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Fundamentals of Photovoltaic Systems

 

The basic element of a photovoltaic system is the solar cell. Modern solar cells are made in sizes up to about 6 inches square and are most often made from silicon, a semiconductor.

The photovoltaic effect occurs when sunlight shines on the silicon, freeing electrons and generating an electric current. The electricity is collected and transported by metal contacts on the top and bottom of the cell. The current flows through a wire to provide electricity. 

A single cell produces only about 0.45 volts. It takes 36 cells connected in series to generate the voltage required for charging a 12-volt battery. The power produced in full sunlight is a function of the size of the solar cell and the efficiency (always getting higher as the technology improves).  These cells are usually mounted (laminated with a clear plactic) behind a tempered glass sheet with the cells wired together to form a photovoltaic module (also called a” panel” or “flat-plate collector”). For instance, the most popular design in 2016 is to use 60 cells, each about 6" square, to produce 230 to 310 watts (depending on efficiency) from modules that are about 39" x 66".  The rear of the module is sealed with plastic or glass for protection, long life requires a moisture tight assembly. The modules connected together for one system form an “array.”  Large systems will have these photovoltaic modules connected physically and electrically in "sub-arrays". 

Single cells produce little power and are not often used individually.  They can be found, however, in some items such as small yard lights.    Arrays, depending on the number of panels used, can provide all the electricity for a home or even create a huge generating station.

Modules or arrays are sometimes mounted on tracking systems, which follow the sun across the sky.  These devices help maximize electricity production because sunlight shines directly on the PV modules throughout the day.  Single-axis trackers move as the sun changes position from the east to west.  Two-axis trackers not only follow the suns east to west movement, but also allow for its apparent change in attitude with different seasons. Trackers can increase the energy production of a photovoltaic system by nearly 40 percent, but the tracking equipment costs more than a simple fixed mounting.

Electricity storage is a critical component of many PV systems. If power is needed at night or on cloudy days, solar-generated electricity can be stored in batteries.  It is even possible to have large PV-powered homes, for example, it is not uncommon to find banks of 50 or more batteries.  These battery banks usually store sufficient electricity to power the home through one or two cloudy days and nights.  However, it is generally more cost effective operate larger photovoltaic systems inter-actively with the local electric utility.

All photovoltaic, cells produce direct current (dc) electricity.  That electricity can be used immediately if the PV cell is connected to a device designed for dc power – many refrigerators in recreational vehicles, for example. However most homes and appliances are designed to operate on alternating current (ac) electricity provided by utility companies.  For applications, an inverter, which changes dc electricity to ac must be added to the PV system.

If a system uses batteries, charge controllers are also important components.  These devices protect batteries from excessive charge when the modules produce more electricity than the batteries can store.  They also keep batteries from releasing electricity if their charge is too low.  Without charge controllers, batteries suffer extreme wear-and-tear and become less effective, and last a shorter amount of time.

Insert links from Art 141 Photovoltaics (PV) - Introduction

Solar Electric (PV)

Photovoltaics (PV) covers the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry.

Solar electric has been a gaining strength for a number of years.  In the beginning, early adopters turned to solar for the independence or its environmental benefits: solar homeowners could live beyond the utility lines, and solar was a non-polluting resource.

The earliest adopters were almost exclusively people that lived beyond the reach of the utility lines.  Solar generated electricity and battery storage allowed people to live where ever they wanted. Land was cheaper beyond the power lines and even with the expense of solar, this made economic sense too many people looking to escape urban living. But, the numbers of people living off grid was still a small number.

Environmental concern represented the second wave of solar adopters – as people concerned about the impact their electrical demand was having on the planet turned to solar to lessen their carbon footprint.

According to a 2011 report on renewable energy sources and climate change mitigation, the International Panel on Climate Change calculated the life-cycle global warming emissions associated with renewable energy—including manufacturing, installation, operation and maintenance, and dismantling and decommissioning—as minimal [1].

These findings were repeated in other research and data collected and reported on in peer studies over the past decade and helped fuel the environmental argument for solar energy.

The Union of Concerned Scientists compared the carbon dioxide emissions equivalent per kilowatt-hour for coal and renewable energy resources.

It is no surprise that coal is ranked the most polluting electricity generating resource and renewables the least. Coal emits more than 20 times as much carbon dioxide equivalent per kilowatt-hour of generation compared to the life-cycle carbon emissions for solar PV. The comparison between coal and wind is even greater. Coal emits 71 times more carbon dioxide than wind for each kilowatt-hour of electricity generated [2].

In addition, a study by the U.S. Department of Energy's National Renewable Energy Laboratory explored the feasibility and environmental impacts associated with generating 80 percent of the country’s electricity from renewable sources by 2050 and found that global warming emissions from electricity production could be reduced by more than 80 percent [3].

The healthy alternative extended beyond the concern for the planet, the third wave of solar adopters included people looking for healthy alternatives for humankind.

This wave looked to generating electricity from renewable energy rather than fossil fuels because of the  significant public health benefits.  From reduced premature mortality to lost workdays associated with breathing illnesses, the economic impact of fossil fuels on overall healthcare costs has been estimated at between $361.7 and $886.5 billion [4].

The last wave came was the during the Great Recession of the late 2000s and early 2010s. Solar PV offered the opportunity to re-tool America and create significant jobs as a result.

In 2009, the Union of Concerned Scientists conducted an analysis of the economic benefits of a 25 percent renewable energy standard by 2025; it found that such a policy would create more than three times as many jobs as producing an equivalent amount of electricity from fossil fuels, resulting in a benefit of 202,000 new jobs in 2025 [5].

For the past two or three decades, the reasons for adopting solar have been growing stronger every day.  The tipping point is cost-parity and that day is not far off, and in some cases, it has already been realized.

References:

1.      Intergovernmental Panel on Climate Change (IPCC). 2011. IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation.

2.      Union of Concerned Scientists (UCS). 2009.

3.      National Renewable Energy Laboratory (NREL). 2012. Renewable Electricity Futures Study. Volume 1, pg. 210.

4.      Machol, Rizk. 2013. Economic value of U.S. fossil fuel electricity health impacts. Environment International 52 75–80.

5.      Environmental Protection Agency. 2010. Assessing the Multiple Benefits of Clean Energy: A Resource for States. Chapter 5.

For a more technical introduction to small utility connected PV systems, the interactive PV-Explore presentation presents some details.  This is slightly dated (2012 state of the art), but describes some of the components.  At present it is not integrated with the rest of this website, use your back arrow to return to this page. PV-Explore- Understanding and Troubleshooting Grid Connected PV systems  

For more information, follow the topics below.

Links about larger scale photovoltaic systems:

Some interesting uses of PV:

In Hot Water - Experiences of Solar Hot Water in Arizona

Summary of Presentation given at the World Renewable Energy Forum (WREF)/ASES Conference in Denver, CO in  May 2012
(Full presentation is available for download below.)

During a utility (APS and SRP) funded 2010 Pilot Study to assess SDHW installations for assuring compliance for RECs, it was determined that there was an extremely high rate of failure in meeting basic national guidelines (SRCC), and now with over thousands of audits executed since the Pilot study, there is critical information that needs to be shared with the various solar arenas in Arizona - utilities; governmental code and inspections departments; State licensing agencies; the solar equipment industry; and the design and construction industry; as well as outside Arizona - the nationally growing trade education element; utilities; state and local governmental agencies; and trade organizations in other states.

With the implementation of permanent programs by both Salt River Project (SRP) and Arizona Public Service (APS), the AzSC, acting as a 3rd party neutral resource, has executed over 3000 audits. The findings of this effort are significant, not only for Arizona but also for the larger community - nationally and possibly internationally - for both the solar industry and for the consumer.

The Forum established by the AzSC is intended to share Arizona's experience in various contexts with participation of Daniel Peter Aiello and Geoff Sutton of the AzSC, and Joel Dickinson of Salt River Project. The presentation describes lessons learned, and significant issues discovered that impact the ongoing viability of this technology for government, industry, and the consumer.

The presentation comes from different contexts:

  • The utility experience and viewpoint of lessons learned, issues discovered, and actions taken (and planned) within the context of meeting utility incentives programs requirements.
  • Lessons learned in the trenches, and issues found in the quality of work and industry practice.
  • Conditions and issues involved with the numerous "players" in this arena including the utilities, and those outside the utility context - Registrar of Contractors (ROC), solar equipment organizations and trade associations, building departments and the inspections systems, and the design/construction community.

Full presentation available for download here (7.97 MB PDF).

Handbook of Secondary Storage Batteries and Charge Regulators in Photovoltaic Systems

Photo courtesy NREL

Solar photovoltaic systems often require battery subsystems to store reserve electrical energy for times of zero insolation. This handbook is designed to help the system designer make optimum choices of battery type, battery size and charge control circuits. Handbook of Secondary Storage Batteries and Charge Regulators in PV Systems.

NOTE: All files are PDF format

Complete Handbook (4,337kb)

The following files are divided into sections for easier viewing and download if necessary:


Prepared by: Exide Management and Technology Company, 19 West College Avenue, P.O. Box 336 Yardley, Pennsylvania 19067. Work Performed for The U.S. Department of Energy, Sandia National Laboratories, Albuquerque, New Mexico 87185 Under Contract No. 13-2202. Originally Printed August 1981; Updated 2003 by AzSC Board Members Lane Garrett and Bill Kaszeta.