Difference between revisions of "Solar cell"
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The energy delivered by a solar cell comes in a very specific form. Understanding this form helps with chosing the right solar cell. | The energy delivered by a solar cell comes in a very specific form. Understanding this form helps with chosing the right solar cell. | ||
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Indications are given in the properties of the solar cell, like Voltage and Watt. But also these indications must be understood. These properties Voltage and Watt are most efficient values. Most of the time the efficiency is less. | Indications are given in the properties of the solar cell, like Voltage and Watt. But also these indications must be understood. These properties Voltage and Watt are most efficient values. Most of the time the efficiency is less. | ||
Revision as of 10:54, 30 April 2015
This page is under construction!!!
Intro
A solar cell is a complex device with many details under the hood. Studying a solar cell, optimizing the efficiency is high tech.
Deep Basics
The energy emerging from a solar cell is an effect which can be described with quantum mechanics. This means that it is complicated!
There is a very good lecture course online: [1]
Important figures and ideas which can be discovered in this course are:
The integrated power delivered on the surface of the Earth is 100 mW/cm2
But only wavelength shorter than 1.1Ev are absorbed: so this means only 44 mW/cm2 is possible at best. Why not "all" energy can be used is a quantum effect: this has to do with discontinuity in absorbing energy. Electrons, which catch the energy or send it out again do this in "jumps", not in a nice variable way.
Irradiance
Irradiance is the power delivered at a certin spot of the world at a certain time of year. This is the maximum of the possible energy to be harvested from a solar cell. Tables can be found at this page: http://solarelectricityhandbook.com/solar-irradiance.html
MPPT relation
The energy delivered by a solar cell comes in a very specific form. Understanding this form helps with chosing the right solar cell.
Indications are given in the properties of the solar cell, like Voltage and Watt. But also these indications must be understood. These properties Voltage and Watt are most efficient values. Most of the time the efficiency is less.
- Examples:
Description from this site: The cell is sensitive to a wide range of light wavelengths (430-1100nm), so it should produce power in a number of different settings. The rated open circuit voltage is 350mV (900nm, 1mW/cm2 light source), and short circuit current is 47μA. Angle of half sensitivity: phi = ± 65°. open circuit voltage is 350mV short circuit current is 47μA First remark: you never get these two values together, so this is not Ohms law, neither the Power law!
Description from this site: This unit is rated for 8V open voltage and 650mA short circuit. We actually took a random unit outside and measured 9.55V open voltage and 550mA short circuit. open circuit voltage is 8V short circuit current is 650mA
- Voltage
Indicated solar cell voltage is a maximum. At this maximum (open circuit voltage) there is not current! No current means no energy. So normally the voltage is lower. Under circumstances of the wheather, clouds, the volatge will vary - it is not stable at all. Say we have a stable illumination. From these examples you can see that the voltage is having a maximum. The voltage needed by a microcontroller is also fixed in a range with a minimum and a maximum. The delivered voltage from two equal cells in series can be the sum of these voltages but only if the illumination is the same. The current can even be running backwards. Extra protection can be needed (diodes) to prevent this backward current. In case of a large chain of cells the whole efficiency can deteriorate by a "bad" cell. So pratical context is very important. Say we have a varying illumination. For this situation, which is occuring quite often, or in case of morning and evening situations there are special chips, which can deliver a stable voltage even if the voltage of the cell is lower. For chargers this is important because they only function with a specific voltage.
- Current
The indicated current is running with no voltage (short circuit current). This means that no energy is generated at this current. Normally the current is lower.
- Watt
The number of Watts is multiplying the maxium voltage with the maximum current. So this numbers of Watts is never attained. The real value of energy is depending on the maximum in the power point relation. Keeping the voltage and current this maximum is one of the technical difficulties of working with solar power. If the voltage is not at this maximum can mean losing 30% of the possible energy.
The second lecture: [2] teaches the deep mechanisms behind the MPPT relation.
- MPPT Graph Shape