Increasing heat transfer efficiency for low temperature turbines

[ponderer]

I have a design outline, and I wonder if I could get comment.

 

I have been giving a great deal of thought to very low temperature geothermal power generation. The problem seems to be heat transfer efficiencies at low temperature differentials. Low temperature 190F geothermal uses a binary cycle to turn turbines.

 

I propose here the use of a coupled binary cycle system to generate power at very low temperatures.

 

There are a number of inefficiencies to overcome, to power the compressor for the initial earth loop cycle. No pump is needed for circulation, but a compressor must be powered, in order to provide the heat source. This compressor requires a power source.

 

First let us start with a standard ground loop for DX type geothermal systems in say 56F soil.

 

We may expect to raise the temperature to 108F. We are getting 400% recovery of heat vs energy to run the compressor. It is a geothermal heat pump.

 

Such a temperature differential results in poor enegry recovery for electrical generation, and very inefficient operation. Power output could not run the cycle 1 compressor.

 

Here I propose to possibly increase conversion efficiency, and create a kind of geothermal jet engine.

 

The first part of the unit is a flow through compressor that compresses the cycle 1 refrigerant into a flowing hot liquid, that is pushed backward, to what would be the combustion chamber in a jet engine.

 

Now this is the questionable bit.

 

Here I propose to have many short small parallel tubes, with all the ends gathered to form an overall donut shape. It would be like a bundle of short straws arranged in a donut pattern as seen from the ends. These tubes have a shared corresponding donut shaped reservoir on either end. Through the center of each tube passes a coaxial slightly smaller diameter tube, which passes through the outer walls of the shared reservoirs.

 

On the same end of each of the smaller piercing tubes, a cap containing an atomizer is installed. The other end is left open. All the atomizing caps are fed from a shared donut shaped pressurized cycle 2 liquid refrigerant reservoir. This reservoir might be pressurized by centripetal force by spinning about the main axle.

 

The open ends of the tubes forming a donut are positioned to face turbine blades.

 

Hot cycle 1 refrigerant flows backward from the flow through compressor into the front donut reservoir, and passes as a thin layer between inner and outer tubes, from the front reservoir, to the rear reservoir, where it is drained away, back to the ground loops to become steam again, and pick up heat.

 

On the same axle as the flow through compressor and donut heat exchanger, are the turbine blades powered by the cycle 2 steam. The cycle 2 refrigerant is atomized into the small tubes at the closed end, with small droplets hitting the tubes walls and changing state to a gas instantly, causing steam thrust out the open end of all the little tubes. You have a multitude of steam jets, each with a constantly renewed heat jacket and refrigerant mist, which could be balanced to produce a steady jet stream from each jet.

 

Could such an arrangement break even or even produce excess power on such a low temperature differential?

 

The idea is to increase overall system efficiency by running the whole process on the same axle, and perhaps the steam jet donut might increase energy transfer efficiency.

 

Another ground loop or a condenser might be used to condense the cycle 2 refrigerant.

 

Would such a scheme even increase efficiency over other methods of heat transfer?

 

If such a scheme could work, I would imagine putting a DC motor on the same axle for battery startup, which would then be used as a generator under operation.

 

Output could be governed by cycle 2 refrigerant feed pressure.

 

Total electrical output would have to be a continuous 2-3kW to be useful for microgeneration.

 

The target would be a cost effective, weather, time of year, and time of day independent, and consistent 48-72kWh per day, at 2-3kw, with batteries storing excess power and draining on peak loads.

 

So let's start with some intial numbers.

 

We have a ground temperature of 56F or lower assuming we will be constantly drawing energy from the ground, using a sufficient amount of ground loop, so that the energy is replaced as fast as it is used.

 

Now, we can heat our cycle 1 refrigerant to 110F, and the best cycle 2 refrigerant I can find that will condense in a ground loop of 56F yet boil at a temperature lower than 110F, is R-123 which has a boiling point of 82F.

 

So we would be misting tubes at 110F with a mist that boils at 82F, in order to create our steam jets, to power the turbines. It should be a matter of figuring out the output of one stem jet and then summing their combined output.

 

One would expect that the smaller the diameter of the tubes, the greater the spray contact and so the higher heat transfer, the higher the steam pressure, within practical limits. A mister that projects a circular pattern at the tube walls might be more effcient than a generalized spray.

 

Flow rate of the cycle 1 refrigerant would have to be a factor, as well as the quantity of cycle 2 coolant sprayed. The two would have to balanced to optimize heat transfer.

 

If the cylce 1 refrigerant flows too fast, energy is wasted. If it flows too slow not enough heat is transferred to optimize the steam pressure produced. Further, if too much cycle 2 refrigerant is sprayed, it can cool the tubes and reduce steam production, with cycle 2 refrigerant running out the ends of the tubes. Decreasing the cycle 2 refrigerant spray rate will reduce steam pressure and would allow for slowing turbine speed.

 

The question is, if such a turbine were built, and the DC motor was started up to turn the compressor and start the heat pumping, could the this be self sustaining and even create extra power, turning the DC motor into a generator?

Edited November 22, 2010 by ponderer

[CaptainPanic]

I know (from your responses on other threads) that you dislike negative feedback... So, I am sorry in advance.

Hopefully you still want to read and learn.

I have been giving a great deal of thought to very low temperature geothermal power generation. The problem seems to be heat transfer efficiencies at low temperature differentials. Low temperature 190F geothermal uses a binary cycle to turn turbines.

No, heat transfer is not a problem. It's the thermodynamic efficiency.

 

Heat transfer, mass transfer, geometries, number of tubes, instantaneous evaporation, and all that other stuff is just "design"... but design is irrelevant if the thermodynamics don't add up.

 

Thermodynamic limitations cannot be overcome. You try to create a complicated design to overcome thermodynamics limitations, but that's just a waste of time.

 

If your invention is going to work, it will have to abide the main laws of thermodynamics... so you must be able to describe everything in simple words, and using only terms of energy (not heat transfer).

[ponderer]

I know (from your responses on other threads) that you dislike negative feedback... So, I am sorry in advance.

Hopefully you still want to read and learn.

 

No, heat transfer is not a problem. It's the thermodynamic efficiency.

 

Heat transfer, mass transfer, geometries, number of tubes, instantaneous evaporation, and all that other stuff is just "design"... but design is irrelevant if the thermodynamics don't add up.

 

Thermodynamic limitations cannot be overcome. You try to create a complicated design to overcome thermodynamics limitations, but that's just a waste of time.

 

If your invention is going to work, it will have to abide the main laws of thermodynamics... so you must be able to describe everything in simple words, and using only terms of energy (not heat transfer).

 

 

Sorry, it's not the negative feedback. It's the can't do attitude. I would not employ any of you lot to solve a problem.

 

Here is the beginnings of a solution.

 

Efficiencies of 500% can be acheived with a currently available heat pump.

 

20% thermodynamic efficiency would be needed to sustain a continuously self powered heat pump, if the mechanical energy can be delivered with 100% efficiency to the cycle 1 compressor.

 

If we change the DX ground loop to an insulated tank, we my lower the temperature in the tank to below that of the surrounding 56F geology. We may keep the tank as a reservoir at a constant temperature buy drawing out heat as fast as it is replaced, and keep the tank at -35F.

 

We draw the heat out at 500% efficiency, and heat our cycle 1 refrigerant to 125F.

 

The underground tank acts as a condenser for the cycle 2 refrigerant.

 

We have 235.8K and 324.7, which gives a theorertical thermodynamic efficiency of 27.4%.

 

Hum, imagine that.

Edited November 24, 2010 by ponderer

[swansont]

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[CaptainPanic]

Sorry, it's not the negative feedback. It's the can't do attitude. I would not employ any of you lot to solve a problem.

 

Here is the beginnings of a solution.

 

Efficiencies of 500% can be acheived with a currently available heat pump.

 

20% thermodynamic efficiency would be needed to sustain a continuously self powered heat pump, if the mechanical energy can be delivered with 100% efficiency to the cycle 1 compressor.

 

If we change the DX ground loop to an insulated tank, we my lower the temperature in the tank to below that of the surrounding 56F geology. We may keep the tank as a reservoir at a constant temperature buy drawing out heat as fast as it is replaced, and keep the tank at -35F.

 

We draw the heat out at 500% efficiency, and heat our cycle 1 refrigerant to 125F.

 

The underground tank acts as a condenser for the cycle 2 refrigerant.

 

We have 235.8K and 324.7, which gives a theorertical thermodynamic efficiency of 27.4%.

 

Hum, imagine that.

So you want to build a heat engine which has a warm side of +52C (or 125F) and a cold side of -37C (or -35F). Your temperature difference is therefore 89 degrees Celsius. And you want to operate at 500% efficiency (which we shall call a Coefficient of Performance of 5, to use the heat pump vocabulary).

 

We can immediately conclude (using a table of types of heat engines which I gave you before in a previous thread) that you're in the range of some theoretical (and non-existent) heat pump. Only the theoretical Lorentz Cycle heat pump can achieve this... which means that it may work, using the Lorentz cycle (link to more literature provided in wikipedia) and that you really must push all other efficiencies to nearly 100% (no friction, no losses).

Good luck in designing such a heat pump.

 

But anyway, using your numbers, and your assumptions (which I maintain are overly optimistic), you would be able to generate some heat which would indeed be useful for electricity generation. And indeed you would make more than you use in the cycle.

 

But: you also have your second cycle which requires another heat pump... which has its own COP. As far as I can see, you haven't taken its energy consumption into account at all.

 

But of course, thermodynamics as a whole suffers from the "can't do attitude".

Carnot is such as bastard.

Edited November 24, 2010 by CaptainPanic

[ponderer]

So you want to build a heat engine which has a warm side of +52C (or 125F) and a cold side of -37C (or -35F). Your temperature difference is therefore 89 degrees Celsius. And you want to operate at 500% efficiency (which we shall call a Coefficient of Performance of 5, to use the heat pump vocabulary).

 

We can immediately conclude (using a table of types of heat engines which I gave you before in a previous thread) that you're in the range of some theoretical (and non-existent) heat pump. Only the theoretical Lorentz Cycle heat pump can achieve this... which means that it may work, using the Lorentz cycle (link to more literature provided in wikipedia) and that you really must push all other efficiencies to nearly 100% (no friction, no losses).

Good luck in designing such a heat pump.

 

But anyway, using your numbers, and your assumptions (which I maintain are overly optimistic), you would be able to generate some heat which would indeed be useful for electricity generation. And indeed you would make more than you use in the cycle.

 

But: you also have your second cycle which requires another heat pump... which has its own COP. As far as I can see, you haven't taken its energy consumption into account at all.

 

But of course, thermodynamics as a whole suffers from the "can't do attitude".

Carnot is such as bastard.

 

These people seem to think that have it worked out.

 

http://endlessenergyinc.com/how-it-works/

 

I guess you would call it a scam. Can't be done.

 

Well that is what I'm trying to figure out.

 

Seems you wouldn't buy one.

 

I'm considering a simpler heat transfer process, with "ideal" system efficiencies, to get a grasp if this is even feasible.

 

I'm seeing breakeven maybe, if you factor in real world system losses. I don't see much headroom for excess mechanical energy, and I am considering a wider range of temperature than they are using. Perhaps the pressure differential that they are using should be considered.