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The Brayton-Barton Combined-Cycle Engine

Sunoba

Renewable Energy Systems

In principle, it should be possible to design an engine that would combine the conventional Brayton power cycle with the Barton evaporation cycle. That would give the Brayton-Barton Combined-Cycle Engine, BBCCE. An illustration of the thermodynamic cycle for such an engine is given at the bottom of this web page.

However, there are formidable difficulties in developing such an engine in piston-in-cylinder form. Chief amongst these is the disparity in size and timescales of the two cycles as highlighted below:

Brayton Cycle For the Brayton cycle to operate in a piston-in-cylinder form, you would need two separate cylinders (one for compression, the other for expansion) separated by a constant-pressure heater. For efficiency and to minimise costs, those components would be relatively small and operate fast.

Barton Cycle For the evaporation cycle of the BEE in piston-in-cylinder form, the components need to be large and operate relatively slowly so that water droplets can be injected and have time to evaporate.

Other considerations too complicated to describe here would prevent the BBCCE engine being built in continuous-flow form.

The best option would be to deploy the BEE in piston-in-cylinder form after an Open-Cycle Gas Turbine (Brayton cycle).

Thermodynamic cycle for the BBCCE

The P-V diagram below is based on the following parameters: Brayton pressure ratio 15.6; heating rate 800 kJ/kg; BEE volume expansion ratio 3.0; inlet air at 20°C and saturated; water temperature 20°C. The theoretical efficiency of this combined-cycle is high. The concept allows for heating by renewable sources (solar or geothermal) as well as in-line heating with any fuel (coal, gas, oil or biomass). The exhaust will be approximately 55-60°C and saturated.

© Sunoba Pty Ltd

6 November 2008