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Combined Cycle Power Plant for Hybrid Aeroplane.

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Background

Inverted Gas Turbine

From the Institute of Combustion Technology - DLR

MGT = Micro Gas Turbine

IBC = Inverted Brayton Cycle

In the inverse micro gas turbine process, the flow of the individual components occurs in a different sequence than the conventional MGT cycle (Brayton cycle); see Figure 2. In an IBC-MGT, the process air is fed directly into the recuperator (2), heated and forwarded to the combustion chamber, where it is mixed with the fuel and burned (3). After exiting the combustion chamber, the exhaust gas is led through the turbine, where it expands below ambient pressure (4). Subsequently, the exhaust gas is first cooled in the recuperator (2) and then in the downstream water heat exchanger (5), before being compressed to atmospheric pressure in the compressor (1). In this inverse cycle - as in a conventional cycle the turbine again generates more energy than is required for the compression process, so that a generator (6) can be run. As a result of the compression, the exhaust gas is heated, and can thus be cooled down again in a second water heat exchanger (7) in order to exploit as much of the released energy as possible.

Figure 2

Ignore the blue line in the above diagram for the basic cycle without exhaust gas recirculation.

Kalina Cycle

The Kalina cycle is a rankine cycle like a steam cycle. It uses a mixture of ammonia and water. This reduces the boiling point of the mixture.

Description

Air (the green arrows) passes through a heat exchanger and is heated. This heat exchanger acts as a condenser to a separate closed rankine cycle (water-ammonia). Then the air enters an inverted brayton cycle. It passes into a combustion chamber, and then is expanded to subatmospheric pressure in a turbine. The air is then cooled in a 2nd heat exchanger, then compressed, and finally passed through a final heat exchanger and back out to the atmosphere. The work extracted as it expands through the turbine is less than the work taken to compress it.

Meanwhile the water-ammonia cycle is being progressively heated by the second two heat exchangers. This fluid is then expanded through a turbine before passing into the initial heat exchanger/condenser.

The turbines and compressor are linked through a single shaft to a generator that charges a battery or powers the plane.

Operation in Flight

The plane takes off and climbs to altitude on battery power using an electrically powered propeller. Once at altitude it switches from the battery to the combined cycle power plant.

Motivation and Synergies

At altitude the incoming air is very cold this increases the effectiveness of the condenser, reduces its size, and improves the efficiency of the cycle.

Questions

  1. Are the heat exchangers too heavy?
  2. Could we just use a conventional steam cycle in place of the kalina cycle?
  3. Would the combustion chamber need a catalytic section to stay lit?
  4. What combustion temperature would optimise the system?
  5. Could one remove the generator altogether and use this powerplant for thrust?
  6. Would this be better used as a hybrid electric APU because it is too heavy?
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    $\begingroup$ Very interesting but I doubt anyone can answer this vast amount of questions, most questions require a research to answer. SE usually requires a single question rather than a series of questions. E.g. "heat-exchangers too heavy" could be answered with yes and no as it depends on the type and the effectiveness, the higher the effectiveness, the more surface area the more mass. $\endgroup$ – 0scar Dec 14 '20 at 10:54
  • $\begingroup$ @Oscar Maybe I could just edit the questions to -> question 'Would this powerplant have an adequate power to weight ratio?' $\endgroup$ – Tobe Dec 14 '20 at 11:20
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    $\begingroup$ The bottom-line is whether these questions are applicable for SE at all. Focusing on the fist question, it cannot be answered IMHO, this needs power requirements and more. Generally, heat-exchangers are heavy and voluminous so the answer is probably no, but without a proper research this is guesswork. The subject is interesting and few papers on closed gas turbine cycles have been published. Basically the IBC is a complicated system to generate heat for closed cycle. $\endgroup$ – 0scar Dec 14 '20 at 11:30
  • $\begingroup$ the problem of "would x" questions, in my view, is that there is no hard data to base an answer upon, and thus they require some degree of opinion in answering. $\endgroup$ – Federico Dec 14 '20 at 13:22
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    $\begingroup$ I think it would be best to first ask about the realistic max efficiency of this kind of a contraption on physics SE, perhaps evaluate somehow a realistic minimum weight of such a system and then compare those to modern turbine engine. Practicality in aviation also requires very high reliability, so this is no easy question to answer... in any hybrid system the current limiting factor is the weight of the battery system. Hauling tons of batteries high up in the air and keeping them there is generally not very efficient ;) $\endgroup$ – Jpe61 Dec 14 '20 at 17:17

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