Anything better than boiling water?

[jasonpetras]

Recently, I've seen a number of topics here about energy sources. The one thing they all seem to have in common their energy conversion method: boil water to turn a turbine (of course there are some exceptions hydro-dams just turn the turbine, no boiling necessary, Windfarms use wind to turn turbines). But are there any other methods for first part of the energy conversion? Especially in nuclear energy sources. Although I am a firm believer in KISS (keep it simple stupid) and I know that motors/turbines are high efficient devices (>90%), it seems that there should be higher efficiency ways of doing this. Any ideas from the theoretical physicists?

 

[KenGr]

Water is convenient because of it's phase change properties. It is easy to pump a liquid and bring it back up to pressure after it expands through a turbine. However, the energy transfer required to condense the steam after it leaves the turbine gives you a certain energy loss that does limit the efficiency. More efficient systems are possible based on all gas cycles but these have other limitations. Gas turbines have higher efficiency when coupled with a heat recovery steam cycle (this is called combined cycle and involves two turbines - one a combustion gas turbine and one a steam turbine).

With a given thermodynamic cycle, the secret to high efficiency is ultimately increasing the high temperature. Simplistically (looking at a Carnot cycle, the most simple thermodynamic cycle), cycle efficiency is the working fluid temperature change (supply temperature minus exhaust temperature) divided by the supply temperature (all in absolute temperature units, of course). Since exhaust temperature is ultimately limited by the available heat sink temperature, higher supply tempertures will increase efficiency.

With this we reach material limits as a practical limitation. Finding metals for turbines that will withstand higher temperatures is critical. Gas turbines have internal cooling systems but strength requirements ultimately limit this. Research continues on ceramic turbine materials.

There are nuclear reactors that operate on gas cycles instead of water cycles. The Pebble Bed reactor is the latest version of the High Temperature Gas Reactors. Again materials are the limitation and new turbine and generator designs are required to realize the full potential. The efficiency of HTGRs could theoretically be almost 50% compared the values in the mid-30s we see with light water reactors. (Keep in mind that light water reactor cycles using superheat have been proposed that would probably exceed 40% but were not thought to be cost effective due to the very low cost of nuclear fuel.)

 

[Shalmanese]

Basically, the most efficent way of making electricity is to turn a couple of magnets in a circle and the most efficent way of turning something is via a turbine. A turbine actually only has something like 40 - 50% efficiency at best but its the best way to make electricity.

 

[Demon-Xanth]

The advantages of boiling water is the temperature range is reletively moderate, the material is clean and plentiful, and the boiling produces pressure. This pressure is what makes the whole thing work. Just using hot water wouldn't gain anything at all.

 

[dakels]

Why can't you put magnets in an alternator type device and have them push the cylinder around? Super strong magnets, like superconductors facing in an angle against each other to propel the cylinder around and around. Strips of them so as the one pushes the other, it reaches the next strip and gets another push, etc. There is no way to make this thing perpetual or near perpetual? When a superconductor is repelling another magnet in suspension, some sort of energy is keeping this thing afloat from the forces of gravity no? This energy is nearly perpetual so long as the superconductor retains it's current? Couldn't this be used to propel an object in a constant centrifugal motion to stay power a turbine?

I realize the idea is simple so therfore I imagine it isn't possible in the way I describe it, but why?

 

[f95toli]

You could. But what would be the point? You would need more energy to push the cylinder around than you could get from the cylinder.
In any electrical power plant you will have a rotating turbine, the turbine generates electricity by moving magnets in a circle which induces currents in some type of coil. The basic principle is the same as for the thing you put on the front wheel to power the lights on your bicycle.

About the superconductor idea. You are right that one can use superconducors to make very efficient generators (a 100 MW prototype is beeing built by GE right now) but the principle you are describing does not work for the simple reason that it does not conserve energy, as soon as you "use" some of the generated current to do something uselfull the turbine would stop rotating (unless of course you add more energy to the turbine).
The fact that the generator is superconducing is irrelevant in this case. Superconducting generators are efficent,cheap and small but thats it, there is no way to "make" energy using superconductivity (or anything else).

 

[Demon-Xanth]

I believe a better way to look at a generator is this:

A generator converts mechanical or chemical energy into electrical energy.


The turbine is simply a way of taking moving water or steam and converting it to a rotating force that runs the generator itself.


Edit: damn tab...

 

[dkozloski]

The property of water that is the hardest to improve on is the extremely high latent heat of vaporization, 540cal./gram. The tremendous release of energy that takes place with the transition from steam to water is hard to beat. When the famous inventor Bill Lear was trying to reinvent the steam car/bus he offered an extremely high prize for anyone that could produce a better working fluid no matter how exotic. He never had to pay the prize and there are no practical steam cars on the road.

 

[PowerEngineer]

Originally posted by: dakels
Why can't you put magnets in an alternator type device and have them push the cylinder around? Super strong magnets, like superconductors facing in an angle against each other to propel the cylinder around and around. Strips of them so as the one pushes the other, it reaches the next strip and gets another push, etc. There is no way to make this thing perpetual or near perpetual? When a superconductor is repelling another magnet in suspension, some sort of energy is keeping this thing afloat from the forces of gravity no? This energy is nearly perpetual so long as the superconductor retains it's current? Couldn't this be used to propel an object in a constant centrifugal motion to stay power a turbine?

I realize the idea is simple so therfore I imagine it isn't possible in the way I describe it, but why?

The short answer is no. You may want to consider the difference between force and energy. Two magnets (oriented so that like poles face each other) will exert a replusive force against each other, and as you suggest this replusive force can be used to suspend a magnet (and whatever it's attached to) in mid-air (provided the replusive force at some distance between the magnets matches the gravitational force). That said, this suspension requires no energy to maintain it. Neither does the upward force provided by the chair you are sitting in. Energy (or work) is the product of the distance an object is moved against a force. A suspended object has the upward force of replusion balanced against the force of gravity. Moving the object in the vertical direction will take energy; moving it in the horizontal direction will take no energy (as a result of the replusive or gravitational forces).

Also bear in mind that an electrical generator (rotating magnet producing changing magnetic fields through surrounding coils resulting in induced electrical currents) produces torque on its rotor in opposition to its direction of rotation. That's why that rotor must be connected to some turbine that must apply an opposing torque (i.e. in the direction of rotation) in order to keep the generator rotor turning. This means that a magnetically suspended cylander like the one you describe would be quickly brought to a halt when attached to a generator.

 

[PandaBear]

Guys, remember this, you don't get something for nothing according to the law of physics. The superconductor bounce around idea will just bounce around until it sit comfortably in a place suspended, not going around and generate indefinite amount of energy.

Regarding to efficiency. One can get better efficiency out of fuel cell than any combustion or fuel buring related generation. The problem with that is to find pure hydrogen to get to use so it doesn't pollute the fuel cell. Currently work is being done on methanol conversion but it is still not ideal.

I still think the french way of doing it is the best. Use Uranium to get electricity, and then recycle the waste back into the fuel (plutonium) and extract the unusable and highly dangerous waste like americanium together. It will be more radioactive but the half life is greatly reduced (from 1000s of years to around 30 years). Problem solved.

Only if bush is willing to do the right thing than sponsoring all the Texas based energy company and oil company.