# Solar Energy Questions and Answers – Empirical Equations for Estimating Solar Radiation Availability on Horizontal Surface for Cloudy Skies

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This set of Solar Energy Interview Questions and Answers for freshers focuses on “Empirical Equations for Estimating Solar Radiation Availability on Horizontal Surface for Cloudy Skies”.

1. What is the relationship between insolation and cloud cover in sky?
a) Inverse
b) Directly proportional
c) Square
d) Exponential

Explanation: From ancient time, people knew the relationship between sunshine and cloud cover. There is an inverse relationship between insolation and the amount of sky covered by clouds.

2. Which of the following is a reason for inverse relationship between insolation and cloud cover in sky?
a) Sunshine duration records are caused by sun being obstructed by transparent cloud cover
b) Sunshine duration records are caused by sun being obstructed by opaque cloud cover
c) Sunshine duration records are caused by sunshine being transmitted through opaque cloud cover
d) Sunshine duration records are caused by sun being obstructed by cloud cover

Explanation: There is an inverse relationship between insolation and cloud cover in sky. This is because of sunshine duration records are caused by sun being obstructed by transparent cloud cover.

3. Which does Angstrom-Savinov formula tell?
a) Relationship between local solar radiation and global cloudiness
b) Relationship between local solar radiation and local cloudiness
c) Relationship between global solar radiation and mean cloudiness
d) Relationship between global solar radiation and global cloudiness

Explanation: Angstrom-Savinov formula is a relationship between global solar radiation (H) and mean cloudiness (C). It is given by mathematical relationship, H=H0[1−(1−k)C].
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4. What does the constant “k” signify in the Angstrom-Savinov formula?
a) Global solar radiation
b) Mean cloudiness
c) Reflection of solar radiation within clouds
d) Transmission of solar radiation within clouds

Explanation: The constant “k” signifies the transmission of solar radiation within clouds. It is defined to account for the partial and practical transmission of solar radiation through clouds.

5. The constant “k” in Angstrom-Savinov formula depends on ________
a) latitude of the location
b) longitude of the location
d) cloud opacity

Explanation: The constant “k” accounts for transmission of solar radiation through clouds. It depends on the latitude of the location. It does not depend on the solar radiation and cloud capacity.
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6. How does the constant “k” in Angstrom-Savinov vary in high and low latitudes?
a) Between 0.55 in high latitudes and 0.22 in low latitudes
b) Between 0.55 in high latitudes and 0.33 in low latitudes
c) Between 0.33 in high latitudes and 0.55 in low latitudes
d) Between 0 in high latitudes and 0.9 in low latitudes

Explanation: The constant “k” in Angstrom-Savinov has a range from 0.33 to 0.55. It varies between 0.55 in high latitudes and 0.33 in low latitudes.

7. Which of the following models uses linear regression to estimate solar radiation from sunshine duration?
a) Angstrom-Savinov
b) Ertekin and Yladtz
c) Anstrom-Prescott
d) Sen

Explanation: Anstrom-Prescott model uses linear regression to estimate solar radiation from sunshine duration. H/H0=a+b*(S/S0) is the regression equation. H is global radiation and S is the sunshine duration.

8. Akinoglu and Fcevit uses a _______ to estimate solar radiation.
a) linear model
b) power model
c) exponential model

Explanation: Akinoglu and Fcevit uses a quadratic model to estimate solar radiation. Like Angstom-Savinov, it also estimates solar radiation from sunshine duration.

9. How is an empirical model developed?
a) From data acquired via sensors or measuring instruments
b) From theoretical analysis
c) From deep learning
d) From trial and error

Explanation: Empirical models of any system is developed from data acquired via sensors or measuring instruments. The developers decide the variables and the output and perform various numerical analysis to formulate a relationship.

10. What does the ratio of global solar radiation (H) and estimated daily global radiation for cloudless sky (H0) indicate?
a) Solar radiation transmission ratio
b) Clearness index
c) Solar radiation reflection percentage
d) Turbidity

Explanation: The ratio of global solar radiation (H) and estimated daily global radiation for cloudless sky (H0) indicates clearness index. It neither indicates solar radiation transmission ratio nor does it indicate turbidity. Solar radiation reflection percentage is indicated by albedo.

11. Which of the following is important during data acquisition from measuring instruments? Assume that details about data to be collected is known.
a) Type of data
b) Materials used to manufacture measuring instruments
c) Location, environment and characteristics of measuring instrument
d) Brand of measuring instruments

Explanation: Assuming that details about the data to be collected is known, location, environment and characteristics of measuring instrument is important. This is because instruments can give different values under different conditions. Basically, data acquisition procedure should be consistent and authentic.

12. Which of the following statistical indicator is used to evaluate any type of model?
a) mean
b) median
c) error
d) root mean square error

Explanation: Root mean square error is a commonly used statistical indicator to evaluate any type of model. It basically describes the mean error between simulated/measured/observed values and the true values.

13. Why do ground-based observers overestimate overcast cloud cover?
a) Clouds appear to fill large area of sky when near horizon
b) Clouds appear to fill large area of sky when far from horizon
c) Clouds appear to fill small area of sky when near horizon
d) Clouds appear to fill small area of sky when far from horizon

Explanation: Ground-based observers tend to overestimate overcast cloud covers. This is because clouds with vertical extents appear to fill a greater area of sky when located near the horizon than when they are overhead.

14. Reduction of error between simulated values and the true values increases accuracy of the empirical model.
a) True
b) False

Explanation: Reduction of error between simulated values and the true values increases accuracy of the empirical model. This is because decrement in error indicates that simulated values are moving closer to true/desired values.

15. Cloud cover estimation is free from error.
a) True
b) False

Explanation: Like any other estimation, cloud cover estimation is not free from errors. For instance, a small hole in the cloud could remain open for a long period of time thereby allowing more sunlight to pass through than estimated.

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