Long term trends of solar radiation

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Martin Wild ETH Zürich

[email protected]

6th PV Performance Modeling and Monitoring Workshop, October 24, Freiburg

Long term trends of solar radiation

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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?

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Units Wm-2

Greenhouse gases

Solar Radiation in the Global Climate System

Wild et al. 2013 Climate Dynamics / IPCC AR5

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Units Wm-2

Greenhouse gases

Sun as ultimative energy source

Wild et al. 2013 Climate Dynamics / IPCC AR5


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Clouds Water Vapour

Aerosols

atmospheric attenuation

Wild et al. 2013 Climate Dynamics / IPCC AR5 Units Wm-2


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Clouds Water Vapour

Aerosols

atmospheric attenuation

Units Wm-2


Available solar energy at Earth surface

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•  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 interpretation

Ohmura et al. 1998

Observations of Surface Solar Radiation

Ohmura, Gilgen, Wild 1989

GEBA stations

BSRN stations

BSRN site Payerne

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“dimming”

Potsdam, Germany 1937 – 2014

Inso

latio

n an

omal

ies

(Wm

-2)

Decadal changes in surface solar radiation

Sur

face

sol

ar ra

diat

ion

(Wm

-2)

Wild et al. 2005, Science Wild 2016, WIREs Clim Change

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“dimming”


Inso

latio

n an

omal

ies

(Wm

-2)


Sur

face

sol

ar ra

diat

ion

(Wm

-2)

“dimming”


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“dimming”


Inso

latio

n an

omal

ies

(Wm

-2)


Sur

face

sol

ar ra

diat

ion

(Wm

-2)


“dimming” “brightening”

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Numbers: literature values for changes in Wm-2/decade


Wild, M. 2012: Enlightening Global Dimming and Brightening. Bull. Amer. Meteor. Soc., 93


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Satellite-derived products show brightening since mid-1980s

Hatzianastassiou et al., ACP

surface solar radiation determined from satellites


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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%)

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Wild et al, 2005, Science


Transmission through the cloud-free atmosphere 1950-2000

Global Anthropogenic Sulfur Emissions 1950-2000

Data source: Stern, 2005



Variations in atmospheric transmission in line with variations in anthropogenic emissions

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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


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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

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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


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Total radiation= Direct + Diffuse Dimming 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

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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.

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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

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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

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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.

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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

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Implication for future PV output


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

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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


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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.

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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.

Technology

Long term trends of solar radiation