This article gives you the complete input on how a solar panel work and what are the characteristics,rating of the solar panel to look for before buying it. The functioning of a solar cell is simple if you understand the basics of electron transfer and doping.
The solar cell is a two terminal device. It is made up of a thin slice of semiconducting material. The electrical contact in the solar cells is provided by imprinted metal contacts.
When solar radiation is incident on silicon cell, it generates a DC photo voltage of about 0.5 V and photocurrent of about few tens of mA.cm. The schematic representation of the photocell in which the lines represent the metal contact for electron collection is shown below.
As the photo voltage produced by a single cell is very low, cells are connected together in series and packaged into modules. Solar panel is a collection of solar cells connected to each other in series.
A panel contains 28 to 36 cells in series and under standard insolation (~ 900 W.m-2) it generates a DC voltage of about 12V. Cells within the panel are integrated suitably so that even if one cell fails the panel will deliver power. The view of a solar panel is on the right:
For high current and voltage output, several modules are connected in series and in parallel to form an array. The arrangement of the solar panels to form the array is shown below:
Based on the power requirement, the number of panels can be increased. The array is also called as photovoltaic (PV) generator. Between the PV generator and load, power conditioning unit and battery storage could also be incorporated to maintain constant voltage and also to deliver power during the nights and sun-off/cloudy periods. If the array is directly connected to the Grid, it is termed as grid-interactive PV generator.
Things to look for when purchasing solar panel for home
This system is very ideal for home installation if large space is available in the terrace of the house which could be used for putting up the panels, harnessing the free solar energy into electricity and selling it to the Electricity board.
Meters can be set to measure our input to the grid. In fact such systems should be promoted so that each home installation becomes the producer of electricity meeting their own demand and delivering the excess to the National grid.
While choosing the solar cell one has to look for the following important characteristics:
- Open circuit voltage (VOC), because the operating regime of the solar cell is from 0 to VOC in which the cell delivers power. High the range better would be its applications requiring lesser number of cells to deliver power at a required voltage.
- Short circuit current (JSC), the maximum current when the two terminals of the cell are connected. Though this value is never achieved in the operating condition when the load is included in the circuit, it serves as an important factor to decide about the cell capability and also for cell/material comparisons.
- Power density (P), which is given as the product of current and voltage. When it is maximum, it is referred to as maximum power point. The values of voltage and current density corresponding to this point are denoted as Vm and Jm. They can be identified from the following plot of J vs V.
- Fill factor (FF) is defined as the ratio of peak power density to the product of JJC and VOC.
FF = (Jm x Vm)/(JSC xVOC)
- Efficiency (h) is the power density delivered compared to the incident light power density (PS).
h = (Jm x Vm)/ PS
The four parameters, JSC, VOC, FF and h are the key performance indicators of a cell.
One has to be familiar with the fact that resistance of the contacts and the leakage current from the sides of the cell can result in wastage of power and hence lesser efficiency of the cell. Also heating of the cell by the infrared component of the solar spectrum can result in lowering of efficiency of the cell.
Doping of semi conductor
Elements such as silicon that has semi conducting property is known as intrinsic semi conductor. On doping with pentavalent or trivalent elements silicon which is tetravalent acquires an extra electron or deficiency in valency per atom of the dopant respectively.
This extra electron or hole (deficiency) can efficiently take part in the conduction process when photons impinge on the system. The mobility of electron or hole constitutes current.
Such dopant based power generation is higher than that by intrinsic semi conductivity and it is known as extrinsic semi conductivity. Semiconductors are suitable materials for photovoltaics as they have energy gap between valence and conduction bands matching with the visible photons.
When photon is incident on the semiconductor, it creates free electron from the doped semiconductor with higher energies. These electrons should be guided to flow away from the site of creation or else it would be lost in recombination.
There needs to be an electric field to push the electrons into the external circuit. This is achieved by junction of materials with different electrical properties.
For efficient photon-semiconductor interaction, three important processes are essential. They are,
- The active part of the material should absorb the photon and result in electron generation. The generated electron needs to be excited to the higher energy level.
- The electrons and holes created by the photon should move away from the site of creation by ultrafast kinetics.
- The charge carriers (electrons and holes) should be removed from the cell and directed to useful load before they loose the extra energy.
Essential components of a solar cell
Hence the solar cell needs to have components to meet the requirements of the above mentioned functionalities. The essential components are
- Semiconductor for the creation of electron-hole pairs.
- Region with drift field for charge separation.
- Front and back electrodes for collecting the charges.
Do you have any question regarding the article, don’t hesitate to leave a comment below. I would be happy to help.