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Thursday, 4 May 2017


Efficiency of Solar PV Modules:

Photovoltaic (PV) technology uses semiconductor cells to absorb the solar irradiance and convert it into electrical energy. Commercially available solar PV modules currently have efficiency in the range 15 to 20%. Although the efficiency of the PV modules have increased over the years because of intense Research and Development efforts, there are many factors that lead to degradation of solar PV performance. As more and more MW range solar PV plants are coming up, there is an urgent need to pay attention towards issues that reduce the output of a solar PV module and hence the plant.

Factors affecting Solar PV output:

Among the various factors, the solar irradiance and cell temperature greatly affects the output energy from a solar PV module. Low irradiance and higher cell temperature significantly reduce the PV output. Cell temperature is key issue particularly in countries like India. 

In addition to these factors, the output energy from a solar PV module or solar plant also depends on the reliability and performance of the Balance of System (BoS) and environmental factors. The various losses that occur in a PV system or plant are mismatch loss, wiring loss, inverter loss, sun-tracking loss, shading loss, soiling loss and solar dimming loss.

Soiling Loss in Solar PV Plant:

This article is focused mainly on soiling loss and solar dimming loss that do happen in a solar PV plant. 
"Soiling Loss is the loss in PV output power and hence energy resulting from accumulation of dust, dirt, snow and other particles such as tree leaves and bird’s dropping, etc. over the surface of the PV module." 
Dust is a thin layer that covers the surface of the PV panels and its size depends upon the location of the PV plant and its environment. Accumulation of dust may be due to wind, human activities, vehicular movements, industrial and constructional activities etc. The amount of accumulated dust over the PV module surface reduces the daily, monthly, and annual energy yield of the solar PV plant. Study shows that Middle East and North Africa have the worst dust accumulation areas in the world. India too is prone to heavy dust deposits, particularly the states like Rajasthan. 
In one of the study carried out in Baghdad in the year 2016 showed an 18.7% decline in the efficiency of solar PV modules, when left unattended for a month. A new study, carried out under the guidance of Prof. Bergin of Duke University in one of the IIT campus, has found that dust and particulate matter (PM) are reducing the energy output of solar PV plants in Northern part of India by 17-25% annually. 

The study,  found that half the reduction comes from dust and particles deposited on the surface of solar PV modules, which forms a physical barrier to sun-light entry. Other half of the reduction came from the atmospheric pollution that reduces the sunlight reaching the ground, also known as “solar dimming”. 
Solar dimming or Global Dimming is the gradual reduction in the amount of direct irradiance at the earth’s surface. The effect varies by location and is supposed to have been caused by an increase in particulates such as sulphate aerosols in the atmosphere due to human activities. 
According to a study carried out at the Indian Institute of Tropical Meteorology, Pune, the amount of Solar radiation reaching India’s land mass is reducing by 0.86 Watts/ square metre per year. 

The output of solar PV module or panel is very much dependent on the solar irradiance on the module surface. Or in other words, the output power of the solar PV module is directly proportional to the solar irradiance. The standard solar irradiance for calculating solar PV output and performance is 1000 W/m2. At half the standard irradiance, the output power of the solar PV module is also reduced to half, as can be seen from figure 1. Lower solar irradiance also reduces the output voltage of the PV panel. Figure 1 shows the power-voltage ( P-V) curve of solar PV module at different solar irradiance level  at constant temperature of 25 oC. 

Now a days several software are available for assisting the solar PV designing which can very much predict and asses the PV module behaviour under varrying solar irradiance and other variables.
Fig1: P-V curve of solar PV module at different solar irradiance level ( at 25 oC)

A similar study carried out in Singapore found that the solar PV output has been reduced by 15-25% because of poor air quality. In the worst case the solar irradiance was recorded as low as 50%. 

This means that cleaning of solar PV panels is not enough to ensure maximum energy from a solar PV plant; environmental quality and pollution level also plays a significant role. In future solar PV plant developers will be extra cautious when signing Power Purchase Agreement (PPA) with clients having solar PV plants located in highly polluted areas.