Transcritical cycle
[1] Other typical applications of transcritical cycles to the purpose of power generation are represented by organic Rankine cycles,[2] which are especially suitable to exploit low temperature heat sources, such as geothermal energy,[3] heat recovery applications[4] or waste to energy plants.
[6] This evidences the extreme potential of transcritical cycles to the purpose of producing the most power (measurable in terms of the cycle specific work) with the least expenditure (measurable in terms of spent energy to compress the working fluid).
During the heating phase, which is typically considered an isobaric process, the working fluid overcomes the critical temperature, moving thus from the liquid to the supercritical phase without the occurrence of any evaporation process, a significant difference between subcritical and transcritical cycles.
[11] Due to this significant difference in the heating phase, the heat injection into the cycle is significantly more efficient from a second law perspective, since the average temperature difference between the hot source and the working fluid is reduced.
Modern ultrasupercritical Rankine cycles can reach maximum temperatures up to 620°C exploiting the optimized heat introduction process.
The typical conceptual configuration of a transcritical cycle employs a single heater,[14][15] thanks to the absence of drastic phase change from one state to another, being the pressure above the critical one.
Evaporators accomplish fluid evaporation process (typically up to the saturated vapour conditions) and in superheaters the working fluid is heated form the saturated vapour conditions to a superheated vapor.
In transcritical cycles, the very high maximum pressures and the liquid conditions along the whole compression phase ensure a higher density and a lower specific volume with respect to supercritical counterparts.
Organic cycles are appropriate choices for low enthalpy applications and are characterized by higher average densities across the expanders than those occurring in transcritical steam cycles: for this reason a low blade height is normally designed [18] and the volumetric flow rate is kept limited to relatively small values.
On the other hand in large scale application scenarios the expander blades typically show heights that exceed one meter and that are exploited in the steam cycles.
For this reason, at fixed boundary conditions, power produced and working fluid, a lower mass flow rate is expected in transcritical cycles than in other configurations.
In the last decades, the thermal efficiency of Rankine cycles increased drastically, especially for large scale applications fueled by coal: for these power plants, the application of ultrasupercritical layouts was the main factor to achieve the goal, since the higher pressure ratio ensures higher cycle efficiencies.
In large scale applications, ultrasupercritical Rankine cycles employ up to 10 feedwater heaters, five on the high pressure side and five on the low pressure side, including the deaerator, helping in the increment of the temperature at the inlet of the boiler up to 300°C, allowing a significant regenerative air preheating, thus reducing the fuel consumption.
Studies on the best performant configurations of supercritical rankine cycles (300 bar of maximum pressure, 600°C of maximum temperature and two reheats) show that such layouts can achieve a cycle efficiency higher than 50%, about 6% higher than subcritical configurations.
[19] Organic Rankine cycles are innovative power cycles which allow good performances for low enthalpy thermal sources[20] and ensure condensation above the atmospheric pressure, thus avoiding deaerators and large cross sectional area in the heat rejection units.
Moreover, with respect to steam Rankine cycles, ORC have a higher flexibility in handling low power sizes, allowing significant compactness.
The cost of carbon dioxide is two order of magnitude lower than the ones of the average refrigerant working fluid and the environmental impact of carbon dioxide is very limited (with a GWP of 1 and an ODP of 0), the fluid is not reactive nor significantly toxic.
No other working fluids for refrigeration is able to reach the same environmental favourable characteristics of carbon dioxide.
