They are equipment capable of altering voltage and characteristics of electric current they receive to transform it into suitable for specific uses.
Those that receive direct current and transform it into direct current with a different voltage are called DC-DC converters. They are not widely used in photovoltaic systems.
Those that receive direct current and transform it into alternating current are called DC-AC converters or inverters. The function of an inverter is to change a DC input voltage to a symmetrical AC output voltage, with the magnitude and frequency desired by user.
They allow to transform 12V or 24V direct current that modules produce and store batteries, in 125V or 220V alternating current.
This allows use of electrical devices designed to work with AC.
A simple inverter consists of an oscillator that controls a transistor, which is used to interrupt incoming current and generate a square wave. This square wave feeds a transformer that softens its shape, making it look like a more sinusoidal wave and producing the necessary output voltaje.
The voltage output waveforms of an ideal inverter should be sinusoidal.
This gives rise to different types of inverters:
1) Square wave inverters: they are cheaper, but less efficient. They produce too many harmonics that generate interference (noise). They are not suitable for induction motors.
Recommended if you want AC power only for a TV, a computer or a small electrical device. Inverter power will depend on device nominal power (for a 19 ” TV a 200 W inverter is enough).
2) Modified sine wave inverters: they are more sophisticated and expensive. They use pulse width modulation techniques.
Wave width is modified to bring it as close as possible to a sine wave. Harmonics content is less than in square wave.
They are the ones that offer best quality / price ratio for lighting, television or frequency inverters connection.
3) Pure sine wave inverters: with a more elaborate electronics, a pure sine wave can be achieved.
Until recently these investors were large, expensive and inefficient; but lately, has been developed equipment with 90% or more efficiency, telecontrol, energy consumption measurement and battery selection.
Since only induction motors and most sophisticated devices or loads require a pure sine wave form, it is usually preferable to use modified sine wave inverters; which are cheaper.
Inverters must be dimensioned from two variables.
First is considering electrical power wattages that inverter can continuously supply during its normal operation.
Inverters are less efficient when used at a low percentage of their capacity. For this reason it is not advisable to oversize them and they must be chosen with a power as close as possible to that of load consumption.
Second is starting power. Some inverters can supply more than their nominal capacity for short time periods. This capacity is important when using motors or other loads that require 2 to 7 times more power to start than to stay running once they have started (induction motors, high power lamps).
Incorporating an inverter is not always the best option from energy efficiency point of view. It may seem an easy solution to convert all solar system output to a standard AC power but it has several disadvantages.
First is that it increases system cost and complexity.
An inverter also consumes energy (in addition to 15% for performance loss) and therefore decreases overall system efficiency.
For a small house electrification (light points, TV and a small appliance) it is possible and profitable to do without the inverter.
For lighting it is better to invest in low voltage lights instead of investing in an inverter.
Laying of 2 lines can be interesting: one connected to batteries to feed points of low consumption lighting or LED and devices that consume DC and another connected to inverter to power appliances that consume AC.
The advantage of the inverter is that operating voltage is much higher and therefore the use of thick cables can be avoided. Especially when wiring is extremely long it may be economically feasible to use an inverter.
A feature that incorporates most modern converters is possibility of operating as battery chargers, taking alternating current from a generator or grid.
This is an extract of contents included in Technical-Commercial Photovoltaic Solar Energy Manual and Sopelia e-learning training.
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