PV system.

Reducing cost with low cost micro controllers[edit]

Arduino is an open-source electronics prototyping platform based on flexible, easy-to-use hardware and software. Lot of designers and hobbyists are using these micro controller boards. Developers can use these boards with arduino shield which are separate modules providing hardware capabilities like temperature sensing, wireless communication, etc. Using Arduino uno or Arduino pro would be a good option.

Arduino Uno costs $24.95 and Arduino Pro costs $14.95. Arduino Uno is faster but Pro which has 8Mhz speed is also enough fast for solar tracking application.

These boards can be programmed using Arduino IDE software. It is very easy to write programs using this software as wide range of libraries are available for various sensor applications. [6]

Low power, low cost micro controllers[edit]

MSP430 LaunchPad gives developers low cost low power option. These are very low power consuming processor boards with high speed processing micro controller boards. These processors are specially designed for applications like Energy Harvesting, Portable consumer, medical, security, Smart grid and sensor monitoring.

Energia is open source software to program MSP430 Launch Pad boards. It is very easy to use and has lot of online examples for coding and large library available.

Code Composer Studio (CCS) is TI's software development environment for writing and debugging programs on MSP430 Launch Pad. Code Composer Studio supports all of TI's embedded processing and debugging capabilities. Free evaluation versions are available for 30 day trials.

There are more software code compilers available namely IAR workbench, Keil, MSPGCC and Mentor graphics which are more like code composer than Energia.

MSP430 LaunchPad can be used with modules which add more hardware capability like wireless, LED lighting, etc. These modules are called as BoosterPacks.

MSP-EXP430G2 LaunchPad comes with internal temperature and smoke detectors. Its cost is just $9.99.[7] And it can be used with various kind of BoosterPacks.

H-Bridge DC motor driver[edit]

H-Bridge Drivers are needed for driving the motor in both the clockwise and anti-clockwise directions. It is possible because of the bi-directional current flow design of the circuit. The Leegoal L298N Stepper Motor Driver Controller Board Module comes with a price tag of $4.99.

H-Bridge consists of transistor switches which changes state between on and off to flow the current in particular direction.[8]

Low cost H-Bridge DC or 4 wire 2 phase Motor Driver[edit]

SODIAL(R) L9110S DC Stepper Motor Driver H-Bridge For Arduino 800mA 2.5-12v TTL/CMOS is a H-Bridge motor driver with a price of just $2.49. This module can simultaneously drive two DC motors or a 4-wire 2-phase stepper motor. Works under low static current which means it consumes low power. Power supply varies form 2.5 V to 12 V. Input levels match to TTL/CMOS output because of which it can be connected directly to the CPU. Which means no extra power consumption for power level conversion. Can drive 800mA of continuous current per channel. Compatible with low power micro controller boards.

Step towards low cost sun tracking[edit]

This solar tracker is good example for the power systems that could be used for portable power applications which require 12 volt range of power input. The support and the pivot are made of still and aluminum to make it low weight system. Stepper motors are used to move the PV panel support in particular direction. It uses worm gear system and timing belts for translating the rotating motion from small gear to bigger gear.

System uses LDR's to track the sun and orients the PV panal in the solar radiation direction. Arduino Uno has been used with power regulator, H-bridge driver ICs to get the operation of the circuit as expected for tracking the sun.

Structure design uses bar, plates and angle with some bearing for the support of the pivot. This is a dual axis tracker so it move in both Azimuthal and Zenithal direction. The weight of the tracker has been kept low so that the could have low specifications and would be cheaper. The system uses limit switches to turn off the rotation in E-W direction on both ends.

Solar Panel mount[edit]

Changing Declination Angle

1. Because of its unique design and few installation steps installation time reduces
significantly to half the time of competitive products.

2. Requires only 7 parts to install and only 1 wrench required.


3. It has flexible design to meet all weight, height and wide range of wind conditions
and accommodate majority of PV modules.


4. Its is also highly compatible for roof mounting with or without penetrations.


5. Design saves up to $10k on 250kw project.

Why use 3D printing[edit]

Contrary to traditional manufacturing technique where material gets subtracted from a block of material to form a design 3D printing uses additive manufacturing. 3D models are digital files so these could be sent anywhere around the world over the internet. On demand production can be done without inventory management. For printing new type of design each time no new setup is needed. It has a potential to reduce global economic imbalance as every country could produce their own products. As no fossils are used for 3D printing carbon footprint decreases which means 3D printing is completely green technology. Production and distribution de-globalizes as production is brought near to consumer. 3D printing reduces steps like casting, forming molding and machining which require computer and robots which is used in traditional manufacturing.

3D printing process: A 3D CAD model is prepared which then could be converted to a stereo lithographic file. A 3D stereo lithographic file can be sliced into layers. These layers then get printed using 3D printer which additively prints each layer on top of the earlier one to print a 3D object.

Disadvantages : Some of the 3D manufacturing methods require high powered energy sources e.g. metal 3D printing. Injection molding is quick and produces several objects while 3D printing takes longer time almost an hour to print 1.5 inch cube with good quality print. There is lack of good material and methods to produce objects with 3D printing which could have more strength and speed of production.

Advantages : Best option for rapid prototyping as it just require 3D modeling and printing. Lot of metal objects are being 3D printed using metal 3D printers which are fully functional design. So no need of extra processes. 3D printers are available for home now-a-days with a cost of under $1000 approximately. Very complex parts also could be printed with just one process. Digital design and manufacturing ensures overnight production without monitoring. Complexity is free, which means there is no extra cost for production for complex objects. Instant production anywhere in the world is possible. As this is a additive process it doesn't produce any waste once the printer is well calibrated.

Plastic 3D printed parts are much cheaper than metal 3D printed parts. So for applications where metal parts are not necessary plastic 3D printed part can be used as low cost solution. That way cost reduction for that part could be 80-90%.

Selection of Solar Panel[edit]

For high power application or low power applications using 30 Watt solar panel is better option. Reason for this is that the 30 Watt solar panel is best fit if we need to mount it in array because of its size and if it is considered for low power application then using a single 30 Watt solar panel is good option as it is light weight and gives significant power for household or portable applications. Following table shows the price comparison of the solar cells available in the market.

Company name Model name Power(W) Price($)
General Electric GEPV-030-M 30 Watt Commercially available on contact **
Suntech Power Suntech STP030P 30 Watt $99.00 **
Tektrum Development Corporation HQRP 30 Watt $149.91 **
Tycon Power Systems TPS-24-30 30 Watt $169.95 **
British Petroleum BP-330J 30 Watt $284.95 **
Goal Zero Boulder 30 30 Watt $199.99 **

** Prices keep on changing.

Understanding Photovoltaics[edit]

There are two types of solar technologies concentrated solar power(CSP) and solar Photo voltaic technology. Solar photo voltaic is a popular technology based on photo diodes which produce power when hit by photons from sun whereas in CSP the solar heat is captured using parabolic reflectors and focused on the rod or pipe through which liquid flow and conduct heat to run the steam or gas turbines. Because the power can be produced in any scale it can be used for home utility and Grid like large applications and to small applications like mobile computing.

Currently 90% of the market is dominated by crystalline silicon solar cell. Its just a photo cell with larger photon receptor area which converts light into electrical energy. Maximum power of the PV is the product of Imax and Vmax. Maximum power point point is a point where the power(V-I) curve becomes squared for given irradiance.

Fill Factor, FF = A2/A1 see figure 4 in -- [9]

Efficiency = Pmax/Pein(light irradiated power) = (Voc x Isc x FF)/Pein

Cell Structures : These are multi-junction solar cells made by stacking individual single junction cells with the band-gap descending from the top to the bottom of the cell. Designing cells this way makes the highest energy photons absorbed by the top junction and the lower energy photons transmitted to be absorbed by the lower band-gap solar cells. Multi-junctions are usually built using compound semiconductor hetero-structures using III-V[10] and II-V materials. Concentrated PV are emerging cells which uses multi-junction with intensity 10-1000 times of traditional cells because of which it enjoys lowest cost per watt but currently costs higher. In future the cost of new systems will go down making $/W lesser and efficiency will increase.

Solar Panel Power Characteristics, Shading Effects and Tilt Angles[edit]

The current and power output of the solar panel depends on the internal resistance of the load and the intensity of the solar radiation. Depending on battery's internal resistance the operating voltage of the solar panel will change(lower) than the max voltage specification. The current will be the same always which the panel is suppose to produce, e.g. If the panel is 30 Watt panel, the current will be 1.5 ampere for both 20 V and 10 V output voltage producing 30 watt and 15 watt power.

PV panels face shading due to soft or hard sources like tree branch, roof vent, chimney or blanket, tree branch, bird dropping sitting directly on top of the glass. If even one full cell is hard shaded, the voltage of that module will drop to half of its un-shaded value in order to protect itself. If enough cells are hard shaded, the module will not convert any energy and will, in fact, become a tiny drain of energy on the entire system.

Partial shading will reduce its power output if even one cell on a 36-cell solar panel is soft shaded. As all cells are connected in series the total amount of power level will go down by the amount of powered reduced from the cell which is in shade.

When a full cell is shaded it consumes energy produced by rest of the cells on module. The solar panel will route the power(more specifically current) around that series string. If one full cell in a series string shades it will likely cause the module to reduce its power level to half of full power available. If a row of cells on the solar panel is fully shaded the power output may drop to zero because now the power on the solar panel is not enough to drive the current through the shaded region. The best way to avoid a drop in output power is to avoid shading whenever possible.

Carbon Fiber and ABS filament for 3D Prints[edit]

For 3D printing a new type of filament is available in the market produced by Filabot, made from two strong materials Carbon Fiber and ABS plastic, to make an even stronger combination. They mix a Carbon Fiber Masterbatch and our ABS pellets together to a 5% Carbon Fiber ration and extrude that into filament. Even better it still prints just the same as normal ABS.

Design and development of a low-cost solar tracker[edit]

This low cost solar tracker uses maximum power point tracking for maximum electrical output and sun tracker for maximum incident light through out the day which gives maximum overall efficiency for the solar power.

Crystalline silicon solar cells have theoretical maximum efficiency of 28%.

Sun Power Corp. has already achieved an efficiency of the panels of 24%.

Using MPPT increases the gained efficiency to about 20% - 30%.

Annual gains of energy of solar trackers over fixed panels are around 30% - 50% depending on the site of installation and type of tracker.

Photovoltaic Systems Maximum Power Point Tracking[edit]

1. Constant Voltage
The constant voltage measured which represents VMP is the simplest method. VMP value can be programmed by an external resistor connected to a current source pin of the control IC. In this case, this resistor can be part of a network that includes a NTC thermistor so the value can be temperature compensated(which means temperature variation won't have any effect on VMP measurement). This method can collect upto 80% of maximum power.

2. Open Circuit Voltage
This is indirect calculation of VMP by calculating VOC. The k value which is a function of the logarithmic function of the irradiance, increasing in value as the irradiance increases. The k value is typically between 0.70 to 0.80
VMP = k x VOC
Frequent VOC sampling helps in correcting its value with change in temperature and radiance.

3. Short Circuit Current
This is a similar method to Open Circuit Voltage only difference is voltage replaces current and k value varies from 0.9 to 0.98. IMP = k x IOC
This method uses a short load pulse to generate a short circuit condition. During the short circuit pulse, the input voltage will go to zero, so the power conversion circuit must be powered from some other source. One advantage of this system is the tolerance for input capacitance compared to the VOC method.

4. Perturb and Observe
Perturb and Observe (P and O) searches for the maximum power point by changing the PV voltage or current and detecting the change in PV power output. In this step size should be optimal so that it reaches the maximum power point quickly and stops perturbing soon.

5. Incremental Conductance
Incremental conductance (IC) locates the maximum power point when:

(dIPV/dVPV) + (IPV/VPV) = 0.

This condition simply states that the maximum power point is located when the instantaneous conductance,(IPV/VPV), is equal to the negative value of incremental conductance,(dIPV/dVPV). The IC uses a search technique that changes a reference or a duty cycle so that VPV changes and searches for the condition of above equation and at that condition the maximum power point has been found and searching will stop.When the left side of Equation 5 is greater than zero, the search will increment VPV. When the left side of above equation is less than zero, the search will decrement VPV. Incremental Conductance (IC) is good for conditions of rapidly varying irradiance. However, noise may cause continuous searching so some amount of noise reduction may be needed.

Combinations of above methods are good to use since only one method would not give the expected results.

How Buck Converter Works[edit]

Buck Converter

Buck converter is used to regulate/step down the voltage for the load. It can be used with AC/DC power supply. It uses bridge rectifier when the input is AC.

It uses MOSFET as a switch to allow the input voltage to go across the load. Load voltage is given as:

VOUT = VIN x (On time of switching waveform (tON) / periodic time of switching waveform(T))

During the time tOFF the stored power in the Inductor and Capacitor flow through the Load maintaining the voltage at a certain level and smoothing out the output voltage waveform using L-C filter. Output voltage can be varied by varying duty cycle of the PWM signal given to the MOSFET as bias.

The combination of the diode, inductor and capacitor is called as Flywheel circuit.

Current Sense Amplifier[edit]

Current sense amplifiers outputs a voltage proportional to the current flowing in a power rail. It utilizes a "current-sense resistor" to convert the load current in the power rail to a small voltage, which is then amplified by the current-sense amplifiers.

Current sensing of 1 A to 20 A can be done using current-sense resistor resistance range between 1 to 100 mΩ.

Voltages in the range of 10 to 100 mV can be sensed in the presence of very large common-mode voltages of 5 to 30 V.

Some current sensing amplifier are unidirectional sensors and some are bidirectional sensors.

Over voltage protection is necessary because if the voltage sensed is out of the range then the internal ESD protection diodes conduct high current and amplifier burns.

LTC6101, MAX4080, AD8210, TS1100, INA193 and MAX9938 are some examples of Current sense amplifiers.

Solar tracking power generator[edit]

This project discusses a new IR camera based tracking option for solar radiation tracking. By using special algorithm in MCU it can determine the location of the sun by analyzing the IR image. As soon as MCU knowns location of the sun it tracks the sun using azimuthal and altitudal position control motors. This method is the most accurate method and has no effect on sensing if the whether is cloudy. It could only be beneficial to use this method if it is light weight, power efficient and cost effective.

MPPT Algorithm: Perturb and observe method for MPPT tracking is used in this project which works well. For Monitoring the current, voltage and temperature sensors are used. This sensed values also improve efficiency of the PV system. Different testing methods have been used to measure the efficiency, tracking system performance, MPPT tracking performance, etc.

Wireless connectivity for the Internet of Things[edit]

For maintaining the cold chain cool the wireless connectivity is important for monitoring the temperature of the vaccine refrigerator. Connecting the remote appliances with mobile network/web network requires reliable wireless communication technology. Since this is closely related with lowe power and open source energy consumption considering IoT technology is good option for minimal use of energy.

Wireless monitoring could be done using combination of techinologies like WiFi, Bluetooth,Zigbee, 6LoWPAN and 4G-LTE network. New ZigBee Pro devices are very low power consuming devices and con be integrated with all kind of electrical appliances.[11] All these technologies use there respective standard IEEE protocols and are available with free or paid user licenses for wireless access. For example 2.5Ghz is a free user licence spectrum and 4G-LTE is not a free to use licence and owned by the companies which provide you with there mobile network.

Thermoelectric Coca cola refrigerator[edit]

This refrigerator runs on 12V DC/AC 110-volt.
It has 4 liter capacity and cools up to 32 degree F below ambient temperature(Around 5oC).
It costs $65.
Thermoelectric cooling technology allows the cooler to operate silently.

Thermoelectric refrigerator driven by solar cells[edit]

In this experiment the refrigerator was treated in ways during daytime and nighttime. Solar cells were used during the day and battery stored power was used during night and cloudy or rainy day with the help; and the battery was assisted by a.c. rectifier. Using this technique for thermoelectric refrigerator the temperature could be maintained between 5-10oC.

The cooling output of refrigerator and the solar radiation reach maximum levels at the same season and time, so its a good option to use solar power for refrigeration. The thermoelectric refrigerators are usually light, reliable, noiseless, rugged, low cost in mass production and uses electrons rather than refrigerant as a heat carrier. The thermoelectric refrigerator discussed here was filed for patent in china.[12]

During the day the refrigerator was driven directly by solar cells and in the night it was driven by battery. The auto switching between day and night configuration was done by the controller. The data logger was used to perform temperature monitoring during the test. The system was designed and analysed assuming that the hot side temperature of the thermoelectric module was 13 °C higher than that of the ambient temperature, and the heat conduction from outside to the internal refrigerator was neglected. Experiment shows that the cold side temperature of the module decreases quickly in the first 20 min, and stays steadily below 5°C after one and half an hours.

The refrigerator can maintain the temperature in refrigerated space at 5~10 °C, and has a COP about 0.3. The power output of the solar cells presented in this paper are really impractical and low. Today's solar cells give more power than the solar cells used in this experiment.

How does thermoelectric refrigeration work?[edit]

In 1834 Jean Peltier observed that when an electrical current is applied across the junction of two dissimilar metals, heat is removed from one of the metals and transferred to the other. Thermoelectric refrigeration is based on this fact and uses metals cubes to transfer the heat. Solid-state thermoelectric modules are capable of transferring large quantities of heat when connected to a heat absorbing device on one side and a heat dissipating device on the other. The heat dissipating device uses a small fan to disperse the heat into the air. These cooling systems are compact, light weight, lower priced and requires low battery as it is simpler than other refrigeration techniques hence require smaller cooling system construction. Periodic servicing and maintenance is required for effective thermoelectric cooling such as dusting and vacuuming. The modules do not deteriorate with useand can run with 12V battery and motor which can be replaced easily.

Enhancement of an Electronic Solar Tracking System[edit]

LDRs, photo transistors and IR sensors were used to test the performance of tracking system with these sensors. With different testing voltages, different irradiance and different alignment distances between sensors the photo transistors were found to be giving best output which made the tracking system better than with LDRs or IR sensors as solar irradiance sensors.

Infrared Photo Transistor[edit]

Infrared Photo Transistor could be better option than using LDRs as it can provide accurate and and more output voltage/power for sensing the sun light.

ABSOLUTE MAXIMUM RATINGS (25 Degrees C)

Collector to Emitter Sustaining Voltage (Vce) 30 V
Emitter to Collector Breakdown Voltage 5 V
Collector Current 25 mA
Operating Temperature Range -40 to +85 Degrees C
Storage Temperature Range 5 V
Emitter to Collector Breakdown Voltage -40 to +85 Degrees C
Lead Soldering Temperature (1/16 inch from case for 5 sec) 240 Degrees C
Relative Humidity at 85 Degrees C 85%
Power Dissipation at or below 25 Degrees C Free Air Temperature 100 mW

ELECTRICAL CHARACTERISTICS

Dark Current (Vce = 15 V) 100 nA
Light Current (Vce = 5 V, H = 20 mW/cm) 20 nA
Collector to Emitter Saturation Voltage 0.4 V
Rise Time (10 to 90%) 5 microS
Fall Time (90 to 10%) 5 microS

Microprocessor-Controlled New Class of Optimal Battery Chargers for Photovoltaic Applications[edit]

Performance Characteristics

Method Circuit Hardware Software Simulation Algorithm Cost Efficiency
VMPPT, Continuously adjusting charging current Complex, 1524 IC, 8085 and lot of other components Simulink, DC/DC buck converter, Battery Parameter calculation Non-linear battery model, control Open circuit voltage & varying current using PWM signal $15 - $20 Shorter charge time, 40% to 75% faster charge time, Low efficiency

FPGA Implementation of MPPT Using Variable Step-Size P&O Algorithm for PV Applications[edit]

Performance Characteristics

Method Circuit Hardware Software Simulation Algorithm Cost Efficiency
Perturbation and Observation method, Using FPGA XC2C384 Chip. No integrated temperature sensor, But no need of current and voltage sensor. Complex circuit Architecture on FPGA. Non-linear model of PV panel power output, Variable step size P & O algorithm. RioRand FPGA costs $25-$30, FPGA chip $40 - $50. 96% efficiency

A Fast and Accurate Maximum Power Point Tracker for PV Systems[edit]

Performance Characteristics

Method Circuit Hardware Software Simulation Algorithm Cost Efficiency
Open circuit voltage & short circuit current monitoring. Buck-Boost Charge unit MPPT; VOC, IMP & ISC monitored & sent to MPPT Eqns(RHS) to control MPPT. Simulink, Faster than other MPPT like P&Q & more accurate than approximate methods that use linearity between voltage (current) at maximum power point and open-circuit voltage(short-circuit current). Non-linear V-I characteristics of PV panel are used in developing the algorithm and adjusting the MPPT according to the OCV and SC current. Circuit construction is simple. So this is low cost application. It is fast, good in different insolation, temperature and level of degradation; and Max deviation in max power is less than 1.5%.

Accurate, Compact, and Power-Efficient Li-Ion Battery Charger Circuit[edit]

Performance Characteristics

Method Circuit Hardware Software Simulation Algorithm Cost Efficiency
Two high gain voltage and current feedback loops. High efficiency circuit is good for optimal charging but not fit for MPPT tracking. Not specified. Uses a diode to smoothly{i.e., continuously) transition between two high-gain linear feedback loops and control a single power MOS device, automatically

charging the battery with constant current and then constant voltage.

Small circuit and low cost. 1.7 Hr charge time. 83% power efficient. Due to fast response it is accurate and efficient. Constant current output.

A dynamic lithium-ion battery model considering the effects of temperature and capacity fading[edit]

While doing research on battery operated systems the system models are studied using simulation before putting together the functional hardware for the successful operation of system under consideration. For the successful simulation results battery model has to be giving similar results as the real battery under consideration. Lithium-ion batteries have effect of temperature and capacity fading on there performance over the life cycle. Generally a battery is considered to be usable until 80% of its capacity. The fading effect doesn't depend on whether the battery is in operation or not. Even if it is not in use it still fades. Calender life and cycle life loss of the battery increase linearly with time and increasing temperature.

The simulation model presented in this paper using Matlab/Simulink gives similar results to the real test performed with lithium-ion batteries at 25oC and 50oC as per the feedback of all the authors who did practical research on the lithium ion batteries. According to the results the charging and discharging characteristics are the same as shown by the simulation. The simulation modeling and design methodology has a modular structure that can be implemented, simulated and analyzed with different power and parameters of any desired lithium-ion battery system. The developed model has a proven result to be able to evaluate the battery performance under several different operating conditions. Because of proposed model's similar performance results, it can be directly used in different simulation models including battery systems.