Recycling Heat Energy for Engines: Exploring Countercurrent Exchange

[max efficient]

When thinking about how everyday engines could be made to be more efficient . I remembered a biology lecture, where I was taught that the blood in the gills of a fish, move in the opposite direction of water flow when they swim, allowing them to absorb a higher level of oxygen from the water. The apposing blood to water flow maintains a gradient of greater oxygen concentration in the water the whole time it passes through the gill causing much more oxygen exchange than that which would occur if the blood and water flowed in the same direction.

The principle fish take advantage of is known as countercurrent exchange and not only can it be utilised to oxygenate blood but also to exchange heat between two fluid bodies. See the wiki for more http://en.wikipedia.org/wiki/Countercurrent_exchangeSo can this principle be used to make more efficient engines?

Here is my attempt with "The Recycling Heat Engine System" see diagram one http://i1098.photobucket.com/albums/g362/max_efficient/P1020268.jpg

Components of the system

The spotted fluid = fluid which is pumped along a pathway in the system to carry heat from one component to another.

The box with H = the heat source (constant temperature maintained by energy input).

The horizontal lined junction = Where the spotted fluid enters and exits the heat engine.

The box with E = the heat engine, where heat energy is converter to useful forms.

The vertical lined junction = where the clear fluid enters and exits the heat engine.

The clear fluid = fluid which is pumped along a pathway in the system to carry heat from one component to another. (does not mix with the spotted fluid)

The box with R = the recycler, where heat energy is conducted from from the clear fluid to the spotted fluid.

The box with C = the sink, where the clear fluid is cooled to a specific temperature

How the system operates

(1) The spotted fluid is heated by the heat source to a specified temperature.

(2) The Spotted fluid enters the heat engine.

(3) The heat engine or high conduction compound heat engine (see diagram 2 http://i1098.photobucket.com/albums/g362/max_efficient/P1020272.jpg) has the spotted fluid move along a series of stirling heat engines. Heat energy moves through the heat engines from the spotted fluid into the clear fluid with some of the energy converted by the heat engine into useful forms. Note this is a countercurrent exchange and that if maximised the exiting clear fluid is hotter than the exiting spotted fluid, which is essential for the next process.

(4) due to the heat gradient between the two fluids, the recycler uses countercurrent exchange to ensure that a maximum amount of heat energy moves from the clear fluid to the spotted fluid.

(5) the clear fluid is then cooled more, down to a specific temperature, to maintain a consistent temperature gradient for the system in the sink.

In a normal heat engine system, the clear fluid would dump all the heat energy it receives in the sink. Due to this the spotted fluid is not pre-warmed, before reaching the heat source and then requires extra heat energy, for it reach the specific temperature.

The Logic of system seems right. But the second law of thermodynamics suggests otherwise.

So is my logic bad or do I misunderstand the second law?

P.S
Sorry about the quality of the diagrams, I'm ill in a hostel in Spain, with a lousy little laptop with no programs to draw things. Also i don’t see any reason why the recycler couldn’t just be another high conducting compound heat engine. Not sure if there would be an advantage.

 

[K^2]

You are only loosing energy. The only thing an exchanger does, is allowing you to maintain a temperature difference between two locations despite current exchange between them. That's not what you want. You want the hot fluid to be as hot as possible and cold fluid to be as cold as possible. No exchanger is needed for that, if you already have a heat source and cold source. Anything else you do will only reduce the efficiency.

 

[netheril96]

No.
To recycle heat, you have to put energy in which in effect will reduce the overall energy output.

 

[max efficient]

I'm not getting both, can you both elaborate? The intention of the exchanges is to use energy that is usually exhausted from the system into the sink, to lessen the amount of heat energy required to make the hot component of a heat a specific temperature without reducing the heat gradient. can you see how this is achieved? what is the specific problem in this situation?

 

[Curl]

The problem is that the exhaust heat is at a lower temperature than the hot source.

Heat moving spontaneously from a colder object to a hotter object macroscopically is improbable (in other words impossible).

 

[max efficient]

At no point during the process do i see a deffience of the clausius statement occurring, nor is the exhaust heat hotter than the hot source. The heat source is the hottest thing in the system.

Imagine a system that is very standard (different to the on depicted). fluid passes through a heat source which heats it. It then flows over a plate of a stirling engine which then absorbs some of the heat. the fluid which is now cooler is pumped back to the heat source to continue the cycle. The stirling engine conducts the majority of the heat energy to the cooling loop and the rest is converted to usefull forms. In the cooling loop, fluid is heated by the other striling engine plate. the fluid then flows to the sink, where it is cooled to a particular teperature, before moving on back to the plate to absorb heat energy from the engine.

In the standard system, the exiting heating loop fluid can only be hotter or the same temperature as the exiting cool loop fluid. As the reycyling system has a type of engine that makes use of countercurrent flow, the exiting cool cycle is hotter than the the exiting heat cycle. What this means is that the energy passing though the engine is higher.

So all this heat energy that passed through the system isn't dumped in the sink. One can take advantage of countercurrent flow, to swap the temperatures of the cooling loop and the heating loop, so this way less heat energy has to be added to the system, to warm the heating fluid to the specific temperature and less has to be lost to the sink.

 

[Curl]

The "counter-current flow" is useless and does nothing to improve efficiency of an ideal engine. Messing around with the outflow will only reduce efficiency. You need to see the whole picture instead of focusing on one part of the engine; this will cause you to think you've improved efficiency somewhere but neglect to see that you reduced it in some other way.

The more familiar you are with engine cycles, the more apparent it is.

 

[Drakkith]

In the standard system, the exiting heating loop fluid can only be hotter or the same temperature as the exiting cool loop fluid. As the reycyling system has a type of engine that makes use of countercurrent flow, the exiting cool cycle is hotter than the the exiting heat cycle. What this means is that the energy passing though the engine is higher.

If the heated fluid gets heated to the same temp, or close to the same temp as the heat source, simply adding in your recycler will not accomplish anything. Putting heat back into the fluid that's to be heated won't accomplish anything in this circumstance, as it will only get as hot as the heat source.

However, if your heated fluid is not reaching the temp of the heat source, then adding the recycler could improve the efficiency by causing the fluid to be warmer when it reaches the heat source and enable it to be heated to a higher temp. However you can design a heat engine to easily work around this by ensuring that the fluid is heated as high as possible while it is in contact with the heat source.

 

[max efficient]

If the heated fluid gets heated to the same temp, or close to the same temp as the heat source, simply adding in your recycler will not accomplish anything. Putting heat back into the fluid that's to be heated won't accomplish anything in this circumstance, as it will only get as hot as the heat source.

The recycler adds efficiency to the system by heating fluid closer to the heat souce temp than it would otherwise be without the recycler. As the fluid is already pre warmed it takes less energy to to warm the fluid to the temperature specified for it, in the heat source.

It takes less energy to boil water that is a temp of 70c, rather than 4c before it is boiled. Can you understand how the intent behind matches with principles used in the system?

 

[netheril96]

The recycler adds efficiency to the system by heating fluid closer to the heat souce temp than it would otherwise be without the recycler. As the fluid is already pre warmed it takes less energy to to warm the fluid to the temperature specified for it, in the heat source.

It takes less energy to boil water that is a temp of 70c, rather than 4c before it is boiled. Can you understand how the intent behind matches with principles used in the system?

Well, I admit that I had a knee-jerk reaction discounting such efficiency claims as violating second law. But really I have a hard time understanding your design. Please re-explain your idea in a concise manner and cut out all those technical details.

 

[Drakkith]

The recycler adds efficiency to the system by heating fluid closer to the heat souce temp than it would otherwise be without the recycler. As the fluid is already pre warmed it takes less energy to to warm the fluid to the temperature specified for it, in the heat source.

It takes less energy to boil water that is a temp of 70c, rather than 4c before it is boiled. Can you understand how the intent behind matches with principles used in the system?

Yes. And my post was agreeing with you. IF the fluid is heated to the same temp, or very near the same temp, as the heat source, then there is no need for the recycler. If it is not however, then the recycler will improve efficiency by pre-heating the fluid so that it can then be pass through the heat source and exit at a higher temp than it would otherwise.

 

[max efficient]

IF the fluid is heated to the same temp, or very near the same temp, as the heat source, then there is no need for the recycler.

I'm not getting you on this! After going through the heat engine the recycler heats the fluid and then it passes to the heat source to get even hotter. I don't get how if the fluid exits the same temp as the heat source, from the heat source you wouldn't need the recycler. when it exits the heat engine it is colder than after the recycler regardless of what temperature it was heated to in the heat source. If you cut out the recycler you have a lot more energy being dumped in the sink and a lot more energy being consumed by the heat source.

You can though get rid of the recycler if you use just one fluid and have it operate a figure 8flow, but I wanted to drive the effect of countercurent flow into every ones minds. Having two different fluid loops also made it a little easier to explain what is going on in the engine itself. I was also thinking of nuclear power systems when i came up with the system, which would have separate loops as a safety measure.


netheril96, maybe you will understand the figure 8 system better. The important thing is that you understand a Stirling engine doesn't work like a steam turbine. you simply heat one end and have the other end kept cooler to have it operate. have a look at the wiki if needed.

Fluid is heated in a heat source then it is pumped into the high conduction compound engine at junction A. In the engine the heat movies from this fluid into the Stirling engines. the fluid exits at junction B. the fluid is then cooled in the sink and passes to junction C where it then absorbs heat energy from the Stirling engines until it exist at junction D. From there it then moves on to be heated at the heat source once again.

The Important thing is the countercurent flow allows for the fluid coming out of junction B to be cooler than the fluid coming out of junction D. Energy has passed though the engines without the heat gradient diminishing as it would usually, it remains partially intact.

The multiple Stirling engines are needed, for if one were used, the temperature for a give location would not translate to the opposite side, which would cause their to be no effect to result from countercurrent flow. The more engines the lest diminished the heat gradient will be.

When i came up with this recycling system, i was thinking nuclear power, but quickly i realized that you could replace the sink and heat source with a heat pump that uses less energy to form the gradient, then the engine extracts. This would be type two perpetual motion, very much in violation of the second law.

 

[QuantumPion]

This already exists, it's called a http://en.wikipedia.org/wiki/Feedwater_heater" .

 

[max efficient]

"The energy used to heat the feedwater is usually derived from steam extracted between the stages of the steam turbine. Therefore, the steam that would be used to perform expansion work in the turbine (and therefore generate power) is not utilized for that purpose." wikipedia on feedwater heater

the system depicted on wikipedia is not the same system. Its a steam turbine system and heat is being extracted from the heat engine rather than from the exhausted steam. Extracting heat from the engine is going to reduce useful energy output, where as from the exhaust steam, no reductions in output would occur as a consequence.

How I think countercurrent exchange could be used in a steam power system for greater efficiency.

see this
http://en.wikipedia.org/wiki/Therma...of_a_typical_coal-fired_thermal_power_station

If you look at 8 the condenser, its probably using counter current flow; but the reason it is using it, is to cool the steam as much as possible to create a heat gradient; Dumping all the energy the cooling water receives in the cooling tower. How i would change the system, would be to get rid of the standard feedwater heater and replace it with a two part feedwater heating condenser.

The idea is that you pump water at the point of the feedwater heater through part one of the condenser that uses countercurrent flow for maximum heat energy transfer, heating the water before the economiser. Then the ex steam moves to part two where it is cooled by cooling tower water to a specified temperature before it then is pumped to the deaerator.

That system would result in less heat energy being dumped into the cooling tower and more being recycled. Such a system also appears to be beyond the efficiency that the second law allows.

 

[russ_watters]

Without reading the entire thread, it sounds from the first post like you are describing something similar to a recouperator: http://www.turbec.com/products/combustion.htm

 

[Gabe21]

the simple fact is this application of heat transfer is impractical to use on an engine. just using the radiator on a vehicle engine is about twice as efficient as they need to be. that's y all vehicles have thermostats that regulate the cooling system. if the radiator cooling system ran all the time the car engine would never warm up properly. so the heat transfer system is already the most practical. if you wanted to increase effenciency try to make a motor with a lower operating temperature.

your example with the gills on the fish absorbing oxygen is not the same as heat transfer. heat flows from hot to cold. the only way to increase the efficency of heat transfer is to make whatever is absorbing the heat colder, or make it out of something that radiates heat at a faster rate. e.g. aluminum radiates heat faster than steel.

 

[Drakkith]

Gabe he's talking about a heat engine, not a combustion engine.

 

[Drakkith]

I'm not getting you on this! After going through the heat engine the recycler heats the fluid and then it passes to the heat source to get even hotter. I don't get how if the fluid exits the same temp as the heat source, from the heat source you wouldn't need the recycler. when it exits the heat engine it is colder than after the recycler regardless of what temperature it was heated to in the heat source. If you cut out the recycler you have a lot more energy being dumped in the sink and a lot more energy being consumed by the heat source

Ok. Let's call the fluid running through the heat source Fluid A and the fluid running through the sink Fluid B.

First, imagine that there is no recycler/countercurrent. Fuid A exits the heat source at say 400 C when the heat source is at 550 C. It passes through the heat engines, transfers heat to Fluid B via the heat engine and then exits. The fluid is now at say 200 C. Fluid B exits the heat engine at 200 C also and flows to the sink and is cooled to 50 C. Fluid A flows to heat source while Fluid B flows to heat engine and the cycle repeats.

Now, let's look at one with the recycler/countercurrent. Let's imagine there is near perfect heat transfer. Fluid A exits the heat engine at 50 C. Meanwhile, Fluid B exits the engine at 400 C. Both fluids run into the recycler and fluid B is cooled to 50 C while Fluid A is heated to 400 C. So, now you have fluid A at 400 C instead of 200 C when it enters the Heat Source. This warmer fluid is then heated to say 500 C instead of 400 C. On the next go around it is heated even higher, let's say to 550 C, the temp of the heat source. These last 2 cycles will definitely increase the efficiency of your heat engine. But let's look at the 4th cycle.

Fluid A, now at 550 C, enters the heat engine and is cooled to 50 C before exiting. Fluid B is heated to 550 C and exits as well. Both flow into the recycler where Fluid A is heated back to 550 C. Fluid A exits the recycler and enters the Heat Source. What happens now? NOTHING! The fluid is at the same temp as the heat source and cannot be heated any higher! You have hit your maximum efficiency for this setup.

Thats what I mean when I say that if the fluid exiting the heat source is at or near the temp of the heat source, then the recycler and countercurrent exchange do not matter. You can easily design an engine and heat source to get the maximum temperature of Fluid A when it leaves the heat source without the need for the extra components.

 

[max efficient]

It seems you are not considering, that heat energy is lost from the system, when heat energy passes through the heat engine. The biggest problem i have with what your saying is how you describe effciency.

My understanding of efficiency is that it takes a cirtain amount of energy to create a heat gradient and a cirtain amount of energy can be extracted from it. When a heat pump or heat engine is ideal according to theory. It takes as mch energy to create a heat gradient as can be taken from it.

The system raises the problem that you can have energy pass through a heat engine and have it extracted without deminishing a heat gradient as normal. altough the heat gradient has diminished due to the absorbtion of energy even in an ideal system, it can just be topped up, with all the heat energy added to the system converted to useful forms rather than a portion as normal.

In an ideal system you would only require a heat source and you could get rid of the sink; but you could only cut out the recycler in a figure 8 system.

So the minimum components you could have, in an ideal figure 8 system, is the heat source, th engine and the piping.

The major point i want to make, is using countercurent flow in an ideal heat engine system, heat energy can pass through the heat engine where part is extacted without the gradient used being entirely deminished, allwing one to use this gradient once more to operate the heat engine and extract more energy. In an ideal system taking advantage of countercurrent flow, carnot efficiency appears to be exceeded, as all the energy added by a heat source is converted to useful forms, rather than a portion.

 

[max efficient]

your example with the gills on the fish absorbing oxygen is not the same as heat transfer. heat flows from hot to cold. the only way to increase the efficiency of heat transfer is to make whatever is absorbing the heat colder, or make it out of something that radiates heat at a faster rate. e.g. aluminum radiates heat faster than steel.

Oxygen absorption is similar to heat absorption. Oxygen diffuses from places of high concentration to low concentration. Transfer is a result of a concentration gradient.

russ_watters, Yes a Recuperator is essentially what the recycler is. I i don't think in combustion engines they can be used to maximum effect as, you would have problems with them overheating and issues with pressure. Sterling engines can operate at low heats and small gradients, with very high efficiency and would be better at using the effect to a maximum.

Someone needs to show that the countercurrent system would not extract as much energy from a heat gradient as a normal system for the sake of the second law.

This could be done quantitatively. Consider 3 boxes that have a heat capacity of 1 kj per kelvin. one is the hot box and the other 2 are the cool boxes, As depicted
[H]-[1c]
-[2c]

H connects to 1c which is 10k cooler than H, via a Carnot engine, heat is transferred reaching equilibrium with heat part heat energy taken out. H then moves onto 2c which is 10k cooler than H, when it connects after it had formed equilibrium with 1c. heat is transferred via Carnot engine once again.

If one calculates (which i do not know how to) the energy converted into useful from the Carnot engines in this hypothetical. then compares it to if H had connected to just one cool box but reached the same temperature as it does in equilibrium with c2, one can determine if the countercurrent system has an advantage.

The countercurrent system steps energy transfer where as a regular system does not.This stepping has to be less efficient than regular energy transfer in one go, for the sake of the second law.

 

[Drakkith]

The countercurrent system steps energy transfer where as a regular system does not.This stepping has to be less efficient than regular energy transfer in one go, for the sake of the second law

I think you are misunderstanding it. Each heat engine can only produce a certain amount of power or work depending on the temperature difference between its hot and cold sides and various other effects. Your countercurrent simply increases the amount of heat transferred between heat engines. You are still limited to the carnot efficiency in every heat engine.

The system raises the problem that you can have energy pass through a heat engine and have it extracted without deminishing a heat gradient as normal. altough the heat gradient has diminished due to the absorbtion of energy even in an ideal system, it can just be topped up, with all the heat energy added to the system converted to useful forms rather than a portion as normal.

The process of converting heat energy to work in a heat engine WILL result in a loss of energy. Meaning that if you heated the fluid and then removed the heat source, you could NOT run the heat engine forever since every cycle would result in a loss of energy.

 

[max efficient]

I agree that each heat engine inside will only operate at carnot efficiency, but the system as a whole appears not to. I also agree with your second statement, heat energy is lost from the system hence the need for a heat source. The problem is that entropy is reduced.

If you had the fluid flow concurrent in the compound engine. The out flows would be the same temperature. you couldn't flow them into another compound engine and extract power out of them because their is no haet gradient between the exiting fluids. where as the countercurrent flow there is, so you would be then able to harness more energy.

Do you understand the signifcance of the heat gradient existing between the exhausts?