Transcritical cycle

A transcritical cycle is a closed thermodynamic cycle where the working fluid goes through both subcritical and supercritical states. In particular, for power cycles the working fluid is kept in the liquid region in the compression phase and in vapour and supercritical conditions in the expansion phase. The ultrasupercritical steam Rankine cycle represents a widespread transcritical cycle in the electricity generation field from fossil fuels, where water is used as working fluid. Other typical applications of transcritical cycles for power generation are represented by Organic Rankine cycles, especially suitable for low temperature heat sources, as geothermal energy, heat recovery applications or waste to energy plants. With respect to subcritical cycles, the transcritical cycle exploits by definition higher pressure ratios, hence higher efficiencies for the majority of the working fluids. On the other hand, considering supercritical cycles as alternative, the transcritical cycle can reach higher specific works due to the limited relative importance of the compression work.

While in supercritical cycles both pressure levels are above the critical pressure of the working fluid, in transcritical cycles one pressure level is above the critical pressure and the other is below. In the refrigeration field carbon dioxide, CO₂, is increasingly considered of interest as refrigerant.

Transcritical conditions of the working fluid

Phase diagram of a generic fluid

TQ diagram of heat introduction to a subcritical and transcritical cold source

In trascritical cycles, the pressure of the working fluid at the outlet of the pump is higher than the critical pressure, while the inlet conditions are close to the saturated liquid pressure at the given minimum temperature.

During the heating phase, an isobaric process, the working fluid overcome the critical temperature, going form the liquid phase to the supercritical phase, with no evaporation process, a significant difference between subcritical and transcritical cycles. 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.

As a consequence, the maximum temperatures reached by the cold source can be higher, at constant hot source characteristics, and the expansion can exploit highest pressure ratios, hence higher power. Modern ultrasupercritical Rankine cycles can reach maximum temperatures up to 620°C exploiting the optimized heat introduction process.

Characterization of the power cycle

Ts diagrams of subcritical (yellow), transcritical (blue) and supercritical (red) power cycles

Plant layouts of transcritical power cycles (top left), supercritical power cycles (top right), subritical power cycles (bottom)

As in any power cycle, the most important indicator of its performance is the thermal efficiency. The thermal efficiency of a transcritical cycle is computed as:

$\eta _{cycle}={\frac {W_{Cycle}}{Q_{in}}}$

where $Q_{in}$ is the thermal input of the cycle, provided by either combustion or with an heat exchanger, and $W_{Cycle}$ is the power produced by the cycle.

The power produced is considered comprehensive of the produced power during the expansion process of the working fluid and the one consumed during the compression step.

The typical conceptual configuration of a transcritical cycle employs a single heater, thanks to the absence of drastic phase change from one state to another, being the pressure above the critical one. In subcritical cycles, instead, the heating of the working fluid occurs in three different heat exchangers: in economizers the working fluid is heated in the liquid phase, up to a condition close to the saturated liquid, in Evaporators the fluid evaporates and in Superheaters the working fluid is heated form the saturated vapour conditions to a superheated vapor. Moreover, if Rankine cycles used as bottom cycles in combined cycles are considered, their configuration is always subcritical, with multiple pressure levels and hence multiple evaporators, economizers and superheaters, introducing a significant complication to the heat injection process in the cycle .

Characterization of the compression process

Pump diagram

Compressor Stage GE J79

Along an adiabatic and isentropic process, as the one theoretically accounted for in pumps of transcritical cycles, the enthalpy difference across both a compression and an expansion is computed as: $\Delta h=\int _{Pmax}^{Pmin}v\cdot dP$

Consequentlty, a working fluid with a lower specific volume (hence higher density) can inevitably be compressed spending a lower mecanical work than one with low density (more gas like). 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. Considering the different physical phases where the compression occurs, transcritical cycles employs pumps and supercritical ones uses compressors in the compression step.

Characterization of the expansion process

Traditional classification of expansions with pure working fluids

TurbineBlades

In the expansion step of the working fluid in transcritical cycles, as in subcritical ones, the working fluid can be discharged either in wet or dry conditions.

Typical dry expansions are the ones involving organic or other unconventional working fluid, characterized by a not negligible molecular complexity and high molecular weight.

The expansion step occurs in turbines: depending on the application and on the nameplate power produced by the power plant, both axial turbines and radial turbines can be exploited in the expansion. Axial turbines favours lower rotational speed and higher power production, while radial turbines are suitable for limited powers produced and high rotational speed.

Organic cycles, typical for low enthalpy applications, are characterized by a higher average density across the expansion, with respect to transcritical steam cycles: for this reason a low blade height is expected and the volumetric flow rates is limited. On the other hand, for large scale applications, blade heights over one meter be exploited in steam cycles, where the density at the outlet of the last stage is significantly low.

In general, the specific work of the cycle si expressed as: $w_{Cycle}={\frac {Power_{expansion}-Power_{compression}}{{\dot {m}}_{compression}}}$

Even though the specific work of any cycle is strongly dependant on the actual working fluid considered in the cycle, it is expected for transcritical cycles to have higher specific work than the respective subcritical and supercritical cycle exploiting the same working fluid. For this reason, a lower mass flow rate is expected in transcritical cycles than in other configurations, at constant boundary conditions, power produced and working fluid selection.

Applications in power cycles

Ultrasupercritical Rankine cycles

Simplified layout of a coal fired power plant

In the last decades, the thermal efficiency of Rankine cycles increased drastically, especially for large scale applications fueled by coal: for these power plant, the application of ultrasupercritical layouts was the main factor to achieve the goal, since the higher the pressure ratio ensures higher cycle efficiencies.

The increment in thermal efficiency of power plants fueled by dirty fuels became crucial also in the reduction of the specific emmissions of the plants, both in therms of greenhouse gas and for pollutant such as sulfur dioxide or NOx. 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.

Organic Rankine cycles

Internal view of Plug-and-Play Micro-ORC

Ts Diagram of subcritical and transcritical R134a cycles

Organic Rankine cycles are innovative power cycles which allow good performances for low enthalpy thermal sources and ensure condensation above the atmosferic 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 flexibilty in handling low power sizes, allowing significant compactness. Typical applications of ORC cover: waste heat recovery plants, geothermal plants, biomass plants and waste to energy power plants.

Organic Rankine cycles use organic fluids (such as hydrocarbons, perfluorocarbons, Chlorofluorocarbon, and many others) as working fluids. Most of them have a critical temperature in the range of 100-200°C, for this reason perfectly adaptable to transcritical cycles in low temperature applications. Considering organic fluids, having a maximum pressure above the critical one can more than double the temperature difference across the turbine, with respect to the subcritical counterpart, and significantly increase both the cycle specific work and cycle efficiency.

Applications in refrigeration cycles

Ts diagram of a transcritical heat pump: cold source (yellow), hot source (red), transcritical cycle (blue)

A subcritical refrigeration cycle, where heat rejection occurs at a pressure lower than the critical one

A refrigeration cycle, also known as heat pump, is a thermodynamic cycle that allow the removal of heat from a low temperature heat source and the rejection of heat into a high temperature heat source, thanks to mechanical power consumption. Traditional refrigeration cycles are subcritical, with the high pressure side (where heat rejection occurs) below the critical pressure.

Innovative transcritical refrigeration cycles, instead, should use a working fluid whose critical temperature is around the ambient temperature. For this reason, carbon dioxide is chosen due to its favourable critical conditions. In fact, the critical point of carbon dioxide is 31°C, reasonably in between the hot source and cold source of traditional refrigeration applications, thus suitable for a trascritical applications. In transcritical refrigeration cycle, the heat is dissipated through a gas cooler instead of a desuperheter and a condenser, like in subcritical cycles, thus limiting the plant components, plant complexity and costs of the power block.

The advantages of using carbon dioxide as working fluid, instead of traditional refrigerant fluids (like HFC of HFO), in refrigeration cycles is represented both by economic aspects and environmental ones. On one hand, the cost of carbon dioxide is two order of magnitude lower than the ones of the average refrigerant working fluid, on the other hand the environmental impact of carbon dioxide is very limited (with a GWP of 1 and an ODP of 0), the fluid is not toxic nor reactive. No other working fluids for refrigeration is able to reach the environmental favourable characteristics of carbon dioxide.