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thermal engineering Thermal engineering is a specialized sub-discipline of mechanical engineering that deals with the movement of heat energy and transfer. The energy can be transferred between two mediums or transformed into other forms of energy. A thermal engineer ...
, the Organic Rankine Cycle (ORC) is a type of
thermodynamic cycle A thermodynamic cycle consists of a linked sequence of thermodynamic processes that involve transfer of heat and work into and out of the system, while varying pressure, temperature, and other state variables within the system, and that eventu ...
. It is a variation of the
Rankine cycle The Rankine cycle is an idealized thermodynamic cycle describing the process by which certain heat engines, such as steam turbines or reciprocating steam engines, allow mechanical work to be extracted from a fluid as it moves between a heat sou ...
named for its use of an organic, high- molecular-mass fluid whose vaporization temperature is lower than that of
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
. The fluid allows heat recovery from lower-temperature sources such as biomass combustion, industrial
waste heat Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing work. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste heat has lower utilit ...
,
geothermal heat Geothermal heating is the direct use of geothermal energy for some heating applications. Humans have taken advantage of geothermal heat this way since the Paleolithic era. Approximately seventy countries made direct use of a total of 270 PJ of ...
, solar ponds etc. The low-temperature heat is converted into useful work, that can itself be converted into electricity. The technology was developed in the late 1950s by Lucien Bronicki and Harry Zvi Tabor.Harry Zvi Tabor
Cleveland Cutler, Encyclopedia of the Earth, 2007.
Israeli Section of the International Solar Energy Society
, edited by Gershon Grossman, Faculty of Mechanical Energy, Technion,
Haifa Haifa ( he, חֵיפָה ' ; ar, حَيْفَا ') is the third-largest city in Israel—after Jerusalem and Tel Aviv—with a population of in . The city of Haifa forms part of the Haifa metropolitan area, the third-most populous metropoli ...
; Final draft.
Naphtha engines, similar in principle to ORC but developed for other applications, were in use as early as the 1890s.


Working principle of the ORC

The working principle of the organic Rankine cycle is the same as that of the
Rankine cycle The Rankine cycle is an idealized thermodynamic cycle describing the process by which certain heat engines, such as steam turbines or reciprocating steam engines, allow mechanical work to be extracted from a fluid as it moves between a heat sou ...
: the working fluid is pumped to a
boiler A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, centr ...
where it is evaporated, passed through an expansion device (turbine, screw, scroll, 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: * 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 drop Pressure drop is defined as the difference in total pressure between two points of a fluid carrying network. A pressure drop occurs when frictional forces, caused by the resistance to flow, act on a fluid as it flows through the tube. The main d ...
s 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 In thermodynamics, the exergy of a system is the maximum useful work possible during a process that brings the system into equilibrium with a heat reservoir, reaching maximum entropy. When the surroundings are the reservoir, exergy is the po ...
destruction and reduces the cycle performance.


Applications for the ORC

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. Among them, the most widespread and promising fields are the following:


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 plant A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the generation of electric power. Power stations are generally connected to an electrical grid. Many p ...
s (for example a small scale
cogeneration Cogeneration or combined heat and power (CHP) is the use of a heat engine or power station to generate electricity and useful heat at the same time. Cogeneration is a more efficient use of fuel or heat, because otherwise- wasted heat from elec ...
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 Solar may refer to: Astronomy * Of or relating to the Sun ** Solar telescope, a special purpose telescope used to observe the Sun ** A device that utilizes solar energy (e.g. "solar panels") ** Solar calendar, a calendar whose dates indicate t ...
parabolic trough A parabolic trough is a type of solar thermal collector that is straight in one dimension and curved as a parabola in the other two, lined with a polished metal mirror. The sunlight which enters the mirror parallel to its plane of symmetry is fo ...
technology in place of the usual steam Rankine cycle. The ORC allows electricity generation at lower capacities and lower collector temperature, and hence the possibility for low-cost, small scale decentralized
CSP CSP may refer to: Education * College Student Personnel, an academic discipline * Commonwealth Supported Place, a category in Australian education * Concordia University (Saint Paul, Minnesota), US Organizations * Caledonian Steam Packet Compa ...
units. The ORC also enables hybrid CSP-PV systems equipped with
thermal energy storage Thermal energy storage (TES) is achieved with widely different technologies. Depending on the specific technology, it allows excess thermal energy to be stored and used hours, days, months later, at scales ranging from the individual process, ...
to provide on-demand recovery of up to 70% of their instantaneous electricity generation, and can be a fairly efficient alternative to other types of electrical storage.


Windthermal energy

Recently so called windthermal energy turbines are discussed that could convert wind energy directly into medium temperature heat (up to 600°C). They can be combined with a thermal storage and could suitably be matched with ORC to generate electricity.


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 The ozone depletion potential (ODP) of a chemical compound is the relative amount of degradation to the ozone layer it can cause, with trichlorofluoromethane (R-11 or CFC-11) being fixed at an ODP of 1.0. Chlorodifluoromethane (R-22), for examp ...
(ODP) and the
global warming potential Global warming potential (GWP) is the heat absorbed by any greenhouse gas in the atmosphere, as a multiple of the heat that would be absorbed by the same mass of carbon dioxide (). GWP is 1 for . For other gases it depends on the gas and the time ...
(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 Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are fully or partly halogenated hydrocarbons that contain carbon (C), hydrogen (H), chlorine (Cl), and fluorine (F), produced as volatile derivatives of methane, ethane, and propan ...
: Banned by
Montreal Protocol The Montreal Protocol is an international treaty A treaty is a formal, legally binding written agreement between actors in international law. It is usually made by and between sovereign states, but can include international organization ...
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 High-fructose corn syrup (HFCS), also known as glucose–fructose, isoglucose and glucose–fructose syrup, is a sweetener made from corn starch. As in the production of conventional corn syrup, the starch is broken down into glucose by enzy ...
(e.g. R134a,
R245fa 1,1,1,3,3-Pentafluoropropane (HFC-245fa) is a hydrofluorocarbon is a colorless gas used primarily for closed-cell spray foam insulation. HFC-245fa is also known as pentafluoropropane and by its chemical name 1,1,1,3,3-pentafluoropropane. Environ ...
) * HCs:
Flammable A combustible material is something that can burn (i.e., ''combust'') in air. A combustible material is flammable if it ignites easily at ambient temperatures. In other words, a combustible material ignites with some effort and a flammable mat ...
, common
by-product A by-product or byproduct is a secondary product derived from a production process, manufacturing process or chemical reaction; it is not the primary product or service being produced. A by-product can be useful and marketable or it can be consid ...
s of gas processing facilities (e.g. isobutane,
pentane Pentane is an organic compound with the formula C5H12—that is, an alkane with five carbon atoms. The term may refer to any of three structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, pentane means exclusively the ' ...
,
propane Propane () is a three-carbon alkane with the molecular formula . It is a gas at standard temperature and pressure, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as ...
) * PFCs


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 & 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 The Rankine cycle is an idealized thermodynamic cycle describing the process by which certain heat engines, such as steam turbines or reciprocating steam engines, allow mechanical work to be extracted from a fluid as it moves between a heat sou ...
*
Thermodynamic cycle A thermodynamic cycle consists of a linked sequence of thermodynamic processes that involve transfer of heat and work into and out of the system, while varying pressure, temperature, and other state variables within the system, and that eventu ...
*
Relative cost of electricity generated by different sources Different methods of electricity generation can incur a variety of different costs, which can be divided into three general categories: 1) wholesale costs, or all costs paid by utilities associated with acquiring and distributing electricity to ...
* Naphtha launch * Working fluids


References


External links


Knowledge Center on Organic Rankine Cycle
{{Thermodynamic cycles, state=uncollapsed Thermodynamic cycles