Week 7 - Short- and long-wave radiation

Question 1

What does the term ‘circumsolar activity’ mean and what is its significance in the modelling of solar radiation?

Answer 1

Circumsolar activity defines the apparent ring around the sun from which there emanates highly directional diffuse radiation. This requires that the true direct be augmented by the circumsolar component, which is then subtracted from the measured diffuse radiation to give a truly background diffuse irradiance.

Question 2

Differentiate between isotropic and anisotropic sky conditions.

Answer 2

An isotropic sky has uniform brightness (luminance) while an anisotropic sky has non-uniform brightness (luminance), which gives rise to complexity in the calculation of the diffuse component of solar heat gain.

Question 3

Explain with the aid of a sketch how solar radiation finds its way into a building to become a cooling load.

Answer 3

A - reflected shortwave flux

B - flux emission by convection and longwave radiation

C - shortwave flux transmission to cause opaque surface insolation

D - shortwave transmission to cause transparent surface insolation

E - shortwave transmission to adjacent zone

F - enclosure reflections

G - shortwave loss

H - solar energy penetration by transient conduction

I - solar energy absorption prior to retransmission by the processes of B.

Question 4

Identify the most effective shading device based on the numerical values of the contributing components. Explain your choice.

Answer 4

The most effective shading device would be one that blocks the direct component because this is the dominant portion of the total surface irradiance. Although usually not cost effective, this is best done by employing a device that tracks the solar position.

Question 5

Based on theoretical considerations, describe how the direct component of the calculated total irradiance might be reduced without the use of a shading device. Give a specific example.

Answer 5

By increasing the angle of incidence of the direct beam by making appropriate adjustments to the surface azimuth and/or elevation angles, e.g. by reducing the azimuth to <180º or the elevation to <15º.

Question 6

What is a view factor?

Answer 6

A view factor is a measure of the geometric relationship between two surfaces and is used in calculations relating to radiation exchange. A value of 0 indicates zero exchange potential while a value of 1 indicates that all radiation leaving one surface will arrive at the other.

Question 7

How might view factors be determined numerically for an energy system of arbitrary geometric complexity?

Answer 7

Inter-surface view factors may be determined numerically by dividing both surfaces into a number of finite elements and then for each element in surface 1:

place a unit hemisphere at the element centre;
discretise the hemisphere into patches;

for each patch: extend a ray from the centre point of the hemisphere base circle through the patch centroid;
determine the point of intersection (element) with surface 2 (if any) and calculate the point view factor from the two elemental areas and angles;
sum over all patches;
sum over all elements in surface 1.