State and explain principal, construction and working of flat plate collector used for solar energy.
The Earth receives an incredible supply of solar energy. The sun, an average star, is a fusion
reactor that has been burning over 4 billion years. It provides enough energy in one minute
to supply the world's energy needs for one year. The amount of solar radiation striking the
earth over a three-day period is equivalent to the energy stored in all fossil energy sources.
Global installed capacity for solar-powered electricity has seen an exponential growth,
reaching around 227 GW at the end of 2015. It produced 1% of all electricity used globally.
Solar energy is used in thermal and photovoltaic systems:
- Thermal systems concentrates sunlight, converts it into heat, and applies it to a steam
generator or engine to be converted into electricity in order to warm buildings, heat
water, generate electricity, or destroy dangerous waste. For example, solar thermal
- Photovoltaic systems are composed of cells made of semiconductor material,
(silicon). It can produce power when sunlight strikes the semiconductor material and
creates an electric current. For example, solar cells.
Flat Plate collectors (Solar thermal collectors):
- The flat-plate solar collectors are probably the most fundamental and most studied
technology for solar-powered domestic hot water systems.
- The basic principle for this device is that the sun heats a dark flat surface, which
collects as much energy as possible, and then the energy is transferred to water, air,
or other fluid for further use.
Construction and Working:
These are the main components of a typical flat-plate solar collector:
- Black surface : absorber plate that absorbs the incident solar energy (copper or aluminium sheet coated with selective coating)
- Glazing cover: transparent layer that transmits radiation to the absorber, but prevents radiative and convective heat loss from the surface (plastic or glass)
- Tubes: contain heating fluid to transfer the heat from the collector
- Support structure: protect the components and hold them in place
Insulation: cover sides and bottom of the collector to reduce heat losses (polymeric material)
- The schematic of a flat plate solar collector with liquid transport medium is given here.
- The black absorber plate absorbs radiant heat from sunlight.
- The black absorber plate is covered by transparent screens to reduce the heat loss
due to convection and radiation to the atmosphere.
- There are tubes carrying water, which gets heated due to the heat absorbed.
- The thermal insulation prevents heat loss during heat transfer.
- The flat-plate systems normally operate and reach the maximum efficiency within the
temperature range from 30 to 80 °C, however some new types of collectors that
employ vacuum insulation can achieve higher temperatures (up to 100°C).
- Due to introduction of selective coatings, stagnant fluid temperature in flat-plate
collectors has been shown to reach 200 °C.
- Flat-plate collectors need to face the sun to obtain maximum sunlight exposure. The
installation angle should be equal to or up to 15° higher than the latitude of the
- This angle ensures optimal heat output throughout the year. The flat plate solar
collectors are highly useful for low temperature heating. The main use of this
technology is in residential buildings where the demand for hot water has a large
impact on energy bills. Commercial applications include car washes, military laundry
facilities and eating establishments.