Photovoltaics

Photovoltaics Technical Information

Solar cells, also called photovoltaics (PV), convert sunlight directly into electricity. When sunlight is absorbed by these materials, the solar energy knocks electrons loose from their atoms, allowing the electrons to flow through the material to produce electricity. This process of converting light (photo) to electricity (voltage) is called the photovoltaic (PV) effect.
Solar cells are typically combined into modules that hold about 40 cells; about 10 of these modules are mounted in PV arrays that can measure up to several feet on a side. About 10 to 20 PV arrays can provide enough power for a household; arrays are interconnected to form a single PV system. These PV arrays can be mounted at a fixed angle facing south, or they can be mounted on a tracking device that follows the sun, allowing them to capture approx. one and a half times the sunlight over the course of a day.

There are different technologies available for making solar cells.Silicon wafer-based solar (crystalline silicon) technology accounts for more than 86% of the solar cell commercial production today. It is different from amorphous silicon, which is used for thin film technology. In a controlled environment, crystalline silicon is much more efficient than amorphous silicon, but if any part of a crystalline silicon panel is shaded, the whole system stops producing energy. This is not true for amorphous silicon. Amorphous silicon is cheaper and also more readily available.
Gallium is the rarest component of new photovoltaic compounds recently announced by South African researchers for use in solar panels as a more efficient alternative to crystalline silicon.

Efficiency

The performance of a solar cell is measured in terms of its efficiency at turning sunlight into electricity. Only sunlight of certain wavelengths will work efficiently to create electricity, and much of it is reflected or absorbed by the material that make up the cell. Because of this, a typical commercial solar cell has an efficiency of 15%—about one-seventh of the sunlight striking the cell generates electricity. Low efficiencies mean that larger arrays are needed, and that means higher cost. Improving solar cell efficiencies while holding down the cost per cell is an important goal of the PV industry. Production of solar panels has risen sixfold since 2000 and is doubling every 2 years today.

Locating Photovoltaics

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.)

Sun/Clouds: It is important to estimate the sun availability and cloud cover. Sometimes you can obtain this information on the web (eere.gov) 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. 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.

Building-integrated Photovoltaics (BIPV)

Instead of putting a convention PV array on a roof. There are now ways to gather solar energy through the use of conventional building materials. Thin film solar cells use layers of semiconductor materials only a few micrometers thick make it possible for solar cells to now double as rooftop shingles, roof tiles, building facades, or the glazing for skylights. Many buildings are taking advantage of this new method of incorporating PV. BIPV systems are aesthetically more popular than conventional arrays as they can blend seamlessly into the design. They also can help save an owner on building costs by replacing building materials that would usually be used with BIPV materials that do double duty. These advantages make BIPV one of the fastest growing segments of the solar industry.

 
Solar Roof Shingles

PV Shingles are made with mono or polycrystalline solar cells directly integrated with regular asphalt shingles. They have a deep, dark, purplish-blue color, and look similar to other shingles. They are usually more expensive to install than typical PV panels, but the added value is in the aesthetics as they are not obviously solar collectors. Also, the solar shingles offset the cost of traditional shingles for the roof and are just as reliable. Some can be applied directly  intermixed with regular asphalt shingles while others may need special installation. The typical power output spans several watts (roof shingles) to about 50 watts (roof tiles with crystalline solar cells). They are manufactured by only a few companies worldwide including SunPower Corporation, Solar Components Corporation, and Atlantis Energy Systems.

 
PV Advantages

Independence: Many homeowners feel that energy independence from utilities is their primary motivation for adopting PV.
Reliability: Even in harsh conditions PV systems are reliable and sturdy. They also prevent costly power failures when constant power is essential. They are durable and work for at least 20 years.
Low Maintenance Cost: Systems don’t need much attention once installed since there are no moving parts. They usually run without needed tune-ups like other energy systems.
Safety: PV systems do not require the use of combustible fuels and are very safe when properly designed and installed
No Fuel Cost: There is no costs associated with storing, purchasing or transporting fuel, because no source is required.
Reduced Sound Pollution: This system operates silently and with minimal movement.

PV Disadvantages

Initial Cost: An array can be a sizable investment, but the ROI (return on investment) is getting shorter and the cost of energy from the grid continues to rise and incentives are provided.
Solar Orientation: In order to function as their best, panels need direct sunlight. Shadows from trees or other objects can greatly reduce the systems efficiency. Unlike solar hot water, PV panels can face other directons—East or West—and still proved over 80% of the performance of south-facing collectors.
Energy Storage: Some PV systems need batteries if they are to provide power when the utility grid is down, to store power generated during the day. This can increase the size, cost and complexity of a system.
Efficiency Improvements: In order to use the new power generated as efficiently as possible, electrical loads need to be reduced through efficiency and old outdated appliances need to be replaced.
Manufacturing: Panels have a high embodied energy and can made from some toxic materials. There are health and safety issues only involving in the manufacturing process. Typically PV panels will produce enough energy to offset their embodied energy in 7-10 years.

Sizing Photovoltaics

Sizing a residential photovoltaic power system is not particularly complex. Sizing is based on the user’s projected needs (loads), goals and budget.

To optimize your system these areas need to be addresses:
Siting.
The site should be clear of shade to increase the systems efficiency.
Orientation
The array orientation with respect to true south and proper inclination is important for maximizing annual photovoltaic output based on local climate conditions.
Mounting Options
The optimal mounting system can maximize gain either in summer, winter or both by the angle relative to horizontal.
Modules
PV modules should be selected according to the systems design parameters.
Wiring
System wiring should be designed to minimize voltage drop, meet safety codes, and provide protection from the environment.
Controllers
The controller must operate a system efficiently while meeting the needs of the user.
Battery Storage
The battery bank must be sized to the specific installation and time desired to be self-reliant.
Loads
The system loads determine the size of the system and should be minimized by intelligent planning.

Most photovoltaic dealers help designers size systems. Each module manufacturer has a design method based on their product specifications, ranging from simple analysis to full simulations. If needs are not critical, a more general sizing method may be adequate.

Residential Renewable Energy Tax Credits

Consumers who install solar electric systems can receive a 30% tax credit for systems placed in service from January 1, 2006 through December 31, 2016; the previous tax credit cap of $2,000 no longer applies.
For more information, go to:
http://www.energy.gov/taxbreaks.htm

There are additional state and local rebates available for photovoltaic installation. Contact you local energy provider to see what is available in your area.

Power Storage

You may want a way to store your power:

Batteries allow you to store energy directly from the energy generated by the PV Array.
Batteries store DC energy and allow you to utilize the energy during the night, or when there is a blackout (if you are connected to the grid). Batteries are an extremely important power supply for critical electrical loads that consistently require usage. If you are wishing to power a load only during the day, a battery may not be required, i.e. to power a fan on sunny days inside of a greenhouse. Utility grid-connected PV systems do not require the use of batteries, though they can be used as an emergency backup power supply.