Solar Radiation and Solar Energy Collectors
1. Define solar collector.
A solar collector is a device for collecting solar radiation and transfer and the energy to a fluid passing in contact with it. Utilization of solar energy required solar collectors.
2. What are the different types of solar collectors.
Solar collectors are classified into two types. They are
i. Non-concentrating or Flat plate type solar collector.
ii. Concentrating (Focusing) type solar collector.
3. What is the advantages of concentrating type solar collector over Non-concentrating type solar collector.
In concentrating collector, the area intercepting the solar radiation is greater sometimes hundred of times greater than absorber area.
By means of concentrating collectors, much higher temperature can be obtained than with non-concentrating type. Concentrating collectors may be used to generate medium pressure steam.
They use many different arrangement of mirrors and lenses to concentrate the sun ray’s on the boiler. This type shows better efficiency than the flat plate type.
4. Define flat plate collectors
Solar collectors for home heating usually called flat plate collectors, almost have one or more glass covers, although various plastic and other transparent materials are often used instead of glass.
5. State Wien’s Law
Wien’s Law state that, the emission increases with temperature. The re-emitted light if so progressively shorter wavelength and greater energy as the temperature of blockbody increases. This is expressed by Wien’s Law, which can be written as,
6. State Plank’s Law
Planck’s Law state that the spectral emissive power of a black surface is give by,
Where, C1 and C2 are constants whose values are 0.596 x 10-16 W-m2 and 0.014387m-K respectively, l is the wavelength and T is the temperature of the black surface in K.
7. State Stefan – Boltzmann Law.
The Stefan – Boltzmann law is obtained by integrating Planck’s Law over all the wavelength from 0 to µ and state that the emissive power of a black surface is given by
8. Define Solar Constant.
The Solar Constant ISC is the rate at which energy is received from the Sun on a unit area perpendicular to the rays of the Sun, at the mean distance o the earth from the Sun. Based on experimental investigation and subsequent measurement, the value of ISC is 1367 W/m2 has been recommended.
9. Draw the spectral distribution of extraterrestrial solar radiation.
10. Define the following terms.
i. Direct radiation
ii. Diffuse radiation, and
iii. Global radiation
i. Direct radiation: Solar radiation received at the earth’s surface without change of direction, i.e., in the with the Sun, is called Beam or Direct radiation.
ii. Diffuse radiation: The radiation received of the Earth’s surface from all parts of the sky’s hemisphere, after being subjected to scattering in the atmosphere is called Diffuse radation.
iii. Global radiation: The sum o the Beam radiation and Diffuse radiation is referred as Total or Global radiation.
11. Define Air Mass(AM)
Air Mass(AM) is often used as a measure of the distance travelling by beam radiation through the atmosphere before it reaches a location on the earth surface. It is defined as the ratio o the mass of the atmosphere through which the beam radiation passes to the mass it would pass through if the Sun directly overhead(i.e., at its zenith).
12. Name the instruments used to measure the Solar Radiation and Sunshine hours.
Solar Radiation Flux is usually measured with the help of a Pyranometer or Pyrheliometer.
The duration of bright sunshine in a day measured by means of a Sunshine Recorder.
13. What are types of the Flat- plate solar collectors?
Flat plate solar collectors may be divided into two main classification based on the type of heat transfer fluid used. They are,
i. Liquid Heating Collector
ii. Solar Air Heater
14. Give the main components of Flat-Plate Collector.
The majority of the Flat-Plate Collector have five main components as follows:
i. A transparent cover.
ii. Tubes, fins, passages or channels.
iii. Absorber plates
v. Casing or Container.
15. Draw the typical structure of Liquid Collector.
16. Draw the typical cross section of typical Air Collector or Solar Air Heaters.
17. What are the different types of Air Heaters?
Basically air heaters are classified into the following two categories
i. Non-porous type air heaters.
ii. Porous type air heaters.
18. What are the different losses occurs in Solar Collector?
i. Conductive Loss
ii. Convective Loss
iii. Radiative Loss
19. Define Collector Efficiency
Collector Efficiency (μc) is the collector performance and is defined as the ratio of the useful gain over any time period to the incident solar energy over the same time perioid.
20. Define fin efficiency
Fin efficiency is used to indicate the effectiveness of a fin in transferring a given quantity of heat. In efficiency is defined as,
Fin efficiency = Actual heat transferred / Heat which would transferred if entire fin area were at base temperature.
21.What are the different types of concentrating collectors?
As per the number of concentrating collectors are
i. Parabolic through collector
ii. Mirror strip reflector
iii. Fresnel lens collector
iv. Flat plate collector with adjustable mirrors.
v. Compound parabolic concentrator(C.P.C)
22. What are the different types of heat transfer mechanism?
Heat transfer occurs by three basic mechanisms, they are
23. Define concentration Ratio
It is the ratio of the effective area of the aperture to the surface area of the absorber.
24. What are the main advantage of concentrator system over flat plate system.
i. Concentration system can be used for electric power generation, because of high temperature of working fluid obtained.
ii. The heat storage cost will be less causes of the high temperature attainable and high temperature storage made possible.
iii. Solar heating and cooling applications system can attain higher efficiencies.
iv. Anti-freeze and other liquid to protect the absorbers may not be required.
25. What are the properties of materials used in Flat Plate Collector ?
The following three properties considered for selecting material for Flat Plate Collector:
i. Thermo-physical properties
ii. Physical properties
iii. Environment properties
26. Define Transmissivity – Absorptiviy Product.
The efine Transmissivity – Absorptiviy Product is defined as the ratio o the flux absorbed in the absorber plate to the flux incident on the cover system, and is denoted by the symbol (ta), an appropriate subscript (b to d) being added to indicate the type of incident radiation.
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