Matthew Panzer, an assistant professor of chemical engineering, lays out the options
Meeting the world’s ever-growing energy demands in an environmentally responsible and sustainable manner is one of the most pressing issues facing us. Solar energy—sunlight—is an abundant, clean, safe and free resource, providing approximately 1,000 watts of power per square meter to Earth’s surface on a sunny day. In fact, the total amount of solar energy that hits Earth in just two hours is more than enough to meet current global energy consumption for an entire year.
How can we most effectively capture, convert and store this tremendous natural resource? First, it is important to recognize that sunlight consists of a spectrum of wavelengths. About half of it is lower energy infrared radiation that we cannot see, but we feel as heat. The rest is higher energy visible light or ultraviolet light. Some technologies for harnessing solar energy target the entire spectrum, while others use only a portion of the available wavelengths.
One of the first technologies that comes to mind when discussing solar energy is the growing use of solar cells, also known as photovoltaics, which convert sunlight directly into electricity. Solar cells are silent, non-polluting and long-lived devices that typically convert 10 to 15 percent of the energy received into energy that can be used.
They are not the only way to get electricity from solar energy, though. Sunlight can also be intensely focused onto a small area, using an array of mirrors or lenses to heat water and create steam. High-pressure steam can be driven through a turbine to generate electricity.
When the sun shines, we can store the electricity generated by solar cells or steam-driven turbines by using batteries (technically energy stored as electrochemical potential) or supercapacitors (energy stored in an electric field, due to the spatial separation of positive and negative charges). Then we can release electrical energy when it is cloudy or at night.
There are at least two other ways to store solar energy for use later. First, the thermal energy of concentrated sunlight can be stored in the heat capacity of a molten salt (the liquid form of an ionic compound like sodium chloride) at a high temperature. When electricity is needed later, heat is transferred from the molten salt to water, using a heat exchanger to generate steam to drive a turbine.
A second method of harnessing and storing solar energy is to employ sunlight to produce a fuel. For example, a photoelectrochemical cell uses solar energy to split water into hydrogen and oxygen gases, which can be stored as fuels. These gases are then recombined to generate electricity in a device known as a fuel cell. An attractive feature of this approach is that the byproduct of the fuel cell reaction is simply water.
We should not forget that sunlight can also be used to directly heat a tank of water located outside the home, and that solar heated water can be used for washing or showering; this is common in parts of the developing world.
While many of the technologies described here are in use on a small scale today, we must continue to develop innovative methods of storing solar energy and promote sustainable energy policies that benefit generations to come.