Solar Geometry Flashcards
What is the difference between daylight and sunlight?
Daylight:
- The diffused natural light from a completely overcast sky.
Sunlight:
- The direct solar radiation received in clear sky conditions.
What are solar gains?
Solar gains are the energy we capture/reject from the sun as and when we need.
What is a sun path diagram?
What is zenith?
A point which is directly overhead (perpendicular to the Earth’s surface).
What is the meridian?
A circle of constant longitude which passes through a given place on Earth’s surface.
What is a great circle?
A great circle divides a sphere into two equal halves.
What is obliquity?
- Obliquity is the tilt of the Earth with respect to the plane of revolution of the Earth around the sun (23.4o).
- The obliquity of the Earth causes the seasons depending on whether the Earth is tilted towards/away from the sun.
What is declination (D)?
- Declination is the angle made by the Sun’s rays at a given time of the year with the Earth’s equatorial plane.
- It varies over the year from ‘- obliquity (-23.4o)’ in winter to ‘+ obliquity (+23.4o)’ in summer. It is 0o at the equinox.
- It is a measure of where you are in the seasons.
What is the equation for declination?
What is hour angle (H)?
The angle that the Earth needs to rotate in order to bring the meridian of a place to its position at noon.
What is the equation for hour angle?
What is latitude (L)?
- Latitude is how far/close you are to the equator, measured using the angle subtended by the place under consideration from the centre of Earth.
- It is important in determining how much solar radiation a place receives.
What is altitude (y)?
The altitude is the angle of the sun above the horizon plane. It is zero at sunrise and sunset, and at its maximum value when the sun is due south (in the Northern hemisphere).
What is azimuth (z)?
- The azimuth is the compass direction of the Sun.
- This is the angle of the Sun measured from South.
- Azimuth is 0o when it is due South (at noon).
Which solar geometric properties apply to Earth as a whole, and which apply to single places on Earth?
Earth as a whole:
- Hour Angle (H)
- Latitude (L)
- Great Circle
- Obliquity
- Declination (D)
Single place on Earth:
- Altitude (y)
- Azimuth (z)
What is the equation for altitude (y)?
What is the equation for azimuth (z)?
What are the solstices/equinox?
- The obliquity is the tilt of the Earth towards/away from the sun effecting the length of a day.
- Summer solstice = Longest day of the year.
- Winter solstice = Shortest day of the year.
- Equinox = Between the two.
What is the declination at equinox?
Zero
What are orthographic projections?
What are stereographic projections?
What are the features of orthographic/stereographic projections?
What are the solar features of the tropics and arctic circle?
- In the tropics, the sun is at zenith at least once in a year.
- On the tropics, the sun is at zenith exactly once in a year.
- In the arctic circle, there will be no sunrise or sunset at least once a year.
- On the arctic circle, there will be no sunrise or sunset exactly once a year.
How do you derive the altitude at noon equation?
- y*noon = 90 - ß
but. .. ß = L - D
so. .. ynoon = 90 - (L - D) - y*noon = 90 - L + D
How do you plot a sunpath diagram?
- Calculate the Azimuth at sunrise (East) and sunset (West), altitude will be 0o. (Remember, azimuth is measured from South, and so you have to add/subtract the azimuth from 180o)
- Calculate the altitude at noon, azimuth will be 180o.
- Connect the dots in the arc.
- Plot the same for summer solstice, winter solstice and the equinoxes.
- You can then plot hour lines (shown in blue).
What is the azimuth at noon?
The azimuth is due South and so 0o.
What are the features of sunpath diagrams?
- Radial lines indicate the solar azimuth.
- Concentric rings indicate the solar altitude.
- Length of arcs show the length of days.
- If there is a complete arc/circle, the sun never sets.
- If there is no arc, the sun never rises.
- For Southern Hemispheres, the arcs appear to be upside down.
What are the features of sunrise and sunset azimuth?
- They are equal and opposite.
- At the equinox, the sun always rises at exactly due East, and sets at exactly due West.
What is the formula for solar azimuth at sunrise/sunset?
What is the equation for glazing characteristics?
What factors affect glazing characteristics?
- The angle of incidence of radiation causes the reflectance/absorption/transmit range of glazing to vary.
- Therefore, if the sun is lower in the sky (and so perpendicular to the glass pane), there is less reflection, and so more radiation is absorbed/transmitted.
- If the sun is higher in the sky (and so parallel to the glass pane), there is more reflection, and so less radiation is absorbed/transmitted.
NOTE: The angle of incidence is measured between the incident radiation and the normal to the plane (0o is perpendicular to the plane).
What is the role of shading?
Shading is used to:
- Control solar gains by:
Reducing cooling loads in the summer.
Allowing passive heating in the winter.
- Control glare from the sky.
How do you calculate the shading transmittance?
