☀️Concentrated Solar Power Systems Unit 2 – Solar Radiation & Resource Assessment

Key Concepts

• Solar radiation the electromagnetic energy emitted by the sun that reaches the Earth's surface
• Solar spectrum the distribution of solar radiation across different wavelengths (ultraviolet, visible, and infrared)
• Direct Normal Irradiance (DNI) the amount of solar radiation received per unit area by a surface perpendicular to the sun's rays
• Global Horizontal Irradiance (GHI) the total amount of shortwave radiation received by a surface horizontal to the ground
• Diffuse Horizontal Irradiance (DHI) the amount of radiation received by a surface that has been scattered by molecules and particles in the atmosphere
• Pyrheliometer an instrument used to measure DNI by tracking the sun's position
• Pyranometer an instrument used to measure GHI and DHI
• Solar resource assessment the process of quantifying and characterizing the available solar energy at a specific location

Solar Spectrum and Radiation Basics

• Solar radiation originates from nuclear fusion reactions in the sun's core, which convert hydrogen into helium
• The solar spectrum spans a wide range of wavelengths, with the majority of energy concentrated in the visible and near-infrared regions
  • Ultraviolet (UV) radiation (wavelengths shorter than 400 nm) accounts for ~5% of the total solar energy
  • Visible light (wavelengths between 400 and 700 nm) accounts for ~43% of the total solar energy
  • Infrared (IR) radiation (wavelengths longer than 700 nm) accounts for ~52% of the total solar energy
• The Earth's atmosphere attenuates solar radiation through absorption and scattering by gases (water vapor, ozone, and carbon dioxide), aerosols, and clouds
• The solar constant the average amount of solar radiation received at the top of the Earth's atmosphere (~1,361 W/m²)
• Air mass (AM) the path length of solar radiation through the atmosphere relative to the shortest possible path (AM1.5 is the standard reference for solar cell testing)
• Spectral irradiance the power of electromagnetic radiation per unit area per unit wavelength (W/m²/nm)

Measurement Techniques and Instruments

• Pyrheliometers measure DNI by tracking the sun's position and using a collimated detector with a narrow field of view (5-6°)
  • Thermopile pyrheliometers use a series of thermocouples to measure the temperature difference between a black absorber and a reference
  • Cavity pyrheliometers use a blackbody cavity to absorb solar radiation and measure the resulting temperature increase
• Pyranometers measure GHI and DHI using a hemispherical glass dome to collect radiation from the entire sky
  • Thermopile pyranometers use a series of thermocouples to measure the temperature difference between a black absorber and a reference
  • Silicon pyranometers use a photodiode to convert solar radiation into an electrical current
• Shading devices (shadow bands or shading balls) are used to block direct sunlight when measuring DHI with a pyranometer
• Rotating shadowband radiometers (RSRs) use a rotating band to alternately measure GHI and DHI with a single instrument
• Spectroradiometers measure the spectral distribution of solar radiation using a monochromator and a detector array

Data Analysis and Interpretation

• Quality control procedures are used to identify and remove erroneous or inconsistent data points (sensor drift, shading, or equipment malfunction)
• Temporal aggregation involves averaging or summing solar radiation data over specific time intervals (hourly, daily, or monthly)
• Spatial interpolation techniques (kriging or inverse distance weighting) estimate solar radiation values at unsampled locations based on nearby measurements
• Decomposition models (DISC or DIRINT) estimate DNI from GHI measurements when direct DNI data is not available
• Transposition models (Perez or Hay-Davies) convert horizontal irradiance (GHI) to irradiance on tilted surfaces (GTI)
• Long-term averages and interannual variability are used to characterize the solar resource and assess the risk of energy production shortfalls
• Typical Meteorological Year (TMY) datasets represent the typical solar radiation and weather conditions for a specific location based on historical data

Solar Resource Mapping

• Satellite-based methods estimate solar radiation by analyzing images from geostationary satellites (Meteosat or GOES)
  • Heliosat algorithm derives surface solar radiation from satellite images by comparing the observed cloud albedo to a reference clear-sky albedo
  • Physical models (SUNY or BRASIL-SR) simulate the radiative transfer of solar energy through the atmosphere based on satellite-derived atmospheric parameters
• Ground-based measurements from weather stations or dedicated solar monitoring networks are used to validate and calibrate satellite-derived estimates
• Geographical Information Systems (GIS) are used to integrate solar radiation data with other geospatial data layers (terrain, land use, or grid infrastructure)
• Solar resource maps provide a visual representation of the spatial distribution of solar energy potential (DNI, GHI, or GTI) over a region
• Bankable datasets (TMY or P50/P90) are used by project developers and investors to assess the long-term economic viability of solar energy projects

Forecasting and Modeling

• Numerical Weather Prediction (NWP) models simulate the dynamics of the atmosphere to forecast solar radiation and other meteorological variables
  • Global models (GFS or ECMWF) provide coarse-resolution forecasts for the entire Earth
  • Mesoscale models (WRF or NAM) provide higher-resolution forecasts for specific regions by downscaling global model outputs
• Cloud motion vectors derived from satellite images are used to forecast short-term (0-6 hours) changes in cloud cover and solar radiation
• Statistical methods (regression or machine learning) are used to post-process NWP model outputs and improve the accuracy of solar radiation forecasts
• Ensemble forecasting combines multiple model runs with slightly different initial conditions to quantify the uncertainty of solar radiation predictions
• Probabilistic forecasts express the likelihood of different solar radiation levels occurring within a given time frame (P10, P50, or P90)

Impact on CSP System Design

• The available DNI determines the optimal size and configuration of the solar field (heliostat layout or trough orientation) to maximize energy capture
• The solar multiple the ratio of the solar field aperture area to the power block capacity is optimized based on the local DNI resource and storage capacity
• Thermal energy storage (molten salt or phase change materials) allows CSP plants to generate electricity during periods of low or no sunlight
• The capacity factor the ratio of the actual energy output to the theoretical maximum output is influenced by the DNI resource, storage capacity, and power block efficiency
• Site selection for CSP plants considers the DNI resource, land availability, water access, grid connectivity, and environmental impacts
• Performance modeling tools (SAM or Greenius) simulate the energy output and economic performance of CSP plants based on the local solar resource and system design parameters

Challenges and Future Developments

• Improving the accuracy and spatial resolution of solar resource assessment methods, particularly in regions with complex terrain or high aerosol loading
• Developing low-cost, high-accuracy instrumentation for measuring DNI and spectral irradiance at remote sites
• Integrating solar radiation measurements with other meteorological data (temperature, humidity, or wind speed) to improve the accuracy of CSP performance models
• Optimizing CSP plant design and operation strategies based on advanced solar resource forecasting and machine learning techniques
• Reducing the cost and increasing the efficiency of CSP components (heliostats, receivers, or power blocks) to improve the economic competitiveness of CSP technology
• Developing hybrid CSP-PV or CSP-fossil fuel systems to provide dispatchable renewable energy and support grid stability
• Exploring advanced CSP technologies (supercritical CO2 power cycles or particle receivers) to achieve higher operating temperatures and efficiencies