The case for a direct current grid

Most countries in the world currently use alternating current in their grid. That AC has become the global standard is a relic of the past. In order to prevent losses due to resistance in electric wires, electricity has to be transported at high voltage. Only most electricity is produced and consumed at low voltage. Consequently the voltage has to be transformed when it is transported.

In the beginning of the 20th century, it was impossible to transform the voltage of direct current. Hence in early DC grids electricity had to be transported and be consumed at the same voltage as it was produced. And because of the inability of low voltage DC transmission, DC was rejected in favour of AC grids, since AC could be easily transformed at the time.

A century ago most applications of electricity could be performed with devices that used either AC or DC. Electric light and motors were the most used electric devices those days.

Times have changed, however. With the introduction of power electronics, and of DC-to-DC converters in particular, it has been become possible to modify the voltage of a DC current. And further, most electronic devices work on DC rather than AC nowadays. For instance integrated circuits (a.k.a. chips) cannot work on AC, and require a rectifier to convert AC into DC. On the hand there are only a very few applications for which DC devices are not suitable. And for those few situation an DC-to-AC converter might solve problems with a DC grid [1].

Because power electronics were developed by the time that most countries had already built an extensive AC grid, switching to a DC grid would take an enormous investment. What is the main reason we still stick to AC grids. Nowadays, DC power transmission is only used for long distance transport.

In space settlements, however, we will need to build the entire electric power transmission from scratch. Therefore it would be a good opportunity to build a DC grid rather than an AC one.

It is well-known that Solar power is the principal source of energy for space settlements. And photo-voltaic cells generate a direct current. Then the generated power is converted into high energy laser or microwave beams, which are transmitted to the space settlement, where the beam is converted back into electricity. And in the settlement most electrical devices require an DC input.

Opting for a DC grid in space settlements would be the logical thing to do.

[1] In fact many devices that run on AC, first convert to current they get from the grid into DC, and use an inverter to change the current back to AC. The reason for this that the frequency of the grid is not the same as the desired frequency for a certain application. In case of a DC grid only an inverter would be needed, instead of both a rectifier and an inverter.