Final Report: Increasing Photovoltaic Panel Energy Output by Having the Solar Cells Track the SunEPA Grant Number: SU835299
Title: Increasing Photovoltaic Panel Energy Output by Having the Solar Cells Track the Sun
Investigators: Diong, Bill , Chaulagain, Manoj , Daly, Alex , Francis, Teshaun , Guevara, Pedro , Halim, Marco , Hsu, David , Nasseri, Simin , Tippens, Scott , Vanegas, Sonia , Webb, Michael
Institution: Southern Polytechnic State University
EPA Project Officer: Hahn, Intaek
Project Period: August 15, 2012 through August 14, 2013
Project Amount: $15,000
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2012) RFA Text | Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , P3 Challenge Area - Built Environment , P3 Challenge Area - Energy , P3 Awards , Sustainability
Standard photovoltaic panels are constructed by affixing solar cells to the flat surface of its transparent layer. Most of these panels are then mounted on the ground or on roofs at a fixed angle of tilt. With a fixed tilt, the amount of energy collected by the panel will not be maximal throughout the day since optimal performance requires the solar cells to always face the sun directly. To address this problem, numerous tracking systems have been developed that rotate the entire weighty module to track the sun’s movement across the sky, and this results in collecting 29% to 40% more energy than a fixed-tilt panel. However, these solutions generally require significant investments in materials and equipment (such as sensors, controllers and motors), more extensive and complex installation procedures, and additional structural requirements for the mounting surface and for the roofs (should they be mounted there). The wide range of motion also limits how close each module can be placed to other modules so as to avoid undesirable shading of the solar cells.
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%.
The team’s approach consisted of the following key steps:
- Determine the type and design of passive actuators (not motors) that would be most appropriate for rotating lightweight groups of cells in an inexpensive yet reliable and robust photovoltaic module. This will focus on, but is not limited to, methodologies based on the deformation of bimetallic coils (due to direct radiational heating by the sun).
- Work on the design and placement of shadow plates or gratings relative to the bimetallic coils that will act to ascertain the direction of sunlight and determine the proper amount of rotation required from the actuator.
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.
Bimetallic Coil Actuator
Since the means of rotating the groups of cells was key to the proposed concept, much effort was directed at analysis, simulation and testing of bimetallic coils. Due to the project’s short time-frame, off-the-shelf coils that we estimated might be up to the requirements were purchased and evaluated. However, we also developed a computer-aided-design (CAD) model of such coils using SolidWorks® software. The purpose was two-fold: first, to replicate in simulation the behavior of the given coils using different parameters, and comparing the results to real-life test data to show that the coils could behave as needed; secondly, using this model during Phase II for custom design to fine-tune and optimize the coil’s performance and cost.
The best out of the three bimetallic coils was selected because of the following:
- It provides about 2° of displacement per °F of temperature change, when unloaded, as labeled.
- It produces sufficient torque to overcome another passive coil attached to it via rod, and rotate six 52mm x 52mm cells attached to the rod. Moreover, the resulting torque appears adequate to rotate a 2nd rod connected via links in parallel to the 1st rod, although further testing under varying sunlight conditions is needed.
The other two coils did not perform as well as the selected coil.
Arrangement of Cells on the Rods
Our initial concept calls 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. Clearly bypass diodes will be needed, but we also 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).
Our Phase I proposal envisioned each rod being ‘powered’ by a pair of opposing coils. However, our work has allowed us to reduce the required number of coils by 50%, by linking two parallel rods together near their end-points. Hence the rotation of a ‘powered’ rod is transferred to rotate the second rod.
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)
We first characterized the standard panel, although under non-ideal conditions due to various constraints. The obtained i-v characteristic indicates in particular that the open-circuit voltage is about 18V, and the maximum power point occurs at 16.5V and 0.4A, yielding 6.6W. This helped guide us in setting the load value (an adjustable power resistor set to 41 ohms) for the subsequent energy collection tests.
Due to project delays and unfavorable weather conditions, only 1.5 days of outdoor testing was completed before the report deadline. 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 shadow-casting objects, and tilted at an angle of about 10 degrees from the horizontal with the available fixtures. However, we were able to collect data only until about noontime the first test day, when the wind (sustained speed of 16 mph, and gusts up to 29 mph were recorded about that time) managed to overturn the Standard panel, and it broke apart; we found its components scattered around the roof.
Furthermore, we discovered an issue with one of the adjustable loads used for the testing, which meant that only 4.7 hours (on a partly cloudy day, from 2:55pm to 7:39pm - just after sunset) of data from the 2nd day of testing was valid for comparison. Based on the limited test data currently available (we will collect more data between now and the Expo date), we found that the energy collected by the prototype module during those 4.7 hours does exceed that of the standard panel, but by only 1.4%: 1,580 J compared to 1,558 J.
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 bimetallic coil actuators 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 4.7 hours of data which are valid for comparison. Those indicated that the energy collected by the prototype module during that time exceeded that of the standard panel by 1.4%. But further outdoor tests need to be performed.
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.