lesson 3

Question 1

advantage of concentrating collector over the flat collector

Answer 1

1. the size of the absorber can be reduced giving high concentrating ratio
2. thermal losses are less than FPC, However, small losses occur in the concentrating collector because of its optical system as well as by reflection absorption by mirrors and lenses.
3. the efficiency increases at high temperatures.
4. in these collectors the area intercepting the solar radiation is greater than the absorber area
5. these collectors are used in high-temperature applications
6. Reflectors cost less per unit area than a flat-plate collector
7. A focusing or concentrating system can be used for electrical power generation when not used for heating or cooling
8. little or no anti-freeze is required to protect the absorber in a concentrated system

Question 2

disadvantage of concentrating collector over the flat collector

Answer 2

1. only the beam component is collected in the case of focusing collectors because the diffuse component cannot be reflected and thus is lost
2. in some stationary reflecting systems it is necessary to have a small absorber to track the sun image; in others, the reflector may have to be adjustable to more than one position if year-round operation is desired; in other words, costly orienting systems have to be used to track the sun.
3. additional requirement of maintenance particularly to retain the quality of reflecting surface against dirt, weather, oxidation etc.
4. non-uniform flux on absorber whereas flux in flat-plate collectors in uniform
5. additional optical losses such as reflectance and intercept loss, so they introduce additional factors in energy balance
6. high initial cost

Question 3

disadvantage of solar air heater

Answer 3

1. need to handle a larger volume of air than a liquid due to the low density of air as the working substance
2. thermal capacity for air is low
3. they have relatively high fluid circulation cost
4. they have relatively high volume of storage

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