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Attenuation of the Solar Radiation by Aerosols


Aerosols Attenuating the Solar Radiation Collected by Tower STE Plants: the Slant Path in the Surface Layer

Aerosols attenuate the solar radiation collected by tower Solar Thermal Energy plants (STE), along two pathways: 1) the atmospheric column pathway, between the top of the atmosphere and the heliostats, resulting in Direct Normal Irradiance (DNI) changes; 2) the slant path close to surface level, between the heliostats and the optical receiver (Fig. 1). The attenuation along the surface-level slant path can be significant in tower STEs as aerosols mostly lay within the surface atmospheric layer, called the boundary layer, and the attenuation increases with the distance between the heliostats and the tower. In tower STEs of 100 MW or more, the distance between the heliostats can be larger than 1 km.



Figure 1. The two pathways of the collected solar radiation in a tower STE, with the boundary layer height (BLH) which can approximate the Aerosol Layer Height, the distance between the heliostat and the receiver, DHR, and the solar zenith (SZA) and the slant path (SPA) angles.


Accurate Slant Path Attenuation Estimated by a Ground-Based Instrument

We used measured aerosol optical thickness (AOT) and computed boundary layer height (BLH) to estimate the attenuation of the solar radiation in the slant path at Ouarzazate (Morocco). High variabilities in both the aerosol amount and the vertical layering generated a significant magnitude in the annual cycle and also significant inter-annual changes. Indeed the annual mean of the attenuation at 550 nm caused by aerosols over a 1-km heliostat-receiver distance was 3.7% in 2013, and 5.4% in 2014 because of a longest desert dust season. Also, the monthly minimum attenuation of less than 3% is usually observed in winter while the maximum of more than 7% is usually observed in summer (Fig. 2).  The method and first resultas were presented at the 2015 Solar Paces Conference at Cape Town, RSA.

AOT is measured by the ground-based radiometers of the AERONET network, and BLH is delivered by the ECMWF operational analysis. AOT was smaller than 0.1 in November-February 2012-2014 while it was larger than 0.2 in June-August and even larger than 0.3 in August 2012 and 2014 and in May-June 2006.



Figure 2. Attenuation at surface level in Ouarzazate, at 4 wavelengths and for a distance of 1 km, caused by aerosols. Monthly averages were computed for 2012/02-2012/12.


Maps of the Slant Path Attenuation Estimated by Satellite Instruments

AOT is delivered by AERONET but also by instruments onboard satellite platforms.  The same method allows to estimate of the slant path attenuation at a regional scale and also its spatial variability, by analysing the data set provided by MODIS.  The solar broadband transmittance, Tsurf, (= 1 -attenuation) depends on both AOT and BLH.  On 16 August 2012, Tsurf was little varying from 0.75 to 0.85 inland and strongly decreased along the coasts because of decreasing BLH (Fig. 3).  Tsurf is larger on the Iberian Peninsula because AOT was as small as on the coast but BLH larger.  The results were presented at the 2016 Solar Paces Conference, at Abu Dhabi, UAE.



Figure 3. Solar resource parameters computed with MODIS AOT (Deep Blue algorithm), MODIS water vapour content, MODIS ozone content, ECMWF ERAinterim BLH, on 16 August 2012 at around noon.  The contribution of aerosols in reducing the DNI accumulated in one day is showed on the left, and Tsurf on the right.  No clouds are considered in the coloured pixels, while white pixels indicate cloud presence.  AERONET stations are also indicated.



At least one year of ground-based measurements of the aerosol extinction coefficient (made by a nephelometer or a visibilimeter) is necessary to perform a thorough validation of the method. Special care must be taken for sites not respecting the hypothesis of a unique and uniform aerosol layer. Indeed we show that the method would over estimate the aerosol attenuation at surface level at Banizoumbou (Niger) in the Sahel region.  We are studying the impact of other sources of data for the aerosol layering.