Introduction to Photovoltaics

Transcription

1 INTRODUCTION Objectives Understand the photovoltaic effect. Understand the properties of light. Describe frequency and wavelength. Understand the factors that determine available light energy. Use software to simulate the photovoltaic effect.

2 Introduction to Photovoltaics Solar energy is emitted from sunlight that radiates onto the earth. Solar energy output each hour is a billion times more than the amount of electricity used by the world s population. Not all of this energy reaches the earth s surface. Some of this energy is absorbed by the atmosphere and some energy is reflected into space. However, each day enough energy radiates onto the United States that it could provide power to the country for more than a year. The amount of energy radiated to a specific area is dependent on the time of day, the season, the amount of cloud cover, and the distance from the earth s equator. Photovoltaics is the generation of electric current from radiant light. A photovoltaic (PV) cell is a device that converts solar energy to electricity; it is often called a solar cell. PV cells do not produce noise or pollution, and they do not require the use of fossil fuels or other natural resources to create electricity. Therefore, photovoltaics is an important technology that will help us reduce pollution and conserve the earth s natural resources.

3 History of Photovoltaics In 1839, Edmond Becquerel found that a small amount of current, or the flow of electrons, occurs when materials are illuminated with light between electrodes immersed in an acidic solution. Heinrich Hertz observed that the same phenomenon occurs in metals. This phenomenon is called the photovoltaic effect. The photovoltaic effect is the production of current that develops from the absorption of light through two materials. In the late 1800s, R.E. Day observed the photovoltaic effect with selenium, and then C. E. Fritts created thin selenium films that were inexpensive yet still produced the photovoltaic effect. However, selenium did not become a common PV material because selenium cells did not produce enough power to make up for their cost.

4 In the early 1900s, research in physics and light theory contributed to the advancement of photovoltaics. Through the 1940s and 1950s, J. Czochralski developed a method to produce highly-pure crystalline silicon. This method led Bell Telephone Laboratories to produce a highly-efficient silicon PV cell. In the late 1950s, the United States began to use PV cells to power satellites and their success led to PV cell use in other space programs. Eventually, photovoltaics became efficient enough for commercial usage to power large buildings, and economic enough for individual usage to power small devices such as a water heater or a calculator.

5 Light Energy To understand photovoltaics, it is necessary to understand how light interacts with PV cells. Sunlight that reaches the earth s surface is the energy source used by PV. Each part of the sun s radiation from visible to invisible light has a different amount of energy. The electromagnetic spectrum is a range of all possible frequencies of radiation. The electromagnetic spectrum characterizes light as a wave which has a specific wavelength or frequency. The different colors of light can be arranged by their wavelength or energy level. In the visible part of the spectrum, red light has the longest wavelength and the lowest energy; and violet light has the shortest wavelength and highest energy. Infrared and ultraviolet light are invisible. Infrared light has a lower energy than visible light, and ultraviolet light has a higher energy than visible light. Sunlight contains a large amount of infrared and ultraviolet light; however, most of the invisible light is blocked by the earth s atmosphere. The energy produced by PV cells is generated from visible light.

6 In the 1800s, many scientists experimented with light rays and began to characterize light as a wave. It is useful to describe light as a wave because the light rays interact with objects in a wave pattern. James C. Maxwell began experimenting with electromagnetic waves and derived mathematical constants for electricity and magnetism. Maxwell used these constants to develop frequency and wavelength for the electromagnetic spectrum. All waves have a certain amount of space between them which is measured from each peak. Wavelength is the distance between two peaks of a wave. Frequency is the number of waves in a specified distance or time. Therefore, a long wavelength means there is a large distance between peaks which occur at a low frequency. Inversely, a short wavelength has a small distance between peaks which occur at a high frequency.

7 In the early 1900s, Max Planck determined that light is emitted from atoms as small packets of energy which he called quanta. A quantum is the smallest possible amount of a physical property, such as a specific quantity of electromagnetic radiation. In 1905, Albert Einstein showed that light behaved in localized packets and acted like particles. He also determined the values of the light energy quanta which he called photons. A photon is a quantum of light. The amount of energy in a photon is dependent on the wavelength of the light. Einstein showed that only photons with a short wavelength produce current. Light is now described as behaving as waves and as particles when observed in nature; therefore, photons are now described as a wave packet. A wave packet is a pulse of waves that travel as a unit.

8 Not all sunlight reaches the earth s surface. The earth s atmosphere and clouds absorb, reflect, and scatter sunlight. A majority of x-rays are absorbed before they reach the earth s surface and most ultraviolet light is filtered by the atmosphere. Sunlight may be direct or diffuse. Direct light is radiated directly from the sun without reflecting from any other objects. Diffuse light is scattered from clouds, dust, objects, or the earth s surface. Direct and diffuse light usually have different distributions of color and energy levels. Some photovoltaic systems can use both direct and diffuse light, but most of them use only direct light.

9 Air mass (AM) is a measure of the length of the path which light travels through the atmosphere to a surface. Light that reaches the earth s surface is measured against the light that enters the outer edge of the atmosphere where air mass is zero to determine the amount of light energy lost in the atmosphere. This energy loss varies with the thickness of the atmosphere. When the sun is positioned lower in the sky, the light passes through a thicker part of the atmosphere and loses more energy.

10 Solar Radiation When sunlight reaches the earth s surface, it is distributed unevenly in different geographic regions. Global radiation is the total amount of sunlight that reaches a surface; it is composed of direct and diffuse light. The actual amount of sunlight falling on a specific geographic region is known as insolation or incident solar radiation. Regions near the equator receive more incident solar radiation than other regions on earth. The amount of sunlight also depends on the seasons, climate, time of day, cloud cover, and air pollution. The angle at which sunlight passes through the atmosphere determines the intensity of light energy. When the sun is directly overhead, the light rays have a shorter path through the atmosphere with less obstructions than when the sun is positioned at a low angle to the horizon. The position of a region on earth relative to the sun is determined by three angles. Two angles are variable, the hour angle and the declination angle; the third angle is a fixed angle, which is the region s latitude. The hour angle is determined by the position of the earth in its current axial rotation. The declination angle is the position of the sun at its highest point in the sky relative to the equator; this angle changes with the position of the earth s rotation about the sun. These angles indicate how a region on earth moves relative to the sun. The sun s position in the sky can be calculated with these three angles. However, to calculate the sun s position relative to earth requires two different angles; elevation angle the azimuth angle. The elevation angle is a measure of the sun s angle relative to the horizon; it is also known as the altitude angle. The azimuth angle is a measure of the sun s angle from the north-south meridian.

11 To gain the most solar energy, a surface must be at right angles to direct sunlight at all times. The efficiency of most PV cells is reduced as the elevation angle increases. Therefore, the PV surface is set perpendicular to the sun s angle to receive the maximum amount of direct light. In the northern hemisphere, a PV cell surface must face south to receive maximum exposure to direct light. The solar spectrum changes throughout the day and with geographic location. The standard spectrum has been designated to measure the performance of PV cells from different manufacturers. The largest percentage of energy loss from a PV cell is due to the inability of the material to absorb light energy from the full spectrum.

12 Photovoltaic Cells Electricity is created in PV cells by the absorption of photons. PV cells are composed of two or more semiconductor materials. A semiconductor is a material that has the electrical properties of a conductor (such as metal) and an insulator (such as plastic or rubber). In a PV cell, one layer of semiconductor material has a positive charge (p-type) and the other layer has a negative charge (ntype). As light passes through the PV cell, photons interact with the semiconductor atoms, freeing electrons which flow from the negatively charged layer to the positively charged layer. This flow of electrons produces electric current. When semiconductor materials are layered, photons that pass through the PV cell cause electrons to move from the n-type material to the p-type material. The area where these layers meet is called the pn junction. The electron flow at the pn junction creates an electric field this movement of electrons across the pn junction is the central process of the photovoltaic effect.

13 Photovoltaic Systems A PV cell produces a small amount of voltage, therefore multiple solar cells are connected to form a module, which can produce enough voltage to power a large electrical device. An array is a panel that is composed of multiple modules.

14 Most solar arrays produce direct current and require an inverter to convert the power to alternating current. A PV power system is usually composed of a solar array that converts sunlight to electricity and an inverter that converts the electricity to alternating current which powers a load. The power generated from a PV power system is dependent on multiple environmental factors; therefore, to remain efficient the system requires a device to store electricity, such as a battery, and a controller to regulate the current. Most PV power systems also have a device, which connects to a power grid, to access external power from a commercial power system. The following figure shows the common configuration of a PV power system.