What is Solar Insolation?

Insolation is a measure of the solar radiation energy received on a given surface area in a given time. It is commonly expressed as an the irradiance in watts per square meter (W/m²) at any moment or averaged over time as kilowatt-hours per square meter (kWh/m² ) either per day or over a whole year. The largest radiation values are over the equatorial zone because the Sun's rays are more concentrated, whereas lower values are achieved towards the poles as the solar radiation is projected over a larger area. Outside the Earths atmosphere the solar constant gives an irradiance figure of 1367W/m²

Solar Radiation on the Earths Surface

The atmosphere absorbs some of this radiation. By the time the radiation reaches the Earth's surface surface, this value has decreased to between 800 to 1000 Watts/m² on a good clear day. This decrease is due to a variety of mechanisms, namely

• Reflection back into space by the atmosphere
• Absorption of energy by various molecules in the atmosphere (see the marked dips in the graph below)
• Mie scattering by dust and pollutants in the air
• Rayleigh scattering by interaction with the air molecules

The spectrum outside the atmosphere is often termed AM0 air mass 0. At the equator with the sun directly overhead the reduction in light intensity due to passing this shortest distance through the atmosphere is termed AM1. Over the course of our year the air mass in Ireland would vary from a value of about 1.2 in mid-June to about 4 in mid-December. Thus the reduction or attenuation factor due to the Sun' radiation travelling through the atmosphere and being absorbed and scattered, would give have a value of between 20-40% in June and about 50-85% in December. The Mie scattering component is particularly susceptible to pollution in industrial areas and is primarily responsible for the variations in these total attenuation values. The clean air that sweeps in over Ireland and Britain usually keeps Mie attenuation at the low end of the values given, except around major cities or when air masses stagnate over the country for apreciable times. The graph above gives an impression of the effect of these factors across the wavelengths of radiation in the Sun's spectrum.

One very interesting thing to note on this graph. Our eyes have evolved to take advantage of the most energy rich (i.e. visible) part of the sprectrum, and secondly, the significant amount of energy in the near infrared part of the graph. Good solar panel glazing takes advantage of as much of this energy as possible by letting through Ultraviolet and the Near Infrared parts of the sprectrum. Specifically low iron content in the glass helps the performance.

Historically ordinary window glass was not manufactured to be transparent to the non-visible parts of the spectrum hence has worse performance than solar glass.

As eluded to in the history page, Clear glass has a peculiar property: it easily allows sunshine to pass through, but inhibits thermal radiation from doing the same. Basically the visible part of the spectrum passes through the plate glass. It heats up the solar panel and because even a very hot solar panel is much cooler than the surface of the sun, it tries to emit radiation with a (huge) wavelenght above 3,000nm. This wavelenght is so big that it cannot pass between the atomic spaces of the glass and so is trapped inside.

Solar Radiation values are generally expressed in kWh/m²day. This is the amount of solar energy that strikes a square metre of the earth's surface in a single day. Of course this value is averaged to account for differences in the days' length. There are several units that are used throughout the world.

The conversions based on surface area as follows: 1 kWh/m2/day = 317.1 btu/ft2/day = 3.6MJ/m2/day

The raw energy conversions are: 1kWh = 3412 Btu = 3.6MJ = 859.8kcal

Met Eireann Website.

To convert to more useful units.

1 m2 = 10,000 cm2
3,600,000 Joules in 1 kWh

to give an idea on what this energy can do.
12 kWh is required to heat 200 litres of water from 10°C to 60°C

Or expressed as an average each day.

Is Ireland's insolation level low, moderate or high?

The following scale is a basic guide for insolation levels. Although a value of 5 is not considered very high during the summer months, as an average annual value this is very high. Central Australia one of the hottest sunniest places on earth has an annual average insolation of 5.89 kWh/m² day.

Average annual insolation levels:
Central Australia = 5.89 kWh/m² day - Very High
Dublin, Ireland = 2.52 kWh/m² day – Moderate

Compared to central Europe we have very respectable Solar energy values, particularily in the Spring and Autumn. More importantly our cool summers help reduce panel overheating problems in summer while still giving a very good output.

Efficiency of low inertia systems

Because the radiation in Ireland is often diffuse. Systems that heat up quickly can better take advantage of short burst of sunlight and weak sunshine. By minimising the thermal inertia of an installation, the efficiency and response time improve.

This can be done by using smaller diameter pipework & lower volumes of liquid in the panel and by adjusting the pump flow rate.