Solar Irradiance and Irradiation
Solar Irradiance, what is it and what does it mean when dealing with solar photovoltaic systems. We use many different words and meanings such as solar radiation (electromagnetic), solar irradiance (for power), solar irradiation (for energy), as well as solar insolation to describe the amount of sunlight that is available at any particular location. We use the suns energy to generate electricity, by using photovoltaic panels, or use it to heat water with the help of solar thermal panels, so having a good supply of solar energy is important.
Our sun is both a heat source and a light source, giving us the warmth and sunlight we need to survive. The sun is an excellent source of energy and we can harness it in so many different ways, but how do we know if there is enough radiant energy for a solar photovoltaic (PV) panel to generate electricity.
The sun is an excellent source of radiant energy. The amount of solar energy per unit area arriving on a surface at a particular angle is called irradiance which is measured in watts per square metre, W/m2, or kilowatts per square metre, kW/m2 where 1,000 watts equals 1 kilowatt.
However, the direct distance measured between the Earth and the Sun varies annually thereby causing variations in the amount of solar irradiance energy received during the natural cyclic rotation of the Earth over one full year period.
According to NASA, the average irradiance value measured on the edge of space and outside the Earth’s atmosphere on a flat surface positioned perpendicular to the sun is about 1,370 watts per m2 (that is 1.37 kilowatts). This irradiance value given by NASA is called the Solar Constant and is used to determine the solar values down on the Earth’s surface.
But the values of irradiance measured across the surface of Earth are much lower than the solar constant. The scattering and reflecting of the sunlight when passing through the atmosphere decreases the amount of energy which reaches the Earth’s surface due to the climatic conditions at that point.
Solar Irradiance through the Atmosphere
For example, annual conditions such as the time of year, seasonal and temperature variations, cloudy or overcast conditions, as well as the angle at which the sun’s solar rays strike the ground all influence the amount of solar irradiance available at a particular location.
By the time it pass through our atmosphere and reaches the ground at sea level, the maximum solar irradiance across a 1m2 horizontal surface at an equatorial location on a clear day around solar noon is close to 1,000 watts, that is 1000W/m2 (or 1kW/m2). When dealing with solar PV panels, this solar irradiance level of 1kW/m2 is known as a “Full Sun” or commonly “Peak Sun” with the “Peak Sun Hours” (PSH) being the number of hours of time that the solar irradiance received at the surface is at this peak level of 1kW/m2.
However, when the solar radiation is averaged over the entire 24 hour day and night cycle as well as over a whole year of 365 days, even the best locations receive on average per day only 250–300 W/m2. That’s less than 30% of what arrives at the top of the Earth’s atmosphere. So there is a lot of what is called “solar attenuation”, that is the loss of solar irradiance, as it passes through the Earth’s atmosphere before it reaches the Earth’s surface with solar attenuation being greater during the winter months.
We could plot the daily, monthly or even annual amounts of solar irradiance (power) available for any given location giving us a clearer idea of the minumum and maximum levels available for the generation of electrical energy using photovoltaic panels as shown.
Solar Irradiance During the Day
We can see from our daily example, that the solar irradiance available during the brighter sunnier and longer summer days is greater than that of the shorter, duller winter days as we would expect. So the peak sun hours available during the summer is clearly longer than the winter period allowing a PV panel to operate at its peak rated output longer.
So for example, if the average solar energy which falls on a surface during the summer months is 800 W/m2 and is available for a full 8 hours per day, the daily amount of solar irradiance received during the summer months will be:
800 W/m2 x 8 hours = 6400 Wh/m2 or 6.4 kWh/m2
So from above, we can see that if 1 kWh/m2 is equal to one peak sun hour (PSH), then 6.4 kWh/m2 is equal to 6.4 peak sun hours, or 6.4 PSH.
Now if we assume that during the winter months the average solar energy available drops by half, that is to 400 W/m2 and is only available for half as many hours, that is 4 hours compared to the summer months, then the amount of solar irradiance received during the winter months would be:
400 W/m2 x 4 hours = 1600 Wh/m2 or 1.6 kWh/m2 = 1.6 PSH
Then we can see from this very simple example that the amount of solar energy collected during the sunnier summer months is four times greater at 6.4 kWh/m2, than the solar energy collected during the duller winter months at only 1.6 kWh/m2. Again according to NASA, the worldwide daily average value of solar irradiance across the whole planet over one day is approximately equal to 5 kWh/m2 or 5 PSH.
Solar Irradiance Example
Photovoltaic (PV) panels convert solar irradiance into electricity. If we assume we have a single 200 watt photovoltaic panel, how much energy could be potentially produced by the panel per day during the summer and winter months using the peak sun hours values from our example above.
Solar panel output during the summer days:
200 Watts x 6.4 FSH = 1280 Wh/day or 1.28 kWh/day
Solar panel output during the winter days:
200 Watts x 1.6 FSH = 320 Wh/day or 0.32 kWh/day
Thus if we assume we need 1000 watts per day of solar energy to power our home, we could do this during the summer months with just one 200 watt photovoltaic panel, but would require four 200W panels during the winter months. Therefore, a higher solar energy availability (by mean of PSH) will result in a smaller solar PV wattage requirement, while a lower PSH value, will require a much higher PV wattage.
However this is a very simplistic example, in reality the PV panel requirement to power a particular home or charge a battery bank will ultimately be determined by the connected load and consumption rather than in higher or lower PSH values. But it gives us a good example of how the solar irradiance and peak sun hours at a particular location not only varies based on geographic location and weather throughout the year, but determines the size and cost of any planned solar photovoltaic system.