Organic Rankine vs. Steam Rankine Cycle Efficiency

I sugegst that you contact ORMAT TURBINES which is the leader in this type of technology.

The "thermodynamic bottleneck" is the temperature of heat rejection (condensation) - most commonly ambient air/water. Please review "Carnot Efficiency".

Thanks for the input, folks.

Looking at the problem from the standpoint of Carnot efficiency resolves the issue for all low-temp differential applications (steam or otherwise); and apparently resolves the whole matter without further discussion.

On this basis, most ORC systems would seem to be pegged for generally lower efficiency by nature; as Carnot resolves that efficiency is pinned to the overall cycle's temperature differential: A relatively small difference in Tin - Tout = a small capacity to transform that available heat into work.

Correct?

On the basis of simple Carnot efficiency (1-T_c/T_h) taken from Rankine values (T_c & T_h), it's a real "read 'em and weep" scenario for most practical heat engine efficiencies in general: The picture becomes clear.

There has to be a better way out there somewhere . . .

Thanks for the information; and cheers!

I might have missed it in the thread, but the Rankine cycle as normally used in plants and ships, including nuclear boilers, most often includes a vacuum at the condensor end, so that more energy may be derived from the working fluid, with normal sea temperatures. This also prevents the after part of the turbine from being impinged by saturated steam causing blade erosion. The overall efficiency of the system is increased by using the saturated steam in other parts of the system, such as evaporators, eductors to create vacuum, steam heating in the living spaces, preheating incoming boiler water or bunker fuel, etc.

I liked working on a steamship like the Del Orleans (as the Training Ship Golden Bear), because it was a working thermodynamics lab.

Heat Recovery Steam Generators are also used on the exhaust of diesel and gas turbine plants for the equivalent reasons.

Insatiable said... "On the basis of simple Carnot efficiency (1-T_c/T_h) taken from Rankine values (T_c & T_h), it's a real "read 'em and weep" scenario for most practical heat engine efficiencies in general: The picture becomes clear.

There has to be a better way out there somewhere . . ."

Have you had any success in finding a "better way?"

I'm interested because there are a number of processes out there (Focardi & Rossi & Black Light Power as two examples) where the enthalpies are greater than those associated with "normal" chemical reactions.

Black Light has a hydrino theory; Focardi says "cold fusion" but without a sensible theory.

These appear to be reproducible processes, but the Heat generated is relatively low temperature -- 100C or a bit more at or near atmospheric pressure.

Thermal efficiency (heat energy in vs heat energy out) is around 3:1 up to maybe 10:1. Complicating the situation is the fact that a percentage of the output energy must be fed back to sustain the reaction.

With ORC efficiencies of 10%, there simply isn't enough energy to keep the whole process running AND get energy out.

None of my searches shows any way out, but I just started looking. You've had 2 years! :-)

Hence the question.

For anyone who might be interested:

http://www.cbsnews.com/video/watch/?id=4955212n

And where does "latent heat of vaporization" come in? Since the exhaust has to be a gas, the heat of vaporization is lost for power generation, though it can be used for heating, etc. Wouldn't an organic fluid which vaporized easily, compared with water, and behaved more like an ideal gas, lead to greater efficiency?

Why the concern over "efficiency"? If you are capturing waste heat from other processes, even 10% is a gain over what would normally go up the stack. The question really should not be efficiency, but whether the limited wasted energy can be recovered cost-effectively. If it takes 50 years to recover the investment with a piece of equipment with a 40 year life expectancy, obviously this is not a good investment...

Ah! since you are talking about cost-efficiency, Power plants are talking about waste heat recovery because the cost of making their own electricity and maybe get renewable certificates is lesser than buying electricity from the grid.