Atmospheric aerosols affect the generation from solar tower plants like the 100 MW Tower CSP project, Jinta ZhongGuang was originally planned by the largest renewable energy firm in the world, Three Gorges Renewables, that built the 22 GW Three Gorges Dam. Cosin Solar bought the project from them after they failed to meet a milestone, and is now constructing it.
Atmospheric attenuation is projected to increase by varying amounts – depending on how fast we transition from fossil fuels and that impact on the content of atmospheric aerosols, according to the IPCC sixth assessment report (AR6) which features new state-of-the-art CMIP6 models.
This projected increase in dust will affect the optical performance of central tower concentrated solar power plants, especially for large plants where the heliostat-receiver distance may be greater than a kilometer, as atmospheric attenuation intensifies with aerosol concentration and the light’s path through the atmosphere.
The turbidity in the air over the solar field would weaken the light intensity that hits the receiver. But how much, and where and by when?
Jesus Polo and his team developed a model to calculate the trends in atmospheric attenuation expected worldwide from now to 2060.
The estimation is based on a model developed by the team and evaluated with six years of atmospheric attenuation monitored in southeast Spain, from June 2017 to July 2023, at the Plataforma Solar de Almería (PSA) the biggest experimental facility for testing CSP systems on-sun. Their latest results Solar tower power generation under future attenuation and climate scenarios are published in the January 2025 issue of Renewable and Sustainable Energy Reviews:
The study is important because CSP must perform projected generation over 25- to 35-year contracts, and developers must accurately estimate expected annual generation over the contract duration. Any projects with contracts signed in 2025 will still be generating energy in 2050, and some for longer. The most recent DEWA CSP tower project came online in 2024 and has a 35-year contract.
Developers would lose money under these contracts if their calculations were off, so it is crucial to know future changes affecting generation over time.
Build on CMIP6 atmospheric attenuation models
Polo told SolarPACES in a call from Spain that in the study researchers from University of New South Wales collaborated by supplying the aerosol retrievals from the 6th Coupled Model Intercomparison Projects (CMIP) The Spanish team incorporated that information in their attenuation model for different scenarios. These scenarios run until 2060.
“Many believe atmospheric attenuation is probably one of the most influencing factor in the optical efficiency of a central tower solar field,” Polo noted.
“So in those places where the negative impact on the solar field is expected to be significant, it is quite important to have reliable and good knowledge of atmospheric attenuation as a key parameter to know the potential impact in the plant performance.”
Atmospheric attenuation increase and its impact on future solar tower CSP plants; IMAGE © Solar tower power generation under future attenuation and climate scenarios In the maps, white dots represent locations of existing solar tower plants (50 MW or higher)
Polo et al. developed the theoretical model in 2016 and tested it outdoors at PSA’s testing facility, which revealed that some changes were needed.
“So this is now the second version of the model,” he noted. “We saw some deviations when we evaluated the 2016 model with experimental data at PSA in 2020. So now we correct our model using the experimental data to consider this fact.”
However, the results are site-specific to southeastern Spain. Atmospheric attenuation varies by location, and regions with higher current levels are expected to experience a more pronounced increase in the future.
“There’s uncertainty in different climatic conditions associated to the uncertainty of climate models predicting atmospheric aerosols distributions,’ he explained.
“The model is validated at PSA, in Almeria with the characteristic range of aerosol loading in southeast Spain, and this could be extrapolated to many similar sites. However, the assessment of the model for sites with much larger values of the aerosol optical depth (Middle-East for instance) is still pending. Polo expects the aerosol load to be quite a bit higher in the Middle East or India.
How the Polo model works
The model uses two main inputs: the distance between the heliostat and receiver (S) and the Aerosol Optical Depth (AOD). The model calculates atmospheric attenuation as a percentage using a complex mathematical formula. This formula considers the distance between the heliostat and receiver, measured in kilometers. It also uses four coefficients determined by the Aerosol Optical Depth (AOD).
These four coefficients are not fixed; they change based on the AOD value. Each coefficient is calculated using its own specific formula that depends on the AOD.
“Instead of the instant extinction coefficient, we use the Aerosol Optical Depth,” he explained.
“AOD is a key parameter regarding the suitability of the extinction coefficient because the higher the extinction coefficient, the higher the energy attenuation, and vice versa, and this extinction coefficient is a parameter that is related to the high degree of turbidity of particles in the boundary layer of the solar field. AOD is well estimated worldwide by satellite and meteorological models. This model is very powerful because we have this input information everywhere in the world and it allows to model the attenuation dynamics into the CSP modeling tools.”
Finally, the model applies a correction factor to fine-tune the result. This correction factor is calculated in two ways, depending on whether the AOD is higher or lower than a specific threshold (0.05). Combining all these elements – the distance, the four coefficients based on AOD, and the correction factor – the model estimates the atmospheric attenuation percentage.
This model approximates the physical behavior of atmospheric attenuation, which follows an exponential law based on the extinction coefficient at the boundary layer.
Full paper:
Solar tower power generation under future attenuation and climate scenarios
