lecture 7: photovoltaic applications · 2011. 5. 24. · npre 201: advanced energy systems, lecture...

NPRE 201: Advanced Energy Systems, Lecture 7. Photovoltaic Applications

Lecture 7: PHOTOVOLTAIC APPLICATIONS The greatest allure of solar energy is the conversion of this electromagnetic radiation directly into our most efficient and readily available form of energy: electricity. Photovoltaic (PV) cells make this practice possible. This lecture-discussion will present how PV cells work, how they can be improved, and what fraction of our energy mix we can expect them to contribute. Though there are no moving parts in the PV cells and no cost for the fuel, this technology is not without drawbacks. Among the constraints to widespread incorporation of PV cells in energy markets are that the Sun shines approximately half a day, cells can degrade or break, cells are costly to make compared to the energy they produce, the weather in upper latitude climates is more detrimental to cells, and sunlight's energy density is low compared to the burning of a fossil fuel. Nevertheless, scientists have made considerable progress with photovoltaics over the last two decades, and they will fill certain markets in the years ahead. One of the greatest areas of PV use continues to be space applications, in that they harness solar energy for satellites.

7.1 Overview and Basic Physics

Discussion Questions for Section 7.1

D7.1.1 What is the 1.14 eV silicon energy gap? D7.1.2 In a p-n junction, how does potential vary? D7.1.3 How does current flow in a photocell circuit? D7.1.4 Where does the solar spectrum peak?

In many respects solar photovoltaic applications (PV) look more promising, going by current market trends, than its elder cousin, solar thermal. If you've ever used a solar-powered calculator or watch, you've seen PV in action. As the name suggests, photovoltaic cells produce electricity directly from sunlight. It's very different from other types of solar devices in that it used sun's light, rather than its heat. Although these panels are conceptually simple, their physical complexity is a big drawback for many applications. The history of PV's dates back to 1839 when Edmund Becquerel (this one is different from the one who discovered radioactivity), a French experimental physicist, observed the photovoltaic effect. He discovered this effect while experimenting with an electrolytic cell--the current generation increased when exposed to light. A similar effect was observed in a solid (selenium) several decades later. In the1880's Selenium PV cells were built that converted visible light into electricity and were 1% to 2% efficient. Even today light sensors for cameras are still made from selenium. Another big development came with Einstein publishing a paper on photoelectric effect in 1905 (along with the seminal paper on his theory of relativity), which gave a theoretical basis for the production of photoelectrons (electric current) by incident photons (light). The culmination of decades of experimental work was Bell Lab's production of a silicon PV cell with 4% efficiency in 1954. Bell's researchers, D.M. Chapin, C.S. Fuller, and G.L. Pearson, published the results of their discovery in the "Journal of Applied Physics,"

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entitled 'A New Silicon p-n Junction Photocell for Converting Solar Radiation into Electrical Power.' In 1958 the US Vanguard space satellite used a small (less than one watt) array to power its radio. The space program has played an important role in the development of PV's ever since. During the 1973 oil crisis the US Department of Energy funded the Federal Photovoltaic Utilization Program, resulting in the installation and testing of over 3,100 PV systems, many of which are in operation today. The 1970s through the 1990s have seen a relative disinterest in solar power with majority ownership of many United States PV manufacturers transferring to German and Japanese interests. However, The Gulf war of 1990 again sparked US interest in energy alternatives. International markets for solar take off in the mid-1990s.

To understand how photocells respond to light, we first look at the properties of pure silicon in the dark, then of a simple p-n junction in the dark, and then of the p-n junction in the light. Then we look at design improvements aimed at increasing efficiency of production of photoelectricity from sunlight. (For a more complete discussion, see Chapter 7 of Renewable Energy Resources by John W. Twidell and Anthony D. Weir (E. & F. N. Spoon, New York.) Silicon is a semiconductor, and like a conductor it has a valence band and a conduction band. Electrons with high enough energy to be in the conduction band can move from atom to atom in the material, being impeded only by scattering off of variations in the potential in the medium. Unlike for a conductor, in a material that is a semiconductor at room temperature there is a comparatively large energy gap between the valence band and the conduction band. For pure silicon at 27 C (300 K) this gap is Eg=1.14 electron volts (eV). By contrast, measured in electron volts, the energy kT for 300 K is about 0.03 eV, where Boltzman's constantcan be expressed k=(1/11,400) eV/(degree K) if the energy units one wants kT in are eV. The fraction of the outer electrons in the conduction band is exp[-Eg/(kT)]. This is very small for pure silicon and gives it a fairly high electrical resistivity of about 2500 ohm-meters.

Figure 7.1a: Energy Levels in a Pure Semiconductor at Low Temperature illustrates the energy gap.

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DESCRIPTION — The energy gap between the valence band and conduction band in a semiconductor depends on both the material and its temperature. For pure silicon at 27 C it is 1.14 eV and at c. 200 C it is 1.11 eV. Values for various semiconductors typically range from 1 eV to 2 eV.

Adding a small concentration of higher valence electronegative (n) material (e.g. phosphorous or arsenic in periodic table group V) introduces loosely bond electrons. These will migrate readily to a nearby region that has instead and excess of electropositive (p) material (such as boron or gallium in periodic table group III). The missing electrons leave positively charged regions called "holes" behind when the migrate to into the "P" region doped with electropositive impurities.

Figure 7.1b: Energy Levels in a Pure Semiconductor at Low Temperature illustrates electron energy difference between N and their energy in a P type materials.

DESCRIPTION —Doping with atoms with a valence lower than the background material produces a P-type semiconductor. Doping with atoms with a higher valence produces N-type. This decreases the electrical resistivity. "Common values for silicon photovoltaics are 0.01 ohm-meters for a dopant concentration of 1022 m-3 and 0.1 ohm-meters for 1021 m-3.

Thus, in an electrically isolated p-n junction a positive charge builds up on the N-material side and a negative charge on the P-material side, leading to an electrostatic potential gradient. For a silicon junction doping densities of 1022m-3 the width of non-neutral regions is about 0.5 microns and the potential gradient across it peaks at about two million volts/meter.

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Figure 7.1c: Charge Potential and Energy Across a P-N Junction illustrates the charge, potential, and energy differences across a dark junction.

DESCRIPTION — This figure illustrates charge accumulation and electrostatic potential variation across a p-n junction.The energy of electrons increases as they move upward their energy gradient (to the right), and vice versa for postively charged holes (localized regions where a higher valence dopant is missing one of its electrons).

When a p-n junction is illuminated by sufficiently energetic photons, the excess electrons on the "p" side of the junction can be knocked over to the "n" side. These electrons typically diffuse about 100 microns and can thus be carried well beyond the effective junction width to knock other electrons towards the back of the N-type material. If a conduction wire is attached to the back of the N-type material then these electrons will flow into the wire and can be directed through an external load back to the P-type material to complete the circuit.

Figure 7.1d: How Solar Cells Work illustrates the electron flow in an illuminated solar cell.

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DESCRIPTION — Light enters form the left and is absorbed in the p-n junction area. When that happens the charge is separated and an electron and hole are formed. The electron displaces other electrons, which eventually flow through the external circuit and back into the material to recombine with a hole.

Two properties of photocells complicate production of devices with high energy efficiency (the ratio of electrical energy out to electromagnetic energy in). First, electrons must jump between specific energy levels in order to absorb a photon. The range of allowed photon energies is broadened by the possibility of simultaneous emission or absorption of a low energy phonon in the material, but still absorption in given materials is only efficient in specific ranges of photon energy. This difficulty can be addressed by sandwiching different materials. However, transparency is very limited in gallium arsenide all the way from the near infrared to the ultraviolet, and even in silicon the e-folding length for absorption falls to almost as low as 10 microns at the blue end of the spectrum. (This is compared to 100—300 microns towards the red end of the visible spectrum from 0.5 to 0.7 micron light wavelengths.) Most of the energy in sunlight reaching the earth near sea level is in the visible part of the spectrum, but a substantial portion of this is at wavelengths below 0.5 microns.

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Figure 7.1e: Solar Energy Spectrum shows how atmospheric absorption by ozone and water preferentially depletes the longer wavelength part of the solar spectrum.

DESCRIPTION —Notice that the visible band corresponds to the maximum radiation of our sun and also the ability of that radiation to transmit through our water-laden atmosphere and water laden eyeballs. That's not accidental. Another difficulty is the higher opacity of the conducting electrical contacts, which without careful engineering can lead to additional significant absorption in a complicated structure.

7.2 Advances


7.2.1 How does mulilayering increase energy efficiency? 7.2.2 How can energy be concentrate on photocells in a flat array?

Low efficiency photovoltaic arrays take up a lot of space per unit average power production. Nevetheless, area requirements per unit power for large scale photovoltaics are less than for hydropower. There is, however, still considerable incentive to develop high efficiency devices. However, high efficiency devices are expensive, which in turn produces an incentive to concentrate light upon them using devices constructed of

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cheaper materials. In a sandwich device, material designed to utilize the less penetrating blue light is place on the sunward part of the device.

Figure 7.2a: Illustration of Multi-layer Technology is a schematic of the photoelectric components of a three layer device.

DESCRIPTION — Note how different wavelengths of light will be absorbed by different layers. This allows the highest energy light waves (blue light) to have its entire energy captured. Then each successively less energetic wavelength of light can still be captured but it will also be able to leave, nearly, its full amount of energy behind.

In between each of the semiconductor materials in a multilayer photocell are thin conductor layers and transparent insulators. The whole device is mounted on a substrate, which may be made of glass to protect the materials and allow sunlight through onto them.

Three types of modern photocell types are

a) Mono-crystalline (highest efficiency and cost)

b) Poly-crystalline

c) Amorphous (lowest efficiency and cost)

Solar concentrators can use grooved flat plates call Fresnell Lenses to concentrate light on carefully arranged photocell arrays. This is in contrast to solar thermal devices, where viscous losses in heat transfer fluids restrict require minimum size cooling channels and thus limit geometric flexibility.

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Figure 7.2b: Picture of Solar Concentrator Made out of Flat Fresnell Lenses shows an array of Fresnell lense devices.

DESCRIPTION — Fresnell lenses with small concentric circular grooves can concentrate light onto photovoltaic cells using an essentially flat concentrator rather than more complicated parabolic or heliostat geometries.

7.3 Markets


7.3.1 In a practical application, how are photocells connected? 7.3.2 How were costs expected to vary with production volume? 7.3.3 Which type of order dominated recent U.S. production? 7.3.4 How did global installed capacity grow in the 1990s?

Because the hardware needed for photovoltaic applications is entirely solid-state electronics, solar cells are relatively free from maintenance and have a comparative longer lifetime. These advantages make photovoltaics competitive in some markets

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where solar thermal technology isn't economically viable. With an annual failure rate of 1 in 10,000, photovoltaic cells are ideal candidates for remote locations with absolutely no infrastructure–roads or grids.

Figure 7.3a: Lamp Post shows a small photovoltaic installation with storage batteries

DESCRIPTION — For remote areas a solar power system which charges a battery which is in the "DANGER: high voltage" box during the day then provides light for the area at night. I doubt that there's high voltage inside but high capacity battery probably run at a low voltage because it's much easier to charge at low voltages to solar cells without having to go through too many transformer steps.

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Figure 7.3b: Practical Application of Solar PV Panels shows a practical application of solar PV panels

Figure 7.3c: Circuit Diagram of a PV System shows how photovoltaic modules are connected to form an array.

DESCRIPTION — Photovoltaic modules can be connected in series and parallel to form an array, thus increasing total available power output to the needed voltage and current for a particular application. This figure shows a circuit diagram of a typical photovoltaic system. Globally, PV cell and module production registered a 43% growth from 201 MWe installed peak capacity in 1999 to 288 MWe in 2000. Japan topped the list with 128.6 MWe, followed by U.S. (75 MWe), for the year 2000. ("The PV Boom," Renewable Energy World July/August 2001, p. 145.) Economies of scale in manufacturing are an important factor in the economics of photovoltaics, due to the large capital cost of engineering and building a production facility with low unit costs in the solid state industry (c.f. Scientific American, September 1990, p. 150). Earlier estimates perhaps somewhat optimistically projected a decrease to less than $2/We of peak installed direct current capacity by the time 150MWe/year production was reached.

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Table 7.3a: Comparison of Costs Involved in Manufacture of Photovoltaic Cells gives a breakdown of production costs estimates for production rates 1 MWe vs 150 MWe with earlier and advanced technology.

Now 1 MW Current

Techonlogy 150 MW/year

Advanced Technology 150MW/year

Module Lens 206 12 12 Housing 91 37 37 Cells 156 15 75 Reciever 91 40 52 Other and labor 64 13 13 Subtotal 608 117 189

Tracking Structure 110 25 25 Drive 60 25 25 Controls 15 7 7 Subtotal 185 57 57

Balance of Systems

Site work 70 43 43 Installation 40 25 25 Subtotal 110 68 68 Total $/m2 903 242 314 DC field rating (W/m2) 127 145 230

Field $/WDC 7.11 1.67 1.36 Inverter $/W 0.40 0.10 0.10 System $/WDC 7.51 1.77 1.46

DESCRIPTION — This table shows prices estimates reported in 1994 in $/m2 for the module including the photovoltaic cells, the tracking system needed to keep it pointed towards the sun, site work and installation costs, and the total cost in $/m2. It also shows estimates of rated (presumably peak) power density in W/m2, and the total field costs and costs of a final usable system in $/W of direct current (DC) power. Strong economies of

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scale were expected when moving from production levels of 1 MW/year to 150 MW/year using available technology, with some additional improvements from new technology developments. In fact in the year 2000, in Japan led with Sharp producing 50.4 MWe and Kyocera producing 42 MWe. The 150 MWe production level was surpassed globally, nearly attained in Japan, and was within a factor of three of being attained by individual companies. However, subsidies were still needed to get costs to customers down to about $3/We.

There are two conditions that suggest a public subsidy to overcome the initial hurdle producing the investment and experience necessary to reduce energy production costs with a new technology. One of these occurs when the required investment is too large for private capital to manage, a problem which may be compounded if results of the investment can't or shouldn't be kept proprietary in order to recoup the investment. They can't be kept proprietary if the basic question is what production costs can be achieved, rather than how to achieve them. They shouldn't be kept proprietary (from the point of view of national or global interest) if there are significant external benefits that don't accrue to the company (like pollution reduction or increased energy security). A perceived public good in California has prompted such action on the latter grounds ("The PV Boom," Renewable Energy World July/August 2001, p. 148.).

The Sacramento Municipal Utility District (SMUD) launched phase two of its PV Pioneer programme by offering subsidized photovoltaic systmes to its customers at prices below $3.50 per watt. SMUD installed 1.6 MW of PV systems in 2000. Most of the systems were installed on commercial and institutional buildings. About two hundred 2 kW amorphous silicon systems were installed in the Pioneer II mode, where the customer purchases the system.

Of course a single municipal utility district of modest size like Sacramento's is unlikely to be able to significantly influence the overall economics of photovoltaic manufacturing. For this national policy or coordinated international policies are needed. One danger of this, however, is the creation of politically powerful vested interests that successfully lobby for continuing subsidies even after the initial barriers to establishing economies of scale are overcome. The role of the ethanol industry in helping to thwart an earlier U.S. national policy of reducing overall agricultural subsidies is one example of many possible cautions in this regard. Even with a fairly patchwork set of government policies, photovoltaic use in the United States has been growing rapidly.

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Table 7.3b: U.S. Photovoltaics Module Installations by Market Sector (MWe) shows penetration in various U.S. market segments from 1990—2000.

Year 1992 1994 1996 1998 2000

Grid-connected 0.8 1.2 2.0 2.2 5.5

Off-grid consumer 2.6 3.0 4.0 4.5 6.0

Government projects 0.6 0.6 1.2 1.5 2.5

Off-grid other 2.6 3.3 4.4 5.2 7.5

Consumer 2.2 0.7 2.2 2.4 2.5

Exports 9.2 16.2 25.1 37.9 55.0

Total, less imports 18.2 25.6 38.9 53.7 75.0

DESCRIPTION — This table gives estimates of domestic consumption and exports to peak capacity of photovoltaic installations. No imports were recorded before 1999, but in the year 2000 imports of 4.0 MWe have to be subtracted from the sum of the installed amounts to get a total production in the United States of 75.0 MWe. Other totals may differ from column sums due to rounding. "Off-grid other" includes both industrial and commercial sectors. World-wide photovoltaic market segments (according to Siemens) in 1997 were: Power grid 34%, Industrial 28%, rural habitation 27%, Consumer 8%, and Indoor 3%.

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Figure 7.3d: Cumulative Installed Photovoltaic Power provides global totals from 1992 through 1999 for countries most active in photovoltaic installations.

DESCRIPTION — This figure show estimates of cumulative global photovoltaic peak electrical production capacity in the International Energy Agency Photovoltaic Power Systems Program (IEA PVPS) member countries. These included Australia, Austria, Canada, Denmark, Finland, France, Germany, Israel, Italy, Japan, Korea, Mexico, Netherlands, Norway, Portugal, Spain, Switzerland, UK, and USA. Individual reports on these countries were available from the IEA PVPS web site as of the date indicated in that reference.

lecture 7: photovoltaic applications · 2011. 5. 24. · npre 201: advanced energy systems, lecture...

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