Organic Rankine Cycle
Overview
 
The Organic Rankine cycle (ORC) is named for its use of an organic, high molecular mass fluid
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...

 with a liquid-vapor phase change
Phase transition
A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....

, or boiling point
Boiling point
The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....

, occurring at a lower temperature than the water-steam phase change. The fluid allows Rankine cycle
Rankine cycle
The Rankine cycle is a cycle that converts heat into work. The heat is supplied externally to a closed loop, which usually uses water. This cycle generates about 90% of all electric power used throughout the world, including virtually all solar thermal, biomass, coal and nuclear power plants. It is...

 heat recovery from lower temperature sources such as biomass combustion, industrial waste heat
Waste heat
Waste heat sometimes called Secondary heat or Low-grade heat refers to heat produced by machines, electrical equipment and industrial processes for which no useful application is found. Energy is often produced by a heat engine, running on a source of high-temperature heat...

, geothermal heat, solar ponds etc. The low-temperature heat is converted into useful work, that can itself be converted into electricity.
Encyclopedia
The Organic Rankine cycle (ORC) is named for its use of an organic, high molecular mass fluid
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...

 with a liquid-vapor phase change
Phase transition
A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....

, or boiling point
Boiling point
The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....

, occurring at a lower temperature than the water-steam phase change. The fluid allows Rankine cycle
Rankine cycle
The Rankine cycle is a cycle that converts heat into work. The heat is supplied externally to a closed loop, which usually uses water. This cycle generates about 90% of all electric power used throughout the world, including virtually all solar thermal, biomass, coal and nuclear power plants. It is...

 heat recovery from lower temperature sources such as biomass combustion, industrial waste heat
Waste heat
Waste heat sometimes called Secondary heat or Low-grade heat refers to heat produced by machines, electrical equipment and industrial processes for which no useful application is found. Energy is often produced by a heat engine, running on a source of high-temperature heat...

, geothermal heat, solar ponds etc. The low-temperature heat is converted into useful work, that can itself be converted into electricity. A prototype was first developed and exhibited in 1961 by solar engineers Harry Zvi Tabor
Harry Zvi Tabor
Harry Zvi Tabor is an Israeli physicist. He is known as the father of Israeli solar energy. He is generally credited with having brought Israel's solar energy program to international prominence.- Background :...

 and Lucien Bronicki.

Working principle of the ORC

The working principle of the organic Rankine cycle is the same as that of the Rankine cycle
Rankine cycle
The Rankine cycle is a cycle that converts heat into work. The heat is supplied externally to a closed loop, which usually uses water. This cycle generates about 90% of all electric power used throughout the world, including virtually all solar thermal, biomass, coal and nuclear power plants. It is...

: the working fluid is pumped to a boiler where it is evaporated, passes through a turbine and is finally re-condensed.

In the ideal cycle, the expansion is isentropic
Isentropic process
In thermodynamics, an isentropic process or isoentropic process is one in which for purposes of engineering analysis and calculation, one may assume that the process takes place from initiation to completion without an increase or decrease in the entropy of the system, i.e., the entropy of the...

 and the evaporation and condensation processes are isobaric
Isobaric process
An isobaric process is a thermodynamic process in which the pressure stays constant. The term derives from the Greek isos, , and barus,...

.

In the real cycle, the presence of irreversibilities
Irreversibility
In science, a process that is not reversible is called irreversible. This concept arises most frequently in thermodynamics, as applied to processes....

 lowers the cycle efficiency
Thermal efficiency
In thermodynamics, the thermal efficiency is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, a boiler, a furnace, or a refrigerator for example.-Overview:...

. 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
    Pressure drop is a term used to describe the decrease in pressure from one point in a pipe or tube to another point downstream. "Pressure drop" is the result of frictional forces on the fluid as it flows through the tube. The frictional forces are caused by a resistance to flow. The main...

    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
    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. When the surroundings are the reservoir, exergy is the potential of a system to cause a change as it achieves equilibrium with its...

     destruction and reduces the cycle performance.

Improvement of the organic Rankine cycle

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
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

 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

The organic Rankine cycle technology has many possible applications, and counts more than 250 identified power plants worldwide . Among them, the most widespread and promising fields are the following:
  • Waste heat
    Waste heat
    Waste heat sometimes called Secondary heat or Low-grade heat refers to heat produced by machines, electrical equipment and industrial processes for which no useful application is found. Energy is often produced by a heat engine, running on a source of high-temperature heat...

     recovery

Waste heat recovery is the most important development field for the Organic Rankine Cycle (ORC). It can be applied to heat and power plants (for example a small scale cogeneration
Cogeneration
Cogeneration is the use of a heat engine or a power station to simultaneously generate both electricity and useful heat....

 plant on a domestic water heater), or to industrial and farming processes such as organic products fermentation, hot exhausts from ovens or furnaces, flue gas condensation
Flue gas condensation
Flue gas condensation is a process, where flue gas is cooled below its water dew point and the heat released by the resulting condensation of water is recovered as low temperature heat....

, exhaust gases from vehicles, intercooling of a compressor, condenser of a power cycle, etc.
  • Biomass
    Biomass
    Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....

     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
    Geothermal power
    Geothermal energy is thermal energy generated and stored in the Earth. Thermal energy is the energy that determines the temperature of matter. Earth's geothermal energy originates from the original formation of the planet and from radioactive decay of minerals...


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
    Solar thermal energy
    Solar thermal energy is a technology for harnessing solar energy for thermal energy . Solar thermal collectors are classified by the United States Energy Information Administration as low-, medium-, or high-temperature collectors. Low-temperature collectors are flat plates generally used to heat...


The organic Rankine cycle can be used in the solar parabolic trough
Parabolic trough
A parabolic trough is a type of solar thermal energy collector. It is constructed as a long parabolic mirror with a Dewar tube running its length at the focal point. Sunlight is reflected by the mirror and concentrated on the Dewar tube...

 technology in place of the usual steam Rankine cycle. The ORC allows a lower collector temperature, a better collecting efficiency (reduced ambient losses) and hence the possibility of reducing the size of the solar field.

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
    Saturation Vapor Curve
    The saturation vapor curve usually states for the separation curve between the two-phase state and the vapor state in the T-s diagram. When used in a power cycle, the fluid expansion depends strongly on the nature of this saturation 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
Ozone depletion potential
The ozone depletion potential of a chemical compound is the relative amount of degradation to the ozone layer it can cause, with trichlorofluoromethane being fixed at an ODP of 1.0. Chlorodifluoromethane , for example, has an ODP of 0.055...

 (ODP) and the global warming potential
Global warming potential
Global-warming potential is a relative measure of how much heat a greenhouse gas traps in the atmosphere. It compares the amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar mass of carbon dioxide. A GWP is calculated over a specific 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

See also

  • Rankine cycle
    Rankine cycle
    The Rankine cycle is a cycle that converts heat into work. The heat is supplied externally to a closed loop, which usually uses water. This cycle generates about 90% of all electric power used throughout the world, including virtually all solar thermal, biomass, coal and nuclear power plants. It is...

  • Thermodynamic cycle
    Thermodynamic cycle
    A thermodynamic cycle consists of a series of thermodynamic processes transferring heat and work, while varying pressure, temperature, and other state variables, eventually returning a system to its initial state...

  • Relative cost of electricity generated by different sources
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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