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Sunday, March 21, 2010

DC voltage: 12, 24, or 48 volts?

When designing a distributed power generator (solar panels, micro wind turbines, etc.) a choice have to made: the DC voltage of the system.

In other words, the generator (panels or wind turbine) will produce DC electrical current at a given voltage, let's say 24V. A cable will carry this power to the batteries, the charge controller and to the inverter, that have to be designed for the same voltage (24V). Then the inverter converts it to alternate current at the prescibed grid voltage, 110 V in the US, 220 V in Europe for example.

On the market you can find generators, batteries, and inverters, working at different voltages: 12V, 24V, 48V, but also higher, like 250V.

How should you choose the DC voltage in your design?

To answer this question, you have first to understand pro and cons of different voltages.

Why higher voltage is better?

A higher voltage is better because with a higher voltage you have a lower current on the cable. The current on the cable goes like the inverse of the voltage. In other words, to carry the same power (for example 400 Watts), at 12 V you'll have 400/12 = 33.3 A flowing in your cable; while at 48 V you will have only 400/48 = 8.33 A flowing.

The reasons why a smaller current is preferable are:
  1. the dissipated power on the cable is R times I^2, where R is the cable resistance, while I^2 is the square of the flowing current. Suppose that the cable going from the solar panel or wind generator has a resistance of 1/4 of Ohm. Then in the previous example we would have a power loss of about 17 Watt for the 48 V cable, while a power loss of 278 W for the 12 V cable! In other words more then half of the generated power would be lost if you use a 12 V voltage in your system, while less than 5% is lost in the case of a 48 V system.
  2. larger current means larger electronic devices in the inverter, as the size of electronics roughly depends on the current, not on the voltage.
Why smaller voltage is better?

The only reasons why a smaller voltage is better, is to save on insulation in some parts of the system (wind turbines, inverter), and to save on batteries.
Indeed, batteries have a rated voltage of 12 V. Therefore you have to connect two of them in series to obtain 24 V, and four of them to obtain 48 V. On very small systems, you may want to keep the voltage low, and therefore put only fewer batteries.

As you've seen, it's a sort of tradeoff. The larger the power produced by the system, the higher should be the voltage, to avoid power losses. However, if the power is not too big, and the cables are quite short (that is the resistance R is small), you can keep the voltage a little bit lower, saving something on the devices.