Photovoltaic Cells
Introduction
The sun produces 3.9 x 1026 watts of energy every second. Of that amount, 1,386 watts fall on a square meter of
Earth’s atmosphere and is known as the solar constant. The amount of this energy that reaches the Earth’s surface
on a sunny day varies according to the time of year, from about 1,000 watts/meter2 on a sunny summer day to less
than 700 watts/meter2 on a sunny winter day. A watt is a unit used to measure power. This energy can be used to
generate electricity with a device called a photovoltaic cell.
Solar cells, also called photovoltaic (PV) cells by scientists, convert sunlight directly into electricity. PV gets its
name from the process of converting light (photons) to electricity (voltage), which is called the PV effect. The PV
effect was discovered in 1954, when scientists at Bell Labs in New Jersey discovered that silicon (an element
found in sand) created an electric charge when exposed to sunlight. Soon solar cells were being used to power
space satellites and smaller items like calculators and watches.
The performance of a photovoltaic cell (solar cell) is measured in terms of its efficiency at turning sunlight into
electricity. Much of the light energy is reflected or absorbed by the material that makes up the cell. Because of
this, a typical commercial photovoltaic cell has an efficiency of 15% or about one-sixth of the sunlight striking the
cell generates electricity.
According to the Solar Energy Industries Association (SEIA), in 2016 solar installations represented the largest
source of new energy generating capacity in the United States, beating out both natural gas and wind. During this
time, the U.S saw more than 10.5 gigawatts (10,500 megawatts) of utility-scale solar photovoltaic installations
added to the grid, pushing total photovoltaic capacity to nearly 40 gigawatts. This capacity firmly positions the
United States as the world’s 4th largest solar power (behind China, Germany and Japan), and the U.S. is rapidly
increasing its PV capacity.
Figure 1. United States photovoltaic installation capacity 2010-18, including expected installation capacity 20192024
Source: Wood Mackenzie Power & Renewables
Figure 2. Anatomy of a photovoltaic cell
Source: Samlex Solar
Questions
1. In 2009 the total installed capacity of solar PV in the United States was 1.64 GW. In 2019, the U.S.
installed 2.7 gigawatts (GW) of solar PV capacity in the first quarter of the year to reach 67 GW of total
installed capacity, enough to power 12.7 million American homes. Use the formula below to calculate the
percentage increase over this 10-year period. Show all of your work and clearly label each step.
% Change Formula
New Value - Initial Value
---------------------------------- X 100 = % Change
Initial Value
2. Assume you decided to build a cabin off the electrical grid in a remote region of the United States, and
your cabin would require 720 kWh per month to easily meet your needs. You decide to produce your
electricity using photovoltaic cells and store it using a battery system so that it is available when you wish
to use it. If every photovoltaic cell has the potential to produce .25 kilowatts (250 watts), and they
produced electricity for 24 hours a day, how many photovoltaic cells would you need to meet your
electricity needs for a month? Show all of your work and clearly label each step.
3. In practice, darkness, various weather conditions, maintenance and the angle of the Sun limit the
production of each photovoltaic cell to about 15% efficiency. Assuming the photovoltaic cells you are
using to power your cabin operate at 15% efficiency, how many would you need to meet your electrical
needs? Show all of your work and clearly label each step.
4. Using a life-cycle analysis, producing 1 kWh of electricity using photovoltaic cells generates 105 g CO2.
Producing 1 kWh of electricity using natural gas produces 465 g of CO2. How much less CO2 is produced
each month if you produced if you generate 720 kWh of electricity using photovoltaic cells versus using
natural gas? Show all of your work and clearly label each step.
Figure 3. Average number of homes powered by a MW of photovoltaic cells in each U.S. state
Source: Solar Energy Industries Association
5. Megawatts are used to measure the output of a power plant or the amount of electricity required by an
entire city. One megawatt (MW) = 1,000 kilowatts = 1,000,000 watts. For example, a typical coal plant is
about 600 MW in size. For a typical fixed-tilt PV installation, the general rule of thumb is that for every
1kW of photovoltaic cells needed, the area required is approximately 100 square feet. This means that for
a 1mW solar PV power plant, the area required is approximately 2.5 acres (1 hectare) or 100,000 square
feet. However, it’s important to keep in mind that this amount of land is just for the panels themselves,
and doesn’t include the space required for other solar equipment, which can bring the total to
approximately 4 acres (1.6 hectares) for a 1mW solar farm. Suppose you live in a state where 1 MW of
photovoltaic cells powers 125 homes, how much land would be needed to power 25,000 homes?
Figure 4. Average daily total solar radiation at land surface in the United States
6. Based upon the information provided in figure 3, list the 10 states would you expect to produce the
greatest amount of electricity using photovoltaic cells?
7. In 2018, the top ten states (in order) in terms of installed photovoltaic capacity are: California, North
Carolina, Arizona, Nevada, Texas, New Jersey, Massachusetts, Florida, Utah and Georgia. What reasons
might explain the difference between the lists?
Figure 5. Average daily total solar radiation at land surface in the World
Figure 6. Human population density in persons per square kilometer
Source: Wikimedia Commons
8. Based upon the information provided in figure 5 and figure 6, construct an argument that identifies the
best areas to place large arrays of photovoltaic cells to provide electricity for humans.
9. Design an experiment that measures how changing the angle of a photovoltaic cell affects its electricity
production.
a. Identify the independent variable.
b. Identify the dependent variable.
c. State the hypothesis.
d. Describe the procedure you would use to test your hypothesis.
10. Why does the angle of the photovoltaic cell affect the amount of electricity it produces?
11. Why would the season of the year possibly affect the efficiency of the photovoltaic cell?
12. Besides angle, what other factors affect the energy output of a photovoltaic cell?
13. Based on the results of this investigation would the power output of the photovoltaic cell be greater or
less just beyond Earth’s atmosphere? Defend your claim with evidence and reasoning.
References/Resources
April 25, 1954: Bell Labs Demonstrates the First Practical Silicon Solar Cell. (n.d.) (2009). Retrieved from
https://www.aps.org/publications/apsnews/200904/physicshistory.cfm
Frangoul, Anmar. (2018). From California to Texas, these are the US states leading the way in solar. Retrieved from
https://www.cnbc.com/2018/09/19/the-us-states-leading-the-way-in-solar.html
How is electricity measured? (n.d.) Retrieved from https://www.ucsusa.org/clean_energy/our-energy-choices/how-iselectricity-measured.html
1 kilowatt-hour. (n.d.) Retrieved from https://blueskymodel.org/kilowatt-hour
Klatz, Cheryl. (2017). Northern Lights: Large-Scale Solar Power Is Spreading Across the U.S. In Yale Environment
360.
https://e360.yale.edu/features/northern-lights-utility-scale-solar-power-spreading-across-the-us
U.S. Energy Information Administration. (2018). Most utility-scale fixed-tilt solar photovoltaic systems are tilted 20
degrees-30 degrees [Data file]. Retrieved from
https://www.eia.gov/todayinenergy/detail.php?id=37819
U.S. Energy Information Administration. (2019). Photovoltaics and Electricity, U.S. Energy Information Administration
[Data file]. Retrieved from https://www.eia.gov/energyexplained/index.php?page=solar_photovoltaics
Runyon, Jennifer. (2019). U.S. solar market tops 10 GW in 2018, again. In Renewable Energy World. Retrieved from
https://www.renewableenergyworld.com/articles/2019/03/us-solar-market-tops-10-gw-in-2018-again.html
National Renewal Energy Laboratory. Solar Photovoltaic Technology Basics [Data file]. Retrieved from
https://www.nrel.gov/research/re-photovoltaics.html
Perea, A., Honeyman, C., Kann, Shayle, Mond, Allison, Shiao, MJ, Jones, Jade, Moskowitz, Scott, Smith, Colin,
Gallagher, Benjamin. (2016). Solar Market Insight Report 2016 Year in Review. Retrieved from
https://www.seia.org/research-resources/solar-market-insight-report-2016-year-review
McFadden, Christopher. (2017). Top 10 Performing Countries for Solar Energy. Retrieved from
https://interestingengineering.com/top-10-performing-countries-for-solar-energy
Solar Energy Industries Association. (2018). What’s in a Megawatt? Retrieved from
https://www.seia.org/initiatives/whats-megawatt