Organic Rankine cycle

T-s diagram for the ideal/real ORC

The working principle of the organic Rankine cycle is the same as that of the Rankine cycle: the working fluid is pumped to a boiler where it is evaporated, passed through an expansion device (turbine,[3] screw,[4] scroll,[5] or other expander), and then through a condenser heat exchanger where it is finally re-condensed.

In the ideal cycle described by the engine's theoretical model, the expansion is isentropic and the evaporation and condensation processes are isobaric.

In any real cycle, the presence of irreversibilities lowers the cycle efficiency. Those irreversibilities mainly occur:[6]

• During the expansion: Only a part of the energy recoverable from the pressure difference is transformed into useful work. The other part is converted into heat and is lost. The efficiency of the expander is defined by comparison with an isentropic expansion.
• In the heat exchangers: The working fluid takes a long and sinuous path which ensures good heat exchange but causes pressure drops that lower the amount of power recoverable from the cycle. Likewise, the temperature difference between the heat source/sink and the working fluid generates exergy destruction and reduces the cycle performance.

In the case of a "dry fluid", the cycle can be improved by the use of a regenerator: since the fluid has not reached the two-phase state at the end of the expansion, its temperature at this point is higher than the condensing temperature. This higher temperature fluid can be used to preheat the liquid before it enters the evaporator.

A counter-current heat exchanger (gas to liquid) is thus installed between the expander outlet and the condenser inlet. The power required from the heat source is therefore reduced and the efficiency is increased.

75 kW ORC turbogenerator used at an experimental power plant in Lappeenranta, Finland

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The organic Rankine cycle technology has many possible applications, and counts more than 2.7 GW of installed capacity and 698 identified power plants worldwide.[7] Among them, the most widespread and promising fields are the following:[8]

The selection of the working fluid is of key importance in low temperature Rankine Cycles. Because of the low temperature, heat transfer inefficiencies are highly prejudicial. These inefficiencies depend very strongly on the thermodynamic characteristics of the fluid and on the operating conditions.

In order to recover low-grade heat, the fluid generally has a lower boiling temperature than water. Refrigerants and hydrocarbons are two commonly used components.

Optimal characteristics of the working fluid :

• Isentropic saturation vapor curve :

Since the purpose of the ORC focuses on the recovery of low grade heat power, a superheated approach like the traditional Rankine cycle is not appropriate. Therefore, a small superheating at the exhaust of the evaporator will always be preferred, which disadvantages "wet" fluids (that are in two-phase state at the end of the expansion). In the case of dry fluids, a regenerator should be used.

• Low freezing point, high stability temperature :

Unlike water, organic fluids usually suffer chemical deteriorations and decomposition at high temperatures. The maximum hot source temperature is thus limited by the chemical stability of the working fluid. The freezing point should be lower than the lowest temperature in the cycle.

• High heat of vaporisation and density :

A fluid with a high latent heat and density will absorb more energy from the source in the evaporator and thus reduce the required flow rate, the size of the facility, and the pump consumption.

• Low environmental impact

The main parameters taken into account are the Ozone depletion potential (ODP) and the global warming potential (GWP).

• Safety

The fluid should be non-corrosive, non-flammable, and non-toxic. The ASHRAE safety classification of refrigerants can be used as an indicator of the fluid dangerousness level.

• Good availability and low cost
• Acceptable pressures

Simulating ORC cycles requires a numerical solver in which the equations of mass and energy balance, heat transfer, pressure drops, mechanical losses, leakages, etc. are implemented. ORC models can be subdivided into two main types: steady-state and dynamic. Steady-state models are required both for design (or sizing) purpose, and for part-load simulation. Dynamic models, on the other hand, also account for energy and mass accumulation in the different components. They are particularly useful to implement and simulate control strategies, e.g. during transients or during start & Another key aspects of ORC modeling is the computation of the organic fluid thermodynamic properties. Simple equation of states (EOS) such as Peng–Robinson should be avoided since their accuracy is low. Multiparameter EOS should be preferred, using e.g. state-of-the-art thermophysical and transport properties databases.

Various tools are available for the above purposes, each presenting advantages and drawbacks. The most common ones are reported hereunder.

• Rankine cycle
• Thermodynamic cycle
• Relative cost of electricity generated by different sources
• Naphtha launch
• Working fluids

• Knowledge Center on Organic Rankine Cycle