Sunlight and the Photovoltaic Effect
The photovoltaic effect , is the process of using sunlight to produce DC electricity in a silent, clean, and autonomous way. The equipment required to produce this electricity is commonly called a “solar panel,” and are modular and require minimum maintenance. Combined with their long durability solar systems are increasing in popularity in remote areas or when an installation is expected to last.
Solar panels are devices able to transform light radiation into electricity through a process of trapping the photons and using them to excite P-type and N-Type semiconductors to move free electrons. Modern photovoltaic panels can generally convert around 15-20% of energy directly into electricity. There are panels that are more efficient, but they are very costly and , easy to damage, and are generally not accessible in places where humanitarian organisations might work.
Light enters the device through an anti-reflective coating that minimises the loss of light by reflection; it . The device then effectively traps the light striking the solar cell by promoting its transmission to the three energy-conversion layers below.
- N-Type Silicon layer; Provides extra electrons (negative).
- P-N junction layer. The absorber The absorbion layer, which constitutes the core of the device orienting the electrons in one direction.
- P-Type Silicon layer; Creates vacancy of electrons (positive).
Most solar cells are a few square centimetres in area and are protected from the environment by a thin coating of glass or transparent plastic. Because a typical 10cm×10cm (4 inch × 4 inch) solar cell generates only about two Watts of electrical power, cells are usually combined in series to boost the voltage or in parallel to increase the current. A solar, or photovoltaic (PV), module generally consists of 36 or more interconnected cells laminated to glass within an aluminium frame.
One or more of these PV modules may be wired and framed together to form a solar panel, and multiple panels can be combined to form a solar array, together supplying power as a single unit.
All solar cells - and by extension solar panels - degrade over time. While solar systems draw energy from the sun, the sun also slowly breaks down the components of solar cells. Most commercially available solar panels degrade at an average rate of 2% per year of usage. The duration of use of an installation must be factored for planning and budgeting purposes. For example, a solar array installed in direct sunlight degrading at 2% a year means that after 10 years, the panels will only be roughly 80% as efficient as they were at the time of installation. Less efficiency means less Wattage output from the array, meaning longer periods of of time to charge batteries and less optimal charging times throughout the day. Humanitarian agencies planning to use solar arrays longer than 10 years in a single location may want to consider budgeting for the replacement of panels after 12-15 years if the the overall output is no longer meeting the needs of the location.
A complete photovoltaic system may consist of one solar module or many – solar panel or array - , depending on the power needed. While batteries can be used as back-up of any main power supply, solar systems need a battery system to storage store the energy produced. Therefore, a solar system always includes some form of battery system, either small or big. These batteries are specifically designed to deliver limited current over long period of time.
A power system can accommodate different electrical loads by regulating the voltage and/or current coming from the solar panels going to the battery to prevent overcharging. Most "12 volt" panels can put out about 16 to 20 volts in optimal conditions, so if there is no regulation the batteries can and will be damaged from overcharging. Most batteries need around 14 to 14.5 volts to get become fully charged. Like any other electrical system, proper assessment and cabling is are required.
A solar system is usually composed by:
Solar systems can accommodate almost any specific need because they are modular in nature. This makes it possible connect PV modules directly do many devices, such as submersible pumps or standalone freezer units, or as a complete solar power plants arrays able to produce energy for distribution purposesentire offices or compounds.
Solar modules are rated in Watt-peak, represented as nominal peak power (P max), derived from multiplying peak power voltage (Vmp) with its peak power current (Imp):
A 100Wp solar panel produces 100W under standards standard test conditions (STC). The STC exist only in laboratories, applying a solar irradiance to panels of 1,000W/m2 with a cell temperature of 25ºC. In a real installation, the actual production of electricity is usually far lower than the peak-power, but however the measures remain useful as qualitative reference to compare sizes and capacities as every panel is rated under those the same conditions.
Example: Label that Comes with Solar Panel
Daily Irradiance: The quantity of energy provided by the sun in one day is the most important parameter. Areas closes close to the equator have the best average irradiance, however this parameter general rule may vary greatly from one place to the other and from one season to the other. Average The average performance of a PV system expressed in KwH/m2/day can be referenced in the chart below.
Long Term Average Daily Sum
Long Term Average Yearly Sum
Shade, haze, haze and and cloudy weather: any obstacle blocking sun radiation light will decrease the energy production of the module. In addition, if a solar panel is partially shaded, the electricity production may stop as the shaded cells will consume the energy produced by the rest of the panel. In some cases, a phenomenon called “hot spot heating” occurs when the shaded portions of a single panel rapidly heat up as they consume electricity from the an unshaded part, and can rapidly destroy the panel. This can be prevented by using by-pass diodes which are commonly included in PV modules, but it is highly recommended to check on this feature.
Panel orientation: a poorly oriented panel - from for example facing the north in the northern hemisphere - will produce far less energy than the panel is rated for, or even no energy at all.
Temperature: Temperature above 25oC also can decrease the amount of energy produced by a solar panel.
Daylight Hourshours: Solar panels produce more electricity when the vertical rays of sunlight are closer together, providing more energy per square cm. By result, solar panels will produce less electricity as the sun is near the horizon than it will when the sun is directly overhead. A simple way to think about this: I practical terms, a solar panel near the equator that is outside for a 12 hour day will only produce the equivalent of 6 hours worth of peak electricity, and this is only under optimal conditions. Changes to the seasons or bad weather will drop this production even further.
WPV modules combined to create solar panels, and solar panels combined mounted together to create solar arrays are possible using standard junction boxes - MC3/MC4 type - that are waterproof and easy to connect. Like batteries, panel arrays should only use solar modules with the same characteristics, the same model, and as far a possible the same history.
Solar trackers - devices that orient panels towards the sun - are complex, expensive and not recommended outside of industrial uses and/or high latitudes where the sun moves considerably. Some mounts are designed to allow seasonal adjustment, giving the ability to switch manually between 2 two positions during the year, which should be more than enough to orient the modulesfor most installations.
There are essentially 2 two types of solar mounts available: Ground and Roof mounts. Ground mounted solar panels are easier to install and maintain than roof mounted systems. Roof mounted systems are difficult or impossible to adjust and can cause structural damage due to weight and wind pressure. However, ground mounts have their own problems; they occupy usable space, are more prone shade, and run the risk of accidental damage from cars and people. Mounting decisions should be made depending on the location and infrastructure available.
While more expensive upfront, the MPPT Charge Controller will give more power (and potentially reduce the size of the PV module) and extend the lifespan of the batteries connected to it. Certain controllers even allow to connect it connection to smart devices for remote control and monitoring.
Pulse Width Modulation (PWM): PWM charge controllers can be considered an electric switch between the solar panel and battery packs, programmed to only allow a pre-determined current into the battery. The controller slowly reduces the amount of power going into the battery as they the batteries approach maximum capacity. PWM Charge Controllers do not adjust voltage, meaning the batteries and panels must have compatible voltages in order to operate properly. This makes this type of charge controller suitable for smaller solar applications, or for installations that feature lower voltage panels and limited size battery banks. PWMs are a more affordable option but will result in a lower power production from the PV .
Turn solar Solar to electricityElectricity
small Small solar system
Large power system