7.4.7: Inverters

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The PV panels generate electric power – and what next?...

A homeowner can use the power at home only, or sell it to the utility company. But the electricity, how it flows out from the panels, is not good for any of the two purposes. The thing is that all American home installations deliver 110 V (essentially, all wall outlets, and all lightening delivers/uses such voltage) or 240 V (used by kitchen ovens, or, e.g., well pumps) – and, which is also important, of alternating current. If, say, at some moment the right slot in the wall outlet is plus, and the left slot is minus, then after 1/120 second the left is plus and the right is minus – and after another 1/120 second again the right is plus and the left is minus. So, the voltage oscillates with a period of 1/60 second, or with the frequency of 60 Hertz.

On the other hand, if the output voltage from a solar panel is delivered by two wires, then one of them is always plus, and the other is always minus. In other words, the solar panels generate direct current. It’s completely incompatible with a household installation – and there is absolutely no chance to sell direct current to the utility company.

The solution of the problem is an electronic device called a solar inverter (or PV inverter, or solar converter ). Such device converts the output DC power from the PV panels to AC power with the same voltage and frequency as the power delivered by the utility company – so that the output can be used at home for powering household devices without changing anything. Also, the surplus power can be sent to the utility company (if it’s ready to purchase it) – it’s then sent out of the home by the same power line through which the company delivers power to the house.

There are many types of inverters, depending of the specific needs of the owner of the PV installation. Some people may opt to cut the connection with the grid whatsoever – they will need an “off-grid-type” converter, most likely with storage batteries that can deliver power after the sunset and be recharged during the sunlight hours. An intelligent off-grid converter will take care of all such needs. People who want to sell the surplus power to the utility company need some other type of specialized inverter. And so on.

The usage of PV solar installation at a practical scale has begin about 40 years ago. The “heart” of an inverter at that time was a transformer with an iron core, a bulky and very heavy device. The weight of an inverter capable of converting, say, 5 kW of power, could be as much as 200 pounds. Fortunately, the progress in semiconductor technology – and specifically, the development of special transistors called “power MOS FET” – made it possible to eliminate the bulky iron-core transistors and to reduce the weight considerably – as well as the price.

The efficiency of modern solar inverters is very high – typically, in the product specification cards it’s given as 98% or even 98.5%.