Photograph by Ruslan Dashinsky/Shutterstock
for National Geographic
There are several types of photovoltaic, or PV, cells available that power small devices such as calculators, phones, music players, computers, and lights.
The PV cell that has been used for years in calculators and watches is made from a thin layer of silicon on top of any number of surfaces that conduct electricity, and is called an amorphous silicon cell. These cells convert about 12 percent of available solar energy into electricity, providing only small amounts of power.
The most common type of PV cell is the crystalline silicon cell, which is made from thin wafers of silicon either sliced off a single crystal or made of many crystals and impregnated with other elements that help the cell generate and conduct electricity. These cells approach 20 percent efficiency.
Other types of PV cells include thin-film cells, which can be printed onto flexible backings and integrated into clothing, cells made from organic materials, and combinations of cell types to increase efficiency to almost 40 percent.
Batteries: Most devices use solar panels to charge an internal battery, which is used to charge another device, such as a phone. There are several types of rechargeable batteries, each with its own limitations. Nickel-cadmium (NiCd) batteries, the earliest version of rechargeable batteries, “remember” their ability to hold a charge, which means they must be fully discharged before they can recharge all the way. They also lose their charge over time in storage, especially in warmer weather. Nickel-metal hydride (NiMH) batteries have similar limitations but don’t lose their charge quite as fast. Lithium ion and lithium polymer batteries are lighter and can hold more energy and hold a charge for a longer time, but are susceptible to overheating and still need to be replaced after a certain number of charging cycles.
Rechargeable batteries can contain toxic cadmium, lead, and lithium. A law mandating that these batteries be recycled, and that mercury in batteries be phased out, has been in effect in the U.S. since 1996. There are many recycling programs that reclaim the battery materials and use them to manufacture more batteries.
Surface Area, Charging Time, and Portability The amount of electricity a solar panel generates depends on several factors: the surface area, the efficiency of the cells, and the quality of the sunlight. The angle of the panel toward the sun, cloud cover, and time of year all have an effect on the quality of the sunlight that reaches the panel. Bright sunlight at a 90 degree angle to the panel will allow the cells to convert the most light into electricity and charge devices (or the internal battery) the quickest.
The surface area has an impact on how portable the device is. Chargers range from pocket-sized, which aren’t much bigger than the devices they’re designed to charge, to multiple square feet that fold or roll up.
Multiple Devices: Many chargers work for multiple devices, but you may have to buy the right adapters. Pay attention to the wattage rating, which affects which devices you can charge. A small, handheld charger may not be able to charge a laptop—you may need a large panel with more surface area to charge larger devices.
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