Uncovering the Secrets of the Solar Panel
Zeyang Ren
University of Southern California
Currently, solar panel technology has the potential to revolutionize how we produce
electricity from nature without using traditional equipment, such as thermal
generating plants. Requiring no generators and emitting no pollutants, the use of solar
panels can be involved not only in commercial and residential applications, but also in
most aerospace productions. Although solar panels are still partially in the
development stage, it promises vast opportunities in the future.
Introduction
Because of the world’s growing population, there will be more demand for energy in
the future. Currently there is too much pressure on fossil fuels and wood sources of
energy, which could deplete easily and are not renewable, to produce electricity.
With the development of technology, people finally invented the solar panel to
convert solar power, a natural and renewable energy, to electricity. A solar panel,
also called a solar module, has a simple but meticulous structure. As shown in
Figure 1.1, the external structure is apparently several pieces of glasses, which
allows light to pass through while protecting the semiconductors inside from
oxidation and corrosion.
Figure#1.1 Solar Panels
Inside the glasses, we can see that a solar panel is an assembly of solar cells. These
solar cells could achieve a satisfied output voltage when connected in series or get a
desired current capability when connected in parallel. Most of the connecting wires
between solar cells are made of non-magnetic conductive transition metals, such as
silver and copper. Although solar panels are still in a discovering state, they are
available for a wide range of applications to save our environment from getting
polluted to a great extent.
Smallest unit: solar cell
A solar cell, also called photovoltaic cell, is the most significant part of a solar panel.
It is an electrical device that is used to convert light energy directly into electricity.
The electricity does not depend on any external generators but natural light. The
Figure 2.1 below is the most basic solar cell, which is called a p-n junction solar cell.
The solar cells are connected in a circuit with a load [1].
Figure#2.1 A p-n junction solar cell with resistive load [1]
Basically, materials of solar cells are made of silicon or silicon oxide. In the solar
cells, the positive and negative carriers are separated into the P region and the N
region. The region between two opposite carriers is called the space charge region.
In this region, the left edge would be some negative charges due to the P region and
vice versa. In this situation, even with zero bias applied to the junction, an electric
field in the free space charge region exists due to the separate carriers on the sides.
When photons in sunlight hit the solar cells and are absorbed by the space charge
region, the electrons in this region are knocked loose relatively because the
concentration of the positive charges is increased [1]. This would cause an electric
potential difference. At this moment, current 𝐼𝐿 is created when positive charges
flow from right to left in Figure 2.1 [1]. According to the special composition of solar
cells, most current 𝐼𝐿 flows in the single direction and creates the direct current
electricity.
The most recent generation of solar cell is the Amorphous Silicon Solar Cell (ASSC),
which introduced the largest lights-silicon contacting area, and manufacture
relatively inexpensive solar cell systems. From now to achieve more and more
energy transmission, each ASSC is about 40 cm wide and many meters long. The
fundamental process to make ASSC is simple, as shall be explained.
When silicon is deposited by specific techniques at the temperature below 600
degrees, an amorphous film (a thin film) is formed regardless of the type of
substrate. And when adding hydrogen in the thin film, a new material is created,
which is called hydrogenated amorphous silicon. Amorphous Silicon Solar Cell is
just a normal solar cell by adding the hydrogenated amorphous silicon on the top to
achieve a high optical absorption coefficient. By using it, most sunlight is absorbed
within approximately 1 um of the surface. Although the ASSC’s conversion
efficiencies are smaller than the traditional solar cells, ASSC’s price makes it more
attractive [2].
Because a single solar panel can produce only a limited amount of power, we have to
install about 40 of these to create a solar panel [3]. Moreover, in order to increase
the efficiency and their multiple uses, multiple panels can compose a photovoltaic
system with a DC-AC inverter and solar tracker.
Application
Based on the report from U.S. Department of Energy, solar panels can generate from
10 watts power to 300 watts power [4]. This broad range would depend on different
kinds of solar cells on the market. While some small panels are available to support
small consumer devices, large panel systems are enough for residency applications.
For example, the roof-mounted solar panels are available for vehicles to power
electronic systems. Lightweight portable solar panels, as shown in Figure 3.1, are
available to power small batteries or some other electronic devices, such as laptops
and cell phones [3]. These devices are currently on the market due to its flexibility.
Figure#3.1 Lightweight portable solar panel and Taiwan’s dragon-shaped arena
The most amazing application of solar cells is Taiwan’s dragon-shaped arena (Figure
3.1), designed by Toyo Lio [5]. This arena has a capacity of 50,000 seats, and 100% of
its electricity is created by solar panels. About 14,155 sq. meter solar roofs with
8,844 solar panels are used to generate the electricity power to support 3,300 lights
and two jumbo vision screens [5]. Although building such a new high-tech stadium is
always a massive undertaking that requires millions of dollars and highly skilled
labors, it is symbolic of construction that uses nature energy and saves the
environment from being polluted.
Moreover, high-tech products are always used on other high-tech products. Solar
panels play a significant role in the field of aerospace. In order to complete more
missions in space or other planets, how long a device can operate is one of the most
considerable problems. Even though the device can bring a super charged battery to
space and produce electricity, batteries eventually become depleted. However, with
the innovation of solar panels, the problem could be solved easily because devices
could convert the solar power in space to electricity automatically to maintain their
normal operation without any battery. By using solar panels, devices can operate as
long as they are required to. For instance, the International Space Station (Figure 3.2)
uses many solar panels on its wings to produce electricity to support the whole
station operating functionally [6]. International Space station total has 8 solar array
wings, which consists of 168 solar panels in each wing. These solar panels can
generate maximum 246 kilowatts to support to operation of the international space
systems. [6]
Figure# 3.2 solar panels on International Space Station
Which one is better, tradition or innovation?
Among several decades, the most traditional method to produce electricity has been
thermal generating plants, such as fossil fueled plants, and cogeneration. Thermal
generating plants are highly efficient and profitable to operate, because most
utilities in this world have relied on the thermal generating plants for decades.
However, thermal generating plants do cause serious environment problems,
because burning fossil fuels such as coal generates carbon dioxide, nitric oxide and
sulfur dioxide and cause severe air pollution. Even though most developed countries
have good pollution controls, it is still a critical part to consider that fossil fuels are
insufficient on earth and waste materials.
However, solar panels are really fantastic devices to convert the clean energy-light
to electricity without using any generators. Because sunlight is the only power they
need, they emit no pollutants and need little maintenance. Therefore for a long-term
consideration, solar panels would be the majority way to generate electricity instead
of thermal generating plants.
Limitations
As is known to all, the biggest issue to make the better solar panel is its conversion
coefficient. Currently, the most efficient solar panel on the market can only convert
10%-13% of solar power [7]. Also, the costs should be taken into consideration
when making solar panels into the market, because this high technology mainly
requires high quality silicon semiconductor as its raw material. Thus, when
considering the rate of return of investment, solar panels are always the last choice
compared with thermal generating plants. Furthermore, when using solar panels, its
natural limitation is sporadic availability, which would be the biggest limitations
people cannot change. Solar panel would stop working and producing electricity
when clouds and nightfall interrupt [7].
Looking Ahead
Future development of solar panel will focus on the concentrating sunlight and
multiple junctions of solar cells. For example, the Solar Junction, a company based in
California, has set a record of 43.5% from concentrating sunlight. This high ratio
would make more energy when this conversion rate is a constant.
Furthermore, the most important step to improve its efficiency may be using new
nanotechnology. Firstly, solar panel’s efficiency could be improved by changing the
semiconductor materials. Currently, professors of the Viterbi School of Engineering
at the University of Southern California are seeking new nano material to substitute
the original semiconductor, which could achieve solar panel’s conversion coefficient
up to 50%. Also, by using nano technology, people could create solar panels to fit
into any materials. For example, plastic solar panels are recently being developed.
Each solar cell uses organic nanotechnology, which could be incorporated into
different kinds of materials such as plastics and canvas. This kind of plastic solar
panel could be used as military tents and clothing, so that soldiers do not need to
carry many redundant power supply during the mission. Everything is possible in
the future. People can still speculate that the transparent solar panels will be
invented by changing their materials. People could use it as the transparent solar
windows, which not only can be used as normal windows, but also can produce
electricity for the house use at the daytime.
Conclusion
Solar panels are a technology that has been developed frequently in the past 20
years, but it is still questionable due to its efficiency, high costs and sporadic
availability. Due to its range of applications and its upcoming development, however,
solar panels show great potential to substitute the original thermal generating
plants to improve our polluted environment. With any luck, solar panels will soon
become efficient and reliable devices to make human beings’ life better.
References
[1]. D.A. Neamen, “An Introduction to Semiconductor Devices”, 1st Ed, Boston, 2006, ch 12.2, pp.596-604.
[2]. Fuji Electric Systems Co.(2004 Oct 1st) “Introducting Amorphous-Silicon Solar Cells” [online]. Available:
http://www.fujielectric.com/company/news/2004/04100101.html
[3]. S. Grey, (2004), ”Introducing to Solar Panels” [online]. Available:
http://greenliving.nationalgeographic.com/introduction-solar-panels-2347.html
[4]. Sunpower Corporation (2011) “E20/327 Solar Panel” [online]. Available:
http://us.sunpowercorp.com/cs/Satellite?blobcol=urldata&blobheader=application%2Fpdf&blobheadername3=C
ontentDisposition&blobheadervalue3=attachment%3B+filename%3D11_317_sp_e20_327_sf_ds_en_ltr_p.pdf&blobke
y=id&blobtable=MungoBlobs&blobwhere=1300275818929&ssbinary=true
[5]. D. Pham (2011 May 27th), “Dragon-Shaped Solar Stadium in Taiwan is 100% Powered by the Sun” [online].
Available: http://inhabitat.com/taiwans-solar-stadium-100-powered-by-the-sun/
[6]. Kauderer (2000 September 11th ), “STS-106 Shuttle Mission Imagery” [online]. Availability:
http://spaceflight.nasa.gov/gallery/images/shuttle/sts-106/html/s106e5116.html
[7]. P. Duxbury, “Theoretical and Practical Limits on Solar Energy Conversion: Why use Nano-structured material ”
[online]. Availability: http://www.pa.msu.edu/cmp/CORE-CM/SolarEfficiency.pdf