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The Hydraulic Gas Turbine uses hydraulics to improve the efficiency of gas turbines. Meaning the combustors are replaced by hydraulically powered compressors and the heating of the airflow is done by compressing the airflow to a very high compression ratio, and not by burning fossil fuel.

How it works


The starter engine (1) turns the compressor (1) which takes in air (20ºC), compresses it to 17 bar (increasing its temperature to ~ 400ºC) and directs the compressed air to a cylinder (3).

A hydraulic pump/motor (4) drives, via a gear box / flywheel (5) and a crankshaft (6), a piston (7). The piston compresses the air in one stroke to a third of its volume, increasing the pressure to 51 bar. Consequently the air temperature will increase the to at least 1000ºC.

Next, the hot and highly pressurized air expands in the turbines, which drive the compressor (8) and generator (9) and restarts the cycle. At this point the starter engine automatically disconnects itself from the compressor. The air leaving the turbines is still hot enough for use in a (organic) Rankine cycle and generate extra electricity. See figure I, II and III.

Note: The piston can also be powered by a hydraulic cylinder, unfortunately the length of a cylinder stroke sometimes varies a few millimeters, which might cause performance / efficiency issues. E.g. if the designed stroke length is 200 mm, the first stroke may reach 200 mm, the second 203 mm, the third 198 mm and so on. Also a low-speed and high-torque hydraulic motor and gear-box might be more efficient. For instance, a simple epicyclic gear train could (without little energy loss) increase the 20 rpm speed of the Haegglunds CB1120 hydraulic motor (figure 5) to 60 or 600 rpm.

Figure 1
Figure II
Hydraulics powered piston 1
When the piston (1) moves up, the air from the compressor enters via a one way valve (2), e.g. a Tesla valve, the cylinder. The pressure valve (3) is closed and opens at > 50 bar pressure.
Figure III
Hydraulics powered piston 2
When the piston moves down the air is compressed to 50 bar (the one-way valve (2) stops the air being pushed back to the compressor), at which point the pressure valve (3) opens and the hot compressed air expands in the turbines which drive the compressor and generator.

Energy Efficiency Estimate


Take for instance the Siemens SGT 400 gas turbine (figure IV). It has a 16,8 bar gas flow of  39,4 kg/s (= 30.000 liters/s or its volume is reduced to 1.786 liters when compressed to 16,8 bar), generates 12,9 MW, has six combustors and a electrical efficiency of 34,8% (fuel consumption 37 MWh).

Suppose its 6 combustors where replaced by 6 cylinders (with the same inner volume as the SGT-400's combustors), and 6 pistons powered by hydraulic motors. How much energy would be needed to compress the SGT-400 gas turbine's air flow to the temperature and pressure needed to generate 12,9 MW electric output.

Figure IV
Engineer performing maintenance on a Siemens SGT-400 gas turbine

To be on the safe side calculating energy efficiency, the pistons are powered by 12 Haegglunds CB1120 hydraulic motors. Each hydraulic motor has a maximum torque of 370 kNm (37.792 kgfm), a maximum pressure of 350 bar and a power requirement of 980 kW. See figure V.

If the energy for the 12 hydraulic motors, 11,8 MW (12 x 980 kW), would be generated by a gas turbine with the same electrical efficiency as the SGT-400 gas turbine (34,8%), 3,1 MWh can be saved. [ SGT-400 fuel consumption of 37 MWh - 33,9 MWh (energy requirement of 12 hydraulic motors: 11,8 MW / 34,8% x 100) = 33,9 MWh].

Actual energy savings are likely to be higher. The combined torque of 12 Haegglunds CB1120 hydraulic motors is 4.440 kNm, or 740 kNm (75.459 kgfm) per cylinder. This kind of force could easily lift a  M1 Abrahams tank which ways 54.886 kg!

A torque of 540 kNm (55.064 kgfm) should be enough power to compress a 1.786 liters (or 298 liters per cylinder) air flow with a 16,8 bar pressure, so the real power requirement to drive a piston will be closer to 1500 kW. This would result in total power requirement of 9.000 kW (9 MW).

Figure V

Overview specifications Haegglunds hydraulic motors

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