Photovoltaics (PV) is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic power generation employs solar panels comprising a number of cells containing a photovoltaic material. Materials presently used for photovoltaics include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium selenide/sulfide.[1]Due to the growing demand for renewable energy sources, the manufacturing of solar cells and photovoltaic arrays has advanced considerably in recent years.[2][3][4]

As of 2010, solar photovoltaics generates electricity in more than 100 countries and, while yet comprising a tiny fraction of the 4.8 TW total global power-generating capacity from all sources, is the fastest growing power-generation technology in the world. Between 2004 and 2009, grid-connected PV capacity increased at an annual average rate of 60 percent, to some 21 GW.[5] Such installations may be ground-mounted (and sometimes integrated with farming and grazing)[6] or built into the roof or walls of a building, known as Building Integrated Photovoltaics or BIPV for short.[7] Off-grid PV accounts for an additional 3–4 GW.[5]Driven by advances in technology and increases in manufacturing scale and sophistication, the cost of photovoltaics has declined steadily since the first solar cells were manufactured.[8] Net metering and financial incentives, such as preferential feed-in tariffs for solar-generated electricity, have supported solar PV installations in many countries.

Every day across the globe, the sun shines down on the earth. The energy in the photons from the sun can be converted to electrical energy. The term for this process is the ´Photovoltaic Effect´.

Since the first commercially available solar panel in the 1960´s, photovoltaic (PV) technology has continued to be explored and developed throughout the world (Pratt & Schaeffer 51). The constant development of this technology has resulted in an increasing level of efficiency and PV panels that are more affordable than ever before, though still initially expensive. Today, humans continue to search for new ways to make photovoltaic technology a viable option for everyone throughout the world. Since most of us are not studying the atomic level of this technology, we can help in other ways - by gaining an understanding and spreading that understanding of photovoltaics, as well as by helping others to gain access to solar, or photovoltaic, systems.

This article explores the components of a photovoltaic system, describes their role and importance, and works as a beginning guide to those wishing to invest in a photovoltaic system.

Projects on Appropedia[edit | edit source]

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Photovoltaic System Components[edit | edit source]

Wiring[edit | edit source]

To Catch the Sun is the first book created from this much exclusive Appropedia content on photovoltaics. It is was successfully crowdfunded on Kickstarter. See http://tocatchthesun.com to get your own copy.

Color Coding[edit | edit source]

Color Coding of Wire
DC Wiring 120 AC Wiring
Red = Positive Black = Hot
Black = Negative White = Neutral
Green or Copper = Ground

Wire Size[edit | edit source]

  • Ampacity: The current carrying ability of a wire. Hence, the larger the wire, the more capacity it has to carry current.
  • Voltage Drop: The loss of voltage due to a wire´s resistance and length.
  • Wire sizing must be based on the maximum current through the length of the wiring.

Solar Array Wired in Parallel versus Series[edit | edit source]

Parallel Wiring

Photovoltaic system wired in parallel

Wiring solar panels in parallel is when you connect all the positive wires or terminals of the solar panels together, and all of the negative wires together. Parallel wiring is used most in households.

Series Wiring

Wiring solar panels in series is when you connect the positive wire or terminal of the first solar panel to the negative wire of the next one, and so on for as many panels you have.

The following table can be useful to think about when deciding whether to wire in parallel or series

Series Parallel
VT=V1+V2+… V stays same
I stays same Itotal=I1+I2+…
RT=R1+R2+… 1/RT=(1/R1)+(1/R2)+…

Solar Installation Site Analysis[edit | edit source]

Solar Radiation[edit | edit source]

When the sun hits the earth at a particular time and place, it is called INSOLATION. Insolation can be described as power density, and is expressed as watts per meter squared (W/m2) and, in PV, is often presented as average daily values per month. We receive 1,000 W/m2 when we have 100% full sun insolation. (Pratt & Schaeffer 56).

When analyzing a site to install a PV system, it is important to know which month has the lowest and highest rates of insolation, or the lowest and highest average amount of sun that particular site will receive in that month. This information will be important when you are trying to determine the tilt angle of your PV array. Considering all of the months that you will be utilizing your pv operation, it is best to know the daily insolation, or average hours per day of full sun, for the worst weather month of the year. The insolation data will allow you to find an angle that is most appropriate, allowing your panel to sit at an angle that will provide the highest potential for power to your system.

In other words, determine which month has the least amount of sun on average. This is the month that you want to use if you are building a system that will be used year-round. (if you are only going to be using it for summer or winter, find month with least sun during months that you will use the system.)

PV Array Location[edit | edit source]

Sun/Clouds: It is important to estimate the sun availability and cloud cover. Sometimes you can obtain this information on the web if it is a large enough town.

Shade: You want to choose a location that is on or near the place where you loads will be. The MOST IMPORTANT thing to consider when choosing a location for your Array is shading obstacles. Shade covering just one PV cell can reduce the current dramatically. A small amount of shade covering the panel can reduce the panel performance by 80%. As a general rule, the array should be free of shade (during each month in use) from 9am to 3pm. This is the optimum timeframe a panel has to receive light and is called the Solar Window.

Peak Sun Hours[edit | edit source]

Peak sun hours are the number of hours during one day when full sun is available.

Solar Noon[edit | edit source]

Solar noon refers to the time during the day when the sun is the highest in the sky; it is the moment when the sun is the strongest. To find Solar Noon, calculate the length of the day from sunrise to sunset and divide by two.

Sizing a PV System[edit | edit source]

To size your system requires seven main steps:

  1. Estimating your electrical load
  2. Estimating solar energy available
  3. Sizing an array
  4. Sizing batteries
  5. Specifying a controller
  6. Sizing an inverter
  7. Sizing system wiring and switches

These worksheets from Sandia Labs will lead you through the first four steps, and these will lead you through the last three steps. Here is an example AC/DC residence design.

You can also refer to Photovoltaics: Design and Installation Manuel, by SCI.

For more detailed PV systems design it may be useful to use a Solar photovoltaic software simulation program such as RETScreen.

System Sizing Calculation Example

Calculating number of panels:

  1. First, determine average daily building load (Edaily). Electrical utilities generally report energy use in kilowatt hours (kWh) per time. For this calculation, convert to watthours per day (Wh/day). For example, if it is known that a building used 9,125kWh in the last 365 days, Edaily can be calculated as follows:
    Whday.jpg
  2. Determine solar energy available for site location (Full Sun). PVWATTS will provide an average daily insolation value. For example, Arcata, CA is 4.29kWh/m2/day. However, hours per day is more useful. To convert simply divide by peak irradiance of the sun (1kW/m2).

    Pvsizing fullsun.jpg
  3. What is the panel wattage? How about 240W. PPV = 240W
  4. What is the expected balance of systems or AC to DC derate factor. How about nbos = .77. PVWATTS can also help with this.
  5. Using Edaily, Full Sun, PPV and nbos - number of panels (NPV) can be solved by using this general equation:
    Pvsizing npv.jpg
    For example scenario,
    Pvsiizing npv.jpg

Likewise, calculating the space required for a PV system would utilize the equation: A= PVmax/(nPV*full sun (1000 W/m2)). Using this equation, a 3,000 watt system with a PV efficiency of 20% would yield a required space of 15 m2.

Photovoltaic solar thermal[edit | edit source]

Pvt.gif
Photovoltaic solar thermal (PVT) hybrid systems purpose is to produce both heat and electricity in a smaller area than if you were to have both a photovoltaic panel and a solar thermal system. The current design for PVT is to have a solar panel glued to a solar thermal system. PVTs purpose is to use the solar thermal system to cool the photovoltaic cells to perform better, as solar cells degrade with temperatures greater than 25C. This means however, that the thermal aspect of the PVT has a significantly lower efficiency compared to just a solar thermal system (max 50% eff compared to 70+% eff).

Advantages of Photovoltaic Technology[edit | edit source]

Photovoltaic technology holds a number of unique advantages over conventional power-generating technologies. PV systems can be designed for a variety of applications and operational requirements, and can be used for either centralized or distributed power generation. PV systems have no moving parts, are modular, easily expandable and even transportable in some cases. Sunlight is free, and no noise or pollution is created from operating PV systems. PV panels do not require the use of fossil fuels such as coal, oil or natural gas in the energy production process. Alternatively, conventional fuel sources have created an array of environmental problems, namely global warming, acid rain, smog, water pollution, rapidly filling waste disposal sites, destruction of habitat from oil spills, and the loss of natural resources (Solar Energy International 2004). PV modules use silicon as their main component. The silicon cells manufactured from one ton of sand produce as much electricity as burning 500,000 tons of coal (Solar Energy International 2004). PV systems that are well designed and properly installed require minimal maintenance and have long service lifetimes. If properly maintained [9](cleaned and protected), pv panels can last up to thirty years or longer. Other aspects of the system, such as the battery, have much shorter life spans and may need to be replaced after several years of use. Solar Energy International (2004) indicates that there are many other benefits to consider when choosing photovoltaic technology:

  • Reliability: Even under the harshest of conditions, PV systems maintain electrical power supply. In comparison, conventional technologies often fail to supply power in the most critical of times.
  • Durability: In general modules are carry a warranty of 80% of their rated power for 20 or more years. Thus the worst case is an expected 1% decrease in performance per year. There have been several studies showing even less degradation than this at around 0.2%/year. PV modules produce more energy in their lifetime than it takes to produce them.[10]
  • Low Maintenance Cost: PV systems do not require frequent inspection or maintenance. Transporting supplies may get costly, but these costs are usually less than with conventional systems.
  • No Fuel Cost: Since there is no fuel source, there is no required expenditure on the purchasing, storing, or transporting fuel.
  • Reduced Sound Pollution: PV systems operate silently and with minimal movement.
  • Photovoltaic Modularity: Unlike conventional systems, modules may be added to photovoltaic systems to increase available power.
  • Safety: PV systems do not require the use of combustible fuels, and are very safe when properly designed and installed.
  • Independence: PV systems may operate independent of grid systems. This is a large advantage for rural communities in nations lacking basic infrastructure.
  • Electrical Grid Decentralization: Small-scale decentralized power stations reduce the possibility of power outages, which are often frequent on the electric grid. See:Distributed generation
  • High Altitude Performance: When using solar energy, power output is optimized at higher elevations. This is very advantageous for high altitude, isolated communities where diesel generators must be de-rated due to the loss in efficiency and power output.

By offsetting the need for conventional power, distributed solar power delivers measurable benefits from a grid perspective, including:[11]

  • Lower conventional electricity market prices due to reduced peak demand
  • Valuable price hedge from using a free, renewable fuel rather than variably-priced fossil fuels
  • Avoided costs of new transmission and distribution infrastructure to manage electricity delivery from centralized power plants;
  • Reduced need to build, operate and maintain natural gas generating plants
  • Reduced outages due to a more reliable, distributed electric power system
  • Reduced future costs of mitigating the environmental impacts of coal, natural gas, nuclear, and other generation
  • Enhanced tax revenues associated with local job creation, which is higher for solar than conventional power generation. See this example of revenue generation for the Canadian government by supporting PV manufacturing.[12]

Disadvantages of Photovoltaic Technology[edit | edit source]

Solar energy is a fairly inexhaustible source of energy, but that does not necessarily translate to PV being the same. PV systems are:

  • Must pay all at the begining. Prices for modules, the generators, are going down 20-40% every year for the last 7 years. What was once an expensive tech and subsidized (1995-2009) is now affordable and cost effective in sunny regions of even USA and Southern Europe (2012). System components are expensive to replace. The cost of a typical PV system in the U.S. is running between $2 and $6 per Wp[13] To power a typical US home with a 5kW system thus costs between $10,000 and $30,000. For more efficient homes and those that have more modest electric loads (e.g a few high efficiency CFLs) the costs are considerably less. For current prices on PV see http://www.solarbuzz.com/. A recent shortage in solar grade silicon halted the decades long reduction in cost as manufacturing capacity increased. The trend in costs decreasing is being restored as more solar grade siliconW plants come online. In addition, as truly large scale manufacturing utilizing industrial symbiosis is initiated solar PV is slated to be cost competitive with grid provided electricity..[14]
  • High Tech- Require a skilled labor force to create, although operation and maintenance of PV cells themselves is relatively easy. There are currently no good methods for people to make their own PV systems from local materials. The high tech nature gives a large advantage to scale of production with current technologies.
  • Some PV materials are toxic. E.g. the Cadmium in Cadmium Telluride solar cells. Many authors have argued that in the panel itself the Cd is secure from the environment -- but then it demands careful end of life treatment.
  • Intermittent- Solar cells only produce electricity when the sun is shining. At night or in bad weather, you need either storage batteries or a secondary power source. (On the other hand, solar panels are excellent for load balancing because maximum electricity usage and peak solar generation both occur on hot sunny days.)

There are two disadvantages often used in the environmentalist camps concerning high tech PV:

  • Production Pollution- Fossil fuels are extensively utilized to extract, produce and transport PV panels. These processes also entail corresponding sources of pollution. This is true of just about any product made today. Fortunately, the life cycle analysis of a PV system is a net positive for the environment because it can offset fossil fuel energy production over its approximately 25+ year lifetime.
  • High energy cost- Require much energy to produce. In the past it was even argued that it took more energy to produce than they consume. This is just wrong.[15] In this paper the authors clearly show that the three types of photovoltaic (PV) materials, which make up the majority of the active solar market: single crystal, polycrystalline, and amorphous silicon solar cells pay for themselves in terms of energy in a few years (1-5 years). They thus generate enough energy over their lifetimes to reproduce themselves many times (6-31 reproductions)depending on what type of material, balance of system, and the geographic location of the system.

Education about Solar Photovoltaic Cells[edit | edit source]

See also[edit | edit source]

Notes[edit | edit source]

There are far more cost-effective changes than implementing PV-systems in domestic houses. For example, spending it on solar hot water and energy efficiency, and possibly even on carbon offsets has a far greater effect in reducing your carbon/ecologic footprint.[verification needed]

External links[edit | edit source]

Web sites for current information on PV[edit | edit source]


Current production: 10.7GW in 2009 [5]

Useful government web sites on PV[edit | edit source]

How to Afford PV Now[edit | edit source]

Designing Your Own PV system[edit | edit source]

General Resources[edit | edit source]

Misc[edit | edit source]

References[edit | edit source]

  • Pratt, Doug & John Schaeffer. Solar Living Source Book. Tenth. NV: Chelsea Green Publishing Company, 1999.
  1. Cite error: Invalid <ref> tag; no text was provided for refs named jac
  2. German PV market
  3. BP Solar to Expand Its Solar Cell Plants in Spain and India
  4. Large-Scale, Cheap Solar Electricity
  5. 5.0 5.1 REN21. Renewables 2010 Global Status Report p. 19.
  6. GE Invests, Delivers One of World's Largest Solar Power Plants
  7. Building integrated photovoltaics
  8. Richard M. Swanson. Photovoltaics Power Up, Science, Vol. 324, 15 May 2009, p. 891.
  9. For a very good summary article of O&M of large systems see part 1: [1]and part 2: [2].
  10. Joshua Pearce and Andrew Lau, "Net Energy Analysis For Sustainable Energy Production From Silicon Based Solar Cells", Proceedings of American Society of Mechanical Engineers Solar 2002: Sunrise on the Reliable Energy Economy, editor R. Cambell-Howe, 2002.
  11. Summary; http://web.archive.org/web/20121110185859/http://www.onlinetes.com:80/solar-energy-bargain-nj-pa-11912.aspx
  12. K. Branker and J. M. Pearce, "Financial Return for Government Support of Large-Scale Thin-Film Solar Photovoltaic Manufacturing in Canada", Energy Policy 38, pp. 4291–4303 (2010). Open access
  13. A good role of thumb is to double the price of the module to account for system components and installation - for uptodate averages on panel costs see the retail price survey here.
  14. Pearce, J.M. 2008. "Industrial Symbiosis for Very Large Scale Photovoltaic Manufacturing", Renewable Energy 33, pp. 1101–1108. [3]
  15. For a detailed analysis of the life cycle energy costs of solar cells see: Joshua Pearce and Andrew Lau, "Net Energy Analysis For Sustainable Energy Production From Silicon Based Solar Cells", Proceedings of American Society of Mechanical Engineers Solar 2002: Sunrise on the Reliable Energy Economy, editor R. Cambell-Howe, 2002.
  16. Barefoot Solar Engineers