On a sunny day, solar energy—sunlight—provides around 1,000 watts of power per square meter to Earth’s surface, making it a plentiful, clean, safe, and free resource. The total amount of solar energy that reaches Earth in just two hours is more than enough to cover current world energy consumption for a year.
Sunlight comprises a spectrum of wavelengths. Roughly half of it is infrared radiation with a lower energy that we cannot see but feel the heat. The rest is visible or ultraviolet light with higher energy. Some solar energy harvesting systems utilize the entire spectrum, while others only use a fraction of it.
When it comes to solar energy, one of the first technologies that come to mind is the rising use of solar cells, also known as photovoltaics, which converts sunlight directly into electricity. Solar cells are quiet, non-polluting, and long-lasting devices that convert 10 to 15% of the energy they receive into usable electricity.
However, they aren’t the only technique to use solar radiation to generate electricity. Sunlight can also be strongly focused onto a tiny area, heating water and creating steam using a series of mirrors or lenses. To generate electricity, high-pressure steam can be propelled via a turbine.
When the sun shines, we can use batteries (technically energy stored as electrochemical potential) or supercapacitors to store the electricity created by solar cells or steam-driven turbines (energy stored in an electric field, due to the spatial separation of positive and negative charges). When it’s cloudy or late at night, we can release electrical energy.
There are at least two other methods for storing solar energy for later use. First, the heat capacity of molten salt (the liquid form of an ionic chemical like sodium chloride) at a high temperature can be used to store the thermal energy of concentrated sunlight. Heat is transported from the molten salt to water via a heat exchanger to generate steam, which drives a turbine when power is required later.
The use of sunshine to make fuel is a second method of harnessing and storing solar energy. A photoelectrochemical cell splits water into hydrogen and oxygen gases, which can then be stored as fuels. In a fuel cell device, these gases are recombined to generate electricity. The fuel cell reaction’s output is essentially water.
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