The Role of Solar Module Efficiency

In last 10 years, solar module manufacturing technology has been significantly advanced. From 2010, customers of panels tend to expect more and more power from panel year over year. As all the panels were of the same size, power generated by the panel was the key to measure for how advanced the cell technology was. Now, the question is why efficiencies are increasing more and more.

Generally, efficiency is calculated by the ratio of output to input. But, in case of solar modules, efficiency calculation is slightly different. Talking about solar panel’s efficiency, it is simply the measure of the amount of irradiation that falls on the surface of a solar panel and converted into electricity. Due to recent advances in technology, panel efficiency has increased from 15 to 22% over the years. The huge difference between two efficiencies has changed power rating of the panel from 250 watts to 600 watts.

Solar cell efficiency depends on cell design and type of silicon whereas solar panel efficiency depends on cell configuration and panel size. Solar panel’s efficiency is measured under Standard Test Conditions (STC) based on cell temperature of 25°C, solar irradiance of 1000W/m2 and Air Mass of 1.5. Solar panel efficiency can be calculated by the ratio of maximum power to the total panel area measured in m2 .
 

Where, Pmax=Maximum Panel power (Watts)

Area= Panel area (m2 )

Panel’s efficiency depends on many factors like temperature, irradiance, cell type, interconnection of cells, Irradiance (W/m2), Shading, Panel orientation, Temperature, Location (latitude), Time of year, Dust and dirt etc. Also, the colour of back sheet affects the panel efficiency. A black back sheet might look more aesthetically pleasing, but it absorbs more heat resulting in higher cell temperature which increases resistance, this in turn slightly reduces total conversion efficiency.

Increasing the panel size can increase solar panel’s efficiency as larger surface area will be available to capture sunlight. Nowadays, bigger size modules are available to generate up to 700 watts. Cell efficiency is calculated by what is known as the fill factor (FF), which is the maximum conversion efficiency of a PV cell at the optimum operating voltage and current.

The cell design plays key role in panel’s efficiency. Some of the parameters are type of silicon, configuration of bus bar, junction and passivation etc. Right now, panels made using ‘Interdigitated back contact’ (IBC Cells) are the most efficiency (20-22%) due to N type silicon and no losses from busbar shading. Additionally, panels made using Mono PERC cells, N type TOPcon technology and advanced heterojunction (HJT) cells have achieved efficiency above 21%.

While choosing solar panel, one needs to keep in mind factors like performance, reliability, service of manufacturer, warranty conditions etc. Higher efficiency of panels implies lesser payback period, mostly silicon panels have payback period of 2 years and with the new panels with efficiency of above 20%, the payback period has nearly come down to 1.5 years in many locations. In simple words, increased efficiency means solar panel will generate more electricity over 20+years (average life cycle of solar panel) and so can repay the upfront cost sooner. Amount of rooftop area required also changes when efficiency is increased. High efficient same. For example, Mono PERC panels (Navitas Bonito Max) with 21.67% efficiency will require less space than 335 watts polycrystalline panels (Navitas Navisol) with 16.87% efficiency. Rooftop area requirement will be lesser because high efficiency panels can generate more energy per m^2.
 

 

If a panel is facing shading issues, power output will be lower. Shading can lower panel’s as well as system’s efficiency. Partial shading over several cells on a single panel can reduce power output by 50% or more, which in turn can reduce the entire string power by a similar amount since most panels are connected in series and shading one panel affects the whole string. Temperature also affects solar panel’s efficiency. As mentioned earlier, power rating of solar panel is tested under STC (temperature of 25°C), and in sunny days, internal cell temperature reaches to 20-30°C higher than the ambient air temperature which results in 8-15% reduction in total power output depending on the type of solar cell and its temperature co-efficient. To provide an average real estimate of solar panel’s performance during sunny days, Navitas Solar specifies the power rating under Nominal Operating Cell Temperature conditions (NOCT)-cell temperature of 45°C and irradiance of 800W/m2.

Navitas Solar produces all range of panels from 5 to 600 watts per panel with different efficiencies. Talking about the cost, high power-high efficiency panels are usually costlier than low power-low efficiency panels, but extra cost is worth because high power panels will deliver higher performance with lower degradation rates for a long time. Having just larger size does not always equate into higher efficiency, but larger panels using larger size cells increases cell surface area which increases overall efficiency of the panel. Navitas Bonito Max (10 BB, 156 cells) has panel size of about 1.1*2.4 m which can generate from 560 to 605 watts whereas Navitas Navisol (5 BB, 72 cells) has panel size of about 1*1.9 m which can generate from 335 to 345 watts. From the difference in dimensions, space saving can be assumed. Concluding everything, higher efficiency panels can reduce payback period time, reduce space requirement, increase power generation.

- Sanjeev Gupta, Plant Head, Navitas Solar