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Sunday, August 21, 2005

The DC Generation

Since August is inventor's month, I thought I'd focus on two of the big stars of the field. Ben Franklin, aside from his inventions, also discovered electricity. Thomas Edison invented the incandescent light bulb, which made it a useful household technology.

Edison may have lost the war, however. He envisioned multiple DC power plants each serving a small community. Westinghouse pushed for the long haul possibilities of AC distribution. The conflict between the two ended with big and wide winning in the first "not in my backyard" power plants.

This is important because AC and DC are different delivery systems. Neither is inherently "bad" or "good," but the characteristics of each compliment currently technology in different ways. As the size and type of power demands change, the use of current changes as well.

Three important terms in electricity are volts, amps and watts. A watt is equal to volts multiplied by amps. Voltage is electrical potential difference and amperes describe current draw. Using the example of water flowing through a pipe, voltage would be the speed of the water, while current would be the size of the pipe. It's a flawed analogy, but I have a diagram describing it anyway.

It turns out that sending thousands of volts at small currents produces less heat loss over miles and miles of electrical cable (less water turbulance at high speeds). Alternating current makes it possible to send 50,000 volts down a power line and get it down to household voltage without much energy loss.

Modern electronics is based on the transformer. It can change the voltage/current ratio of an alternating power source by any multiple using wires wrapped around an iron core. This cannot be done with DC. Step down transformers take high voltage power and bring it down to 120V home current. There's also the issue of three phase power and AC generators, but I digress.

After these transformers, most solid state electronics have a transformer and bridge rectifier to get AC voltage to 12 volts DC. While the conversion is almost lossless, putting your hand over the one in your VCR (in the corner where the power cord comes out) gives you an idea how much heat loss results. When homes were wired for power, it was used for light bulbs. It moved on to heating systems, fans and radios. The adoption of television started to turn the tide. The last half century has gone from AC friendly uses to electronics that run almost exclusively on DC.

Therein lies the irony. A hinderance to solar and wind power is that their DC power output is incompatible with AC powered appliances. Inverters bridge the gap expensively and wastefully. When the current is produced, it becomes DC all over agin in the computer and the TV and the clock radio. Electric lights and ovens work well enough in AC. Fans are less efficeint. Electronics are worse.

Micro-generation could have the dual advantages of lossless power delivery and directly compatible DC power. Laptops, portable phones, travel TVs and VCRs already run off 12V DC. Cell phones and CD players that run off batteries have chargers that can be used in a 12V car outlet. If you could power a house without an inverter, it would significantly lower the cost of a renewable energy system.


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