Reasons for Monitoring Solar Radiation Data

There are many reasons that prompt a country to monitor solar radiation and weather data in general. First and foremost, as in all countries around the world, it is a norm that such data is collected to help identify any changes in the climate on local and global levels. This data is collected and stored automatically in an electronic medium, so that it can be easily accessed. The availability of data in one country could also be used to a certain extent, to predict possible weather trends in neighbouring countries, thus increasing mutual collaboration between nations.

Solar radiation and weather data are essential for sizing solar energy applications such as solar water heaters and solar electric systems. Without the availability of local data, one can only make estimates which would result in an undersized or oversized system. This would clearly affect the output of the system and in the case of undersizing, the result would cause bad reputation to solar systems on a large scale.

Solar Radiation: Its Nature and Measurement:

Solar radiation is a mixture of electromagnetic radiation of different wavelengths that have different percentage composition. The most energetic part is termed the ultra-violet region and constitutes about 9% of the total solar radiation on a sunny day. The visible light region, which is made up of the 7 colours of a rainbow, has a 45% share, while the infra-red region takes up the remaining 46%.

The total radiation reaching an object on the surface of the earth can be divided into 2 parts. First, the direct radiation, which is that energy coming directly from the sun’s disc and its immediate surrounding. Second, the indirect or diffuse radiation, which is the amount of radiation reaching

 the object from the surrounding or reflected by clouds. The diffuse component is about 20% of the total solar radiation on a sunny day and could reach up to 100% on a cloudy day.

Between 1957 and 1972, solar radiation was being measured in Malta by the Meteorological Office, during the British Rule. A solar radiation instrument, known as a pyranometer, was situated within the perimeter of Qrendi airstrip and hourly data readings of solar radiation falling on a horizontal surface, were collected. The data was then processed to yield average monthly values. Based on this data, the mean annual solar radiation falling on a 1x1 m² horizontal surface is 5 kWh/day.

During the years 1972 to 1982 the spectral distribution of solar radiation, i.e. the percentage wavelength compositions, were measured at the Malta College of Arts, Science and Technology and a scientific paper was published.

Present Situation in Malta

In June 1993, the Institute for Energy Technology of the University of Malta took the initiative to re-launch a solar radiation monitoring programme in June 1993. A weather station and 2 pyranometers were placed on the roof of the Chemistry building at the University of Malta, to measure weather parameters as well as the total radiation on the horizontal plane and on a plane inclined at an angle of 36° (equal to the latitude of Malta) to the horizontal. At this inclination, a surface would absorb the maximum annual amount of solar energy.

In June 1995, the instruments were moved to the Institute’s premises at Marsaxlokk and upgraded. At the moment, solar radiation, air temperature, humidity, wind speed and direction and rainfall, are being monitored. Data is taken every 10 seconds and averaged for 10 minute intervals according to the international standards of the World Meteorological Organisation. The data is checked for any systematic errors and automatically stored for future use. This brings Malta up to date with other countries regarding monitoring of weather parameters for scientific purposes.

Total solar radiation is being measured both on the horizontal plane and on an angle of 36° to the horizontal. The former is used as a standard mode of monitoring solar radiation around the world, while the latter gives information that could be used to size solar systems that are placed at that inclination. Diffuse solar radiation is also measured on the horizontal surface and this could be an indicator for the long-term change in cloudiness and turbidity in the atmosphere.

The Institute adopts a daily close supervision programme on all instruments and hardware, to ensure the highest standard of data quality. It includes checking of power and back-up power supplies, cleanliness of instruments, protection against humidity, collection of data, adjustments and recalibration of instruments.

The data has already been used in several scientific papers that were accepted by international bodies and this shows that our monitoring is up to date. The Institute can supply detailed weather and solar radiation data as well as wind data. Services and consultancy in the fields of energy conservation, applications of solar thermal, solar photovoltaic systems and wind energy applications, are offered.

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