Long term trends of solar radiation

1. ETH
Martin Wild
ETH Zürich
martin.wild@env.ethz.ch
6th PV Performance Modeling and Monitoring Workshop, October 24, Freiburg
Long term trends of solar radiation

2. ETH
Content
• How stable is the amout of solar radiation received at
the Earth surface over the years?
• How did solar radiation vary over past decades?
• What are the reasons for the decadal variations?
• How will solar radiation change over future decades?
• How will PV performance be affected by changes in
solar radiation and climate?

3. ETH
Units Wm-2
Greenhouse gases
Solar Radiation in the Global Climate System
Wild et al. 2013 Climate Dynamics / IPCC AR5

4. ETH
Units Wm-2
Greenhouse gases
Sun as ultimative energy source
Wild et al. 2013 Climate Dynamics / IPCC AR5
Solar Radiation in the Global Climate System

5. ETH
Clouds
Water Vapour
Aerosols
atmospheric attenuation
Wild et al. 2013 Climate Dynamics / IPCC AR5Units Wm-2
Solar Radiation in the Global Climate System

6. ETH
Clouds
Water Vapour
Aerosols
atmospheric attenuation
Units Wm-2
Solar Radiation in the Global Climate System
Available solar energy at Earth surface

7. ETH
• Worldwide measurements of energy fluxes at
the surface (2500 sites)
• Solar radiation data at many sites since 1950s,
some back to 1930s
• Database at ETH Zurich
• WCRP initiative, starting in 1992
• Highest measurement quality at selected sites
worldwide (currently 51 anchor sites)
• Minute values
• Ancillary data for radiation interpretationOhmura et al. 1998
Observations of Surface Solar Radiation
Ohmura, Gilgen, Wild 1989
GEBA stations
BSRN stations
BSRN site Payerne

8. ETH
“dimming”
Potsdam, Germany 1937 – 2014
Insolationanomalies(Wm-2)
Decadal changes in surface solar radiation
Surfacesolarradiation(Wm-2)
Wild et al. 2005, Science
Wild 2016, WIREs Clim Change

9. ETH
“dimming”
Potsdam, Germany 1937 – 2014
Insolationanomalies(Wm-2)
Decadal changes in surface solar radiation
Surfacesolarradiation(Wm-2)
“dimming”
Wild et al. 2005, Science
Wild 2016, WIREs Clim Change

10. ETH
“dimming”
Potsdam, Germany 1937 – 2014
Insolationanomalies(Wm-2)
Decadal changes in surface solar radiation
Surfacesolarradiation(Wm-2)
Wild et al. 2005, Science
Wild 2016, WIREs Clim Change
“dimming” “brightening”

11. ETH
Numbers: literature values for changes in Wm-2/decade
Decadal changes in surface solar radiation
Wild, M. 2012: Enlightening Global Dimming and Brightening. Bull. Amer. Meteor. Soc., 93
“dimming” “brightening”

12. ETH
Satellite-derived products show brightening
since mid-1980s
Hatzianastassiou et al., ACP
surface solar radiation determined from satellites
Decadal changes in surface solar radiation

13. ETH
What causes dimming and brightening?
Sun cannot explain dimming /
brightening
Variations in solar output more than an
order of magnitude smaller than dimming/
brightening and uncorrelated
Variations in solar output
measured from satellites
Sunspot numbers
± 1 Wm-2 (< ±0.1%)

14. ETH
Wild et al, 2005, Science
“dimming” “brightening”
Transmission through the
cloud-free atmosphere 1950-2000
Global Anthropogenic
Sulfur Emissions 1950-2000
Data source: Stern,
2005
“dimming” “brightening”
What causes dimming and brightening?
Variations in atmospheric transmission in line
with variations in anthropogenic emissions

15. ETH
Surface solar radiation in Italy 1959-2013
under cloud free and cloudy conditions
Similar variations under clear and all weather conditions
Manara et al. 2016
based on 29 radiation sites
Manara V., Brunetti M., Celozzi A., Maugeri M., Sanchez-Lorenzo A., Wild, M., 2016: Detection of dimming/brightening
in Italy from homogenized all-sky and clear-sky surface solar radiation records (1959–2013), Atmos. Chem. Phys 16
What causes dimming and brightening?

16. ETH
Can aerosol explain the solar radiation changes?
=> sensitivity studies with climate model ECHAM5-HAM with sophisticated
treatment of aerosol and cloud microphysics
Europe, 1950-2000
constant aerosol
emissions
variable aerosol
emissions
Folini and Wild 2011 / 2015 JGR
surface solar radiation simulated in a climate model
Transient simulations with constant and historic (variable) aerosol emissions
Modelling studies of dimming/brightening

17. ETH
Aerosol from air pollution reduce the amount of
solar radiation reaching the ground through direct
and indirect effects (cloud albedo/cloud lifetime)
Direct and indirect aerosol effects
What causes dimming and brightening?

18. ETH
Total radiation= Direct + DiffuseDimming total radiation
diffuse fraction
Brightening total radiation
diffuse fraction
Dimming and brightening affects quantity & quality of radiation
Abakumova et. al. 1996
Impact on diffuse / direct solar radiation
Total solar
radiation
Direct solar
radiation
Diffuse solar
radiation
Solar radiation during dimming phase
in Odessa 1960-1986
More pollution leads to
less direct and more
diffuse solar radiation

19. ETH
diffuse/direct radiation at German sites since 1990
Global (“total”) horizontal
+1.6 % per decade
Diffuse horizontal
-3.9 % per decade
Direct horizontal
+7.6 % per decade
Müller et al. 2014
During brightening with less pollution, total (“global”) radiation increases
due to more direct radiation, while diffuse decreases
Müller, Wild, Driesse, Behrens, 2014: Rethinking solar
resource assessments in the context of global dimming and
brightening, Solar Energy, 99, 272-282.

20. ETH
Brightening and effect of panel orientation
Global tilted 30° south
+2.1% per decade
Direct normal (tracked plane)
+8.5% per decade
Global normal (tracked plane)
+3.7% per decade
Global (“total”) horizontal
+1.6 % per decade
Brightening enhances with panel exposed
towards Sun, since contribution from direct
radiation increases
Müller et al. 2014

21. ETH
Source: Berner Fachhochschule Burgdorf
Insolation on horizontal and tilted (45°) panels 1992-2011
Measured at Burgdorf (Switzerland)
Brightening increased solar resources by 8 % on horizontal
and 13 % on southward tilted (45°) panels from 1992 to 2011
Tilted 45° South
Horizontal plane
Brightening and effect of panel orientation

22. ETH
Implications for solar resource assessments
In resource assessments, instead of using the longest possible
observation period as the estimator for future solar irradiance,
only the 10 most recent years should be used
Different possibilities to estimate future solar resources
Müller et al. 2014
Müller, Wild, Driesse, Behrens, 2014: Rethinking solar resource assessments in the context of global
dimming and brightening, Solar Energy, 99, 272-282.

23. ETH
Projections of future changes in solar resources
Projections by
40 climate models
used in the latest
IPCC report
Consistency in solar radiation changes till 2050
consistent
increase
consistent
decrease
How consistent are solar radiation changes projected
for coming decades by various climate models?
Wild et al. 2015, Solar Energy

24. ETH
Implication for future PV output
Wild et al. 2015, Solar Energy
Scenario RCP 8.5, up to 2050
Changes in solar radiation and temperature taken into account
according to Crook et al. (2011)
Fractional changes in PV output

25. ETH
Fractional change in PV output over Europe
Winter Spring
Summer
Autumn
Scenario RCP 8.5 to 2050, based on 40 climate models (CMIP5)
Implication for future PV output
Wild et al. 2015, Solar Energy

26. ETH
Summary
• Solar radiation at Earth’s surface is not stable over the years, but
undergoes substantial changes on decadal timescales
• Observed changes show widespread decline from the 1950s to 1980s
(“global dimming”), followed by a partial recovery (“brightening”)
• Caused by changes in the transparency of the atmosphere, not by the
Sun itself. Air pollution important contributor
• Changes measured on horizontal planes are enhanced when planes
are oriented towards the sun
• Changes have to be taken into account in solar resource assessments
• Substantial impact on climate and environmental change, and solar
power production
• Global climate model projections of solar radiation and temperature
changes up to 2050 suggest some increase in PV performance in
Europe (summer/autumn) , SE USA and SE Asia, while a decrease
particularly in higher latitudes.

27. ETH
Selected related Publications
Wild, M. et al. 2015. Projections of long-term changes in solar radiation
based on CMIP5 climate models and their influence on energy yields
of photovoltaic systems. Solar Energy 116,12–24
Müller, B., Wild, M., Driesse, A., Behrens, K., 2014. Rethinking solar
resource assessments in the context of global dimming and
brightening. Solar Energy 99, 272–282.
Wild, M., 2012. Enlightening global dimming and brightening. Bull. Am.
Meteorol. Soc. 93 (1), 27–37.
Wild, M., 2009. Global dimming and brightening: a review. J. Geophys.
Res. – Atmos. 114, D00d16.
Wild, M. et al., 2005. From dimming to brightening: decadal changes in
solar radiation at Earth’s surface. Science 308 (5723), 847–850.