[nethawk]

I have heard of the Organic Rankine Cycle and other similar systems which use the waste heat from a steam turbine to boil a liquid of lower boiling point (for example, an organic compound like pentane or isobutane) and turn a second turbine to generate extra power.

It seems like this would be a great thing for our nuclear, and even coal-fired power plants, because it is such a shame how we waste two-thirds of the energy in the fuel when generating electricity. If our nuclear units were outifitted with this system, fossil plants could be closed, reducing greenhouse gas emissions. Even a 20% increase in efficiency would be great. It would also obviously be good for smaller-scale power plants, like waste-to-energy facilities and wood-fired plants.

Obviously that organic fluid has to come from somewhere (oil), but it is constantly recycled, not consumed.

What are the issues with this technology? Is the initial investment too high or is the technology not mature enough? I do believe it is being used in many geothermal power stations where the water is not hot enough to make superheated steam, obviously at a smaller scale than the typical nuclear unit.

[strider3700]

usually the investment to get it set up makes it barely worth while doing. It adds complexity to the system and that increases the costs. It is being done in a few places though, our local pulp mill needs to massively heat the pulp the steam is then fed through a generator and they produce about 1/3 of their own power off of it.

Nuke plants would be far better off if they took the waste fuel and ran it through another plant designed for that and then took that waste through another and so on until you're getting lowish level waste left over. American plants don't do this for some reason that I'm unsure of. Mostly politics I believe. The canadian candu reactors burn some of the american nuclear waste but it's so hot we can't burn it very quickly is my understanding.

[nethawk]

Yes strider, we don't reprocess our nuclear waste for political reasons. They were afraid that it could fall into the wrong hands. President Bush has made mentions of restarting it, whether we'll start doing it again or not is a whole other story. I agree that that is the best way to reduce waste and extend uranium supplies, since only about 1% of the total energy in the nuclear fuel is ever put to use in a once-through system.

On the subject of ORC, if they could install a system in a 1 GW plant that would recover say 300 megawatts of power from the total 2 GW of waste heat. Over a 40-year operating lifetime, that's

7200 MWh/day
at 3 cents per kWh, that's $216,000 worth of electricity per day.
$78,840,000 per year
$3,153,600,000 over a 40-year operating time plus the hidden benefits such as reduced GHG emissions, definitely a good thing.

I don't know if this accounts for the costs to install and maintain the ORC over its life, but it sure is a lot of money! There may be limits to large-scale setups with regards to maintenance and operation.

I have done some looking, and for nuclear - CBC, or Closed Brayton Cycle may be more practical. An inert gas such as CO2 or helium is heated using a high-temperature nuclear reactor, and it is then used to spin a gas turbine. The gas is cooled back down in a water heat exchanger, making steam which flows to a second turbine, just like the combined-cycle concept used in gas-fired power plants. The inert gas is recycled back through the reactor loop and used again.

The problem there is that our present nuclear power stations can't simply be retrofitted to use it. It's a whole new design.

The South African Pebble-Bed reactor design uses the Brayton Cycle.

More about advanced power cycles