Cogeneration (CHP) engine electrical efficiency

[gadz]

Hello to you all,

Does anyone know if gas engines running on a cogeneration (combined heat & power) mode can achieve electrical efficiency rates as high as 40% as it is http://www.biomassenergy.gr/en/articles/technology/biogas/15-biogas-plants-electric-and-thermal-energy-production-from-the-cogeneration-system" ?

How realistic is that?

 

[brewnog]

Yes they absolutely can.

 

[Allenman]

My father is the manager of one such plant. They burn Naptha fuel in two 30,000 hp jet engines (both connected to 20MW generators). The exhaust is then directed at a boiler which powers a steam turbine and a third 20MW generator. I think he mentioned the efficiency if the plant is very close to 90% (I may have to doublecheck on that one though)... Most gas burning power facilities, if you look at the smoke stacks you can see the wavy heat coming out; that's wasted energy. Where my dad works it looks like they aren't even using them.

 

[gadz]

Thanks for your response.

Well, of course CHP gas engines can achieve higher total efficiencies. My point is how they can reach such high electrical efficiency.
I mean CHP steam turbines which burn solid fuels (wood etc) they also have high total efficiency but their electrical one seems to be quite low (15-20 %).

 

[xxChrisxx]

Superheat + Reheat, multistage turbines and regenerative heating. 40% is an easily realisable figure for a Rankine cycle with all the trimmings.

 

[brewnog]

It's achievable for a reciprocating engine too, as the OP showed in his link. The manufacturers at the top of their game are selling piston spark ignition generators now with electrical efficiencies well in excess of 40%. The technologies used are lean burn, Miller cycle, high compression ratios, clever mixture preparation, heavy turbocharging and aftercooling, and a good ignition and control strategy.

 

[cmb]

During winter, when my gas central heating system is running, I use a whistle kettle on my gas stove and fill it right up (instead of an electric kettle with only just enough water for my coffee mug).

This is because with the whistle kettle, I get 100% thermal efficiency from the gas, and I also get a kettle full of boiling water too!

If your purpose is to generate combustion heat, then anything else you do with the work available while converting the chemical energy to heat is just 'free energy'!

 

[brewnog]

That's not the point though, is it? The point is that these systems can generate electricity at efficiencies above 40%, and allowing enough usable waste heat to be recovered for overall efficiency to be in the 90% region.

Take an end user with a CHP plant with, say, 30% electrical efficiency. Sure, they can use the heat, and they can sell the power to the grid, and the overall efficiency may still approach 90%. But compare that with a 40% efficient system, where the additional available electricity will more than pay for the difference in start up cost for the more efficient scheme, and you can see that the 30% system couldn't even be given away. The electricity is the by-product but it's also the key traded commodity.

 

[cmb]

That's not the point though, is it? The point is that these systems can generate electricity at efficiencies above 40%, and allowing enough usable waste heat to be recovered for overall efficiency to be in the 90% region.

Depends on what is meant by 'gas engine'. There are Stirling engine CHP systems that are commercially available that get that high, I believe, like the WhisperGen product. You might want to punt into 'CHP gas engines' hydrogen fuel cells, which'd get you up towards 60% electrical conversion efficiency.