Renewable Energy - Solar (Part 2)

Solar energy is the energy derived from the sun in the form of solar radiation (light photons and heat). There are two methods of conversion from sunlight into electricity, either directly using photovoltaics (PV), or indirectly using concentrated solar power (CSP). Concentrated solar power systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. Photovoltaics convert light into electric current using the photoelectric effect.

Concentrated Solar Power (CSP)

Are systems that use mirrors or lenses to concentrate a large area of sunlight or solar thermal energy, onto a small area. Electrical power is produced when the concentrated light is converted to heat which drives a heat engine (usually a steam turbine or Stirling engine) connected to an electrical power generator. CSP technologies exist in four common forms; parabolic trough, Fresnel reflector, dish Stirling and solar power tower.

a) Parabolic Trough: Consists of a linear parabolic reflector that concentrates light onto a receiver positioned along the reflector's focal line. The receiver is a tube positioned directly above the middle of the parabolic mirror and filled with a working fluid. The reflector follows the sun during the daylight hours by tracking along a single axis. A working fluid (molten salt) is heated to 150–350 °C as it flows through the receiver and is then used as a heat source for a power generation system. Trough systems are the most developed CSP technology.

b) Fresnel Reflectors: Made of many thin, flat mirror strips to concentrate sunlight onto tubes through which working fluid is pumped. Flat mirrors allow more reflective surface in the same amount of space as a parabolic reflector, thus capturing more of the available sunlight, and they are much cheaper than parabolic reflectors. Fresnel reflectors can be used in various size CSPs.

c) Dish Stirling: Also known as dish engine system consists of a stand-alone parabolic reflector that concentrates light onto a receiver positioned at the reflector's focal point. The reflector tracks the Sun along two axes. The working fluid in the receiver is heated to 250–700 °C and then used by a Stirling engine to generate power. Parabolic-dish systems provide the highest solar-to-electric efficiency among CSP technologies, and their modular nature provides scalability.

d) Solar Power Tower: Consists of an array of dual-axis tracking reflectors (heliostats) that concentrate light on a central receiver atop a tower; the receiver contains a fluid deposit. The working fluid in the receiver is heated to 500–1000 °C and then used as a heat source for a power generation or energy storage system. Solar power tower development is less advanced than trough systems, but they offer higher efficiency and better energy storage capability.

Photovoltaic (PV)

Is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit photovoltaic effects. Photovoltaic power generation employs solar panels composed of a number of solar cells containing photovoltaic material. Photovoltaic power generation employs solar panels composed of a number of solar cells containing a photovoltaic material. Materials presently used for photovoltaics include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium gallium selenide/sulfide.