PV Panels are built from PV Modules connected either in series and/or in parallel. A PV Module is built up of "cells" which is the base building block of the system. PV (Photo Voltaic) cells are semiconductor devices that directly convert sunlight to DC electricity. They do not consume any fuel other than sunlight to generate power and typically have a life span of at least 25 years. Most cells produce approximately one-half of a volt. A typical module will have 36 cells in series which gives it an operating voltage of 18Volts and a nominal voltage of 12 volts. The current output of the module is determined by the amount of surface area of the module. The PV cells are encapsulated in the module frame to protect them from the elements. Modules are typically flat and rectangular and produce between 5 and 500 watts of power.
There are essentially 3 different types of PV Panels, Crystalline Silicon, Amorphous Silicon and other Thin Film technology PV Panels. Crystalline Silicon panels are the oldest, most reliable and highly efficient PV panels in the market today.
Crystalline Panels can be further divided into Mono-Crystalline and Poly-Crystalline panels. Mono-Crystalline or Single-Crystalline PV panels are the oldest commercially available PV panels and have proven to be durable and efficient-up to 14%. Multi-Crystalline cells are slightly less efficient when compared to Mono-Crystalline cells but are less expensive as well. The most common manufacturing approach is to process discrete cells on wafers sawed from silicon ingots. Ingots can be either single-crystal or multi-crystalline.
Amorphous Silicon and other thin film technology PV panels are manufactured by depositing of thin layers of non-crystalline-silicon materials on inexpensive substrates. It is the least energy intensive of the manufacturing approaches for commercial photo voltaics. This includes amorphous silicon cells deposited on stainless-steel ribbon, cadmium telluride (CdTe) cells deposited on glass, and copper indium gallium di-selenide (CIGS) alloy cells deposited on either glass or stainless steel substrates.
Amorphous Silicon Modules have a shorter history compared to Crystalline Modules and are generally less efficient as well at standard test conditions.. Uni-Solar is the most popular Amorphous Silicon manufacturer and their cells typically are in the 8-10% efficiency range. They are durable and light weight and can be made flexible which gives them a distinct advantage over crystalline Silicon modules for certain applications. Another distinct advantage of Amorphous and Thin Film modules over Crystalline modules is their capacity to generate power with diffused light or when there is shading or on cloudy days. Even though at Nominal conditions Crystalline modules have a higher efficiency of converting light to DC Power, for practical every day performance thin film modules may generate more power than crystalline modules due to their ability to work on cloudy days with diffused sun light.
Other thin-film technology based modules, either based on Cadmium Telluride, Gallium Arsenide or CIGS have moderate efficiency, again around 8% and are durable but some must be recycled at the end of their life as they contain hazardous material.
Another interesting option available now is called BIPV or Building integrated Photo Voltaics. They are aesthetically pleasing as well as perform a "building" function in addition to generating electricity. A BIPV roof replaces the need for a traditional roof thereby adding to the financial savings of a renewable energy system.
The most common and basic descriptor of a PV Module performance is the current-voltage (I-V) curve. An example of an IV curve is shown below:
PV Device performance is given by the following parameters:
- Open Circuit Voltage: Voc
- Maximum power Voltage: Vmp
- Short Circuit current: Isc
- Maximum Power Current: Imp
- Maximum Power: Pmp
The power produced by the panel is dependant on the solar irradiation. The Open Circuit voltage and Maximum power voltage is not effected greatly by the amount of irradiance but the Short circuit current and the Maximum power current are directly dependant on the amount of solar irradiation striking the surface of the PV module. The fact that the voltage varies little with changing sun light levels makes PV devices well suited for battery charging applications.
When designing a PV system it is important to ensure that the PV modules are operating at their Maximum power condition.Three major factors affect the performance output of PV modules: Load resistance, Amount of Sunlight and PV module/Cell temperature.
Load Resistance: The system should be designed to ensure that the PV system operates at voltages close to the maximum power voltage of the array. If a load's resistance is well matched to a module's IV curve, the module will operate at or near the maximum power point, resulting in highest possible efficiency. As the load's resistance increases, the module will operate at voltages higher than the maximum power voltage causing efficiency and current output to decrease. Efficiency also decreases as the voltage drops below the maximum power point.
Amount of sunlight: A module's current output is directly proportional to the amount of sunlight it is exposed to. As the amount of solar insolation drops the power generation of the module drops accordingly. Refer to the IV curve above which also shows the effect of decreasing solar insolation. Shading is another contributor to loss of efficiency. Even partial shading will result in dramatic loss of power production.
It is important to do a site analysis before installing PV panels. Crystalline cells are more affected by the sunlight and shading than the modules based on thin-film technologies.
PV Module/Cell Temperature: as the cell temperature rises above the standard operating temperature of 25 degrees C, the module operates less efficiently and the voltage decreases. As the temperature rises, the shape of the I-V curve remains the same but shifts to the left indicating lower voltage output and energy. Generally between 80 and 90 degrees C, a module loses approximately 0.5% of efficiency per degree rise in temperature. Airflow under and over the modules is crucial to remove heat to avoid high cell temperatures.
As a rule of thumb the PV Panel temperate is generally 25-40 degrees Centigrade above ambient temperature. Amorphous silicon and other thin-film technology panels are less efficient but have better temperature sensitivity and either do not loose power production capacity or sometimes improve their efficiency under higher temperatures.
Most Module manufacturers specify data at both Standard test conditions (1000 watts/Square meter irradiance, 25 degrees cell temp, 1.5 air mass) and the Nominal operating cell temperature (800 watts/Square meter irradiance, 20 Degrees ambient temperature and wind velocity 1m/s).
Here is a sample of specification data available for commercial modules: