Steam Energy Transfer Question

The main reason is specific volume. The energy of steam is given in KJ/kg, and specific volume is m3/kg. You want to maximize the energy given per relative size of equipment/pipes.

As a side note - steam calculations are based on enthalpy (h), not internal energy (u) ............ NOTE also that h = u + PV; where PV is considered "flow work"

Super heated steam has more energy because the higher the temperature the higher the velocity it can travel through pipe work, and more velocity per mass to do work in a turbine, hence the reason you have reheat turbines. Not sure if this is what you are after, in simple terms?

Regards JD.

In a steam turbine driven power plant.

Energy is used is the chemical energy in coal.

The chemical energy of fuel is absorbed by steam.

The heat energy of steam is converted to mechanical energy by turbine.

The turbine in turn rotates a generator to produce electricity.

The major efficiency loss is between conversion of heat energy to mechanical energy of turbine.

The turbine rotates at a high speed of 3000 RPM.
You have to think how to do the energy conversion.

At present we do not have a better method of this conversion.
For gas fuel it is directly used for driving a turbine where it has a higher efficiency with another waste heat boiler and steam turbine.

Thanks, everyone, for your input.

So, if I'm getting the general idea here, the energy which we put into a given mass of saturated water (steam) is not necessarily reflected in the amount of energy which can be communicated as work in a turbine or expansion engine setting. The factor which determines the relative suitability of any thermodynamic fluid for the task of providing energy transfer in a turbine or expansion engine is the change in specific volume which that fluid undergoes within the temperature/pressure range spanned by the desired enthalpy or internal energy drop for the fluid.

Therefore, in the case of steam, we must push the pressure/temperature of the saturated water up beyond that which, on the basis of pure energy in/energy out, would seem to be adequate for the task at hand; and into a temperature/pressure region where the specific volume change is greatest for the enthalpy or internal energy drop which we wish to exploit.

Is this correct?

Thanks again . . .

@XMech:

Clear, lucid, thorough: What else could you ask for . . .

Well, maybe if you're me, there's one more thing

On the basis of these facts (and all things being equal), it would seem as though water could be as efficient as any other thermofluid in a thermomechanical setting; the constraint being a matrix of scaling effects. Yes?

Assuming this is indeed so, what is the key motivating factor for those instances which utilize refrigerants as the working fluid in closed-cycle expansion-engine or turbine applications? In other words, what material-specific property/value(s) should generally be maximized (or minimized) if one was to choose a synthetic thermofluid in place of saturated H₂O?

(Pleeeeeease don't say that it is inextricably tied up with a full understanding of entropy, as the whole concept is hopelessly baffling to me; and may induce vomiting)

Thanks again for ALL of the help!

It seems you are jumping between different topics/issues

1. Thermofluids (eg. oil based) are used because of their high boiling points and can reach higher temperatures without falling into the category of "expansible fluids" and thus avoiding pressure vessel/boiler regulations.
2. Water can be used as a refrigerant such as in the lithium bromide vapor absorption cycle.
3. Water/Steam is used in thermo cycles due to the following advantages:
1. Non-toxic
2. Readily available
3. Non-corrosive (just have to treat and remove O₂, CO₂, salts, etc)
4. Cheap
5. Non-flammable
6. High specific heat capacities
7. Fairly low viscosities
8. High heat transfer coefficients
   
9. Steam is easy to control due to temperature/pressure relationship (does not require pumps in order to transport it)
   
10. Steam gives up a lot of heat at constant temperature

But, if I understand your main question:

"what material-specific property/value(s) should generally be maximized (or minimized) if one was to choose a synthetic thermofluid in place of saturated H₂O?"

As mentioned above, hot oil systems are often used to avoid pressure/boiler regulations. In this case the properties of the thermofluid are:

1. Cheap and readily available
2. Non-toxic
3. Non-flammable
4. Non-corrosive
   
5. High boiling point (high vapour pressure)
6. High specific heat capacity
7. Low viscosity
8. High heat transfer coefficients
9. Low density

Of course many of those ideal properties are inversely related to each other, for example - it is usually the case that in order have high specific heat you compromise with higher viscosities and higher densities.

I hope that the above answers your questions, and if not, please reply for clarification.

-------

Oh, and by the way, the link in your first post isn't really working - I can't see anything other than a simple schematic which says "low entropy engine" - so I can't comment on it, nor elaborate on your specific reference to (avoid) entropy.

........... and then, after going over your question again, I see I missed the point regarding "closed-cycle expansion-engine or turbine applications", often referred to as turboexpanders.

In which case most of the above properties are the same: cheap, readily available, non-toxic, non-flammable, non-corrosive, .........

But the specific properties you want in the refrigerant (as opposed to thermofluid) are:

1. Low boiling point (within reason to what you are cooling)
2. High heat of vaporization
3. High critical temperature
4. Low density (liquid) / high density (vapour)
5. High heat transfer coefficient
   

The main purpose of turboexpanders are to recover some energy during the throttling process (make the cycle more efficient)

Sorry about the mixup in nomenclature; as I was under the impression that refrigerants and the like were all considered "thermofluids" in this setting.

Sorry, also, that my link to the USPTO was a fiasco; as it pointed to an early patent for (what I now understand as) an Organic Rankine Cycle system using R-11 as the preferred working fluid.

As an aside, I found one of your comments here quite interesting:

"Thermofluids (eg. oil based) are used because of their high boiling points and can reach higher temperatures without falling into the category of 'expansible fluids' and thus avoiding pressure vessel/boiler regulations."

I can't imagine what a headache it must be for industry to deal with the regulatory bureaucracies which have grown up around boilers and the like: Are refrigerants such as R-134a in turboexpanders considered "expansible fluids" by regulatory decree? As I understand things, ORC is a more efficient cycle than its saturated-water counterpart (as is seemingly represented in the above-cited patent).

And finally, after clearing up the mess which I created through my ignorance of proper nomenclature, could you comment on why the likes of R-134a could be considered desirable over saturated H₂O in a turboexpander setting? What are the efficiency-distinguishing properties which a refrigerant would have vs. steam in a closed-cycle system?

Little by little, I think I'm getting something of a handle on this; and hope I'm not being too much trouble along the way.

Whew! Thank you again for your help; and have a great day

Great discussion! Thanks for the information. I believe that I've taken home some significant value here; and I hope the thread is useful to other visitors who stop by in the future. I think that the collective matter which we've explored has sunk the last nail in this for me.

Again, I apologize for having been an ongoing confusing influence vs. proper nomenclature: The term "ORC" is what I was seeking to communicate to describe power-generation settings which utilize refrigerants as the working fluid of choice.

Well, that's about it for me here without going way off-topic. I might open a new thread to better understand why, as I have discovered today, overall efficiencies for power generating systems utilizing the ORC are seemingly so low: 10% looks like the norm

Thanks again, and have a great day!

Ahhhhhhhhhhh - yes .......... the Organic Rankine Cycle - it seems I lost touch with that topic amidst the other topics.

The properties of the working fluids there (including refrigerants) are low boiling point so that a lower temperature input can be used with a sufficient pressure drop to obtain reasonable work output, and still keep the fluid in a vapour state.

XMech.

This site on recovered steam energy you may find interesting.

Regards JD.

Tremendous!

From Wiki:

"There is considerable work being done to develop an enhanced version of a gas-turbine power production cycle to operate at temperatures near 550°C. This is a significant usage, which could have large implications for bulk thermal and nuclear generation of electricity, because the supercritical properties of carbon dioxide at above 500°C and 20 MPa enable very high thermal efficiencies, approaching 45 percent. This could increase the electrical power produced per unit of fuel required by 40 percent or more. Given the huge volume of extremely polluting fuels used in producing electricity, the potential environmental impact of such an efficient cycle could be very large.^[1]"

"Supercritical carbon dioxide is also an important emerging natural refrigerant, being used in new, low carbon solutions for domestic heat pumps.^[6] These systems are undergoing continuous development with supercritical carbon dioxide heat pumps already being successfully marketed in Asia. The EcoCute systems from Japan, developed by consortium of companies including Mitsubishi, develop high temperature domestic water with small inputs of electric power by moving heat into the system from their surroundings. Their success makes a future use in other world regions possible.^[7]"

"Liquid carbon dioxide (industry nomenclature R744 / R-744) was used as a refrigerant prior to the discovery of R-12 and is likely to enjoy a renaissance due to environmental concerns. Its physical properties are highly favorable for cooling, refrigeration, and heating purposes, having a high volumetric cooling capacity. Due to its operation at pressures of up to 130 bars, CO₂ systems require highly resistant components that have been already developed to serial production in many sectors. In car air conditioning, in more than 90% of all driving conditions, R744 operates more efficiently than systems using R-134a. Its environmental advantages (GWP of 1, non-ozone depleting, non-toxic, non-flammable) could make it the future working fluid to replace current HFCs in cars, supermarkets, hot water heat pumps, among others. Some applications: Coca-Cola has fielded CO₂-based beverage coolers and the US Army is interested in CO₂ refrigeration and heating technology.^[14][15]"

"By the end of 2007, the global car industry is expected to decide on the next-generation refrigerant in car air conditioning. CO₂ is one discussed option (see The Cool War)."

Exciting! Thanks for bringing this to my attention.

Great fodder to throw in on my next thread dealing with steam vs. other working fluids in the Rankine Cycle . . .

Ciao!