A high-efficiency solar Rankine engine with isothermal expansion

Review of low-temperature vapour power cycle engines with quasi-isothermal expansion

“…Cascading multiple cycles would give better energy recovery than single closed or open cycles; however, the open cycle is the most applicable. For an operating pressure of 10 bar, the potential performance of the liquid nitrogen open cycle cryogenic engine can have an ideal efficiency of 19% and specific work of 200 kJ/kg; but this performance is quite low in comparison to the 27% ideal efficiency predicted for a surface heated engine operating on steam with similar pressure [44], and the reported 318 kJ/kg specific work achieved for a real surface heated (steam jacketed) engine operated at 5 bar [43]. Furthermore, unlike the surface heated engine, although the cryogenic engine has the advantage of not losing heat to the ambient (since it can be at the same or even lower temperature potential with the ambient), it could suffer evaporative losses and safety (explosion) risks at the storage tank; as it slowly absorbs heat from the ambient, which could result to excessive pressure rise [72].…”

“…The speed of heat transfer is a limiting factor in the steam jacket design, but for certain applications speed is not considered critical. Igobo and Davies [44] proposed to use a slow-moving machine to drive a high pressure reverse osmosis process for desalination that works on a cycle time of the order of minutes. In this case, it was proposed to eliminate the traditional boiler (evaporator) of the steam engine and rely on the jacket (which may be thermal oil fed) alone for heat transfer to vaporise the working fluid and to maintain quasi-isothermal expansion.…”

“…Fig 4a shows a case where the expansion starts from a super-heated state [42]; while in fig [43]; while in Fig 4b, the expansion starts from a saturated state [43]. [44]. As a result of the heat addition during the expansion process, the spent vapour becomes significantly superheated at the end of expansion, thus necessitating regeneration (utilising the heat available at the end of expansion to preheat the feed working fluid) to improve the thermal efficiency.…”

A cost-effective steam-driven RO plant for brackish groundwater

“…For ease of calculation it is nonetheless assumed to be isothermal. A similar study was performed using a hot oil jacket surrounding the power cylinder leading to the following expression for efficiency [17].…”

Low-temperature organic Rankine cycle engine with isothermal expansion for use in desalination

“…The system required a starting driving force of 28 kN for a 200 mm bore power cylinder and a cycle time of about 4 minutes. Igobo and Davies [13] reported a water/steam Rankine cycle power cylinder with isothermal expansion operated at cylinder wall temperature of 240 o C; the study indicated output force of up to 13 kN for 100 mm bore cylinder, and it was concluded that the isothermal Rankine cycle engine can be easily scaled-up to drive DesaLink. The isothermal expansion was preferred over the conventional adiabatic (isentropic) expansion for higher thermal efficiency.…”