ME 430 Fundamentals of Solar Energy Conversion for heating and Cooling Applications

Transcription

1 ME 430 Fundamentals of Solar Energy Conversion for heating and Cooling Applications Lecture (1 of 2) Solar Energy Resource and Availability C. Cruickshank and S. Harrison 2008 The Solar Constant 1

2 Variation in Extraterrestrial Solar Radiation Due to variation in the radiation emitted by the sun (less than 1% variation) Due to variation of the earth-sun distance (up to 3% variation) For engineering purposes, in view of uncertainties and variability of atmospheric transmission, the energy emitted by the sun can be considered to be fixed. Orientation and Tilt Angle Sun-earth geometric relationship: motion of the earth about the sun From Solar Energy Engineering, Jui Sheng Hsieh 2

3 Orientation and Tilt Angle Sun-earth geometric relationship: location of artic and antarctic circles and the tropics From Solar Energy Engineering, Jui Sheng Hsieh Solar Energy Availability in Canada Annual Mean Daily Global Solar Radiation and Variability of Solar Radiation Incident on a Horizontal Surface Source: 3

4 Solar Energy Availability in Canada Annual Mean Daily Global Radiation, Incident on Inclined Surfaces of 90 o and 60 o with a South Orientation Source: Solar Energy Availability The sun s path at different times of the year at central European latitude (London, Berlin) The amount of solar energy available on the earth depends on the geographical latitude and the time of day and year at a given location. Because of the inclination of the earth s axis, the sun reaches high solar altitudes in the summer than in the winter. 4

5 Solar Energy Availability From Solar Energy Engineering, Jui Sheng Hsieh Solar Energy Availability Daily courses and daily totals for irradiation in London 5

6 Solar Time Solar time Local Clock Time Solar time is used in all of the sun-angle relationships. It is based on the apparent angular motion of the sun across the sky with solar noon the time that the sun crosses the meridian of the observer. Apply 2 corrections: -difference in longitude between observer s meridian and the meridian on which local standard time is based on (4 mins to transverse 1 o ) - equation of time account for perturbations in the earth s rotation which affect the time the sun crosses the observer s meridian Solar time is calculated from: Solar Time Solar Time LST = 4(LSM - LON) + ET where: LST is the local standard time LSM is the local standard meridian LON is the local longitude ET is the equation of time given by: ET= 229.2*( cos(b) sin(b) cos(2b) sin(2b)) Where B = (n-1)*(360/365), n = day of the year All equations use degrees not radians! 6

7 Solar Time Example At Madison, WI, what is the solar time corresponding to 10:30 AM central time on February 3? Solar Time LST = 4(LSM - LON) + ET In this case: LST is the local standard time (10:30) LSM is the local standard meridian (90 o W) * LON is the local longitude (89.4 o W) * ET is the equation of time given by: ET= 229.2*( cos(b) sin(b) cos(2b) sin(2b)) Where B = (n-1)*(360/365), n = 34 therefore B = Thus ET = minutes Solar Time 10:30 = 4( ) + (-13.5) = -11 minutes Solar Time = 10:19 * This information would be provided. Solar Radiation Definitions G G = G dir + G dif + G ref Global solar irradiance and its components The radiation from the sun that meets the earth without any change in direction is called direct or beam radiation, G dir. The radiation from the sun after its direction has been changed by scattering in the atmosphere is called diffuse radiation, G dif. The radiation from the sun after it is reflected on the ground is called the ground reflected radiation, G ref. The sum of the beam, diffuse and reflected solar radiation on a surface is called the global solar irradiance, G G. 7

8 Solar Radiation Sun s level at midday within the course of a year in London and Berlin (latitude 52 o N) The air mass factor (AM) is a measure of the length of the path of the sunlight through the earth s atmosphere in terms of one atmosphere thickness. Solar Spectrum Solar irradiance outside atmosphere Direct solar irradiance at sea level Sun spectrum AM 0 in space and AM 1.5 on the earth with a sun elevation of 41.8 o 8

9 Solar Radiation Global solar irradiance and its components with different sky conditions Solar Radiation The average annual global solar irradiance is significantly higher at lower than at higher latitudes. Monthly solar irradiation (kwh/m 2 per day on a horizontal surface) around the world 9

10 Solar Radiation Monthly sum of global solar irradiance (diffuse and direct) for Miami, USA. Orientation and Tilt Angle 10

11 Orientation and Tilt Angle From Solar Energy Engineering, Jui Sheng Hsieh Orientation and Tilt Angle From Solar Energy Engineering, Jui Sheng Hsieh 11

12 Orientation and Tilt Angle The declination angle: n δ = 23.45sin Angles are to be specified in degrees not radians!! Orientation and Tilt Angle The intensity of the beam radiation on a tilted surface or horizontal surface, G s, is equivalent to the direct normal beam radiation, G DN, multiplied by the cosine of the angle of incidence of beam radiation on the surface: G = G cosθ s DN 12

13 Orientation and Tilt Angle Direction of Beam Radiation 13

14 Effects of Receiving Surface Orientation Sun Chart Solar altitude diagram with example silhouettes of objects (for a latitude of about 50 o ) 14

15 Sun Chart 15