Energy extraction in power plants

[sid_galt]

So, I have basically four questions

1) What is the efficiency of converting the heat energy generated by a power plant due to combustion/fusion to electric energy?

2) What are the major ways in which energy is lost while converting it?

3) Is there any way to improve the efficiency?

4) Are their efficient methods to increase the temp. difference so as to increase the Carnot efficiency and therefore the operating efficiency?

 

[brewnog]

1) What is the efficiency of converting the heat energy generated by a power plant due to combustion/fusion to electric energy?

Currently, fusion is not a viable source of power. Fusion has not yet (afaik) been performed to give net energy output. For coal, maximum efficiencies are somewhere around 35%. Apparently there's a new design in Denmark which gets up to 45%, using sea water as coolant, very high temperatures (steam at 580 Celcius) and pressures (29MPa), and with two reheat stages.

2) What are the major ways in which energy is lost while converting it?

Cooling towers (~45% total heat lost), chimneys (~10%), auxiliary services, (~3%) and process inefficiencies (turbines, piping) (~1%) are all eating away at Carnot.

3) Is there any way to improve the efficiency?

As mentioned above, using higher pressures and temperatures can increase the overall efficiency, but at a cost of more general engineering expenditure. Good cooling can help (since the efficiency is based upon the difference in maximum and minimum temperatures), and supercritical cycles give efficiency advantages too.

4) Are their efficient methods to increase the temp. difference so as to increase the Carnot efficiency and therefore the operating efficiency?

Yes, but at a cost. The higher you go in terms of pressure and temperature, the more it costs to make, run and maintain the plant. Engineers must strike a balance between efficiencies and capital costs, - in reality this is what engineering is all about.

 

[sid_galt]

Currently, fusion is not a viable source of power

Sorry, I meant fission.

 

[russ_watters]

An added loss for fission is that it requires two loops of working fluid (so as not to send radioactive water to the cooling towers). This makes the thermodynamic efficiency somewhat lower than a fossil fuel plant.

 

[brewnog]

Also, nuclear plants are often restricted from using very high temperatures or pressures in their steam cycles, for safety reasons. It's not a direct inefficiency, but it means that Carnot cannot be improved this way.

 

[Clausius2]

Some things to add to these good answers:

-Maybe the most efficient power plants for electricity generation are the Combinated Cycle ones (using a compound of Brayton+Rankine cycle). Its efficiency reaches 50%.

-There is a limit in the efficiencies of power plants: the 2nd principle or Carnot theorem. None power plant can surpass the efficiency of a Carnot engine.

- I think the main problem is transform the fuel chemical energy to thermal energy. This kind of conversions are coupled with a loose of availability due to the 2nd principle. There are some machines, as Hydrogen Cells which directly converts chemical energy into work without loosing availability.

 

[Astronuc]

1) What is the efficiency of converting the heat energy generated by a power plant due to combustion/fusion to electric energy?

Nuclear (fission) plants have technical safety limits imposed on temperature and pressure which limit the efficiency. Current US plants obtain thermal to electrical efficiencies around 31-33%. Framatome claims the new EPR with greatly improved steam turbines may realize about 37% efficiency. All nuclear plants use steam (Rankine) cycle for thermal to mechanical conversion.

Back in the 1970's there were plans for at least 8 advanced High-Temperature Gas Cooled Reactors in the US, and the efficiencies of those plants was targeted in the range 39-42%, due to high temperatures of the He-cooled, graphite-moderated core. None of the plants were built, and the only gas cooled reactor in commercial operation - Ft. St. Vrain (Colorado) had significant technical problems (http://fsv.homestead.com/FSVHistory.html).

Coal/oil plants can operate at much higher temperatures and pressure (superheat), and so their efficiencies can be in the mid-to-upper 30's%.

The most efficient plants to date are combined-cycle plants which operate with an aero-derivative gas-fired turbine (Brayton cycle) with a steam cycle using the exhaust heat of the gas turbine. The gas turbine efficiency is about 42-45%, and the steam cycle gets about 30-33% of the exhaust for a total efficiency of about 62%.

For additional info on congen plants - see http://www.poweronsite.org/Tutorial/Cogeneration.htm

Some basics on thermodynamic cycles - https://wrc.navair-rdte.navy.mil/warfighter_enc/SHIPS/shipeng/Thermo/thermo.htm

2) What are the major ways in which energy is lost while converting it?

Plant efficiency is primarily determined by the maximum temperature of the working fluid (steam in most cases, or combustion temperature in a combustion turbine). Think of Carnot efficiency. In the case of steam, it must be condensed and pumped back to the steam generator (boiler).

3) Is there any way to improve the efficiency?

Efficiency can be improved by raising the temperature and pressure of the steam, but that comes with a trade off with respect to corrosion and erosion of the metals (alloys) in the boiler and steam generator. Corrosion in nuclear plants is complicated by the fact that corrosion products from the core and in the primary coolant circuit are radioactive, and must be collected by a filter system and ultimately disposed as radwaste.

Improvements have been made in steam turbine blade design - see the Olkiluoto-3 (EPR) thread - which enable more efficient thermal to mechanical conversion without increasing T_hot. Similarly, the manufacturers of aeroderivative turbines (e.g. GE, Siemens, Alsthom) have greatly improved efficiencies of compressors and gas turbines, so system efficiencies have also improved.

http://www.gepower.com/prod_serv/products/plant_perf_software/en/gatecycle/library.htm
Siemens bought the Westinghouse gas turbine business, IIRC, and ABB may have sold their turbine business as well.
http://www.siemenswestinghouse.com/en/gasturbinesitem/index.cfm

4) Are their efficient methods to increase the temp. difference so as to increase the Carnot efficiency and therefore the operating efficiency?

New alloys have been developed that may enable higher temperatures. I believe fossil plants may be maxed out, but there are several designs for advanced (Gen-IV) nuclear plants that may allow higher temperatures and therefore higher efficiencies. However, the advanced plants are still in design phase.

 

[sid_galt]

Thank you for the replies. BTW, what do you think of Tesla turbines in comparision to the conventional ones? I heard that they are much more effective.