Resistive Losses of Electric Energy
Energy is useless! Wait a bit! Don’t jump! Let me qualify. Energy is useless unless and until we can convert it to power. Electrical power means a flow of current caused by a voltage driving it. This process is inherently inefficient. Current can only flow through a conducting path, and a path will always have a finite resistance. The path resistance will invariably demand an octroy on transport of energy. We know this energy loss in terms of the I2R power loss. When a device is generating power, or converting power to a useful form, losses occur not only in the interconnecting path but also in the devices themselves- because current must flow through them. Octroy again!
Specifically, in solar energy systems resistive losses of energy occur everywhere. Right in the solar cell, in the cabling, the protection and control network, and the inverter and associated equipment.
Losses in the Solar Cell
Losses within the solar cell will affect the fill factor which is a quality indicator. Losses inside the cell are of two types-the local octroy, and pilferage. We say the cell has a series resistance, and a shunt resistance. The series resistance charges octroy on flow-through of energy, and the shunt resistance causes an inherent pilferage.
• The Leakage Resistance-The pilferage will take place in the shunt resistance whether useful current flows through the cell or not. It is caused by shunt leakage currents bypassing the cell. Pilferage or leakage losses are represented by an equivalent ‘shunt resistance’. Its value will depend on the cell construction.
• The Series Resistance– the series resistance charges an octroy depending on the current value and occurs at three locations.
o Within the semiconductor material of the cell
o At the metal semiconductor contacts
o In the top and bottom contacts.
Losses in the Cabling
Cabling has resistance. All cabling causes an I2R loss. In addition, poor contacts- not tightened properly, oxidized, or corroded terminals will cause a voltage drop, and hence power loss. The conductor loss can be reduced by selecting larger conductor sizes, but that increases cost of the installation. Guidelines should be followed in selecting conductor sizes according to the level of current expected. At the same time, minimum length of conductors compatible with easy connection and disconnection must be used.
Losses in the protective and Control Circuitry
All circuit breakers and fuses in series with the current path cause a voltage drop resulting in a power loss of the octroy type (dependent on current value). These can be represented by an equivalent loss resistance.
Losses in the Inverter
DC from the strings is fed to the inverter where it is inverted to obtain alternating current (AC). The inversion process will invariably cause a loss in power with respect to the input. Some of this loss can also be attributed to the resistance of the switching semiconductors and the cabling.
Fill Factor of a cell is not a loss, but a measure of the loss in a cell due to its series resistance, More here. A higher series resistance discussed above will result in a lower fill factor. In simple words it is the ratio of the maximum power obtained from the cell by a maximum power point device (Vmp. Imp) to the theoretical maximum available from the product Voc. Isc.
The Fill factor has a value around 80% for a normal silicon PV cell.