AC/DC without the wailing
Tuesday, February 19, 2008 at 10:22PM
Toby Considine in Microgrids and Distributed Systems, Re-thinking things

The American Scientist has an excellent summary on issues of AC/DC this month, including some confirmations of points made here earlier, sometimes eliciting howls and snarky email.

One interesting account is that of the municipal ISP in Gnesta, Sweden which opted to build a DC data center years ago. They did not order specialized equipment. They replaced their existing UPS systems with one supplying a steady 350 watts of power into existing systems. As odd as it seems, they did not fry any hardware; they simply saw a continuing 28% reduction in energy use. The worst problems they faced were over-riding some safeties during equipment start-up. One can only imagine the savings if the system were actually designed.

This account leaves out that since their system runs on the batteries of their UPS, the batteries now have 28% longer life. As the electricity in data centers is converted directly to heat, the air conditioning needs of the data center have a similar drop.

One last interesting point is that after Westinghouse and Tesla won out, Consolidated Edison continued to offer legacy DC services in Manhattan to support DC elevator motors and such. The surprising thing, to me, is that this service was finally phased out just this last fall (or possibly the fall before depending upon the publication lead times for the American Scientist).

To reprise the argument, now with more confidence, most modern equipment is DC (Direct Current) internally. Power coming over the lines is AC (Alternating Current). . Those little warm bricks and “wall warts” on the plugs convert AC power to DC. Batteries are DC. Solar power is DC. Wind Power is DC. AC has an advantage in long lines transmission. If power were converted once when it came to the house to DC, then the heat of conversion, now separated onto so many systems, could be better used.

Every DC source today must be converted to AC so it can be converted back to DC. This wastes energy. If the internal distribution of power in the home and office were DC, energy storage would be more effective. On-site generation would be more effective.

In any case, I recommend reading the article.

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