Brayton cycle

A heat engine moves heat from a high temperature area (Th), to a lower temperature sink (Tc), in order to generate power. The Brayton cycle is a thermodynamic engine that can be used for power (usually electric) generation. It uses a gas as a working medium and both temperature and pressure changes to move the energy in the cycle.

In this cycle a hot gas leaves a heat source, turns a turbine, is cooled in a heat exchanger, runs through a compressor and returns to the heat source. The turbine powers the compressor, and part of the turbine power is used to power a turbo-alternator to make electricity.

• An airplane turbo engine is an example of an open Brayton cycle.

• A nuclear reactor power plant using helium cooling is an example of a closed Brayton cycle.

• The ideal equation for a Brayton cycle is efficiency(ef) = 1-Tc/Th.

• A real Brayton engine has a number of inefficiencies and non-reversible, so the actual efficiency is significantly lower.

For a high efficiency Brayton cycle, Th needs to be significantly higher than Tc.

• For example, with an efficiency of 40% Tc/Th is 0,6. If Tc is 500K, then Th is 830K.

• For the same example at 60% efficiency, Tc/Th=0,4, and for a TC of 500K Th is 1250K.

If applied to a closed cycle used on Mars, Tc is the radiator temperature, and Th would be the temperature of the reactor core. Note that Mars has large cold sinks, so in principle, heat engines would be more efficient on Mars than on Earth. However, dumping heat on Earth is easier (into the air and rivers), so it may be an engineering challenge to dump large amounts of waste heat on Mars.

The Rankine cycle is a common alternative cycle, using phase change from gas liquid and back to gas. It is used in most thermal electric plants and in many cooling/refrigeration cycles.