By Siddharth Gangal and Aarushi Dave
If you’ve come across the word “solar cell”, ample number times during your pre-purchase research, and are confused what exactly it does, here’s a brief guide to it!
By definition, a solar cell is an electronic device, made of semiconducting material, that absorbs the rays of the sun, and converts them to useful electricity, thanks to the photovoltaic effect. Grouped together, they form solar modules, which eventually form solar panels together, that form a part of solar installations. Similar to batteries, they convert solar radiation into DC direct current electricity, unlike the chemical energy to electricity conversion.
While we come from a generation using mostly conventional sources such as the utility grid or regular batteries, the world is fortunately shifting towards conventional sources of energy such as solar. As apart of this rising change, and most importantly, as a conscious solar buyer, we want to help you understand how solar cells work.
Let’s break down that definition to understand what solar cells really are:
1.Composition and structure: The cells must absorb the radiation of the sun
- Solar cells are made of silicon which is a semiconducting material i.e. neither insulator nor conductor.
- Silicon can be made positive type (p-type) i.e. fewer electrons, or negative type (n-type) i.e. more electrons, by specially treating them chemically or doping.
- When a combination of a layer of p type and n type silicon is placed together, a barrier is formed at the junction of the two. This is where photovoltaic effect takes place.
- While many materials satisfy the needs of a photovoltaic conversion, practically, all photovoltaic energy conversions use semiconductor materials as p-n junction.
2. Photovoltaic Effect and Functioning: The photovoltaic effect is what produces electricity from the absorbed solar radiation:
- At the p-n junction barrier between the two layers, sunlight falling on it, ‘photons’ are generated.
- These photons provide energy to the electrons, knocking from the p-layer across the barrier to the n-layer, and from the n-layer into the attached circuit.
- This is how DC electricity is generated and flows through the circuit. This is called the Photovoltaic effect.
- Hence a semiconducting material is required, as it absorbs light and promotes electrons to a higher energy state, and then transfers it to the external circuit.
3. Efficiency: What percent of received energy do they convert to useful electricity
- In other words, the energy output from the solar cell for the energy input from the sun rays.
- Practically, majority solar cells convert around 10-20% energy received into electricity. Theoretically, a typical, single-junction silicon solar cell has maximum efficiency of about 30 % called the Shockley-Queisser limit. While cutting edge technology can create such cells in laboratories, operating in perfect conditions give 46% efficiency, this is practically impossible
- In real world domestic solar installations, around 15% efficiency maybe achieved by the panels. This is drop is due to real world factors influencing the cells such as photons of different wavelengths producing different amounts of energy, intensity of the incident sunlight and the temperature of the solar cell.
- In addition, ensure you minimise the following factors, to improve efficiency:
i. How the panels are positioned and their tilt
iii.Keep the panels clean
iv.Provide ventilation to the panels to keep them cool as increasing temperature lowers their efficiency.
- Efficiency is often an important parameter to judge the performance of the cells and compare with other cells. Hence you must measure efficiency carefully, without the above-mentioned factors influencing the panels, especially when used for comparison.
- Particularly essential parameter when you have limited space for installation
4. Cells, Modules, Arrays: How one forms another:
- Cells form Modules. Modules form Arrays. It’s simple.
- As mentioned earlier, a solar cell is an electronic device which converts solar radiation into
electricity. These cells are actually the building blocks of solar modules.
- While each cell generates around 0.5 V, any number of cells can form a solar module.
Similarly, modules put together form arrays, which are installed in sites.
Number of cells used in modules have the following uses:
i. 36 cells: charge a 12 V battery
ii. 60 cells: the typical residential grid connected system uses solar modules
iii. 72 cells: large commercial and utility scale solar systems
- On increasing the number of solar cells per module, the voltage and wattage increases. Smaller custom size modules with lesser cells can also be created.
- Panels typically have a standard size of 6” * 6”
5. 60 cells vs 72 cell modules
- 60 cell panels are commonly used for domestic purposes such as residential rooftop solar installations.
- 72 cell panels are commonly used for ground-mounted, commercial, or utility sized solar installations.
- While 72 cell panels are bigger in size, the have a higher power output rating for the same material, because of more cells. The extra 12 cells make these panels significantly larger (2m x 1m compared to 1.65m x 0.95m for 60 cell) and heavier (about 28kg compared to around 20kg in 60 cell).
Reasons why 72 cells panels can be unsuitable for residential installations:
i. Due to larger dimensions and weight, in most cases, they require different mounting than standard panels i.e. 3 rails instead of 2.
ii. Majority 72 cell panels found in the market currently are designed with commercial or utility grade installation features. Hence, they are transported in bulk, lifted onto rooftops by crane instead of manual lifting and usually need to be installed flat because (hence little or no requirement for wind resistance).
iii. Since they are not structurally designed to be manhandled individually, the possibility of flexing the panels while carrying and installing them on a rooftop may reduce the long-term viability of the backing sheet (consequently of the entire panel).
iv. Small, material differences in the positive tolerance levels, efficiency and temperature coefficient and a significantly reduced level of hail resistance have also been observed on comparing the two types.
v. However, 72 cell panels are usually cheaper due to different manufacturing than resident standard for other panels
- 72 panels are not more efficient than 60 panels! They only produce greater output due to more number of cells. From around 270W per panel on 60 cells, up to around 310W or 315W in 72 cells is seen but at the same efficiency.
6. Three busbar model:
- Busbars are, by definition, is a metallic strip or bar, typically housed inside switchgear, panel boards, and busway enclosures for local high current power distribution.
In solar panels, busbars are thin flat strips made of copper or aluminium, which allow heat to dissipate more efficiently because of their high surface area to cross-sectional area ratio.
- Insulators can support busbars, or insulation may completely surround them.
- They separate the solar cells and are used to carry the direct current generated from the cells to the inverters.
With this article we hope to have clarified some of your doubts regarding solar cells and hope you have lesser confusions in your pre purchase research!