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Solar: future energy
13 November 2015

National Meteorological Services around the world use complicated models, which consist of millions of lines of computer code, to predict the weather from hours to days or weeks ahead. As well as predicting everyday weather parameters such as temperatures, winds, rainfall and cloud cover, the computer models can also be used to predict the amount of energy from the sun (solar energy) that reaches any given location.

Within the European Union, at least 20% of each country’s total energy consumption must come from renewable sources by the year 2020, increasing to at least 27% by 2030. Solar energy is currently one of the least expensive forms of clean energy. With the growing interest in and use of solar power comes the need for reliable solar radiation or energy forecasts.

Solar energy forecasting is complicated because it depends on getting a lot of different things correct including each of the following:

1. The cloud cover
2. The amount of liquid water and ice in the clouds
3. The size of the water droplets and ice crystals
4. The shape of the ice crystals (e.g. hexagonal, spherical)
5. Optical properties of the clouds – how they interact with the solar radiation (e.g. How opaque are they?  How much of the Sun's radiation do they scatter?)
6. Aerosols in the atmosphere (e.g. sea salt, soot, desert dust, volcanic ash)
7. Gases in the atmosphere (Carbon Dioxide, Oxygen, Methane, Nitrogen Oxide)
8. The reflectivity of the surface (also known as the surface albedo e.g. snow has an albedo of ~85% and is a reason why skiers get sunburnt)
9. The approximations made in the model's mathematical equations that describe radiation transfer through clouds and aerosols (i.e. how much they transmit and reflect the Sun's radiation).

Errors in any one of the above can lead to errors in the amount of solar energy forecast by the model. Solar forecasting is therefore complicated. Met Éireann is involved in developing the solar energy science in the ALADIN-HIRLAM system in collaboration with Denmark, Finland, Estonia, the Czech Republic and the Netherlands.

Figure 1 shows the daily average solar radiation over Europe for each month of the year, calculated using climate reanalysis data from ECMWF, the European Centre for Medium Range Weather Forecasts. For the months of April through to August, the average amount of solar radiation for Ireland is quite similar to much of western and central Europe.

Figure 1. Surface global (i.e. direct and diffuse) downward shortwave (solar) radiation. Monthly means of daily mean shortwave radiation are shown.

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