Grantee Research Project Results
Final Report: Achieving Increased Photovoltaic Panel Energy Collection with Cell-Strings that Track the Sun
EPA Grant Number: SU835691Title: Achieving Increased Photovoltaic Panel Energy Collection with Cell-Strings that Track the Sun
Investigators: Diong, Bill , McFall, Kevin , Tippens, Scott
Institution: Southern Polytechnic State University
EPA Project Officer: Page, Angela
Phase: I
Project Period: August 15, 2014 through August 14, 2015
Project Amount: $15,000
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2014) RFA Text | Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , P3 Challenge Area - Sustainable and Healthy Communities , P3 Challenge Area - Air Quality , P3 Awards , Sustainable and Healthy Communities
Objective:
The SPSU Sun Seekers team will develop a photovoltaic module that encloses small groups of solar cells that can track the sun. This will allow the module itself to be mounted simply at a fixed tilt but still reap the substantial energy collecting benefits associated with active solar tracking panels, while avoiding their significant additional complexity, cost and weight. The project’s key technical objective is to design a fixed-tilt photovoltaic module, enclosing groups of cells that can track the sun, which improves upon the amount of energy collected by a similarly-sized fixed-tilt solar panel by at least 25%.
Summary/Accomplishments (Outputs/Outcomes):
During the course of this project, analysis, simulation and testing were performed to obtain a thorough understanding of the fundamental issues involved in designing and prototyping a fixed tilt passive tracking photovoltaic (PV) module that can improve upon the amount of energy collected by a similarly-sized standard fixed-tilt panel.
Power Transmission (Linkage) Design The power transmission linkage is the assembly which transmits rotation of the motor shaft to the rotation of the other shafts in the solar panel assembly that support the PV cells. A design was sought to permit 70° of rotation in either direction from a horizontal orientation. Three concepts were generated (rack and pinion, belt and chain link, and parallel linear linkage), a couple were prototyped, and the best of those was selected via a design matrix for small-scale prototype demonstration. The linear linkage concept was the one that would require the least number of parts to construct, and also would require the least lubrication. It was also the only concept that would theoretically allow some angular limits to be imposed physically, as opposed to electronically which would require periodic recalibration. Motor Selection In order to rotate the rods via the linkage system, a motor is required. Three types of motors were considered: stepper motors, DC gear motors, and servo motors. DC gear motors have enough gear reduction (in combination with the intrinsic friction of the rods and linkage system) to allow the motor to stay in place even when power is removed from it. This results in much lower energy consumption than a stepper motor since the DC gear motor only needs to be powered when actively rotating the solar cell rods. It is also less costly than a comparable servo motor
Arrangement of Cells on the Rods
Our initial concept called for the cells to be grouped into columns affixed to rotatable rods (think shish-kebab) that will be oriented in a North-South direction when the new module is installed. The baseline arrangement was to line up the cells side-to-side along the rods as these cells are presently laid out on panels. Then a key technical issue presents itself when these cells are turned towards the early-morning sun or the late-afternoon sun, and cast a shadow on the cells behind them. Hence we asked the question - is there a cell arrangement that would produce less shading. Coming up with an interleaved ‘diamondback’ cell arrangement, with the cells lined up corner-to-corner along the rods, the team calculated that shading could be reduce by as much as 25% over a reasonably-defined range of sun angles, and of cell angles (allowing for some sun-tracking imprecision). Sunlight Direction Sensing The team chose a photodiode to detect the direction of maximum solar intensity. The photodiode was chosen over similar elements like photoresistors and phototransistors because photodiodes produce a current in proportion to increasing light intensity. Furthermore, photodiodes act as micro solar cells, requiring no extra energy to function. This of course furthers the goal of a power saving design solution. Then, the sun is tracked by adjusting the direction of the rotatable photodiode - affixed to the motor shaft - to the angle that yields the highest output current from the photodiode as measured by the microcontroller, which controls the motor’s rotation.
Motor Control
Two microcontrollers, the ATMega328p and the C8051F34x, were evaluated for controlling the DC gear motor, which rotates the rods supporting the PV cells (and the photodiode). Both the ATMega328p and the C8051F34x provided sufficient capability to carry on the simple functions of basically rotating the motors clockwise and counter-clockwise, and measuring the photodiode current. Ultimately, the ATMega328p was selected because it was much less expensive. The algorithm used to control the motor is essentially a guess and check method, but done with some intelligence so that the ATMega328p only guesses and checks when required so that it can minimize power usage. The entire algorithm depends on three pieces of information: a single analog reading that yields the light intensity at the current angle/direction of the photodiode (and cells), and two limit switch outputs that indicate when the cells are pointed toward the western or eastern horizon. The analog reading is the primary source of control during the day, whereas the limit switches assist with re-setting the panel after sunset to point the cells towards the eastern horizon in readiness for the next morning’s sun.
PV Cell Encapsulation
Solar cells are extremely fragile and require extra protection if the forces acting on it are not evenly distributed across the entire cell. Silicone provides a protective shield that will increase the solar cells’ durability. Also, silicone is very transparent to UV-wavelength. Solar cells and soldered tabbing wires often undergo temperature damage over time and lose efficiency. The additional step of encapsulating the solar cells prevents temperature damage. The encapsulant that the team decided to use was the Sylgard 184 silicone elastomer kit which consisted of a base compound to be combined with a curing compound. Even though this curing agent could be cured with either heat or at room temperature, the decision was made to let the encapsulant be cured at room temperature to ensure it would spread evenly around all sides of the PV cells.
Testing and Results
Two panels were constructed: the first being a standard fixed-mount type panel and the second being a prototype of our design concept. To ensure a valid comparison of their energy collection performance, we kept several panel parameters the same including the following:
● both include 36 cells of the same material (16% efficient multi-crystalline Silicon solar cells) and dimensions (52mm x 52mm) connected in series, so the total cell area and the panel voltage are practically identical;
● both had their cells wired together using the same type of tabbing wire
● both include the same type, number and connection of diodes (bypass and blocking)
The test data to be obtained from the standard solar panel and the cell-tracking solar module are: voltage and current. From those two values, the total electrical energy collected can be determined. The team decided to use LabView SignalExpress software from National Instruments, together with the PCI-6259 DAQ board and the SCB-68 connector block hardware for this purpose. Due to the input voltage restrictions of this PCI board, the output voltage of the panel had to be reduced with a voltage divider (10 kΩ in series with 5 kΩ) sense circuit. The current is measured via a current sense resistor (0.25 Ω) connected in series with the load; current is found as the voltage of the current sense resistor divided by its resistance. The power is then calculated by multiplying the voltage and the current at each sampling time instant (sampling rate is 1 Hz). The energy collected daily is then determined as the integral of the power (using the trapezoidal approach) over the daylight hours. We first characterized the standard panel, although under non-ideal conditions due to various constraints. The obtained current-voltage characteristic helped guide us in setting the load value (an adjustable power resistor set to 39.5 ohms) for the subsequent energy collection tests. Both the standard panel and the prototype module were moved to the roof of the SPSU Engineering Technology Center, which has a small but conveniently located instrumentation room with portholes for running cables between the inside data collection instruments and the outside device under test. Both devices were positioned facing the South away from shadowcasting
objects, and tilted at an angle of about 30° from the horizontal with the available fixtures. Due to project delays and unfavorable weather conditions, only 3 days of outdoor testing was completed before the report deadline, with motor control problems encountered during the first 2 days. On the third day of testing, a cloudy day, the motor control appeared to be working and the energy data for that day was computed. The results indicate that the prototype collected almost 8 times more energy than the standard panel; 3,842 J compared to 620 J. Unfortunately, it appears that the standard panel has a problem, one that we are in the process of resolving. Once this problem is fixed, we will continue to collect comparative outdoor test data, and report on these at the National Sustainable Design Expo.
Conclusions:
Our student team, the SPSU Sun-Seekers, has designed and prototyped a photovoltaic module that encloses small groups of solar cells that can track the sun. Indoor testing validated the concept of using a small motor and simple linear linkage to rotate those cells, which are to have an interleaved ‘diamondback’ cell arrangement, with the cells lined up corner-to-corner along the rods. The limited outdoor testing on the baseline standard panel and the prototype module that could be performed generated 1 day of possibly usable data. Those indicated that the energy collected by the prototype module during that time exceeded that of the standard panel by almost 700%, which may not be reasonable. But further outdoor tests need to be performed. Also, it should be noted that the present prototype requires components that cost about 30% more than for the standard panel, and it has a footprint that is about 50% greater than the standardpanel.To conclude, increasing the amount of energy collected by innovative photovoltaic panels, such as the one that our Sun-Seekers group has proposed, should lead to more widespread adoption of solar energy for electric power production in this country and around the world. Consequently, less electric power produced by coal-fired and gas-fired power plants will result in less pollution from those plants, leading to healthier living conditions for all mankind.
Supplemental Keywords:
Sun-tracking solar cells; solar panel; maximizing solar energy collection; cell‐strands.The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.