Due to the fact, that the topic of solar energy in Armenia is new, since the beginning of Summer we have faced some misunderstanding from our customer’s side who installed solar panels on their roof or ground.  The main question is: “Why does solar PV – Power station in a sunny summer day do not achieve its peak-power?” Let’s get started in order.

What is the peak power of the station? By this, it is usually understood the total power of all installed panels. But how is this power determined? After all, the power of the panel depends on the intensity of the light incident on it, the temperature on the surface of the solar cells, etc. At which intensity of the incident light is the power of the panels determined?

In most of countries (everywhere in Europe) it is common to test the panels at the, so called, Standard Test Condition (STC), it means incident light intensity of 1000 W / sq.m., temperature 25ºC on the surface of the solar cells and spectrum AM1.5. Please believe me, that such an intensity of solar radiation never happens above of the Tropic of Cancer (or below the Tropic of Capricorn). Temperature on the solar cell surface, is of course, can be 25ºC, but not always. Therefore, the power indicated on the panel’s label is more suitable for comparing the panels with each other, but it has no direct relation to their real power. These are just the standard test conditions (STC) required for all manufacturers.

In reality, the panel power drops down approximately by 0.4% with each degree above 25 ° C.

In a summer sunny day, the surface temperature of the panels can reach 60C (this also depends on the color of the cells). In that case, the temperature difference is 60ºC-25ºC= 35ºC and the power drop can reach 14%.

But when, actually, the solar station shows its peak-power ? After all, those of our customers who have installed panels in Winter or Spring have seen that it is possible. This happens in two cases. First, at the panel temperature below of 25°C and secondly, there are cases when the sun peeks out of the spaces in between the clouds, the direct light combined with the reflective light will boost the panels’ power production. This, so called the edge of cloud effect, of course, may happen, but not so often and in very short time intervals.

It doesn’t mean, that solar power plants do not work well in the summer, because the main parameter is not the peak power, but the generated energy (measured in kWh), in other hand,  itself, is much higher in summer time due to long light days.

The daily output of energy of the solar power plant with 147kW capacity on March 5, 2018․

The daily energy output of the same solar power plant (see picture above) on June 30, 2018․ Figures І and ІІ show the daily energy production at the same power plant at the 5th of March and 30th of June. As you can see, in cold, cloudy weather, the station twice showed the power higher than the established, but the actual energy production (area) was only 402 kWh, and on a hot summer day at 30th of June the station, even though didn’t reach the power of 100 kW, but energy output (the area under the bell) was almost twice as higher, about 734 kWh.

Author: Vahe Sharafyan