Passive and active solar technologies use sunlight to produce useful forms of energy. Passive solar technology uses no mechanical or electrical devices—no pumps, fans, or electrical controls—to convert sunlight or distribute energy. For example, passive solar home design can use windows and walls to absorb solar energy and distribute heat. A trombe wall is a type of wall that is made from a material (such as stone, metal, or concrete) that can store a lot of heat and is painted a dark color to absorb as much of the Sun's energy as possible. During the day, sunlight shines on the wall and warms it up. As the air temperature cools during the night, the wall then radiates heat. Passive heating technologies utilize natural heat transfer mechanisms (radiation, conduction, and convection) to redistribute energy gained from sunlight. Passive home design can be modified with fans to make it more active and help circulate heat.
Active solar technologies use equipment to convert solar energy into usable energy. For example, a common type of solar collector called a flat-plate collector uses sunlight to heat air or fluid (usually water or an antifreeze solution) flowing inside a box; pumps or fans usually circulate the air or fluid. Photovoltaics—special cells made of semiconductor materials that convert sunlight into electricity—are another example of an active solar technology. Photovoltaics are useful for a wide range of applications, from powering calculators and road signs, to homes and buildings, and even solar power plants. Concentrating solar power (CSP) systems are generally used for utility-scale electricity production. CSP uses reflective surfaces to concentrate sunlight onto receivers (tubes with fluid flowing through them); the hot fluid is then used to produce electricity in the same way as a conventional steam turbine generator. CSP can also be used for lower-tech applications, such as solar cookers in regions with few other resources.
The Sun is an environmentally friendly source of energy, but solar technology suffers from one major obstacle: What happens when there is no sunlight available? Nighttime and clouds prevent both passive and active technologies from functioning, and so researchers are working on finding ways to store energy that has been captured during sunlight hours to be used later. One promising technology for solar power plants is a new kind of battery that is made from all liquid materials (two electrode liquids that are separated by an electrolyte liquid). This battery can operate at high electrical currents, is capable of storing large amounts of electricity, and does not degrade as quickly as traditional batteries. Another potential solution involves molten salts. Molten salts can be heated to very high temperatures and can hold this heat for days. Solar power plants use sunlight to heat the salts, which can then heat water to produce steam to turn turbines.
