An In-Depth Look Into Solar PV

In the previous article, I gave you an introduction to solar PV. In this article, we will take an in-depth look at solar PV.

Broadly speaking, there are two main types of solar PV modules: crystalline silicon (c-Si) and thin film.

There are two types of c-Si modules: polycrystalline and monocrystalline. Polycrystalline silicon contains crystals of varying sizes and orientations. On the other hand, monocrystalline silicon contains a homogenous crystalline framework, i.e. the entire sample is one single, continuous, and unbroken crystal without any grain boundaries. It is very easy to tell the two apart from their looks; polycrystalline silicon has a metal flake effect, while monocrystalline has an even colouring, as can be seen in the figure.
Thin film modules are usually categorized based on the photovoltaic material used. The four main types of materials used are: amorphous silicon and other types of silicon, cadmium telluride (CdTe), copper indium gallium selenide (CIS or CIGS), and dye-sensitized solar cells (DSC) or other organic solar cells.

        Thin film solar PV modules can also be categorized in a completely different way: rigid and flexible. Rigid modules are created by using a glass substrate, while flexible thin film solar PV modules are created on a flexible metallic or plastic substrate. Both types of modules can have any of the above mentioned four types of photovoltaic materials.

       The advantages of different types of modules are:

The disadvantages are given in the table below:

          So how much energy do PV modules generate? As a rule of thumb, 1 kW solar PV modules generate 5 units (kWh) of energy per day, give or take a few percent, at most places in India, and they can do that for around 300 days in an year. So 1 kW solar PV modules generate around 1,500 units per year. This of course assumes that the PV modules are working optimally, for which they have to be cleaned regularly. Also, the PV modules have to be installed in shadow-free areas since shadows can reduce the generation significantly. This deserves a little bit of an explanation.

          Let’s say that only 5% of one module is covered by shadow. How much do you think will that impact generation? Common sense would suggest that the generation would reduce by 5%. But alas, that’s not true! If the generation of the solar cell that is covered by shadow reduces by 50%, then the generation of the entire chain will be reduced by 50%! Why? Because the solar cells are connected in series in a PV module, and the PV modules in a panel are also connected in series in most cases, remember? So the rule that “the chain is as strong as the weakest link in it” applies here as well. The current flows through the chain and if the current carrying capacity of one solar cell is reduced by 50%, the current carrying capacity of the entire chain is reduced by 50%!

         This has one other disastrous side-effect as well. Since all the other solar cells – that are not in the shadow – are capable of generating a lot more current but aren’t allowed to do so, all the extra energy, which could have been generated but isn’t, has to be dissipated as heat. Therefore, the PV modules heat up which impacts their long-term reliability adversely.

     I know a person who got a solar installation done on his rooftop but wasn’t getting anywhere close to the expected generation. Turned out, his housemaid was putting up clothes for drying on the PV modules! 🙂 I rest my case.

         One other condition has to be satisfied for PV modules to work optimally. They have to be installed facing the South direction. Why? Because that way, they get the maximum exposure to the Sun as it rises in the East and travels in the sky during the day, before it finally sets in the West. Of course, this is true for India and all the other countries in the Northern Hemisphere. For countries in the Southern Hemisphere, the PV modules have to be facing the North direction. In the above figure, the perfect scenario is if the house roof – on which the PV modules are installed – is facing the South direction.

         The modules also have to be elevated at an angle equal to latitude plus 10° in winter and latitude minus 10° during summer. This is to ensure that the Sun’s rays are as perpendicular to the PV module’s surface as possible. However, actually doing this isn’t always feasible. So the modules are installed at an angle equal to latitude in most cases, which is considered to be a “good compromise”. So in the above figure, the slope of the roof would have to be equal to the latitude of the place where it is located.

     PV modules have a very long life, assuming of course that people don’t do crazy things – like the one mentioned above – with them. Many people, especially the ones who know nothing about them, find it very hard to believe that PV modules can last for 25 years. Granted that it is a quarter of a century, but good quality PV modules really do last that long. The proof is in the typical warranty given by most manufacturers:

      What the power warranty means is that a 300W module will generate at least 270W in the first 10 years, and at least 240W in the next 15 years. In reality, good quality PV modules will generate more than these minimum values, and continue to generate power for as long as 30, maybe even 40, years, although their capacity would have probably fallen to half their rated power by then.

    A solar PV module manufacturer called REC Solar gives a linear power output warranty as can be seen in the figure above. It guarantees at least 97% power in the first year, and a maximum of 0.7% reduction in power from the 2nd till the 25th year. This warranty, as you can see from the diagram, is a lot better than the typical warranty. They are guaranteeing a lot more power compared to the typical warranty.

   There’s one more thing that deserves an explanation. A 300W solar PV module is rated as “330Wp”. The “p” in the rating stands for “peak”. What that means is that it will generate 300W in STC, short for Standard Test Conditions.

There are three components to STC:

1. Irradiation: This is the intensity of the radiation that is incident on the solar PV cell, and it is 1000 watts per square meter in STC.
2. Temperature: This is the “cell” temperature, and is 25°C in STC. Note that the cell temperature is anywhere 10°C to 20°C higher than the ambient temperature.
3. Air mass: This is a measure of the optical path length through which light travels before it hits the solar PV cell. As light passes through the atmosphere, it gets attenuated due to scattering and absorption. For STC, the air mass has to be 1.5

This is a bit counter-intuitive to understand because if you take a 10W CFL bulb, it will consume 10W anywhere in the world at any time. However, that’s not the case with solar PV cells. The amount of electricity that they generate very much depends on the above-mentioned three factors, and in all probability is less than the rated peak power. In India, the amount of electricity that solar PV modules generate in actual conditions, called NOCT which is short for Normal Operating Cell Temperature, could be as much as 25% less than the rated peak power.

I hope this article gave you an in-depth look into solar PV modules as promised. In the next article, I will give you an overview of the different types of solar PV systems.

Sustainably yours,

Prashant Karhade.
Writer, Publisher, Entrepreneur