29 May 2015

Run Away from These People

On a web site dedicated to people who do consumer electrical work, someone posits that Tesla's new home battery packs will force homes to go voltage DC current. This is absolutely and completely wrong, because fails to get even a single fact right, as would be clear to anyone who knew the most basic physics:
CE pros might be sitting in the proverbial catbird seat when it comes to the future of wiring homes.

With the solar revolution seemingly in full swing all across the nation, consumers are enamored with “going off the grid.” And many homeowners are counting on home batteries to be the next phase of their off-the-grid plans. Companies likes Tesla and RoseWater Energy Group are leading the way in the development of these new power storage devices for homes.

But if the battery power trend takes off, it must lead to a new paradigm in which homes will be powered more with low voltage wiring than line voltage electrical, according to a blog by CE veteran Paul Self on Buildz.com.

Indeed, will the pure science limitations of AC/DC conversion eventually force a gigantic sea change from builders, electricians and the National Electric Code itself in the way homes are constructed and wired? The answer is “Yes” that might have to happen, says Self.

Here is Self’s logic:
[Or more accurately, here are his completely incoherent thoughts]
“An underlying issue with solar power and the Tesla battery is the fact that they run on DC while the power infrastructure in buildings is AC. Stepping power up and down from AC to DC and vice-versa wastes energy, about 20 percent is lost in the conversion. Some converters do a better job than others, but resolving this 20 percent loss is very important when working on a battery stored energy supply.
Ummmm. No. Inverters operate at efficiencies of about 95% at their design points, and if you are connected to the grid, as Tesla envisions, you will only use the inverter .
Many devices in a home could run on DC. Almost all non-incandescent light bulbs can run on DC and require a transformer to step the 110 VAC down to a 12 – 5 VDC signal for the bulb.
No. The a fluorescent light ballast operates as follows: It uses a transformer to jump UP the voltage (not down), then rectifies it to DC, and then uses an inverter (square wave, because unlike motors, that is fine for gas tubes) to hop the frequency from 60hz in the wall, to 1+ Khz to eliminate flicker.  (For incandescents, it's not an issue, because even as the AC current goes through them they hot.  CFLs are generating lights from individual sparks).
Other devices like computers, TVs, cable boxes, and cell phone chargers all operate similarly.
No again. They cannot use line current, they need to condition the power, and it is easier to do this with AC current.
Appliances like electric ovens, electric water heaters, and air conditioners will require 110VAC, but most of the house is on DC.
Let me paint you a picture.
They reacquire 110VAC because they are high power, and at low voltage, the amperage, and wire size go through the roof. (Details follow)
  • Sun generates 12VDC via the solar panel
Yes, a panel will generate relative low voltage DC, but if arranged in series, you can generate 100s of volts, and have smaller and cheaper wiring with less transmission losses.
  • Solar panels push power to a battery
Again, note arranging in series. In their cars, for example, Tesla battery packs operate at DC voltages between 250V and 425V.
  • The battery or the solar panel push 12VDC to a DC to AC converter (20% loss of power).

From an info page for an inverter for solar vendor
Again. Inverters are around 95% efficient these days.
  • AC is distributed throughout the house
  • Many devices then convert the power BACK to DC (20% loss of power)
Again, the efficiencies of rectifiers is about 95%, not 80%, and the devices that have to convert it back to DC would have to do so anyway, because power from the batteries is not as uniform as their electronics requires.
  • This all seems pretty silly to lose this much power. I am sure it is less than a cumulative loss of 40 percent power, but when your trying to free yourself of the power company, this really adds up.
Also note that he knows no math.  Two devices operating at a 20% loss (80% efficiency) would give us losses of 80% x 80% = 64%; 100% -64% = 36%.

This guy cannot even do basic multiplication.

You know what also "really ads up"?


Let's take a 40 W light bulb.

Power is Voltage x Current = V x I, so for a 120V source, the current is ⅓ amp. For his 12 Volt DC system, the current would be 3⅓ amps, 10 times more current.

You can also express power in terms of current and resistance, P=I2R, so losses in transmission would go through the roof.

Well, they would, except if you look up the charts, it turns out for a typical (I chose 50 feet) length, the DC wiring has much lower resistance, because it is much thicker.

How much thicker? Well the table for wiring for 12VDC gives us 6 Ga wire, and the worksheet for 120VAC gives us 14 Ga wire, and the wire gauge size table gives us conductor diameters of 0.17" and 0.064" respectively.

That is a lot of copper you end up paying for, and for many applications.

He also ignores the fact that DC Motors (the kind with brushes) are less efficient, run hotter, and wear out more quickly.

The motor on your furnace, and dryer, and washing machine, are typically induction motors, that operate directly off of  your house current, and with a DC house you would need inverters for the motors.  (So called "Brushless DC motors" contain their own power circuitry linked to an induction motor).

If you see this guy, do not let him near your electronics.  He will kill himself, or kill you, or set your house on fire.

Note that I am NOT an electrical engineer, though I got the basics in school, but this is so ignorant, and so wrong, that I felt compelled to Fisk this bit of  bovine scatology.

H/T /.


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