Organic Rankine cycle

Thermodynamics
The classical Carnot heat engine
Branches Classical Statistical Chemical Equilibrium / Non-equilibrium
Laws Zeroth First Second Third
Systems State Equation of state Ideal gas Real gas State of matter Equilibrium Control volume Instruments Processes Isobaric Isochoric Isothermal Adiabatic Isentropic Isenthalpic Quasistatic Polytropic Free expansion Reversibility Irreversibility Endoreversibility Cycles Heat engines Heat pumps Thermal efficiency
System properties Note: Conjugate variables in italics Property diagrams Intensive and extensive properties Functions of state Temperature / Entropy (introduction) Pressure / Volume Chemical potential / Particle number Vapor quality Reduced properties Process functions Work Heat
Material properties Property databases Specific heat capacity  Compressibility  Thermal expansion
Equations Carnot's theorem Clausius theorem Fundamental relation Ideal gas law Maxwell relations Onsager reciprocal relations Bridgman's equations Table of thermodynamic equations
Potentials Free energy Free entropy Internal energy Enthalpy Helmholtz free energy Gibbs free energy
History Culture History General Heat Entropy Gas laws "Perpetual motion" machines Philosophy Entropy and time Entropy and life Brownian ratchet Maxwell's demon Heat death paradox Loschmidt's paradox Synergetics Theories Caloric theory Theory of heat Vis viva ("living force") Mechanical equivalent of heat Motive power Key publications "An Experimental Enquiry Concerning ... Heat" "On the Equilibrium of Heterogeneous Substances" "Reflections on the Motive Power of Fire" Timelines Thermodynamics Heat engines Art Education Maxwell's thermodynamic surface Entropy as energy dispersal
Scientists Bernoulli Carnot Clapeyron Clausius Carathéodory Duhem Gibbs von Helmholtz Joule Maxwell von Mayer Onsager Rankine Smeaton Stahl Thompson Thomson Waterston
Book:Thermodynamics

The organic Rankine cycle (ORC) is named for its use of an organic, high molecular mass fluid with a liquid-vapor phase change, or boiling point, occurring at a lower temperature than the water-steam phase change. The fluid allows Rankine cycle heat recovery from lower temperature sources such as biomass combustion, industrial waste heat, geothermal heat, solar ponds etc. The low-temperature heat is converted into useful work, that can itself be converted into electricity.

Naphtha engines, similar in principle to ORC but developed for other applications, had been in use since the 1890s.

Working principle of the ORC

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 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:^[1]

• 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.

Improvement of the organic Rankine cycle

ORC with Regenerator

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 evaporator inlet. The power required from the heat source is therefore reduced and the efficiency is increased.

Applications for the ORC

75-kilowatt ORC turbogenerator originally used at an experimental power plant in Kuopio, Finland.

Part of a series about
Sustainable energy
Energy conservation
Cogeneration Efficient energy use Green building Heat pump Low-carbon power Microgeneration Passive solar building design
Renewable energy
Anaerobic digestion Geothermal Hydroelectricity Solar Tidal Wind
Sustainable transport
Carbon-neutral fuel Electric vehicle Fossil fuel phase-out Green vehicle Plug-in hybrid
Sustainable development portal Renewable energy portal Environment portal

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.^[2] Among them, the most widespread and promising fields are the following:^[3]

Waste heat recovery

Waste heat recovery is one of the most important development fields for the organic Rankine cycle (ORC). It can be applied to heat and power plants (for example a small scale cogeneration plant on a domestic water heater), or to industrial and farming processes such as organic products fermentation, hot exhausts from ovens or furnaces (e.g. lime and cement kilns), flue-gas condensation, exhaust gases from vehicles, intercooling of a compressor, condenser of a power cycle, etc.

Biomass power plant

Biomass is available all over the world and can be used for the production of electricity on small to medium size scaled power plants. The problem of high specific investment costs for machinery, such as steam boilers, are overcome due to the low working pressures in ORC power plants. Another advantage is the long operational life of the machine due to the characteristics of the working fluid, that unlike steam is non eroding and non corroding for valve seats tubing and turbine blades. The ORC process also helps to overcome the relatively small amount of input fuel available in many regions because an efficient ORC power plant is possible for smaller sized plants.

Geothermal plants

Geothermic heat sources vary in temperature from 50 to 350 °C. The ORC is therefore perfectly adapted for this kind of application. However, it is important to keep in mind that for low-temperature geothermal sources (typically less than 100 °C), the efficiency is very low and depends strongly on heat sink temperature (defined by the ambient temperature).

Solar thermal power

The organic Rankine cycle can be used in the solar parabolic trough technology in place of the usual steam Rankine cycle. The ORC allows power generation at lower capacities and with a lower collector temperature, and hence the possibility for low-cost, small scale decentralized CSP units.^[4]

Choice of the working fluid

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

Examples of working fluids

• CFCs: Banned by Montreal Protocol due to ozone depletion (e.g. R-11 R-12)
• HCFCs: Phasing out due to Copenhagen Amendment to Montreal Protocol (e.g. R-22 R-123)
• HFCs (e.g. R134a R245fa)
• HCs: Flammable, common by-products of gas processing facilities (e.g. isobutane, pentane, propane)
• PFCs^[5]

Modeling ORC systems

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 & stop.

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.

See also

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

External links

• Knowledge Center on Organic Rankine Cycle

References