Lecture 2 – Sun and solar radiation Photovoltaics

Photovoltaics

Lecture 2 – Sun and solar radiation Photonics - Spring 2020

dr inż. Aleksander Urbaniak

1. FUNDAMENTALS

The Sun

distance

1.5*10¹¹ m = 1 AU

mass:

≈2*10³⁰ kg (333 M_Earth)

radius 109 R_Earth

composition H: 73.46%

He: 24.85%

O: 0.77%

C: 0.29%

Fe: 0.16%

other: 0.37%

surface temperature 5778 K

source: Wikipedia

Total power reaching the atmosphere: 174 000 TW

Power reaching the land surface:

35 300 TW

Human annual power use

≈ 50 TW

Solar Resource

1 TW ≈ 8*10¹² kWh/year 1 kWh = 3.6 MJ

Power reaching the land surface:

720 HEC

Human Energy Consumption unit

= 1 HEC

Total power reaching the atmosphere: 3400 HEC

Solar Resource

average mid to late XX

century

Black Body Radiation

Total power density

(Stefan-Boltzman’s law):

T

H = 

4 2 8 2

3 4 5

10 67 . 15 5

2

K m

W c

h

k





= 





u(  ,T) = 2  hc



[e

hc

kT

−1]

Planck distribution:

Wien’s law

] 2900 [

T m

peak



 

Solar radiation in space

Solar radiation intensity:

2 4 59.6

m T MW

H_Sun = 

HSun

D H R₂

2 0 =

source: pvedurom.org

Solar radiation outside the atmosphere

R

=6.96*10

km R

=6.35*10

km

D=1.5*10⁸ km ± 1.7 %

1353 2

m H_const = W

( )



 



 −

+

= 365

2 cos 360

033 . 0

1 n

H H

const

n – day of the year

source: G. D. Rai, “Solar Energy Utilisation”, Khanna Publishers, 1980, p. 44.

Solar constant:

While the solar radiation incident on the Earth's atmosphere is relatively constant, the radiation at the Earth's surface varies widely due to:

• atmospheric effects, including absorption and scattering;

• local variations in the atmosphere, such as water vapour, clouds, and pollution;

• latitude of the location

• the season of the year and the time of day.

Solar radiation at the Earth surface

source: nasa.gov

Atmospheric effects

source: MIT OpenCourseWare

Incoming Solar Radiation -Insolation

100 = 45 +25 +25 +5

45 = 29 + 104 - 88

70 = 45 +25

25

45

Atmospheric effects

source: Wikipedia

AM1.5 Global: Used for testing of Flat Panels (Integrated power intensity: 1000 W/m²) AM1.5 Direct: Used for testing of concentrators (900 W/m²)

AM0: Outer space (1366 W/m²)

Solar spectra

The above charts, in Excel files: http://www.pveducation.org/pvcdrom/appendicies/standard-solar-spectra

Air Mass

The Air Mass is the path length which light takes through the atmosphere normalized to the shortest possible path length. The Air Mass quantifies the reduction in the power of light as it passes through the atmosphere and is absorbed by air and dust.

( ) ^

cos

= 1

AM

• valid up to around 75^o

AM0: Just above atmosphere (space applications)

AM1: Sun directly overhead AM1.5G: “Conventional”

G (Global): Scattered and direct sunlight D (Direct): Direct sunlight only

AM2.5: Northern Europe

( )

(

)

cos

1

− −

= +





• close to the horizon:

F. Kasten and Young, A. T., “Revised optical air mass tables and approximation formula”, Applied Optics, vol. 28, pp. 4735–4738, 1989

Direct, diffused and scattered light

direct

diffused

scattered

Direct, diffused and scattered light

Rayleigh scattering

The scattering from molecules and very tiny particles (< 0.1 wavelength) is predominantly Rayleigh scattering. The strong wavelenght dependance of Rayleigh scattering enchances blue wavelengths giving us blue sky.

Red wavelenghts are larger than air particles making red light mostly diffuse

( ( ) ^ )







2 4

2 4

0

8 1 + cos

= I R

I

Rayleigh and Mie Scattering

For particle sizes larger than a wavelength, Mie scattering predominates. This scattering produces a pattern like an antenna lobe, with a sharper and more intense forward lobe for larger particles.

Mie scattering is weakly wavelength dependent and produces the white glare around the sun when a lot of particles is in the air. It also gives us the white light from mist and fog.

source: hyperphysics.com

Motion of the Sun

source: pvedurom.org

Solar radiation on a tilted surface









 +

−

=

+

= 90

)

module

S

sin(

S

 – latitude

 – declination angle

( )

 +

−

= ^o 10 day

365 cos 360 45

.

 23

source: pvedurom.org

https://www.pveducation.org/pvcdrom/properties-of-sunlight/solar-radiation-on-a-tilted-surface

Insolation

Solar radiance

• instantaneous power density [kW/m

] Insolation

• incoming solar radiation,

• per area per time unit [kW/m

/day]

Solar radiation can be given in several ways including:

• Typical Mean Year data for a particular location

• Average daily, monthly or yearly solar insolation for a given location

• Global isoflux contours either for a full year, a quarter year or a particular month

• Sunshine hours data

• Solar Insolation Based on Satellite Cloud-Cover Data Calculations of Solar Radiation

Calculation of solar insolation

https://www.pveducation.org/pvcdrom/properties-of-sunlight/solar-radiation-on-a-tilted-surface

January 1 February 27 June 20

Measuring the solar radiation

PYRANOMETERS

• thermopile

temperature difference between black and white areas

• photodiode / photovoltaics pyranometer

Insolation

https://neo.sci.gsfc.nasa.gov/

Peak Sun hours

The average daily solar insolation can be refered as "peak sun hours". It refers to the solar insolation which a particular location receive if the sun were shining at its maximum value for a given number of hours. The peak solar radiation is 1 kW/m², so the number of peak sun hours is identical to the average daily solar insolation. For example, a location that receives 5 kWh/m² per day can be said to have received 5 hours of sun per day at 1 kW/m²

source: pvedurom.org

Seasonal variations of insolation

https://www.pveducation.org/pvcdrom/properties-of-sunlight/calculation-of-solar-insolation

0 degress 30 degress 60 degress

The trajectory of the sun relative to a fixed ground position is important when mounting a fixed solar array.

• Local weather patterns may limit exposure of sun at certain times of day.

• When do you want more power? Summer vs. winter?

• Not only does the length of the day change, but so does the position of the sun in the sky throughout the seasons.

• Important when considering shading effects!

Seasonal and diurnal variations of insolation

http://astro.unl.edu/naap/motion3/animations/sunmotions.html

Fixed Vs tracking systems

Fixed mount

Mechanical Advantages: Simple to manufacture, lower installation and maintenance costs.

Wind-loading: all mounts other than fixed flush-mounted panels must be carefully designed having regard to wind loading due to greater exposure.

Indirect light: approximately 10% of the incident solar radiation is diffuse light, available at any angle.

Tolerance to misalignment: effective

collection area for a flat-panel is relatively insensitive to quite high levels of

misalignment with the Sun

source: wikipedia

source: https://solargis.info

Tracking systems

200-kilowatt CPV modules on dual axis tracker in Qingdao, China

4MW horizontal single axis tracker in Vellakoil, Tamil Nadu, India

Weather – long time

source: http://neo.sci.gsfc.nasa.gov blue – no clouds

white – totally cloudly January 2016

Weather – short time

source: https://pl.sat24.com/pl/pl

Weather

1. Short time constant (less predictable): Cloud cover. Relevant to predicting power supply

reliability.

2. Long time constants (more predictable):

Diurnal & seasonal variations. Relevant to

calculating total annual energy output.

Not only the weather

source: https://solargis.info

source: Wikipedia