Wikipedia:Cleanup Taskforce/String inverter

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String Inverter - A relatively new term in the world of alternative energy, it applies primarily to solar electric generating equipment. Generically, an inverter is a device which changes electricity from DC (direct current) to AC (alternating current). Prior to the popularization of string inverters, most photovoltaic (PV) electicity-generating systems were based on one or more solar panels which produced 12, 24 or 48 volts nominal DC power. In situations where more power (more panels) were used, the panels would be connected in parallel, to increase the available current flow while keeping the voltage output at 12, 24 or 48 volts, depending on the nominal panel output voltage.

In many systems, the solar panels are used to charge batteries and a "non-string" inverter was then used to invert the DC voltage from the panel/battery array to the more useful common AC voltages (117VAC in North America and 234VAC in most of Europe, etc.). As things evolved, it became practical to "grid-tie" the AC output of these inverters to the normal AC power lines in a home or commercial business. In other words, to hook the power you were generating right up to the power lines so that your locally-generated solar electricity would be perfectly synchronized with the utility company's power grid (a task done by the inverter; specifically by a "grid-tie" rated inverter).

For systems where utility power was available, this arrangement provided several benefits, not the least of which was the ability to sell-back your solar power to the utility company (in fact, local laws now require the utility companies to buy it in many areas). So, if you were generating more than you used at any given time, you were, in effect "banking" the excess. This fact led to the realization that, for grid-tied systems where the grid was always available, there was really no need for a huge battery bank...the utility company effectively stored your excess and gave it back free (or sold it to someone else and paid you). Since the batteries were a major cost factor and had limited life, this approach became more and more popular.

The string inverter was born of this evolution with the emerging popularity of grid-tied systems and the 12V/24V/48V DC approach was seen as cumbersome and inefficient. One of the primary laws of physics regarding electrical power is Ohm's law, which defines the relationships between the units Volts/Amperes/Watts/Ohms, etc. Power in Watts is equal to Voltage times Amperes, so lower-voltage systems draw more "amps" for the same amount of power. For instance, a 12VDC system producing 4800 Watts (a whole roof full of solar panels) would have to produce a current of 400 Amperes (12 x 400 = 4800). To carry 400 Amperes of current without significant losses (from resistance) would require gigantic wires or copper bars from the roof to the location of the inverter. The power losses in an electical conductor rise as the square of the current per the expression of Ohm's law P = I^2 x R, where R is the resistance of the wire. This "IR loss" factor applies to AC and DC alike and is the reason why power companies string their long-distance transmission lines at very high voltages; to reduce the current and thus the losses for a given amount of power.

So...the string inverter was born. Simply put, a string inverter is a device for converting DC to AC power and which is designed for high voltage DC inputs. Using a string inverter, the solar panel array, still typically rated at 12V, 24V or 48V each panel (although higher voltage panels are now coming out) is wired in series, rather than in parallel. It's that simple: The panels are arrayed in a "string" to produce the same amount of total power but at higher voltages (typically 200-800V), thus lower current, allowing much smaller wiring and much smaller and lighter weight inverter construction. The IR losses encountered in wiring are also present in the inverter equipment's electronics, thus higher voltage DC input circuitry can be built more economically and with lower internal losses, improving efficiency in the electronics as well as in the cabling between the solar array and the inverter.

The popularity of this new approach in PV (photovoltaic solar power) applications has not fallen on deaf ears in other arenas, as well. Small-scale wind generators, for instance, have followed a similar evolution. Starting with 12V and 24V automotive-type generators or other low voltage DC units and batteries, the more current crop of home wind generator systems are now using higher-voltage principals and, for grid-tied systems, are skipping the batteries, too. The term "string inverter" is somewhat nonsensical in this context because the small-scale wind generators are not typically done in arrays like solar panels and there is typically only a single generator per installation (for small systems). Now high-DC-voltage "grid-tie string inverters" are all the rage for use in wind systems, too.

So, string or no string, the bottom line is that the budding homebrew power industry is getting hip to high voltage as a means of improving system and inverter efficiency and lowering the costs of getting the power from one place (the roof or tower, etc.) to another (the location of the inverter). String inverters should more accurately be called simply "high input voltage inverters", probably, since the "string" moniker really applies only to multi-panel PV solar systems and these same units are very popular for wind and hydo-generation systems where there is no "string" of series-connected sources, per se.