The Sun Chaser Energy-Harvesting System

When Sjoerd Brandsma entered the 2012 Renesas Green Energy Challenge, he wanted to create a fun project that would take advantage of his experience at a company that heavily uses GPS.

Brandsma, who lives in Kerkwijk, The Netherlands, has worked as a software engineer and is currently an R&D manager at CycloMedia, which produces 360° street-level panoramic images with geographic information system (GIS) accuracy.

Ultimately, Brandsma’s Sun Chaser project won third prize in the Renesas Green Energy Challenge. The Sun Chaser is an energy-harvesting system that automatically orients a solar panel to face the sun.

Photo 1: The Sun Chaser’s stepper motor controls the solar panel‘s “tilting.”

Photo 1: The Sun Chaser’s stepper motor controls the solar panel‘s “tilting.”

“The Sun Chaser perfectly follows the sun’s path and keeps the battery fully charged when there’s enough sunlight,” Brandsma says in his article about the project, which appears in Circuit Cellar’s June issue. ”It can power a small electronics system as long as there’s enough sunlight and no rain, which would damage the system due to lack of protection. This project also demonstrates that it’s possible to build an interesting green-energy system with a tight budget and a limited knowledge of  electronics.”

A registered GPS calculates the orientation of the Sun Chaser’s solar panel, which is mounted on a rotating disc. “You can use an external compass, the internal accelerometer, a DC motor, and a stepper motor to determine the solar panel’s exact position,” Brandsma says. “The Sun Chaser uses Renesas Electronics’s RDKRL78G13 evaluation board running the Micriµm µC/OS-III real-time kernel.”

The following article excerpt describes the GPS reference station and evaluation board in greater detail. The issue with Brandsma’s full article is available online for membership download or single-issue purchase.

Whenever you want to know where you are, you can use a GPS receiver that provides your position. A single GPS receiver can provide about 10 to 15 m (i.e., 33’ to 50’) position accuracy. While this is sufficient for many people, some applications require positioning with significantly higher accuracy. In fact, GPS can readily produce positions that are accurate to 1 m (3’), 0.5 m (18”), or even 1 to 2 cm (less than 1“). A technique called “differential GPS” can be used to achieve higher accuracy.

The differential technique requires one GPS receiver to be located at a known position (often called a control or reference point) and a second “rover” receiver at the location to be measured. The information from the two GPS receivers (rover and control) is combined to determine the rover’s position. That’s where a GPS reference station comes in. It functions as the control point and serves potentially unlimited users and applications. Leica Geosystems has published an excellent introductory guide about GPS reference stations (Refer to the Resources at the end of this article.)

The GPS reference station should always be located at a position with a broad sight. In some situations it can be difficult to provide a decent power supply to the system. When regular power isn’t available, a solar panel can power the GPS reference station.

My Sun Chaser GPS reference station uses a 10-W solar panel connected to a 12-V battery to provide enough power. To increase the energy harvesting, the solar panel is mounted on a rotating disc that can be controlled by a DC motor to point in the desired direction. A stepper motor controls the solar panel’s “tilting.” Photo 1 highlights the main components.

The RDKRL78G13 is an evaluation and demonstration tool for Renesas Electronics’s RL78 low-power microcontrollers. A set of human-machine interfaces (HMIs) is tightly integrated with the RL78’s features. I used several of these interfaces to control other devices, read sensors, or store data.

Most of the system’s hardware is related to placing the solar panel in the correct position. Figure 1 shows the top-level components used to store the GPS information and position the solar panel.

Figure 1: The Sun Chaser’s components include a Renesas Electronics RDKRL78G13 evaluation board, a GPS receiver, a stepper motor, and an SD card.

Figure 1: The Sun Chaser’s components include a Renesas Electronics RDKRL78G13 evaluation board, a GPS receiver, a stepper motor, and an SD card.

The RDKRL78G13 evaluation board has an on-board temperature and light sensor. Both sensor values are stored on the SD card. The on-board light sensor is used to determine if rotating/tilting makes sense (at night it’s better to sleep). For this project, the temperature values are stored just for fun so I could make some graphs or do some weather analysis.

A micro-SD memory card slot on the RDKRL78G13 evaluation board provides file system data storage. I used it to store all incoming data and log messages using the FAT16/FAT32 file system.

The on-board Renesas Electronics RQK0609CQDQS MOSFET controls the DC motor that rotates the evaluation board. The DC motor can be controlled by applying a PWM signal generated from one of the RL78’s timers. The MOSFET is controlled by the RL78’s TO05 port and powered from the 12-V battery. A PWM signal is generated on TO05 by using Timer4 as a master and Timer5 as a slave. It’s only necessary to rotate clockwise, so additional hardware to rotate the platform counterclockwise is not required.

A digital compass is needed to determine the evaluation board’s rotated position or heading (see Figure 2). The Honeywell HMC5883L is a widely used and low-cost compass. This I2C-based compass has three-axis magnetoresistive sensors and a 12-bit ADC on board. It can read out values at a 160-Hz rate, which is more than enough for this project.

Figure 2: A Honeywell HMC5883L digital compass verifies the evaluation board’s rotated position or heading.

Figure 2: A Honeywell HMC5883L digital compass verifies the evaluation board’s rotated position or heading.

The compass uses the RL78’s IICA0 port through the Total Phase Beagle debug header, which is mounted on the RDKRL78G13 evaluation board. The Beagle analyzer provides easy access to this I2C port, which increases the flexibility to change things during prototyping.

The HMC5883L compass turned out to be a very sensitive device. Even the slightest change in the hardware setup seemed to influence the results when rotating. This meant some sort of calibration was needed to ensure the output was consistent every time the system started. [Brandsman’s full article descibes how how the HMC5883L can be calibrated. It’s important to know that every time the system starts, it makes a full turn to calibrate the compass.

A GPS module must be connected to the system to provide the system’s current location. I wanted the GPS module to be inexpensive, 3.3-V based, and have an easy and accessible interface (e.g., UART).

Figure 3 shows a schematic of a Skylab M&C Technology SKM53 GPS module, which is based on the MediaTek 3329 GPS receiver module. This module supports NMEA messages and the MTK NMEA Packet Protocol interface to control things such as power saving, output message frequency, and differential global positioning system (DGPS).

Unfortunately, the 3329 receiver can’t output “raw” GPS data (e.g., pseudorange, integrated carrier phase, Doppler shift, and satellite ephemeris), which would significantly improve the GPS reference station’s capabilities. Due to budget and time limitations (it takes some more software development effort to handle this raw data), I didn’t use a receiver that could output raw GPS data.

Figure 3:A Skylab M&C Technology SKM53 GPS receiver obtains the system’s current location.

Figure 3:A Skylab M&C Technology SKM53 GPS receiver obtains the system’s current location.

The SKM53 GPS receiver is connected to the RL78’s UART2. All data from the GPS receiver is stored on the SD card. As soon as a valid GPS position is received, the system calculates the sun’s position and moves the platform into the most ideal position.

A compact stepper motor is needed to tilt the platform in very small steps. The platform had to be tilted from fully vertical to fully horizontal in approximately 6 h when the sun was exactly following the equator, so speed wasn’t really an issue. I wanted to do very fine tilting, so I also needed a set of gears to slow down the platform tilting.

I used an inexpensive, easy-to-use, generic 5-V 28BYJ-48 stepper motor (see Figure 4). According to the specifications, the 28BYJ-48 stepper motor has a 1/64 gear reduction ratio and 64 steps per rotation (5.625°/step) of its internal motor shaft.

An important consideration here is that you don’t want to retain power on the stepper motor to keep it in position. This particular stepper motor has some internal gears that prevent the platform from flipping back when the stepper motor is not powered.

The stepper motor can be controlled by the well-known ULN2003 high-voltage high-current Darlington transistor array. The ULN2003 is connected to P71-P74. Each of the ULN2003’s four outputs is connected to one of the stepper motor’s coils. When two neighbor coils are set high (e.g., P72 and P73), the stepper motor will step in that direction.

When it comes to solar panels, you can build your own panel out of individual solar cells or buy a fully assembled one with known specifications. I used a no-name 10-W solar panel. The size (337 mm long × 205 mm wide × 18 mm high) was acceptable and it delivered more than enough energy. I used a charge controller to protect the battery from overcharging and to prevent it from supplying power to the solar panel at night.

Like solar panels, many charge controllers and battery protectors can be used in such a system. I chose the lazy approach: Just take one off the shelf. The CMP12/24 charge controller is specially designed for small solar systems. It has a stabilized 12-V output, which is taken from the connected battery. It can handle up to 12 A of charging or load current and, according to the specifications, it consumes about 20 mA of quiescent current. There is some room for improvement, but it worked for my project.

I had some 7805 voltage regulators lying around, which I figured could do the job and supply just enough power when the system was starting up. However, when it comes to power saving, the 7805 is not the way to go. It’s a linear regulator that works by taking the difference between the input and output voltages and burning it up as wasted heat.

What I needed was a switching regulator or a buck converter. I used a National Semiconductor (now Texas Instruments) LM2596. Note: The LM2596 is made by several companies and is available in inexpensive, high-quality modules (most cost a little more than $1 per converter). These ready-to-use modules already have the necessary capacitors, diodes, and so forth on board, so it’s really a matter of plug and play.

I used a lead acid RT1219 12-V 1.9-AH battery for power storage. You can use any 12-V battery with sufficient capacity.

Editor’s Note: Check out other projects from the 2012 Renesas RL78 Green Energy Challenge.

Eco-Friendly Home Automation Controller

The 2012 DesignSpark chipKIT Challenge invited engineers from around the world to submit eco-friendly projects using the Digilent chipKIT Max32 development board. Manuel Iglesias Abbatemarco of Venezuela won honorable mention with his autonomous home-automation controller. His design enables users to monitor and control household devices and to log and upload temperature, humidity, and energy-use sensor data to “the cloud” (see Photo 1).

The design comprised a Digilent chipKIT board (bottom), my MPPT charger board (chipSOLAR, middle), and my wireless board (chipWIRELESS, top).

Photo 1: The design comprised a Digilent chipKIT board (bottom), my MPPT charger board (chipSOLAR, middle), and my wireless board (chipWIRELESS, top).

The system, built around the chipKIT Arduino-compatible board, connects to Abbatemarco’s custom-made “chipSOLAR” board that uses a solar panel and two rechargeable lithium-ion (Li-on) cells to provide continuous power. The board implements a maximum power point tracking (MPPT) charger that deals with a solar panel’s nonlinear output efficiency. A “chipWIRELESS” board integrating a Quad Band GSM/GPRS modem, an XBee socket, an SD card connector, and a real-time clock and calendar (RTCC) enables home sensor and cloud connectivity. The software was written using chipKIT MPIDE, and the SD card logs the data from sensors.

“Since the contest, I have made some additions to the system,” Abbatemarco says. “The device’s aim is uninterrupted household monitoring and control. To accomplish this, I focused on two key features: the power controller and the communication with external devices (e.g., sensors). I used DesignSpark software to create two PCBs for these features.”

Abbatemarco describes his full project, including his post-contest addition of a web server, in his article appearing in Circuit Cellar’s May issue. In the meantime, you’ll find descriptions of his overall design, power management board, and wireless board in the following article excerpts.

The system’s design is based on a Digilent chipKIT Max32 board, which is an Arduino-compatible board with a Microchip Technology 32-bit processor and 3.3-V level I/O with almost the same footprint as an Arduino Mega microcontroller. The platform has all the computational power needed for the application and enough peripherals to add all the required external hardware.

I wanted to have a secure and reliable communication channel to connect with the outside world, so I incorporated general packet radio service (GPRS). This enables the device to use a TCP/IP client to connect to web services. It can also use Short Message Service (SMS) to exchange text messages to cellular phones. The device uses a serial port to communicate with the chipKIT board.

I didn’t want to deal with cables for the internal-sensor home network, so I decided to make the system wireless. I used XBee modules, as they offer a good compromise between price and development time. Also, if properly configured, they don’t consume too much energy. The XBee device uses a serial port to communicate with the chipKIT board.
To make the controller”green,” I designed a power-management board that can work with a solar panel and several regulated DC voltages. I chose a hardware implementation of an MPPT controller because I wanted to make my application as reliable as possible and have more software resources for the home controller task.

One board provides power management and the other enables communication, which includes additional hardware such as an SD card, an XBee module, and an RTCC. Note: I included the RTCC since the chipKIT board does not come with a crystal oscillator. I also included a prototyping area, which later proved to be very useful.

I was concerned about how users inside a home would interact with the device. The idea of a built-in web server to help configure and interact with the device had not materialized before I submitted the contest entry. This solution is very practical, since you can access the device through its built-in server to configure or download log files while you are on your home network.

To make the system eco-friendly, I needed to enable continuous device operation using only a solar panel and a rechargeable Li-ion battery. The system consumes a considerable amount of power, so it needed a charge controller. Its main task was to control the battery-charging process. However, to work properly, it also had to account for the solar panel’s characteristics.

A solar panel can’t deliver constant power like a wall DC adapter does. Instead, power varies in a complex way according to atmospheric conditions (e.g., light and temperature).
For a given set of operational conditions, there is always a single operating point where the panel delivers its maximum power. The idea is to operate the panel in the maximum power point regardless of the external conditions.

I used Linear Technology’s LT3652 MPPT charger IC, which uses an input voltage regulation loop. The chip senses the panel output voltage and maintains it over a value by adjusting the current drawn. A voltage divider network is used to program the setpoint.
You must know the output voltage the panel produces when operated at the maximum power point. I couldn’t find the manufacturer’s specification sheet for the solar panel, but the distributor provides some experimental numbers. Because I was in a hurry to meet the contest deadline, I used that information. Based on those tests, the solar panel can produce approximately 8 V at 1.25 A, which is about 10 W of power.

I chose 8 V as the panel’s maximum power point voltage. The resistor divider output is connected to the LT3652’s VIN_REG pin. The chip has a 2.7-V reference, which means the charge current is reduced when this pin’s voltage goes below 2.7 V.

I used a two-cell Li-ion battery, but since the LTC3652 works with two, three, and four cells, the same board with different components can be used with a three- or four-cell battery. The LT3652 requires an I/O voltage difference of at least 3.3 V for reliable start-up, and it was clear that the panel’s 8-V nominal output would not be enough. I decided to include a voltage step-up stage in front of the LT3652.

I used Linear Technology’s LT3479 DC/DC converter to get the panel output to around 18 V to feed the MPPT controller. This only works if the LT3562’s voltage control loop still takes the VIN_REG reference directly from the panel output. Figures 1 and 2 show the circuit.

Power management board

Figure 1: Power management board

Figure 2: Power management board

Figure 2: Power management board

I could have fed the chipKIT on-board 5-V linear regulator with the battery, but I preferred to include another switching regulator to minimize losses. I used Linear Technology’s LTC3112 DC/DC converter. The only problem was that I needed to be able to combine its output with the chipKIT board’s 5 V, either through the USB port or the DC wall adapter option.

The chipKIT board includes a Microchip Technology MCP6001 op-amp in comparator configuration to compare USB voltage against a jack DC input voltage, enabling only one to be the 5-V source at a given time. Something similar was needed, so I included a Linear Technology LTC4411 IC, which is a low-loss replacement ORing diode, to solve the problem.

To my knowledge, when I designed the board a battery gauge for two-cell lithium batteries (e.g., a coulomb counter that can indicate accumulated battery charge and discharge) wasn’t available. The available options needed to handle most of the computational things in software, so I decided it was not an option. I included a voltage buffer op-amp to take advantage of the LTC3112’s dedicated analog voltage output, which gives you an estimate of the instantaneous current being drawn. Unfortunately, I wasn’t able to get it to work. So I ended up not using it.

Building this board was a challenge, since most components are 0.5-mm pitch with exposed pads underneath. IC manufacturers suggest using a solid inner ground layer for switching regulators, so I designed a four-layer board. If you have soldering experience, you can imagine how hard it is to solder the board using only a hot air gun and a soldering iron. That’s why I decided it was time to experiment with a stencil, solder paste, and a convection oven. I completed the board by using a commercially available kitchen convection oven and manually adjusting the temperature to match the reflow profile since I don’t have a controller (see Photo 2).

Photo 3: Custom chipSOLAR board

Photo 2: Custom chipSOLAR board

The wireless board has all the components for GPRS communication and the 802.15.4 home network, as well as additional components for the SD file system and the RTCC. Figure 3 shows the circuit.

Figure 3: The communication board schematic is shown.

Figure 3: The communication board schematic is shown.

At the time of the contest, I used a SIMCom Wireless Solutions SIM340 GPRS modem. The company now offers a replacement, the SIM900B. The only physical differences are the board-to-board connectors, but the variations are so minimal that you can use the same footprint for both connectors.

During the contest, I only had the connector for the SIM340 on hand, so I based almost all the firmware on that model. Later, I got the SIM900B connector and modified the firmware. The Project Files include the #if defined clause for SIM900 or SIM340 snippets.

A couple of things made me want to test the SIM900B module, among them the Simple Mail Transfer Protocol (SMTP) server functionality and Multimedia Messaging Service (MMS). Ultimately, I discovered that my 32-MB flash memory version of the SIM900B was not suitable for those firmware versions. The 64-MB version of the hardware is required.
The subscriber identity module (SIM) card receptacle and associated ESD protection circuitry are located on the upper side of the board. The I/O lines connected to the modem are serial TX, RX, and a power-on signal using a transistor.

The chipKIT Max32 board does not have a 32,768-Hz crystal, so Microchip Technology’s PIC32 internal RTCC was not an option. I decided to include Microchip Technology’s MCP79402 RTCC with a super capacitor, mainly for service purposes as the system is already backed up with the lithium battery.

I should have placed the SD card slot on the top of the board. That could have saved me some time during the debugging stage, when I have had some problems with SD firmware that corrupts the SD file system. When I designed the board, I was trying to make it compatible with other platforms, so I included level translators for the SD card interface. I made the mistake of placing a level translator at the master input slave output (MISO), which caused a conflict in the bus with other SPI devices. I removed it and wire-wrapped the I/O lines.

Another issue with this board was the XBee module’s serial port net routing, but it was nothing that cutting some traces and wire wrap could not fix. Photo 3 shows all the aforementioned details and board component location.

Photo 3: This communication board includes several key components to enable wireless communication with sensors,  the Internet, and cellular networks.

Photo 3: This communication board includes several key components to enable wireless communication with sensors,the Internet, and cellular networks.

Editor’s Note: Visit here to read about other projects from the 2012 DesignSpark chipKIT Challenge.

Solar Cells Explained (EE Tip #104)

All solar cells are made from at least two different materials, often in the form of two thin, adjacent layers. One of the materials must act as an electron donor under illumination, while the other material must act as an electron acceptor. If there is some sort of electron barrier between the two materials, the result is an electrical potential. If each of these materials is now provided with an electrode made from an electrically conductive material and the two electrodes are connected to an external load, the electrons will follow this path.

Source: Jens Nickels, Elektor, 070798-I, 6/2009

Source: Jens Nickels, Elektor, 070798-I, 6/2009

The most commonly used solar cells are made from thin wafers of polycrystalline silicon (polycrystalline cells have a typical “frosty” appearance after sawing and polishing). The silicon is very pure, but it contains an extremely small amount of boron as a dopant (an intentionally introduced impurity), and it has a thin surface layer doped with phosphorus. This creates a PN junction in the cell, exactly the same as in a diode. When the cell is exposed to light, electrons are released and holes (positive charge carriers) are generated. The holes can recombine with the electrons. The charge carriers are kept apart by the electrical field of the PN junction, which partially prevents the direct recombination of electrons and holes.

The electrical potential between the electrodes on the top and bottom of the cell is approximately 0.6 V. The maximum current (short-circuit current) is proportional to the surface area of the cell, the impinging light energy, and the efficiency. Higher voltages and currents are obtained by connecting cells in series to form strings and connecting these strings of cells in parallel to form modules.

The maximum efficiency achieved by polycrystalline cells is 17%, while monocrystalline cells can achieve up to 22%, although the overall efficiency is lower if the total module area is taken into account. On a sunny day in central Europe, the available solar energy is approximately 1000 W/m2, and around 150 W/m2 of this can be converted into electrical energy with currently available solar cells.

Source: Jens Nickels, Elektor, 070798-I, 6/2009

Source: Jens Nickels, Elektor, 070798-I, 6/2009

Cells made from selenium, gallium arsenide, or other compounds can achieve even higher efficiency, but they are more expensive and are only used in special applications, such as space travel. There are also other approaches that are aimed primarily at reducing costs instead of increasing efficiency. The objective of such approaches is to considerably reduce the amount of pure silicon that has to be used or eliminate its use entirely. One example is thin-film solar cells made from amorphous silicon, which have an efficiency of 8 to 10% and a good price/performance ratio. The silicon can be applied to a glass sheet or plastic film in the form of a thin layer. This thin-film technology is quite suitable for the production of robust, flexible modules, such as the examples described in this article.

Battery Charging

From an electrical viewpoint, an ideal solar cell consists of a pure current source in parallel with a diode (the outlined components in the accompanying schematic diagram). When the solar cell is illuminated, the typical U/I characteristic of the diode shifts downward (see the drawing, which also shows the opencircuit voltage UOC and the short-circuit current ISC). The panel supplies maximum power when the load corresponds to the points marked “MPP” (maximum power point) in the drawing. The power rating of a cell or panel specified by the manufacturer usually refers to operation at the MPP with a light intensity of 100,000 lux and a temperature of 25°C. The power decreases by approximately 0.2 to 0.5 %/°C as the temperature increases.

A battery can be charged directly from a panel without any problems if the open-circuit voltage of the panel is higher than the nominal voltage of the battery. No voltage divider is necessary, even if the battery voltage is only 3 V and the nominal voltage of the solar panel is 12 V. This is because a solar cell always acts as a current source instead of a voltage source.

If the battery is connected directly to the solar panel, a small leakage current will flow through the solar panel when it is not illuminated. The can be prevented by adding a blocking diode to the circuit (see the schematic). Many portable solar modules have a built-in blocking diode (check the manufacturer’s specifications).

This simple arrangement is adequate if the maximum current from the solar panel is less than the maximum allowable overcharging current of the battery. NiMH cells can be overcharged for up to 100 hours if the charging current (in A) is less than one-tenth of their rated capacity in Ah. This means that a panel with a rated current of 2 A can be connected directly to a 20-Ah battery without any problems. However, under these conditions the battery must be fully discharged by a load from time to time.

Practical Matters

When positioning a solar panel, you should ensure that no part of the panel is in the shade, as otherwise the voltage will decrease markedly, with a good chance that no current will flow into the connected battery.

Most modules have integrated bypass diodes connected in reverse parallel with the solar cells. These diodes prevent reverse polarization of any cells that are not exposed to sunlight, so the current from the other cells flows through the diodes, which can cause overheating and damage to the cells. To reduce costs, it is common practice to fit only one diode to a group of cells instead of providing a separate diode for each cell.

—Jens Nickels, Elektor, 070798-I, 6/2009

Q&A: Scott Potter (Engineering a Way To Clean Solar Mirrors)

Designer and technology executive Scott Potter won first prize in the 2012 RL78 Green Energy Challenge, presented by Renesas Electronics in partnership with Circuit Cellar and Elektor magazines. The global contest called on participants to develop green energy designs utilizing Renesas’s RL78 microcontrollers. Scott won with his solar-powered electrostatic cleaning robot, which removes dust and debris from the tracking mirrors of large-scale concentrating solar power plants.—Mary Wilson, Managing Editor

Scott Potter

MARY: Where do you live and what is your current occupation?

SCOTT: I live in Los Gatos, CA, and I’m a senior director at Jasper Wireless, a company providing machine-to-machine (M2M) data communications services. I have been with Jasper since the beginning in 2005 when the company started with four people and a plan. Now Jasper is approaching 150 employees and we are a global company. I have served many roles at Jasper, working on location technology, device middleware, back-end reporting, and front-end software.

My other job is as an inventor at Taft Instruments. We are just now forming around the technology I developed for the RL78 design challenge. We are finding there is a big need for this solution in the solar industry, which is poised for tremendous growth in the next few years.

MARY: How did you first become interested in embedded electrical design? What is your educational background?

SCOTT: I started working for my father at his startup in the basement of our home in Long Island when I was a teenager (child labor laws were more lax back then). We were doing embedded electronics design along with mechanical modeling and prototyping. I learned from the best and it has stuck with me all these years. I went on to get a BSEE from Tufts University and I toyed with the idea of business school, but it never gripped me like engineering.

MARY: Why did you enter the 2012 Renesas RL78 Green Energy Challenge? What about its focus appealed to you?

SCOTT: The green energy design challenge came along at the perfect time. I had been working on the cleaning robot for a few months when I saw the challenge. The microcontroller I had originally picked was turning out to be not a great choice, and the challenge made me take a look at the RL78. The part was perfect, and the challenge gave me a goal to work toward.

MARY: How did the idea of designing a robot to clean solar-tracking mirrors (i.e., heliostats) for solar power plants come to you?

SCOTT: I can’t say it came to me all at once. I have participated in solar technology development sporadically throughout my career, and I have always tried to stay abreast of the latest developments. After the lessons learned from the parabolic trough concentrators, the move to high-concentration concentrating solar power (CSP) plants, which more efficiently convert solar power to electrical power, struck me as the right thing to do.

The high-concentration CSP plant utilizes hundreds of thousands of mirrors spread over many acres. The mirrors reflect sunlight onto a centrally located tower, which creates intense heat that drives a steam turbine generator.

The efficiency gains from the higher temperatures will make this the dominant technology for utility scale power generation. But there is a high maintenance cost associated with all of those mirror surfaces, especially in environments where water is scarce. A number of people have realized this and proposed various solutions to keeping the surfaces clean. Unfortunately, none of the proposed solutions will work well at the scale of a large utility plant.

I experimented with quite a few waterless cleaning techniques before coming back to electrostatics. It was my wife, Dia, who reminded me that NASA had been cleaning dust off panels on space missions for years using electrostatic principles. She convinced me to stop working with the forced-air concept I was doing at the time and switch to electrostatics. It was definitely the right choice.

MARY: What does the system do? What problems does it solve for power plants? How is the device different from what is already available for the task of cleaning heliostats?

SCOTT: Our patent-pending device is unique in many ways. It is completely autonomous, requiring no external power or water. The installation time is less than 10 s per heliostat, after which the device will remain attached and operating maintenance free for the life of the plant. We borrowed a marketing term from the military for this: “Set it and forget it.”

Most of the competing products have a long installation time and require some external wiring and maintenance. These can be logistical problems in a field of hundreds of thousands of mirrors.

Our device is also unique in that it cleans continuously. This prevents accumulation of organic materials on the surface, which can mix with dew and make a bio-film on the surface. That film bakes on and requires vigorous scrubbing to remove. We also have a feature to handle the dew, or frost, if it’s present.

MARY: What were some of your design challenges along the way and how did you address them?

SCOTT: They were numerous. The first challenge was the power source. It is important that this device be entirely self-powered to avoid having to install any wiring. I had to find a solar-panel configuration that provided enough power at the right voltage levels. I started with lower voltages and had a lot of trouble with the boost converters.

I also couldn’t use any battery storage because of the life requirement. This means that everything has to operate intermittently, gracefully shutting down when the sun fades and then coming up where it left off when the sun returns.

The next challenge was the mechanical drive. This had to grip the mirror tightly enough to resist a stream of water from a cleaning hose (infrequent cleaning with water will probably still be performed). And it had to do this with no power applied.

Another big challenge was the high-voltage electronics. It turns out there is little off-the-shelf technology available for the kind of high-voltage circuitry I needed. Large line output power transformers (LOPTs) for old cathode ray tubes (CRTs) are too large and expensive.

Some of the resonant high-voltage circuits used for cold cathode fluorescent lighting (CCFL) can be used as building blocks, but I had to come up with quite a few innovations to be able to control this voltage to perform the cleaning task. I had more than a few scorched breadboards before arriving at the current design, which is very small, light, and powerful.

MARY: You recently formed Taft Instruments (click here for Taft website). Who are the players in the company and what services does it provide?

SCOTT: We formed Taft instruments to commercialize this cleaning technology. We have been very fortunate to attract a very talented team that has made tremendous progress promoting the company in industry and attracting investment.

We have Steve Gluck and Gary Valinoti, both highly respected Wall Street executives who have galvanized the company and provided opportunities I could never have imagined. They are now recruiting the rest of the team and we are talking to some extremely qualified people. And of course my wife, Dia, is making numerous contributions that she will probably never get credit for.

MARY: How’s business? How would you describe the market for your product and the potential for growth and reach (both domestically and globally)?

SCOTT: We are not at the commercial deployment stage just yet. Our immediate focus is on the field trials we are starting with a number of industry players and the US Department of Energy National Laboratories. We fully expect the trials to be successful and for our large-scale rollouts to begin in about a year.

The market potential for this is tremendous. I’m not sure anyone fully realizes yet the global transformation that is about to take place. Now that the “grid parity” point is near (the point where the cost of solar power is competitive with fossil fuels), solar will become one of the fastest-growing markets we have seen in a century.

Entire national energy pictures will change from single-digit percentages to being dominated by solar. It is a very exciting time in the solar industry, and we are very happy to be part of it.

MARY: Are you individually—or is your company—developing any new designs? If so, can you tell us something about them?

SCOTT: Yes. I can’t say much, but we are working on some very interesting new technologies that will improve on the electrostatic cleaning principles. This technology will vastly expand the base that we can work with.

MARY: You describe yourself as a “serial entrepreneur” with a strong technical background in electronics, software, hardware, and systems design. That combination of skills comes in handy when establishing a new business. But it also helped you land your day job eight years ago as Director of Location Technology at Jasper Wireless. What do you see as future key trends in M2M communications?

SCOTT: M2M has really taken off since we began in 2005. Back then, there were only a few applications people had envisioned taking wireless. That list has exploded, and some analysts are predicting volumes of M2M endpoints that exceed the human population by tenfold!

We have seen large growth in a number of different verticals over the years, the most apparent one right now being automotive, with all the car companies providing connected services. Jasper is uniquely positioned to offer a global solution to these companies through our carrier partners.

MARY: Over the years, you have gained expertise in areas ranging from embedded electronics and wireless, to applications of the global positioning and geographic information systems (GPS and GIS). What do you enjoy most and what are some career highlights? Is one your involvement in the development of a GPS for the New York fire department’s recovery operations after the collapse of the World Trade Center?

SCOTT: What I enjoy most is working with motivated teams to create compelling products and services. One of my proudest moments was when our team at Links Point rose to the 9/11 challenge. At the time, I was a founder and the chief technology officer of Links Point, which provided GPS and location mapping.

When the request came from the New York fire department for a solution to locating remains at the recovery site, the team dedicated themselves to providing a solution no first responder had ever had access to previously. And we did that in record time. We had to come up with a proposal in a half-day and implement it within three days. You have to realize that GPS and PDAs were very new at the time and there were a lot of technical challenges. We also had to compete with some other companies that were proposing more accurate surveying equipment, such as laser ranging.

Our product, a PDA with a GPS attachment, won out in the end. The advantages of our handheld devices were that they were rugged and that firefighters could easily carry them into Ground Zero. We got the opportunity and honor of serving the  FDNY because of the extreme talent, dedication, and professionalism of my team. I would like to mention them: Jerry Kochman, Bill Campbell, Murray Levine, Dave Mooney, and Lucas Hjelle.

MARY: What is the most important piece of advice you would give to someone trying to make a marketable product of his or her design for an electrical device?

SCOTT: Whatever the device, make sure you are passionate about it and committed to seeing it come through. There is a quote that Dia framed for me hanging in my lab—this is attributed to Goethe, but there is some question about that. Anyway, the quote is very inspirational:

“Until one is committed, there is hesitancy, the chance to draw back. Concerning all acts of initiative (and creation), there is one elementary truth that ignorance of which kills countless ideas and splendid plans: that the moment one definitely commits oneself, then Providence moves too. All sorts of things occur to help one that would never otherwise have occurred. A whole stream of events issues from the decision, raising in one’s favor all manner of unforeseen incidents and meetings and material assistance, which no man could have dreamed would have come his way. Whatever you can do, or dream you can do, begin it. Boldness has genius, power, and magic in it. Begin it now.” I

Editor’s note: For more details, schematics, and a video of Scott Potter’s solar-powered electrostatic cleaning robot, click here.

Electrostatic Cleaning Robot Project

How do you clean a clean-energy generating system? With a microcontroller (and a few other parts, of course). An excellent example is US designer Scott Potter’s award-winning, Renesas RL78 microcontroller-based Electrostatic Cleaning Robot system that cleans heliostats (i.e., solar-tracking mirrors) used in solar energy-harvesting systems. Renesas and Circuit Cellar magazine announced this week at DevCon 2012 in Garden Grove, CA, that Potter’s design won First Prize in the RL78 Green Energy Challenge.

This image depicts two Electrostatic Cleaning Robots set up on two heliostats. (Source: S. Potter)

The nearby image depicts two Electrostatic Cleaning Robots set up vertically in order to clean the two heliostats in a horizontal left-to-right (and vice versa) fashion.

The Electrostatic Cleaning Robot in place to clean

Potter’s design can quickly clean heliostats in Concentrating Solar Power (CSP) plants. The heliostats must be clean in order to maximize steam production, which generates power.

The robot cleaner prototype

Built around an RL78 microcontroller, the Electrostatic Cleaning Robot provides a reliable cleaning solution that’s powered entirely by photovoltaic cells. The robot traverses the surface of the mirror and uses a high-voltage AC electric field to sweep away dust and debris.

Parts and circuitry inside the robot cleaner

Object oriented C++ software, developed with the IAR Embedded Workbench and the RL78 Demonstration Kit, controls the device.

IAR Embedded Workbench IDE

The RL78 microcontroller uses the following for system control:

• 20 Digital I/Os used as system control lines

• 1 ADC monitors solar cell voltage

• 1 Interval timer provides controller time tick

• Timer array unit: 4 timers capture the width of sensor pulses

• Watchdog timer for system reliability

• Low voltage detection for reliable operation in intermittent solar conditions

• RTC used in diagnostic logs

• 1 UART used for diagnostics

• Flash memory for storing diagnostic logs

The complete project (description, schematics, diagrams, and code) is now available on the Challenge website.


DIY Green Energy Design Projects

Ready to start a low-power or energy-monitoring microcontroller-based design project? You’re in luck. We’re featuring eight award-winning, green energy-related designs that will help get your creative juices flowing.

The projects listed below placed at the top of Renesas’s RL78 Green Energy Challenge.

Electrostatic Cleaning Robot: Solar tracking mirrors, called heliostats, are an integral part of Concentrating Solar Power (CSP) plants. They must be kept clean to help maximize the production of steam, which generates power. Using an RL78, the innovative Electrostatic Cleaning Robot provides a reliable cleaning solution that’s powered entirely by photovoltaic cells. The robot traverses the surface of the mirror and uses a high voltage AC electric field to sweep away dust and debris.

Parts and circuitry inside the robot cleaner

Cloud Electrofusion Machine: Using approximately 400 times less energy than commercial electrofusion machines, the Cloud Electrofusion Machine is designed for welding 0.5″ to 2″ polyethylene fittings. The RL78-controlled machine is designed to read a barcode on the fitting which determines fusion parameters and traceability. Along with the barcode data, the system logs GPS location to an SD card, if present, and transmits the data for each fusion to a cloud database for tracking purposes and quality control.

Inside the electrofusion machine (Source: M. Hamilton)

The Sun Chaser: A GPS Reference Station: The Sun Chaser is a well-designed, solar-based energy harvesting system that automatically recalculates the direction of a solar panel to ensure it is always facing the sun. Mounted on a rotating disc, the solar panel’s orientation is calculated using the registered GPS position. With an external compass, the internal accelerometer, a DC motor and stepper motor, you can determine the solar panel’s exact position. The system uses the Renesas RDKRL78G13 evaluation board running the Micrium µC/OS-III real-time kernel.

[Video: ]

Water Heater by Solar Concentration: This solar water heater is powered by the RL78 evaluation board and designed to deflect concentrated amounts of sunlight onto a water pipe for continual heating. The deflector, armed with a counterweight for easy tilting, automatically adjusts the angle of reflection for maximum solar energy using the lowest power consumption possible.

RL78-based solar water heater (Source: P. Berquin)

Air Quality Mapper: Want to make sure the air along your daily walking path is clean? The Air Quality Mapper is a portable device designed to track levels of CO2 and CO gasses for constructing “Smog Maps” to determine the healthiest routes. Constructed with an RDKRL78G13, the Mapper receives location data from its GPS module, takes readings of the CO2 and CO concentrations along a specific route and stores the data in an SD card. Using a PC, you can parse the SD card data, plot it, and upload it automatically to an online MySQL database that presents the data in a Google map.

Air quality mapper design (Source: R. Alvarez Torrico)

Wireless Remote Solar-Powered “Meteo Sensor”: You can easily measure meteorological parameters with the “Meteo Sensor.” The RL78 MCU-based design takes cyclical measurements of temperature, humidity, atmospheric pressure, and supply voltage, and shares them using digital radio transceivers. Receivers are configured for listening of incoming data on the same radio channel. It simplifies the way weather data is gathered and eases construction of local measurement networks while being optimized for low energy usage and long battery life.

The design takes cyclical measurements of temperature, humidity, atmospheric pressure, and supply voltage, and shares them using digital radio transceivers. (Source: G. Kaczmarek)

Portable Power Quality Meter: Monitoring electrical usage is becoming increasingly popular in modern homes. The Portable Power Quality Meter uses an RL78 MCU to read power factor, total harmonic distortion, line frequency, voltage, and electrical consumption information and stores the data for analysis.

The portable power quality meter uses an RL78 MCU to read power factor, total harmonic distortion, line frequency, voltage, and electrical consumption information and stores the data for analysis. (Source: A. Barbosa)

High-Altitude Low-Cost Experimental Glider (HALO): The “HALO” experimental glider project consists of three main parts. A weather balloon is the carrier section. A glider (the payload of the balloon) is the return section. A ground base section is used for communication and display telemetry data (not part of the contest project). Using the REFLEX flight simulator for testing, the glider has its own micro-GPS receiver, sensors and low-power MCU unit. It can take off, climb to pre-programmed altitude and return to a given coordinate.

High-altitude low-cost experimental glider (Source: J. Altenburg)

DIY Solar-Powered, Gas-Detecting Mobile Robot

German engineer Jens Altenburg’s solar-powered hidden observing vehicle system (SOPHECLES) is an innovative gas-detecting mobile robot. When the Texas Instruments MSP430-based mobile robot detects noxious gas, it transmits a notification alert to a PC, Altenburg explains in his article, “SOPHOCLES: A Solar-Powered MSP430 Robot.”  The MCU controls an on-board CMOS camera and can wirelessly transmit images to the “Robot Control Center” user interface.

Take a look at the complete SOPHOCLES design. The CMOS camera is located on top of the robot. Radio modem is hidden behind the camera so only the antenna is visible. A flexible cable connects the camera with the MSP430 microcontroller.

Altenburg writes:

The MSP430 microcontroller controls SOPHOCLES. Why did I need an MSP430? There are lots of other micros, some of which have more power than the MSP430, but the word “power” shows you the right way. SOPHOCLES is the first robot (with the exception of space robots like Sojourner and Lunakhod) that I know of that’s powered by a single lithium battery and a solar cell for long missions.

The SOPHOCLES includes a transceiver, sensors, power supply, motor
drivers, and an MSP430. Some block functions (i.e., the motor driver or radio modems) are represented by software modules.

How is this possible? The magic mantra is, “Save power, save power, save power.” In this case, the most important feature of the MSP430 is its low power consumption. It needs less than 1 mA in Operating mode and even less in Sleep mode because the main function of the robot is sleeping (my main function, too). From time to time the robot wakes up, checks the sensor, takes pictures of its surroundings, and then falls back to sleep. Nice job, not only for robots, I think.

The power for the active time comes from the solar cell. High-efficiency cells provide electric energy for a minimum of approximately two minutes of active time per hour. Good lighting conditions (e.g., direct sunlight or a light beam from a lamp) activate the robot permanently. The robot needs only about 25 mA for actions such as driving its wheel, communicating via radio, or takes pictures with its built in camera. Isn’t that impossible? No! …

The robot has two power sources. One source is a 3-V lithium battery with a 600-mAh capacity. The battery supplies the CPU in Sleep mode, during which all other loads are turned off. The other source of power comes from a solar cell. The solar cell charges a special 2.2-F capacitor. A step-up converter changes the unregulated input voltage into 5-V main power. The LTC3401 changes the voltage with an efficiency of about 96% …

Because of the changing light conditions, a step-up voltage converter is needed for generating stabilized VCC voltage. The LTC3401 is a high-efficiency converter that starts up from an input voltage as low as 1 V.

If the input voltage increases to about 3.5 V (at the capacitor), the robot will wake up, changing into Standby mode. Now the robot can work.

The approximate lifetime with a full-charged capacitor depends on its tasks. With maximum activity, the charging is used after one or two minutes and then the robot goes into Sleep mode. Under poor conditions (e.g., low light for a long time), the robot has an Emergency mode, during which the robot charges the capacitor from its lithium cell. Therefore, the robot has a chance to leave the bad area or contact the PC…

The control software runs on a normal PC, and all you need is a small radio box to get the signals from the robot.

The Robot Control Center serves as an interface to control the robot. Its main feature is to display the transmitted pictures and measurement values of the sensors.

Various buttons and throttles give you full control of the robot when power is available or sunlight hits the solar cells. In addition, it’s easy to make short slide shows from the pictures captured by the robot. Each session can be saved on a disk and played in the Robot Control Center…

The entire article appears in Circuit Cellar 147 2002. Type “solarrobot”  to access the password-protected article.

Living & Working Off the Grid

Interested in engineering your own solar panel system installation? If so, you’ve likely begun researching photovoltaic technology, construction materials, and test equipment on the Internet. Have you been satisfied with the information you’ve found? Probably not. There’s simply a scarcity of reliable electronics engineering advice out there about serious solar panel installation projects. Enter Circuit Cellar. Over the past several years, we’ve published articles by professional engineers about their own installations.

Three panels are wired in series and run into the MPPT controllers. Their capacity is 170 W each, 510 W total, to charge the batteries and put off running the generator. (Source: George Martin CC218)

So, before you get sidetracked with another 3-minute video or bullet-point tutorial, do yourself a favor and read columnist George Martin’s two-part article series “Living and Working Off the Grid.” Here’s an excerpt from Part 1:

“First, I’m an engineer—and an electrical one at that (except my degree is so old that it reads “DC ONLY!”). In addition, my neighbors in New Mexico already have systems up and running. Jeff and Pat live up the road in a handmade log home. Jeff is a former engineer for General Motors (Pontiac GTO, Avanti, and Saturn to his credit). That makes for some interesting discussions about how electronics will revolutionize the car industry, but I digress. He’s a mechanical engineer who doesn’t fully embrace all of this electrical stuff. He has a minimal system with four panels of about 150 W each. Another neighbor’s system has about 3 kW of panels. Armed with the idea that it could be done, I started to match up equipment with our requirements.

The equipment I selected falls into four main categories: solar panels, inverters, charge controllers, and batteries. In fact, you could consider each independently and not get too far off an ideal system. There are, however, some areas of concern when mating equipment from different manufactures, so I stayed with one manufacturer for the control devices.


Solar panels convert solar energy into electrical energy. Again, there is a lot of literature available about how this is accomplished. But what about some hard code conversion details? Standard test conditions require a temperature of 25°C and an irradiance of 1,000 W/m² with an air mass of 1.5 (AM1.5) spectrum. They correspond to the irradiance and spectrum of sunlight incident on a clear day on a sun-facing 37° tilted surface with the sun at an angle of 41.81° above the horizon. This condition approximately represents solar noon near the spring and autumn equinoxes in the continental U.S. with the surface of the cell aimed directly at the sun. Thus, under such conditions, a solar cell with a 12% efficiency and a 100 cm2 (0.01 m2) surface area can be expected to produce approximately 1.2 W of power.[1] This gives you an idea of what’s involved in rating and selecting solar panels. Look at the University of Western New Mexico’s weather site for solar radiation and you’ll get a feeling for the actual solar radiation for the area.

There is another consideration when selecting a panel, namely cost per watt. If you start looking, you will find panels of different wattages and different prices. In March 2004, I started a spreadsheet listing panels from 125 to 195. Note pricing from March 2004, purchased equipment in 2005, installed in 2006–2007, and operational in October 2007. Then, I added the costs different suppliers were charging for each panel and calculated a price/watt number.

My results range from $4.35 to $4.76 per watt. I estimated that I would need 3,000 W of panels, and came up with $13,320 for the cost of the BP Solar SX 170B.

More polysilicon is currently being used in solar panel manufacturing than all other usages combined, so this is big business. It also seems that the larger-power-rating panels command a higher price per watt. It is sort of like the CPU business where chips are speed graded and priced accordingly.

My cost estimates are a bit old, so you’ll need to run the numbers with today’s prices. Let me add that I found solar panels to be in tight supply, so when you begin your design, look to secure the panels at a good price early in the game.

The 3,000 W in my design was derived from the sun’s availability in the winter. Figure 2 represents the solar radiation for an actual cloudless winter day.

Figure 2: The actual solar radiation recorded by Western New Mexico University in Silver City about 30 miles to the west of the house site. (Source: George Martin CC 216)

The peak radiation is 600 W/m2. Let’s estimate that the shape of the curve is a sine function so that the area under the curve is its average value (2/pi, or 0.6366 times the peak value) multiplied by its width. So, that is 600 W/m2 × 0.6366 × 8 h (9 A.M. to 5 P.M.), or 3,055.7 Wh/m2. Therefore: Close to 10 kWh per day is good enough for the workshop, but not enough for the house when it’s built. And 3 kW of panels is what one neighbor is using.

We also need to account for cloudy days. The energy to run the workshop would need to come from the battery or backup generator. Another concern is hot summer days when the panel efficiency drops because of the heat. But the days are longer in the summer. Actually, it’s still a 24-hour day, but there is more available sunlight each day. I don’t have test results for summer generation (because I’m writing this in February 2008 after getting the system put together in October 2007), so stay tuned. The last point to watch out for in panel selection is cold weather open-circuit voltage going into the charge controller.

In the cold, with no current drawn, the open-circuit voltage of the solar panel will rise. If several panels are connected in series (for efficiency), this voltage may damage the input to the charge controller. This is a well-known situation and your equipment dealer will be able to guide you in this area.


I must confess that I find inverters boring. They are not as exciting as solar panels, charge controllers, or even batteries. I thought I would not find much difference in available inverters and that probably was due to my lack of enthusiasm. I selected inverters from OutBack Power Systems. I wanted the inverter/charge controller combination to be from one manufacturer. As I looked at the literature, OutBack seemed to have covered all of the issues for my installation. I ended up with two OutBack VFX3648 inverters (see Photo 2).

Photo 2: The placement of the Outback System next to the feed into the normal house distribution panel. (Source: George Martin CC216)

They are 3.6 kW (continuous) with connections for a 48-V battery and vented. You will find vented and sealed inverters. I selected vented because they typically have a larger power rating and I’m not in a harsh environment.

Also, the inverters are located in an area that is protected from the elements. Another option is a fan on the inverter. The fan also gives you more capacity, but what will you do when the fan fails, and you know it will? Our system is a normal 220-V home application. So, there are two inverters, one for each phase. OutBack has a neat option that includes a transformer to supply the second phase so the second inverter can remain in a low-power operating mode. When the power requirements become large enough, the main inverter will signal the second (slave) inverter to start up and handle the increased load. This is a good setup for our application. We can install a normal commercial heating/cooling system and power up only the second inverter when the load is calling for it.

Click here to read the entire article series.