Looking For Electricity Info

Back when I was in my wide-eyed twenties I read an exciting book about technology, and in it was a section devoted to electricity production. It showed that with modern concepts such as supercritical operation and recovering waste heat to provide hot water for entire cities, coal-burning generating stations could operate at over 50% efficiency. And yesterday I read about an outstanding installation in Europe said to be reaching 90% use of fuel energy.

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Then I turned to the US Energy Information Administration’s heating-rate data for 2015 and learned that the average efficiency of US coal-fired generating stations was 32.5% that year, and that of thermal stations using nuclear fission heating, 32.6%. The bright side is that in the same year natural-gas-fired generating operated at an average of 43% efficiency.

OK, next I wanted to know how much of that power arrives at the point of use. Here’s what I found as losses:

Step-up transformer from alternator voltage to transmission line voltage - 1-2%
Transmission line loss (conductor resistance, radiation, corona) - 2-4
Step-down transformer from line to distribution network - 1-2
Distribution network – transformers and cables - 4-6%

We can see evidence of some of this loss by looking at transformers; they usually have external heat-radiating arrays of tubes or fins to dissipate their resistive and magnetic hysteresis heating (the hum you hear from a transformer arises from vibration in its magnetic core laminations as the current rapidly reverses). Walking 0ne humid evening in Carmel, CA, I could hear transmission lines crackling and snapping.

Averaging, I get a total loss of 11% - a transmission efficiency of 89%. If we are charging our electric vehicle’s battery we have the charge/discharge efficiency which for nickel-metal hydride cells is said to be 70% and for lithium-ion between 80 and 90%. We feel this loss every time we actually have a laptop on our laps as the warm place on the bottom of the case, next to the battery. Think of the lithium ions during charge/discharge as passengers, getting onto or off of a large commercial airliner – there is definitely some friction (whose electrical analogy is resistance) either way.

Advanced electric motors these days seem to be brushless three-phase variable-speed AC, so we need a power supply that can change the DC at the battery terminals to AC at a frequency and phase continuously adjusted to the rotation of the motor’s armature (the motor has an angular encoder on it just as do modern combustion engines). We now have the high efficiency of IGBT, or Insulated-Gate Bipolar Transistors, to supply the necessary AC power at an efficiency of 90%, a large improvement over the previous generation of “switches.”

Finally we have the electric traction motor itself. Available literature informs us that 94% efficient motors are available over-the-counter, and efficiencies as high as 97% can be achieved. Motor losses arise from conductor resistance, magnetic hysteresis, windage, and bearing friction.

Now we’re ready to find overall system efficiency by multiplying together all of the above efficiencies, as follows:

In the case of coal or nuclear - .325 x .89 x .85 x .90 x .96 = 21.2%
In the case of natural-gas - .43 x .89 x .85 x .90 x .96 = 28.1%

The percentage of US electricity supplied by “stuff that comes out of the ground” – coal, natural gas, and nuclear – was 87% in 1996, 90% in 2006, 89.6 in 2009, and 84.9 in 2016. The big change over these years has been the increase in use of natural gas (13% in 1996 and 33.8% in 2-16) and the decline of coal (down from 52% in 1996 to just 30.4% last year).

The driving forces behind the above change have been EPA’s encouragement of the power industry to convert from coal to natural gas, and the fall in natural gas price made possible by the combination of horizontal drilling and hydraulic fracturing of gas-bearing but otherwise impermeable rock strata (industry jargon for this is “tight plays”). It probably also helps that natural gas can be delivered by pipeline (sorry to utter that “trigger-word”) whereas coal travels by the 10,000-ton trainload in hopper cars.

We’d love to see a rapid upswing in renewable energy from such sources as wind, solar, and geothermal power, and there have been increases. Last year wind supplied 5.6% (up from 4.13% in 2013) and solar 0.9% (up from 0.23% from 2013).

California is the leader in solar, getting almost 10% of its power from that source last year. In terms of more efficient conventional power, the 600 mW John W. Turk coal-fired station in Arkansas, completed at a cost of $1.8 billion, operates at 40% efficiency.

Germany got 12.3% of her power from wind and 6% from PV solar in 2015, but still took 42.3% from coal.

An especially attractive recent source of electric power in Europe has been combined-cycle gas/steam turbines. Natural gas drives a gas turbine/alternator and the waste heat from the turbine then raises steam to drive a bottoming cycle turbine/alternator. Efficiency is now just touching an impressive 60% in the latest installations.

The Energy Information Administration is a part of the US Federal Statistical System, whose purpose is to provide data that can be a reliable basis for sound policy-making. It publishes tremendous statistics on all of the above and it is great fun to put on one’s Magic Decoder Ring and try to figure out what it might mean.