Photovoltaics

Photovoltaic Basics

 

How is electricity is produced by a photovoltaic — what we often call a PV or solar electric — system? To help you understand, SolarFAQs.com covers the basics of PV technology, which includes the underlying physics, how various PV devices are designed and become fully functional systems, and what’s happening today in PV research and development.

The Solar Energy Technologies Program of the U.S. Department of Energy (DOE) and its partners are adding to our fundamental knowledge and expertise in this area while improving the technologies that put the abundant energy of sunlight to work for us.

PV Physics

What is meant by the word photovoltaics? First used in about 1890, the word has two parts: photo, derived from the Greek word for light, and volt, relating to electricity pioneer Alessandro Volta. So, photovoltaics could literally be translated as light-electricity. And that’s what photovoltaic (PV) materials and devices do — they convert light energy into electrical energy (Photoelectric Effect), as French physicist Edmond Becquerel discovered as early as 1839. Commonly known as solar cells, individual PV cells are electricity-producing devices made of semiconductor materials. PV cells come in many sizes and shapes — from smaller than a postage stamp to several inches across. They are often connected together to form PV modules that may be up to several feet long and a few feet wide. Modules, in turn, can be combined and connected to form PV arrays of different sizes and power output. The size of an array depends on several factors, such as the amount of sunlight available in a particular location and the needs of the consumer. The modules of the array make up the major part of a PV system, which can also include electrical connections, mounting hardware, power-conditioning equipment, and batteries that store solar energy for use when the sun isn’t shining. PV systems are already an important part of our lives. Today, PV systems provide power for many calculators and wristwatches. More complicated systems provide power for communications satellites, water pumps, and the lights, appliances, and machines in some people’s homes and workplaces. Many road and traffic signs along highways are now powered by PV. In many cases, PV power is the least expensive form of electricity for performing these tasks.

PV Devices

Photovoltaic devices can be made from various types of semiconductor materials, deposited or arranged in various structures, to produce solar cells that have optimal performance. The three main types of materials used for solar cells are—

  • Silicon, which can be used in various forms, including single-crystalline, multicrystalline, and amorphous.
  • polycrystalline thin films, Copper indium diselenide (CIS), cadmium telluride (CdTe), and thin-film silicon.
  • single-crystalline thin film, Cells made with gallium arsenide. We then discuss the various ways that these materials are arranged to make complete solar devices. The four basic structures we describe include homojunction, heterojunction, p-i-n and n-i-p, and multijunction devices.

PV Systems

A photovoltaic (PV) or solar cell is the basic building block of a PV (or solar electric) system. An individual PV cell is usually quite small, typically producing about 1 or 2 watts of power. To boost the power output of PV cells, we connect them together to form larger units called modules. Modules, in turn, can be connected to form even larger units called arrays, which can be interconnected to produce more power, and so on. In this way, we can build PV systems able to meet almost any electric power need, whether small or large. PV systems can be classified into two general categories: flat-plate systems or concentrator systems. We will talk about the differences between these two types of systems later on. By themselves, modules or arrays do not represent an entire PV system. We also need structures to put them on that point them toward the sun, and components that take the direct-current electricity produced by modules and “condition” that electricity, usually by converting it to alternate-current electricity. We might also want to store some electricity, usually in batteries, for later use. All these items are referred to as the “balance of system” (BOS) components. Combining modules with the BOS components creates an entire PV system. This system is usually everything we need to meet a particular energy demand, such as powering a water pump, or the appliances and lights in a home, or, if the PV system is large enough, all the electrical requirements of a whole community.

Energy Payback Times for Photovoltaic Technologies

Energy payback time (EPBT) is the length of deployment required for a photovoltaic system to generate an amount of energy equal to the total energy that went into its production. Roof-mounted photovoltaic systems have impressively low energy payback times, as documented by recent (year 2004) engineering studies. The value of EPBT is dependent on three factors: (i) the conversion efficiency of the photovoltaic system; (ii) the amount of illumination (insolation) that the system receives (about 1700 kWh/m2/yr average for southern Europe and about 1800 kWh/m2/yr average for the United States); and (iii) the manufacturing technology that was used to make the photovoltaic (solar) cells.

With respect to the third factor, i.e., manufacturing technology, there are three generic approaches for manufacturing commercial solar cells. The most common approach is to process discrete cells on wafers sawed from silicon ingots. Ingots can be either single-crystal or multicrystalline. However, in either case, the growing and sawing of ingots is a highly energy intensive process. A more recent approach which saves energy is to process discrete cells on silicon wafers cut from multicrystalline ribbons. The third approach involves the deposition of thin layers of non-crystalline-silicon materials on inexpensive substrates. It is the least energy intensive of the three generic manufacturing approaches for commercial photovoltaics. This last group of technologies includes amorphous silicon cells deposited on stainless-steel ribbon, cadmium telluride (CdTe) cells deposited on glass, and copper indium gallium diselenide (CIGS) alloy cells deposited on either glass or stainless steel substrates.

Recent research has established battery-free, grid-tied EPBT system values for several (year 2004-early 2005) photovoltaic module technologies (see Table 1). In Table 1, the values in the last column are the reciprocals of the respective values in the third column. It is seen that, even for the most energy intensive of these four common photovoltaic technologies, the energy required for producing the system does not exceed 10% of the total energy generated by the system during its anticipated operational lifetime. Future research will extend the table to include amorphous silicon and CIGS alloys.

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