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Detailed treatment covers: *scientific basis of the photovoltaic effect and solar cell operation
*the production of solar silicon and of silicon-based solar cells and modules
*the science and technology of up-and-coming thin film PV technologies
*how choice of semiconductor materials and their production influence costs and performance
*high-performance approaches for concentrators and space applications
*new types of organic dye-based solar cells
*making measurements on solar cells and modules and how to relate results under standardised test conditions to real outdoor performance
*photovoltaic system installation and operation of components such as inverters and batteries.
*architectural applications of building-integrated PV
*finance and the role of investors: public funding and policy in promoting PV world-wide Each chapter is written to be partially accessible to beginners while providing detailed information of the physics and technology for experts. Encompassing a review of past work and the fundamentals in solar electric science, this outstanding reference provides an invaluable resource to practitioners, consultants, researchers and students in the PV engineering industry.
Steven S. Hegedus and Antonio Luque
1.1 THE BIG PICTURE
Congratulations! You are reading a book about a technology that has changed the way we think about energy. Solar electricity, also known as photovoltaics (PV), has shown since the 1970s that the human race can get a substantial portion of its electrical power without burning fossil fuels (coal, oil or natural gas) or creating nuclear fission reactions. Photovoltaics helps us avoid most of the threats associated with our present techniques of electricity production and also has many other benefits. Photovoltaics has shown that it can generate electricity for the human race for a wide range of applications, scales, climates, and geographic locations. Photovoltaics can bring electricity to a rural homemaker who lives 100 kilometers and 100 years away from the nearest electric grid connection in her country, thus allowing her family to have clean, electric lights instead of kerosene lamps, to listen to a radio, and to run a sewing machine for additional income. Or, photovoltaics can provide electricity to remote transmitter stations in the mountains allowing better communication without building a road to deliver diesel fuel for itsgenerator. It can help a major electric utility in Los Angeles, Tokyo, or Madrid to meet its peak load on hot summer afternoons when air conditioners are working full time. It allows homes and businesses a new level of guaranteed energy availability and security, and photovoltaics has been powering satellites orbiting the Earth or flying to Mars for over 30 years.
Photovoltaics is an empowering technology that allows us to do totally new things, as well as, do old things better. It allows us to look at whole new modes of supplying electricity to different markets around the world and out of the world (in outer space). It also allows us to do what we already do (generate electricity, which is distributed over the transmission grid) but to do it in a sustainable, pollution-free, equitable fashion. Why is photovoltaics equitable? Because nearly every one has access to sunlight!
Electricity is the most versatile form of energy we have. It is what allows citizens of the developed countries to have nearly universal lighting on demand, refrigeration, hygiene, interior climate control in their homes, businesses and schools, and widespread access to various electronic and electromagnetic media. Access to and consumption of electricity is closely correlated with quality of life. Figure 1.1 shows the Human Development Index (HDI) for over 60 countries, which includes over 90% of the Earth's population, versus the annual per capita electricity use (adapted from ref 1). The HDI is compiled by the UN and calculated on the basis of life expectancy, educational achievement, and per capita Gross Domestic Product. To improve the quality of life in many countries, as measured by their HDI, will require increasing their electricity consumption by factors of 10 or more, from a few hundred to a few thousand kilowatt-hrs (kWh) per year. How will we do it? Our choices are to continue applying the answers of the last century such as burning more fossil fuels (and releasing megatons of C[O.sub.2], S[O.sub.2], and N[O.sub.2]) or building more nuclear plants (despite having no method of safely disposing of the high-level radioactive waste) or to apply the new millennium's answer of renewable, sustainable, nonpolluting, widely available clean energy like photovoltaics and wind. (Wind presently generates over a thousand times more electricity than photovoltaics but it is very site-specific, whereas photovoltaics is generally applicable to most locations.)
1.2 WHAT IS PHOTOVOLTAICS?
Photovoltaics is the technology that generates direct current (DC) electrical power measured in Watts (W) or kiloWatts (kW) from semiconductors when they are illuminated by photons. As long as light is shining on the solar cell (the name for the individual PV element), it generates electrical power. When the light stops, the electricity stops. Solar cells never need recharging like a battery. Some have been in continuous outdoor operation on Earth or in space for over 30 years.
Table 1.1 lists some of the advantages and disadvantages of photovoltaics. Note, that they include both technical and nontechnical issues. Often, the advantages and disadvantages of photovoltaics are almost completely opposite of conventional fossil-fuel power plants. For example, fossil-fuel plants have disadvantages of: a wide range of environmentally hazardous emissions, parts which wear out, steadily increasing fuel costs, they are not modular (deployable in small increments), and they suffer low public opinion (no one wants a coal burning power plant in their neighborhood). Photovoltaics suffers none of these problems. The two common traits are that both PV and fossil fueled power plants are very reliable but lack the advantage of storage.
Notice that several of the disadvantages are nontechnical but relate to economics and infrastructure. They are partially compensated for by a very high public acceptance and awareness of the environmental benefits. During the late 1990s, the average growth rate of PV production was over 33% per annum.
What is the physical basis of PV operation? Solar cells are made of materials called semiconductors, which have weakly bonded electrons occupying a band of energy called the valence band. When energy exceeding a certain threshold, called the band gap energy, is applied to a valence electron, the bonds are broken and the electron is somewhat "free" to move around in a new energy band called the conduction band where it can "conduct" electricity through the material. Thus, the free electrons in the conduction band are separated from the valence band by the band gap (measured in units of electron volts or eV). This energy needed to free the electron can be supplied by photons, which are particles of light. Figure 1.2 shows the idealized relation between energy (vertical axis) and the spatial boundaries (horizontal axis). When the solar cell is exposed to sunlight, photons hit valence electrons, breaking the bonds and pumping them to the conduction band. There, a specially made selective contact that collects conduction-band electrons drives such electrons to the external circuit. The electrons lose their energy by doing work in the external circuit such as pumping water, spinning a fan, powering a sewing machine motor, a light bulb, or a computer. They are restored to the solar cell by the return loop of the circuit via a second selective contact, which returns them to the valence band with the same energy that they started with. The movement of these electrons in the external circuit and contacts is called the electric current. The potential at which the electrons are delivered to the external world is slightly less than the threshold energy that excited the electrons; that is, the band gap. Thus, in a material with a 1 eV band gap, electrons excited by a 2 eV photon or by a 3 eV photon will both still have a potential of slightly less than 1 V (i.e. the electrons are delivered with an energy of 1 eV). The electric power produced is the product of the current times the voltage; that is, power is the number of free electrons times their potential. Chapter 3 delves into the physics of solar cells in much greater detail.
Sunlight is a spectrum of photons distributed over a range of energy. Photons whose energy is greater than the band gap energy (the threshold energy) can excite electrons from the valence to conduction band where they can exit the device and generate electrical power. Photons with energy less than the energy gap fail to excite free electrons. Instead, that energy travels through the solar cell and is absorbed at the rear as heat. Solar cells in direct sunlight can be somewhat (20-30°C) warmer than the ambient air temperature. Thus, PV cells can produce electricity without operating at high temperature and without mobile parts. These are the salient characteristics of photovoltaics that explain safe, simple, and reliable operation.
At the heart of any solar cell is the pn junction. Modeling and understanding is very much simplified by using the pn junction concept. This pn junction results from the "doping" that produces conduction-band or valence-band selective contacts with one becoming the n-side (lots of negative charge), the other the p-side (lots of positive charge). The role of the pn junction and of the selective contacts will be explained in detail in Chapters 3 and 4. Here, pn junctions are mentioned because this term is often present when talking of solar cells, and is used occasionally in this chapter.
Silicon (Si), one of the most abundant materials in the Earth's crust, is the semiconductor used in crystalline form (c-Si) for 90% of the PV applications today (Chapter 5). Surprisingly, other semiconductors are better suited to absorb the solar energy spectrum. This puzzle will be explained further in Section 1.10. These other materials are in development or initial commercialization today. Some are called thin-film semiconductors, of which amorphous silicon (a-Si) (Chapter 12), copper indium gallium diselenide (Cu(InGa)[Se.sub.2] or CIGS) (Chapter 13), and cadmium telluride (CdTe) (Chapter 14) receive most of the attention. Solar cells may operate under concentrated sunlight (Chapter 11) using lenses or mirrors as concentrators allowing a small solar cell area to be illuminated with the light from larger area. This saves the expensive semiconductor but adds complexity to the system, since it requires tracking mechanisms to keep the light focused on the solar cells when the sun moves in the sky. Silicon and III-V semiconductors (Chapter 9), made from compounds such as gallium arsenide (GaAs) and gallium indium phosphide (GaInP) are the materials used in concentrator technology that is still in its demonstration stage.
For practical applications, a large number of solar cells are interconnected and encapsulated into units called PV modules, which is the product usually sold to the customer. They produce DC current that is typically transformed into the more useful AC current by an electronic device called an inverter. The inverter, the rechargeable batteries (when storage is needed), the mechanical structure to mount and aim (when aiming is necessary) the modules, and any other elements necessary to build a PV system are called the balance of the system (BOS). These BOS elements are presented in Chapters 17 to 19.
1.3 SIX MYTHS OF PHOTOVOLTAICS
Borrowing a format for discussing photovoltaics from Kazmerski, in this section, we will briefly present and then dispel six common myths about photovoltaics. In the following sections, we identify serious challenges that remain despite 40 years of progress in photovoltaics.
The six myths are as follows:
1. Photovoltaics will require too much land area to ever meet significant fraction of world needs:
Solar radiation is a rather diffuse energy source. What area of PV modules is needed to produce some useful amounts of power? Let's make some very rough estimates to give answers that will be accurate within a factor of 2. Using methods described in detail in Chapter 20 (especially equations 20.50 and 20.51 and Table 20.5), one can calculate how much sunlight falls on a square meter, anywhere in the world, over an average day or a year. We will use an average value of 4 kilowatt-hrs (kWh) per [m.sup.2] per day to represent a conservative worldwide average. Now, a typical PV module is approximately 10% efficient in converting the sunlight into electricity, so every square meter of PV module produces, on average, 4 x 0.1 = 0.4 kWh of electrical energy per day. We can calculate the area in [m.sup.2] needed for a given amount of electrical energy E in kWh by dividing E by 0.4 kWh/[m.sup.2]. (Chapter 20 contains much more detailed methods to calculate the incident sunlight and the PV module output as a function of time of day, month of year, etc.)
Let us consider three different-sized PV applications: a family's house in an industrialized country, replacing a 1000 MW (megawatt) coal or nuclear powered generating plant, or providing all the electricity used in the USA.
First, for a typical family, let us assume that there are four people in the house. Figure 1.1 shows a range of electricity usage for the industrialized countries. Let us use 6000 kWh/person/year as an average. But, this includes all their electrical needs including at work, at school, as well as the electricity needed for manufacturing the products they buy, powering their street lights, pumping water to their homes, and so on. Since people spend about a third of the day awake in their home, let us assume that a third of their electrical needs are to be supplied in their home, or 2000 kWh/person/year. Dividing this by 365 days in a year gives about 5 kWh/person/day, or 20 kWh/day per family of four. This is consistent with household data from various sources for the US and Europe. Thus, they would need 20 kWh/0.4 kWh/[m.sup.2] or 50 [m.sup.2] of solar modules to provide their electrical power needs over the year. Thus, a rectangular area of solar modules of 5 by 10 meters will be sufficient. In fact, many roofs are about this size, and many homes have sunny areas of this size around them, so it is possible for a family of four, with all the conveniences of a typical modern home, to provide all their power from PV modules on their house or in their yard.
Next, how much land would it take to replace a 1000 MW coal or nuclear power plant that operates 24 hours/day and might power a large city? This would require [10.sup.6] kW x 24 hr/(0.4 kWh/[m.sup.2])or 6 x [10.sup.7] [m.sup.2]. So, with 60 [km.sup.2] (or 24 square miles) of photovoltaics we could replace one of last century's power plants with one of this century's power plants. This is a square 8 km (or 5 miles) on a side. For the same electricity production, this is equivalent to the area for coal mining during the coal powered plant's life cycle, if it is surface mining, or three times the area for a nuclear plant, counting the uranium mining area. This is also the same area required to build a 600 km (373 miles) long highway (using a 100 m wide strip of land).
Finally, we can calculate how much land is needed to power the entire US with photovoltaics (neglecting the storage issue). The US used about 3.6 x [10.sup.12] kWh of electricity in 2000. This could be met with 2 x [10.sup.10] [m.sup.2]. If we compare with the area of paved roads across the country, of about 3.6 x [10.sup.6] km and assume an average width of 10 m this leads to 3.6 x [10.sup.10] [m.sup.2].
Excerpted from Handbook of Photovoltaic Science and Engineering Copyright © 2003 by John Wiley & Sons, Ltd. Excerpted by permission.
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|List of Contributors|
|1||Status, Trends, Challenges and the Bright Future of Solar Electricity from Photovoltaics||1|
|2||Motivation for Photovoltaic Application and Development||45|
|3||The Physics of the Solar Cell||61|
|4||Theoretical Limits of Photovoltaic Conversion||113|
|5||Solar Grade Silicon Feedstock||153|
|6||Bulk Crystal Growth and Wafering for PV||205|
|7||Crystalline Silicon Solar Cells and Modules||255|
|8||Thin-film Silicon Solar Cells||307|
|9||High-Efficiency III-V Multijunction Solar Cells||359|
|10||Space Solar Cells and Arrays||413|
|12||Amorphous Silicon-based Solar Cells||505|
|13||Cu(InGa)Se[subscript 2] Solar Cells||567|
|14||Cadmium Telluride Solar Cells||617|
|15||Dye-sensitized Solar Cells||663|
|16||Measurement and Characterization of Solar Cells and Modules||701|
|18||Electrochemical Storage for Photovoltaics||799|
|19||Power Conditioning for Photovoltaic Power Systems||863|
|20||Energy Collected and Delivered by PV Modules||905|
|21||Economic Analysis and Environmental Aspects of Photovoltaic Systems||971|
|22||PV in Architecture||1005|
|23||Photovoltaics and Development||1043|
|24||Financing PV Growth||1073|