Electricity generation and characteristics

There are lots of ways to generate electricity, but many of the big ones really just involve boiling water and using it in a big steam, or Rankine, engine.  The spinning engine rotates a magnetic field, which cranks out alternating current.  The setup is just like a motor in reverse, and in fact, if you put electrical energy into the generator, it acts like a motor instead.  Another major system, hydroelectric turbines, use moving or falling water to spin the rotor, and are otherwise essentially identical.  A third system burns fuel -- usually natural gas these days -- in what amounts to a jet engine turbine.  The fancy name for this is a Brayton cycle engine.

One of the big advantages to a hydro-power system is that the `fuel' is practically free, at least once you have built a dam.  Another is that the generators can be `turned on' nearly instantly.  Brayton engines are also quick to start and stop.  The Rankine engines tend to be slower to work up to speed, as they have to `build up a head of steam'.  For the same reason, they are harder to shut down.  Nuclear plants have this same `head of steam' problem, but have very cheap fuel; as a result, nuclear plants will sometimes actually pay someone else to take their power, just so that they can keep chugging along and sell more power later.

Combined cycle generators and efficiency

Because fuel is not free, Brayton engines can be expensive to run.  But there is a technological trick: the output from the jet engine is still very hot.  You can use that hot exhaust to boil water and combine the Brayton cycle jet engine with the Rankine cycle steam engine, into one `combined cycle' gas turbine (CCGT) generator.  This raises the efficiency.  (See this page for technical details about combined cycle efficiency.)  The main penalty is that the combined-cycle plant is not quite as quick to start up or shut down (but as it turns out, it is not that bad -- nothing like a nuclear plant, for instance).

Efficiency in a gas-burning generator is much like miles-per-gallon in a car.  A low `mpg' generator has to burn a lot more gas to generate the same number of kWh.  Just like an inefficient car needs more gasoline to go a thousand miles, an inefficient burner needs more natural gas to produce a thousand kWh.  An efficient burner will use less gas, and since these generators use such prodigious quantities of fuel, efficiency is a big deal.  The efficiency of any heat-fired system can be described in terms of a heat rate, measured in BTUs (British Thermal Units) per kWh.  This measure is sort of upside down -- gallons-per-mile instead of miles-per-gallon -- so a bigger number is worse.

The absolute maximum efficiency, or rock-bottom number for the heat rate, is 3414 BTU/kWh.  No one will ever get there, but the smaller the number, and the closer it is to 3414, the more efficient the generator is.  In practice, a modern CCGT runs around 7000 BTU/kWh or better (49% efficiency or better) in cool, dry weather.  The 60% efficiency that is theoretically possible would lower this to just under 6000 BTU/kWh.

By contrast, some of the old plants in California have absolutely horrible heat rates: 12, 13, or even 14 thousand BTU/kWh (see this long PDF file).  According to other data, some are as bad as only 19% efficiency -- a heat rate of about 18000 BTU/kWh!

Efficiency and dollars

When you buy natural gas, you normally pay by the `therm' -- 100,000 BTU -- or by the `million BTU', which for no particularly good reason is spelled MMBtu.  (You would think M would stand for mega, so that you could write MBTU, but I guess with these archaic British units, M gets used as the Roman numeral M, meaning thousand.  MM means a thousand thousand, or a million.)  When you buy electricity on the wholesale market, you normally pay by the megawatt hour, or MWh.  If a good CCGT needs 7000 BTU to make one kWh, it should make one MWh from 7 MMBtu.  Thus, the output from a modern CCGT, in MWh, costs about seven times as much as the fuel, in MMBtu, just to pay for the gas.

The output from an old 14000 BTU/kWh plant, on the other hand, would cost 14 times as much as the fuel, just to pay for the gas.  That makes it twice as expensive as the new plant, even before looking at anything else.

Although natural gas burns very cleanly (compared to coal or oil), it is not completely clean.  A big generating plant has to buy pollution credits, or spend money to install special pollution control equipment, or sometimes even both.  As you might expect, since an efficient plant burns less gas, it produces less pollution too.  So a modern plant is not just more reliable, it also uses less fuel and produces less pollution than an old plant.  For various reasons, the new plants are often much better than `twice as clean' -- and pollution credits can get expensive, so the old plant costs more than twice as much as the new ones, in pollution output.

All in all, then, an old plant can cost well over twice as much to run as a new one.  When the new one costs $70 to run long enough to put out one MWh, the old one costs over $140.  When the new one costs $105 to run, the old one costs well over $210.  A 19% efficient burner would cost over $270/MWh in fuel alone!

Microturbines and Fuel Cells

These days, anyone can buy a super-small gas turbine generator, called a `microturbine'.  One manufacturer is Capstone.  They say their device is 29% efficient.  This is probably a marketing number, meaning you will never do better than 29% (or about 11775 BTU/kWh).  If you need heat, though, you can use the warm exhaust.  In colder areas, combining heat and electrical generation in this manner brings up the efficiency tremendously: you burn the gas for the heat, and create electricity as a side effect.  Instead of buying a furnace, you buy a turbine.  Microturbines are highly practical, as long as you can use the extra heat; otherwise the relative inefficiency generally makes utility power cheaper.  Since heat can be used to cool things (consider propane-powered refrigerators, for instance), I think microturbines combined with heating and air conditioning could catch on in many places, such as supermarkets.

The fuel cell uses a similar idea.  It `burns' anything containing hydrogen, including natural gas, producing electricity and heat.  The process is completely different, though: instead of one moving part, there are no moving parts.  There are a number of types of fuel cells; the usual one promised for a `hydrogen economy' is the Proton Exchange Membrane or PEM cell.  This has a special membrane (hence the name) that does the trick.  The chemical processes and ion transport mechanisms are rather similar to those that occur in living cells.  Fuel cells are still very expensive, though.


Wind turbines use mechanical energy to produce electricity just like any other turbine.  Instead of burning fuel, though, they capture energy from the wind.  Wind power is cheap and effective, and the `fuel' is free.  Their main drawback is that sometimes the wind does not blow.

Solar Photovoltaic

Photovoltaics use a completely different trick to produce electric power.  In the 1800s a number of people observed odd effects of sunlight on electricity.  Eventually Albert Einstein was able to explain the mechanism, and in the late 1980s, solar PV became practical and cost-effective in remote locations, where stringing up utility wires would be prohibitive (e.g., $30000 for the poles and wires, to a single customer).  The main problem with solar PV is that it is expensive, and not very concentrated.  It also produces DC power, rather than AC, so to fit in with existing systems it requires conversion, and of course it only produces power when the sun shines.

In dry sunny desert climates, however, solar PV has the advantage that it produces power exactly when is needed most: during hot sunny summer afternoons.  That makes it exceptionally well-suited to installation on individual building roofs, to offset their loads.  This space is otherwise entirely unused, so the need to use a lot of square footage to get much electricity is irrelevant -- that square footage is otherwise wasted anyway.  That leaves the problem of expense: `roof electric' still costs $0.25/kWh or so today.  Some states have buydown or tax credit programs that reduce this cost, though, and the price is already half what it was ten years ago, and is poised to drop quite a bit more in the next ten years.


All contents are copyright © 2001 Chris Torek.