Thermodynamics and Energy


Heat, Work and Illusion: the Case of the Rankine Cycle

Authors: Sosale Chandrasekhar

It is argued that no net conversion of heat to work occurs in the Rankine engine, and that the putative work obtained originates in the higher pressure at which the vaporized liquid is maintained. Thus, although heat is employed to convert the working liquid to vapour, this remains as such upon performance of the work (typically, rotation of a turbine). The heat is let out to the surroundings upon condensing the vapour, which is then compressed for the next cycle of operations. This ensures that the vapour is again produced at a relatively high pressure. Thus, the expansion work obtained would only depend on the pressure drop experienced by the vapour within the turbine. Therefore, the above compression stage is the sole driver of the process. The invalidity of the Rankine cycle – much like that of the Carnot cycle – indicates the improbability of converting heat to work in a sealed system operating within a closed loop. In fact, the traditional steam engine is based on the build-up of pressure brought on by the rapid vaporization of a mass of water in a confined space. The resulting steam then expands directly within a chamber designed to capture the work. The process is driven by the transfer of a large mass of steam, with a concomitant pressure drop, through a turbine. In the Rankine cycle, however, the vapour produced initially is allowed to expand at constant pressure – thus expending its thermal energy acquired during vaporization – before being led into the turbine.

Comments: 8 Pages.

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[v1] 2015-12-18 04:33:18

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