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.
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!
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!
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.
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.