The design of the V-2 rocket that terrorized London in the last days of WWII was, in substantial part, determined by Roman legislators around 450 BC. The V-2 rocket was designed to be transported on the back of a truck for launch. The width of the Meillerwagen was determined, of course, by the width of the roads available, many of which dated to Roman times. Roman roads were designated by law to be 2.45 metres wide. The V-2 therefore had to be built to comply with a 2500-year-old engineering standard! (This isn’t my original insight, by the way, I just can’t find the source).
A much younger, but perhaps even more anachronistic engineering standard just passed into history a couple of days ago: the last remnants of the DC power grid in New York City.
Thomas Edison, the famous inventor, was the first to start selling grid electricity to customers. His system transmitted power at 110 volts, using Direct Current (DC). This worked quite well – provided the generator was within a kilometre or so of the customer. In an electrical distribution system, voltage is the rough equivalent of water pressure – it’s the amount of energy transferred by a unit of electricity (such as the charge carried by a single electron) as it travels from place to place. The higher the voltage, the more energy carried by each electron. If you want to transmit electricity a long way across a small cable, you need to use a high voltage to minimise the loss of energy. However – like high-pressure water – high-voltage electricity will also escape through the wire and travel through other things very easily – including humans. That’s why long-distance electrical cables have to be separated such a long way from people, and from each other. And that’s not practical in home appliances. So, what was required was a way to change the voltage – make it high for long-distance transmission, and drop it down again for the end use. But – at least with the technology of the time – DC voltage couldn’t be changed – high voltage for long-distance transmission, low voltage for in the home.
Several engineers and businesspeople – chief amongst them George Westinghouse and Edison’s former assistant Nikola Tesla – gradually developed a different transmission system called Alternating Current, or AC for short. In AC transmission, the voltage increased and decreased in a regular pattern, depending on the system somewhere between 25 and 100 times per second – and the current flow actually reversed direction, hence the name AC. The advantage of AC distribution was that by the use of a gadget known as a transformer, the voltage (technically, the size of the peaks and troughs, thus increasing the “average” voltage) could be increased as much as desirable for long-distance transmission, and decreased closer to the end user.
The competition between the two competing standards became known as the War of Currents. Edison resorted to marketing stunts that would make the most ruthless contemporary CEO think twice, even constructing the first electric chair to promote the view that AC was unacceptably dangerous. Like all good scare campaigns, there is a certain element of truth to this – at the same (averaged-out) voltage level, AC is indeed more dangerous than DC. Any practical distribution voltage, however, will be dangerous whether it’s DC or AC, and the massive efficiency advantages of the AC system won out. Edison started selling AC equipment within a decade.
However, the existing DC grid in New York didn’t shut down. DC had some advantages for some customers; notably, DC motor technology was more advanced than AC motors. But by 1928, plans were being made to phase out the DC grid. On-site AC-DC converters – a scaled-up version of the gadgets that supply DC power to all your electronics – were gradually installed. And, 79 years later, that process is complete.
It’s fascinating to think how things might have turned out if DC had have been the standard power transmission technology. Edison’s scheme relied on having power generation close to where it is consumed to avoid excessive transmission losses. Essentially, that’s what the various proponents of distributed electricity generation are arguing for right now. Or would high-voltage DC power transmission – which, if you can efficiently increase voltages to similar levels as AC, is actually slightly more efficient – have become commonplace?
But there’s one last thing to take out of this tale. It will take a really long time to change the architecture of our energy supply systems. If I may again get on my hobby horse for a moment, that makes drop-in replacements for coal-fired power stations – large-scale energy sources that run reliably – that don’t emit CO2 rather useful things.
The link about DC power was originally from Slashdot, arguably the mother of all blogs