Solar Energy

To induce the built-in electric field within a PV cell, two layers of somewhat differing semiconductor materials are placed in contact with one another. One layer is an “n-type” semiconductor with an abundance of electrons, which have a negative electrical charge. The other layer is a “p-type” semiconductor with an abundance of “holes,” which have a positive electrical charge.

Although both materials are electrically neutral, n-type silicon has excess electrons and p-type silicon has excess holes. Sandwiching these together creates a p/n junction at their interface, thereby creating an electric field.
When n- and p-type silicon come into contact, excess electrons move from the n-type side to the p-type side. The result is a buildup of positive charge along the n-type side of the interface and a buildup of negative charge along the p-type side.

Because of the flow of electrons and holes, the two semiconductors behave like a battery, creating an electric field at the surface where they meet—what we call thep/n junction. The electrical field causes the electrons to move from the semiconductor toward the negative surface, where they become available to the electrical circuit. At the same time, the holes move in the opposite direction, toward the positive surface, where they await incoming electrons.

How do we make the p-type (“positive”) and n-type (“negative”) silicon materials that will eventually become the photovoltaic (PV) cells that produce solar electricity? Most commonly, we add an element to the silicon that either has an extra electron or lacks an electron. This process of adding another element is called doping.

The sun emits almost all of its energy in a range of wavelengths from about 2×10-7to 4×10-6 meters. Most of this energy is in the visible light region. Each wavelength corresponds to a frequency and an energy: the shorter the wavelength, the higher the frequency and the greater the energy (which is expressed in electron-volts, or eV).

Red light is at the low-energy end of the visible spectrum and violet light is at the high-energy end, where it has half again as much energy as red light. In the invisible portions of the spectrum, radiation in the ultraviolet region, which causes the skin to tan, has more energy than that in the visible region. Likewise, radiation in the infrared region, which we feel as heat, has less energy than the radiation in the visible region.

Solar cells respond differently to the different wavelengths, or colors, of light. For example, crystalline silicon can use the entire visible spectrum, plus some part of the infrared spectrum. But energy in part of the infrared spectrum, as well as longer-wavelength radiation, is too low to produce current flow. Higher-energy radiation can produce current flow, but much of this energy is likewise not usable.

In summary, light that is too high or low in energy is not usable by a cell to produce electricity. Rather, it is transformed into heat.

www.google.com/sky/

1.2 Air Mass

The sun is continually releasing an enormous amount of radiant energy into the solar system. The Earth receives a tiny fraction of this energy; yet, an average of 1367 watts (W) reaches each square meter (m2) of the outer edge of the Earth’s atmosphere. The atmosphere absorbs and reflects some of this radiation, including most X-rays and ultraviolet rays. Still, the amount of the sun’s energy that reaches the surface of the Earth every hour is greater than the total amount of energy that the world’s human population uses in a year.

How much energy does light lose in traveling from the edge of the atmosphere to the surface of the Earth? This energy loss depends on the thickness of the atmosphere that the sun’s energy must pass through. The radiation that reaches sea level at high noon in a clear sky is 1000 W/m2 and is described as “air mass 1” (or AM1) radiation. As the sun moves lower in the sky, the light passes through a greater thickness (or longer path) of air, losing more energy. Because the sun is overhead for only a short time, the air mass is normally greater than one—that is, the available energy is less than 1000 W/m2.

Scientists have given a name to the standard spectrum of sunlight at the Earth’s surface: AM1.5G (where G stands for “global” and includes both direct and diffuse radiation, described next) or AM1.5D (which includes direct radiation only). The number “1.5” indicates that the length of the path of light through the atmosphere is 1.5 times that of the shorter path when the sun is directly overhead.

The standard spectrum outside the Earth’s atmosphere is called AM0, with no light passing through the atmosphere. AM0 is typically used to predict the expected performance of PV cells in space. The intensity of AM1.5D radiation is approximated by reducing the AM0 spectrum by 28%, where 18% is absorbed and 10% is scattered. The global spectrum is 10% greater than the direct spectrum.

These calculations give about 970 W/m2 for AM1.5G. However, the standard AM1.5G spectrum is “normalized” to give 1000 W/m2, because of inherent variations in incident solar radiation.

Flat-plate collectors, which typically contain a large number of solar cells mounted on a rigid, flat surface, can make use of both direct sunlight and the diffuse sunlight reflected from clouds, the ground, and nearby objects.

• Direct light consists of radiation that comes straight from the sun, without reflecting off of clouds, dust, the ground, or other objects. Scientists also talk about direct-normal radiation, referring to the portion of sunlight that comes directly from the sun and strikes the plane of a PV module at a 90-degree angle.
• Diffuse light is sunlight that is reflected off of clouds, the ground, or other objects. It obviously takes a longer path than a direct light ray to reach a module. Diffuse light cannot be focused by the optics of a concentrator PV system.
• Global radiation refers to the total radiation that strikes a horizontal surface. Global sunlight is composed of direct-normal and diffuse components of sunlight. Additionally, diffuse and direct-normal sunlight generally have different energy spectra or distributions of color.

NASA’s Surface Meteorology and Solar Energy Data

This is a renewable energy resource web site sponsored by NASA’s Earth Science Enterprise Program that contains over 200 satellite-derived meteorological and solar energy parameters, monthly averaged from 10 years of data, and data tables for a particular location.

When sunlight reaches the Earth, it is distributed unevenly in different regions. Not surprisingly, the areas near the Equator receive more solar radiation than anywhere else on the Earth. Sunlight varies with the seasons, as the rotational axis of the Earth shifts to lengthen and shorten days with the changing seasons. For example, the amount of solar energy falling per square meter on Yuma, Arizona, in June is typically about nine times greater than that falling on Caribou, Maine, in December. The quantity of sunlight reaching any region is also affected by the time of day, the climate (especially the cloud cover, which scatters the sun’s rays), and the air pollution in that region. Likewise, these climatic factors all affect the amount of solar energy that is available to PV systems.

Although the quantity of solar radiation striking the Earth varies by region, season, time of day, climate, and air pollution, the yearly amount of energy striking almost any part of the Earth is vast. Shown is the average radiation received on a horizontal surface across the continental United States in the month of June. Units are in kWh/m2/day.

2. The Crystalline Silicon Solar Cell

PV cells can be made of many different semiconductors. But we’ll use crystalline silicon as an example, for three reasons.

First, crystalline silicon was the material used in the earliest successful PV devices.

Second, and more important, it’s still the most widely used PV material.

Third, although other PV materials and designs exploit the photoelectric effect in slightly different ways, if you know how the effect works in crystalline silicon, then you’ll have a basic understanding of how it works in all PV devices.

3. 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.

In this section, we first review the three main types of materials used for solar cells. The first type is silicon, which can be used in various forms, including single-crystalline, multicrystalline, and amorphous. The second type is polycrystalline thin films, with specific discussion of copper indium diselenide (CIS) cadmium telluride (CdTe), and thin-film silicon. Finally, the third type of material is single-crystalline thin film, focusing especially on 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.

4. 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.

The basic photovoltaic or solar cell typically produces only a small amount of power. To produce more power, cells can be interconnected to form modules, which can in turn be connected into arrays to produce yet more power. Because of this modularity, PV systems can be designed to meet any electrical requirement, no matter how large or how small.