A Three-Dimensional Approach to Photovoltaic Cells


By Andy Chan, THINK 2013 Winner

Abstract

Solar energy is seen as a source of clean, alternative energy. However, costs of photovoltaic cells (solar panels) are high. Most commercial systems are costly and need complex equipment to manufacture and maintain, which makes PV cells an impractical source of sustainable energy in comparison to fossil fuels. In addition, hundreds of acres are needed to be set aside for solar farms, and that real estate increases the overall costs.

For this experiment, various structures were constructed out of thin-film cells and tested. Their outputs were compared to flat thin-film cells in terms of footprint space and number of panels. To simulate the different times of day, they were tested are different angles of light incident on the surface. The three-dimensional system performed better than traditional flat panels, with higher power output at all angles of light. Some 3-D systems were better than others, especially those that incorporated reflective surfaces and mirrors. The 3-D systems are more aesthetically attractive, reduce footprint space, and can be mass-produced without expensive, high-temperature, high-vacuum processes. This makes their design feasible and cost-effective.


Original Concept

My research on PV cells began with the goal to produce a sustainable and clean source of alternative energy, helping reduce and some day replace the energy dependency on fossil fuels. Originally I wanted to create a new type of PV cells that mimicked the photosynthetic process in plants, but I lacked the mentorship, supplies, and knowledge to do so. But, inspired by nature's design, I realized that leaves have a certain curvature and overlapping structure, and I started exploring the idea of 3-D solar panels. I first experimented with a tower design, but the power output was low compared to traditional flat panels.


Design Process

This led me to investigate designing and creating 3-D panels that can experience larger internal light reflection than current panels. My first hypothesis was that curved PV cells would produce more power output than flat cells. I used thin-film panels because of their ability to bend and flex into different shapes and structures. I began by setting up a series of experiments that tested the hypothesis. I had a controlled environment of 21.7 °F and 9 W LED light, and I varied the circuit'sresistance to see changes in voltage and current. The power output of the PV cell was then circuit × voltage. I compared this to the power output from a flat PV cell that covered the same surface area (same panel costs).

To my surprise, the curved panels had lower power output than the flat panels; in fact, more curvature led to less power output. I reasoned that although the light incident on the surface of the panel has a normal angle of 0°, the surface area for which that holds is very small. As the distance increases, that area exponentially decreases.

I then experimented with basic geometric shapes, such as cube, pyramid, and trapezoid, using the same experimental procedures, but the results revealed that these structures still had lower power output than flat panels with the same surface area. After careful analysis, I stumbled across the idea of creating structures similar to bubble wrap.


Bubble Wrap System

In the Bubble Wrap (BWr) system, two cells are stacked and suspending by string attached to the sides of a clear container. Mirrors are positioned underneath the cubicle to reflect the overhead light onto the bottom cell. In later iterations of the design, curved mirrors are used instead of flat mirrors because they allow light to be focused on the bottom cell. The two cells are wired together in series to produce the same current but greater voltage. A plastic covering helps shield the cells from corrosion and particle settlement. This maintains PV cell efficiency and sustainability.


Results

The BWr system generally had more power output than the flat PV cells. The lowest increase was 2.00% while the highest increase was 166%.

 

  Power (μW)  
Load/resistance (Ω) 2 flat thin film panels BWr system Power increase
Open circuit 0 0 0.00%
540 53.3 54.9 3.00%
100 10 10.2 2.00%
50 3.8 4.7 24.10%
27 2 2.4 19.10%
Short circuit 0 0 0.00%

Test: Load/resistance varies while light angle is held constant at 90° overhead. The 2 flat panels have the same space footprint as the BWr system. Result: The BWr system outperforms the flat panels across all loads and resistances, reaching a peak of 24.1% power output increase.

 

  Power (μW)  
Angle of light overhead 2 flat thin film panels BWr system Power increase
90° 10.1 26.9 166%
60° 6.0 14.4 139%
30° 2.3 5.8 150%

Test: Light angle varies while load/resistance is held constant at 100 Ω. Concave mirrors are implemented in the BWr system to increase power output. Result: The BWr system with concave mirrors outperforms the flat panels across all angles of light, reaching over 100% power output increase. However in hot and bright conditions, concave mirrors can increase the cells' surface temperature and decrease their efficiency, negating some of the power output increase.

 

  Power (μW)  
Angle of light overhead 2 flat thin film panels BWr system Power increase
90° 10.1 26.9 166%
60° 10.1 14.4 43%
30° 10.1 5.8 –43%

Test: Light angle varies while load/resistance is held constant at 100 Ω. Flat panels are equipped with tracking devices to have optimal power out at all angles. Concave mirrors are implemented in the BWr system to increase power output. Result: At angles less than 60°, the flat panels with a tracking device is able to maintain 0° from the normal line, which is smaller than the angle of the BWr system with concave mirrors. Therefore the BWr system's power output decreases. Still, it outperforms the traditional flat panels in the majority of tests.


Peltier Plates

The next set of experiments utilized Peltier plates to cool the cells' surface temperature, since PV cells are more efficient at lower temperatures. This complicated the design and increased its construction and maintenance costs. To prevent heat transfer from the Peltier plate's hot side to its cold side, I attached heat sinks to expel heat from the hot side. Unfortunately, this required an external battery for extra power. I ultimately found that the changes in power output were negligible.


Cost Analysis

Compared to traditional flat panels, the BWr system increases power output per footprint space. To achieve the same power output, flat panels can require costly tracking systems (as I simulated in my tests). Building the traditional system costs $18,000 – $48,000, while building a comparable BWr system costs $13,545 – $23,545.

There are some drawbacks with my approach to be considered, however. Solar energy is overly dependent on its source of light and its location relative to the light. When I tried to conduct experiments outdoors, the data had large margins of errors. Small amounts of cloud coverage caused the power output to greatly decline. The commercially available PV cells I used are still relatively inefficient and expensive.


Conclusions

This project demonstrates that there are unconventional ways to make solar energy more practical and inexpensive than current systems. Solar energy is still inefficient and costly compared to fossil fuels. But, as shown through my BWr system, further investigation in 3-D solar energy systems can make it more competitive.


Acknowledgments

I thank the following people and organizations for their incredible support and dedication:

  • THINK team (MIT) for giving me this once-in-a-lifetime opportunity to pursue my research and providing the funding and guidance needed complete the project
  • John Kymissis (Columbia University) for initial guidance on PV cells and experimental setup
  • Shawn Richard (Cinco Ranch HS), mentor throughout THINK and Intel ISEF
  • Somak Das (MIT) for showing me around MIT and reaching out to MIT faculty and staff

 

I also thank the sponsors for funding my visit to MIT and my research:

  • Thomson Reuters
  • Chevron